Heteroatom substitution effect on electronic structures, photophysical properties, and excited-state intramolecular proton transfer processes of 3-hydroxyflavone and its analogues: A TD-DFT study

Effect of different substituent on the ESIPT process and fluorescence features of 2-(2-hydroxyphenyl)benzoxazole derivatives: A DFT/TD-DFT study

In this contribution, four derivatives of 5'-(para-R-Phenylene) vinyl-2-(2'-hydroxyphenyl) benzoxazole (PVHBO) were ingeniously designed by introducing two electron-withdrawing substituents and two electron-donating substituents, aiming to investigate the influence of different substituents on the photophysical properties of PVHBO and the excited state intramolecular proton transfer (ESIPT) process via the density functional theory (DFT) and time-dependent DFT (TD-DFT) methods. By utilizing the geometric parameters and the simulated infrared (IR) spectra, we compared the intramolecular hydrogen bonds (IHBs) strengths in the S0 and S1 states of the molecules. Via conducting the hole-electron analysis, the reduction in fluorescence intensity for the enol and keto forms of PVHBO, PVHBO-MeO, and PVHBO-NH2 were also well explicated. Besides, the potential energy curves (PECs) and corresponding transition state (TS) structures for both S0 and S1 states were also constructed to accurately obtain energy barriers of forward and reversed proton transfer processes. The calculated absorption and fluorescence spectra also show that PVHBO-NH2 has the largest Stokes shifts of 158 nm and 219 nm in both the enol and keto states, with a significant increase in fluorescence intensity observed upon the induction of electron-withdrawing groups. Through this work, it can provide the theoretical basis for the design of novel luminescent materials.

Excitation wavelength-dependent fluorescence anisotropy of 3-hydroxyflavone: revisiting the solvation processes and high-energy state excitation in ESIPT-active compounds

To gain a more comprehensive understanding of the phenomenon of high-fluorescence anisotropy of the normal form emission of ESIPT-active compounds in protic solvents, excitation wavelength dependence of emission anisotropy was investigated for 3-hydroxyflavone (3HF) using steady-state spectroscopic technique and quantum chemical calculations. It was shown for the first time that the anisotropy of 3HF normal form emission is characterized by significant dependence on excitation energy. Experimental results indicate that the fluorescence anisotropy of 3HF in methanol (at 20 °C) changes abruptly from about 0.18 to about 0.10 with a decrease in excitation wavelength. This spectroscopic phenomenon can be explained by two factors: (1) breaking of intermolecular solute-solvent hydrogen bonds upon photoexcitation and (2) excitation of ESIPT-active fluorophores to the second singlet state (S2). The results of quantum chemical calculations clearly indicate that specific hydrogen bonding solvation interactions can lead to the formation of 3HF-methanol complexes with larger molecular volumes than the volume of free 3HF molecule. High excitation energy can reform and break solute-solvent bonds, which leads to a decrease in molecular system volume. This results in a decrease in rotational correlation time and fluorescence anisotropy. As is known, the fluorescence lifetime of small-sized molecules is closely correlated with the conformational changes in the excited state, and in the case of ESIPT-active compounds, the lifetime of normal form emission is almost fully determined by the ultrafast ESIPT process. Therefore, although in general, fluorescence lifetime is considered independent of excitation energy, but because the timescale of ESIPT processes is of the same order as the timescale of internal conversion, in the case of ESIPT-active compounds, fluorescence lifetime changes caused by high-energy state excitation cannot be neglected. The emission anisotropy of the normal form of an ESIPT-active compound will decrease with an increase in fluorescence lifetime caused by an increase in excitation energy sufficient to excite molecules to higher electronic states. In this work, both hypotheses are discussed and verified using experimental data and quantum chemical calculations for 3HF in methanol.

Excited-state relaxation mechanisms of 2,2'-(1-phenyl-1H-1,2,4-triazole-3,5-diyl)diphenol: single- or double-proton transfer?

