Quantum Chemical Benchmark Studies of the Electronic Properties of the Green Fluorescent Protein Chromophore. 1. Electronically Excited and Ionized States of the Anionic Chromophore in the Gas Phase

We present the results of quantum chemical calculations of the electronic properties of the anionic form of the green fluorescent protein chromophore in the gas phase. The vertical detachment energy of the chromophore is found to be 2.4-2.5 eV, which is below the strongly absorbing pi pi* state at 2.6 eV. The vertical excitation of the lowest triplet state is around 1.9 eV, which is below the photodetachment continuum. Thus, the lowest bright singlet state is a resonance state embedded in the photodetachment continuum, whereas the lowest triplet state is a regular bound state. Based on our estimation of the vertical detachment energy, we attribute a minor feature in the action spectrum as due to the photodetachment transition. The benchmark results for the bright pi pi* state demonstrated that the scaled opposite-spin method yields vertical excitation within 0.1 eV (20 nm) from the experimental maximum at 2.59 eV (479 nm). We also report estimations of the vertical excitation energy obtained with the equation-of-motion coupled cluster with the singles and doubles method, a multireference perturbation theory corrected approach MRMP2 as well as the time-dependent density functional theory with range-separated functionals. Expanding the basis set with diffuse functions lowers the pi pi* vertical excitation energy by 0.1 eV at the same time revealing a continuum of "ionized" states, which embeds the bright pi pi* transition.