Photophysical and spectroscopic manifestations of the low-lying pi sigma* state of 4-(dimethylamino)benzethyne: solvent-polarity dependence of fluorescence and excited-state absorptions

A concerted experimental and computational study of 4-(dimethylamino) benzethyne, DMABE, has been carried out to probe the low-lying pi sigma* state and the role it plays in the photophysics of the molecule. The subpicosecond transient absorption spectra reveal the presence of a strong excited-state absorption at about 700 nm and a weaker absorption at about 520 nm. The observed absorption maxima are in excellent agreement with the TDDFT calculations that place a strongly allowed pi sigma* <- pi sigma* transition at 750 nm, and a weaker pi pi* <- pi pi* (LE) transition at 528 nm. This agreement combined with the differing decay times, and differing solvent shifts of the two transients, allow assignments of the 700 nm absorption to the pi sigma* state and the 520 nm absorption to the LE (pi pi*) state. The bifurcation of the initially excited L-a (pi pi*) state into the pi sigma* state and the LE state, as probed by transient absorption, is strongly influenced by solvent polarity, with polar environments favoring the L-a -> pi sigma* decay channel over the competing L-a -> LE decay channel. The nanosecond radiationless decay of the LE state to the dark pi sigma* state is also strongly enhanced in polar environments, thus accounting for the dramatic quenching of fluorescence in solvents of high polarity.