EFFECTS OF J-AGGREGATION ON THE REDOX LEVELS OF A CYANINE DYE

The effects of J-aggregation on the redox thermodynamics for a cationi c cyanine spectral sensitizing dye are investigated. When adsorbed to the surfaces of cubic AgBr microcrystals, both the monomeric and J-agg regate forms of the cyanine dye can be reversibly oxidized with redox solutions containing ferricyanide or molybdicyanide complex. Diffuse r eflectance spectra recorded for thin gelatin coatings of the dyed micr ocrystals after treatment with redox solution show distinct spectral b ands associated with the dye and a stable, monooxidized dye radical io n, The fraction of dye oxidized can be accurately varied by simple adj ustment of the electrochemical potential of the redox buffer solution. For the J-aggregated dye, the reflectance band for the dye shifts to shorter wavelengths and becomes significantly broadened with increasin g fractional degree of oxidation. The formal oxidation potential for t he adsorbed dye can be obtained from a Nernstian plot of redox-solutio n potential E vs log [oxidized dye]/[dye] as constructed from the refl ectance spectral data. The results indicate that the one-electron oxid ation potential of the monomer on cubic AgBr to be lower than that of the J-aggregate by 74 mV. The energy of the singlet excited state of t he J-aggregate is calculated to be lower than that of the monomer by a much larger amount. For the cationic dye in this study, the aggregati on-induced changes in redox potential result in dramatic differences i n the comparative photoresponses of the monomer and aggregate dye syst ems.