Charge-Transfer States Determine Iron Porphyrin Film Third-Order Nonlinear Optical Properties in the near-IR Spectral Region

Femtosecond degenerate four-wave mixing (DFWM) was used to study third-order nonlinear optical properties of electropolymerized Fe(III) tetrakis( p -hydroxyphenyl)porphyrin films. Third-order nonlinear susceptibility χ 1111 (3) (−ω;ω,ω,−ω) was determined at 760−1350 nm. Susceptibility χ (3) values for a 500 nm thick porphyrin film ranged between (2−15)×10 -20 m 2 V -2 ((1.4−10.7)×10 -12 esu). The time response of electronic nonlinearities was faster than 120 fs. The sum-over-states model was applied to determine real and imaginary components for χ (3) and molecular second hyperpolarizability, γ. At wavelengths <800 nm, porphyrin film χ (3) is enhanced due to the 700 nm one-photon charge-transfer (CT) band. Nonlinear properties at longer wavelengths are determined by a two-photon resonance with a 17 400 cm -1 state. This state was not observed for other porphyrins and is attributed to a two-photon CT transition. Due to the resonance with the two-photon state, porphyrin Im χ (3) and two-photon absorption cross-section δ increased about 10 times when compared to values in the long-wavelength limit. Because of the contributions of the two-photon state, real component Re χ (3) had a negative sign in the near-IR spectral range; Re χ (3) ≈ −6×10 -20 m 2 V -2 at 9000−13 000 cm -1 . Results suggest that transition metal CT states can improve third-order nonlinear properties of organic materials.