Triazole compounds are important organic systems with excellent electronic properties, which have diagnostic potential in the fields of organic electronics and organic photovoltaics. The important photophysical nature of these systems is the transformation between the enol and keto forms after excited-state proton transfer. In this study, the IR vibrational spectrum, ESIPT mechanism, and excited-state decay dynamics of 2,2'-(1-phenyl-1H-1,2,4-triazole-3,5-diyl)diphenol (ExPh) were explored using electronic structure calculations and non-adiabatic dynamics simulations. Two S1/S0 conical intersections with distinct proton transfer (ESIPT-I and ESIPT-II) involved were obtained. The associated two-dimensional S1 minimum-energy potential energy surface indicated that the dynamical roles of these two S1/S0 conical intersections in the S1 excited-state decay were quite different. The ESIPT-I reaction was more favorable to occur than the ESIPT-II process. Our dynamics simulations supported this hypothesis with the whole trajectories decaying to the ground state via the S1S0-1 conical intersection, which involved the ESIPT-I process. The ESIPT-Involved efficient deactivation pathway could be partially responsible for the decrease in fluorescence emission. These results and ESIPT mechanisms are helpful for understanding the decay pathways of similar systems.

A Theoretical Study of Solvent Effects on the Structure and UV-vis Spectroscopy of 3-Hydroxyflavone (3-HF) and Some Simplified Molecular Models

Solvent effects on the UV-vis spectra of 3-hydroxyflavone and other structurally related molecules (3-hydroxychromen-4-one, 3-hydroxy-4-pyrone, and 4-pyrone) have been studied by combining time-dependent density functional theory (TDDFT) and the polarizable continuum method (PCM). Among the first five excited states of the four considered molecules, electronic states of n → π* and π → π* nature appear. In general, the stability of the n → π* states decreases as the π space becomes larger in such a way that only for 4-pyrone and 3-hydroxy-4-pyrone are they the first excited states. In addition, they become less stabilized in ethanol solution than the ground state, and this causes blueshift transitions in solution. The opposite trend is found for the π → π* excited states. They are less energetic with the π-system size and when passing from gas phase to solution. The solvent shift also depends strongly on the size of the π systems and on the formation of an intramolecular hydrogen bond; thus, it decreases when going from 4-pyrone to 3-hydroxyflavone. The performance of the three versions (cLR, cLR², and IBSF) of the specific-state PCM method in predicting transition energies are compared.

Scutellarein in organic solvents: changes in spectroscopic properties caused by solute-solvent interactions

In this work, the spectroscopic properties of scutellarein (6-hydroxyapigenin) were studied in three organic solvents (methanol, acetonitrile and N,N-dimethylformamide) taking into account possible ionization and isomerization (tautomerization) processes. Significant visible colour changes were reported in the case of scutellarein in N,N-dimethylformamide. It was shown that isomerization processes can be one of the reasons for the observed changes in absorption spectrum, because some scutellarein isomers have an absorption band at about 623 nm while other forms of scutellarein show no absorption in this region. Moreover, spectroscopic properties were studied for cases of scutellarein in acetonitrile and methanol. The molar extinction coefficient has been found in the case of methanol solution which could be used to determine scutellarein concentration in this solvent using spectroscopic methods in future studies. The quantum-chemical calculations were performed for neutral and anionic forms and for two types of possible isomers of scutellarein in each solvent. The results help explain the experimentally observed rising absorption in the 500-750 nm wavelength range. Another important result of the quantum-chemical calculations is a prediction of excited state intramolecular proton transfer (ESIPT) in scutellarein. This result has been obtained for free molecule in vacuum and in the cases of methanol, acetonitrile and N,N-dimethylformamide solution. It was found that the excited state energy of the normal molecular form is higher than the excited state energy of the tautomer form of scutellarein.

Ultrafast nonadiabatic excited-state intramolecular proton transfer in 3-hydroxychromone: A surface hopping approach

We employ the ab initio molecular dynamics within the surface hopping method to explore the excited-state intramolecular proton transfer taking place on the coupled "bright" S₁ (ππ*) and "dark" S₂ (nπ*) states of 3-hydroxychromone. The nonadiabatic population transfer between these states via an accessible conical intersection would open up multiple proton transfer pathways. Our findings reveal the keto tautomer formation via S₁ on a timescale similar to the O-H in-plane vibrational period (<100 fs). Structural analysis indicates that a few parameters of the five-membered proton transfer geometry that constitute the donor (hydroxyl) and acceptor (carbonyl) groups would be adequate to drive the enol to keto transformation. We also investigate the role of O-H in-plane and out-of-plane vibrational motions in the excited-state dynamics of 3-hydroxychromone.