Nonlinear optical spectroscopy of new squaraine derivatives with high potential for multidisciplinary applications.

New symmetrical anilinosquaraine derivatives with four hydroxyl groups in the molecular core, namely, 2,4-bis[4-[4-(3,5-didecoxyphenoxy)butyl-methyl-amino]-2,6-dihydroxy-phenyl]squaraine (1) and 2,4-bis[4-[6-(3,5-didecoxyphenoxy)hexyl-methyl-amino]-2,6-dihydroxy-phenyl]squaraine (2), were synthesized. Both compounds were found to exhibit manifold linear spectroscopic and nonlinear optical properties with high potential for a number of important multidisciplinary applications. Linear photophysical and photochemical properties of 1 and 2 were assessed in liquid solvents of different polarities at room temperature, including steady-state absorption, emission, and excitation anisotropy spectra, along with fluorescence quantum yields and lifetimes. The fast intramolecular relaxation processes in 1 and 2 were analyzed using femtosecond transient absorption pump-probe spectroscopy, and unusually strong optical amplification (gain) was observed in their narrow fluorescence contours without any evidence of solvent reorganization phenomena. The degenerate two-photon absorption (2PA) spectra of 1 and 2 were measured in a broad spectral range, and a maximum 2PA cross-section of ≈2000 GM was observed. The effect of the superluminescence of 1 and 2 was observed under femtosecond pumping, which extended their potential for photonic applications.