The influence of sigma and pi acceptors on two-photon absorption and solvatochromism of dipolar and quadrupolar unsaturated organic compounds.

Two-photon absorption cross sections delta and solvatochromic properties were determined for a series of quadrupolar and dipolar compounds by using femtosecond excitation in the spectral range between 710 and 960 nm. The compounds investigated were distyrylbenzenes and polyenes bearing appropriate pi or sigma acceptors. The delta values for the centrosymmetric compounds trans, trans- 1,4-bis[2-(2',5'-dihexyloxy)phenylethenyl]-2,3,5,6-tetrafluorobenzene (6), trans, trans-1,4-bis[2-(4'-dibutylamino)phenylethenyl]- 2,3,5,6-tetrafluorobenzene (2), trans, trans-1,4-bis[2-(4'dimethylamino)phenylbutadienyl]- 2,3,5,6-tetrafluorobenzene (7), trans,-trans-1,4-bis[2-(4'-dimethylamino)phenylethenyl]2,5- dicyanobenzene (4) and trans,trans-1,4-bis[2-(4'-dimethylamino)phenylethenyl]-2- propylsulfonyl-5-(2-ethylhexyl)sulfonylbenzene (3) are on the order of 600, 1400, 1700, 3000, and 4100 x 10(-50) cm4 s photon-1, respectively. The corresponding dipolar compounds trans-2-(4'- dimethylaminophenyl)ethenyl-2,3,4,5,6-pentafluorobene (8), trans-4-(4'-dimethylaminophenyl)butadienyl-2,3,4,5,6-pentafluorobenzene (9), trans-6-(4'-dimethylaminophenyl)hexatrienyl-2,3,4,5,6- pentafluorobenzene (10) were additionally investigated. All centrosymmtric compounds are good fluorescent materials, while the dipolar chromophores 8-10 exhibit low fluorescence quantum yields. Solvatochromism was also observed for the fluorophores 2-10 as a result of intramolecular charge transfer (ICT). Furthermore, a reasonable correlation was obtained between measured and calculated delta. Quantum chemical calculations were performed by using the INDO Hamiltonian with a MRDCI scheme. The results show that the sum over states (SOS) expression for the second hyperpolarizability gamma is appropriate to describe the mechanism of two-photon absorption. Mechanistic investigations of quadrupolar compounds showed that the energy of the two-photon excited state is higher than S1.