Excited State Dynamics of 2-MPT-Derived Fluorescent Molecular Switches

Excited state dynamics from molecular fluorescent switches of the donor- or acceptor-derived 5-Methoxy-2-(2-pyridyl)thiazole (2-MPT) fluorescent molecules has been studied by ultrafast spectroscopy. The twisted intramolecular charge transfer (TICT) excited state is found to be created within 1 ps. Then the relaxation of TICT excited state causes further twist of the molecular conformation (especially the moiety 2-MPT) and reduces the transition moment. It takes about 600–700 ps to reach a state with ten nanoseconds of lifetime. This final state is nearly a dark state, which is attributed as the main reason for the drastic decrease of the fluorescence quantum field with respect to the moiety 2-MPT. For MPTEA with a strong electron donor in high polarity solvents, TICT's excited state partially transfers to a planar intramolecular charge transfer (PICT) excited state within 10 ps due to the strong dipole-dipole interaction with solvent molecules. PICT's excited state gives a relatively strong emission with a large red shift. The results demonstrate that the excited state dynamics of these molecular fluorescent switches provides deep insight to their photophysical processes, which is important for exploring novel molecular switches as potential candidates for optical biolabels, sensors and logic gates applications.