Circularly Polarized Luminescence in Helicene and Helicenoid Derivatives

Abstract

In this chapter, we discuss the circularly polarized luminescence (CPL) of helicene and helicenoid derivatives. The organic helicenic derivatives are classified according to the type of atom (heteroatom or carbon) incorporated within the helical backbone. Transition-metal complexes and chiroptical devices incorporating helicene derivatives and exhibiting CPL activity are also presented.

4.1 Introduction

Circularly polarized luminescence (CPL) is a fascinating property of many classes of chiral emissive molecules. The well-known luminescence dissymmetry factor (g _lum) as a measure of the degree of circular polarization of emitted light is defined as g _lum = 2ΔI/I = 2(I _L − I _R)/(I _L + I _R), where I _L and I _R denote the left- and right-handed circularly polarized emission intensities, respectively. Various types of chiral fluorescent conjugated organic molecules exhibiting CPL are known and generally possess g _lum values on the order of 10⁻⁴–10⁻² [1,2,3,4,5,6]. Among them, helicenes are a special class of chiral molecules which consist of n ortho-fused aromatic rings combining a helical topology with an extended π-conjugation [7,8,9]. Since the first preparation of enantioenriched helicenes in 1956 [10], their chirality has essentially been characterized through their very large optical rotation (OR) values, and their intense and characteristic electronic circular dichroism (ECD). In more rare cases, vibrational circular dichroism (VCD) and Raman optical activity (ROA) of helicenes have been measured. More recently, strong interest has grown on the elucidation of the chirality-related emission properties of helicenes. Indeed, helicene derivatives can be regarded as helically shaped polycyclic aromatic hydrocarbons (PAHs); they thus usually display organic semiconducting behavior and are efficient chiral emitters. With the development of circularly polarized emission techniques, the CPL activity of helicenes and helicenoids is therefore being more and more investigated. In this chapter, we present the helicene derivatives that have shown to display CPL. Helically twisted acenes will not be considered here.

Theoretically, the luminescence dissymmetry factor g _lum for the electronic transition i → j can be expressed by the following equation: ${g}_{\mathrm{lum}}=4\frac{\left|{\boldsymbol{\upmu}}_{ij}\right|\bullet \left|{\mathbf{m}}_{ji}\right|\bullet \mathrm{cos}\uptheta \boldsymbol{\upmu}, \mathbf{m}}{{\left|{\boldsymbol{\upmu}}_{ij}\right|}^2+{\left|{\mathbf{m}}_{ji}\right|}^2}$, where μ _ij and m _ij are, respectively, the electric and magnetic transition dipole moment vectors, while θμ,m is the angle between them. In the case of an electronic transition, the |m _ji| term is usually small with respect to |μ _ij| so the equation becomes${g}_{\mathrm{lum}}=4\frac{\left|{\mathbf{m}}_{ji}\right|}{\left|{\boldsymbol{\upmu}}_{ij}\right|\ }\cos\ \uptheta \boldsymbol{\upmu}, \mathbf{m}$. High g _lum values are therefore usually obtained for magnetic dipole-allowed transitions. For this reason, CPL spectroscopy is widely applied to chiral lanthanide(III) complexes: the magnetically allowed intraconfigurational f → f transitions of lanthanide metal ions often provide extraordinary g _lum values [11]. In general, chiral organic π-conjugated systems display luminescence dissymmetry factors significantly lower than lanthanide(III) compounds, due to their electronic transitions with strong electric dipole character. However, their easy processability, the wide range of emission wavelengths accessible, and good quantum yields of fluorescence, together with their propensity to self-assemble into chiral supramolecules or aggregates makes chiral π-conjugated molecules appealing systems for improved materials with CPL activity [4] . Therefore, there is a growing interest in the investigation of the CPL properties of chiral π-conjugated systems.

In 2018, Mori et al. tried to see whether there was a correlation between excitation and emission dissymmetry factors; they examined the experimental ratio g _lum/g _abs (where g _abs = Δε/ε) for a series of chiral organic emissive molecules among which helicenes. They found that this ratio significantly depended on the structure of the helicenic molecule and varied between 0.16 and 28 [12].

This chapter is structured via the different types of CPL-active helicene derivatives, i.e., the N-, O-, S-, B-, Si-, P-, and C-based helicenes and helicenoids, together with transition metal complexes of helicenes exhibiting CPL.

4.2 CPL-Active N-Containing Helicenes

4.2.1 Helicenic Bridged Triarylamines

In 2003, Venkataraman et al. described the preparation of helical triarylamines. These compounds were among the first helicenic structures displaying clear CPL-activity [13]. The two diastereomers of (P,S)-1a and (M,S)-1b displayed identical absorption spectra (Fig. 4.1a, b and Table 4.1) in the UV-vis region with maximum absorption and emission occurring at 434 nm and 453 nm, respectively. These pseudoenantiomeric compounds showed mirror-image ECD and CPL spectra, revealing that the (1S)-camphanate substituent had no influence on the chiroptical properties. Fluorescence dissymmetry factors of ±0.001 were obtained in chloroform. For the longer and more π-conjugated helicenes (P,S)-2a and (M,S)-2b, the maximum emission occurred at longer wavelength, i.e., 478 nm, and slightly smaller fluorescence dissymmetry factors (±0.0008) were obtained. For both compounds 1 and 2, and for the same transition, g _abs and g _lum have essentially the same value showing no significant geometry change upon population of the emitting state. This is corroborated by the small Stokes shifts of the emission maxima. In 2016, the CPL activity of sulfurated systems 3 and 4 was measured by Longhi et al. [14] who found a g _lum as high 0.9 × 10⁻² at ~510 nm (positive for (M) and vice versa, Table 4.1) for 4 while racemization was observed upon excitation of 3. Post-functionalization of heterohelicene 5 with formyl (6) and diacyanovinyl (7) groups enabled to extend the π-conjugation [15]. The tails of UV-vis and ECD spectra together with emission and CPL bands in CH₂Cl₂ are highlighted in Fig. 4.2. These compounds are strongly emissive, with quantum yields up to 0.86 for 5 in CH₂Cl₂ (Table 4.1). A clear red shift of absorption and emission was observed upon increasing the conjugation. The strong charge transfer and high polarity of these molecules were evidenced experimentally through the high solvent polarity dependence of the emission properties (emission wavelength and quantum yield). Regarding the chiroptics, similar g _abs and g _lum magnitudes were found for each compound with g _abs/g _lum of +5.6 × 10⁻³/+4.7 × 10⁻³ for (M)-5, +2.1 × 10⁻³/+1.4 × 10⁻³ for (M)-6, and + 0.9 × 10⁻³/+0.9 × 10⁻³ for (M)-7. These properties were also compared in liquid state/solid state and in nanoparticles obtained by rapid precipitation in water. Respective g _lum values of 4.5 × 10⁻³, 1.5 × 10⁻³, and 2.8 × 10⁻³ were measured for nanoparticles of 5, 6, and 7 dispersed in water, showing that CPL activity was conserved. Note that the influence of solvent polarity was studied on the quantum yields but not on the absolute CPL values.

