Intramolecular Singlet Fission: Insights from Quantum Dynamical Simulations

We investigate the dynamics of intramolecular singlet fission in a dimer consisting of two pentacene-based chromophores covalently bonded to a phenylene spacer using an approach that combines high-level ab initio multireference perturbation theory methods and quantum dynamical simulations. The results show that the population of the multiexcitonic state, corresponding to the first step of singlet fission, is facilitated by the existence of higher-lying doubly excited and charge transfer states that participate in a superexchange-like way. The important role played by high-frequency ring-breathing molecular vibrations in the process is also discussed

Low-Lying Electronic Excited States of Pentacene Oligomers: A Comparative Electronic Structure Study in the Context of Singlet Fission

The lowest-lying electronic excited states of pentacene and its oligomers are investigated using accurate multireference wave function methods (CASPT2/CASSCF) and the many-body Greens’s function approach (GW/BSE). The results obtained for dimers and trimers of different geometry reveal a complex electronic structure, which includes locally excited, charge transfer, and multiexciton states. For singlets of single-excitation character, both approaches yield excitation energies that are in good overall quantitative agreement. While the multiexciton states are located relatively high in energy in all systems investigated, charge transfer states exist in close proximity to the lowest-lying absorbing states. The implications of the results for the mechanisms of singlet fission in pentacene are discussed

Relationship between the Excited State Relaxation Paths of Rhodopsin and Isorhodopsin

The pigment Isorhodopsin, an analogue of the visual pigment Rhodopsin, is investigated via quantum-mechanics/molecular-mechanics computations based on an ab initio multiconfigurational quantum chemical treatment. The limited -1 error found for the spectral parameters allows for a nearly quantitative analysis of the excited-state structure and reactivity of its 9-cis-retinal chromophore. We demonstrate that, similar to Rhodopsin, Isorhodopsin features a shallow photoisomerization path. However, the structure of the reaction coordinate appears to be reversed. In fact, while the coordinate still corresponds to an asynchronous crankshaft motion, the dominant isomerization component involves a counterclockwise, rather than clockwise, twisting of the 9-cis bond. Similarly, the minor component involves a clockwise, rather than counterclockwise, twisting of the 11-trans bond. Ultimately, these results indicate that Rhodopsin and Isorhodopsin relax along a common excited-state potential energy valley starting from opposite ends. The fact that the central and lowest energy region of such valley runs along a segment of the intersection space between the ground and excited states of the protein explains why the pigments decay at distinctive conical intersection structures

Theoretical Insight into the Intrinsic Ultrafast Formation of Cyclobutane Pyrimidine Dimers in UV-Irradiated DNA: Thymine versus Cytosine

The higher formation yields measured in the ultrafast photoinduced formation of cyclobutane thymine dimers (TT) with respect to those of cytosine (CC) are explained, on the basis of ab initio CASPT2 results, by the existence in thymine of more reactive orientations and a less efficient photoreversibility, whereas in cytosine the funnel toward the photolesion becomes competitive with that mediating the internal conversion of the excited-cytosine monomer

Origin of the Absorption Maxima of the Photoactive Yellow Protein Resolved via Ab Initio Multiconfigurational Methods

We discuss the role of the protein in controlling the absorption spectra of photoactive yellow protein (PYP), the archetype xanthopsin photoreceptor, using quantum mechanics/molecular mechanics (QM/MM) methods based on ab initio multireference perturbation theory, combined with molecular dynamics (MD) simulations. It is shown that in order to get results in agreement with the experimental data, it is necessary to use a model that allows for a proper relaxation of the whole system and treats the states involved in the electronic spectrum in a balanced way, avoiding biased results due to the effect of nonrepresentative electrostatic interactions on the chromophore

Dynamical Simulation of Electron Transfer Processes in Alkanethiolate Self-Assembled Monolayers at the Au(111) Surface

