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 io dide were adsorbed onto microcrystalline cellulose by two different me thods: by deposition from ethanolic solutions, followed by solvent eva poration, and also from ethanolic solutions in equilibrium with the po wdered solid. Within experimental error, both methods provided the sam e 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 ce llulose and some dye molecules become entrapped within the natural pol ymer chains and in close contact with the substrate. The use of dichlo romethane, a solvent which does not swell microcrystalline cellulose, provides samples which exhibit a smaller fluorescence quantum yield. T his is consistent with a larger degree of mobility (and also the forma tion of nonplanar and less emissive conformers) of the cyanines adsorb ed on the surface of the solid substrate, while entrapment provides mo re rigid, planar, and emissive fluorophers. At the same time, the adso rption isotherms of 2,2'-cyanine on cellulose from alcoholic and dichl oromethane solutions show that the specific cellulose surface area acc essible for dye adsorption is larger when adsorption is from ethanol r ather than from dichloromethane. For 2,2'-cyanine the fluorescence qua ntum yields (Phi(F)) determined were about 0.08 when dichloromethane ( a solvent which does not swell cellulose) was used for sample preparat ion, while with ethanol Phi(F) was approximately 0.30. These values ar e about 3 orders of magnitude higher than those in solution, showing t he importance of the rigid dry matrix in reducing the nonradiative pat hways of deactivation of the (pi, pi) first excited singlet state of this cyanine. X-ray photoelectron spectroscopic studies present eviden ce for hydrogen bonding of 2,2'-cyanine to cellulose at low loadings a nd for the formation of aggregates at higher loadings adsorbing from b oth 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 c ompatible with dye molecules being firmly bonded to the substrate by o ne of the nitrogen atoms, with the other unbound.