Theoretical insight into the quadratic nonlinear optical response of organics: derivatives of pyrene and triaminotrinitrobenzene (Invited Paper)

Results of the present work show that: 1) The AM1 finite field approach provides a useful tool to screen the quadratic nonlinear optical potentialities of large organic molecules. 2) The investigation of three sets of pyrene push-pull derivatives points out that the detailed nature of the electronic coupling between the push-pull groups and the conjugated link, plays an important role in the nonlinear response. 3) the charge transfer in the excited state is far from reaching a unit value. In the pyrene derivatives, the charge given by the donor or acquired by the acceptor is in all cases smaller then 0.45 |e|. As the conjugated link extends from benzene to pyrene, the electronic overlap decreases between the donor-dominated upper occupied MO's and the acceptor-dominated lower unoccupied MO's, which weakens the charge transfer all the way from the donor to the acceptor. 4) The case of 1,3,5-triamino-2,4, 6-trinitrobenzene exmplifies that octupolar contributions to the secone-order polarizability can be important.