Artur Nenov

MOLCAS 8: New Capabilities for Multiconfigurational Quantum Chemical Calculations across the Periodic Table

In this report, we summarize and describe the recent unique updates and additions to the Molcas quantum chemistry program suite as contained in release version 8. These updates include natural and spin orbitals for studies of magnetic properties, local and linear scaling methods for the Douglas–Kroll–Hess transformation, the generalized active space concept in MCSCF methods, a combination of multiconfigurational wave functions with density functional theory in the MC‐PDFT method, additional methods for computation of magnetic properties, methods for diabatization, analytical gradients of state average complete active space SCF in association with density fitting, methods for constrained fragment optimization, large‐scale parallel multireference configuration interaction including analytic gradients via the interface to the Columbus package, and approximations of the CASPT2 method to be used for computations of large systems. In addition, the report includes the description of a computational machinery for nonlinear optical spectroscopy through an interface to the QM/MM package Cobramm. Further, a module to run molecular dynamics simulations is added, two surface hopping algorithms are included to enable nonadiabatic calculations, and the DQ method for diabatization is added. Finally, we report on the subject of improvements with respects to alternative file options and parallelization.

Molecular Driving Forces for Z/E Isomerization Mediated by Heteroatoms: The Example Hemithioindigo

A combined experimental and theoretical investigation of photoinduced Z/E isomerizations is presented. Unsubstituted Hemithioindigo is selected as a representative minimal model to unravel the reaction mechanism in the presence of heteroatoms on an atomic level. Time-resolved spectroscopy reveals multiexponential reaction dynamics on the few picoseconds time scale, which are interpreted by quantum chemical calculations at the CASSCF/CASPT2 level of theory. Detailed insight into the processes governing the ultrafast decay from the first excited state, mediated by a number of conical intersections, is provided. Charge separation and charge balance recovery on the reaction pathway play the leading role and are controlled by the electron-donating or -withdrawing character of the heteroatoms. The electronic and geometric structures of the individual minimum energy conical intersections governing the reaction are rationalized, and an extended energetically low lying conical intersection seam is extracted. Comparison to the experimental results permits linking the observed time constants to molecular intermediates and pathways. An explanation is provided for the pronounced differences of Z → E and the E → Z photoreactions upon excitation to the first excited singlet state.

Accelerated and Efficient Photochemistry from Higher Excited Electronic States in Fulgide Molecules

The photoinduced electrocyclic ring-opening of a fluorinated indolylfulgide is investigated by stationary and ultrafast spectroscopy in the UV/vis spectral range. Photoreactions, initiated by optical excitation into the S(1) (570 nm) and S(N) (340 nm) absorption band of the closed isomer, lead to considerable differences in reaction dynamics and quantum yields. Transient absorption studies point to different reaction pathways depending on the specific excitation wavelength: excitation into the S(1) state leads to the known reaction behavior with a picosecond decay to the ground state and a small quantum yield of 7% for the photoproduct. The S(N) state shows an unexpected long lifetime of 0.5 ps. The photoreaction starting from the S(N) state leads to a large extent directly to the product ground state and back to the educt ground state. This results in an increased reaction quantum yield of 28%. In contradiction to Kashas rule, the S(1) state is only populated with an efficiency of 38%. The observed behavior strongly differs from the expected picture with fast relaxation into the S(1) state and a subsequent ring-opening reaction starting from the lowest excited electronic state. Quantum chemical calculations confirm and complement the experimental findings allowing a sound molecular interpretation to be obtained.

Making fast photoswitches faster--using Hammett analysis to understand the limit of donor-acceptor approaches for faster hemithioindigo photoswitches.

Hemithioindigo (HTI) photoswitches have a tremendous potential for biological and supramolecular applications due to their absorptions in the visible-light region in conjunction with ultrafast photoisomerization and high thermal bistability. Rational tailoring of the photophysical properties for a specific application is the key to exploit the full potential of HTIs as photoswitching tools. Herein we use time-resolved absorption spectroscopy and Hammett analysis to discover an unexpected principal limit to the photoisomerization rate for donor-substituted HTIs. By using stationary absorption and fluorescence measurements in combination with theoretical investigations, we offer a detailed mechanistic explanation for the observed rate limit. An alternative way of approaching and possibly even exceeding the maximum rate by multiple donor substitution is demonstrated, which give access to the fastest HTI photoswitch reported to date.

