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    Large third-order nonlinear optical response of porphyrin J-aggregates oriented in self-assembled thin films

    Year: 2006

    Journal: J. Mater. Chem., 2006, 16, 1573–1578, 20111221

    Authors: Elisabetta Collini, Camilla Ferrante, Renato Bozio, Andrea Lodi and Glauco Ponterini

    Organizations: Department of Chemical Sciences, University of Padova and INSTM, Via Marzolo 1, I-35131 Padova, Italy. Chemistry Department, University of Modena and Reggio Emilia and INSTM, via Campi 183, I-41100 Modena, Italy

    The preparation and characterization of a self-assembled material showing a high nonlinear response and good photostability to ultrashort laser pulses is presented. The material is built by alternate deposition of tetrakis(4-sulfonatophenyl)porphyrin diacid (H4TPPS22) and poly(diallyldimethylammonium chloride) (PDDA) forming electrostatically self-assembled multilayers (ESAMs). UV-visible absorption and emission experiments show that in this matrix H4TPPS22 is present mainly in its J-aggregated form. Furthermore, linear dichroism experiments on a 3 bilayer film show a preferential alignment of the porphyrin aggregate with the J-band transition dipole moment parallel to the film surface. The two photon absorption (TPA) properties of these films are investigated with the Z-scan technique at 806 nm, employing 130 fs pulses. The samples exhibit strong nonlinearities with a very large two-photon absorption coefficient bTPA of 50 cm GW21. The origin of this large response is investigated. It has been already demonstrated that aggregation enhances the molecular TPA cross section of H4TPPS22 from 30 to 1000 GM in water solution thanks to cooperative effects. In a 20 bilayer film a further increase by a factor of 1.7 is observed and explained in terms of preferential alignment of J-aggregates in the multilayers.