Linking molecular structure with excited-state photochemical dynamics is crucial for engineering efficient organic solar cells and related photochemical applications. Two molecular dye systems are currently under investigation in our lab: carboalkoxyphenylporphyrins in solution-cast thin films and a new family of highly fluorescent thiazolothiazole viologen dyes. The singlet exciton diffusion lengths of solution-cast porphyrin thin films with various alkyl chain lengths have been examined. Modifications of peripheral solubilizing groups help orientate porphyrins in solution processed thin films and influence molecular orientation. The photoluminescent singlet decay lifetime of pristine porphyrin films and films lightly doped with [6,6]-phenyl-C61-butryic acid methyl ester (PCBM) were used in a 3D exciton diffusion Monte Carlo simulation to extract the exciton diffusion parameters and the nanocomposition. Longer alkyl chain derivatives yielded increased PL decay lifetimes and lengthened exciton diffusion lengths (LD) for octyl and hexyl containing porphyrin derivatives. GIWAXS and XRD data indicates that molecular organization is strongly dependent upon the peripheral carboalkoxy substituent, and that nematic molecular organization resulted in an increase in the exciton diffusion length. Our findings are an important step toward a deeper understanding of the exciton diffusivity and molecular packing relationship. We have also synthesized a class of extended viologen dye structures that incorporate a thiazolo[5,4-d]thiazole backbone. The dyes exhibit both reversible yellow to dark blue electrochromism and high fluorescence quantum efficiency that is deactivated upon electrochemical reduction. The fused bicyclic thiazolothiazole heterocycle allows the alkylated pyridinium groups to remain planar, strongly affecting their electrochemical properties. The electrochromic and strongly fluorescent properties make these materials attractive for molecular electronics, biomolecular sensing, and other photochemical applications.
