Herein, we report complementary computational and experimental evidence supporting the existence, for indan-1-ylidene malononitrile and fluoren-9-ylidene malononitrile, of a non-radiative decay channel involving double bond isomerisation motion. The results of UV-Vis transient absorption spectroscopy highlight that the decay takes place within hundreds of picoseconds. In order to understand the related molecular mechanism, photochemical reaction paths were computed by employing multiconfigurational quantum chemistry. The results indicate that the excited state deactivation occurs via concerted double bond twisting of the dicyanovinyl (DCV) unit coupled with a pyramidalisation of its substituted carbon. It is also shown that the observed differences in the excited state lifetimes when passing from indan-1-ylidene malononitrile to fluoren-9-ylidene are associated with the change in the topography of the conical intersection driving the decay from intermediate to sloped, respectively.

The effect of different conformations and substitutions on the photoisomerization of a retinal protonated Schiff base model is investigated by nonadiabatic molecular dynamics simulations. Three groups of retinal analogues are studied: (i) conformational isomers, (ii) methyl-substituted retinals, and (iii) C11-C12 bond locked retinals. In total 259 trajectories are calculated in the gas phase starting from different initial conditions. The effect on bond selectivity, the directionality of the isomerization, excited-state lifetime, and product distribution is derived from the ensemble of trajectories. Among the group of four isomers (9-, 11-, 13-cis, and all-trans) the 11-cis analogue is the most selective in terms of isomerizing double bond, while the other three produce a mixture of isomers. However, there is no preference for isomerization directionality and the product formation for the 11-cis isomer. In the group of analogues with different methylation patterns, it is found that a methyl group at position C10 can introduce unidirectionality. This methyl group also speeds the photoisomerization. In case of the analogue that is demethylated at the positions C10 and C13, all trajectories isomerize successfully from cis to trans conformation. The three C11-C12 bond locked retinals are found to have very different properties, which depend on the number of methylene units bridging this bond. The five-membered ring imposes a too-large restriction; hence, all trajectories remain on the excited state in the simulation time of 300 fs. The seven-membered ring is more flexible with preference for isomerization of the C9-C10 bond. Interestingly, the eight-membered ring leads to the fastest isomerization time and full directionality of C11-C12 bond isomerization. The trends observed in these simulations can help to understand whether the effects are intrinsic to the chromophore or are induced by the protein environment, by comparing to the trends from experiment. Furthermore, the derived understanding can support design of molecular motors to achieve high product yield and unidirectionality. © 2016 American Chemical Society.

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.

A series of vitamin-A aldehydes (retinals) with modified alkyl group substituents (9-demethyl-, 9-ethyl-, 9-isopropyl-, 10-methyl, 10-methyl-13-demethyl-, and 13-demethyl retinal) was synthesized and their 11-cis isomers were used as chromophores to reconstitute the visual pigment rhodopsin. Structural changes were selectively introduced around the photoisomerizing C₁₁C₁₂ bond. The effect of these structural changes on rhodopsin formation and bleaching was determined. Global fit of assembly kinetics yielded lifetimes and spectral features of the assembly intermediates. Rhodopsin formation proceeds stepwise with prolonged lifetimes especially for 9-demethyl retinal (longest lifetime τ₃ = 7500 s, cf., 3500 s for retinal), and for 10-methyl retinal (τ₃ = 7850 s). These slowed-down processes are interpreted as either a loss of fixation (9dm) or an increased steric hindrance (10me) during the conformational adjustment within the protein. Combined quantum mechanics and molecular mechanics (QM/MM) simulations provided structural insight into the retinal analogues-assembled, full-length rhodopsins. Extinction coefficients, quantum yields and kinetics of the bleaching process (μs-to-ms time range) were determined. Global fit analysis yielded lifetimes and spectral features of bleaching intermediates, revealing remarkably altered kinetics: whereas the slowest process of wild-type rhodopsin and of bleached and 11-cis retinal assembled rhodopsin takes place with lifetimes of 7 and 3.8 s, respectively, this process for 10-methyl-13-demethyl retinal was nearly 10 h (34670 s), coming to completion only after ca. 50 h. The structural changes in retinal derivatives clearly identify the precise interactions between chromophore and protein during the light-induced changes that yield the outstanding efficiency of rhodopsin.