Molecular investigations for spectroscopic properties and photoreactions of fluorescent dyes

Abstract

Photoactive organic molecules have been widely used in state-of-the-art chemical and biological applications. Photoreactions of such molecules bring about a beneficial outcome that is instrumental in elucidating photoinduced biological processes and advancing modern scientific techniques for a wide range of applications. Therefore, this dissertation employs a molecular simulation to elucidate the photoreactions of synthesized and naturally occurring fluorescent dyes. In Paper I, the influences of the hydrogen (H)-bonding interaction on fluorescence quenching of 7-methoxycoumarin (7MC) via photoinduced electron transfer from a guanine (Gua) base are studied by excited-state dynamics simulations. The simulations are performed using a hybrid quantum mechanics/molecular mechanics scheme with full multiple spawning dynamics. The computational results demonstrate favorable consistency with reported experimental data of solvated 7MC in terms of absorption and emission properties. In the case of the H-bonded 7MC-Gua complex, the simulation results suggest that the electron transfer occurs on the second excited state and is influenced by both the H-bond length and the intermolecular planarity between the two molecules. In Paper II, the UV photoresponse of the modeled anionic green fluorescent protein (GFP) chromophore, is also investigated by a dynamically weighted complete active spaces self-consistent field (DW-CASSCF) electronic structure, a recently reformulated method. The results calculated by DW-CASSCF are in good agreement with the results from higher level of theory calculations and previous experiments. In addition, the calculations suggest that the isolated GFP chromophore undergoes ultrafast internal conversion from the third to second excited state upon UV photo-excitation, which could outcompete autoionization.

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