Theoretical Investigations of the Effects of J-Aggregation on the Linear and Nonlinear Optical Properties of E-4-(4-Dimethylaminostyryl)-1-methylpyridinium [DAMS+]

J-type aggregation of organic chromophores into inorganic host matrixes provides a useful route toward materials showing strong second-order nonlinear optical (NLO) response. The increased NLO response of J-aggregates is related to the peculiar arrangement of the NLO-phores into the host matrix, which produces the appearance of a narrow and intense band in the material electronic absorption spectrum, red-shifted with respect to the main absorption band of the isolated NLO-phore. A theoretical investigation, based on DFT, TDDFT, and ZINDO calculations on the relationship between the structural features of various [DAMS+] ([DAMS+] = E-4-(4-dimethylaminostyryl)-1- methylpyridinium) dimeric or oligomeric aggregates and their linear and nonlinear optical properties shows that the appearance of a new red-shifted absorption band, typical of J-aggregation, is associated with interchromophoric transitions of charge-transfer character, due to the splitting of HOMO and LUMO levels. The intensity of this latter band increases by increasing the number of NLO-phores in a model of oligomeric arrangement of J-aggregates. The calculated quadratic hyperpolariz-abilities for the mostly responsive J-type trimeric aggregates of [DAMS+] are found to largely exceed that of three isolated NLO-phores, confirming a cooperative NLO strong contribution due to J-aggregation. Finally, our DFT and TDDFT calculations on eclipsed or with opposite dipole dimeric H-aggregates of [DAMS+] show a splitting of HOMO and LUMO levels, which gives place to interchromophoric transitions of charge-transfer character but blue-shifted, as observed experimentally.