Lipophilic anilino squaraine dye as a novel superluminescent probe with efficient two-photon transition and high photostability

A new squaraine structure with large lipophilic fragments and four -OH groups in the molecular core, 2,4-bis[4-[4-[3-decoxy-5-(2-hexyldecoxy)phenoxy]butyl-[6-(3,5-didecoxyphenoxy)hexyl]amino]-2,6-dihydroxyphenyl]squaraine (1), was synthesized as an efficient near-infrared fluorescent probe for two-photon fluorescent microscopy applications. Linear photophysical and photochemical properties of 1 were investigated in liquid media at room temperature, including excitation anisotropy and photodecomposition quantum yields. The nature of the electronic structure and fast vibronic relaxation processes in 1 were revealed using the femtosecond transient absorption pump-probe spectroscopy technique. Large two-photon absorption (2PA) cross-sections of ∼400 GM were shown for 1 at ∼800 nm using the intensity-modulated femtosecond laser beam technique. Efficient superluminescence was observed in a concentrated dichloromethane solution of 1 under femtosecond transverse pumping. The high fluorescence quantum yield (∼0.85-0.95), 2PA cross section, and photochemical stability show the promising potential of 1 for applications in laser-scanning fluorescence microscopy.