Langmuir-Blodgett films of a cationic zinc porphyrin-imidazole-functionalized fullerene dyad: Formation and photoelectrochemical studies

Electron donor-acceptor dyad ensembles of a water-soluble cationic zinc porphyrin (viz., zinc tetrakis(N-methylpyridinium)porphyrin tetrachloride, Zn(TMPyP)) and a C-60 derivative that bears an imidazole ligand (viz., 2-(phenylimidazolyl)fulleropyrrolidine, C(60)im) were assembled during the formation of Langmuir and then Langmuir-Blodgett (LB) films. Surface pressure versus surface area isotherms and surface pressure time profiles, as well as Brewster angle microscopic images documented that the Langmuir films formed were remarkably stable. Subsequently, these Langmuir films were transferred onto different solid substrates, by using the LB technique, for spectroscopic and photoelectrochemical characterization. The UV-vis spectroscopic investigations confirmed that the water-soluble Zn(TMPyP) was, indeed, transferred together with C(60)im in the LB films. Upon visible light illumination of these LB films, deposited on the ITO transparent conductive supports, a photocurrent generated in the C(60)im-Zn(TMPyP) system is ascribed to an efficient photoinduced electron transfer from the electron donor, porphyrin singlet excited-state to the electron acceptor, C-60. Overall, internal photon-to-current efficiency, IPCE, of the photoanodic current generation (with ascorbate as a sacrificial electron donor) in the ITO/C(60)im-Zn(TMPyP)/ascorbate/Pt construct is over 5x larger than that of the photocathodic system (with methyl viologen, MV2+, as a sacrificial electron acceptor) in the ITO/Zn(TMPyP)-C(60)im/MV2+/Pt construct. Highly ordered film stacking favors vectorial electron transfer within the dyad, giving rise to the highest IPCE values of 2.5% determined for a photoanode that was composed of around 20 monolayer films.