π-Conjugated dendritic nanosized chromophore with enhanced two-photon absorption

A novel pi-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-5H-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 x 10(-20) cm(4)/GW), 15.6 times larger than that for the one-branched dipolar chromophore (0.35 x 10(-20) cm(4)/GW), [9,9-dibutyl-7-(2-{4-[2-(9,9-dibutyl-9H-fluoren-2-yl)-vinyl]-phenyl}-vinyl)-9H-fluoren-2-yl]-diphenylamine (1). The three-branched chromophore, 2,7,12-tris(2-{4-[2-(9,9-dibutyl-7-diphenylaniino-9H-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 x 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 pi-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.