Ultrafast dynamics intersection

As mentioned in the introduction, only few vibrational degrees of freedom are usually actively involved in the ultrafast dynamics at the conical intersection. To account for the effect of energy transfer from the active modes to the many inactive modes of the polyatomic molecule or a condensed-phase environment, the reduced-density-matrix formalism may... 

The ab initio molecular dynamics study by Hudock et al. discussed above for uracil included thymine as well [126], Similarly to uracil, it was found that the first ultrafast component of the photoelectron spectra corresponds to relaxation on the S2 minimum. Subsequently a barrier exists on the S2 surface leading to the conical intersection between S2 and Si. The barrier involves out-of-plane motion of the methyl group attached to C5 in thymine or out-of-plane motion of H5 in uracil. Because of the difference of masses between these two molecules, kinematic factors will lead to a slower rate (longer lifetime) in thymine compared to uracil. Experimentally there are three components for the lifetimes of these systems, a subpicosecond, a picosecond and a nanosecond component. The picosecond component, which is suggested to correspond to the nonadiabatic S2/S1 transition, is 2.4 ps in uracil and 6.4 ps in thymine. This difference in the lifetimes could be explained by the barrier described above. 

As a last example of a molecular system exhibiting nonadiabatic dynamics caused by a conical intersection, we consider a model that recently has been proposed by Seidner and Domcke to describe ultrafast cis-trans isomerization processes in unsaturated hydrocarbons [172]. Photochemical reactions of this type are known to involve large-amplitode motion on coupled potential-energy surfaces [169], thus representing another stringent test for a mixed quantum-classical description that is complementary to Models 1 and II. A number of theoretical investigations, including quantum wave-packet studies [163, 164, 172], time-resolved pump-probe spectra [164, 181], and various mixed... 

Transient absorption experiments have shown that all of the major DNA and RNA nucleosides have fluorescence lifetimes of less than one picosecond [2—4], and that covalently modified bases [5], and even individual tautomers [6], differ dramatically in their excited-state dynamics. Femtosecond fluorescence up-conversion studies have also shown that the lowest singlet excited states of monomeric bases, nucleosides, and nucleotides decay by ultrafast internal conversion [7-9]. As discussed elsewhere [2], solvent effects on the fluorescence lifetimes are quite modest, and no evidence has been found to date to support excited-state proton transfer as a decay mechanism. These observations have focused attention on the possibility of internal conversion via one or more conical intersections. Recently, computational studies have succeeded in locating conical intersections on the excited state potential energy surfaces of several isolated nucleobases [10-12]. 

The qualitative picture suggested by these calculations has recently been confirmed by new experiments on the UV photochemistry of phenol. Lee and collaborators have shown that loss of H atoms is the major process in phenol at 248 nm excitation [36], Ashfold and co-workers have applied high-resolution photofragment translational spectroscopy to obtain uniquely detailed insight into the photodissociation dynamics of phenol [37], The phenoxy radical is produced in a surprisingly limited subset of its available vibrational states [37], Such vibrational mode-specific dynamics is a signature of ultrafast radiationless decay through directly accessible conical intersections. Similar results have been obtained for pyrrole, imidazole and indole [38 10],... 

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. 

All this information about the S, and S0 states suggests that aminopyrimidine will present ultrafast decay through puckered conical intersections. The excited-state lifetime and the actual conical intersection that participate in the deactivation process can only be found by dynamics simulations (see below). 

Benchmark ab initio quantum dynamical studies are carried out for the prototypical naphthalene and anthracene radical cations of the PAH family aiming to understand the vibronic interactions and ultrafast decay of their low-lying electronic states. The broadening of vibronic bands and ultrafast internal conversion through conical intersections in the Da — — D2 electronic states of these species is... 

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