Exploring the electronic structure of unsymmetrical squaraine dyes through synthesis and functionalization

Abstract

In this study, we report the synthesis of four unsymmetrical squaraines and their dicyanomethylene derivatives, incorporating hydroxyalkyl chains. Their electronic properties were systematically investigated in CHCl₃, EtOH and DMSO solutions using UV-Vis absorption and steady-state fluorescence spectroscopy. The dyes exhibit narrow absorption bands (631–689 nm), nonlinear solvent-dependent behavior, and a pronounced tendency to aggregate, as evidenced by blue-shifted absorption bands even at dilute concentrations. Introduction of a chalcogen atom induces a modest bathochromic shift of up to 14 nm, whereas substitution with dicyanomethylene groups produces a more pronounced shift of approximately 40 nm. In the excited state, these squaraines exhibit small Stokes shifts and solvent-dependent fluorescence quantum yields (0.02 to 0.2), consistent with their ground-state electronic behavior across different structural modifications. By combining experimental measurements with computational analyses, we provided a comprehensive understanding of the structural and electronic factors governing their photophysical properties. Importantly, we demonstrate that this series of unsymmetrical squaraines bearing hydroxyalkyl substituents can be further functionalized without compromising their intrinsic electronic and optical characteristics.