Aggregation-enhanced Raman scattering of a cyanine dye in homogeneous solution

We have conducted Raman investigations on the cyanine dye 3,3'-dimethyl-9-phenyl-4,5:4',5'-dinaphthothia-carbocyanine (NTC) for which an enhanced Raman scattering occurs that is concomitant with the formation of aggregates, and for which resonance Raman and surface-enhanced Raman scattering can be excluded. The formation of aggregated molecules is determined through characteristic changes in absorption and fluorescence spectra. Also, fluorescence lifetime measurements for monomeric and aggregated NTC are reported and independently confirm aggregate formation. Off-resonance Raman measurements for NTC in various physical states confirm that molecular aggregation leads to an enhancement of Raman scattering of Vibrational modes of the monomeric species (i.e., intramolecular modes) and an even greater relative enhancement of certain modes of the molecules involved more intimately with formation of the aggregate (i.e., intermolecular modes). Additionally, we make empirical band assignments for NTC from which we are able to exclude structural distortion due to intermolecular interaction as the source for Raman band intensity enhancement.