Time-resolved emission of retinoic acid

Steady-state and time-resolved emission spectra were measured to study the relaxation of all-trans retinoic acid (ATRA) from its excited-states in a number of solvents. We found that the time-resolved emission signal is composed of three decay components arising from three different excited-states. The ultrafast component that is shorter than 80 fs is dominant at λ ≤ 500 nm, whereas the intermediate component is dominant at longer wavelengths and its lifetime is solvent dependent. For acetonitrile, methanol and 1-pentanol the average decay times of the intermediate component are 0.9, 1.3 and 4 ps respectively, and the overall decay is non-exponential. The third decay component has a very small amplitude of ∼0.05 in polar solvents and a longer lifetime of ∼10 ps. We assign the fast decaying emission component to the allowed transition from the upper 1Bu+ state to the ground 1Ag state. The intermediate lifetime component is assigned to the forbidden transition from the excited 1Ag ππ* state to the ground-state. In n-octane, a non-polar solvent, the amplitude of the intermediate time component is smaller than in polar solvents, and its lifetime is 600 fs. This lifetime is shorter than in acetonitrile, whose viscosity is half that of n-octane. Moreover, the amplitude of the long time component in this solvent at long wavelengths is dominant. We therefore propose that the excited state ordering may be different in non-polar solvents. We also conducted a temperature dependence study on ATRA emission in 1-propanol and 1-pentanol, both of which are glass forming liquids. We found that the average emission lifetime increases as the temperature decreases. The value of the activation energy of the non-radiative process is half that of the dielectric relaxation process of the solvent.