Photoprocesses of thiamonomethinecyanine monomers and dimers

Abstract

The photophysical properties and the monomer–dimer equilibrium of four thiamonomethinecyanine dyes (1–4) were studied by time-resolved and steady-state spectroscopic methods. In ethanol and aqueous solution at room temperature the behaviour is mainly due to a monomer and dimer, respectively. The dimerization constant for 2, the triethylammonium salt of 3,3′-di-(γ-sulfopropyl)-4,5-benzo-5′-chlorothiamonomethinecyaninebetaine, in aqueous solution is K_D = 7 × 10⁵ M⁻¹ and lower in the presence of ethanol. Fluorescence occurs from both monomer and dimer; the quantum yield is Φ_f⩽0.006 for 1–4 in ethanol and 10–20 times higher in aqueous solution. The triplet state of the dimers was characterized; the lifetime is 0.1–0.9 ms and the yield ⩽0.2. With increasing temperature the amount of dimer is reduced, resulting in a correspondingly smaller value for T–T absorption. In microheterogeneous media, e.g. cetyltrimethylammonium bromide (CTAB), sodium dodecyl sulfate (SDS) or Triton X-100, the equilibrium is shifted at 25°C towards the solubilized monomer above the critical micellization concentration (c.m.c). The CTAB concentrations for dimer deaggregation of 1, 3 and 4 are much lower (10–30 μM) than the c.m.c. The amount of T–T absorption decreases with the CTAB concentration and resembles that of the dimer ground state. For 2 the dimers are converted into J-aggregates at [CTAB] in the 10–20 μM range and split into monomers around the c.m.c.