RAMAN EXCITATION-SPECTRA OF COUPLED INTRAMOLECULAR INTERMOLECULAR VIBRONIC MODES OF AGGREGATED 4,4'-CYANINE

We show that excitation wavelength behavior of the resonance Raman enhanced band of 4,4'-cyanine attributed to intramolecular-intermolecular coupling, when referenced to bands attributed to intramolecular modes, provides evidence for the presence of aggregated molecules as well as, through the position of an excitation maximum, the frequency at which the aggregate lattice mode has the strongest "coupling" with the particular intramolecular mode. The frequencies of the excitation maxima are related to the frequency at which peak absorption would appear in the absorption spectrum, and it is advanced that for the case of 4,4'-cyanine adsorbed onto a silver electrode at room temperature, the aggregate absorption maximum is near 650 nm. The latter determination is not possible using absorption and emission spectroscopies under the given solution conditions.