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Zusammenfassung:
In commonly studied GFP chromophore analogues such as 4-(4-hydroxybenzylidene)-1,2-dimethyl-1H-imidazol-5(4H)-one (PHBDI), the dominant photoinduced processes are cis–trans isomerization and subsequent S1 → S0 decay via a conical intersection characterized by a highly twisted double bond. The recently synthesized 2-hydroxy-substituted isomer (OHBDI) shows an entirely different photochemical behavior experimentally, since it mainly undergoes ultrafast intramolecular excited-state proton transfer, followed by S1 → S0 decay and ground-state reverse hydrogen transfer. We have chosen 4-(2-hydroxybenzylidene)-1H-imidazol-5(4H)-one (OHBI) to model the gas-phase photodynamics of such 2-hydroxy-substituted chromophores. We first use various electronic structure methods (DFT, TDDFT, CC2, DFT/MRCI, OM2/MRCI) to explore the S0 and S1 potential energy surfaces of OHBI and to locate the relevant minima, transition state, and minimum-energy conical intersection. These static calculations suggest the following decay mechanism: upon photoexcitation to the S1 state, an ultrafast adiabatic charge-transfer induced excited-state intramolecular proton transfer (ESIPT) occurs that leads to the S1 minimum-energy structure. Nearby, there is a S1/S0 minimum-energy conical intersection that allows for an efficient nonadiabatic S1 → S0 internal conversion, which is followed by a fast ground-state reverse hydrogen transfer (GSHT). This mechanism is verified by semiempirical OM2/MRCI surface-hopping dynamics simulations, in which the successive ESIPT-GSTH processes are observed, but without cis–trans isomerization (which is a minor path experimentally with less than 5% yield). These gas-phase simulations of OHBI give an estimated first-order decay time of 476 fs for the S1 state, which is larger but of the same order as the experimental values measured for OHBDI in solution: 270 fs in CH3CN and 230 fs in CH2Cl2. The differences between the photoinduced processes of the 2- and 4-hydroxy-substituted chromophores are attributed to the presence or absence of intramolecular hydrogen bonding between the two rings.
