The photophysics and photochemistry of DNA is of great importance due to the potential damage of the genetic code by UV light. Quantum mechanical studies have played a key role in interpretating the results of modern time-resolved pump-probe spectroscopy, and in elucidating the main photoactivated reactive paths. This review provides a concise, complete picture of the computational studies carried out, approximately, in the past decade. We start with an overview of the photophysics of the nucleobases in the gas phase and in solution. We discuss the proposed mechanisms for ultrafast decay to the ground state, that involve conical intersections, consider the role of triplet states, and analyze how the solvent modulates the photophysics. Then we move to larger systems, from dinucleotides to single- and double-stranded oligonucleotides. We focus on the possible role of charge transfer and delocalized or excitonic states in the photophysics of these systems and discuss the main photochemical paths. We finish with an outlook on the current challenges in the field and future directions of research.

Singlet fluorescence lifetimes of adenosine, cytidine, guanosine, and thymidine, determined by femtosecond pump-probe spectroscopy (Pecourt, J.-M. L.; Peon, J.; Kohler, B. J. Am. Chem. Soc. 2000, 122, 9348. Pecourt, J.-M. L.; Peon, J.; Kohler, B. J. Am. Chem. Soc. 2001, 123, 10370), show that the excited states produced by 263 nm light in these nucleosides decay in the subpicosecond range (290-720 fs). Ultrafast radiationless decay to the ground state greatly reduces the probability of photochemical damage. In this work we present a theoretical study of isolated cytosine, the chromophore of cytidine. The experimental lifetime of 720 fs indicates that there must be an ultrafast decay channel for this species. We have documented the possible decay channels and approximate energetics, using a valence-bond derived analysis to rationalize the structural details of the paths. The mechanism favored by our calculations and the experimental data involves (1) a two-mode decay coordinate composed of initial bond length inversion followed by internal vibrational energy redistribution (IVR) to populate a carbon pyramidalization mode, (2) a state switch between the pi,pi* and nO,pi* (excitation from oxygen lone pair) excited states, and (3) decay to the ground state through a conical intersention. A second decay path through the nN,pi* state (excitation from the nitrogen lone pair), with a higher barrier, involves out-of-plane bending of the amino substituent.

The decay paths on the singlet excited-state surface of 9H-adenine and the associated energy barriers have been calculated at the CAS-PT2//CASSCF level. There are three fundamental paths for the photophysics: two paths for the (1)L(b) state which are virtually barrierless at the present level of theory and correspond to formation of the (n,pi) intermediate and direct decay to the ground state and a third path for ground-state decay of the (n,pi) state with an activation barrier of approximately 0.1 eV. The (1)L(a) state, which has the largest oscillator strength, either decays directly to the ground state or contributes indirectly to the excited-state lifetime by populating the two other states. The results are used to interpret the photophysics in terms of an excited-state plateau for the (1)L(b) state that corresponds to the short-lived excited-state component (approximately 0.1 ps) and a well (i.e., a proper minimum) for the (n,pi) state that gives rise to the long component (1 ps or more). The direct decay to the ground state of the (1)L(b) state is probably the decay channel invoked to explain the experimental wavelength dependence of the relative amplitudes of the two components. In addition to that, the excited-state component in the nanosecond range detected in the time-resolved photoelectron spectrum is proposed to be a triplet (pi,pi) state formed after intersystem crossing from the singlet (n,pi) state.

We address the possibility of populating the lowest triplet state of cytosine by an "intrinsic"mechanism, namely, intersystem crossing (ISC) along the ultrafast internal conversion pathway of the electronically excited singlet species. For this purpose, we present a discussion of the ISC process and triplet-state reactivity based on theoretical analysis of the spin-orbit strength and the potential energy surfaces for the relevant singlet and triplet states of cytosine. High-level ab initio computations show that ISC is possible in wide regions of the singlet manifold along the reaction coordinate that controls the ultrafast internal conversion to the ground state. Thus, the ISC mechanism documented here provides a possibility to access the triplet state, which has a key role in the photochemistry of the nucleic acid bases.

CASSCF and CAS-PT2 calculations are performed for the ground and excited states of radical cations consisting of two and three nucleobases. The generalized Mulliken-Hush approach is employed for estimating electronic couplings for hole transfer in the pi-stacks. We compare the CASSCF results with data obtained within Koopmans' approximation. The calculations show that an excess charge in the ground and excited states in the systems is quite localized on a single base both at the CASSCF level and in Koopmans' picture. However, the CASSCF calculations point to a larger degree of localization and, in line with this, smaller transition dipole moments. The agreement between the CAS-PT2 corrected energy gaps and the values estimated with Koopmans' theorem is better, with the CAS-PT2 calculations giving somewhat smaller gaps. Overall, both factors result in smaller CASSCF/CAS-PT2 couplings, which are reduced by up to 40% of the couplings calculated using Koopmans' approximation. The tabulated data can be used as benchmark values for the electronic couplings of stacked nucleobases. For the base trimers, comparison of the results obtained within two- and three-state models show that the multistate treatment should be applied to derive reliable estimates. Finally, the superexchange approach to estimate the donor acceptor electronic coupling in the stacks GAG and GTG is considered.

