Photoinduced charge-transfer association of tetracyanoquinodimethane with aminobiphenyls

The molecular association of acceptors with electron donors is studied in the highly-polar solvent CH3CN. Tetracyanoquinodimethane (TCNQ) forms a stable charge-transfer complex with donor molecules such as 4-aminobiphenyl (4-AB), benzidine (BD) and 2-aminobiphenyl (2-AB) with high association constants. The complexes of TCNQ with 4-AB or BD show new absorption bands at around 800 and 500 nm, which can be identified as reduced TCNQ•- and TCNQ 2- species, respectively. These bands grow quickly upon photo-irradiation, implying that the charge-transfer complexes are easily formed in an excited state. Conversely, a small spectral manifestation of the charge transfer was observed in the case of 2-AB complex. It is demonstrated that the structural orientation between the geminate ion pairs could play an important role in building a stable complex.