Solvent-polarity tuning excited-state charge coupled proton-transfer reaction in p-N,N-ditolylaminosalicylaldehydes

Detailed insight into the excitation behavior for charge versus proton transfer in p-NN-ditolylaminosalicylaldehyde (Ia) has been gained via luminescence spectroscopy and femtosecond dynamics. In cyclohexane, following an ultrafast rate (similar to2.0 x 10(12) s(-1)) of excited-state intramolecular proton transfer (ESIPT), fast equilibrium takes place between normal (N*) and tautomer excited states (T*), resulting in dual fluorescence maximized at 450 and 540 nm, respectively, with a common population decay rate of 360 ps(-1). The normal emission exhibits drastic solvent-polarity dependence and has been concluded to originate from a charge-transfer species incorporating excited-state intramolecular charge transfer from ditolylamine to carbonyl oxygen. In dipolar solvents, competitive rates between ESIPT and solvent relaxation were observed, and the solvated charge-transfer state is thermodynamically more favorable, so that the T* --> N* reverse proton transfer takes places. Supplementary support was provided by the corresponding experiments for the methoxy derivative of la as well as other relevant analogues. The results shed light on detailed proton/charge transfer coupled dynamics as well as the associated solvent-relaxation dynamics at an early time domain.