Fig. 4.1

(a) Chemical structures of helical bridged triarylamines 1a,b; 2a,b; and 3–4. (b) UV-vis (A), ECD (B), fluorescence (C), and CPL (D) spectra of (P,S)-1a and (M,S)-1b in CHCl₃. Reproduced with permission [13]. Copyright 2003, American Chemical Society

Table 4.1 Photophysical data of helical triarylamines and of azahelicenes

Fig. 4.2

UV-vis absorption (solid) and fluorescence (dashed) spectra (top) and CD and CPL spectra (bottom) for 5, 6, and 7 in CH₂Cl₂. The red and blue bars show the calculated ECD bands (CAM-B3LYP/6-31G(d)) for the (P)- and (M)-helices, respectively. The transition energies have been calibrated using a factor of 0.88. Photographs show the emission of 5-7. Reproduced with permission [15]. Copyright 2017, American Chemical Society

4.2.2 Emission Properties and CPL Activity of Azahelicenes

In 2014, Abbate et al. reported the CPL activity of blue-fluorescent 5-aza[6]helicene (8) enantiomers in relation to their ECD spectrum (Figs. 4.3 and 4.4) [20]. The sign of the CPL signal was controlled by the sign of the lower energy ECD named S-type band in relation with Inoue and Mori’s nomenclature [22]. This effect was also shown in carbo[6]helicenes (M)-10 and (M)-11 which exhibited positive CPL signals at ~410 and 415 nm, respectively, in CHCl₃ (vide infra) [20]. In 2016, Longhi and Santoro reported the vibronically resolved calculated UV-vis, ECD, emission, and CPL spectra of 8. A CPL dissymmetry factor g _lum of +0.59 × 10⁻³ at 444 nm was experimentally measured for (M)-8 in CHCl₃ (Table 4.1) and used as a helically shaped chiral model to test the validity of advanced theoretical calculations of chiroptical techniques [16]. Note that the g _lum value of corresponding 1-aza[6]helicene 9 was evaluated to be between 10⁻⁴ and 10⁻³ by Fuchter, Campbell, and coworkers who used this helicene as a chiral inducer in organic light-emitting diodes (OLEDs, see Sect. 4.10) [21]

Fig. 4.3

Structures of aza[6]helicenes 8,9 and carbo[6]helicenes 10,11 [20, 21]

Fig. 4.4

ECD and CPL spectra of 8 in CHCl₃. Reproduced with permission [20]. Copyright 2014, American Chemical Society

4.2.3 Double Vs. Single Azahelicenes

In 2014 Tanaka and coworkers reported the enantioselective synthesis of azahelicenes 12 and 13 and of S-shaped double azahelicenes 15 and 16 (Fig. 4.5) [17, 18]. Their photophysical properties are summarized in Table 4.1. Double azahelicenes 15 and 16 showed red shifts of absorption and emission maxima as compared with their corresponding single azahelicenes 12 and 13. They also showed higher quantum yields in CHCl₃ solution. Interestingly, the CPL activity of the S-shaped double azahelicenes was significantly higher than that of the single azahelicenes. Indeed, CPL measurements showed intensities for azahelicenes 12 and 13 were below their measurable limit (g _lum < 0.001), whereas double azahelicenes 15 and 16 exhibited strong CPL activities, with g _lum = 0.028 at 492 nm for (+)-15 and g _lum = +0.011 at 454 nm for (+)-16 in CHCl₃ [17]. In 2016, (−) and (+)-aza[10]helicenes 14 were found to display |g _abs| = 4.5 × 10⁻³ at 303 nm which correspond to a smaller value than for S-shaped 16 (|g _abs| = 6.5 × 10⁻³ at 331 nm) but no CPL activity could be measured for 14 [18].

Fig. 4.5

Chemical structures of enantioenriched single aza[6]helicenes 12–14 and S-shaped double aza[6]helicenes 15,16 [17, 18]

To explain the enhancement of CPL in S-shaped double helicenes, Mori et al. proposed in 2018 a protocol for rationally aligning multiple chiral units to boost the chiroptical responses, using hexahelicene 10 as a prototype [23]. To do so, they aligned two hexahelicenes in various orientations and examined by theoretical calculations which orientation resulted in the highest chiroptical performance from X-shaped or S-shaped double hexahelicenes (see Sect. 4.8.2).

4.2.4 Polyaza[7]helicenes

In 2017, Shibata et al. reported the synthesis of enantiopure polyaza[7]helicenes such as 17 (Fig. 4.6) possessing a 6-5-6-6-6-5-6 skeleton [19] which showed high fluorescence quantum yields under both neutral (Φ _F = 0.39) and acidic conditions (Φ _F = 0.80). The ECD spectrum of (+)-17 showed several positive Cotton bands in the longer wavelength region with very similar shapes for both neutral and acidic forms. These observations imply that the electronic transitions have similar features in the ground state. Under both conditions, helicene 17 also shows strong CPL activity with a g _lum value under neutral conditions of 0.009 at 473 nm which is quite large for a heptahelicene derivative. Upon addition of 200 equivalent amounts of TFA, the g _lum value remained high (0.008 at 514 nm). Overall, these systems combine both high g _lum value with high quantum yields.

Fig. 4.6

Chemical structure, X-ray structure, ECD, and CPL spectra of neutral and acidic form of 17 enantiomers. Adapted with permission [19]. Copyright 2017, Wiley

4.2.5 Azahelicenes with Fused Carbazole Cycles (Pyrrolohelicenes)

Pyrrole-incorporating PHAs have been shown to possess remarkable physical properties such as effective hole-transporting ability and bright emission. In 2012, Hiroto et al. reported pyrrole-fused system 18 displaying an aza[5]helicenic structure with a stable helical conformation, thanks to the presence of bulky ethynylsilylated groups (Fig. 4.7) [24]. Good fluorescence quantum yields were obtained for 18 (Φ _F = 0.36) with a Stokes shift of 2220 cm⁻¹ reflecting the distorted conformation of 18; a CPL anisotropy factor |g _lum| of 3 × 10⁻³ was measured for both enantiomers. The corresponding bis-butadiyne bridged azahelicene dimer (M,M)- and (P,P)-20 with a figure-eight shape was prepared and exhibited red-shifted absorption and emission, with a higher fluorescence quantum yield (Φ _F = 0.55) and higher g _abs and g _lum values as compared to (M)- and (P)-18 (with opposite signs, see Table 4.2). This enhancement was attributed to the rigid conformation of the dimer [25]. See Sect. 4.3 for the corresponding CPL-active oxygen-containing helicene derivative 19.

Fig. 4.7

Chemical structures of pentahelicenes incorporating a carbazole (18) together with a dimer (20) and a dibenzofurane analogue (19); pentahelicenes including imide functions (21a,b and 22, 22.H⁺) together with pentacarbohelicene 23

Table 4.2 Photophysical data of helicenes fused with pyrroles or imide cycles

4.2.6 Helicene Imide Derivatives

Aromatic diimides are known to display bright emission properties. In 2016, Hasobe and coworkers reported [5]carbohelicene derivatives 21a,b fused with an electron-withdrawing maleimide and substituted with electron-donating methoxy groups [26]. Compared to pristine [5]carbohelicene 23, the introduction of an electron-withdrawing maleimide group onto a [5]carbohelicene core contributes to the stabilization of the LUMO level in 21a, whereas the energy level of HOMO level in MeO-substituted 21b increases due to the electron donation. As a result, the HOMO-LUMO gap of 21b is smaller than that of 21a and of carbo[5]helicene 23, giving bathochromic shift of absorption and emission bands. The absolute fluorescence quantum yield of 21a was found higher (0.37) as compared to [5]carbohelicene 23 (0.04), whereas Φ_F of 21b was slightly smaller (0.22). The chirality of these [5]carbohelicene derivatives 21a,b was evidenced by their ECD and CPL activities. In particular, 21a and 21b gave good CPL activity with anisotropy factors g _lum estimated to be ±2.4 × 10⁻³ and ± 2.3 × 10⁻³ in THF. This example was the first observation of CPL in [5]carbohelicene derivatives which are usually thought to be configurationally unstable. Here, the authors verified that the CPL signal was stable with time in solution. Note that this negative CPL signal for the (P)-(+) isomer is a S-type one (vide supra and [22]), i.e. corresponding to the small negative ECD signal at 435 nm with g _abs values of −4.8/−5.7 × 10⁻³ for (P)-(+)-21a/b. The same authors also reported a carbo[5]helicene 22 bearing a fused benzimidazole and its protonated form 22.H⁺ (see Fig. 4.7 and Table 4.2) [27].

In 2016, Chen and coworkers reported the preparation of configurationally stable helical aromatic imides displaying full-color CPL responses [28]. For this purpose, they prepared enantiomers (P)-(−)- and (M)-(+)-24a-e with high ee’s (98.4–99%). Each pair of enantiomers displayed mirror-image ECD spectra of moderate magnitude and absolute configurations opposite to those of classical heterohelicenes. (P)-(−)- and (M)-(+)-24a-e exhibited full color fluorescence emission (from 445 to 617 nm) and mirror-image CPL signals in THF (Fig. 4.8). The g _abs values of the enantiomers fell within the range of ±1.5 × 10⁻³ to ±3.5 × 10⁻⁴ and the g _lum values between ±0.2 × 10⁻³ and ± 1.5 × 10⁻³ (Table 4.2).

