Primary photophysical properties of A2E in solution

Time-resolved spectroscopic techniques are used to determine the primary photoprocesses of A2E in solution. Comparison of the absorption and excitation spectrum of A2E in methanol solution indicates excitation 400 nm populates the S-2 excited state. Transient absorption signals decaying with a time constant of 0.9 ps were observed probing around 800 nm. These signals are attributed to the S-2-->S-n transition and reveal the S-2-->S-1 relaxation occurs on the subpicosecond time scale. Transient absorption data probing at shorter wavelengths (480 and 550 nm) are attributed to the S-1-->S-n absorption. These signals exhibit an exponential decay with a time constant of 11 and 13 ps, respectively. Time-resolved emission measurements of the corresponding S-0<--S-1 decay reveal a nonexponential decay; however, > 95 % of the signal amplitude is described by an exponential decay with a time constant of 12.4 ps. Both time-resolved emission and absorpti.on experiments therefore indicate repopulation of the ground electronic state occurs with a time constant of similar to 12 ps. A weak transient absorption probing in the blue (400 nm) persists onto the nanosecond time scale and is attributed to the T-1-->T-n absorption of A2E. Photoacoustic spectroscopy establishes the quantum yield for intersystem crossing of A2E in methanol solution is at most 0.03. The emission quantum yield of A2E in ethanol is determined to be 0.01, and so, nonradiative relaxation is the dominant primary event. The quantum yield for the generation of singlet molecular oxygen following 355 mn excitation of A2E in acetonitrile was determined to be 0.02, consistent with a low production of the excited triplet state. These results establish A2E is not an efficient photogenerator of reaction oxygen species in solution.