1 Introduction

Triplet photosensitizers (PSs) are compounds that show strong absorption of visible light and efficient intersystem crossing (ISC) [1,2,3]. Triplet PSs are useful for the study of fundamental photochemistry, and the compounds have been widely used in photocatalysis [4,5,6], photodynamic therapy [7,8,9], phosphorescence molecular probes [10,11,12], photovoltaics [13] and photon upconversion [14,15,16,17]. Most of the triplet PSs contain heavy atoms, such as Ru, Pt, Ir, I, or Br, to enhance the ISC [1, 2]. However, usage of these heavy atoms is a disadvantage due to the high cost and toxic nature of the triplet PSs. Therefore, it is highly desired to develop heavy atom-free triplet PSs [8]. However, from a photochemistry point of view, it is difficult to develop heavy atom-free triplet PSs because the ISC ability of the compounds becomes elusive without the heavy atom to enhance the spin–orbit coupling (SOC) [18]. Recently, a few approaches to design heavy atom-free triplet PSs have been reported, for instance, using exciton coupling [19, 20], electron spin converter [21,22,23,24], radical enhanced ISC (REISC) [25, 26], singlet fission [27], etc. Unfortunately, the triplet PSs based on these ISC mechanisms are usually synthetically demanding. For instance, the two identical chromophores of the triplet PSs based on exciton coupling have to be connected with rigid linkers and the two chromophores have to adopt a specific mutual orientation [19, 20, 28]. For this reason, a simple molecular structure motif for heavy atom-free triplet PSs is highly desired. Concerning this aspect, the charge recombination (CR)-induced ISC has attracted much attention recently, especially for the compact electron donor–acceptor (D-A) dyads, i.e. the donor and acceptor moieties are connected with short, simple linkers [29,30,31].

CR-induced ISC has been known for decades, especially for the molecular D-A systems mimicking natural photosynthesis centers [21, 32,33,34,35]. In those electron D-A dyads, the electron donor and acceptor are separated at large distances by the long and rigid linkers [36, 37]. The purpose of this molecular structure motif is to reduce the electronic coupling between the donor and acceptor and also the electron exchange energy of the electrons in the radical anion and cation; thus, the radical-pair ISC (RP-ISC) will occur. However, these electron D-A dyads are challenging to prepare from a synthesis point of view.

On the other hand, the compact electron D-A dyads are feasible to synthesize, but the electron exchange energy for the radical pairs in these dyads is large, as a result, the RP-ISC is inhibited [35]. However, given the electron donor and acceptor units in the compact dyads adopting orthogonal mutual orientation, then during CR, the molecular orbit angular momentum change offsets the electron spin angular momentum change of the ISC [38,39,40,41,42,43]. As a result, the CR-induced ISC will occur in these orthogonal compact electron D-A dyads, which is termed as spin–orbit charge transfer ISC (SOCT-ISC) [39,40,41,42,43,44,45].

Recently, the SOCT-ISC mechanism was reported in some compact electron D-A dyads, for instance, anthracene-phenothiazine [46], Bodipy-anthracene [47], Bodipy-phenothiazine [48], perylene-phenothiazine [49], perylenebisimide-carbazole [50], perylene-monoimide-phenothiazine [51, 52], naphthalimide-phenothiazine [53], etc. Moreover, the linkers in these compact orthogonal dyads are C–C or C–N single bond, which is able to rotate to some extent, although confirmation restriction was also imposed. Therefore, much room is left for designing novel molecular structural motifs to attain effective molecular conformation restriction and to study its effect on the SOCT-ISC efficiency.

Herein, we report novel spiro compact electron D-A dyads with lactam rhodamine as the electron donor and coumarin moiety as the electron acceptor [54, 55]. The spiro structure of the dyads is different from the previously reported compact D-A dyads showing the SOCT-ISC. Coumarin derivatives are well-known electron acceptors (reduction potential, ERed =  − 1.58 to  − 1.74) V, vs Fc/Fc+) with a high energy triplet state (ET1 = 2.47 to 2.65 eV) [56,57,58,59], which can be used in the construction of compact electron D-A dyads showing electron or charge transfer processes [59]. Coumarin was widely used in fluorescence probes, fluorescence bioimaging, and light-harvesting arrays [60, 61]. However, its application in the formation of triplet excited states in heavy atom-free organic compounds is rare. It was attached to a transition metal coordination center (Re(I), Ru(II), Ir(III), etc.) [62,63,64], to prepare triplet PSs. We synthesized two aminocoumarin derivatives containing two different electron-withdrawing groups (–CN, –CF3) and linked them with rhodamine B by amidation reaction to obtain the target dyads RB-Cou-CN and RB-Cou-CF3 (Scheme 1). Various steady-state, time-resolved spectroscopic methods, electrochemical characterization, and density functional theory (DFT) computatiosn were used to unravel the photophysical properties. The formation of the triplet state of the dyads was investigated by nanosecond transient absorption (ns-TA) spectroscopy.

