Influence of π-Stacking on the Resonant Enhancement of the Second-Order Nonlinear Optical Response of Dipolar Chromophores

The wavelength-dependent second-harmonic generation (SHG) efficiency of two simple dipolar chromophores,
4-NO2C6H4N(H)Bun (1) and 4-NO2C6H4SN(H)But (2), was compared in solution and in the solid state. Hyper-Rayleigh scattering measurements at 532 nm provided comparable molecular first hyperpolarizabilities. Both
compounds crystallize in non-centrosymmetric space groups, but a more efficient arrangement of dipole
moments results in a significantly larger deff value for 2. Kurtz−Perry experiments from 450 to 700 nm
revealed an important difference in the resonant component of the nonlinear optical responses of these
compounds; the SHG efficiency of crystalline 1 depends more strongly on the incident wavelength than that
of 2. This would be in contradiction with the TD-DFT excitation energies calculated for these molecules, but
the observation can be explained by the resonant contribution from low-energy interchromophore excitations
enabled by π-stacking in the crystal of 1.