Fig. 4.8

(a) Chemical structure of 24a-e ((P)-(−) enantiomers); (b) emission color panel of 24a-e. (c) ECD spectra in THF of pure enantiomers. (d) CPL spectra in THF of pure enantiomers. Adapted with permission [28]. Copyright 2016, Royal Society of Chemistry

4.2.7 Carbocationic Azahelicenes

Lacour and coworkers prepared functionalized carbocationic [4]helicene (25–30) and [6]helicene (31–33) derivatives and studied their photophysical and chiroptical properties (Figs. 4.9 and 4.10 and Table 4.3) [29,30,31]. Interestingly, these compounds exhibit fluorescence emissions in the red to near infrared region (with Φ _F ranging from 0.01 to 0.445), which corresponds to an unusual spectral range for helicene-based chromophores, especially for fully organic ones. As a result, these helical derivatives may be interesting for chiral bioimaging. The same authors investigated different diaza[4]helicenium chiral dyes functionalized with different donor and acceptor groups in order to tune their chiroptical properties in terms of ECD and CPL responses (Fig. 4.9) [29]. These helical derivatives present ECD signatures up to 750 nm with moderate intensity in the visible-red region (Δε ~ 10 M⁻¹ cm⁻¹), resulting from partial charge-transfer transitions involving the nitrogen atoms and the central carbocation. Furthermore, CPL emissions were recorded between 650 and 700 nm and were characterized by a g _lum of ~10⁻⁴ to 10⁻³. Such cationic chiral dyes were also used as pH-triggered ECD and CPL chiroptical switches when they possessed pH-sensitive group such as carboxylic acid. For instance, zwitterionic [4]helicene 29 was reported in 2016 as a reversible pH-triggered ECD/CPL chiroptical switch (Fig. 4.10) [30]. Protonated 30 displayed g _lum of around 5 × 10⁻⁴ and of similar order as g _abs (4 × 10⁻⁴), while the carboxylate derivative 29 displayed no CPL, probably due to very low emission. Overall it represented an on-off CPL switch. Similarly, longer O-containing and N-containing [6]helicenium derivatives 31–33 displayed CPL signals (with opposite signs compared to their optical rotation values) with |g _lum| between 0.32 × 10⁻³ and 2.1 × 10⁻³ in the infrared region [31].

Fig. 4.9

Chemical structures of carbocationic diaza[4]helicenes 25–28 and their UV-vis, ECD and CPL activities. Adapted with permission [29]. Copyright 2016, Royal Society of Chemistry

Fig. 4.10

Chemical structures of carbocationic diaza[4]helicenes 29–30 (pH-triggered switch) and O,N-containing carbocationic [6]helicenes [30, 31]

Table 4.3 Photophysical data of carbocationic azahelicenes

4.3 CPL-Active Oxygen-Containing Helicene Derivatives

In 2011, Tanaka et al. reported a phthalhydrazide-functionalized [7]oxahelicene derivative 34 (Fig. 4.11), displaying a strong increase of g _lum, i.e., one order of magnitude, as compared to other helicenic derivatives [32]. This strong CPL enhancement was attributed to the presence of multiple-hydrogen-bonding sites enabling the formation of a trimeric structure which further organizes into chiral fibers (Fig. 4.12). These chiral fibers were 200 nm wide and 3–4 μm long in chloroform solutions, as characterized by SEM and AFM images. While UV-vis and ECD spectra seemed hardly sensitive to the formation of such aggregates, CPL measurements of 34 afforded g _lum of −0.035 for the (M) enantiomer in chloroform solutions, which was larger than in methanol solutions (−0.021). Similar systems, namely, [7]oxahelicene 35 (71% ee) and [9]oxahelicene derivative 36 (88% ee) were reported in 2017 [33]. Comparison of the photophysical data of these 9-oxahelicene 36 compared to 35 shows a redshift of both the absorption and luminescence spectra by ~20 to 50 nm, along with a decrease in fluorescence quantum yield (0.23–0.18). ECD and CPL spectra followed the same trend as for unpolarized UV-vis and fluorescence measurements, but with more intense ECD and CPL signals for 36. Here the g _lum values around 10⁻³ in chloroform are classical for helicenic solutions (see Table 4.4). Similarly to 18 (see Fig. 4.7), Hiroto and coworkers prepared P-(+) and M-(−) enantiomers of 19 which displayed strong orange fluorescence (Φ _F = 0.66 in dichloromethane) and moderate CPL activity (±1.2 × 10⁻³) [34]. Note that different emission properties between racemic and pure enantiomers were obtained for this compound in the solid state but no CPL in the solid state was reported.

Fig. 4.11

Chemical structures of oxahelicenic derivatives

Fig. 4.12

AFM (b, c) and SEM images of (M)-34 (a) and (rac)-34 (e) prepared in toluene solutions. (d) Postulated mechanism for the formation of supramolecular chiral aggregates from a trimeric association. Reproduced with permission [32]. Copyright 2011, Wiley

Table 4.4 Photophysical data of oxahelicenic derivatives

In 2018, Bedekar and coworkers reported the preparation of (P)-(+) and (M)-(−) enantiomers of two 5,13-dicyano-9-oxa[7]helicene derivatives 37 and 38 and reported their photophysical and chiroptical properties [35]. These helical compounds are also active in CPL and mirror-like CPL spectra were measured with positive sign for (P) enantiomers, and g _lum values of 3–5 × 10⁻³ in DMSO solutions, i.e., falling within the classical range of helicene compounds. In 2014, Bedekar and coworkers also reported helicene-like bis-oxazines 39 and 40 from atropisomeric 7,7′-dihydroxy BINOL derivatives displaying good CPL activity, with g _lum of +0.0015/−0.0009 and + 0.0014/−0.0013 for (P)−/(M)-39 and (P)−/(M)-40, respectively [36].

4.4 CPL-Active Sulfur-Containing Helicene Derivatives

There are few examples of CPL-active sulfur containing helicenes and helicenoids reported in the literature. First of all, it is worth to compare results obtained for 34 with those reported by Katz and coworkers in 2001 on thia[7]helicene-bisquinone derivative 41 decorated with four dodecyloxy groups [37]. Indeed, enantiopure [7]helicene 41 aggregated into columnar structures depending on the solvent type. The aggregation occurred in dodecane and in pure materials, but not in chloroform. The specific rotation showed strong enhancement with increasing concentration, i.e., [α]_D = 2800 and 10,400 at 2 × 10⁻⁵ M and 2 × 10⁻² M, respectively, in dodecane. The same behavior was observed by ECD spectroscopy showing significant change for concentrated solutions in dodecane and for films obtained by drop casting from nonane solutions. Interestingly, for a concentration higher than 2 × 10⁻³ M, the solution was viscous and at 0.05 M it became a gel. CPL measurement was carried out for a solution with 1 × 10⁻³ M in dodecane (Fig. 4.13). Emission from enantiomer aggregates between 600 and 700 nm was found to be mirror image for enantiomeric systems. Interestingly, aggregation offered a strong dissymmetry factor (g _lum = 0.01 at around 630 nm). Likewise, the g _abs was large (g _abs = 0.01 between 500 and 550 nm); thus, the good similarity suggests that the aggregates adopt the same chiral geometry in the ground and the excited states. However, the increased ordering of these aggregates also resulted in a large degree of linear polarization (P = 0.39) which can greatly affect the CPL measurement [38].