Scheme 1
scheme 1

Synthesis route of the spiro electron donor/acceptor dyads, reagents and conditions: a K2CO3, (CH3)2SO4, acetone, reflux, 4 h, yield: 98%. b DIBAL–H (1 M in DCM), dry THF, 0 °C, 1 h, yield: 90%. c MnO2, CHCl3, reflux, 2.5 h, yield: 92%. d BBr3, dry DCM, − 78 °C to rt, 5 h, yield: 92%. e Malononitrile, NaHCO3 (aqueous, 0.1 M), rt, 2 h; HCl (con.), reflux, 1 h; yield: 92%. f TBAB, SnCl2⋅H2O, 90 °C, 1 h, yield: 73%. g POCl3, dry 1,2-dichloroethane, reflux, 4 h; Cou-CN, dry ACN, Et3N, 55 °C, 30 h; yield: 13%. h o-aminophenol, ZnCl2, EtOH, reflux, 8 h, yield: 10%. i POCl3, dry 1,2-dichloroethane, reflux, 4 h; Cou-CF3, ACN, Et3N, 50 °C, 19 h; yield: 30%

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2 Experimental section

2.1 General methods

UV–vis absorption spectra were measured on an 8453 UV–vis spectrophotometer (Agilent Ltd., USA). Fluorescence emission spectra were recorded on an RF-5301PC spectrofluorometer (Shimadzu Ltd., Japan). C13534-11 UV-NIR absolute PL quantum yield spectrometer (Hamamatsu Ltd., Japan) was used to measure the absolute photoluminescence quantum yields. Luminescence lifetimes were measured on an OB920 photoluminescence lifetime spectrometer (Edinburgh Instruments, U.K.). For film preparation, the solution of RB-Cou-CN (1.0 × 10−4 mmol) was injected into polyacrylate emulsion (0.5 mL), and then the transparent and flexible film was formed after drying in air.

2.1.1 Synthesis of RB-Cou-CN

Under N2 atmosphere, rhodamine B (240 mg, 0.5 mmol) was dissolved in dry 1,2-dichloroethane (15 mL), then POCl3 (0.5 mL) was added dropwise and the mixture was refluxed for 4 h. The solvent was evaporated under reduced pressure, then the crude product was dissolved in dry ACN (10 mL). Then Cou-CN (60 mg, 0.32 mmol) and Et3N (0.7 mL) were added and the reaction mixture was stirred at 55 °C for 30 h. On completion of the reaction, the solvent was removed under reduced pressure and the crude product was purified by column chromatography (silica gel; DCM/ethyl acetate, 50:1, v/v). RB-Cou-CN was obtained as a yellow solid (26 mg, yield: 13%). 1H NMR (400 MHz, CDCl3, ppm) δ = 8.03 (d, J = 8.0 Hz, 2H), 7.71 (s, 1H), 7.43 (dd, J = 8.0 Hz, J = 8.0 Hz, 4H), 7.09 (d, J = 8.0 Hz, 1H), 6.56 (s, 2H), 6.39 (s, 2H), 6.25 (s, 2H), 3.31–3.33 (m, 8H), 1.16 (t, J = 16.0 Hz, 12H); 13C NMR (CDCl3, 126 MHz): δ = 168.86, 157.05, 154.82, 154.11, 152.44, 151.21, 149.06, 144.94, 134.15, 129.02, 128.41, 128.09, 127.75, 123.76, 123.67, 120.81, 113.94, 113.66, 110.76, 108.39, 105.62, 100.91, 98.11, 67.73, 44.34, 12.60; TOF–HRMS [(C38H34N4O4)+]: calcd, m/z = 611.2658; found, m/z = 611.2658.

