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The solvent-polarity dependence and temporal characteristics of the transient absorption of 4-(dimethylamino)benzonitrile, DMABN, and 4-(dimethylamino)benzethyne, DMABE, demonstrate the presence of the πσ*-state absorption at about 700 nm and the ππ* (LE)-state absorption at about 520 nm and 450 nm. The rise and decay times of the πσ*-state transient differ from those of the ππ*-state transients in both compounds. Moreover, the peak position of the πσ*-state absorption is blue-shifted and more intense in acetonitrile as compared to n-hexane, whereas the band positions of the ππ*-state absorptions are essentially the same in the two solvents. For DMABN in acetonitrile, the rise time (∼4.3 ps) of the twisted intramolecular charge transfer (TICT)-state transient at 330 nm is identical to the decay time of the πσ*-state transient. The 4.8 ns decay time of the TICT-state absorption of DMABN is longer than the 2.9 ns decay time of the intramolecular charge-transfer (ICT) fluorescence, indicating that the fluorescent ICT state differs from the TICT state observed in transient absorption. These results are consistent with the presence of a low-lying πσ* state in DMABN (and DMABE), and the role the πσ* state plays in the formation of the TICT state of DMABN.

A concerted experimental and computational study of 4-(dimethylamino)benzethyne, DMABE, has been carried out to probe the low-lying pisigma* 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 pisigma* <--pisigma* transition at 750 nm, and a weaker pipi* <--pipi* (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 pisigma* state and the 520 nm absorption to the LE (pipi*) state. The bifurcation of the initially excited L(a) (pipi*) state into the pisigma* state and the LE state, as probed by transient absorption, is strongly influenced by solvent polarity, with polar environments favoring the L(a)-->pisigma* decay channel over the competing L(a)--> LE decay channel. The nanosecond radiationless decay of the LE state to the dark pisigma* state is also strongly enhanced in polar environments, thus accounting for the dramatic quenching of fluorescence in solvents of high polarity.

5,6-Trimethylenecytosine (TMC) and 5,6-trimethyleneuracil (TMU), in which the twist of the C5-C6 bond (or the pyrimidalization of C5) is strongly hindered, do not exhibit the subpicosecond excited-state lifetime characteristic of the naturally occurring pyrimidine bases. This result demonstrates the important role the out-of-plane deformation of the six-membered ring plays in the ultrafast (subpicosecond) internal conversion of photoexcited nucleobases. The dramatically shorter fluorescence lifetime of TMU ( approximately 30 ps) relative to TMC ( approximately 1.2 ns), in aqueous solution at room temperature, is attributed to the presence in TMU of an efficient, secondary nonradiative decay channel of S(1)(pipi*) involving a low-lying (1)npi* state.