Photophysics of cyanine dyes on surfaces - A new emission from aggregates of 2,2'-cyanines adsorbed onto microcrystalline cellulose

Strong aggregation has been detected using diffuse reflectance ground-state absorption techniques to study both 1,1'-diethyl-2,2'-cyanine iodide and 1,1'-diethyl-2,2'-carbocyanine iodide adsorbed onto microcrystalline cellulose. For the former dye, H and J aggregates were formed, while for the latter only H aggregates were observed. The fluorescence quantum yields determined for both dyes adsorbed onto microcrystalline cellulose are about three orders of magnitude greater than those observed in ethanolic solutions being 0.30 for 2,2'-cyanine and 0.33 for the 2,2-carbocyanine. Laser-induced time-resolved absorption and emission spectra of these samples were obtained as a function of the laser fluence using the technique of diffuse-reflectance laser flash photolysis using a photomultiplier for detection for the absorption measurements and a diode array spectrometer with nanosecond time resolution for the emission experiments. A new fluorescence emission from molecular aggregates of the 1,1'-diethyl-2,-2'carbocyanine iodide was detected. This new emission was only observed when excitation was with high laser fluences and from concentrated samples (greater than or equal to 0.50 mu mol of the dye per gram of microcrystalline cellulose) where monomers and H aggregates coexist in the ground state. The new emission was absent in the case of 1,1'-diethyl-2,2'-cyanine iodide adsorbed onto microcrystalline cellulose, where the use of low or high fluences of the pulsed laser with low or high concentrations of this dye produced the same fluorescence spectrum as obtained with steady-state excitation.