2.1.2 Synthesis of RB-Cou-CF3

Under N2 atmosphere, rhodamine B (240 mg, 0.18 mmol) was dissolved in dry DCM (5 mL) and then POCl3 (0.5 mL) was added dropwise. The reaction mixture was refluxed and stirred for 4 h. The solvent was removed under reduced pressure; then crude product was dissolved in dry ACN (8 mL). After that, a solution of Cou-CF3 (115 mg, 0.5 mmol) in dry ACN (8 mL) was added to the reaction mixture via syringe, and Et3N (0.7 mL) was also added, then the mixture was refluxed for 19 h. On completion of the reaction, the solvent was removed under reduced pressure and the crude product was purified by column chromatography (silica gel; DCM as eluent). Compound RB-Cou-CF3 was obtained as a white-yellow solid (99 mg, yield: 30%). 1H NMR (400 MHz, CDCl3, ppm) δ = 7.99 (d, J = 8.0 Hz, 1H), 7.43–7.52 (m, 4H), 7.09 (d, J = 8.0 Hz, 1H), 6.64 (s, 1H), 6.61 (s, 1H), 6.59 (s, 1H), 6.39 (s, 2H), 6.28 (d, J = 8.0 Hz, 2H), 3.30–3.35 (q, 8H), 1.16 (t, J = 16.0 Hz, 12H); 13C NMR (CDCl3, 126 MHz): δ = 168.57, 154.38, 154.06, 152.56, 149.02, 133.81, 128.62, 128.31, 127.97, 125.06, 123.76, 123.62, 121.05, 114.45, 112.38, 110.45, 108.36,105.75, 98.07, 67.51, 44.33, 12.59. TOF–HRMS [(C38H34 F3N3O4 + H+)]: calcd, m/z = 654.2580; found, m/z = 654.2579.

2.2 Nanosecond time-resolved transient absorption spectroscopy

LP980 laser flash photolysis spectrometer (Edinburgh Instruments Ltd., UK) was used to record the ns-TA spectra of the compounds. An Opolette TM HE 355 UV nanosecond pulsed laser (OPOTEK, USA; pulse duration: 3–20 ns; typical laser power: 5 mJ per pulse; tuning: 210–2400 nm) was used as the excitation source, and xenon probe source was a 150 W ozone-free Xe arc lamp (pulsed frequency is up to 10 Hz). The transient signal was digitized with an oscilloscope (TDS 3012C, 100 MHz). Before measurements, all sample solutions were bubbled with N2 for ca. 15 min. The recorded kinetic traces and transient spectra have been analyzed using L900 software.

2.3 DFT calculations

Density functional theory (DFT) was used to optimize the molecular structures of the compounds at the B3LYP and with range-separated functionals i.e. ωB97XD or CAM-B3LYP and 6-31G(d) basis set. The excitation energy or energy gaps between the S0 state and the excited triplet states of the compounds were computed based on the optimal ground state geometry. All the calculations were performed with Gaussian 09W [65].

3 Results and discussion

3.1 Molecular structure design and synthesis scheme

Rhodamine is well known for the closed-form (lactam form) and the opened normal amide structural tautomerization [66]. In the opened form, the xanthene moiety is positively charged. However, rhodamine's closed-form (lactam structure) is neutral and acts as an electron donor. To our surprise, this structure was rarely used for the construction of electron donor–acceptor dyads for the study of electron transfer and charge separation [54, 67].

Recently we used this novel molecular structural motif to study charge transfer [54], although similar compounds were studied for aggregation-induced emission properties [55]. In this study, the rhodamine moiety acts as an electron donor and coumarin unit is an electron acceptor because the spiro lactam structure is beneficial for constructing rigid structures. The congested structure is beneficial for achieving an orthogonal orientation between the xanthene plane and the electron acceptor plane. Previously a cyclic peptide framework was used for the preparation of a rigid electron D-A dyad, but the synthesis was difficult, and the purpose was  to produce a charge transfer state, not to attain a localized triplet state (3LE) [68].

The synthesis of compounds 1–5, Cou-CN and Cou-CF3 are based on the reported methods [69,70,71,72,73,74], The synthesis procedures along with molecular structures of the compounds are presented in Scheme 1. The molecular structures are fully verified with 1H NMR, 13C NMR and HR-MS spectra (see the Experimental Section and Supporting Information).

3.2 UV–vis absorption and luminescence emission spectra

The steady-state UV–vis absorption spectra of the compounds were studied (Fig. 1a). Cou-CN shows an absorption band centered at 390 nm, whereas Cou-CF3 shows an absorption band at 360 nm. For RB-Cou-CN and RB-Cou-CF3 dyads, due to the formation of amide structure, the electron-donating ability of the amino substitution on coumarin moiety is reduced. As a result, we observed a blue-shifted absorption band for RB-Cou-CN and RB-Cou-CF3 at 370 nm and 350 nm, respectively, as compared with the reference Cou-CN (λabs = 390 nm) and Cou-CF3 (λabs = 360 nm). It is known the rhodamine chromophore can undergo reversible closed form-opened form tautomerism in the presence of acid and base, respectively. In the presence of trifluoroacetic acid (TFA), a strong absorption band at 558 nm was observed for both RB-Cou-CF3 and RB-Cou-CN, indicating the stable nature of dyads in an acid-free environment (Fig. S14, ESI†).

