Evolution of the Excitonic State of DNA Stacked Thymines: Intrabase ππ* → S₀ Decay Paths Account for Ultrafast (Subpicosecond) and Longer (>100 ps) Deactivations

Monomer-like ring puckering decay paths for two stacked quantum mechanical thymines inside a solvated DNA duplex described at the molecular mechanics level are mapped using a hybrid CASPT2//CASSCF/MM protocol that accounts for steric, electronic and electrostatic interactions within the nucleobases native environment. Asymmetric stacking between nucleobases open ups different intrabase ππ* decay paths accounting for distinctive excited state lifetimes, spanning the subps to subns time window

Photoelectrochromism in the Retinal Protonated Schiff Base Chromophore: Photoisomerization Speed and Selectivity under a Homogeneous Electric Field at Different Operational Regimes

The spectral tunability, photoisomerization efficiency and selectivity, of the native all-trans retinal protonated Shiff base (PSB) chromophore driven by a homogeneous electric field is systematically investigated. By analyzing the absorption wavelength dependence, charge distribution, and PES profiles along selected torsional angles, as well as the electronic structure, energetics, and topography of the CI seam in the presence of strong positive and negative electric fields, we recognize the existence of qualitatively/fundamentally different photophysics and photochemistry with respect to the unperturbed (i.e., absence of an electric field) chromophore. We rationalize the findings within the scope of molecular orbital theory and deliver a unified picture of the photophysics of the retinal PSB chromophore over a wide, even beyond the usually observed, spectral regime, ranging from the near-infrared to the ultraviolet absorption energies. This work has a 3-fold impact: a) it accounts for, and extends, previous theoretical studies on the subject; b) it delivers a rationale for the ES lifetimes observed in retinal proteins, both archeal and visual rhodopsins, as well as in solvent; and c) the transferability of the discovered trends on PSB mimics is demonstrated

Semiclassical Path Integral Calculation of Nonlinear Optical Spectroscopy

Computation of nonlinear optical response functions allows for an in-depth connection between theory and experiment. Experimentally recorded spectra provide a high density of information, but to objectively disentangle overlapping signals and to reach a detailed and reliable understanding of the system dynamics, measurements must be integrated with theoretical approaches. Here, we present a new, highly accurate and efficient trajectory-based semiclassical path integral method for computing higher order nonlinear optical response functions for non-Markovian open quantum systems. The approach is, in principle, applicable to general Hamiltonians and does not require any restrictions on the form of the intrasystem or system–bath couplings. This method is systematically improvable and is shown to be valid in parameter regimes where perturbation theory-based methods qualitatively breakdown. As a test of the methodology presented here, we study a system–bath model for a coupled dimer for which we compare against numerically exact results and standard approximate perturbation theory-based calculations. Additionally, we study a monomer with discrete vibronic states that serves as the starting point for future investigation of vibronic signatures in nonlinear electronic spectroscopy

Fine Tuning of Retinal Photoinduced Decay in Solution

Single methylation at position C₁₀ of the all-trans retinal protonated Schiff base switches its excited-state decay in methanol from a slower picosecond into an ultrafast, protein-like subpicosecond process. QM/MM modeling in conjunction with on-the-fly excited-state dynamics provides fundamental understanding of the fine-tuning mechanics that “catalyzes” the photoinduced decay of solvated retinals. Methylation alters the interplay between the ionic S₁ and covalent S₂ states, reducing the excited-state lifetime by favoring the formation of a S₁ transient fluorescent state with fully inverted bond lengths that accounts for the recorded transient spectroscopy and from which a space-saving conical intersection seam is quickly (<1 ps) reached. Minimal and apparently innocent chemical modifications thus affect the characteristic intramolecular charge-transfer of the S₁ state as well as the interaction with the covalent S₂ excited state, eventually providing the high tunability of retinal photophysics and photochemistry and delivering a new concept for the rational design of retinal-based photoactive molecular devices

Photoelectrochromism in the Retinal Protonated Schiff Base Chromophore: Photoisomerization Speed and Selectivity under a Homogeneous Electric Field at Different Operational Regimes

The spectral tunability, photoisomerization efficiency and selectivity, of the native all-trans retinal protonated Shiff base (PSB) chromophore driven by a homogeneous electric field is systematically investigated. By analyzing the absorption wavelength dependence, charge distribution, and PES profiles along selected torsional angles, as well as the electronic structure, energetics, and topography of the CI seam in the presence of strong positive and negative electric fields, we recognize the existence of qualitatively/fundamentally different photophysics and photochemistry with respect to the unperturbed (i.e., absence of an electric field) chromophore. We rationalize the findings within the scope of molecular orbital theory and deliver a unified picture of the photophysics of the retinal PSB chromophore over a wide, even beyond the usually observed, spectral regime, ranging from the near-infrared to the ultraviolet absorption energies. This work has a 3-fold impact: a) it accounts for, and extends, previous theoretical studies on the subject; b) it delivers a rationale for the ES lifetimes observed in retinal proteins, both archeal and visual rhodopsins, as well as in solvent; and c) the transferability of the discovered trends on PSB mimics is demonstrated

Photoinduced Formation Mechanism of the Thymine–Thymine (6–4) Adduct

The photoinduced mechanism leading to the formation of the thymine–thymine (6–4) photolesion has been studied by using the CASPT2//CASSCF approach over a dinucleotide model in vacuo. Following light absorption, localization of the excitation on a single thymine leads to fast singlet–triplet crossing that populates the triplet ³(nπ*) state of thymine. This state, displaying an elongated C₄O bond, triggers (6–4) dimer formation by reaction with the C₅C₆ double bond of the adjacent thymine, followed by a second intersystem crossing, which acts as a gate between the excited state of the reactant and the ground state of the photoproduct. The requirement of localized excitation on just one thymine, whose main decay channel (by radiationless repopulation of its ground state) is nonphotochemical, can rationalize the experimentally observed low quantum yield of formation for the thymine–thymine (6–4) adduct

Relationship between Excited State Lifetime and Isomerization Quantum Yield in Animal Rhodopsins: Beyond the One-Dimensional Landau–Zener Model

We show that the speed of the chromophore photoisomerization of animal rhodopsins is not a relevant control knob for their light sensitivity. This result is at odds with the momentum-driven tunnelling rationale (i.e., assuming a one-dimensional Landau–Zener model for the decay: Zener, C. Non-Adiabatic Crossing of Energy Levels. <i>Proc. R. Soc. London, Ser. A</i> <b>1932,</b> 137 (833), 696–702) holding that a faster nuclear motion through the conical intersection translates into a higher quantum yield and, thus, light sensitivity. Instead, a model based on the phase-matching of specific excited state vibrational modes should be considered. Using extensive semiclassical hybrid quantum mechanics/molecular mechanics trajectory computations to simulate the photoisomerization of three animal rhodopsin models (visual rhodopsin, squid rhodopsin and human melanopsin), we also demonstrate that phase-matching between three different modes (the reactive carbon and hydrogen twisting coordinates and the bond length alternation mode) is required to achieve high quantum yields. In fact, such “phase-matching” mechanism explains the computational results and provides a tool for the prediction of the photoisomerization outcome in retinal proteins