An artificial molecular switch that mimics the visual pigment and completes its photocycle in picoseconds

Single molecules that act as light-energy transducers (e.g., converting the energy of a photon into atomic-level mechanical motion) are examples of minimal molecular devices. Here, we focus on a molecular switch designed by merging a conformationally locked diarylidene skeleton with a retinal-like Schiff base and capable of mimicking, in solution, different aspects of the transduction of the visual pigment Rhodopsin. Complementary ab initio multiconfigurational quantum chemistry-based computations and time-resolved spectroscopy are used to follow the light-induced isomerization of the switch in methanol. The results show that, similar to rhodopsin, the isomerization occurs on a 0.3-ps time scale and is followed by <10-ps cooling and solvation. The entire (2-photon-powered) switch cycle was traced by following the evolution of its infrared spectrum. These measurements indicate that a full cycle can be completed within 20 ps.

Tracking the excited-state time evolution of the visual pigment with multiconfigurational quantum chemistry

The primary event that initiates vision is the photoinduced isomerization of retinal in the visual pigment rhodopsin (Rh). Here, we use a scaled quantum mechanics/molecular mechanics potential that reproduces the isomerization path determined with multiconfigurational perturbation theory to follow the excited-state evolution of bovine Rh. The analysis of a 140-fs trajectory provides a description of the electronic and geometrical changes that prepare the system for decay to the ground state. The data uncover a complex change of the retinal backbone that, at approximately 60-fs delay, initiates a space saving "asynchronous bicycle-pedal or crankshaft" motion, leading to a conical intersection on a 110-fs time scale. It is shown that the twisted structure achieved at decay features a momentum that provides a natural route toward the photoRh structure recently resolved by using femtosecond-stimulated Raman spectroscopy.

The role of the intersection space in the photochemistry of tricyclo[3.3.0.0(2,6)]octa-3,7-diene

CASSCF and CASPT2 methods were used to study the photochemistry of tricyclo[3.3.0.0(2,6)]octa-3,7-diene (TOD). The analysis of different S1 reaction paths as well as the topology of the S1/S0 intersection space allows us to establish two novel properties associated with the photochemical behavior of this compound: (i) simple low-lying intersection space domains can mediate different photoproducts, and (ii) TOD photochemistry is probably mediated by two disconnected intersection space domains, related to the formation of cyclooctatetraene and semibulvalene in different time-scales. It is shown that these domains are chemically distinct since the first, leading to COT, mediates barrierless pericyclic reactions while the second, leading to SBV, is accessed through the formation of an excited-state biradical intermediate. To the best of our knowledge, in the domain of single molecule photochemistry, TOD represents the first example where a different chemical role of distinct low-lying intersection spaces has been computationally documented. The observed photoproducts can be rationalized in terms of branching space diagrams, constructed by determining the branching space (derivative coupling and gradient difference vectors) for each conical intersection involved in the photochemical process.

Photostability versus Photodegradation in the Excited-State Intramolecular Proton Transfer of Nitro Enamines: Competing Reaction Paths and Conical Intersections

The phototautomerization mechanism of a model nitro enamine (NEA) chromophore (incorporated in the structure of a highly photolabile pesticide, tetrahydro-2-(nitromethylene)-2H-1,3-thiazine) has been studied using complete active space self-consistent field reaction path computations. The optically accessible 1pipi* excited state of NEA involves separation of charge and correlates diabatically with the ground state of the tautomerized acinitro imine (ANI) form. For optimum photostabilization, the 1pipi* state of NEA should be S1: in this case, the tautomer would be efficiently formed via a diabatic intramolecular proton-transfer pathway passing through an S1/S0 conical intersection, followed by a facile thermal back proton-transfer reaction. However, in NEA itself the lowest excited states correspond to nitro group 1npi* states, and there are additional surface crossings that provide a mechanism for populating the 1npi* manifold. The above results indicate that the high photolability observed for the pesticide [Kleier, D.; Holden, I.; Casida, J. E.; Ruzo, L. O. J. Agric. Food Chem. 1985, 33, 998-1000] has to be ascribed to photochemistry originating on the 1npi* manifold of states, populated indirectly from the 1pipi* state.

The color of rhodopsins at the ab initio multiconfigurational perturbation theory resolution

We demonstrate that "brute force" quantum-mechanics/molecular-mechanics computations based on ab initio (i.e., first principles) multiconfigurational perturbation theory can reproduce the absorption maxima of a set of modified bovine rhodopsins with an accuracy allowing for the analysis of the factors determining their colors. In particular, we show that the theory accounts for the changes in excitation energy even when the proteins display the same charge distribution. Three color-tuning mechanisms, leading to changes of close magnitude, are demonstrated to operate in these conditions. The first is based on the change of the conformation of the conjugated backbone of the retinal chromophore. The second operates through the control of the distance between the positive charge residing on the chromophore and the carboxylate counterion. Finally, the third mechanism operates through the changes in orientation of the chromophore relative to the protein. These results offer perspectives for the unbiased computational design of mutants or chemically modified proteins with wanted optical properties.

The Ring-Opening Reaction of Chromenes: A Photochemical Mode-Dependent Transformation

The efficiency of the photochemical ring-opening of chromenes (or benzopyrans) depends on the vibronic transition selected by the chosen excitation wavelength. In the present work, ab initio CASPT2//CASSCF calculations are used to determine the excited-state ring-opening reaction coordinate for 2H-chromene (C) and 2,2-diethyl-2H-chromene (DEC) and provide an explanation for such an unusual mode-dependent behavior. It is shown that excited-state relaxation and decay occur via a multimodal and barrierless (or nearly barrierless) reaction coordinate. In particular, the relaxation out of the Franck-Condon involves a combination of in-plane skeletal stretching and out-of-plane modes, while the second part of the reaction coordinate is dominated exclusively by a different out-of-plane mode. Population of this last mode is shown to be preparatory with respect to both C-O bond breaking and decay via an S(1)/S(0) conical intersection. The observed mode-dependent ring-opening efficiency is explained by showing that the vibrational mode corresponding to the most efficient vibronic transition has the largest projection onto the out-of-plane mode of the reaction coordinate. To support the computationally derived mechanism, we provide experimental evidence that the photochemical ring-opening reaction of 2,2-dimethyl-7,8-benzo(2H)chromene, that similarly to DEC exhibits a mode-dependent photoreaction, has a low ( approximately 1 kcal mol(-1)) activation energy barrier.

Properties of the Emitting State of the Green Fluorescent Protein Resolved at the CASPT2//CASSCF/CHARMM Level

A "brute-force" ab initio CASPT2//CASSCF/CHARMM computational approach is employed to investigate the properties of the emitting state of the wild-type green fluorescence protein. The results indicate that the emitting moiety corresponds to a slightly perturbed H2O- - -chromophore complex. Thus, the protein matrix seems to be designed in such a way to mimic an environment that is more similar to gas-phase than water solution.