ИСТИНА

Following excitation in the visible range, the relaxation dynamics of the electronically excited anionic chromophore of the Green Fluorescent Protein (GFP) is known to occur on an ultrafast picosecond timescale. At the same time, a meta-analogue of the native para-chromophore exhibits a much longer excited-state lifetime and appears to be non-fluorescent in gas phase and in solution. Here, we aim at understanding the origin of such a remarkably long excited-state lifetime of the meta GFP chromophore in the gas phase. By using the CASSCF(16,14)/(aug)-cc-pVDZ method, we show that the S1 state of the meta-chromophore is optically dark in the Franck-Condon region, and photoexcitation results in the S0-S2 transition in the visible region. Three conical intersections are found that interconnect the S2, S1 and S0 states, which are thought to be relevant to the relaxation dynamics of the meta-chromophore in the gas phase. The initial relaxation from the S2 state occurs barrierless and results in the population of the S1 state along one of the branches. The excited-state population is shown to be trapped in S1 due to the presence of a barrier along the minimum-energy pathway that leads from the planar S1 minimum structure to the highly twisted S1/S0 conical intersection. By using the RRKM theory, we predict an excited-state lifetime of 13 ns. This estimation is consistent with the experimental results obtained through gas-phase time-resolved photoelectron measurements.