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.CASPT2//CASSCF ͉ mid-IR ͉ photochemical switch ͉ time resolved spectroscopy ͉ UV-vis M olecular switches based on photochemical E/Z isomerizations have been used in different contexts to convert light energy into ''mechanical'' motion at the molecular level (1-3). For instance, switches based on azobenzene have been used to control ion complexation (4, 5), electronic properties (6), catalysis (7), and the folding of peptides (8-13) whereas diarylidenes have provided the framework for the construction of rotary motors and transmissions (14). The computer modeling of switches that differ in size, polarity, and isomerization mechanism represents an attractive research target (15) yielding building blocks to be used in diverse molecular environments. However, this cannot be limited to the computation of equilibrium properties but requires the description of the entire photocycle. In other words, one needs to compute the potential energy surfaces controlling the switch E 3 Z and Z 3 E excited-state evolution, its decay and ground state relaxation, and the competing thermal E/Z isomerization in the proper environment (e.g., in solution or in a biomolecule backbone). The complexity of these calculations impedes the study of candidates that are intractable with accurate quantum chemical methods (allowing comparison with spectroscopic data) or that feature, as for azobenzene and diarylidenes (16), more than 1 low-lying excited state, leading to a plethora of reaction paths to be computed.The retinal protonated Schiff-base chromophore of rhodopsins (17-19) constitutes an example of an E/Z switch shaped by biological evolution that can be modeled with quantitative computations (20). In bovine rhodopsin (Rh), a selective photoisomerization of the 11-cis chromophore (PSB11) occurs via evolution of a single 3 * excited state (S 1 ) that survives for only 150 fs and yields, upon decay, the all-trans ground state (S 0 ) product with a 67% quantum yield (16,20). Although these properties make Rh an excellent reference for the design of E/Z switches, irradiation of PSB11 in solution (26, 27) features an unselec...

The coherent photoisomerization of a chromophore in condensed phase is a rare process in which light energy is funneled into specific molecular vibrations during electronic relaxation from the excited to the ground state. In this work, we employed ultrafast spectroscopy and computational methods to investigate the molecular origin of the coherent motion accompanying the photoisomerization of indanylidene-pyrroline (IP) molecular switches. UV/Vis femtosecond transient absorption gave evidence for an excited- and ground-state vibrational wave packet, which appears as a general feature of the IP compounds investigated. In close resemblance to the coherent photoisomerization of rhodopsin, the sudden onset of a far-red-detuned and rapidly blue-shifting photoproduct signature indicated that the population arriving on the electronic ground state after nonadiabatic decay through the conical intersection (CI) is still very focused in the form of a vibrational wave packet. Semiclassical trajectories were employed to investigate the reaction mechanism. Their analysis showed that coupled double-bond twisting and ring inversions, already populated during the excited-state reactive motion, induced periodic changes in π-conjugation that modulate the ground-state absorption after the non-adiabatic decay. This prediction further supports that the observed ground-state oscillation results from the reactive motion, which is in line with a biomimetic, coherent photoisomerization scenario. The IP compounds thus appear as a model system to investigate the mechanism of mode-selective photomechanical energy transduction. The presented mechanism opens new perspectives for energy transduction at the molecular level, with applications to the design of efficient molecular devices.

The light-induced double-bond isomerization of the visual pigment rhodopsin operates a molecular-level optomechanical energy transduction, which triggers a crucial protein structure change. In fact, rhodopsin isomerization occurs according to a unique, ultrafast mechanism that preserves mode-specific vibrational coherence all the way from the reactant excited state to the primary photoproduct ground state. The engineering of such an energy-funnelling function in synthetic compounds would pave the way towards biomimetic molecular machines capable of achieving optimum light-to-mechanical energy conversion. Here we use resonance and off-resonance vibrational coherence spectroscopy to demonstrate that a rhodopsin-like isomerization operates in a biomimetic molecular switch in solution. Furthermore, by using quantum chemical simulations, we show why the observed coherent nuclear motion critically depends on minor chemical modifications capable to induce specific geometric and electronic effects. This finding provides a strategy for engineering vibrationally coherent motions in other synthetic systems.

