INTRODUCTION

Fluorescence, which is spontaneous light emission coming from excited electronic states after absorption of UV/visible light, has received great attention in many disciplines, such as chemistry, physics, material science, medicine, and biology [1–4]. Salicylaldehyde Schiff bases were important typical fluorescent molecules, which were easy to synthesize and stable. It was also known to undergo excited-state intramolecular proton transfer process (ESIPT), which gives rise to large Stokes shifts and efficient emission [5, 6]. Several groups have demonstrated tuning of the fluorescence wavelength from blue and red, and the most common strategy is to adjust various electron-donating or electron-withdrawing substituents at different positions of the organic fluorescent molecules [7–10]. In 2015, Xiang et al. have reported the synthesis and the application of fluorescent pH probes of a series fluorescent Schiff bases containing different electron-withdrawing substituents (–NO2, –F, and –Cl) and electron-donating substituents (–OMe and –NEt2). The fluorescent molecules display strong blue, green, and red fluorescence as the substituents modified [11]. Recently, they also reported a series of simple and highly fluorescent salicylaldehyde hydrazone molecules with different substituents on the aromatic rings [12]. They found the molecules display strong blue, green, yellow, and orange aggregation-induced emission (AIE) with large Stokes shifts (up to 184 nm) and high fluorescence quantum yields (Ф up to 0.20) as the substituents varied.

On the other hand, computational chemistry has become an important tool for chemists and a well-accepted partner for experimental chemistry in these years. Theoretical quantum chemistry is an important area in determining the mechanisms of chemical reactions [13], structural determination of organic compounds [14], prediction of spectroscopic data such as UV/Vis and fluorescence properties of organic molecules [15, 16]. It is important to understand the relationship between structure and spectral property. In theory, the energy gap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) is often used to analyze the absorption and emission properties in theoretical design for this kind of materials. The theoretical studies (DFT calculations) were often used for interpreting the molecular structures, spectroscopic assignments [16, 17]. In this research, we inspired to used DFT calculations to investigate the ground- and excited-state geometries, HOMO–LUMO gaps, UV–Vis, fluorescence and reorganization energy (λ) of six reported salicylaldehyde hydrazones (SHs) containing different electron-withdrawing substituent (–F, –NO2) and electron-donating substituents (–OMe, –NEt2) at different position of the aromatic ring, and further explored the substituents effect on the electronic structure and spectral properties of the molecules.

COMPUTATIONAL METHODS

In this work, the quantum chemical calculations have performed and optimized six salicylaldehyde hydrazones (SHs) using the density functional theory (DFT) [18] method with 6-31g(d,p) basis sets by the Gaussian 09W program package [19] on a Pentium IV/3.60 GHz personal computer. The ground-state and the excited-state geometries were fully optimized by with PBE1PBE [20]. The Polarized Continuum Model (PCM) [15] was used for calculations. The absorption wavelengths (λabs) and emission wavelengths (λem) of these molecules in a solvent acetonitrile are systematically studied at the theoretical level of PBE1PBE/6-31g(d,p). The electronic properties such as EHOMO, ELUMO, energy gap between HOMO and LUMO and reorganization energy (λ) [21] of the title structures were calculated. The optimized molecular structures, HOMO and LUMO surfaces were visualized by GaussView05 program [22].

RESULTS AND DISCUSSION

Optimized Geometries

The schematics of the structures are depicted in Fig. 1, along with the optimized S0 state geometries by PBE1PBE/6-31g(d,p) of the six salicylaldehyde hydrazones (SHs) plotted in Fig. 2. The important optimized structural parameters including bond lengths and dihedral angle of the molecules are listed in Table 1, and compared with the experimental data [12]. The calculated results are in agreement with the experimental values, the maximum deviation for bond lengths and dihedral angle are 0.0393 Å and 8.82°, respectively, indicating that the functional and basis set used are reasonable and feasible for the system under study.

Fig. 1.
figure 1

The sketch map of the structures for the studied systems.

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Fig. 2.
figure 2

(Color online) The stereograph of optimized structures of the SHs by PBE1PBE/6-31g(d,p).

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Table 1.   Selected optimized important length (Å) and angle (deg) of SHs, as well as experimental data
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The main difference between the studied compounds is that the π-conjugated system and the substitution of the salicylaldehyde are changed in the frameworks, compared to the molecule of SH-Ph. It was found that the substituent effect (SH-Ph, 3-OMe-SH-Ph, 4-NEt2-SH-Ph, 3-F-SH-Ph, 3-NO2-SH-Ph) on the salicylaldehyde may not cause significant changes in the optimized structures of the bonds and dihedral angle. But the dihedral angle of the two aromatic rings in the framework is reduced from 29.43° to 12.87° when the benzene ring is replaced by naphthalene (SH-Ph and Naph-SH-Ph). This indicated that the increasing π-conjugated aromatic rings of the system might increase the planarity of the molecule.

