Mechanistic Basis for Red Light Switching of Azonium Ions.

Azonium ions formed by the protonation of tetra-ortho-methoxy-substituted aminoazobenzenes photoisomerize with red light under physiological conditions. This property makes them attractive as molecular tools for the photocontrol of physiological processes, for example, in photopharmacology. However, a mechanistic understanding of the photoisomerization process and subsequent thermal relaxation is necessary for the rational application of these compounds as well as for guiding the design of derivatives with improved properties. Using a combination of sub-ps/ns transient absorption measurements and quantum chemical calculations, we show that the absorption of a photon by the protonated E-H+ form of the photoswitch causes rapid (ps) isomerization to the protonated Z-H+ form, which can also absorb red light. Proton transfer to solvent then occurs on a microsecond time scale, leading to an equilibrium between Z and Z-H+ species, the position of which depends on the solution pH. Whereas thermal isomerization of the neutral Z form to the neutral E form is slow (∼0.001 s-1), thermal isomerization of Z-H+ to E-H+ is rapid (∼100 s-1), so the solution pH also governs the rate at which E/E-H+ concentrations are restored after a light pulse. This analysis provides the first complete mechanistic picture that explains the observed intricate photoswitching behavior of azonium ions at a range of pH values. It further suggests features of azonium ions that could be targeted for improvement to enhance the applicability of these compounds for the photocontrol of biomolecules.

Hyperpolarizabilities of Push-Pull Chromophores in Solution: Interplay between Electronic and Vibrational Contributions.

Contemporary design of new organic non-linear optical (NLO) materials relies to a large extent on the understanding of molecular and electronic structure-property relationships revealed during the years by available computational approaches. The progress in theory-hand-in-hand with experiment-has enabled us to identify and analyze various physical aspects affecting the NLO responses, such as the environmental effects, molecular vibrations, frequency dispersion, and system dynamics. Although it is nowadays possible to reliably address these effects separately, the studies analyzing their mutual interplay are still very limited. Here, we employ density functional theory (DFT) methods in combination with an implicit solvent model to examine the solvent effects on the electronic and harmonic as well as anharmonic vibrational contributions to the static first hyperpolarizability of a series of push-pull α,ω-diphenylpolyene oligomers, which were experimentally shown to exhibit notable second-order NLO responses. We demonstrate that the magnitudes of both vibrational and electronic contributions being comparable in the gas phase significantly increase in solvents, and the enhancement can be, in some cases, as large as three- or even four-fold. The electrical and mechanical anharmonic contributions are not negligible but cancel each other out to a large extent. The computed dynamic solute NLO properties of the studied systems are shown to be in a fair agreement with those derived from experimentally measured electric-field-induced second-harmonic generation (EFISHG) signals. Our results substantiate the necessity to consider concomitantly both solvation and vibrational effects in modeling static NLO properties of solvated systems.

Accurate Nonlinear Optical Properties of Solvated para-Nitroaniline Predicted by an Electrostatic Discrete Local Field Approach.

A general computational protocol for accurate predictions of nonlinear optical (NLO) properties of solvated molecules based on the rigorous local field (RLF) approach taking all relevant effects into account is presented. para-Nitroaniline (pNA) was taken as a model NLO system dissolved in cyclohexane, tetrahydrofuran, and 1,4-dioxane. Molecular dynamics (MD) simulations employing either non-polarizable or polarizable force fields were used to generate representative sets of structures of the solutions. The static NLO properties of the solute were calculated at the MP2/aug-cc-pVDZ level of theory with the multiplicative scaling method used to account for the frequency dispersion of the properties. Focusing on the electric field-induced second harmonic generation (EFISH) and hyper-Rayleigh scattering (HRS), a good agreement between calculated results and experimental measurements was achieved with a polarizable force field. While the solvent effects on the vibrational contributions to the static molecular properties are significant, they remain small for both EFISH and HRS. Our results show that the proposed approach yields reliable predictions of dynamic NLO properties of solvated chromophores, which paves a route to further applications of the RLF approach to study a wide range of NLO phenomena in heterogeneous environments.