X-ray photoelectron, UV-vis absorption, and luminescence spectroscopic studies of 2,2'-cyanines adsorbed onto microcrystalline cellulose

1,1'-Diethyl-2,2'-cyanine iodide and 1,1'-diethyl-2,2'-carbocyanine iodide were adsorbed onto microcrystalline cellulose by two different methods: by deposition from ethanolic solutions, followed by solvent evaporation, and also from ethanolic solutions in equilibrium with the powdered solid. Within experimental error, both methods provided the same fluorescence quantum yield of the adsorbed dyes in the concentration range 0.01-5.0 mu mol of dye per gram of cellulose. Ethanol swells cellulose and some dye molecules become entrapped within the natural polymer chains and in close contact with the substrate. The use of dichloromethane, a solvent which does not swell microcrystalline cellulose, provides samples which exhibit a smaller fluorescence quantum yield. This is consistent with a larger degree of mobility (and also the formation of nonplanar and less emissive conformers) of the cyanines adsorbed on the surface of the solid substrate, while entrapment provides more rigid, planar, and emissive fluorophers. At the same time, the adsorption isotherms of 2,2'-cyanine on cellulose from alcoholic and dichloromethane solutions show that the specific cellulose surface area accessible for dye adsorption is larger when adsorption is from ethanol rather than from dichloromethane. For 2,2'-cyanine the fluorescence quantum yields (Phi(F)) determined were about 0.08 when dichloromethane (a solvent which does not swell cellulose) was used for sample preparation, while with ethanol Phi(F) was approximately 0.30. These values are about 3 orders of magnitude higher than those in solution, showing the importance of the rigid dry matrix in reducing the nonradiative pathways of deactivation of the (pi, pi*) first excited singlet state of this cyanine. X-ray photoelectron spectroscopic studies present evidence for hydrogen bonding of 2,2'-cyanine to cellulose at low loadings and for the formation of aggregates at higher loadings adsorbing from both ethanol and dichloromethane. This hydrogen bonding is assigned as involving dye molecules entrapped within the cellulose chains. On the other hand, for 2,2'-carbocyanine, evidence exists for an increase of hydrogen bonding with dye loading. This result together with evidence from ground-state diffuse reflectance absorption and luminescence is compatible with dye molecules being firmly bonded to the substrate by one of the nitrogen atoms, with the other unbound.