Theoretical Studies On Excited States Intramolecular Proton Transfer Reaction Of Some Conjugated Organic Molecules

Excited state intramolecular proton transfer(ESIPT) is one of the most fundamental reactions in the biological and chemical processes. Because of such peculiar photophysical and photochemical properties, ESIPT molecules have been widely applied in many optical materials, and have long been a subject of intense interest in both theoretical and experimental investigations. In this paper, we have selected some typical organic molecules to study their equilibrium geometries and spectra properties, using the DFT/TD-DFT and sophisticated ab initio methods, and the mechanism of photophysical and photochemistry processes by photoexcitation has also been explored by constructing proton transfer potential energy profiles and serching the S1/S0 conical intersection. On one hand, our calculations have presented resonable explanations for the experimental observations. On the other hand, by exploring the relationship between stuctures and properties, the computational results can provide useful theoretical predictions for the disign of new ESIPT molecules in the future.(1) Excited state intramolecular proton transfer(ESIPT) of four imidazole derivatives, 2-(2'-hydroxyphenyl)imidazole(HPI), 2-(2'-hydroxyphenyl)benzimidazole(HPBI), 2-(2'-hydroxyphenyl)-1Hphenanthro[9,10-d]imidazole(HPPI) and 2-(2'-hydroxyphenyl)-1-phenyl-1H-phenanthro[9,10-d]imidazole(HPPPI), were studied by the sophisticated CASSCF/CASPT2 methodology. The stateaveraged SA-CASSCF method was used to optimize their geometry structures of S0 and S1 electronic states, and the CASPT2 calculations were used for the calibration of all the single-point energies, including the absorption and emission spectra. A reasonable agreement is found between the theoretical predictions and the available experimental spectral data. The forward ESIPT barriers of four target compounds gradually decrease with the increase of molecular size. On the basis of the present calculations, it is a plausible speculation that the larger the size, the faster is the ESIPT rate, and eventually, HPPPI molecule can undergo a completely barrierless ESIPT to the more stable S1 keto form. Additionally, taking HPI as a representative example, the radiationless decays connecting the S0 and S1/S0 conical intersection structures were also studied by constructing a linearly interpolated internal coordinate(LIIC) reaction path. The qualitative analysis shows that the LIIC barrier of HPI in the keto form is remarkably lower than that of its enol-form, indicating that the former has a big advantage over the latter in the nonradiative process.(2) 1,8-Dihydroxydibenzo[a,h]phenazine(DHBP), is a new synthetic compound possessing two intramolecular hydrogen bonds, however, it has been found to exhibit the excited state intramolecular single proton transfer(ESSPT) behavior in recent experiment. To explain the phenomenon reasonably, two combined methods of CASSCF/CASPT2 and DFT/TD-DFT have been employed to investigate the structural and spectral properties of its three tautomers, corresponding to the non-proton-transferred(E), the single-proton-transferred(SK) and the double-proton-transferred(DK) forms. These studies suggest that the E form is the global minimum in the S0 state, while the SK form is the most stable in the S1 state, both of which are responsible for the experimental absorption peak at 2.54 eV and emission band at 1.64 eV, respectively. Because of the relatively high energy barrier, the DK form will play no important role in the fluorescence emission of DHBP. Our present results lend nice support to the experimental finding of SPT.(3) The photoinduced reaction mechanism of excited state intramolecular proton transfer(ESIPT) of 1,8-dihydroxydibenzo[a,c]phenazine(denoted as DHBP), a bis-phenols displaying ESIPT property, has been explored by employing the CASSCF/CASPT2 and DFT/TD-DFT methods. The two lowest singlet electronic states of three tautomers, namely the enol form(E), a keto form with single proton transferred(SK) and a keto form with double proton transferred(DK), have been optimized. These results indicate that the E tautomer is the system with the lowest energy in the ground S0 state, while the SK tautomer is the most stable structure in the singlet S1 state. Moreover, the structural changes of E from S0 to S1 result in a strengthened intramolecular hydrogen bond, which contributes to facilitate the following ESIPT reaction. In addition, the absorption and emission spectra of DHBP have also been simulated using TD-DFT. The vertical excitation energy of the E form and vertical emission energy of the SK form were found at 2.86 and 1.74 eV, respectively, in good agreement with the available experimental values of 2.89 and 1.72 eV, which implies that the fluorescence detected by Piechowska et al. may only arise from the SK tautomer. To give a clearer picture of the proton transfer process, the constrained energy profiles(CEPs) for the three DHBP tautomers in the S0 and S1 states have been constructed. These calculations of CEPs show that the energy barriers during the PT processes from the E/DK to the SK forms in the S1 state are much lower than those of the others considered here, and support the hypothesis of an ESIPT mechanism of only a single proton transfer rather than double proton transfer.(4) Excited state intramolecular proton transfer reactions of five molecules which possess five/six-membered intramolecualr N-H???N or O-H???N hydrogen bond rings based the quinoline and 2-phenylpyridine skeletons, have been described in detail by density functional theory/time-dependent density functional theory computational approaches. Structural parameters and electron excitation analysis reveal that the intramolecular hydrogen bond has been strengthened by excitation from the ground S0 state to the excited S1 state. The energy barriers estimated by the constrained potential energy profiles in ESIPT reaction coordinates show that the proton transfer is barrierless in six-membered intramolecualr hydrogen bond systems, which is more smooth than that with certain barriers in five-membered ones. Due to the more acid OH proton, the ESIPT is more facile in O-H???N compounds than in N-H???N analogues. In addition, when NH2 is substituted with electron-withdrawing acetyl, the hydrogen bond N-H…N can be strengthened and thus facilitate the ESIPT process but with a only 0.04 eV lower energy barrier.