Excited-state proton transfer (ESPT) in Indigo and its monohexyl-substituted derivative (Ind and NHxInd, respectively) in solution was investigated experimentally as a function of solvent viscosity, polarity, and temperature, and theoretically by time-dependent density functional theory (TDDFT) calculations. Although a single emission band is observed, the fluorescence decays (collected at different wavelengths along the emission band using time-correlated single photon counting (TCSPC)) are biexponential, with two identical decay times but different pre-exponential factors, which is consistent with the existence of excited-state keto and enol species. The femtosecond (fs)-transient absorption data show that two similar decay components are present, in addition to a shorter (<3 ps) component associated with vibrational relaxation. From TDDFT calculations it was shown that with both Ind and NHxInd, the reaction proceeds through a single ESPT mechanism driven by an Arrhenius-type activation through a saddle point, which is enhanced by tunneling through the barrier. From the temperature dependence of the steady-state and time-resolved fluorescence data, the activation energy for the process was found to be ∼11 kJ mol–1 for Ind and ∼5 kJ mol–1 for NHxInd, in close agreement with the values calculated by TDDFT: 12.3 kJ mol–1(Ind) and 3.1 kJ mol–1 (NHxInd). From time-resolved data, the rate constants for the ESPT process in dimethyl sulfoxide were found to be 9.24 × 1010 s–1 (Ind) and 7.12 × 1010 s–1 (NHxInd). The proximity between the two values suggests that the proton transfer mechanism in indigo is very similar to that found in NHxInd, where a single proton is involved. In addition, with NHxInd, the TDDFT calculations, together with the viscosity dependence of the fast component, and differences in the activation energy values between the steady-state and time-resolved data indicate that an additional nonradiative process is involved, which competes with ESPT. This is attributed to rotation about the central carbon–carbon bond, which brings the system close to a conical intersection (CI). The CI is of the sloped type, where the seam is reached through an OH stretching vibration.
J. Pina, Daniela Sarmento, Marco Accoto, Pier Luigi Gentili, Luigi Vaccaro, Adelino Galvão, and J. Sérgio Seixas de Melo