Combined Theoretical and Experimental Study of the Photophysics of Asulam

Abstract

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 (μ(g) < μ(e)). 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 (ϕ(f) = 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)(L(a) ππ*) state and the ground state (gs/ππ*)(CI). Along the main decay path of the (1)(L(a) ππ*) state the system undergoes an internal conversion process that switches part of the population from the bright (1)(L(a) ππ*) to the dark (1)(L(b) 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)(L(a) ππ*) and the ground state has been characterized, which contributes to the nonradiative deactivation of the excitation energy.

Insights into the Complex Photophysics and Photochemistry of the Simplest Nitroaromatic Compound: A CASPT2//CASSCF Study on Nitrobenzene

“…Such a CASPT2 approach has been validated in 4 many different studies on organic molecules, providing a correct prediction, description, and interpretation of photophysical experimental data. [26][27][28][29] The search for the evolution of the excited states has been performed by means of MEP calculations, 30,31 using mass-weighted coordinates. The technique provides the steepest descendent path, in which each step is built by the minimization of the energy on a hyperspherical cross section, whose radius has been chosen by the user, of the potential energy hypersurface centered on the initial geometry within a predefined radius.…”

On the Simulation of Two‐dimensional Electronic Spectroscopy of Indole‐containing Peptides

“…In the RASPT2 computations, an IPEA shift of 0.0 a.u. has been used , a normal choice previously adopted in the description of systems of very diverse size and nature , while the imaginary shift parameter has been scanned in the range 0.2–0.6 a.u. .…”

Direct photolysis mechanism of pesticides in water

“…21,66,67) Recent progress in time-dependent density functional theory (TD-DFT) has made it possible to directly examine excited states by using an exchange-correlation functional such as B3LYP and refined ab initio basis sets, such as 6-311++G(d,p), within reasonable precision. 68) Not only electronic transitions with E S and E T values relevant to direct photolysis 50,[69][70][71] but also threedimensional structures in excited states and of reactive intermediates 41,[70][71][72] can be examined using TD-DFT calculations. Since the calculations generally suppose an isolated molecule in a vacuum, a solvation effect has been taken into account in some cases, indirectly by using a polarized continuum 28,33,50) or Onsager's cavity model, 70,73) or directly by introducing water molecules into the calculations.…”

“…68) Not only electronic transitions with E S and E T values relevant to direct photolysis 50,[69][70][71] but also threedimensional structures in excited states and of reactive intermediates 41,[70][71][72] can be examined using TD-DFT calculations. Since the calculations generally suppose an isolated molecule in a vacuum, a solvation effect has been taken into account in some cases, indirectly by using a polarized continuum 28,33,50) or Onsager's cavity model, 70,73) or directly by introducing water molecules into the calculations. 49) A bond liable to be photocleaved 53,73,74) together with the preference of a homolytic or heterolytic mechanism 33,75) can be conveniently evaluated by its dissociation energy.…”