Synthesis and spectroscopic properties of phthalocyanine-[60]fullerene conjugates connected directly by means of a four-membered ring

New covalently C ₆₀-conjugated phthalocyanine (Pc) analogues in which the Pc and C ₆₀ components are connected by means of a four-membered ring have been synthesized by taking advantage of a [2+2] cycloaddition reaction of C ₆₀ with benzyne units generated from either a phthalocyanine derivative (8) or its precursor (1). The reaction of 1 with PhI(OAc) ₂ and trifluoromethanesulfonic acid (TfOH) followed by the [2+2] cycloaddition of C ