The Influence of σ and π Acceptors on Two‐Photon Absorption and Solvatochromism of Dipolar and Quadrupolar Unsaturated Organic Compounds

Two‐photon absorption cross sections δ 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 π or σ acceptors. The δ 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 4100×10 −50 cm 4  s photon −1 , respectively. The corresponding dipolar compounds trans ‐2‐(4′‐dimethylaminophenyl)ethenyl‐2,3,4,5,6‐pentafluorobenzene ( 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 δ. 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 γ 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 S 1 .