Organometallic Complexes for Nonlinear Optics. 33. Electrochemical Switching of the Third-Order Nonlinearity Observed by Simultaneous Femtosecond Degenerate Four-Wave Mixing and Pump−Probe Measurements

The molecular inorganic compound 1,3,5-{ trans -[RuCl(dppe) 2 ]C⋮C-4-C 6 H 4 C⋮C} 3 C 6 H 3 ( 1 ) is reversibly oxidized in solution using an optically transparent thin-layer electrochemical (OTTLE) cell, the oxidation to 1 3+ resulting in the appearance of a strong absorption band at 11 200 cm -1 in contrast to 1, which is optically transparent at frequencies below 20 000 cm -1 . This linear electrochromic behavior is accompanied by third-order nonlinear optical electrochromic behavior, which has been probed by a combination of femtosecond degenerate four-wave mixing (DFWM) and pump−probe experiments at 12 500 cm -1 . DFWM studies of 1 and 1 3+ reveal an order of magnitude increase in the hyperpolarizability, |γ|, upon oxidation with |γ| values of 2000 × 10 -36 esu and 20 000 × 10 -36 esu, respectively; the DFWM signal of the latter is dominated by a delayed response (relaxation time ca. 1 ps). The transient absorption (TA) data for 1 reveal efficient two-photon absorption (σ 2 ≈ 10 3 GM), whereas the TA data for 1 3+ reveal saturable absorption (decay time ca. 1 ps). The switching of third-order nonlinearity in the 1 / 1 3+ pair represents the first demonstration of femtosecond time-scale processes being responsible for nonlinear electrochromism.