Fig. 1
figure 1

a UV–vis absorption spectra of the compounds. b Fluorescence emission spectra of the compounds (optically-matched solutions were used, A = 0.10, λex = 320 nm). c Normalized fluorescence emission spectra of the compounds. c = 1.0 × 10−5 M in toluene. 20 °C

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The fluorescence of the compounds was studied (Fig. 1b,c). Cou-CN and Cou-CF3 show strong fluorescence (fluorescence quantum yields are ΦF = 29% and 59%, respectively, in n-hexane). In comparison, the fluorescence of the coumarin chromophore in the dyads is almost completely quenched (Fig. 1b), and the ΦF are only 0.5–1.1% (in n-hexane, Table 1). This strong fluorescence quenching even in low polar solvents indicates that energy/electron transfer is feasible upon photoexcitation for both dyads. However, singlet energy transfer from the coumarin moiety to the lactam form of the rhodamine unit can be excluded due to the uphill feature of the singlet energy transfer. The most credible reason for fluorescence quenching is photoinduced electron transfer (PET) between rhodamine and coumarin moieties.

Table 1 Photophysical parameters of the compounds
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Notable here, besides the weak fluorescence bands of coumarin moiety (1LE emission; locally excited) in RB-Cou-CN and RB-Cou-CF3, a broad and weak emission band for the dyads centered at 550 nm and 595 nm was also observed in n-hexane, respectively (Fig. 1c and Fig. S15). The LE emission of RB-Cou-CF3 and RB-Cou-CN dyads is solvent polarity-independent. In contrast, the emission intensity at a longer wavelength sharply decreases and emission bands are red-shifted with increasing the solvent polarity (see Fig. S15, ESI†). Thus, the emission band at a longer wavelength is consistent with the solvent polarity-dependent charge transfer (CT) emission features [75]. On the basis of CT emission bands, we calculate 1CT state energy in different solvents. For instance, the 1CT state of RB-Cou-CF3 is estimated as 2.25 eV and 2.10 eV in n-hexane and toluene, respectively. Similarly, for RB-Cou-CN, the 1CT energy in different solvents are 2.19 eV (in n-hexane) and 1.94 eV (in toluene).

To obtain the triplet state energy of the dyads, we measured the luminescence spectra of the compounds at 77 K in deaerated n-hexane (Fig. 2, Fig. S16, ESI†). When the temperature decreased from 297 to 77 K, the emission intensity of both the dyads was enhanced, and the shape of the emission band changed. For instance, RB-Cou-CF3 shows three vibrational bands at ca. 450 nm, 515 nm and 550 nm (Fig. 2a). The lifetime of the 450 nm band is short, only 12.4 ns, while the other two bands have a long lifetime of 241 ms. For RB-Cou-CN (Fig. 2b), a broad emission band at about 570 nm was observed, but a shoulder band was also present at 520 nm. The lifetimes detected at 520 nm and 560 nm are 452 ms and 365 ms, respectively.

Fig. 2
figure 2

Normalized luminescence spectra of the compounds. a RB-Cou-CF3 and b RB-Cou-CN at 297 K and 77 K, λex = 340 nm, in deaerated n-hexane, c = 5.0 × 10−5 M

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From these observations, we approximate 3LE energy of RB-Cou-CF3 and RB-Cou-CN dyads as 2.23 eV and 2.17 eV, respectively. Moreover, the normalized luminescence spectra taken at 297 K and 77 K show that the 3LE energy of RB-Cou-CF3 is close to the 1CT state (2.25 eV in n-hexane). Although 1CT/3LE states share almost similar energy, the reverse ISC is nonefficient and thermally activated delayed fluorescence was not observed. This is confirmed by the fact that under deaerated conditions, fluorescence intensity/lifetimes of the dyads were not significantly increased (Fig. S17, ESI†). The same trend holds for RB-Cou-CN, which could be the reason for occurring the ISC in n-hexane. However, the energy gaps between 3LE and 1CT states increase with increased solvent polarity, and the 1CT state lies much lower than 3LE state. This mismatching of the states is obviously unfavorable for the efficient 1CT → 3LE process of ISC.

We, furthermore, measured the temperature-dependent (297–77 K) luminescence spectra of RB-Cou-CF3 and RB-Cou-CN dyads in deaerated n-hexane (Fig. 3). For RB-Cou-CF3 (Fig. 3a), with decreasing the temperature, the LE emission band at 410 nm sharply increases and red-shifted. In contrast, the CT emission band at about 550 nm slightly decreased and shifted bathochromically; therefore, it can be easily assigned to an intramolecular charge transfer (ICT) state [76]. It is noteworthy to mention that the reduced fluorescence intensity at lower temperatures is different from the normal fluorophores [18, 77]. When the temperature further decreased to 137 K (Fig. 3b), only one emission band centered at 550 nm was present, and its intensity drastically increased. From 117 to 77 K, the LE emission band in the range of 400–480 nm was raised again, while the band at 550 nm split into two peaks, and the lifetime was prolonged to a millisecond time scale. This can be assigned to the phosphorescence of coumarin moiety in the dyad. Similarly, for RB-Cou-CN (Fig. 3c), upon reducing the temperature, the LE emission decreased and blue-shifted, with the trend being opposite to that of RB-Cou-CF3. Meanwhile, the CT emission band declined sharply and slightly red-shifted, hence it is assigned to a CT state [76]. Moreover, when the temperature dropped from 177 to 77 K (Fig. 3d), emission intensity at 570 nm gradually increased, and its lifetime was enhanced again due to the phosphorescence of coumarin moiety.

