Combined Theoretical and Experimental Study of the Photophysics of Asulam

The photophysics of the neutral molecular form of the herbicide asulam has been described in a joint experimental and theoretical, at the CASPT2 level, study. The unique π → π* aromatic electronic transition (f, ca. 0.5) shows a weak red-shift as the polarity of the solvent is increased, whereas the fluorescence band undergoes larger red-shifts. Solvatochromic data point to higher dipole moment in the excited state than in the ground state (μg < μe). The observed increase in pKa in the excited state (pKa* – pKa, ca. 3) is consistent with the results of the Kamlet–Abboud–Taft and Catalán et al. multiparametric approaches. Fluorescence quantum yield varies with the solvent, higher in water (ϕf = 0.16) and lower in methanol and 1-propanol (approx. 0.02). Room temperature fluorescence lifetime in aqueous solution is (1.0 ± 0.2) ns, whereas the phosphorescence lifetime in glassy EtOH at 77 K and the corresponding quantum yield are (1.1 ± 0.1) s and 0.36, respectively. The lack of mirror image symmetry between modified absorption and fluorescence spectra reflects different nuclear configurations in the absorbing and emitting states. The low value measured for the fluorescence quantum yield is justified by an efficient nonradiative decay channel, related with the presence of an easily accessible conical intersection between the initially populated singlet bright 1(La ππ*) state and the ground state (gs/ππ*)CI. Along the main decay path of the 1(La ππ*) state the system undergoes an internal conversion process that switches part of the population from the bright 1(La ππ*) to the dark 1(Lb ππ*) state, which is responsible for the fluorescence. Additionally, singlet–triplet crossing regions have been found, a fact that can explain the phosphorescent emission detected. An intersystem crossing region between the phosphorescent state 3(La ππ*) and the ground state has been characterized, which contributes to the nonradiative deactivation of the excitation energy.