Aggregation and photobleaching of merocyanine 540 as probed by resonance light scattering

Merocyanine 540 (MC540) aggregation induced by adding KCl has been studied by resonance light scattering (RLS) and absorption spectrophotometry. In water, MC540 exists as H-dimers and monomers with absorption maxima at ～500 and 535 nm, respectively (these species lack RLS). Upon the introduction of the salt in concentrations below 0.15 M, the spectrum undergoes a hypochromic effect, without modification of its general shape. In addition to the hypochromic effect, a new broad RLS band emerges at ～420–460 nm, which arises from large H-aggregates of the dye. The formation of these aggregates does not manifest itself in absorption spectra. At KCl concentrations above the critical one (0.15 M), a new absorption band at ～517 nm emerges; simultaneously, a strong RLS band of a similar shape appears, which proves the formation of large supramolecular aggregates of MC540. Comparison of the spectrophotometry and RLS data shows that large aggregates of MC540 are more photolabile than dimers and monomers.