Synthesis and Investigation of New Solar Cell Photosensitizers Having a Fluorazone Backbone

A synthetic sequence for the preparation of fully conjugated, 2,7-disubstituted fluorazone (9H-pyrrolo[1,2-a]indol-9-one) derivatives was developed, comprising Elming-Clauson-Kaas type pyrrole formation, POCl3-mediated ring closure, selective halogenation and elongation of the conjugate backbone via cross-coupling reactions. As a proof of principle, such methodology was used to prepare for the first time two organic D-?-A dyes containing the fluorazone moiety. The new compounds displayed broad absorption of visible light when adsorbed on nanocrystalline TiO2 and electrochemical properties compatible with their employment as photosensitizers in dye-sensitized solar cells. Small-scale photovoltaic devices fabricated with the fluorazone dyes yielded power conversion efficiencies in the 2.1-2.4% range, corresponding to approx. 70% of the efficiency obtained with reference organic dye DF15 under the same conditions.