The optimized structures of S1 states were also obtained using TD-DFT at PBE1PBE/6-31g(d,p) level. The dihedral angle between salicylaldehyde and benzoylhydrazine of the S0 and S1 state is depicted in Fig. 3. From Fig. 3, the dihedral angle differences between S0 and S1 states of four compounds (Naph-SH-Ph, 3-OMe-SH-Ph, 4-NEt2-SH-Ph, 3-NO2-SH-Ph) are below 15° except SH-Ph. Interestingly, the dihedral angle differences of compound 3-OMe-SH-Ph and 3-NO2-SH-Ph are 0.13° and 2.68°, respectively which are much lower than other compounds. The tiny dihedral angle differences between S0 and S1 states indicate that the non-radiative decay from S1 to S0 is able to be suppressed effectively for the compounds [23].

Fig. 3.
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(Color online) Dihedral angles of the six compounds at their S0 and S1 states.

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The Frontier Molecular Orbitals Analysis

The influence of the electronic and luminescent properties is usually investigated by the molecular orbital analysis, especially excitation and transition properties can be reflected by the frontier molecular orbitals. The influence of a particular substituent on an aromatic system is usually investigated in terms of conjugated and induced effects, and the conjugated effect is usually associated with the sharing of π-electrons between the substituent and the aromatic system. The induced effect is related to the σ-electron system of the aromatic molecule. In general, the electron-withdrawing groups can make the energies of the highest occupied and lowest unoccupied MOs decreased, while the electron-donating groups cause an increase of the HOMOs and LUMOs energies.

The calculated HOMO and LUMO energies and electronic density contours are presented in Fig. 4. In contrast with SH-Ph, it is easy to see that HOMO and LUMO energies increase for the compound with electron-donating group on salicylaldehyde (3-OMe-SH-Ph, 4-NEt2-SH-Ph) while for the electron-withdrawing groups the HOMO and LUMO energies decrease (3-F-SH-Ph, 3-NO2-SH-Ph). The maximum difference of HOMOs energy is 0.84 eV appeared in 4‑NEt2-SH-Ph (HOMO: –5.35 eV) and that of LUMOs energy is 1.05 eV appeared in 3-NO3-SH-Ph (LUMO: –2.68 eV). For 4-NEt2-SH-Ph and 3-NO3-SH-Ph, the HOMO–LUMO gaps (∆H–L) decrease 0.46 and 0.75 eV by comparing to SH-Ph, whereas they increase 0.02 eV for 3-OMe-SH-Ph and 0.11 eV for 3-F-SH-Ph. To explain these phenomena, selecting frontier molecular orbitals of these complexes which are depicted in Fig. 4, for 4-NEt2-SH-Ph, the HOMO is mostly localized on salicylaldehyde and imine while the LUMO is distributed throughout the framework, and the strong electron-donating group of NEt2 could shared the p electron with salicylaldehyde by p–π conjugation, thus the strong p–π conjugated donation from the NEt2 group raises the energy of HOMO level in evidence, resulting in the ∆H–L decrease. For 3-NO3-SH-Ph, the LUMO is focused on the phenyl part of salicylaldehyde, thus the electron-withdrawing substituent of nitro at the phenyl of salicylaldehyde will decrease the LUMO energies more than HOMO energies, resulting in the decreased ∆H-L. Different from 3-NO2-SH-Ph, the LUMO is distributed throughout the framework in the molecular 3-F-SH-Ph which will decrease the HOMO energies more than LUMO energies, so for 3-F-SH-Ph, the ∆H-L is increased. In addition, the expansion of the conjugated salicylaldehyde for Naph-SH-Ph seems increase the HOMO energies and hence reduce the ∆H-L. So the strong electron-donating (electron-withdrawing) substituents and extended conjugate system on salicylaldehyde can make evident decrease of the ∆H–L for these molecules.

Fig. 4.
figure 4

(Color online) The HOMO and LUMO energies and electronic density contours of the six compounds at S0 state.

