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Conical Intersections, charge localization, and photoisomerization pathway selection in a minimal model of a degenerate monomethine dye
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Image of FIG. 1.
FIG. 1.

Examples of monomethine dyes. The molecules resonate between Lewis structures which invert bond alternation and redistribute the formal charge. Examples include, from top to bottom, the symmetric monomethine cyanine dye 1122C, the symmetric monomethine cyanine dye NK88, the asymmetric monomethine cyanine dye thiazole orange, and the chromophore of the GFP, an asymmetric diarylmethine oxonol dye.

Image of FIG. 2.
FIG. 2.

The four isomers of the asymmetrical dye thiazole orange, which differ by isomerism of the bridge. Thiazole orange is the least symmetrical of the three example molecules in Fig. 1, and so all isomers are distinguishable. They are labeled according to usual organic chemistry nomenclature.

Image of FIG. 3.
FIG. 3.

We illustrate three conceivable situations, where the shape of potential energy surfaces, and their interactions, could influence photochemistry. On the left, two states are biased toward the same pathway and ensuing dynamics may converge to a common evolution. In the middle, the electronic states are close in energy at the FC region, and are not biased strongly, so there is ambiguity in the pathways and no clear connection between the dynamics and the initially excited state. On the right, the states are biased differently, so that the dynamics on the different states diverge. Other situations are also conceivable.

Image of FIG. 4.
FIG. 4.

Localized-orbital active space representations for monomethine dye systems. For every monomethine dye, there is a “methine adapted” three-orbital solution to the state-averaged complete active space self consistent field problem. Monomethine cyanine dyes (left) have a two-electron solution, and diarylmethine dyes (right) possess a four-electron solution. In either case, the many-electron state space is six-dimensional and has a natural valence-bond structure in the localized representation (bottom). The energetic ordering of the localized orbitals is inverted in the two dye classes.

Image of FIG. 5.
FIG. 5.

A toy model using simple orbitals, where orbital overlaps have a similar functional dependence on and to the matrix elements in our model Hamiltonian, which are shown.

Image of FIG. 6.
FIG. 6.

Representation of relevant coordinates in terms of the geometrical model in Fig. 4 (top) and as vectors in the plane (bottom). Single bond twists change one angle while leaving the other constant. The conrotatory and disrotatory twist coordinates are antisymmetric and symmetric combinations of the single bond twists, respectively. The parity of the combination coordinates depends on the “handedness” convention used to define the torsion angles, which here uses left and right hand rules for left and right torsion angles. Changing the handedness of the definition for one of the bonds is equivalent to interchanging the parameters and . In the context of an untwisted molecular frame with symmetry, conrotatory twisting preserves symmetry and breaks symmetry, while the disrotatory twist breaks symmetry and preserves symmetry.

Image of FIG. 7.
FIG. 7.

The analytic eigenvalues of a symmetric matrix can be expressed by two parameters, and , which are polynomials of first and higher traces of the matrix. The space of all degeneracies between the eigenvalues can be expressed as a relationship between these two parameters. (Top) Eigenvalues plotted over a plane spanned by the parameters and . The conditions and (region shown at bottom) are sufficient to guarantee real eigenvalues of a matrix and are equivalent to symmetry and positive definiteness of the matrix. When the inequality is strong, three nondegenerate eigenvalues exist. On the boundary (highlighted in black) at least two of the eigenvalues are degenerate. degeneracies occur on the part of the boundary. degeneracies occur on the region of the boundary, and a three-state intersection occurs at .

Image of FIG. 8.
FIG. 8.

Conical intersections in the space spanned by the angles , and the affine parameter [Eq. (36)]. (Top) (red) and (yellow) intersection seams at constant , 1.0 (center), and 1.25 (right). When , there are intersections at the corners of the unit cell for all values of . This is accompanied by a change in the curvature of the seam. (Bottom) The location of the seam at different values of . When , the and intersections lie along perpendicular lines in the plane. When , this behavior changes, and both and intersections lie along a single line. Black arrows indicate the direction of motion of the seams with increasing Intersections were visualized using Eq. (30), with different branches colored according to the sign of [Eqs. (32) and (33)].

Image of FIG. 9.
FIG. 9.

Relationships between conical intersection seams (top) and charge-localization in the adiabatic states of the model. (Left) Conical intersection seams are shown for varying at constant , . Cross sections are shown at , and 1.0. (Right) Population (absolutely squared amplitude) of the fragment diabatic sites are plotted over the torsion plane. Diabatic state populations were used as a measure of the charge distribution, and were used as convex coordinates in a Red-Blue-Green (RGB) color map. Areas where the cross sections intersect the neighborhood of the seam [Eq. (30), ] are shown by filled red and yellow regions. Charge localization is generally twist dependent, and regions of different localizations on the state are separated by intersections impinging on that state.

Image of FIG. 10.
FIG. 10.

Synergistic relationship between energetic biasing of different bond torsions (top) and twist dependence of the charge localization (bottom). (Top) Biasing in the potential energy surface is introduced by asymmetric bridge-end couplings. When the coupling of the bridge to both ends is symmetric (top center), the curvature at the FC point is equivocal with respect to both single-bond twists. Introducing asymmetric coupling alters the curvature to favor one bond over the other (top left and right). (Bottom) Introducing asymmetric bridge-end coupling also exerts changes on the charge distribution over the fragments and its dependence on the twist. When the coupling is symmetric , charge localization follows the twist distribution, for con- and disrotatory twists the charge is spread over the left and right fragments, but for single-bond twists, it localizes on one side or the other in response to small displacements (bottom center). When the dye is not symmetric , the charge is localized at FC. The polarity at FC depends on the sign of . The polarity is insensitive to small displacements from FC—twisting one of the bonds maintains the polarity at FC throughout the available range; twisting about the other induces polarity reversal, but only at large angles. In the context of a solvent reaction field, this means that progress along one coordinate may require a dielectric reorganization event, while the other may not. Note that the domain of the coordinates is out of phase relative to Figs. 8 and 9, so that the Frank-Condon point is in the middle of the square.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Conical Intersections, charge localization, and photoisomerization pathway selection in a minimal model of a degenerate monomethine dye