Ultrafast degenerate four wave mixing studies of third-order nonlinearities in conjugated organic polymers containing azo groups and alkynyl linkages in the polymer backbone

The first series of conjugated organic polymers with the formula {1,4-[2-(RO)-5-(R'O)C6H2]-N=N-C6H4-Cequivalent toC-}(n), where R = methyl, R' = hexyl (7a) and R = hexyl, R' = SiMe2-t-Bu (8), were produced in palladium-catalyzed cross-coupling and homocoupling reactions of selected bis(bromo) and bis(alkynyl) azo monomers. The molecular weights (M-n) for the conjugated polymers ranged from 5500 to 22 000. Decenerate four wave mixing (DFWM) with 780 nm fermosecond pulses was used to investigate electronic third-order nonlinearities for azo monomer 3a and conjugated azo polymer 7a. Both compounds had ultrafast nonlinearities with tau(1) = 100 fs response time. The real part of 7a second hyperpolarizability, gamma(7a), is negative, Re gamma(7a) = -(4.3 +/- 0.3) x 10(-46) m(5) V-2. Polymer 7a's second hyperpolarizability also has an imaginary component, Im gamma(7a) = (1.7 +/- 0.3) x 10(-46) m(5) V-2. Calculated per repeat unit, polymer \gamma(7a)\ = (4.6 +/- 0.5) x 10(-46) m(5) V-2 and is about 780 times larger than that for monomer 3a. gamma(7a) was analyzed with the three-level model; gamma(7a) enhancement is attributed to the conjugation effects in the polymer and contributions from a two-photon state. Polymer 7a's excited state had a 3.4 ps lifetime, as determined by DFWM.