Effects of external electric field and self-aggregations on conformational transition and optical properties of azobenzene-based D-π-A type chromophore in THF solution.

The influence of environments (THF solvents and electric field) and molecular self-aggregations on the structure and optical properties of 4-(4-hydroxyphenylazo)nitrobenzene has been investigated by molecular dynamics (MD) simulations and quantum chemical calculations. Long-range electrostatic effects and the hydrogen bond interactions between the solute and the THF solvent molecules lead to the augments of nonlinear optical (NLO) response by about two times from the gas phase to THF solution, accompanied by considerable red-shift of more than 40 nm in the maximum absorption wavelengths of the ground (S(0)) and low-lying excited states (S(1), S(2), and S(3)). The solvated chromophore reorients quickly (within 300 ps) under external electric field of 1.0 V/nm, even when the direction of the applied electric field is antiparallel to the dipole moment of the solute. Nonequilibrium MD simulations demonstrate that the light-induced cis-trans isomerization in THF solution and external electric field need longer relaxation time (about 1.0 ps) than that in gas phase (about 500 fs). The dipole-dipole interactions and intermolecular hydrogen bonds facilitate the self-aggregations of solute molecules in solution. The V-shaped dimer exhibits higher hyperpolarizability value by about 1.2 times of the monomer, whereas the antiparallel alignment leads to a cancellation of dipole moment and hence dramatic decrease in hyperpolarizability (one-third of the monomer). However, the Boltzmann-weighted contribution to hyperpolarizability from these two aggregations (with 82% V-shaped and 18% antiparallel) is close to that of the monomer. Orientations of D-π-A dipoles in various environments and molecular aggregations are important to modulate the optical properties of materials.