Enhancing the Third-Order Nonlinear Optical Properties of Porphyrins and Molecular Wires

Abstract

ENHANCING THE THIRD-ORDER NONLINEAR OPTICAL PROPERTIES OF PORPHYRINS AND MOLECULAR WIRES By Jonathan Leslie Humphrey, Ph.D. A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at Virginia Commonwealth University. Virginia Commonwealth University, 2006 Major Director: Dr. Darius Kuciauskas Assistant Professor of Chemistry The third-order nonlinear optical (NLO) properties of indium tin oxide (ITO) thin films, ~ e , ~ n , and Co2' 5,10,15,20-tetrakis-(4-hydroxytetraphenyl)porphyrin (TPP) films, and a series of ethynyl-linked azobenzene oligomers were investigated using degenerate four wave mixing (DFWM) with 100 fs laser pulses. To measure the NLO of IT0 thin films, A DFWM method for measuring thin films on thick substrates was refined for the characterization of films less than 100 nm thick and applied to IT0 films -25 nm thick. It was found that the third-order nonlinear susceptibility of ITO, X(~) ITO, is purely electronic xii . . . X l l l at 900 1300 nm (1 1000 7700 cm-') and has a value of (2.16 * 0.18) x 10-l8 m2 v ~ . The X ( 3 ) I ~ 0 value reaches (3.36 % 0.28) x 10-l8 m2 v-? at 1500 nm (6700 cm-I) due to twophoton absorption by free carriers (electrons). Ultrafast electron relaxation was also observed. The -100 fs lifetime of this process could reflect electron scattering in the conduction band. This DFWM method was also used to investigate the two-photon properties of 500 nm thick electropolymerized films of ~e) ' , ~ n ~ ' , and coZt TPP in the near-IR spectral region. Metalloporphyrins with strong charge transfer (CT) transitions in the linear absorption spectra also show enhanced two-photon absorption. (Metalloporphyrin two-photon absorption cross section, 6, increases >10 times over that for the metal free porphyrin.) This effect was attributed to a two-photon induced charge transfer between the metal ion's d orbitals and the n-system of the porphyrin. Correlation of oneand two-photon absorption properties of transition metal porphyrins suggests a new and simple approach to improve organic materials for photonic applications. Finally, a series of oligomers consisting of ethynyl-linked azobenzene units was prepared using Pd- catalyzed cross coupling. The linear and nonlinear optical properties of the oligomers were investigated. The molecular second hyperpolarizability, 3: followed the power law y = n2.1 2+0.05 (n is number of repeat units) for unusually large molecular lengths. The exceptional exciton delocalization length exceeds 360 conjugated bonds (>49 nm) and is attributed to the rigidity of the conjugated backbone.