Fig. 4.13

Chemical structure of (P)-41. (a) Total luminescence and (b) CPL from solutions at 23 °C of (P)-(+) and (M)-(−)-41 in dodecane (1 × 10⁻³ M) after excitation with unpolarized light (λ _ex = 325 nm). Adapted with permission [37]. Copyright 2001, American Chemical Society

In 2016, Yamamoto et al. reported the synthesis of tetrasulfone[9]helicene 43 via the oxidation of tetrathia[9]helicene 42 [39]. Remarkably, it was found that the quantum yield of fluorescence for sulfone[9]helicene 43 (Φ _F = 0.27) was ten times higher than tetrathia[9]helicene precursor 42 (Φ _F = 0.03). The author explain this strong enhancement by a significant increase of the energy gap between the lowest singlet (S₁) and the triplet (T₁) excited states ΔE_ST in the case of 43 (ΔE_ST = 1.02 eV) compared to (ΔE_ST = 0.60 eV) for 42, which may efficiently decrease the intersystem crossing (ISC) rate. Additionally, ECD spectra for 43 were recorded and an anisotropy factor g _abs = −4.7 × 10⁻³ was measured for the (P) enantiomer. Likewise, plotting the fluorescence CPL mirror-image spectra 43 enantiomers gave an estimated anisotropy factor value of g _lum = −8.3 × 10⁻⁴ for the (P)-43 (Fig. 4.14).

Fig. 4.14

Chemical structures of 42 and 43 and their emission data

A series of fluorescent “push-pull” tetrathia[9]helicenes based on quinoxaline (acceptor) fused with tetrathia[9]helicene (donor) derivatives was synthesized for control of the excited-state dynamics and circularly polarized luminescence (CPL) properties [40]. Introduction of a quinoxaline onto the tetrathia[9]helicene skeleton induced a “push-pull” character, which was enhanced by further introduction of electron-releasing or electron-withdrawing groups onto the quinoxaline unit (Fig. 4.15). Significant enhancement in the fluorescence quantum yields (Φ_F) was, for instance, obtained for 44: (Φ_F = 0.30, Table 4.5), which is more than 20 times larger than that of pristine tetrathia[9]helicene (Φ _F = 0.02). Good CPL properties, with an anisotropy factor g _lum of 3.0 × 10⁻³ were measured for 44. In 2017, Avarvari et al. reported the synthesis of (P) and (M)-45, corresponding to Pt(diimine)(dithiolene) connected with a [6]helicene unit [41]. Interestingly, (P) and (M)-45 showed triplet state CPL activity at room temperature with low g _lum values of 3 × 10⁻⁴.

Fig. 4.15

Chemical structures of push-pull systems with improved luminescence and CPL emission and of a [6]helicene Pt(diimine)(dithiolene) complex

Table 4.5 Photophysical data of thiahelicenic derivatives

4.5 CPL-Active Borahelicenes

Due to the electron-accepting and Lewis acidic character of boron, introducing one or several boron atoms into carbohelicenes generally results in strongly blue-emitting fluorophores. For chemical stability reasons, helicenes incorporating boron atoms are azaborahelicenes and oxaborahelicenes, i.e., also including N or O atoms. In 2017, our group prepared enantiopure azabora[n]helicenes 46a-d incorporating one or two boron atoms and with 6,8 or 10 ortho-fused rings (n = 6, 8, 10, Fig. 4.16) [42]. These compounds displayed strong absorption between 250 and 450 nm and blue fluorescence (λ _Em ~ 420–450 nm) with rather strong quantum yields (0.21–0.49) for azaborahexahelicenes 46a,c and more modest ones (~0.07) for the octa- and decahelicenes 46b,d. The introduction of one additional boron atom on 46c strongly increased the emission efficiency compared to 46a, but at the same time strongly decreased the configuration stability (enantiomerization barrier ΔG ^≠ of 27.5 kcal mol⁻¹ at 78 °C, in ethanol) due to the presence of two azaborapentacycles. From the UV-vis spectra, the longer the helicene, the stronger were the absorption coefficients and the more red-shifted the absorption wavelengths. Similarly, the ECD spectra were more red-shifted and more intense for azaboraoctahelicene 46b and azaboradecahelicene 46d as compared to azaborahexahelicenes 46a,c. Note that, except for 46c, the overall ECD signature appeared typical of helicene derivatives and that the (P)-enantiomers display positive optical rotation values. Regarding the CPL responses, g _lum values were found negative for (P)-46a-c and positive for (P)-46d (see Table 4.6). As mentioned above, the sign of CPL greatly varies with the substituents grafted onto the helicenic core and generally follows the sign of the lower energy ECD-active band. The absolute values of g _lum (between 7 × 10⁻⁴ and 10⁻³) for 46a-d are typical of enantiopure organic helicenes.

Fig. 4.16

Chemical structures of azabora[n]helicenes 46a–d and 47–49 and oxaborahelicene 50

Table 4.6 Photophysical data of borahelicenes

Enantiopure azabora[5]helicenes 47–49 were also prepared; they displayed different charge transfer characters and fluorescence quantum yields ranging from 0.13 to 0.30 in toluene, governed by the electron-donor substitution (p-MeO-phenyl, p-Me₂N-phenyl) at the helicene [43]. The dimethylamino-substituted derivative emitted at the most red-shifted wavelength and showed the highest Stokes shift in toluene. These helicenes also showed CPL activity with dissymmetry factors g _lum between 2.5 × 10⁻⁴ and 3.5 × 10⁻³. Their ECD spectra and optical rotation values of 47–49 were very different from azaborahelicenes 46a–d, and it was shown that the sign of the ECD band corresponding to the first transition and the CPL spectrum depended on the electron-donor substitution.

Oxabora[6]helicene 50 was prepared in 2016 by Hatakeyama and coworkers and revealed deep and almost pure blue fluorescence with Commission Internationale de l′Eclairage coordinates of (0.15, 0.08) [44]. Its enantiomers showed CPL high fluorescence quantum yields of 0.65 at 436 nm activity with g _lum of 1.7 × 10⁻³. Achiral structural analogues of 50 have proven efficient B-containing PAH dopants in organic OLEDs and in field-effect transistors [45]. These compounds are indeed known to display good carrier mobilities. Note also that such BN and BO aromatic compounds display increasing interest in the domain of thermally activated delayed fluorescence (TADF) [46].

4.6 CPL-Active Silahelicenes

Silylated π-conjugated molecules also display strong blue emission. In 2013, Nozaki and coworkers reported the synthesis of enantiopure sila[7]helicene 51, bearing a silole as the central cycle (Fig. 4.17) [47]. The UV-vis absorption spectrum of (rac)-51 showed longest absorption at 412 nm, which is much longer than pristine phenanthrene (293 nm) and dibenzosilole (286 nm), due to extended delocalization of the π-electrons over the molecule. The absorption edge of (rac)-51 at 431 nm is similar to that of λ ⁵-phospha[7]helicene (432 nm) and red-shifted compared to the related aza- and oxa-[7]helicenes (425 nm for aza[7]helicene and 409 nm for oxa[7]helicene). Upon excitation at 320 nm, compound (rac)-51 exhibited a strong blue fluorescence with λ_max at 450 nm and good quantum yields in solution and in the solid state (see Fig. 4.17 and Table 4.7). The CPL spectra of enantiopure sila[7]helicene (P)- and (M)-32, which are mirror image of positive and negative sign, respectively; dissymmetry factors of 3.5 × 10⁻⁴ at 470 nm were measured. The authors concluded that the g _lum derives mainly from the helical biphenanthryl moiety, while the heterole moiety plays essential roles in the luminescent properties. In 2015, Tanaka et al. prepared enantioenriched 1,1′-bis-triphenylene-based sila[7]helicenes (P)-52 with 91% ee [48]. Compared to sila[7]helicene 51, 52 displayed red-shifted absorption and fluorescence responses explained by the presence of fused 1,1′-bistriphenylenes resulting in more extended π-conjugation. Probably for the same reason, enantiopure 52 show high g _lum values, i.e., 1.6 × 10⁻², which is uncommonly high for an organic helicene. These values appear larger than that for the 3,3-biphenanthrene-based sila[7]helicene 51 (g _lum = −0.0035 at 470 nm) but smaller than that for the 1,1-bitriphenylene based carbo[7]helicene 61 (g _lum = −0.030 at 428 nm for the (M)-(−) enantiomer; see Sect. 4.8.3) [49].