Fig. 3
figure 3

Temperature-dependent luminescence spectra of RB-Cou-CF3; a 297–177 K, b 157–77 K and RB-Cou-CN; c 297–197 K, d 177–77 K; λex = 340 nm, in deaerated n-hexane, c = 5.0 × 10−5 M. Note: the melting point of n-hexane is 177 K

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We also studied the luminescence properties of dyads in transparent/flexible polyacrylate film under different atmospheres and temperatures (Fig. 4 and Fig. S18, ESI†). For RB-Cou-CN under deaerated conditions, the emission band at 535 nm increased compared with emission measured in an aerated atmosphere (Fig. 4a). Luminescence lifetime is extended from 12.7 μs (Fig. S18, ESI†) to 6.0 ms (Fig. 4b). This emission band is similar to the phosphoresce band (measured at 77 K in deaerated n-hexane) and the CT band of RB-Cou-CN (Fig. 2b).

Fig. 4
figure 4

Luminescence spectra of RB-Cou-CN in transparent/flexible film (polyacrylate emulsion). a Luminescence spectral comparison under aerated and deaerated conditions at 300 K and b decay trace at 300 K detected at 535 nm, in deaerated condition. c Temperature-dependent luminescence spectra and d decay trace at 120 K detected at 535 nm, in deaerated condition. λex = 340 nm.

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Since in aerated condition, this band is also long-lived. We conclude that the phosphorescence of RB-Cou-CN was observed under an aerated atmosphere when measured in film. Moreover, temperature-dependent (300–120 K) luminescence spectra of RB-Cou-CN in the film were also studied (Fig. 4c), and the luminescence intensity of 535 nm band increased with the decline of temperature.

The lifetime of this band is about 478.8 ms, probably the phosphorescence of coumarin moiety (Fig. 4d).

3.3 Electrochemical studies: cyclic voltammograms of the compounds

Cyclic voltammograms were recorded to study the electrochemical properties [21, 78]. In a reference spiro rhodamine with a phenyl attached at its amide position (RB-Ph), the two oxidation waves at + 0.52 V and + 0.67 V (vs. Fc/Fc+) were observed, and there is no reduction wave within the potential range used in the electrochemical measurement [54, 67]. For RB-Cou-CN and RB-Cou-CF3 (Fig. 5), two oxidation waves at + 0.58 V and + 0.73 V (vs. Fc/Fc+) were observed, the first oxidation wave is close to that of reference RB-Ph. An irreversible reduction wave at − 1.67 V was also observed for RB-Cou-CN. Similarly, an irreversible wave at − 1.85 eV was observed for RB-Cou-CF3, which can be assigned to the coumarin unit. These results indicate that rhodamine unit acts as an electron donor and coumarin moiety as an electron acceptor in the dyads.

Fig. 5
figure 5

Cyclic voltammograms of dyads. Conditions: in deaerated DCM containing 0.10 M Bu4NPF6 as supporting electrode, Ag/AgNO3 as reference electrode, redox potential are versus Fc/Fc+. Scan rates: 50 mV/s. c = 1.0 × 10−3 M. 20 °C

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Gibbs free energy changes of the charge separation (ΔGCS) and charge transfer states energy (ECT) of RB-Cou-CN and RB-Cou-CF3 are calculated by using Rehm–Weller equations (Eqs. 1–3) [21], and the results are compiled in Table 2.

