jp311932z_si_001.pdf (1.87 MB)
Combined Theoretical and Experimental Study of the Photophysics of Asulam
journal contribution
posted on 2016-02-19, 18:06 authored by Angelo Giussani, Rosendo Pou-Amérigo, Luis Serrano-Andrés, Antonio Freire-Corbacho, Cristina Martínez-García, M Fernández P., Mohamed Sarakha, Moisés Canle L., J. Arturo SantaballaThe 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.