Photophysical and spectroscopic manifestations of the low-lying πσ* 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 πσ* 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 πσ* ←πσ* transition at 750 nm, and a weaker ππ* ←ππ* (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 πσ* state and the 520 nm absorption to the LE (ππ*) state. The bifurcation of the initially excited La (ππ*) state into the πσ* state and the LE state, as probed by transient absorption, is strongly influenced by solvent polarity, with polar environments favoring the La→πσ* decay channel over the competing La→ LE decay channel. The nanosecond radiationless decay of the LE state to the dark πσ* state is also strongly enhanced in polar environments, thus accounting for the dramatic quenching of fluorescence in solvents of high polarity.