Experimental and Theoretical Investigation on Excited State Intramolecular Proton Transfer Coupled Charge Transfer Reaction of Baicalein

The excited state intramolecular proton transfer (ESIPT) coupled charge transfer of baicalein has been investigated using steady-state spectroscopic experiment and quantum chemistry calculations. The absence of the absorption peak from S₁ excited state both in the experi-mental and calculated absorption spectra indicates that S₁ is a dark state. The dark excited state S1 results in the very weak fluorescence of solid baicalein in the experiment. The fron-tier molecular orbital and the charge difference densities of baicalein show clearly that the S₁ state is a charge-transfer state whereas the S₂ state is a locally excited state. The only one stationary point on the potential energy profile of excited state suggests that the ESIPT reaction of baicalein is a barrierless process.