Fig. 4.17

Chemical structures of silahelicenes (P)-51 and (P)-52. UV-vis/fluorescence spectra and ECD/CPL spectra of sila[7]helicene 51 in CH₂Cl₂. Blue lines in ECD and CPL spectra: (M)-isomer. Red lines: (P)-isomer. The blue and red bars show the calculated ECD spectra. Reproduced with permission [47]. Copyright 2013, American Chemical Society

Table 4.7 Photophysical data of silahelicenes

4.7 CPL-Active Phosphahelicenes

It is now well recognized that phosphorus-containing π-conjugated small molecules, oligomers, polymers, and supramolecular assemblies are important classes of heteroatomic molecular materials for many applications in optoelectronics including OLEDs [50]. P-containing building blocks can indeed lead to materials with unique properties (emission, charge transport, coordination, (anti)aromaticity, etc.). So far, most phosphorus derivatives having helical chirality have displayed polyaromatic (or heteroaromatic) helical scaffolds with pendant phosphorus functions (phosphites, trivalent phosphines and phosphine oxides, helicene-phospholes derivatives) but a few classes of P-containing heterohelicenes have appeared in the literature in the last years. Although several phospholes and helicenes are known to exhibit efficient fluorescence properties, the only example of CPL-active phosphane-containing helicenes has been reported recently [51]. Compounds 53a and 53b are benzooxophosphole derivatives that display a carbo[6]helicene unit that is meta-fused with one terminal oxophosphole ring containing a pendant phenyl ring at position 5 (Fig. 4.18). In both systems a l-menthyl group at the P atom is directed toward the inner groove of the helix (endo isomer). These epimeric compounds can be considered as pseudo-enantiomers since they demonstrate reverse stereochemistry of the helix (P/M) and of the P atom (R _P/S _P) but unchanged stereochemistry of the chiral l-menthyl group. The epimeric helicenes [6]-(P)-endo-53a and [6]-(M)-endo-53b displayed almost identical UV-vis spectra, blue fluorescence with moderate quantum yield (0.07–0.10), and mirror-image ECD spectra. Similarly, they exhibited mirror-image CPL spectra with luminescence anisotropy factor g _lum = +8 × 10⁻⁴ and − 7 × 10⁻⁴ at 452 nm (excitation at 404–416 nm) for (P)-53a and (M)-53b, respectively (Table 4.8). Note that the other phosphahelicenes tested underwent photodegradation under the conditions of the CPL measurement.

Fig. 4.18

CPL-active phosphahelicenes

Table 4.8 Photophysical data of phosphahelicenes

In 2018, Tanaka and coworkers reported the enantioselective synthesis of [7]- and [9]phosphahelicenes 53c and 53d (Fig. 4.18) by [2+2+2] cycloaddition. These heterohelicenes displayed modest quantum yields (0.22 and 0.085) and g _lum values (+8.1 × 10⁻⁴ and + 3.8 × 10⁻⁴) [52].

4.8 CPL-Active Carbohelicenic Derivatives

4.8.1 Pentahelicenic Structures

In 2018, Mori and coworkers reported a combined experimental and theoretical study to elucidate the ECD and CPL behaviors of parent pentahelicene (P)-23 and of D ₃-symmetric triple pentahelicene (P,P,P)-54 [53]. They showed that the pentahelicene unit exhibits absorption and luminescence, with dissymmetry factors g _abs and g _lum that are intrinsically larger than those of higher homologues. Thus, (P)-23 emitted strong CPL with g _lum of −2.7 × 10⁻³ at low temperature (see Table 4.9 and Fig. 4.19), which is about a half value of the g _abs. Due to its photolabile nature, 23 is not suitable to be incorporated in chiroptical materials. However, such undesirable reactivities can be excluded by merging three pentahelicenic units into 54, for which the g _lum and g _abs factors were found to be as high as −1.3 × 10⁻³ and −1.8 × 10⁻³, respectively, for the (P) enantiomer, corresponding to a high g _lum/g _abs ratio of 0.72, indicating moderate excited-state relaxation. Theoretical calculations provided further insights into the improved chiroptical responses at the main band (¹B_b transition) in the triple pentahelicene 23, which was ascribed to its symmetric nature.

Table 4.9 Photophysical data of pentahelicenic derivatives

Fig. 4.19

Chemical structures, fluorescence, and CPL activity of pentahelicenic (P)-23 and of D ₃-symmetric triple pentahelicene (P,P,P)-54. Adapted with permission [53]. Copyright 2018, American Chemical Society

In 2017, Lu and coworkers prepared enantiopure tetrahydrocarbo[5]helicenic derivatives (Fig. 4.20) with high configurational stability, thanks to the presence of phenyl groups placed in the inner groove of the helix [54]. These compounds displayed strong blue fluorescence with quantum yield of up to 0.59. Compounds (P)-(+)-55a-e,56 and (M)-(−)-55a-e,56 also exhibited mirror-imaged ECD and CPL spectra in THF. Due to similar helical backbones, all systems showed similar Cotton effects with strong negative Cotton effects at ~290 nm and positive ones at 315–320 nm for the (P)-(+)-55a-e,56 enantiomers. The g _abs values were found within the range of +2.63 × 10⁻³ to +4.77 × 10⁻³ for the (P) enantiomers and −3.40 × 10⁻³ to −2.73 × 10⁻³ for the (M) enantiomers, respectively. These compounds also showed intense CPL signals, matching with the region of emission spectra and corresponding ECD signals at the longest wavelength (305–400 nm). The pure enantiomers all exhibited relatively high g _lum with values between +3.66 × 10⁻³ and + 6.41 × 10⁻³ for the (P) configurations and −3.40 × 10⁻³ to −6.59 × 10⁻³ for the (M) configurations (Table 4.9) thus showing that chirality existed in both the ground and excited states and could be attributed to their rigid helical structures. Interestingly, absorption dissymmetry factors (g _abs) of (P)-(+)-56 and (M)-(−)-56 were found stronger (+7.9 × 10⁻³ and −7.7 × 10⁻³, respectively), as well as the emission dissymmetry factors (g _lum) as large as 2.8 × 10⁻² and −3.1 × 10⁻² at 466 nm, respectively, considerably higher than values reported for other CPL-active single organic molecules. Similar tetrahydropentahelicenic structures, i.e., (P)-(−)-57a-c and (M)-(+)-57a-c, were prepared in 2018 by Chen et al. [55]. They exhibited mirror-image ECD and CPL spectra in dichloromethane, with g _abs of −4.7 × 10⁻⁴ to −8.3 × 10⁻⁴ for the (P) configuration, and +4.9 × 10⁻⁴ to +8.7 × 10⁻⁴ for the (M) configuration (Table 4.10 and [55]). As for fully conjugated derivative pentahelicene (P)-(+) and (M)-(−)-57d, they display g _abs values of similar magnitude (+3.28 × 10⁻⁴ and −3.57 × 10⁻⁴, respectively). Furthermore, the enantiomers of 57a–c showed CPL signals with g _lum values between −2.5 × 10⁻⁴ and −4.1 × 10⁻⁴ for the (P) enantiomers and +2.6 × 10⁻⁴ and +4.2 × 10⁻⁴ for the (M) ones. (P)-(+) and (M)-(−)-57d in dichloromethane exhibited stronger CPL activity, with g _lum values of −4.52 × 10⁻³ and +4.43 × 10⁻³, respectively. In 2018, Chen et al. also reported the one-pot oxidative aromatization and dearomatization (OADA) reactions of similar tetrahydro[5]helicene diols, with DDQ as the oxidant, which provided a new method for the synthesis of novel CPL-active chiral π-extended diones [58].

Fig. 4.20

Enantioenriched tetrahydrocarbo[5]helicenic derivatives prepared by Chen et al

Table 4.10 Photophysical data of carbon-based helicenes

4.8.2 CPL-Active Hexahelicenic Structures

Hexahelicenic derivatives are prototypic helicenes exhibiting CPL activity. In 2018, our group showed that grafting diketopyrrolopyrrole (dpp) dyes onto a carbo[6]helicene structure through ethynyl bridges (see 58a–c in Fig. 4.21) leads to exciton coupling circular dichroism in the red region arising from the achiral red-absorbing DPP units in the helical environment [56]. Furthermore, red to near-infrared circularly CPL was obtained. Indeed, the association of enantiopure [6]helicene and dpp units provided helical π-conjugated molecules with strong ECD signal in the visible region (~600 nm), intense red and near-infrared fluorescence (ϕ _F ~ 0.4), and CPL activity up to 650 nm with g _lum found to increase from 1 × 10⁻⁴ to 6 × 10⁻⁴ then 9 × 10⁻⁴ with the increase of exciton coupling (i.e., through the series 58a → 58b → 58c). The g _abs values were also found to follow the same increasing trend with the increasing exciton coupling. These results highlighted the synergy between the chiral hexahelicene structure and the organic dye. Thus, decorating carbohelicenes with dyes constitutes an appealing strategy of chemical engineering of a π-helical platform to further improve the chiroptical responses.

