Solvent-Dependent Excited-State Evolution of Prodan Dyes.

Excited-state character and dynamics of two 6-(dimethylamino)-2-acylnaphthalene dyes (Prodan and Badan-SCHCHOH) were studied by picosecond time-resolved IR spectroscopy (TRIR) in solvents of different polarity and relaxation times: hexane, CDOD, and glycerol-. In all these solvents, near-UV excitation initially produced the same S(ππ*) excited state characterized by a broad TRIR signal. A very fast decay (3, ∼100 ps) followed in hexane, whereas conversion to a distinct IR spectrum with a ν(C═O) band downshifted by 76 cm occurred in polar/H-bonding solvents, slowing down on going from CDOD (1, 23 ps) to glycerol- (5.5, 51, 330 ps). The final relaxed excited state was assigned as planar MeN → C═O intramolecular charge transfer S(ICT) by comparing experimental and TDDFT-calculated spectra. TRIR conversion kinetics are comparable to those of early stages of multiexponential fluorescence decay and dynamic fluorescence red-shift. This work presents a strong evidence that Prodan-type dyes undergo solvation-driven charge separation in their S state, which is responsible for the dynamic fluorescence Stokes shift observed in polar/H-bonding solvents. The time evolution of the optically prepared S(ππ*) state to the S(ICT) final state reflects environment relaxation and solvation dynamics. This finding rationalizes the widespread use of Prodan-type dyes as probes of environment dynamics and polarity.