Trans/cis-isomerization of fluorene-bridged azo chromophore with significant two-photon absorbability at near-infrared wavelength

Azo-containing materials have been proven to possess second-order nonlinear optical (NLO) properties, but their third-order NLO properties, which involves twophoton absorption (2PA), has rarely been reported. In this study, we demonstrate a significant 2PA behavior of the novel azo chromophore incorporated with bilateral diphenylaminofluorenes (DPAFs) as a π framework. The electrondonating DPAF moieties cause a redshifted π-π∗ absorption band centered at 470 nm, thus allowing efficient blue-lightinduced trans-to-cis photoisomerization with a rate constant of 2.04×10^-1 min^-1 at the photostationary state (PSS). The open-aperture Z-scan technique that adopted a femtosecond (fs) pulse laser as excitation source shows an appreciably higher 2PA cross-section for the fluorene-derived azo chromophore than that for common azobenzene dyes at near-infrared wavelength (λ_ex=800 nm). Furthermore, the fs 2PA response is quite uniform regardless of the molecular geometry. On the basis of the computational modeling, the intramolecular charge-transfer (ICT) process from peripheral diphenylamines to the central azo group through a fluorene p bridge is crucial to this remarkable 2PA behavior.