Fig. 4.21

Chemical structures, fluorescence, and CPL spectra in CH₂Cl₂ of hexahelicene-dpp derivatives (P)-58a–c. Adapted with permission [56]. Copyright 2018, Royal Society of Chemistry

In 2018, Tanaka and coworkers reported the enantioselective synthesis of fully benzenoid single (59a,b) and double (59c,d) carbo[6]helicenes via efficient gold-catalyzed intramolecular hydroarylation (Fig. 4.22) [57]. Similar to the single (12–14) and double azahelicenes (15–16) described in Sect. 4.2.3, the double carbo[6]helicenes 59c,d exhibited relatively large CPL activities (up to 2.7 × 10⁻³, see Table 4.10), as compared to the single carbo[6]helicenes 59a,b whose CPL was below the limit of the apparatus.

Fig. 4.22

Chemical structures of enantioenriched single carbo[6]helicenes 59a,b and S-shaped double carbo[6]helicenes 59c,d [57]

In 2018, X-shaped and S-shaped pristine double hexahelicenes (60 and 61, Fig. 4.23) were prepared and used as representative molecular models, and a theory-guided, symmetry-based protocol was developed [23]. Compound 60 and 61 exhibited a strong increase in intensity of ECD and CPL. The enhanced chiroptical responses were theoretically assigned to the electric (μ_e) and magnetic (μ_m) transition dipole moments of component hexahelicenes aligned in the correct symmetry. Indeed, 60 and 61, constructed by merging two hexahelicenes in D ₂ and C ₂ symmetry, respectively, showed absorption dissymmetry factors per benzene unit (g _abs/n) for the ¹B_b band that are larger by a factor of up to 1.5 than that of parent 10. This enhancement was well rationalized by μ_e and μ_m and their relative angle (θ) evaluated theoretically. In the double helicenes, μ_e and μ_m were parallel-aligned (θ = 0) to maximize the orientation factor (cos θ) up to unity, which was mere 0.24 (cos 76°) in 10, while |μ_e| and |μ_m| were comparable or only slightly improved. Similarly, the luminescence dissymmetry factor per benzene unit (g _lum/n) was up to 1.7-fold larger for the double helicenes than for 10, for which the increased |μ_e| and θ are responsible. The enhanced g _abs/n and g _lum/n values for double helicenes mean that merging two helicenes is 50–70% more resource efficient than simply assembling them, in favor of the molecular, rather than supramolecular strategy for constructing advanced chiroptical devices.

Fig. 4.23

(a) Experimental and calculated ECD and CPL responses of 10, 60, and 61 ((P) enantiomers). (b) Transition dipole moments in the ground state. Schematic representations of electric (μ_e, blue) and magnetic (μ_m, red) transition dipole moments of the ¹B_b band for X-shaped and S-shaped double hexahelicenes 60 and 61, with the magnitudes relative to parent helicene 10, calculated at the RI-CC2/def2-TZVPP level. Dashed arrows in double helicenes indicate the transition dipole moments of component helicene units. (c) Transition dipole moments in the excited state. Schematic representations of the electric (μ_e, blue) and magnetic (μ_m, red) transition dipole moments of the ¹L_b band of 60 and 61 in the excited states, with the magnitudes relative to those for parent helicene 10, calculated at the RI-CC2/def2-TZVPP level. Adapted with permission [23]. Copyright 2018, Nature Publishers

4.8.3 CPL-Active Heptahelicenic Structures

In 2012, Tanaka et al. reported the preparation of helically chiral 1,1′-bitriphenylenes displaying a central fluorenyl cycle. These compounds correspond to heptahelicenic structures and exhibit among the strongest CPL activity [59]. Indeed, compounds 62 and 63 (Fig. 4.24) obtained with high enantiomeric excesses (93% and 91% ee, respectively) exhibited mirror-imaged CPL spectra with particularly high fluorescence dissymmetry factors (g _lum = −0.030 at 428 nm for (M)-(−)-62 and g _lum = −0.032 at 449 nm for (M)-(−)-63 in chloroform, Table 4.11), which are comparable to phthalhydrazide-functionalized [7]helicene-like molecule 34 (g _lum = −0.035 at 476 nm for the assembly state and −0.021 for the molecularly dispersed state) and significantly larger than those for helically chiral molecules reported to date. However, the g _lum values are found very high compared to the g _abs values, which are around 2.5–2.9 × 10⁻³, i.e., one order of magnitude lower. This is clearly seen in the ECD shape, which displays the typically large bands of (P)-helicenes (a large positive band at ~320 nm, and two large negative bands at ~260 and 285 nm, Fig. 4.25) but small bands between 340 and 400 nm which correspond to the lowest energy transitions. Later on in 2016, Nozaki and coworkers reported the synthesis and photophysical properties of enantiopure [7]helicene-like fluorenyl systems 64a,b [60]. These compounds showed similar absorption spectra with the longest absorption maximum at ~400 nm slightly blue-shifted compared to that of carbo[7]helicene, while they are significantly red-shifted compared to those of fluorene and phenanthrene. Indeed, the π-conjugation is well extended over the whole molecule despite their helically twisted structures. In addition, the longest absorption maximum is slightly red-shifted compared to that of compound 62 despite the fact that 62 possesses triphenylene units [57]. This phenomenon may be attributed to a smaller twist of compounds 64 than compound 62, resulting in more effective π-conjugation along the helical structure. [7]Helicene-like compounds 64a,b exhibited an emission maximum at around 420 nm with a very small Stokes shift and high fluorescence quantum yields (up to 40%) among the highest reported helicenes at that time and slightly higher than those of [7]helicene-like compounds [59]. This makes helicenes incorporating a fluorene unit very appealing for highly emissive chiral molecular materials. ECD spectra of 64a,b enantiomers display a similar shape as for 62 and 63 but their CPL activities were smaller, with dissymmetry factors of 3.0 × 10⁻³ and 2.5 × 10⁻³, respectively. These values are comparable to that of silole-fused compound 51 (vide supra). In 2015, Hasobe and coworkers reported a highly yellow fluorescent [7]carbohelicene fused by asymmetric 1,2-dialkyl-substituted quinoxaline (65) [61]. It displayed a fluorescence quantum yield of 0.25 at 550 nm emission wavelength which is more than 10 times larger than that of the pristine heptahelicene (Φ _F = 0.02). Such a large enhancement of fluorescence in 65 also provided good CPL activity, with g _lum values of ±4.0 × 10⁻³. A negative CPL signal for the (P)-(+) isomer, i.e., an S-type one (vide supra and [20, 22]), corresponding to the small negative ECD signal at 435 nm with a g _abs of −1.3 × 10⁻³. Note also that this compound was successfully used as an emitter in OLEDs but the authors did not report any CPL emission of the OLED.

