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

Abstract

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.