π-Conjugated Dendritic Nanosized Chromophore with Enhanced Two-Photon Absorption

A novel π-conjugated dendritic nanosized chromophore, 1,3,5-tris(7,12-bis(2-{4-[2-(7-diphenylamino-9,9-dibutyl- 9H -fluoren-2-yl)-vinyl]-phenyl}-vinyl)-5,5,10,10,15,15-hexabutyl-10,15-dihydro-5 H -diindeno[1,2- a;1‘,2‘- c ]fluoren-2-yl)-benzene ( 3 ), was synthesized and characterized, together with its corresponding one- and three-branched units. Two-photon absorption (TPA) spectra for these chromophores were measured by the nonlinear transmission spectral technique which utilizes a femtosecond white-light continuum. The TPA peak value for the dendritic chromophore is 5.47 × 10 - 20 cm 4 /GW, 15.6 times larger than that for the one-branched dipolar chromophore (0.35 × 10 - 20 cm 4 /GW), [9,9-dibutyl-7-(2-{4-[2-(9,9-dibutyl-9 H -fluoren-2-yl)-vinyl]-phenyl}-vinyl)-9 H -fluoren-2-yl]-diphenylamine ( 1 ). The three-branched chromophore, 2,7,12-tris(2-{4-[2-(9,9-dibutyl-7-diphenylamino-9 H -fluoren-2-yl)-vinyl]-phenyl}-vinyl)-5,5,10,10,15,15-hexabutyl-10,15-dihydro-5H-diindeno[1,2-a;1‘,2‘-c]fluorene ( 2 ) has a TPA peak value of 2.28 × 10 - 20 cm 4 /GW, 6.5 times as large as that for compound 1 . The enhanced two-photon absorption found in these two multibranched chromophores is attributed to the fact that the dendritic or the three-branched chromophore has both an extended π-conjugated system and an increased intramolecular charge redistribution compared to the one-branched chromophore. With use of comparable structure unit based concentrations, the two-photon excited (TPE) fluorescence intensity for this dendritic chromophore (0.0033 M solution) was found to be enhanced by a factor of 2.9 compared to that for chromophore 1 (0.02 M solution), which would offer a major advantage in TPE fluorescence related applications.