Electron transfer is investigated in a series of self-assembled monolayers (SAMs) consisting of nitrile-substituted short chain alkanethiolate molecules adsorbed at the Au(111) surface. Using first-principles methods and a model electron transfer Hamiltonian, we analyze the main factors controlling, at the molecular level, the electron injection times from donor states localized at the tail group of the SAM into the Au(111) substrate. We show that the donor–acceptor electronic couplings depend significantly on the orbital symmetry of the donor state and the length of the aliphatic spacer chain of the SAM. The dependence on the donor state symmetry and on the molecular structure of the linker can be used to control the electron injection times even in situations where the energy separation between the donor states is smaller than their width

Parallel versus Twisted Pentacenes: Conformational Impact on Singlet Fission

We placed two pentacene chromophores at the termini of a diacetylene linker to investigate the impact of excitation wavelength, conformational flexibility, and vibronic coupling on singlet fission. Photoexcitation of the low-energy absorption results in a superposed mixture of states, which transform on an ultrafast time-scale into a spin-correlated and vibronically coupled/hot delocalized triplet pair 1(T1T1)deloc. Regardless of temperature, the lifetime for 1(T1T1)deloc is less than 2 ps. In contrast, photoexcitation of the high-energy absorption results in the formation of 1(T1T1)deloc lasting 1.0 ps, which then decays at room temperature within 4 ps via triplet–triplet annihilation. Lowering the temperature enables 1(T1T1)deloc to delocalize and vibronically decouple, in turn affording 1(T1T1)loc. In addition, our results suggest that the quasi-free rotation at the diacetylene spacer may lead to twisted conformations with very low SF quantum yields, highlighting the need of controlling this structural aspect in the design of new singlet fission active molecules

Orbital-Symmetry-Dependent Electron Transfer through Molecules Assembled on Metal Substrates

Femtosecond charge-transfer dynamics in self-assembled monolayers of cyano-terminated ethane-thiolate on gold substrates was investigated with the core hole clock method. By exploiting symmetry selection rules rather than energetic selection, electrons from the nitrogen K-shell are state-selectively excited into the two symmetry-split π* orbitals of the cyano end group with X-ray photons of well-defined polarization. The charge-transfer times from these temporarily occupied orbitals to the metal substrate differ significantly. Theoretical calculations show that these two π* orbitals extend differently onto the alkane backbone and the anchoring sulfur atom, thus causing the observed dependence of the electron-transfer dynamics on the symmetry of the orbital

Proton/Hydrogen Transfer Mechanisms in the Guanine–Cytosine Base Pair: Photostability and Tautomerism

Proton/hydrogen-transfer processes have been broadly studied in the past 50 years to explain the photostability and the spontaneous tautomerism in the DNA base pairs. In the present study, the CASSCF/CASPT2 methodology is used to map the two-dimensional potential energy surfaces along the stretched NH reaction coordinates of the guanine–cytosine (GC) base pair. Concerted and stepwise pathways are explored initially <i>in vacuo</i>, and three mechanisms are studied: the stepwise double proton transfer, the stepwise double hydrogen transfer, and the concerted double proton transfer. The results are consistent with previous findings related to the photostability of the GC base pair, and a new contribution to tautomerism is provided. The C-based imino-oxo and imino-enol GC tautomers, which can be generated during the UV irradiation of the Watson–Crick base pair, have analogous radiationless energy-decay channels to those of the canonical base pair. In addition, the C-based imino-enol GC tautomer is thermally less stable. A study of the GC base pair is carried out subsequently taking into account the DNA surroundings in the biological environment. The most important stationary points are computed using the quantum mechanics/molecular mechanics (QM/MM) approach, suggesting a similar scenario for the proton/hydrogen-transfer phenomena <i>in vacuo</i> and in DNA. Finally, the static model is complemented by <i>ab initio</i> dynamic simulations, which show that vibrations at the hydrogen bonds can indeed originate hydrogen-transfer processes in the GC base pair. The relevance of the present findings for the rationalization of the preservation of the genetic code and mutagenesis is discussed