Tetraazachlorin-fullerene conjugates: on-off control of electronic communication enabled by push-pull substituents

Novel tetraazachlorin (TAC)-fullerene (C60) conjugates (TAC-C60) and their analogues (TAiBC-C60 and TABC-C60 where TAiBC = tetraazaisobacteriochlorin and TABC = tetraazabacteriochlorin) have been synthesized by condensing 1,2-dicyanofullerene (1) and phthalonitrile derivatives (2) in the presence of nickel chloride in quinoline, and fully characterized using mass spectrometry and 1H and 13C NMR. By arranging the TAC and C60 units at the minimum distance, and taking also into account the molecular symmetry, the resultant conjugates show on-off electronic communication behavior, depending on the push-pull properties of the peripheral substituents in the TAC moiety. Consequently, the UV-vis absorption spectrum of the electron-releasing butyloxy-substituted TAC-C60 (3a) contains an unusual group of three absorption bands in the Q-band region (500-900 nm) as a result of a strong electronic communication between the two moieties. On the other hand, the absorption spectrum of the electron-withdrawing butylsulfonyl-substituted TAC-C60 (3b) comprises a typically normal TAC spectrum with markedly split two-peak Q-bands. A similar phenomenon is observed between alkoxy-substituted TAiBC-C60 (4a) and butylsulfonyl-substituted TAiBC-C60 (4b). This study reveals that the electron-donating or -withdrawing nature of the peripheral substituents on an azachlorin moiety has an important effect on the electronic structures of our novel azachlorin-C60 conjugates, although the linking carbon atoms are aliphatic sp3 carbon atoms which generally do not contribute to aromaticity. The electronic structures of the conjugates have been investigated in detail using spectroscopic and electrochemical techniques with the aid of DFT calculations.