The low-lying excited singlet states of the keto, enol, and keto-imine tautomers of cytosine have been investigated employing a combined density functional/multireference configuration interaction (DFT/MRCI) method. Unconstrained geometry optimizations have yielded out-of-plain distorted structures of the pi --> pi and n --> pi excited states of all cytosine forms. For the keto tautomer, the DFT/MRCI adiabatic excitation energy of the pi --> pi state (4.06 eV including zero-point vibrational energy corrections) supports the resonant two-photon ionization (R2PI) spectrum (Nir et al. Phys. Chem. Chem. Phys. 2002, 5, 4780). On its S1 potential energy surface, a conical intersection between the 1pipi state and the electronic ground state has been identified. The barrier height of the reaction along a constrained minimum energy path amounts to merely 0.2 eV above the origin and explains the break-off of the R2PI spectrum. The 1pipi minimum of the enol tautomer is found at considerably higher excitation energies (4.50 eV). Because of significant geometry shifts with respect to the ground state, long vibrational progressions are expected, in accord with experimental observations. For the keto-imine tautomer, a crossing of the 1pipi potential energy surface with the ground-state surface has been found, too. Its n --> pi minimum (3.27 eV) is located well below the conical intersection between the pi --> pi and S0 states, but it will be difficult to observe because of its small transition moment. The identified conical intersections of the pi --> pi excited states of the keto cytosine tautomers are made responsible for the ultrafast decay to the electronic ground states and thus may explain their subpicoseconds lifetimes.

The mechanism of the light‐induced spin crossover of the [Fe(bpy)3]2+ complex (bpy=2,2′‐bipyridine) has been studied by combining accurate electronic‐structure calculations and time‐dependent approaches to calculate intersystem‐crossing rates. We investigate how the initially excited metal‐to‐ligand charge transfer (MLCT) singlet state deactivates to the final metastable high‐spin state. Although ultrafast X‐ray free‐electron spectroscopy has established that the total timescale of this process is on the order of a few tenths of a picosecond, the details of the mechanisms still remain unclear. We determine all the intermediate electronic states along the pathway from low spin to high spin and give estimates for the deactivation times of the different stages. The calculations result in a total deactivation time on the same order of magnitude as the experimentally determined rate and indicate that the complex can reach the final high‐spin state by means of different deactivation channels. The optically populated excited singlet state rapidly decays to a triplet state with an Fe d6() configuration either directly or by means of a triplet MLCT state. This triplet ligand‐field state could in principle decay directly to the final quintet state, but a much faster channel is provided by internal conversion to a lower‐lying triplet state and subsequent intersystem crossing to the high‐spin state. The deactivation rate to the low‐spin ground state is much smaller, which is in line with the large quantum yield reported for the process.

We present three formulas for calculating intersystem crossing rates in the Condon approximation to the golden rule by means of a time-dependent approach: an expression using the full time correlation function which is exact for harmonic oscillators, a second-order cumulant expansion, and a short-time approximation of this expression. While the exact expression and the cumulant expansion require numerical integration of the time correlation function, the integration of the short-time expansion can be performed analytically. To ensure convergence in the presence of large oscillations of the correlation function, we use a Gaussian damping function. The strengths and weaknesses of these approaches as well as the dependence of the results on the choice of the technical parameters of the time integration are assessed on four test examples, i.e., the nonradiative S(1) ⇝ T(1) transitions in thymine, phenalenone, flavone, and porphyrin. The obtained rate constants are compared with previous results of a time-independent approach. Very good agreement between the literature values and the integrals over the full time correlation functions are observed. Furthermore, the comparison suggests that the cumulant expansion approximates the exact expression very well while allowing the interval of the time integration to be significantly shorter. In cases with sufficiently high vibrational density of states also the short-time approximation yields rates in good agreement with the results of the exact formula. A great advantage of the time-dependent approach over the time-independent approach is its excellent computational efficiency making it the method of choice in cases of large energy gaps, large numbers of normal modes, and high densities of final vibrational states.

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