PHOTOPHYSICS OF OXACYANINE DYES ON SURFACES - REEXAMINATION OF THE ORIGINS OF THE NEW EMISSION OBSERVED WITH LASER EXCITATION AND HIGH-CONCENTRATIONS OF ADSORBED DYES

Two oxacarbocyanine dyes, 3,3'-diethyloxacarbocyanine iodide (DOCI) an d 3,3'-diethyloxadicarbocyanine iodide (DODCI), have been investigated spectroscopically when adsorbed on microcrystalline cellulose at a ra nge of concentrations from 0.005 to 25 mu mol g(-1). DOCI shows both H and J aggregation in its ground-state absorption spectrum, while DODC I shows only H aggregation. The degree of aggregation is dependent on the degree of hydration of the samples, decreasing with decreasing hyd ration. Steady-state emission spectra show the normal ground-state emi ssion, 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 l aser 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.