$$\Delta G_{{{\text{CS}}}} = e\left[ {E_{{{\text{OX}}}} - E_{{{\text{RED}}}} } \right] - E_{00} + \Delta G_{{\text{S}}}$$
(1)
$$\Delta G_{{\text{S}}} = \frac{{e^{2} }}{{4\pi \varepsilon_{S} \varepsilon_{0} R_{{{\text{CC}}}} }} - \frac{{e^{2} }}{{8\pi \varepsilon_{0} }}\left( {\frac{1}{{R_{{\text{D}}} }} + \frac{1}{{R_{{\text{A}}} }}} \right)\left( {\frac{1}{{\varepsilon_{{{\text{REF}}}} }} - \frac{1}{{\varepsilon_{{\text{S}}} }}} \right)$$
(2)
$$E_{{{\text{CT}}}} = e\left[ {E_{{{\text{OX}}}} - E_{{{\text{RED}}}} } \right] + \Delta G_{{\text{S}}}$$
(3)
Table 2 Electrochemical parameters, Gibbs free energy changes of the charge separation (ΔGCS) and charge transfer states energy levels (ECT) of the compounds
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where ΔGS is the static Coulombic energy which is described by Eq. (1). e = electric charge; EOX = half-wave potential for one-electron transfer oxidation of the rhodamine unit; ERED = half-wave potential for one-electron transfer reduction of the coumarin unit; E00 = excitation energy of that particular excited state; εS = static dielectric constant of the solvent; ε0 = permittivity of free space; RCC = center-to-center separation distance between an electron donor (rhodamine) and electron acceptor (coumarin), RCC (RB-Cou-CN) = 5.38 Å and RCC (RB-Cou-CF3) = 5.37 Å are determined by DFT optimized geometry; RD = radius of electron donor, RA = radius of electron acceptor, εREF = static dielectric constant of the solvent used for the electrochemical studies [21].

For both dyads, ΔGCS values are negative in all solvents (non-polar and polar), which shows that charge separation is thermodynamically allowed even in non-polar solvents such as n-hexane (Table 2). The conclusion agrees well with the luminescence emission spectra of the dyads that CT emission was observed in a non-polar solvent.

3.4 Nanosecond transient absorption (ns-TA) spectroscopy: triplet state properties of the dyads

To study the formation of the triplet state of the dyads upon photoexcitation, nanosecond transient absorption (ns-TA) spectra of the compounds were measured in different solvents. For the reference compounds Cou-CN and Cou-CF3 upon excitation at 355 nm pulsed laser, no transient signals were observed in four representative deaerated solvents (HEX, TOL, DCM and ACN), hence ISC does not occur in the reference compounds and it is consistent with lack of singlet oxygen photosensitizing ability (Table S1, ESI†).

Upon photoexcitation of the dyads with 355 nm pulsed laser, however, transient absorption signals were detected for RB-Cou-CN and RB-Cou-CF3 in n-hexane (Fig. 6). For both the dyads, a ground state bleaching (GSB) band in the range of 300–400 nm and a broad excited state absorption (ESA) band in the range of 400–800 nm were observed (Fig. 6a, c). However, for RB-Cou-CN, a slightly different transient profile was observed upon laser excitation as compared with RB-Cou-CF3, except for the GSB band centered at 360 nm. The ESA band is broader in a 400–800 nm range, while in the case of RB-Cou-CF3, the ESA band is sharp and extended upto 750 nm. The lifetimes of the transient species were determined by monitoring the decay trances at ESA bands of RB-Cou-CN and RB-Cou-CF3 in deaerated n-hexane as 11.0 μs and 15.6 μs, respectively (Fig. 6b,d).

Fig. 6
figure 6

a Nanosecond transient absorption spectra of RB-Cou-CN and b the decay trace at 560 nm. c Transient absorption spectra of RB-Cou-CF3 and d the decay traces at 520 nm. Excited with nanosecond pulsed laser, λex = 355 nm, c = 2.0 × 10−5 M in deaerated n-hexane. 20 °C

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Moreover, when the solutions were exposed to air, the lifetimes of transient species significantly decreased (0.35–0.65 μs), whereas the ESA bands are similar to that observed in N2-saturated solution (Fig. S19, ESI†). This result indicates that the transient species is O2-sensitive and it could be LE triplet state; otherwise, the ESA profile will change under aerated conditions because the radical anion species can be easily quenched by O2 [79]. Furthermore, the coumarin radical anion absorption should appear around 600 nm, but this is not the case detected [59]. The typical triplet ESA absorption range of coumarin moiety is from 450 to 800 nm [80,81,82], thus, we conclude that the ESA bands observed in RB-Cou-CN and RB-Cou-CF3 dyads can be ascribed to the T1 → Tn transition of coumarin triplet state (3Cou*). This is also in agreement with the DFT predictions, the triplet state spin density of the dyads is localized on the coumarin moiety (see the next section), further indicating that the final excited states of RB-Cou-CN and RB-Cou-CF3 in n-hexane should be 3Cou*.

To study the effect of solvent polarity on ISC efficiency of dyads, ns-TA spectra were measured in deaerated TOL, DCM and ACN solvents, however, no triplet signals were detected. This could be due to the mismatching of 1CT and 3LE states in different polarity solvents. For instance, the energy of 1CT state (estimated by the onset CT emission band in TOL) of RB-Cou-CF3 is 2.07 eV, which is lower than that of the 3Cou state (T1 = 2.23 eV). Similarly, in ACN the 1CT state could be lower than the 1CT state in TOL and lies below the 3LE state, and the same trend holds for RB-Cou-CN. In polar solvents, the CT state is non-emissive, and the CT state directly decayed to the ground state via CR.