Fig. 4.24

Chemical structures of CPL-active heptahelicenic derivatives

Table 4.11 Photophysical data of heptahelicenic derivatives

Fig. 4.25

(a) ECD/CPL spectra of heptahelicenic fluorene 63 in CH₂Cl₂. Blue lines in ECD and CPL spectra: (P)-isomer. Red lines: (M)-isomer and (b) UV-vis/fluorescence spectra. Reproduced with permission [59]. Copyright 2012, American Chemical Society

4.9 CPL-Active Transition Metal Complexes of Helicenes

4.9.1 Cycloplatinated Helicenes

Coordination chemistry offers a simple way to tune the optical and electronic properties of the π-ligands since both the coordination sphere geometry and the nature of the metal-ligand interaction can be readily modified by varying the metal center. This will produce a great impact on the properties of the molecule [62]. Recent studies have demonstrated many potential applications of N-containing helicenes in coordination chemistry and in materials science [63]. Indeed, their transition metal complexes may show interesting properties in harvesting (visible) light and re-emitting it at a wavelength that depends on the metallic ion used, thus allowing the development of light-emitting devices, chemosensors, photovoltaic dye-sensitized devices, etc. In 2010, our group prepared the first class of organometallic helicenes incorporating a metallic ion, i.e., Pt, within their helical backbone, named platinahelicenes [64, 65]. Enantiopure platina[6]helicene 66a, platina[8]helicene 66b, bisplatina[6]helicene 66c, and bisplatina[10]helicene 66d (Fig. 4.26), displayed absorption spectra that were strongly red-shifted compared to the starting ligands, with longer absorption wavelengths above 450 nm. Furthermore, platinahelicenes 66a-d are efficient deep-red phosphors, with emission maxima between 630 and 700 nm, quantum yields around 0.05–0.10 in deoxygenated solution at room temperature and luminescence lifetimes of 10–20 μs. Interestingly, platinahelicenes 66a–c displayed circularly polarized phosphorescence with dissymmetry factors as high as 10⁻², which is one order of magnitude bigger than for most of organic helicenes. These g _lum values appeared positive for the (P) enantiomers and negative for the (M), which was not always the case in azaborahelicenes analogues 46a–d [45]. Note that bis-platina[10]helicene 66d also exhibited red phosphorescence at room temperature, but no CPL activity was detected. This can be explained by the weakly chiral environment around the two Pt centers and the high sensitivity to oxygen. Note also that the precursor 1-(2-pyridyl)-hexahelicene 67 displayed fluorescence emission and CPL activity (see Table 4.12). Recently, polyfluorinated platina[6]helicene 66e was prepared by Zheng et al. [66] It displayed similar molecular behavior as 66a–c, namely red phosphorescence, and CPL activity with a g _lum of −3.7 × 10⁻³ in dichloromethane solution for the (P)-(+) enantiomer. The same compound displayed a g _lum of −4.1 × 10⁻³ when incorporated in a DCzppy film (DCzppy: 2,6-bis(3-(9H-carbazol-9-yl)phenyl)pyridine). Interestingly, compounds 66a and 66e were used as chiral dopants in OLED devices to conceive efficient circularly polarized OLEDs (CP-OLEDs; see Sect. 4.10).

Fig. 4.26

Chemical structures of platinahelicenes 66a–e and of precursor 67

Table 4.12 Photophysical data of platinahelicenes and helicene-bipy rhenium complexes

4.9.2 Coordination of Helicene-Bipyridine Ligands

Our group also prepared complexes bearing a helicene-bipyridine-type ligand. Bipyridine ligands are classical N^N chelate ligands but can also act as C^N ones toward different transition metal ions such as platinum. In 2015, we reported the preparation of enantiopure helical cycloplatinated complexes (P)- and (M)-69 from a [6]helicene-bipyridine-type ligand, namely, 3-(2-pyridyl)-4-aza[6]helicene ((P)- and (M)-68 in Fig. 4.27) [67]. Due to the presence of an additional N atom in organometallic species (P)- and (M)-69, the acid-base triggering of UV-vis, ECD, phosphorescence, and CPL were achieved, thus yielding the first acid-based CPL switch (see the increase of g _lum upon protonation in Fig. 4.27). Furthermore, we showed that organic helicene ligand (P)- and (M)-68 was also an efficient chiroptical switch since, after double protonation, it displayed a strong bathochromic shift in emission wavelength while keeping strong CPL fluorescence signal (g _lum = ±2 × 10⁻³ in CH₂Cl₂). TDDFT calculations showed that, upon protonation, the HOMO-to-LUMO transition changed from a π–π∗-type to a charge transfer-type transition.

Fig. 4.27

Synthesis of cycloplatinated helicene (M)-69 from (M)-68 and reversible protonation and deprotonation process of organic and organometallic systems, observed by emission and CPL spectroscopies. (i) Pt(dmso)₂(CH₃)₂, acetone, 50 °C, 5 h, 90%. Variation of emission and CPL responses upon protonation [67]

Rhenium(I)-chloro-tricarbonyl complexes bearing a bipy ligand are known to display efficient luminescence, usually a ³CT emission from an excited state based on the bis-imine ligand. In this context, organic helicene-bipy ligand (P)- and (M)-68 was used as N^N chelate to prepare enantioenriched CPL-active helicene-bipyridine-rhenium complexes 70 (Fig. 4.28) [68]. Starting from (M)-68 ligand, two diastereomeric complexes, i.e., (M,A _Re)-70a ¹ and (P,C _Re)-70a ², were formed, since the Re(I) atom is also a stereogenic center. These stereoisomers were separated by regular silica gel column chromatography and their chiroptical and emissive properties were studied. They revealed strong ECD spectra in CH₂Cl₂ (whose intensity depends on the rhenium stereochemistry; see Fig. 4.28), accompanied by substantial phosphorescence and CPL activity. Indeed (M,A _Re)-70a ¹ and (M,C _Re)-70a ² displayed phosphorescence emission (λ _max ^phos = 673–680 nm, ϕ = 0.13–0.16%, τ  = 27–33 ns) and good g _lum values ((M,C _Re)-70a ²: g _lum ~ −3 × 10⁻³ around 670 nm). Upon reaction with AgOTf and 2,6-dimethylphenyl isocyanide in the presence of NH₄PF₆, (M,C _Re) and (P,A _Re)-71a ² were transformed to cationic complexes (P)- and (M)-71a ^1,2, respectively (see Fig. 4.28). The latter displayed stronger phosphorescence (λ _max ^phos = 598 nm, ϕ = 6%, τ  = 79 μs) and still good CPL activity (g _lum ~ ±1.5 × 10⁻³). However, the stereochemical information at the Re(I) center was lost (epimerization to 50:50 mixture). Nevertheless, the ECD spectrum of (P)-71a ^1,2 displayed an additional positive ECD-active band around 450 nm as compared to (P,C _Re)-114a ¹ and (P,A _Re)-70a ². According to TDDFT calculations, this band does not involve the Re center but corresponds to the HOMO-to-LUMO transition with strong intra-ligand charge transfer from the π-helicene to the bipy moiety [68]. We have thus shown that the incorporation of a rhenium atom within an extended helical π-conjugated bipyridine system can impact the chiroptical and photophysical properties of the resulting neutral or cationic complexes, leading to the first rhenium-based circularly polarized phosphors.

Fig. 4.28

Synthesis of enantioenriched rhenium complexes (M,A _Re)-70a ¹, (M,C _Re)-70a ², and (M)-71a ^1,2 (mixture of two diastereomers). (i) Re(CO)₅Cl, toluene, reflux; (ii) AgOTf, EtOH/THF, then 2,6-dimethylphenyl isocyanide, THF, NH₄PF₆. X-ray crystallographic structure of 70a ². ECD spectra of (P,C _Re)-70a ¹, (P,A _Re)-70a ² and (M)/(P)-71a ^1,2 isomers (CH₂Cl₂). Phosphorescence and CPL spectra of (P,C _Re)-70a ¹, (P,A _Re)-70a ² and (M)/(P)-71a ^1,2 isomers in CH₂Cl₂ [68]

4.9.3 Coordination Chemistry of Bis-Helicene-Terpyridine and Bis-Helicene-Bipy Ligands

In 2016, our group also prepared the bis-helical terpyridine (terpy) ligand 72 which acted as a chiroptical switch upon reversible coordination-decoordination to zinc(II). The strong conformational changes induced led to a multi-responsive chiroptical switch (Fig. 4.29) [69]. The interconversion between the ligand and zinc-complexed states was analyzed via first-principles calculations, which highlighted the change from π-π∗ transitions in the organic ligand to charge transfer transitions in the Zn complex. Overall, this system behaved as a chiroptical switch offering multi-output readout (UV-vis, ECD, luminescence, and CPL). Furthermore, the switching process triggered conformational changes and molecular motion around the Zn center, from a clear trans (W-shape) conformation in the free ligand to a cis (U-shape) one in the Zn-complex 73 (Fig. 4.29, Table 4.13). Recently, we have prepared a novel enantiopure bis-helicenic 2,2′-bipyridine system ligand, 74 [70]. Thanks to the bipyridine unit, the coordination to 75 with Zn^II and protonation processes to 74.2H⁺ were studied revealing efficient tuning of photophysical (UV-vis and emission) and chiroptical properties (ECD and CPL) of the system (Fig. 4.29, Table 4.13). The coordination-decoordination and protonation/deprotonation processes appeared reversible thus constituting novel chiroptical switches.

