Linear and nonlinear optical characterizations of a monomeric symmetricsquaraine-based dye in solution

摘要

The photophysical properties of a symmetric squaryllium dye, namely, 2,4-bis[4-(N,N-dibutyl-amino)-2-hydroxyphenyl] squaraine (SQ), in its monomer form in acetone solution, have beenthoroughly studied by means of one-photon absorption (1PA) and two-photon absorption (2PA),excitation anisotropy, fluorescence emission, fluorescence quantum yield, and excited stateabsorption. The results show that there is a strong one-photon allowed absorption band in the nearIR region associated with intramolecular charge transfer. Higher one-photon allowed and forbiddensinglet excited states were also revealed by absorption and excitation anisotropy. A relatively highfluorescence quantum yield (0.44) was measured for this dye. The nonlinear optical characterizationof SQ in solution confirms the ability of squaraine dyes to be used as good two-photon absorbers.Additionally, it was found that this dye presents both saturable and reverse saturable absorptioneffects. Density functional theory calculations of the 1 PA and 2PA electronic spectra of SQ werecarried out to support the experimental data. A detailed analysis of the symmetry and energy of theorbitals involved in the lowest five electronic transitions is presented and discussed in relation to thebehavior observed experimentally.