Solvent-Dependent Excited-State Evolution of Prodan Dyes

Excited-state character and dynamics of two 6-(dimethylamino)-2-acylnaphthalene dyes (Prodan and Badan-SCH2CH2OH) were studied by picosecond time-resolved IR spectroscopy (TRIR) in solvents of different polarity and relaxation times: hexane, CD3OD, and glycerol-d(8). In all these solvents, near-UV excitation initially produced the same S-1(pi pi*) excited state characterized by a broad TRIR signal. A very fast decay (3, similar to 100 ps) followed in hexane, whereas conversion to a distinct IR spectrum with a nu(C=O) band downshifted by 76 cm(-1) occurred in polar/H-bonding solvents, slowing down on going from CD3OD (1, 23 ps) to glycerol-d(8) (5.5, 51, 330 ps). The final relaxed excited state was assigned as planar Me2N -> C=O intramolecular charge transfer S-1(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-1 state, which is responsible for the dynamic fluorescence Stokes shift observed in polar/H-bonding solvents. The time evolution of the optically prepared S-1(pi pi*) state to the S-1(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.