Nanostructured assembly of porphyrin clusters for light energy conversion

Free base porphyrin molecules form well-defined and ordered nanoclusters in mixed solvents with absorption characteristics that significantly differ from those of the monomer form. These self-assembled crystallites in acetonitrile/toluene (9:1, v/v) can be deposited as thin films on a nanostructured TiO2 electrode using an electrophoretic technique. This porphyrin cluster assembly is highly photoactive and capable of undergoing charge separation under visible light excitation. Photoexcitation of the porphyrin film electrode assembly in a photoelectrochemical cell with visible light produces relatively high photocurrent generation. A maximum photocurrent of 0.15 mA cm(-2) and a photovoltage of 250 mV were attained using the I-3(-)/I- redox couple. The electron flow to the TiO2 electrode can be facilitated by application of a positive potential. An incident photon-to-photocurrent generation efficiency (IPCE) of 2.0% has been achieved at an applied bias potential of 0.2 V vs. SCE. The broad photoresponse of these crystallites throughout the visible range as well as the ease of assembling them on the electrode surface opens up new avenues for harvesting a wide wavelength range of solar light.