Conical intersections chromophores

The first study in which a full CASSCE treatment was used for the non-adiabatic dynamics of a polyatomic system was a study on a model of the retinal chromophore [86]. The cis-trans photoisomerization of retinal is the primary event in vision, but despite much study the mechanism for this process is still unclear. The minimal model for retinal is l-cis-CjH NHj, which had been studied in an earlier quantum chemisti7 study [230]. There, it had been established that a conical intersection exists between the Si and So states with the cis-trans defining torsion angle at approximately a = 80° (cis is at 0°). Two... 

Toniolo A, Olsen S, Manohar L, Martinez TJ (2004) Conical intersection dynamics in solution the chromophore of green fluorescent protein. Faraday Discuss 127 149... 

We have already mentioned in the Introduction (Section 3.7.1) the importance of conical intersections (CIs) in connection with excited electronic state dynamics of a photoexcited chromophore. Briefly, CIs act as photochemical funnels in the passage from the first excited S, state to the ground electronic state S0, allowing often ultrafast transition dynamics for this process. (They can also be involved in transitions between excited electronic states, not discussed here.) While most theoretical studies have focused on CIs for a chromophore in the gas phase (for a representative selection, see refs [16, 83-89], here our focus is on the influence of a condensed phase environment [4-9], In particular, as discussed below, there are important nonequilibrium solvation effects due to the lack of solvent polarization equilibration to the evolving charge distribution of the chromophore. 

Fig. 2 Schematic representation of the Si /Sq conical intersection seam along the torsion coordinate of the central bond of the minimal retinal chromophore model pentadieniminium cation (reprinted with permission from [20])...

Similar organic chromophores (e.g. conjugated hydrocarbons) have similar conical intersection structures. 

The PSB of retinal is the chromophore of rhodopsin proteins (Scheme 2.9). After photon absorption, the S — Sq radiationless decay through a conical intersection results in an... 

Due to the recent availability of suitable computational methods in standard quantum chemistry packages (e.g. GAUSSIAN ), the computation of conical intersections of organic chromophores and, to a certain extent. 

In Sec. 3 we will mainly concentrate on the discussion of the structure (both geometrical, electronic and topographical) of the chemically relevant (low-lying) conical intersections that have been documented in a number of basic organic chromophores. As we will further point out in Sec. 3.1, the majority of these structures have been computed via conical intersection optimizations and do not strictly correspond to the conical intersection points located at the very bottom of the excited-state path. Nevertheless, their general features are representative of the chemically relevant segment of the intersection space and can be used for the mechanistic rationalization of different photoinduced molecular processes. 

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