3.5 Density functional theory: geometry optimization, potential energy surfaces and spin density distribution

To provide insight into the molecule structures and the photophysical property of the dyads, DFT calculations were used to optimize the molecular geometries of the compounds (Fig. 7). The optimized ground state conformations of RB-Cou-CN and RB-Cou-CF3 show that the dihedral angle between the xanthene and the coumarin planes for both dyads is 86.3°, which is beneficial for SOCT-ISC [39, 40, 42]. Moreover, the π-conjugation framework of two dyads is separated by three σ-bonds, thus weak electronic coupling between the donor and acceptor is expected.

Fig. 7
figure 7

a Optimized ground state conformations, HOMO and LUMO of RB-Cou-CN and RB-Cou-CF3, isovalue = 0.03; the yellow and purple color sheets show the planes of the coumarin and the rhodamine moieties, respectively. b Ground state potential energy curves of RB-Cou-CN and RB-Cou-CF3, as a function of the rotational dihedral angle of the selected atoms; the thermal energy (kBT = 0.026 eV) at 300 K is indicated by a dashed line. All the calculations were based on the DFT (B3LYP/6–31G(d)) level with Gaussian 09 in vacuum. Hydrogen atoms are omitted for clarity

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In addition, to investigate the electron transfer in RB-Cou-CN and RB-Cou-CF3 dyads, the distribution of frontier molecular orbitals, highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) were studied (Fig. 7a). HOMO and LUMO are localized on xanthene and coumarin moieties, respectively, indicating the possibility of PET. The ground state potential energy curves (PECs) of RB-Cou-CN and RB-Cou-CF3 dyads against the torsion of the acceptor moiety about the linker are shown in Fig. 7b. The PECs generated by rotation about C − C bond of the selected atoms (RB-Cou-CN: ∠C4–C7–C74–C73, RB-Cou-CF3: ∠C4–C7–C73–C72; the xanthene plane was fixed and the coumarin plane was rotating around the C–C bond of the dyads in the calculations) are shallow, and the energy barriers are small (< 0.15 eV) when the angles vary from 0° to 180°.

This indicates that only limited conformational restriction is applied. However, below the thermal energy range (kBT = 0.026 eV, T = 300 K), the dihedral angles of the dyads can easily vary from 70° to 130°. According to previously reported bodipy-anthracene dyads with high singlet oxygen quantum yields, the PEC was steep and the energy barrier was much larger than the thermal energy at room temperature. Therefore, it was easy to be restricted to orthogonal conformation for those dyads [83]. However, for other bodipy-anthracene dyads with low singlet oxygen quantum yields, the PEC was shallower and the energy barrier was much smaller (< 0.4 eV), which can relax the geometry constrain [83]. In our case, the dihedral angles of RB-Cou-CN and RB-Cou-CF3 dyads are flexible and PECs are shallow, which are detrimental to attaining efficient SOCT-ISC. A significant deviation from the orthogonal geometry can lower triplet quantum yield, which could be the reason for the low singlet oxygen quantum yields of dyads.

The electron spin density distribution of the optimized triplet state geometry, radical anion and radical cation of the dyads are presented in Fig. 8. Spin unpaired electrons of the triplet state are mainly localized on coumarin moiety in both RB-Cou-CN and RB-Cou-CF3, which is supported by the fact that the triplet state signal of coumarin was observed in the ns-TA spectra. The spin density in ACN is also calculated (Fig. S20, ESI†), the triplet spin density of RB-Cou-CF3 was still localized on the coumarin units, however, the triplet spin density of RB-Cou-CN was delocalized to rhodamine unit. Moreover, for both the dyads, the spin densities of the radical anion and radical cation are localized on the coumarin and xanthene moieties (Fig. 8), respectively. These computational results are also supported by an electrochemical study, further representing that the xanthene moiety acts as the electron donor and the coumarin unit as the electron acceptor in the PET process. Moreover, we also optimized and compared the DFT results obtained by B3LYP and using range-separated functionals i.e. ωB97XD and CAM-B3LYP, similar results were observed (Table S2).

