Theoretical Study On Photophysical Properties Of Three Hydroxyl Or Amino Molecules

Excited-state intramolecular proton transfer?ESIPT?is one of the most basic and important processes in chemical and biological fields.Due to the peculiar photophysical and photochemical properties,ESIPT has been widely applied in many optical materials,and has long been a subject of intense interest in both theoretical and experimental investigations.In this paper,we have studied some typical organic molecules including hydroxy-type ESIPT molecules,amino-type ESIPT molecules as well as ESIPT molecule with anti-Kasha rules,which have O-H???N type,N-H???N type and O-H???O type intramolecular hydrogen bonds,respectively.Two combined methods of DFT/TD-DFT and CASSCF/CASSPT2 have been employed to optimize the configuration and construct the potential energy curves of these molecules in their ground and excited states,and their photophysical and photochemical behaviors have been studied.On the other hand,by exploring the relationship between structures and properties,a reasonable explanation of experimentally observed spectroscopic phenomena has been obtained.It is our hope that the computational results can provide a theoretical guidance for the design of novel ESIPT molecules in the future.?1?Enol-keto proton tautomerization and cis-trans isomerization reactions of a novel excited-state intramolecular proton transfer?ESIPT?fluorophore of BTImP and its protonated form?BTImP⁺?were explored using density functional theory/time-dependent density functional theory?DFT/TD-DFT?computational methods with a B3LYP hybrid functional and a 6-31+G?d,p?basis set.In addition,the absorption and fluorescence spectra were calculated at the TD-B3LYP/6-31+G?d,p?level of theory.Our results reveal that both BTImP and BTImP⁺can undergo an ultrafast ESIPT reaction,giving rise to the single fluorescence emission with different fluorescence colors,which are well consistent with the experimental findings.Calculations also show that following the ultrafast ESIPT,BTImP and BTImP⁺can experience the distinctly different cis-trans isomerization processes.The intersystem crossing between the first excited singlet S₁ state and triplet T₁ state is found to play an important role in the photoisomerization process of BTImP⁺.In addition,the energy barrier of the trans-keto?cis-keto isomerization in the ground state of BTImP⁺is calculated to be 10.49 kcal mol^-1,which implies that there may exist a long-lived trans-keto species in the ground state for BTImP⁺.?2?Excited-state intramolecular proton transfer?ESIPT?reactions of a series of N?R?-H???N-type ESIPT compounds with seven-membered-ring intramolecular hydrogen bonds were explored by employing density functional theory/time-dependent density functional theory calculations with PBE0 functional.The target molecules are the four derivatives of o-ABDI with the electron-withdrawing groups of-COCH₃?1?,-COPh?2?,-Ts?3?and-COCF₃?4?,respectively.Topological analysis shows strong intramolecular hydrogen-bonds in the studied molecules of 1 to 3 and the electron density function??r?exhibits good linear relationship with the intramolecular hydrogen-bond length.Our results indicate that the absorption and emission spectra predicted theoretically matched very well the experimental findings.Additionally,the potential energy curves of the S₀ and S₁ states were scanned by using of DFT and TD-DFT to elucidate the ESIPT process.The results show that as the electron-withdrawing strength of R increase,the intramolecular H-bond of the N-S₁ form gradually enhances,and the energy barrier along the ESIPT reaction gradually decreases.For compound 4,its ESIPT reaction is found to be a barrierless process due to the involvement of a strong electron-withdrawing-COCF₃ group.It is therefore a reasonable presumption that the ESIPT efficiency of these N?R?-H???N-type seven-membered-ring H-bonding systems can be improved when a strong electron-withdrawing group in R is introduced.?3?In this work,the ab initio calculation methods of complete active space self-consistent field?CASSCF?and complete active space with second-order perturbation theory?CASPT2?were performed to investivagatethemoleculeof2-??5-?3-hydroxy-4-oxo-4H-chromen-2-yl?thiophen-2-yl?-methylene?malononitrile?3-HTC-DiCN?.The optical properties of 3-HTC-DiCN molecule were analyzed through the configuration optimization,electronic spectrum simulation and the construction of potential energy curve.The results indicate that the second excited singlet S₂ state is found to be an ultrafast process for a novel dye of 3-HTC-DiCN.In addition,it can be speculated that there may exists a crossing point between the S₁ and S₂ potential energy curves along the proton tautomerization reations.Once the3-HTC-DiCN molecule is photoexcited the Franck-CondonS₂^Nregion,it can relax to the crossing point very facilely,and the ESIPT formS₂^NtoS₁^Tis an ultrafast process,which can thoroughly compete with internal conversion from the S₂ to the lowest S₁ state,thus leading to the enhancement of fluorescence quantum yield arising from the proton-transfer tautomer emission as well as the remarkable decrease of the normal emission quantum yield.It is therefore apparent that the present computational results can expain well the non-Kasha behavior observed in 3-HTC-DiCN molecule experimentally.