Effects of pi centers and symmetry on two-photon absorption cross sectionsof organic chromophores

We have theoretically examined a series of organic molecules that exhibit l arge two-photon absorption cross sections in the visible region and that ha ve been synthesized in different laboratories. One- and two-photon absorpti on cross sections of the four lowest excited states of each molecule have b een calculated at the same theoretical level using ab initio response theor y. It is found that the molecular length and the one-photon absorption inte nsity are quite strongly correlated factors, but that a corresponding corre lation for the two-photon absorption is much weaker or is missing. In contr ast, a most crucial role for large two-photon absorption is played by the p i center. For molecules with a given pi center a symmetrical structure with strong donor groups can result in a maximum two-photon absorption cross se ction. Our theoretical findings are consistent with some recent experimenta l observations. The chromophore based on dithienothiophene as pi center att ached with symmetrical N,N-diphenylamine donors is found to have the larges t two-photon cross section in the visible region among all known one-dimens ional two-photon organic materials that have been reported in the literatur e. (C) 2001 American Institute of Physics.