Cooperative self-assembly of squaraine dyes

Abstract

A novel, highly soluble squaraine dye was synthesised in organic solvents by condensation of squaric acid with an appropriately functionalised donor unit and the aggregation behaviour of this squaraine in organic solvents was investigated in detail by concentration- and temperature-dependent UV/vis spectroscopy. The absorption band of this squaraine dye exhibits a hypsochromic shift of more than 100 nm upon aggregation, indicating a H-type excitonic coupling in aggregates. Our detailed studies reveal a cooperative aggregation process with K₂ = 4.87 ± 0.13 × 10⁴ M⁻¹ for the dimerization and an elongation constant of K = 4.87 ± 0.13 × 10⁵ M⁻¹ according to the K₂/K model. The thermodynamics of this process were investigated by applying Meijer's model of nucleation–elongation growth, which provided an elongation temperature of T_e = 311 K with a molecular enthalpy release of h_e = −97 kJ mol⁻¹ and a nucleus size of about two. Furthermore, the structural features of the aggregates were elucidated by AFM and spectroscopic methods, revealing the formation of extended supramolecular fibres.