As a minimal model of the chromophore of rhodopsin proteins, the penta-2,4-dieniminium cation (PSB3) poses a challenging test system for the assessment of electronic-structure methods for the exploration of ground- and excited-state potential-energy surfaces, the topography of conical intersections, and the dimensionality (topology) of the branching space. Herein, we report on the performance of the approximate linear-response coupled-cluster method of second order (CC2) and the algebraic-diagrammatic-construction scheme of the polarization propagator of second and third orders (ADC(2) and ADC(3)). For the ADC(2) method, we considered both the strict and extended variants (ADC(2)-s and ADC(2)-x). For both CC2 and ADC methods, we also tested the spin-component-scaled (SCS) and spin-opposite-scaled (SOS) variants. We have explored several ground- and excited-state reaction paths, a circular path centered around the S1/S0 surface crossing, and a 2D scan of the potential-energy surfaces along the branching space. We find that the CC2 and ADC methods yield a different dimensionality of the intersection space. While the ADC methods yield a linear intersection topology, we find a conical intersection topology for the CC2 method. We present computational evidence showing that the linear-response CC2 method yields a surface crossing between the reference state and the first response state featuring characteristics that are expected for a true conical intersection. Finally, we test the performance of these methods for the approximate geometry optimization of the S1/S0 minimum-energy conical intersection and compare the geometries with available data from multireference methods. The present study provides new insight into the performance of linear-response CC2 and polarization-propagator ADC methods for molecular electronic spectroscopy and applications in computational photochemistry.

Recently, resonant two-photon ionization experiments on isolated adenine and adenosine suggested that adenosine exhibits a significantly shorter excited-state lifetime than adenine, which indicates the existence of an efficient excited-state deactivation mechanism in adenosine that is not existent in adenine. We report on ab initio investigations on a syn and an anti conformer of adenosine exhibiting an intramolecular O-H···N3 hydrogen bond. For both conformers, we have identified the existence of a barrierless excited-state deactivation mechanism that involves the forward-backward transfer of a proton along the intramolecular hydrogen bond and ultrafast radiationless deactivation through conical intersections. The S1/S0 conical intersection associated with the proton-transfer process is lower in energy than the known S1/S0 conical intersections associated with the excited-state deactivation processes inherent to the adenine moiety. These results support the conjecture that the photochemistry of hydrogen bonds plays a decisive role for the photostability of the molecular building blocks of RNA and DNA, which have been selected at the earliest stages of the chemical evolution of life.

The semiempirical orthogonalization-corrected OMx methods have recently been shown to perform well in extensive ground-state benchmarks. They can also be applied to the computation of electronically excited states when combined with a suitable multireference configuration interaction (MRCI) treatment. We report on a comprehensive evaluation of the performance of the OMx/MRCI methods for electronically excited states. The present benchmarks cover vertical excitation energies, excited-state equilibrium geometries (including an analysis of significant changes between ground- and excited-state geometries), minimum-energy conical intersections, ground- and excited-state zero-point vibrational energies, and 0-0 transition energies for a total of 520 molecular structures and 412 excited states. For comparison, we evaluate the TDDFT/B3LYP method for all benchmark sets, and the CC2, MRCISD, and CASPT2 methods for some of them. We find that the current OMx/MRCI methods perform reasonably well for many of the excited-state properties. However, in comparison to the first-principles methods, there are also a number of shortcomings that should be addressed in future developments.

Eumelanin is a naturally occurring skin pigment which is responsible for developing a suntan. The complex structure of eumelanin consists of π-stacked oligomers of various indole derivatives, such as the monomeric building block 5,6-dihydroxyindole (DHI). In this work, we present an ab initio wave-function study of the absorption behavior of DHI oligomers and of doubly and triply π-stacked species of these oligomers. We have simulated the onset of the electronic absorption spectra by employing the MP2 and the linear-response CC2 methods. Our results demonstrate the effect of an increasing degree of oligomerization of DHI and of an increasing degree of π-stacking of DHI oligomers on the onset of the absorption spectra and on the degree of red-shift toward the visible region of the spectrum. We find that π-stacking of DHI and its oligomers substantially red-shifts the onset of the absorption spectra. Our results also suggest that the optical properties of biological eumelanin cannot be simulated by considering the DHI building blocks alone, but instead the building blocks indole-semiquinone and indole-quinone have to be considered as well. This study contributes to advancing the understanding of the complex photophysics of the eumelanin biopolymer.

Kynurenines are UV filters found in the human ocular lens which protect the retina from radiation damage. We report on ab initio investigations of the photochemistry of the cis and trans conformers of kynurenine and of an intramolecularly hydrogen-bonded conformer of 3-hydroxykynurenine O-β-D-glucoside. We have explored the excited-state reaction paths for several radiationless excited-state deactivation processes in kynurenines. We show that electron-driven proton-transfer processes mediated by an excited state of charge-transfer character exhibit negligible barriers and that the relevant potential-energy profiles are lower in energy than the lowest absorbing ππ* state. In these proton-transfer processes, a proton moves from one of the amino groups of kynurenine to the keto group. We also report on nonadiabatic trajectory-surface-hopping molecular-dynamics simulations for photoexcited kynurenine. These simulations show that the cis and trans conformers of kynurenine deactivate on a femtosecond-to-picosecond time scale preferably via electron-driven proton transfer from one of the amino groups to the keto group. Cis kynurenine deactivates via a ring-N-H···O═C proton-transfer process. Trans kynurenine tends to undergo trans → cis isomerization before deactivating via the same process. These results suggest that the deactivation process involving the ring-amino group in the cis conformer of kynurenine is the most efficient excited-state deactivation process in kynurenines. The joint electronic-structure calculations and dynamics simulations provide a new level of mechanistic insight into the efficient UV-filtering capacity of kynurenines.