Ultrafast Degenerate Four Wave Mixing Studies of Third-Order Nonlinearities in Conjugated Organic Polymers Containing Azo Groups and Alkynyl Linkages in the Polymer Backbone

摘要

The first series of conjugated organic polymers with the formula {1,4-[2-(RO)-5-(R'O)C_6H_2]-N=N=C_6H_4-C≡C-}_n, where R = hexyl (7a) and R = hexyl, R' = SiMe_2-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. Degenerate four wave mixing (DFWM) with 780 nm femtosecond pulses was used to investigate electronic third-order nonlinearities for azo monomer 3a conjugated azo polymer 7a. Both compounds had ultrafast nonlinearities with τ_1 = 100 fs response time. The real part of 7a second hyperpolarizability, γ_(7a) = -(4.3 ± 0.3) * 10~(-46) m~5 V~(-2). Polymer 7a's second hyperpolarizability also has an imaginary component, Im γ_(7a) = (1.7 ± 0.3) * 10~(-46) m~5 V~(-2). Calculated per repeat unit, polymer |γ_(7a)| = (4.6 ± 0.5) * 10~(-46) m~5 V~(-2) and is about 780 times large than that for monomer 3a. γ_(7a) was analyzed with the three-level model; γ_(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.