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Absorption Spectra

Absorption spectra were calculated using TD-DFT method in acetonitrile solution on the base of optimized geometries. Table 2 reports the transition energies (E), oscillator strengths (f) and major configurations of the leading excited states with the largest amplitudes, along with their experimental values. As shown in Table 2, the S1 absorption bands of compounds SH-Ph to 3-NO2-SH-Ph are 322.83, 346.21, 319.53, 348.53, 309.86, and 403.12 nm, respectively. In addition, the excitation HOMO → LUMO (SH-Ph: 96.7%; Naph-SH-Ph: 97.8%; 3-OMe-SH-Ph: 95.3%; 4-NEt2-SH-Ph: 98.5%; 3-F-SH-Ph: 95.5%; 3-NO2-SH-Ph: 97.8%) are predominantly responsible for the absorption. Observing Fig. 4 concludes that the transitions of all electrons are typical π → π* types. All these results agree well with the variation tendency of ∆H-L. Taking compound SH-Ph as a reference, the S1 absorption peaks of compound 3-F-SH-Ph and 3‑OMe-SH-Ph are blue-shifted, while others are red-shifted. In comparison with compound SH-Ph (322.83 nm), compounds 3-F-SH-Ph and 3-OMe-SH-Ph show blue-shift by 12 and 3 nm in the S1 absorption wavelength, respectively. The S1 absorption wavelengths of compounds 4-NEt2-SH-Ph and 3‑NO2-SH-Ph are red-shifted by about 25 and 80 nm relative to compound SH-Ph. Therefore, the diethylamino group as electron-donating and nitro group as electron-withdrawing strongly affects the electronic properties, resulting in a larger increase in S1 absorption wavelength than fluorine and methoxy groups. As for compound Naph-SH-Ph, the S1 absorption wavelength is red-shifted compared with compound SH‑Ph due to the increasing π-conjugation length.

Table 2.   Electronic transition data obtained by TD-DFT for the six compounds
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For all the compounds, the largest absorption wavelengths correspond to different excited states. Different from other compounds, the compounds 3‑OMe-SH-Ph and 3-NO2-SH-Ph have absorption peaks localized at 292.05 and 303.94 nm, which mainly come from the transitions from HOMO-1 → LUMO (94.4%) and HOMO-2 → LUMO (67.8%), respectively. The orbital analysis indicates that the HOMO/HOMO-1 mainly distributed on the salicylaldehyde and imine, and the HOMO-2 is distributed throughout the π-conjugated framework. This can explain the particular absorption peaks of compounds 3-OMe-SH-Ph and 3-NO2-SH-Ph, and the theoretical absorption spectra agree well with the experimental values [12] with negligible errors.

Emission Spectra

We have calculated the emission spectra of the studied compounds using the optimized geometries of the excited-state at the theoretical level of TD-DFT. The excited state characteristics of the compounds are summarized in Table 3 and the emission peaks with the lowest emission bands are attributed to the electronic transition of π → π* resulting from the LUMO → HOMO. Neglecting the negligible differences between the calculated and experimental values [12], the calculated results again show the influence of the electron-donating and electron-withdrawing groups with the conjugation of aromatic ring on the properties of fluorescent compounds. Substantially, similar to the absorption spectrum changes, the introduction of fluorine and methoxy group do not significantly change the emission spectrum, whereas the electron-donating diethylamino group and electron-withdrawing nitro group causes red-shift. The large π‑conjugated structure also causes red-shift. This is because that the emission peaks with the strongest oscillator strength are mainly attributed to the electronic transition of π → π* resulting from the LUMO → HOMO. So the HOMO → LUMO gap significantly determined how the emission spectra varied with the substitution and conjugation.

Table 3.   Emission spectra obtained by TD-DFT method for the six compounds
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Ionization Potentials and Electron Affinities

A good device performance of OLED is attributed to the good charge mobility and the comparable balance between the holes and electrons [24]. The ionization potentials (IPs) and electron affinities (EAs) can be used to assess the energy barriers for injecting holes and electrons into compounds, and can be calculated by the DFT on neutral, cation, and anion state geometries. Then, the reorganization energy (λ) is used to approximately estimate the charge transport rate and the balance between hole and electron. The IPs and EAs were obtained with both vertical (\({v}\), at the geometry of the neutral molecule) and adiabatic (a, optimized structures for both the neutral and charged molecules). HEP and EEP were used to evaluate the extraction potentials for hole and electron, respectively. All these calculated results are given in Table 4. The calculated details are similar as given by [25, 26]. By means of the hopping-type mechanism [27–30] and Marcus theory [31, 32], it is known that the efficient charge transfer is mostly dominated by the value of reorganization energy which can be evaluated by the following relations [25]:

