Internal Conversion conical intersection

Surface Hopping, Excited States, Density Functional Theory, Ultrafast Internal Conversion, Conical Intersections, Nucleobases, Base Pairs, Photostability, UV Genetic Damage... 

The prompt dissociation of the fast H atom in the pathway, for which character change and collapse of the 3s Rydberg orbital on the ethyl radical to the Is orbital of the H product are required,121-123 could be assisted by the conical intersection. Also, relaxation and internal conversion from the 3s state to the ground state ethyl can be facilitated by this conical intersection, in addition to other possible vibronic couplings in the A symmetry.39... 

Reflecting personal preferences, we focus in this review on the modeling of ultrafast bound-state processes following photoexcitation such as electron transfer, internal-conversion via conical intersections, and nonadiabatic... 

We demonstrate by using ultrafast time resolved spectroscopy that the photoconversion from dihydroazulene (DHA) to vinylheptafulvene (VHF) is governed by two mechanisms The ring opening proceeds on the excited energy surface on the picosecond time scale. It is followed by an internal conversion to the VHF ground state that is accelerated by the presence of a conical intersection in the case of cyclopenta-DHA. This conical intersection hinders the photoinduced back reaction from the final VHF products. However, we successfully photo-converted the cyanophenyl-VHF-cis back to the DHA in an experiment with two delayed pulses. This opens the route to the development of bistable dihydroazulene switches. 

It is the strong interaction between the excited state and the ground state via the conical intersection that hinders the optical bistability of the CP-DHA compound. In contrast CN-DHA shows a much smaller efficiency of the internal conversion which indicates that no conical intersection is involved. We can therefore raise the question whether a photoinduced backreac-tion from the transient CN-VHF-cis ground state is possible. 

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 present analysis relies on - and extends - the comprehensive theoretical study of Refs. [23,24] on the multi-state interactions in the manifold of the X — E states of Bz+. Like this recent work, it utilizes an ab initio quantum-dynamical approach. In Refs. [23,24] we have, in addition, identified strong coupling effects between the B — C and B — D electronic states, caused by additional conical intersections between their potential energy surfaces. A whole sequence of stepwise femtosecond internal conversion processes results [24]. Such sequential internal conversion processes are of general importance as is evidenced indirectly by the fluorescence and fragmentation dynamics of organic closed-shell molecules and radical cations [49,50]. It is therefore to be expected that the present approach and results may be of relevance for many other medium-sized molecular systems. 

Along this OH dissociation coordinate, we also find a conical intersection between the ttct state,. S , and the ground state, S0, which could act as an efficient route for internal conversion. Such a scenario has been advocated by Domcke and Sobolewski [23, 84, 86] to be responsible for the photostability of nucleobases. However, in the present case, the free energy activation barrier for OH dissociation was computed to be 52 kJ/mol [47], Hence this de-excitation pathway is unlikely to explain the ultrafast nonradiative decay observed experimentally [5, 11, 37], Shukla and Leszczynski [80] find an activation barrier of 154 kJ/mol for the keto-enol tautomerisation of 7H G. However, this result is for tautomerisation in the tttt state, whereas the ROKS study involves two different excited states [47],... 

Two paradigms have been widely used in the past decade to describe the ultrafast relaxation of optically excited tttt states in purine molecules, through internal conversions [69], One of them relies on the existence of a conical intersection (Cl) between the excited state and the ground state, accessible on the excited state surface from the Franck-Condon region [69, 70], The second one, Lim s proximity effect , stems from vibronic coupling between the tttt state and nearby mr states found in these heteroatomic molecules [71]. Excited state quantum calculations have therefore focused recently on a precise characterisation of the strong perturbations and interactions undergone by these tttt or nit states. 

As a matter of fact, a few energetic trends along the several tautomers seem to emerge from theoretical calculations, in particular the energetic of the transitions, even if a consensus between the several methods used has still to be reached to get a precise picture of the excited state. Several types of relaxation mechanisms of the optically excited state have been shown to potentially occur depending upon the tautomer considered, in particular the existence of accessible conical intersections leading to a fast relaxation scheme through internal conversion. 

Figure 13-8. Relevant schematic potential energy curves for the near UV photophysics of the most stable tautomers of guanine. The region of the first singlet tht excited state surface accessible by Franck-Condon excitation is indicated in bold. Excited state internal conversion through a conical intersection (Cl) with S0 is illustrated by curved arrows. Vertical arrows indicate fluorescence emission. The eventual role of excited nir singlet states cannot be ruled out, especially at high energy excess in the excited state (see text)...

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