A comprehensive effort in photodynamical ab initio simulations of the ultrafast deactivation pathways for all five nucleobases adenine, guanine, cytosine, thymine, and uracil is reported. These simulations are based on a complete nonadiabatic surface-hopping approach using extended multiconfigurational wave functions. Even though all five nucleobases share the basic internal conversion mechanisms, the calculations show a distinct grouping into purine and pyrimidine bases as concerns the complexity of the photodynamics. The purine bases adenine and guanine represent the most simple photodeactivation mechanism with the dynamics leading along a diabatic ππ* path directly and without barrier to the conical intersection seam with the ground state. In the case of the pyrimidine bases, the dynamics starts off in much flatter regions of the ππ* energy surface due to coupling of several states. This fact prohibits a clear formation of a single reaction path. Thus, the photodynamics of the pyrimidine bases is much richer and includes also nπ* states with varying importance, depending on the actual nucleobase considered. Trapping in local minima may occur and, therefore, the deactivation time to the ground state is also much longer in these cases. Implications of these findings are discussed (i) for identifying structural possibilities where singlet/ triplet transitions can occur because of sufficient retention time during the singlet dynamics and (ii) concerning the flexibility of finding other deactivation pathways in substituted pyrimidines serving as candidates for alternative nucleobases.photodynamical simulation | photostability | ultrafast photodeactivation | nonadiabatic interactions | ab initio multireference methods O wing to the importance of mutagenic and carcinogenic effects caused by UV radiation on DNA, the UV-induced photochemistry and photophysics of individual bases (1-7), base pairs (8-10), and nuclei acid strands (11-13) have been intensively studied. In each of these levels, it has been found that the genetic code may count on molecular mechanisms to get rid of the energy excess minimizing deleterious effects. Isolated nucleobases (Scheme 1), for instance, are all photostable by returning to the ground state in an ultrafast time scale of a few picoseconds (1, 2, 14-20), which reduces the probability of undergoing photochemical transformations induced by reactive excited states. The ultrafast deactivation of the five natural nucleobases contrasts with the long lifetime of analogous bases not found in DNA and RNA (2, 21), suggesting that it could have been a source of evolutionary pressure in early biotic ages.Ultrafast deactivation occurs through internal conversion mechanisms, where the molecule transfers the photoenergy stored in the electronic system to nuclear vibrational degrees of freedom. This means, the deactivation takes place without photoemission in contrast to what occurs for fluorescent and phosphorescent species. The energy transfer proceeds more intensely near nuclear geometries with stat...

Understanding the properties of electronically excited states is a challenging task that becomes increasingly important for numerous applications in chemistry, molecular physics, molecular biology, and materials science. A substantial impact is exerted by the fascinating progress in time-resolved spectroscopy, which leads to a strongly growing demand for theoretical methods to describe the characteristic features of excited states accurately. Whereas for electronic ground state problems of stable molecules the quantum chemical methodology is now so well developed that informed nonexperts can use it efficiently, the situation is entirely different concerning the investigation of excited states. This review is devoted to a specific class of approaches, usually denoted as multireference (MR) methods, the generality of which is needed for solving many spectroscopic or photodynamical problems. However, the understanding and proper application of these MR methods is often found to be difficult due to their complexity and their computational cost. The purpose of this review is to provide an overview of the most important facts about the different theoretical approaches available and to present by means of a collection of characteristic examples useful information, which can guide the reader in performing their own applications.

Semi-classical simulations of the UV-photoabsorption cross sections of adenine, guanine, cytosine, thymine, and uracil in gas phase were performed at the resolution-of-identity coupled cluster to the second-order (RI-CC2) level. With the exception of cytosine, the spectra of the other four nucleobases show a two band pattern separated by a low intensity region. The spectrum of cytosine is shaped by a sequence of three bands of increasing intensity. The first band of guanine is composed by two pipi* transitions of similar intensities. The analysis of individual contributions to the spectra allows a detailed assignment of bands. It is shown that the semi-classical simulations are able to predict general features of the experimental spectra, including their absolute intensities.