The design of molecules with large two-photon absorptivities

Summary form only given.In recent work we have developed guidelines for the design of both centrosymmetric and dipolar chromophores with large two-photon absorptivities, based upon theoretical, synthetic, and nonlinear spectroscopic studies. We recently found that 4,4'-bis(di-n'-butyl)amino-E-stilbene had a peak two-photon cross section, /spl delta/, of 240/spl times/10/sup -50/ cm/sup 4/ sec/photon, in comparison to stilbene for which the reported value is 12/spl times/10/sup -50/ cm/sup 4/ sec/photon, and is in the range of the largest /spl delta/ values reported for organic compounds. We believe that the large increase in the two-photon absorption for bis(di-n'-butyl)amino-E-stilbene relative to stilbene was related to a symmetrical charge transfer from the amino nitrogens to the conjugated bridge. Therefore, we hypothesized that structural features leading to a significant change in quadrupole moment between the ground and the two-photon state could be beneficial for enhancing the two-photon cross section. We therefore synthesized and studied systems in which: (1) conjugation length was increased by use of pheneylenevinylene groups, (2) the extent of charge transfer from the end of the molecule to the center was increased by substitution of cyano groups on the central ring of a bis-styryl benzene compound, making it a donor-acceptor-donor compound and, (3) the sense of the symmetric charge transfer was reversed by putting alkoxy donors on all three rings bis styryl benzene and the relatively strong dicyano vinyl or thiobarbituric acid accepting groups on both ends, making them acceptor-donor-acceptor compounds. We found that: (1) increasing the length of the molecule leads to an significant increase in /spl delta/, (2) that donor-acceptor-donor or acceptor-donor-acceptor compounds do have enhanced /spl delta/ relative to unsubstituted compounds and bis-donor substituted compounds.