The ultrafast proton transfer dynamics of salicylideneaniline has been theoretically analyzed in the ground and first singlet excited electronic states using density functional theory (DFT) and time-dependent DFT calculations, which predict a (pi,pi( *)) barrierless excited state intramolecular proton transfer (ESIPT). In addition to this, the photochemistry of salicylideneaniline is experimentally known to present fast depopulation processes of the photoexcited species before and after the proton transfer reaction. Such processes are explained by means of conical intersections between the ground and first singlet (pi,pi( *)) excited electronic states. The electronic energies obtained by the time-dependent density functional theory formalism have been fitted to a monodimensional potential energy surface in order to perform quantum dynamics study of the processes. Our results show that the proton transfer and deactivation of the photoexcited species before the ESIPT processes are completed within 49.6 and 37.7 fs, respectively, which is in remarkable good agreement with experiments.

The green fluorescent protein proton wire operating upon photoexcitation of the internally caged chromophore is investigated by means of classical molecular dynamics and multiconfigurational electronic structure calculations. The structure of the proton wire is studied for the solvated protein, showing that the wire is likely to be found in a configuration ready to operate as soon as the chromophore is photoexcited, and leading to a total of three proton translocations in the vicinity of the chromophore. Multiconfigurational CASSCF and CASPT2 calculations provide a detailed overview of the energy landscape of the proton wire for the ground electronic state S0, the photoactive 1pi pi* state, and the charge-transfer 1pi sigma* state. The results allow discussion of the operation of the wire in terms of the sequence of proton-transfer events and the participation of each electronic state.

A study on a modeled version of the complex [Ru(H···H)(C5Me5)(dppm)]+ has been performed both at electronic structure level and including quantum treatment of nuclei. Density functional theory (DFT) electronic structure calculations alone fail to reproduce the experimentally reported geometry of the elongated dihydrogen ligand of the complex, even though the rest of the complex is satisfactorily described. Quantum nuclear motion calculations manage to correctly explain the geometries found experimentally by means of neutron diffraction measurements. Isotopic effects are predicted for the hydrogen−hydrogen distance of the elongated dihydrogen ligand depending on its isotopic composition. Moreover, the temperature dependence of the J(H,D) coupling constant is also interpreted successfully on the grounds of varying population of the vibrational excited states of the Ru−H2 unit.

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Manipulation of neuronal activity using two-photon excitation of azobenzene photoswitches with near-infrared light has been recently demonstrated, but their practical use in neuronal tissue to photostimulate individual neurons with three-dimensional precision has been hampered by firstly, the low efficacy and reliability of NIR-induced azobenzene photoisomerization compared to one-photon excitation, and secondly, the short cis state lifetime of the two-photon responsive azo switches. Here we report the rational design based on theoretical calculations and the synthesis of azobenzene photoswitches endowed with both high two-photon absorption cross section and slow thermal back-isomerization. These compounds provide optimized and sustained two-photon neuronal stimulation both in light-scattering brain tissue and in Caenorhabditis elegans nematodes, displaying photoresponse intensities that are comparable to those achieved under one-photon excitation. This finding opens the way to use both genetically targeted and pharmacologically selective azobenzene photoswitches to dissect intact neuronal circuits in three dimensions.

In the present paper, we consider the formation of rare tautomeric forms of the neutral base pairs adenine−thymine (A−T) and cytosine−guanine (C−G) in low-energy excited singlet electronic states. Ab initio calculations (6-31G basis set) have been carried out at the Hartree−Fock level of theory for the ground electronic states and using a configuration interaction among all single excitations (CIS) technique for the excited electronic states. The obtained results indicate that the double proton transfer is not a feasible process in the ground electronic states. For the excited singlet electronic states, which can be directly accessed upon photoexcitation, the excitation energy is localized in the π system of one of the monomers of the pair. In these states, especially in the A−T base pair, the double proton transfer becomes energetically more accessible. However, it is unlikely that the rare tautomer may live long enough to perturb the duplication of the genetic code. Our theoretical results also show the existence of charge-transfer excited electronic states in both A−T and C−G base pairs. These states are found at a considerable high energy in the region corresponding to the ground-state minimum-energy configuration. These structures, which can be accessed only upon internal conversion from another excited electronic state, have a remarkable minimum of energy in the region corresponding to a single proton transfer that eventually neutralizes the charge separation induced by the electronic transition. We discuss the possibility that such metastable structures may play a key role in altering the DNA unwinding and strand separation (that is, in mutagenesis).

The H-atom transfer and the rotational processes of 2-(2‘-hydroxyphenyl)oxazole derivatives in both ground (S0) and first singlet (S1) excited electronic states have been respectively studied from experimental and theoretical points of view. Experiment and theory support the coexistence of two ground state rotamers, E and ER, with OH···N and OH···O hydrogen bonds, respectively, rotamer E being the most stable and the only one that experiences a photoinduced H-atom motion in the S1 state. The fluorescence of 2-(2‘-hydroxyphenyl)-4-methyloxazole in a rigid polymeric medium suggests that in fluid media the phototautomer of the excited enol rotamer suffers a twisting motion around the C−C bond linking both moieties of the molecule. Ab initio calculations at the Hartree−Fock and CI-all-singles levels reveal (a) the existence of a high-energy barrier to the H-atom transfer in the S0 state, whereas in the S1 state this transfer has a small or null energy barrier, (b) a coupling between a charge transfer and the nuclear rearrangement (OH and N···O modes) that makes the system move from the enol to the keto form, and (c) the presence of excited state rotamers of the keto phototautomer in these oxazole derivatives.

A nuclear quantum dynamical simulation of the proton shuttle operating in the green fluorescent protein has been carried out on a high-quality, high-dimensionality potential energy surface describing the photoactive pipi* excited state, and including motion of both the three protons and of the donor and acceptor atoms of the hydrogen bonds in a closed proton wire. The results of the simulations show that proton transfer along the wire is essentially concerted, synchronous, and very fast, with a substantial amount of the green fluorescent species forming within several tens of femtoseconds. In this regard, analysis of the population of the fluorescent species indicates that at least two dynamical regimes are present for its formation. Within the first hundreds of femtoseconds, dynamics is very fast and impulsive. Later on, a slower pace of formation appears. It is discussed that the two largest decay times for the protonated chromophore reported experimentally (Chattoraj, M.; King, B. A.; Bublitz, G. U.; Boxer, S. G. Proc. Natl. Acad. Sci. U.S.A. 1996, 93, 8362-8367) might correspond to some irreversible process occurring after formation of the fluorescent species, rather than to cleavage of the chromophore's phenolic O-H bond.