Multistep Electron Injection Dynamics and Optical Nonlinearity Investigations of π-Extended Thioalkyl-Substituted Tetrathiafulvalene Sensitizers

Abstract

A comprehensive investigation is presented on the photophysical and third-order nonlinear optical (NLO) properties of two thioalkyl-substituted tetrathiafulvalene molecules (referred here as G1 and G3) to understand their utility as photosensitizers for dye-sensitized solar cell (DSSC) and optoelectronic applications. Both steady-state and time-resolved (in the fs–ns time regime) absorption and photoluminescence (PL) spectroscopy techniques were employed to comprehend the excited-state properties of the molecules in solution as well as in thin films deposited on both quartz and mesoporous TiO2 layers. The spectroscopy measurements in solution and thin films deposited on quartz provided the excited-state properties of dye molecules. Time-resolved PL measurements at the dye–TiO2 interface provided initial evidence of electron injection by fast PL quenching decay dynamics for both the molecules. Detailed target analysis of the femtosecond transient absorption spectroscopy (TAS) data of the dye–TiO2 sample revealed a multistep ultrafast electron injection for both molecules with the fastest injection component being 374 and 314 fs for G1 and G3 molecules, respectively. The ultrafast NLO properties of G1 and G3 were studied using the Z-scan technique with 800 nm, ∼70 fs laser pulses. The open aperture measurements showed three-photon absorption with magnitudes of coefficients 4.7 × 10–5 cm3/GW2 and 5.2 × 10–5 cm3/GW2, and the closed aperture measurements provided second hyperpolarizability (γ) values of 3.5 × 10–31 esu and 4.2 × 10–31 esu for G1 and G3, respectively. Additionally, the onset of optical limiting was estimated to be 5.8 × 10–3 J/cm2 and 5.7 × 10–3 J/cm2 for G1 and G3 molecules, respectively.