Photophysics of oxacyanine dyes on surfaces. Re-examination of the origins of the ‘new emission’ observed with laser excitation and high concentrations of adsorbed dyes

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Abstract
Two oxacarbocyanine dyes, 3,3′-diethyloxacarbocyanine iodide (DOCI) and 3,3′-diethyloxadicarbocyanine iodide (DODCI), have been investigated spectroscopically when adsorbed on microcrystalline cellulose at a range of concentrations from 0.005 to 25 µmol g–1. DOCI shows both H and J aggregation in its ground-state absorption spectrum, while DODCI shows only H aggregation. The degree of aggregation is dependent on the degree of hydration of the samples, decreasing with decreasing hydration. Steady-state emission spectra show the normal ground-state emission, distorted at higher concentrations by absorption by aggregated dye molecules. Laser-excited fluorescence spectra for DOCI show no new features, but for DODCI at higher concentrations and laser fluences a second emission band is observed which shows a square dependence on laser fluence. A similar band can be excited in solution and shows very similar behaviour and is assigned, on this basis, as arising from an emissive photoisomer.