Fig. 8
figure 8

Spin density distributions of RB-Cou-CN (upper panel) and RB-Cou-CF3 (lower panel) at the optimized (a, d) triplet state, (b, e) radical anion, and (c, f) radical cation geometries (isovalues are 0.0004) in cyclohexane. All the calculations were based on the DFT (B3LYP/6–31G(d)) level with Gaussian 09

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Based on the experimental data and DFT results, the Jablonski diagram was constructed to illustrate the photophysical processes involved in RB-Cou-CF3 upon photoexcitation (Scheme 2, MOs involved in the transitions are also shown). The occupied molecular orbitals and unoccupied molecular orbitals are localized on rhodamine and coumarin units, respectively. For the calculated S1, S2 and S3 states of the dyad, the corresponding transitions are HOMO → LUMO, HOMO–1 → LUMO and HOMO–3 → LUMO, respectively (Table S3, ESI†). Moreover, for the T1, T2, T3 and T4 states, the corresponding transitions are HOMO–2 → LUMO, HOMO → LUMO, HOMO–1 → LUMO and HOMO–4 → LUMO. In addition, the T1 and T4 states exhibit 3LE feature, which indicates that T1 and T4 states of dyads are localized on the coumarin unit. However, the T2 and T3 states are with CT features and lie significantly above the 3LE state.

Scheme 2
scheme 2

Simplified Jablonski diagram illustrating the photophysical processes involved in the RB-Cou-CF3: RB-1Cou*-CF3 energy level is obtained from the crossing point of the normalized UV–vis absorption and fluorescence emission spectra, in n-hexane. 1CT energy levels are calculated based on the CT emission band, in n-hexane. The lowest triplet excited state (RB-3Cou*-CF3) energy was obtained from the phosphorescence at 77 K. The energy of Sn and Tn (n = 1,2,3,4) states are also shown with respective transitions, calculated with TDDFT at the B3LYP/6-31G(d) level using Gaussian 09 in vacuum

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We assume that both the dyads can undergo SO-ISC (S1 to the near Tn state and through internal conversion (IC) to form T1) or SOCT-ISC to generate the 3Cou* state (localized on coumarin moiety). However, singlet oxygen quantum yields and CT emission bands (observed in n-hexane) indicate that the formation of 3Cou* state in the dyads is not based on SO–ISC mechanism. Hence, SOCT-ISC mechanism is the only way to form the triplet state in these dyads. Upon photoexcitation, 1Cou* state is first populated subsequently charge separation takes place and leads to populating 1CT state. Herein, there are two optional paths for charge recombination (CR). One is directly decayed to the ground state (with CT emission lifetimes of 4.6 ns for RB-Cou-CF3), and another relaxation path is SOCT-ISC to populate the 3Cou* state. We observed a coumarin localized triplet state via SOCT-ISC mechanism with a lifetime of 15.6 μs. A similar energy diagram was constructed to demonstrate the photophysical processes of RB-Cou-CN (Scheme S1, ESI†).

4 Conclusion

In summary, we prepared spiro rhodamine (RB)-coumarin (Cou) compact electron donor–acceptor dyads, in which the lactam form of the rhodamine unit is the electron donor and the cyano- or trifluoromethyl substituted coumarin as the electron acceptor (RB-Cou-CF3 and RB-Cou-CN), as well as the visible light-harvesting chromophore. The aim of designing such a novel molecular structural motif is to study the spin–orbit charge transfer intersystem crossing (SOCT-ISC) mechanism. CT emission bands were observed for RB-Cou-CF3 (at 550 nm) and RB-Cou-CN (at 595 nm), although there is no significant electronic interaction between electron donor and acceptor at the ground state. Based on the phosphorescence of the dyads observed at 77 K in frozen solution, the energy of the coumarin-localized triplet state of the dyads was determined (2.17 eV for RB-Cou-CN and 2.23 eV for RB-Cou-CF3). The shallow potential energy curves of the dyads against the torsion of the linkers indicate that the flexible molecular conformation may contribute to the insufficient SOCT-ISC efficiency and the low singlet oxygen quantum yields (ΦΔ = 3.2–7.5%). By maintaining the geometry constrain between donor and acceptor as orthogonal dihedral range and 1CT-3LE energy matching could improve the SOCT-ISC efficiency. According to the DFT/TDDFT calculations, electrochemical study and nanosecond transient absorption (ns-TA) spectral data, 3Cou* state is formed only in non-polar solvent via SOCT-ISC, the triplet states lifetimes are 15.6 μs (RB-Cou-CF3) and 11.0 μs (RB-Cou-CN). In polar solvents such as acetonitrile, no 3Cou* state was observed in the ns-TA spectra, because of the low-lying CT states (ca. 1.90 eV). Thus, the proposed photophysical path for RB-Cou compact D-A dyads is as follows: RB-1Cou* → RB+•-Cou–• → RB-3Cou*. Our studies are useful for the design of compact electron donor/acceptor dyads showing spin–orbit charge transfer intersystem crossing (SOCT-ISC) and for the formation of long-lived triplet states.