Abstract
Time-dependent density functional theory (TDDFT) and atoms in molecules (AIM) theory are combined to study the photoinduced excited state intramolecular proton transfer (ESIPT) dynamics for eight anthraquinones (AQs) derivatives in solution. The calculated absorption and emission spectra are consistent with the available experimental data, verifying the suitability of the theory selected. The systems with the excited-state exothermic proton transfer, such as 1-HAQ, 1,5-DHAQ and TFAQ, emit completely from transfer structure (T), while the reactions for those without ESIPT including 1,4-DHAQ and AAAQ appear to be endothermic. Three reaction properties of three systems (1,8-DHAQ, DCAQ and CAAQ) are between the exothermic and endothermic, sensitive to the solvent. Energy scanning shows that 1,4-DHAQ and AAAQ exhibit the higher ESIPT energy barriers compared to 1-HAQ, 1,5-DHAQ and TFAQ with the “barrierless” ESIPT process. The ESIPT process is facilitated by the strengthening of hydrogen bonds in excited state. With AIM theory, it is observed that the change in electrons density ρ(r) and potential energy density V(r) at BCP position between ground state and excited state are crucial factors to quantitatively elucidate the ESIPT.
Highlights
The calculated systems are not stable in ground states (S0(T)), except for 1,4-DHAQ whose stable structures are successfully obtained for both single proton and double protons transfer in ground state (Fig. 2)
Comparison of the hydrogen bond lengths in ground and excited states showed that the hydrogen bonds enhanced in the sequence of 1-HAQ > 1,5DHAQ > 1,8-DHAQ > 1,4-DHAQ
The exothermic reaction and “barrierless” excited state intramolecular proton transfer (ESIPT) process are observed for 1-HAQ, 1,5-DHAQ and TFAQ with the barrier lower than 2 kJ/mol. 1,4-DHAQ have the
Summary
Due to the significance in modern photophysics, photochemistry and biochemistry, such as Green Fluorescent Protein (GFP)[1,2], organic light emitting diodes (OLEDs)[3,4,5] and fluorescent chemosensors[6], the excited state intramolecular proton transfer (ESIPT) phenomenon have attracted numerous experimental and computational interests[7,8,9,10,11,12,13,14,15,16]. The molecules experience an ultrafast intramolecular proton transfer, giving rise to an excited state keto tautomeric form The equilibrium between these two forms lead to various intriguing fluorescence properties for the molecules, including dual emission spectra, double proton transfer and back ESIPT process[15,17,18,19,20,21,22,23]. The PES in excited state confirmed the exothermic reaction and “barrierless” ESIPT process in 1-HAQ, 1,5-DHAQ, DCAQ and TFAQ with the dominant LWE in dichloromethane. The alterable ESIPT properties of 1,8-DHAQ, CAAQ and DCAQ came from their medium energy barriers. The changes of electrons density ρ(r) and potential energy density V(r) at BCP position in ground state and excited state are significant for the ESIPT process. The V(r) at BCP position is better than ρ(r) as a reference for hydrogen bond dynamics
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