Designing and testing biomimetic switches: Multireference perturbation theory is used to model a light-driven molecular switch featuring the same photoisomerization mechanism as the chromophore of the visual pigment rhodopsin (see picture; QM/MM: quantum mechanics/molecular mechanics). By exploiting a synthetic route based on nitrilium-cation cyclization, it was shown that the designed system can indeed be prepared and characterized

We report the results of a multidisciplinary research effort where the methods of computational photochemistry and retrosynthetic analysis/synthesis have contributed to the preparation of a novel N-alkylated indanylidene-pyrroline Schiff base featuring an exocyclic double bond and a permanent zwitterionic head. We show that, due to its large dipole moment and efficient photoisomerization, such a system may constitute the prototype of a novel generation of electrostatic switches achieving a reversible light-induced dipole moment change on the order of 30 D. The modeling of a peptide fragment incorporating the zwitterionic head into a conformationally rigid side chain shows that the switch can effectively modulate the fluorescence of a tryptophan probe.

Protonated Schiff bases (PSBs) of polyenals constitute a class of light-driven switchers selected by biological evolution that provide model compounds for the development of artificial light-driven molecular devices or motors. In the present paper, our primary target is to show, through combined computational and experimental studies, that it is possible to approach the design of artificial PSBs suitable for such applications. Below, we use the methods of computational photochemistry to design and characterize the prototype biomimetic molecular switchers 4-cyclopenten-2'-enylidene-3,4-dihydro-2H-pyrrolinium and its 5,5'-dimethyl derivative both containing the penta-2,4-dieniminium chromophore. To find support for the predicted behavior, we also report the photochemical reaction path of the synthetically accessible compound 4-benzylidene-3,4-dihydro-2H-pyrrolinium. We show that the preparation and photochemical characterization of this compound (together with three different N-methyl derivatives) provide both support for the predicted photoisomerization mechanism and information on its sensitivity to the molecular environment.

While rotary molecular switches based on neutral and cationic organic π-systems have been reported, structurally homologous anionic switches providing complementary properties have not been prepared so far. Here we report the design and preparation of a molecular switch mimicking the anionic p-HBDI chromophore of the green fluorescent protein. The investigation of the mechanism and dynamics of the E/Z switching function is carried out both computationally and experimentally. The data consistently support axial rotary motion occurring on a sub-picosecond time scale. Transient spectroscopy and trajectory simulations show that the nonadiabatic decay process occurs in the vicinity of a conical intersection (CInt) between a charge transfer state and a covalent/diradical state. Comparison of our anionic p-HBDI-like switch with the previously reported cationic N-alkyl indanylidene pyrrolinium switch mimicking visual pigments reveals that these similar systems translocate, upon vertical excitation, a similar net charge in the same axial direction.

Recent experimental and theoretical studies on N-alkylated indanylidene pyrroline Schiff bases (NAIP) show that these compounds exhibit biomimetic photoisomerization analogous to that in the chromophore of rhodopsin. The NAIP compounds studied previously isomerize rapidly and often evolve coherently on the ground-electronic surface after reaction. We present the results of transient electronic absorption spectroscopy on dMe-OMe-NAIP, a newly synthesized NAIP analogue that differs from other NAIP compounds in the substituents on its pyrrolinium ring. Following excitation with 400 nm light, dMe-OMe-NAIP relaxes from the electronic-excited state in less than 500 fs, which is slower than in other analogues, and does not show the prominent oscillations observed in other NAIP compounds. A reduction in the amount of twisting between the rings caused by removal of the methyl group is likely responsible for the slower isomerization. Measurements in solvents of varying viscosity and structure suggest that intramolecular processes dominate the relaxation of nascent photoproducts.