Synthesis of Carboxylated Viologen Cations and Their CT Salts with 1,3-Bis(dicyanomethylidene)Indan Anion

Abstract

Charge transfer (CT) salt by combination of ionic organic donor and acceptor can be an interesting alternative to eliminate large voltage loss in bulk-heterojunction organic solar cells. We have succeeded in synthesis of novel CT co-crystals of 1,3 bis(dicyanomethylidene)indane anion (TCNIH-) donor and N-alkyl substituted viologen cations acceptor to exhibit broad and intense light absorption from visible to NIR. However, it is not straightforward to employ heavily crystallizing organic salts in thin film devices. As an alternative approach to prove the concept, here we propose attachment of the CT pair as a dye to porous oxide semiconductor to dissociate its CT exciton and to harvest it as current in a dye sensitized solar cell (DSSC) fashion. In this work, carboxylated viologen derivatives, N,N’-bis(carboxymethyl)-4,4’-bipyridinium chloride (CE2V) and N-(2-carboxymethyl)N’-methyl-4,4’-bipyridinium iodide (CEMV) (Fig. 1) were synthesized for their attachment to porous ZnO. Their formation of CT salts with TCNIH- has also been studied. CE2V was synthesized by carboxylation of 4,4’-bipyridinium with acrylic acid and subsequently converted to chloride with hydrochloric acid. CEMV iodide was prepared by monomethylating 4,4’-bipyridinium with methyl iodide and carboxylation with acrylic acid. The synthesized viologen was mixed with TCNIH- in 1 : 2 ratio in water, filtrated and recrystallized in ethanol to identify the structure of the co-crystals. Mesoporous ZnO film prepared by doctor blading was sequentially soaked in ethanolic solution of CE2V or CEMV, and acetone/toluene solution of TCNIH-, to obtain ZnO/viologen/TCNIH hybrid structure in the end. The attachment of respective molecules was checked by UV-vis absorption both for the film samples and the ammonia solution dissolving the entire film. Formation of CT salts was confirmed by significant broadening of the absorption towards NIR as compared to that of the Na+ salt of TCNIH- (Fig. 1), so that the presence of the carboxylic acid anchor group in viologens do not disturb their intimate ionic interaction with TCNIH-. Triclinic single crystalline structure of the co-crystals was identified by XRD. CE2V and CEMV were adsorbed at high concentrations (500 μM and 500 μM, respectively) onto ZnO, so that the carboxylic acid group indeed acted as an anchor for their attachment to ZnO. The anionic TCNIH- in fact could directly be adsorbed onto ZnO to yield bluish films, whose spectrum, however, differed from those with pre-attached viologens (Fig. 1). Those of ZnO/viologen/TCNIH indicate clear broadening towards red, which can be attributed to the formation of CT pairs. Photoelectrochemical analysis in the presence of suitable redox system is expected to result in photocurrent from the CT state. Figure 1