Modeling the high-energy electronic state manifold of adenine: Calibration for nonlinear electronic spectroscopy

Pump-probe electronic spectroscopy using femtosecond laser pulses has evolved into a standard tool for tracking ultrafast excited state dynamics. Its two-dimensional (2D) counterpart is becoming an increasingly available and promising technique for resolving many of the limitations of pump-probe caused by spectral congestion. The ability to simulate pump-probe and 2D spectra from ab initio computations would allow one to link mechanistic observables like molecular motions and the making/breaking of chemical bonds to experimental observables like excited state lifetimes and quantum yields. From a theoretical standpoint, the characterization of the electronic transitions in the visible (Vis)/ultraviolet (UV), which are excited via the interaction of a molecular system with the incoming pump/probe pulses, translates into the determination of a computationally challenging number of excited states (going over 100) even for small/medium sized systems. A protocol is therefore required to evaluate the fluctuations of spectral properties like transition energies and dipole moments as a function of the computational parameters and to estimate the effect of these fluctuations on the transient spectral appearance. In the present contribution such a protocol is presented within the framework of complete and restricted active space self-consistent field theory and its second-order perturbation theory extensions. The electronic excited states of adenine have been carefully characterized through a previously presented computational recipe [Nenov et al., Comput. Theor. Chem. 1040-1041, 295-303 (2014)]. A wise reduction of the level of theory has then been performed in order to obtain a computationally less demanding approach that is still able to reproduce the characteristic features of the reference data. Foreseeing the potentiality of 2D electronic spectroscopy to track polynucleotide ground and excited state dynamics, and in particular its expected ability to provide conformational dependent fingerprints in dimeric systems, the performances of the selected reduced level of calculations have been tested in the construction of 2D electronic spectra for the in vacuo adenine monomer and the unstacked adenine homodimer, thereby exciting the Lb/La transitions with the pump pulse pair and probing in the Vis to near ultraviolet spectral window.

Geometrical and substituent effects in conical intersections: Linking chemical structure and photoreactivity in polyenes

The knowledge of the intersection space topography of electronic states is essential for deciphering and predicting photoinduced reactions. Michl and Bonacić-Koutecký developed a two-electron two-orbital model that allowed first systematic studies of the chemical origin of conical intersections in strongly polar systems. We generalize this approach to arbitrary functionalized and unfunctionalized polyene systems. For the extended model, a set of mathematical conditions for the formation of conical intersections are derived. These conditions are translated into geometrical motions and electronic effects, which help to explain and predict the structure and energetics of conical intersections. A three-step strategy for the conceptual search of conical intersections is outlined. Its universal validity is demonstrated using the textbook example cyclohexadiene and its functionalized derivative trifluoromethyl-indolylfulgide, a chromophore studied for possible application as a molecular switch.

Ultraviolet vision: photophysical properties of the unprotonated retinyl Schiff base in the Siberian hamster cone pigment

Abstract The Siberian hamster ultraviolet (SHUV) visual pigment has an unprotonated Schiff-base (SB) retinyl chromophore in the dark state, which becomes protonated after photoexcitation during the early stages of the photobleaching cycle. While the photochemical relaxation processes of the SHUV remain poorly understood, they are expected to show significant differences when compared to those of the protonated SB (PSB) chromophore in visual rhodopsin. Here, we report a study of the photophysical properties of the SHUV unprotonated SB (SHUV-USB), based on multiconfigurational and multireference perturbative methods within a hybrid quantum mechanics/molecular mechanics scheme. Comparisons of multireference and time-dependent density functional theory results indicate that both methodologies predict an ionic excited state (S1), similar to the PSB of rhodopsin, although its minimum has even bond-lengths in the central region of the retinyl polyene chain. The analysis of excited-state manifolds at the Franck–Condon region and S1 minimum configuration indicates that the skeletal relaxation initiated in the S1 surface is likely to involve S1/S2 surface crossing. These results provide valuable insights for future studies of the SHUV-USB photoisomerization mechanism.

