Photoprocesses of thiamonomethinecyanine monomers and dimers

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-(gamma -sulfopropyl)-4,5-benzo-5'-chlorothiamonomethine- cyaninebetaine, in aqueous solution is K-D = 7 x 10(5) M-1 and lower in the presence of ethanol. Fluorescence occurs from both monomer and dimer; the quantum yield is Phi (f) less than or equal to 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 less than or equal to 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 degreesC 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 muM) 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 muM range and split into monomers around the c.m.c.