Exciton and charge-transfer interactions in nonconjugated merocyanine dye dimers: Novel solvatochromic behavior for tethered bichromophores and excimers

A series of merocyanine dye dimers tethered at different sites with spacers ranging from 0 to 5 methylene groups and the corresponding monomer have be en synthesized. The formation, structure, and excited-state properties of t he consequent merocyanine dye "aggregates" in solutions and in rigid glass have been studied by UV/visible absorption spectra, steady-state fluorescen ce, and fluorescence lifetime measurements. The dimers with 0 and 1 methyle ne spacers must exist in more-or-less extended conformations; consequently, they show a very weak and distance-dependent "J"-type exciton coupling, ev ident in both absorption and fluorescence spectra. For the dimers with 2, 3 , and 5 methylene spacers, absorption spectra in nonpolar solvents are cons istent with a largely extended configuration and little or no evident excit on coupling. However, for more polar solvents, a blue shifted absorption sp ectrum is observed, suggesting a folded configuration resulting in an "H" d imer. For the latter dimers, fluorescence spectra in a variety of solvents show a pronounced redshift, which is attributed to a folded "excimer". As a nticipated from its structure, the merocyanine monomer shows a weak positiv e solvatochromic effect that may be correlated using the Taft-Kamlet pi* pa rameter. Remarkably, both the "J" coupled dimer (n = 0) and the dimers havi ng 2-5 methylene groups show a much stronger solvatochromic behavior than t he monomer. The strong solvatochromic effects in these tethered dimers may be attributed to interchromophoric charge-transfer interactions in both gro und and excited states.