Simulation of absorption and scattering spectra of crystalline organic nanoparticles with the discrete dipole approximation: Effects of crystal shape, crystal size, and refractive index of the medium

The effect of the shape (habit) of crystalline organic nanoparticles on their absorption spectra is studied by simulations using the discrete dipole approximation, focusing, in particular, on the vibronic structure of the absorption bands in the spectra. Simulations predict a significant effect that, for sufficiently small particles, can be simply rationalized by the depolarization factor. The crystal size and the refractive index of the medium in which the nanoparticles are embedded are also found to have an effect on the absorption spectra. All factors mentioned are found to influence also the spectra of scattered light. These effects, already broadly documented for metallic nanoparticles, are here demonstrated theoretically for the first time for crystalline organic nanoparticles, providing novel insight into the optical response of such particles. The effects are expected to be displayed by all organic nanoparticles, as long as they have a well-defined crystal structure and are large enough for the optical properties to be understandable using a macroscopic dielectric tensor. The effects demonstrated here should be taken into account when rationalizing differences in absorption spectra of a substance in solution and in nanoparticle form, e.g., in deducing the type of intermolecular packing. The effects are much less pronounced for optically isotropic nanoparticles.