Fig. 4.29

(a) Reversible Zn(II) complexation-decomplexation process of (P,P)-72 to (P,P)-73 using Zn(OAc)₂ and TPEN as the chemical stimuli [69]. (b) Reversible Zn(II) complexation-decomplexation process of (P,P)-74 to (P,P)-75 using Zn(OAc)₂ and TPEN as the chemical stimuli and acid-base triggered switch between (P,P)-74 and (P,P)-74.2H⁺. Emission colors and CPL activity of 72 (black), 75 (red) and 74.2H⁺ (blue). Plain lines are for the (P) helicenes and dotted lines for the (M) ones. Adapted with permission [70]. Copyright 2019, American Chemical Society

Table 4.13 Photophysical data of helicene-bipy and terpy ligands together with heir Zn and proton complexes

4.9.4 CPL-Active Iridium Complexes from Helicene-NHC Ligands

In the last two decades, octahedral cyclometalated iridium(III) complexes have attracted attention due to their appealing properties as phosphors in high-efficiency organic light-emitting devices (OLEDs) [71]. In 2017, the first fused π-helical NHC system was prepared and examined through its diastereoisomerically pure cyclometalated complexes mer-(P,Λ _Ir)-77a ¹ and mer-(P,Δ _Ir)-77a ² from pentahelical imidazolium 76 (Fig. 4.30) [49]. These chiral organometallic species displayed light-green phosphorescence with (i) circular polarization that depends on both the helical-NHC (P)/(M) stereochemistry and the iridium (Δ)/(Λ) one (Fig. 4.30 and Table 4.14) and (ii) unusually long lifetimes (up to 250 μs as compared to 530 ns for model mer-78). The unprecedented features of 77a ^1,2 can be attributed to extended π-conjugation within helical carbenic ligand. A similar cycloiridiated complex, namely, mer-(Δ _Ir/Λ _Ir)-79, bearing a NHC N-substituted with a carbo[4]helicene unit was recently obtained and also displayed long-lived mirror-image circularly polarized phosphorescence (Table 4.14) [72].

Fig. 4.30

Preparation of cycloiridiated complexes 77 ^1,2. Chemical structures and CPL spectra of iridium(III) complexes (P,Λ _Ir)-77a ¹/(M,Δ _Ir)-77a ¹, (P,Δ _Ir)-77a ²/(M,Λ _Ir)-77a ², and (Δ _Ir)-78/(Λ _Ir)-78. Chemical structure of (Δ _Ir)-79. X-ray structure of stereoisomer(P,Λ _Ir)-77a ¹. All complexes have the mer geometry [49, 72]

Table 4.14 Photophysical data of helicene-NHC iridium complexes

4.10 Applications in Optoelectronics

There is an obvious interest to develop the use of helicenes and helicenoids as chiral molecular materials in chiral OLEDs, in chiral sensors, in chiral bioimaging agents, chiroptical switching activity [73, 74] applications that directly take benefit from circularly polarized emission. For instance, there is a strong potential of CP light technologies in the development of OLEDs in which the electroluminescence is directly circularly polarized thus giving CP-OLEDs. Indeed, antiglare filters commonly used for OLED displays exploit the physics of CP light to eliminate glare from external light sources (e.g., the sunlight). Unfortunately, this technology removes approximately 50% of the non-polarized light emitted from the OLED pixels. If the non-polarized OLEDs are replaced with CP-OLEDs (with a comparable device performance), an improved amount of CP light component of the correct handedness would pass through the antiglare filter with less loss, thus increasing the energy efficiency of the display in proportion to the increasing dissymmetry of the light. In addition, the use of CP-OLED will enable to simplify the architecture of the device by avoiding the use of extra filter components, which will directly impact the overall cost of the device.

In 2013, Fuchter and coworkers, reported the use of 1-aza[6]helicene 9 as a chiral dopant in light-emitting polymer, i.e., poly[9,9-dioctylfluorene-co-benzothiadiazole] 80 (Fig. 4.31a) [21]. It was found that blends consisting of a small amount (7%) of enantiopure 1-aza[6]helicene dopant gave a strong CP-photoluminescence response of the 80 films. Increasing the 1-aza[6]helicene blending ratio resulted in improvements of the g _PL factor, up to a significantly high value of 0.5 for the 53% helicene blend (while the starting azahelicene displayed only modest g _lum ~10⁻⁴ to 10⁻³). To explain this behavior, the authors suggested the formation of a chiroptical co-crystalline phase. The authors were then able to fabricate a single-layer polymer LED (PLED) device emitting circularly polarized light from the 80 blends containing 7% of either (−)-1-aza[6]helicene or (+)-1-aza[6]helicene with a dissymmetry factor of electroluminescence (g _EL) factor as high as 0.2. In 2016, Fuchter and Campbell succeeded in preparing a single layer CP-phosphorescent OLEDs (CP-PHOLEDs), using 66a as a chiral emissive dopant; these PHOLEDs displayed strong circularly polarized electrophosphorescence (CPEL), with g _EL reaching −0.38 and + 0.22 at 615 nm for (−)- and (+)-66a, respectively (see Fig. 4.31b) [75]. Although not yet clearly demonstrated, the increase of g _EL as compared to the molecular g _lum value (10⁻²) may be explained by a supramolecular organization of 66a in the solid state. Recently, by decorating the pyridyl-helicene ligands with –CF₃ and –F groups [66], the platinahelicene enantiomers 66e featured good configurational stability as well as high sublimation yield (>90%) and clear circularly polarized phosphorescence, with dissymmetry factors (|g _PL|) of approximately 3.7 × 10⁻³ in solution and about 4.1 × 10⁻³ in doped films. The CP-PHOLEDs with two enantiomers as emitters exhibited symmetric CPEL signals with |g _EL| of (1.1–1.6) × 10⁻³ and good device performances, achieving a maximum brightness of 11,590 cd m⁻², a maximum external quantum efficiency up to 18.81%, which are the highest values among the reported devices based on chiral phosphorescent Pt^II complexes. To suppress the effect of reverse CPEL signal from the cathode reflection, the further implementation of semi-transparent aluminum/silver cathode successfully boosted up the |g _EL| by over three times to 5.1 × 10⁻³.

Fig. 4.31

(a) CP-PLED based on blends between enantiopure 9 and 80. Adapted with permission 19 Copyright 2013, Wiley. (b) CP-PHOLED based on pure enantiopure cycloplatinahelicene 66a. Adapted with permission [75]. Copyright 2016, American Chemical Society

4.11 Conclusion

Since the first examples of helicenes displaying circularly polarized luminescence described in the literature [13, 37], there has been a growing interest in CPL-active helicenes and helicenoids, and the number of reported examples is growing very fast. This is concomitant to the development of helicenes chemistry and the creation of structural diversity of helical structures, either fully carbonated or containing main-group elements, and to the development of CPL instrumentations and CPL technology. However, a much higher degree of circular polarization is still highly desirable for applications, for example, in chiroptical devices. For this purpose, future work will be probably dedicated to the development of supramolecular architectures, sophisticated homogeneous/heterogeneous mixtures, aggregation induced emission (AIE) materials, or chiral TADF systems, to obtain good CPL properties together with overall good quantum yield of photoluminescence. In these domains, there is still plenty of room for fundamental discoveries and for further applications. Finally, although not detailed in the present chapter, theoretical calculations of CPL activity may be of great help to anticipate highly efficient CPL-active systems. In this context, CPL-active helicenes are good models for testing and improving theoretical tools.