No abstract

We report the supermolecular interactions, electronic absorption spectra, and dynamic second-order nonlinear optical polarizabilities β of π-stacking (p-) and hydrogen-bonded (h-) dimers and a trimer in a 2-methyl-4-nitroaniline (MNA) crystal, using the TDB3LYP/3-21G+ coupled with the SOS methods. The calculated results show that the supermolecular interactions and second-order optical polarizabilities are smaller for dimers (p- and h-dimers) than the trimer in MNA crystal, which originates from size effect, and the h-dimer has smaller supermolecular interaction and larger second-order nonlinear optical response compared with that of the p-dimer, which originates from site effect in MNA crystal. The lowest absorption peak comes from the π−π* charge transfers within a MNA molecule for h-dimer, and between the two MNA molecules for the p-dimer in a MNA crystal. The calculated polarizability norm ||β|| of trimer consisting of h- and p-arrangements is 42.0 × 10-30 cm5 esu-1, and the macroscopy susceptibility norm ||χ|| is 6.15 × 10-7 esu as compared with the experimental value ||χ|| of 9.06 × 10-7esu in MNA crystal. The slipped cofacial arrangements of dimer (h-dimer) have larger contributions to the electronic absorption spectrum and nonlinear optical response of MNA crystal.

The optimized configurations and electronic structures have been obtained based on the density-functional theory for finite open single-walled carbon nanotubes of ͑4,4͒ and ͑5,5͒. The calculated results show that the bond lengths, bond angles, and charge distributions at the tube ends are significantly different from those at the tube wall. Then, the electronic absorption spectra of the two tubes are calculated by the time-dependent density-functional theory. The evidences show agreement between the theoretical and available experimental results. A strong and wide absorption band occurs for the case of light polarized parallel to the tube axis; a weak absorption and large transparent range appear as the case of light polarized perpendicular to the tube axis. Finally, the dynamic third-order optical polarizabilities in the three optical processes are obtained in terms of the sum-over-states method combined with the above-mentioned method. It is found that the anisotropy of the polarizabilities is larger for the ͑4,4͒ than ͑5,5͒ tube, and the tube with a larger axial length and smaller diameter has a larger third-order optical polarizabilities at nonresonant frequencies. The largest third-order polarizability is in the direction of both the polarized light and basic light along with the tube axis, and two photon excitation states make main contributions to the polarizabilities of the ͑4,4͒ and ͑5,5͒ tubes.