$$\begin{gathered} {{\lambda }_{{{\text{hole}}}}} = {{\lambda }_{0}} + {{\lambda }_{ + }} = [{{E}^{ + }}({\text{M}})] - {{E}^{ + }}({{{\text{M}}}^{ + }})] \\ + \;[E({{{\text{M}}}^{ + }}) - E({\text{M}})] = [{{E}^{ + }}({\text{M}})] - E({\text{M}})] \\ + \;[{{E}^{ + }}({{{\text{M}}}^{ + }}) - E({{{\text{M}}}^{ + }})] = IP({v}) - HEP, \\ \end{gathered} $$
(1)
$$\begin{gathered} {{\lambda }_{{{\text{electron}}}}} = {{\lambda }_{0}} + {{\lambda }_{ - }} = [{{E}^{ - }}({\text{M}})] - {{E}^{ - }}({{{\text{M}}}^{ - }})] \\ + \;[E({{{\text{M}}}^{ - }}) - E({\text{M}})] = [E({{{\text{M}}}^{ - }})] - {{E}^{ - }}({{{\text{M}}}^{ - }})] \\ + \;[E({\text{M}}) - {{E}^{ - }}({\text{M}})] = EEP - EA({v}), \\ \end{gathered} $$
(2)

where E, E+, and E represent the energies of the neutral, cation and anion, respectively, and M, M+, and M represent the geometries of neutral, cation, and anion, respectively. As is known that if the IP value is smaller, the hole injection is easier, on the other hand, if the EA value is larger, the electron injection is easier. As shown in Table 4, the trends of the IPs and EAs of the studied compounds are positively the same as HOMO and LUMO energies. The value of IP(\({v}\)) obey the order of 3-NO2-SH-Ph > 3-F-SH-Ph > SH-Ph > 3‑OMe-SH-Ph > Naph-SH-Ph > 4-NEt2-SH-Ph, while EA(\({v}\)) obey the order of 3-NO2-SH-Ph > Naph-SH-Ph > 3-F-SH-Ph > SH-Ph > 3-OMe-SH-Ph > 4-NEt2-SH-Ph. Taking compound SH-Ph as a reference, the electron-withdrawing groups (–F and ‒NO2) can increase the IPs and EAs, while the electron-donating groups (–OMe and –NEt2) make the IPs and EAs decrease. The expanded aromatic system (Naph-SH-Ph) can both increase the EAs and decrease the IPs. These changes show that the ability of Naph-SH-Ph to accept an electron and generate a hole is increasing, which should be conducive to enhancing the injection of electrons and holes in the anode and cathode of light emitting diodes.

Table 4.   Ionization potentials (IP), electron affnities (EA), extraction potentials (HEP and EEP), and reorganization energies (λ) for each molecule calculated by DFTa (eV)
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There are many factors known to affect reorganization energy (λ), such as heteroatom nature, heterocyclic substituents and conjugation sizes [24]. It has been demonstrated that the larger the λelectron and λhole of the fluorescent material, the weaker the transmission rates of electrons and holes. In Table 3, it is first observed that most λhole of the six compounds are larger than λelectron with the difference varied from 0.04 to 0.18, which means that the hole transport rate is weaker than electron transport the rate. Secondly, we also learn that compared with SH-Ph, the λhole usually increase by electron-withdrawing groups and decrease by electron-donating groups, while the λelectron always increase by electron-withdrawing and electron-donating groups. Furthermore, the strong electron-withdrawing (–NO2) and strong electron-donating groups (NEt2) can bring big changes for the values of λelectron and λhole which indicates the introduction of these substituents can improve the charge transfer balance of the compounds, and thus to improve the performance of the OLEDs device. Finally, we have compared and found that the discrepancies between λhole and λelectron in the Naph-SH-Ph and SH-Ph are 0.01 and 0.05 eV, respectively. It concludes that these differences are so small that the expanded aromatic system has very little effect on transmission rates of electrons and holes.

CONCLUSION

Since salicylaldehyde Schiff bases were important typical fluorescent molecules, a comprehensive investigation on six compounds of salicylaldehyde hydrazones (SHs) containing different electron-withdrawing substituent (–F, –NO2) and electron-donating substituents (–OMe, –NEt2) at different position of the aromatic ring has been conducted at the level of PBE1PBE/6-31g(d,p) in our work. The calculated geometrical parameters, HOMO, LUMO energies, absorption and emission spectra are well consistent with the reference reports. The calculation results show that different substituents with expanded conjugation of the aromatic system have effect the performance of the optimized geometry, the frontier molecular orbitals, the absorption spectra, the emission spectra and the ionization potentials with electron affinities in different degree. All calculations reveal that introducing strong electron-withdrawing or electron-donating substituents on the base of the expanded aromatic rings is expected to be useful for the luminescent material design.