The photophysics of singlet excited 5-fluorocytosine (5FC) was studied in steady-state and timeresolved experiments and theoretically by quantum chemical calculations. Femtosecond transient absorption measurements show that replacement of the C5 hydrogen of cytosine by fluorine increases the excited-state lifetime by two orders of magnitude from 720 fs to 73 ± 4 ps. Experimental evidence indicates that emission in both compounds originates from a single tautomeric form. The lifetime of 5FC is the same within experimental uncertainty in the solvents ethanol and dimethylsulfoxide. The insensitivity of the S 1 lifetime to the protic nature of the solvent suggests that proton transfer is not the principal quenching mechanism for the excited state. Excited state calculations were carried out for the amino-keto tautomer of 5FC, the dominant species in polar environments, in order to understand its longer excited-state lifetime. CASSCF and CAS-PT2 calculations of the excited states show that the minimum energy path connecting the minimum of the 1 π, π* state with the conical intersection responsible for internal conversion has essentially the same energetics for cytosine and 5FC, suggesting that both bases decay nonradiatively by the same mechanism. The dramatic difference in lifetimes may be due to subtle changes along the decay coordinate. A possible reason may be differences in the intramolecular vibrational redistribution rate from the Franck-Condon active, in-plane modes to the out-of-plane modes that must be activated to reach the conical intersection region.

. (2013) 'Ultrafast above-threshold dynamics of the radical anion of a prototypical quinone electron-acceptor.', Nature chemistry., 5 (8). pp. 711-717. Further information on publisher's website:https://doi.org/10.1038/nchem.1705Publisher's copyright statement:Additional information: Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-pro t purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. Quinones are found throughout nature as key electron acceptor intermediates, 1,2 with examples including plastoquinone which is involved in the electron transfer chain of photosystem II, and ubiquinone (coenzyme Q10) which plays a key role in aerobic cellular respiration. 3 The central moiety responsible for the electron accepting ability in quinones is para-benzoquinone (pBQ), shown in Figure 1c. Electron transfer reactions involving pBQ can be highly exergonic and are therefore often classed as being in the Marcus inverted region. [4][5][6][7][8][9][10] This is shown schematically by the green path in Figure 1a, where a barrier between the Gibbs free energy of the reactants and products lowers the rates of the electron transfer process. However, even in the earliest experimental verifications of the inverted region for intramolecular electron transfer, several electron acceptors based on pBQ showed marked deviations from the expected behaviour, with transfer rates approaching those of a barrierless reaction. 11 It has been proposed that such deviations may involve electronically excited states of the product radical anion of para-benzoquinone (pBQ• -), 12 which could provide reaction pathways that bypass the barrier, as shown in Figure 1a with purple arrows. Figure 1b shows the location of these resonances.From the point of view of an electron approaching pBQ, these anionic resonances in the detachment continuum can capture an electron. Subsequent formation of the anionic ground state through internal conversion would redistribute the excess internal energy amongst all the vibrational modes. In a condensed-phase environment, this energy will be quenched by the surroundings. However, the initially formed excited states of pBQ• -can be unbound with respect to electron loss. For the above picture to be feasible, internal conversion must be able to compete with autodetachment. Electron attachment spectra suggest that it can, 26-28 but how does this occur given that these resonances are in some cases > 1 eV above the detachment threshold?In order to gain a fundamental understanding of the processes involved following electron capture, it is necessary to observe the relaxation dynamics in real t...

A three-state conical intersection between the ground state and the (π,π*) and (n O,π*) singlet excited states of cytosine is the topological feature that dominates the ultrafast decay of singlet excited cytosine. The three-state intersection is associated with seams of intersection between pairs of states (S 1/S 0 and S 2/S 1, respectively), and the resulting topology has been mapped out with CASSCF and CAS-PT2 calculations. The minimum-energy path for the optically active (π,π*) state lies on the S 1 surface, and decay to the ground state takes place at the S 1/S 0 seam. On the other hand, the region of the S 2/S 1 seam must be traversed before accessing the conical intersection with the ground state and recrossing to S 2 becomes possible. Another feature associated with the three-state degeneracy is vibronic coupling between the (π,π*) and (n O,π*) excited states (proximity effect), which lowers the barrier to the S 1/S 0 seam. From a mechanistic point of view, then, the decay is the outcome of the interaction between the three states. The results also suggest that the experimental excited-state lifetime is the effect of two factors, an energetically accessible region of S 1/S 0 degeneracy and a region where the decay can be slowed because of recrossing to S 2.