One-, two- and three-photon spectroscopy of p-conjugated dendrimers: cooperative enhancement and coherent domains

We use wavelength tunable femtosecond pulses to measure intrinsic (simultaneous) two-photon absorption (2PA) and three-photon absorption (3PA) molecular cross section in two series of p-conjugated dendrimers built of identical 4,40-bis(diphenylamino) stilbene (BDPAS) and 4,40-bis(diphenylamino) distyrylbenzene (BDPADSB) repeat units. Record large 2PA cross sections, s2=10 46 cm s are obtained for the largest second-generation BDPAS-based dendrimer, as well as zeroth-generation 4-arm BDPADSB-based dendrimer. In both series, maximum 2PA cross section increases nonlinearly with the number of p-electrons, whereas for higher generations this dependence turns to linear one. 3PA cross section also increases nonlinearly with the size of the system in the series of BDPAS-based molecules, amounting a record large value, s3=10 79 cm s, for the largest, second-generation dendrimer. We interpret these results in terms of direct inter-branch conjugation, which facilitates cooperative enhancement of the nonlinear-optical response. We propose a simple model which allows us to determine the effective size of coherent domains (extent of conjugation), which, in turn, determines the optimum dendrimer size for most efficient nonlinear response. r 2004 Elsevier B.V. All rights reserved. PACS: 73.22. f; 42.50.Hz; 33.80.Wz