Modelling Time-Resolved Two-Dimensional Electronic Spectroscopy of the Primary Photoisomerization Event in Rhodopsin

Time-resolved two-dimensional (2D) electronic spectra (ES) tracking the evolution of the excited state manifolds of the retinal chromophore have been simulated along the photoisomerization pathway in bovine rhodopsin, using a state-of-the-art hybrid QM/MM approach based on multiconfigurational methods. Simulations of broadband 2D spectra provide a useful picture of the overall detectable 2D signals from the near-infrared (NIR) to the near-ultraviolet (UV). Evolution of the stimulated emission (SE) and excited state absorption (ESA) 2D signals indicates that the S1 → SN (with N ≥ 2) ESAs feature a substantial blue-shift only after bond inversion and partial rotation along the cis → trans isomerization angle, while the SE rapidly red-shifts during the photoinduced skeletal relaxation of the polyene chain. Different combinations of pulse frequencies are proposed in order to follow the evolution of specific ESA signals. These include a two-color 2DVis/NIR setup especially suited for tracking the evolution of the S1 → S2 transitions that can be used to discriminate between different photochemical mechanisms of retinal photoisomerization as a function of the environment. The reported results are consistent with the available time-resolved pump–probe experimental data, and may be used for the design of more elaborate transient 2D electronic spectroscopy techniques.

Molecular model of the ring-opening and ring-closure reaction of a fluorinated indolylfulgide.

A combination of experimental and theoretical techniques is used to study the photoinduced ring-opening/closure of a trifluoromethyl-indolylfulgide. Time-resolved UV/vis pump and IR probe measurements are performed in the subpicosecond to 50 ps time range. Probing in the mid-IR between 1200 and 1900 cm(-1) provides mode-specific dynamics and reveals photochemical reaction dynamics as well as the presence of a noncyclizable conformer. Ring-opening occurs with about 3 ps. Experiments on the open isomer confirm that the ring-closure occurs on the subpicosecond time scale. They also show a 10 ps transient that can be assigned to internal conversion of a noncyclizable conformer. Quantum chemical calculations with multireference methods are used to explore the complex potential energy landscape in the excited electronic state showing paths for ring-opening and closure reactions as well as for competing side-reactions. The calculations reveal that photoexcitation induces a charge transfer from the indole to the anhydride. The same charge transfer drives the system toward a low-energy conical intersection seam spreading from the closed to the open ring side and is responsible for the ultrafast relaxation to the ground state. The ultrafast photoreactivity comes at the expense of selectivity.

Multiple Decay Mechanisms and 2D-UV Spectroscopic Fingerprints of Singlet Excited Solvated Adenine-Uracil Monophosphate.

Abstract The decay channels of singlet excited adenine uracil monophosphate (ApU) in water are studied with CASPT2//CASSCF:MM potential energy calculations and simulation of the 2D‐UV spectroscopic fingerprints with the aim of elucidating the role of the different electronic states of the stacked conformer in the excited state dynamics. The adenine 1La state can decay without a barrier to a conical intersection with the ground state. In contrast, the adenine 1Lb and uracil S(U) states have minima that are separated from the intersections by sizeable barriers. Depending on the backbone conformation, the CT state can undergo inter‐base hydrogen transfer and decay to the ground state through a conical intersection, or it can yield a long‐lived minimum stabilized by a hydrogen bond between the two ribose rings. This suggests that the 1Lb, S(U) and CT states of the stacked conformer may all contribute to the experimental lifetimes of 18 and 240 ps. We have also simulated the time evolution of the 2D‐UV spectra and provide the specific fingerprint of each species in a recommended probe window between 25 000 and 38 000 cm−1 in which decongested, clearly distinguishable spectra can be obtained. This is expected to allow the mechanistic scenarios to be discerned in the near future with the help of the corresponding experiments. Our results reveal the complexity of the photophysics of the relatively small ApU system, and the potential of 2D‐UV spectroscopy to disentangle the photophysics of multichromophoric systems.