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 been synthesized. The formation, structure, and excited-state properties of the consequent merocyanine dye "aggregates" in solutions and in rigid glass have been studied by UV/visible absorption spectra, steady-state fluorescence, and fluorescence lifetime measurements. The dimers with 0 and 1 methylene spacers must exist in more-or-less extended conformations; consequently, they show a very weak and distance-dependent "J"-type exciton coupling, evident in both absorption and fluorescence spectra. For the dimers with 2, 3, and 5 methylene spacers, absorption spectra in nonpolar solvents are consistent with a largely extended configuration and little or no evident exciton coupling. However, for more polar solvents, a blue shifted absorption spectrum is observed, suggesting a folded configuration resulting in an "H" dimer. For the latter dimers, fluorescence spectra in a variety of solvents show a pronounced redshift, which is attributed to a folded "excimer". As anticipated from its structure, the merocyanine monomer shows a weak positive solvatochromic effect that may be correlated using the Taft-Kamlet pi* parameter. Remarkably, both the "J" coupled dimer (n = 0) and the dimers having 2-5 methylene groups show a much stronger solvatochromic behavior than the monomer. The strong solvatochromic effects in these tethered dimers may be attributed to interchromophoric charge-transfer interactions in both ground and excited states.