Electronic excitations and first hyperpolarizability of 2‐methyl‐4‐nitroaniline clusters

Abstract

AbstractThe crystal packing effects upon the static and dynamic first hyperpolarizability of 2‐methyl‐4‐nitroaniline (MNA) clusters have been addressed at the TDHF/AM1 level. Due to electrostatic interactions, in 1‐D arrays of MNA unit cells extending along the a crystallographic axis, the effective hyperpolarizability strongly increases whereas stacking unit cells along the b or c axes leads to a reduction of these quantities. These effects are magnified in the case of frequency‐dependent quantities. The first hyperpolarizability variations can mostly be related to bathochromatic and hypsochromatic shifts as well as to changes in transition dipole moments of the low‐energy excited states that dominate the UV/visible absorption spectra. From calculations performed at the CIS/INDO/S level of approximation, these dominant excited states present an important intramolecular charge transfer character and bathochromatic (hypsochromatic) shifts are associated with increase (decrease) of the transition dipole moments. In addition, a large number of intermolecular charge‐transfer excited states have been detected. In the small MNA aggregates as well as in clusters extending only along the b or c crystallographic axes, these excited states possess small transition dipole moments. On the other hand, when stacking MNA unit cells along the a axis the electrostatic interactions induce a decrease of the excitation energies and an increase of the transition dipole moments of these intermolecular charge transfer states. As a consequence, they account for a nonnegligible contribution to the first hyperpolarizability. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002