Conical Intersections, charge localization, and photoisomerization pathway selection in a minimal model of a degenerate monomethine dye

Abstract

We propose a minimal model Hamiltonian for the electronic structure of a monomethine dye, in order to describe the photoisomerization of such dyes. The model describes interactions between three diabatic electronic states, each of which can be associated with a valence bond structure. Monomethine dyes are characterized by a charge-transfer resonance; the indeterminacy of the single-double bonding structure dictated by the resonance is reflected in a duality of photoisomerization pathways corresponding to the different methine bonds. The possible multiplicity of decay channels complicates mechanistic models of the effect of the environment on fluorescent quantum yields, as well as coherent control strategies. We examine the extent and topology of intersection seams between the electronic states of the dye and how they relate to charge localization and selection between different decay pathways. We find that intersections between the S(1) and S(0) surfaces only occur for large twist angles. In contrast, S(2)/S(1) intersections can occur near the Franck-Condon region. When the molecule has left-right symmetry, all intersections are associated with con- or disrotations and never with single bond twists. For asymmetric molecules (i.e., where the bridge couples more strongly to one end) the S(2) and S(1) surfaces bias torsion about different bonds. Charge localization and torsion pathway biasing are correlated. We relate our observations with several recent experimental and theoretical results, which have been obtained for dyes with similar structure.

A three-state effective Hamiltonian for symmetric cationic diarylmethanes

“…The fact that SA3-CAS(4,3) solutions with the same structure can be found for these distinct molecules suggests that the solutions are stable, and that they are a general feature of dyes from this class. 45,47,48 Benchmark studies 49,50 have shown that the excitation energies of anionic GFP chromophore models calculated with multireference perturbation theory and SA-CASSCF models are not sensitive to basis set or active space size within a range of 0.2 eV, which is within quoted reliability limits 43,51,52 of such models. The systems studied here should be easier to deal with than this, because they are all cations.…”

Valence-bond non-equilibrium solvation model for a twisting monomethine cyanine

“…This is also true in non-symmetric methines with very small diabatic gap 37 . The reason why our model predicts a symmetric twist distribution can be traced to the neglect of other intramolecular displacements that could break symmetry in the any real specific case.…”

“…In particular, these geometries are accessible from FC in gas phase. This is contraindicated by ab initio calculations on monomethine cyanines 35 , as well as non-symmetric monomethines with small diabatic gap 37 .…”

“…Ab initio calculations and general models indicate that conical intersections can occur in monomethine dyes in configurations where one or more bonds is twisted, so that the π orbitals on the fragments across the bond overlap weakly 22,35,37 . The non-adiabatic coupling (to nuclear displacement) diverges at conical intersections, so that internal conversion is expected to be dominated by configurations near these points.…”

“…However, twisting one or the other bond will destabilize the diabatic states by different amounts -because the π-bonds are in different places. Accordingly, the polarity of the TICT adiabatic states will depend upon the bond that is twisted 22,37 . The dependence of the TICT polarity on the identity of the twisted bond can also be explained using simple molecular orbital theories.…”