Mar Reguero

The Decay from the Dark nπ* Excited State in Uracil : An Integrated CASPT2/CASSCF and PCM/TD-DFT Study in the Gas Phase and in Water

By integrating the results of MS-CASPT2/CASSCF and TD-PBE0 calculations, we propose a mechanism for the decay of the excited dark state in pyrimidine, fully consistent with all the available experimental results. An effective conical intersection (CI-npi) exists between the spectroscopic pi/pi* excited state (Spi) and a dark n/pi* state (Sn), and a fraction of the population decays to the minimum of Sn (Sn-min). The conical intersection between Sn and the ground-state is not involved in the decay mechanism, because of its high energy gap with respect to Sn-min. On the other hand, especially in hydrogen bonding solvents, the energy gap between Sn-min and CI-npi is rather small. After thermalization in Sn-min, the system can thus recross CI-npi and then quickly proceed on the Spi barrierless path toward the conical intersection with the ground state.

ON THE QUASIDIABATIC CHARACTER OF AVERAGE NATURAL ORBITALS

Abstract The nearly diabatic character of average natural orbitals obtained from linear combinations of density matrices associated with different states is rationalized. These orbitals may be used to build low-dimensional model spaces and effective Hamiltonians which give a nearly diabatic description of the set of interacting states.

Calculation of solvatochromic shifts using MC-SCF theory. The n-π* transition of acetone

Abstract The solvatochromic shift of the n—π* transition of acetone is computed using the MC-SCF method using a continuum representation of the solvent with an elipsoidal cavity. Provided the solvent molecules are not allowed to relax in the field of the excited state, in accordance with the Franck—Condon principle, one obtains qualitative agreement with experimental trends and semi-empirical studies.

Insight into the Mechanisms of Luminescence of Aminobenzonitrile and Dimethylaminobenzonitrile in Polar Solvents. An ab Initio Study

The dual fluorescence of 4-(dimethylamino)benzonitrile (DMABN) has been intensively studied in the last decades, but surprisingly there is not any detailed theoretical study of its photochemistry in polar solvents. In this work, we rationalize the different luminescent behavior of 4-aminobenzonitrile (ABN) and DMABN in acetonitrile by a computational study developed at the CASSCF/CASPT2 level and using the Polarized Continuum Model to reproduce the solvent environment. We present here the critical geometries, energies, and connections between the potential energy surfaces of the low-lying excited states: the locally excited state (LE) and several intramolecular charge transfer states (ICT). The computational results show that the topology of the potential energy surfaces (PES) does not change substantially when the effect of a polar solvent is included, in comparison with the gas phase. For DMABN, though, polar solvents stabilize preferentially the ICT states in such a way that the different interplay with the LE state induces strong qualitative changes in the photochemistry of this compound. Specifically, the planar ICT (PICT) species located on the S2 surface in the gas phase is, in acetonitrile, located on the S1 surface. that is, at the geometry of the PICT minimum, the LE state is higher in energy than the ICT. Now LE and PICT minima are practically degenerate and, given that both correspond to first excited state species, emission can take place from both of them. However, the twisted ICT (TICT) species is still the most favored thermodynamically so it is expected that this species would be preferentially populated. On the other hand, for ABN the equilibrium lies in favor of LE, as the TICT species was found at a much higher energy with a low reaction barrier toward LE. This explains why dual fluorescence cannot be observed in ABN, even in polar solvents.

CALCULATION OF SMALL TRANSITION ENERGIES FROM A DRESSED CONFIGURATION INTERACTION

Abstract Small energy separations between nearly degenerate states in magnetic systems or mixed valence complexes are difficult to calculate. In the recent past difference dedicated CI (DDCI) calculations appeared to be efficient but they omit the 3 rd -order effect of inactive double excitations. A size-consistent dressed CI technique, applied on CAS single and double CI spaces, on model problems, is shown to treat efficiently these effects and to give better results than DDCI. The relative importance of removing unlinked terms from these CI spaces is emphasised.

Phenylazopyridine as Switch in Photochemical Reactions. A Detailed Computational Description of the Mechanism of Its Photoisomerization

Azo compounds are organic photochromic systems that have the possibility of switching between cis and trans isomers under irradiation. The different photochemical properties of these isomers make azo compounds into good light-triggered switches, and their significantly different geometries make them very interesting as components in molecular engines or mechanical switches. For instance, azo ligands are used in coordination complexes to trigger photoresponsive properties. The light-induced trans-to-cis isomerization of phenylazopyridine (PAPy) plays a fundamental role in the room-temperature switchable spin crossover of Ni-porphyrin derivatives. In this work, we present a computational study developed at the SA-CASSCF/CASPT2 level (State Averaged Complete Active Space Self Consistent Field/CAS second order Perturbation Theory) to elucidate the mechanism, up to now unknown, of the cis–trans photoisomerization of 3-PAPy. We have analyzed the possible reaction pathways along its lowest excited states, generated by excitation of one or two electrons from the lone pairs of the N atoms of the azo group (nazoπ*2 and nazo2π*2 states), from a π delocalized molecular orbital (ππ* state), or from the lone pair of the N atom of the pyridine moiety (npyπ* state). Our results show that the mechanism proceeds mainly along the rotation coordinate in both the nazoπ* and ππ* excited states, although the nazo2π*2 state can also be populated temporarily, while the npyπ* does not intervene in the reaction. For rotationally constrained systems, accessible paths to reach the cis minimum along planar geometries have also been located, again on the nazoπ* and ππ* potential energy surfaces, while the nazo2π*2 and npyπ* states are not involved in the reaction. The relative energies of the different paths differ from those found for azobenzene in a previous work, so our results predict some differences between the reactivities of both compounds.

An MCSCF study of the effect of substituents and solvent on the [2 + 2]cycloaddition of tert-butylcyanoketene to phenylethene

The effect of solvent and substituents on the [2 + 2]cycloaddition of tert-butylcyanoketene to phenylethene (styrene) was studied using a CAS-MCSCF method with a four orbital four electron active space in an STO-3G basis. The most favoured reaction path proceeds via a biradical intermediate in which the steric interactions between the three substituents are minimised, an arrangement which nevertheless results in a final cyclobutanone product in which the But and the Ph groups adopt the less stable cis rather than the more stable trans stereochemistry. A continuum solvation model based on a multipolar expansion within an ellipsoidal cavity suggests that benzene solvent enhances this preference. The results agree quantitatively with previous semi-empirical results, although the interpretation differs.

Photosensitization Versus Photocyclization: Competitive Reactions of Phenylphenalenone in Its Role as Phytoanticipins in Plant Defense Strategies

Phenalenone (PN) derivatives are involved in plant defense strategies, producing molecular singlet oxygen in a photosensitization process. Many experimental and theoretical studies determined that PN can performe this process with a quantum yield close to 1. However, it has been observed that the efficiency of some of its derivatives is much lower. This is the case of 9-phenylphenalenone (9-PhPN). To elucidate the factors that determine the different photochemistry of PN and its derivate 9-PhPN, we developed a complete active space self-consistent field/multi-configurational second-order perturbation theory study where several deactivation paths through the lowest excited states were explored. We found that the characteristics of the low-lying excited states are similar for both PN and 9-PhPN in the areas near the geometry of excitation. Consequently, the first processes that take place immediately after absorption are possible in both systems, including the population of the triplet state responsible for oxygen sensitization. However, 9-PhPN can also undergo cyclization by a bond formation between the carbonyl oxygen and a carbon atom of the phenyl substituent. This process competes favorably with population of triplet states and is responsible for the decrease of the quantum yield of oxygen sensitization in 9-PhPN relative to PN.