Strain and hückel aromaticity: driving forces for a promising new generation of electron acceptors in organic electronics.

The tangible possibility of fabricating flexible, lightweight organic photovoltaic devices (OPVs) by using roll-to-roll coaters, similar to those used in the production of print magazines and newspapers, renders this technology a valid alternative to expensive crystalline silicon photovoltaic cells. The most widely used active layer for these OPVs, the so-called bulk heterojunction (BHJ), 5] is based on photoinduced charge transfer from an electron-donating material, such as a light-absorbing and hole-conducting polymer, to an electron-accepting component, typically fullerene[60] and its derivative 1-(3-methoxycarbonyl) propyl-1phenyl-[6,6]-C61 ([C60]PCBM). [6, 7] Several research groups have reported a wide range of new polymeric donor structures that absorb light over a broad wavelength range, and have a narrow energy gap and increased charge transport and collection at the electrode. However, there have been fewer reports on new structures of acceptor components that do not contain fullerene derivatives. C60 and C70 PCBMs are currently considered the most successful acceptor architectures, despite only slight improvements when modifying these functionalized fullerenes. For example, the insertion of electron-donating groups on the phenyl ring of the [C60]PCBM to tune the lowest unoccupied molecular orbital (LUMO) energy levels improved the open-circuit voltage (Voc), while maintaining a relatively unchanged efficiency. Furthermore, [C70]PCBM, which absorbs a wider range of wavelengths than [C60]PCBM, [16] was employed with low-band-gap polymers such as poly[2,6-(4,4-bis-(2-ethylhexyl-4H-cyclopenta[2,1b;3,4b’]-dithiophene)-alt-4,7-(2,1,3-benzothiadizole)] (PCPDTBT), to broaden the photocurrent spectral range. Although encouraging photocurrent and photovoltage values were obtained, a low overall efficiency, which arises from loss mechanisms, was observed. Despite the wide use of these fullerene derivatives, the synthesis of new acceptors with energy levels significantly different from those of current C60 derivatives, and wide versatility in terms of derivatization and functionalization is urgently required. Herein, we report the inherent potential of a new generation of acceptor compounds based on the 9,9’-bifluorenylidene (99’BF) backbone. 99’BF could be considered a tetrabenzofulvalene with an atom numbering that reflects fluorene linked by a double bond between the 9 and 9’ carbon atoms. In the ground state, 99’BF is forced to be coplanar because of the presence of the double bond, but the repulsive interaction between the H1– H1’ and H8–H8’ protons twists the structure of the dimer. The addition of one electron across the C9–C9’ bond is highly favorable for two main reasons: steric (“twist”) strain relief and gain in aromaticity to a 14-p-electron system (Scheme 1).