Enhanced Nonresonant Degenerate Four Wave Mixing in Conjugated Linear Chains through Excited State Population

A new enhancement mechanism for nonlinear optical processes originating from real population of electronic excited states in conjugated linear chains has been presented in previous theoretical studies.1,2 Compared to the ground state,3,4 the calculated nonresonant third-order optical susceptibility γSn(-ω4; ω1, ω2, ω3) of linear chain molecules can be enhanced by orders of magnitude, or even change sign, when the first (S1) or second (S2) electronic excited state is optically pumped and then populated for times suitably long to perform nonresonant measurements of γSn(-ω4; ω1, –ω2, ω3) at frequencies different from the resonant pump frequency. In this study, we report the first experimental observation of excited state enhancement of the degenerate four wave mixing (DFWM) susceptibility γSn(-ω; ω, –ω, ω) of a conjugated linear chain, diphenylexatriene (DPH), when the first π-electron excited state is populated for nanosecond timescales and then probed nonresonantly through picosecond DFWM.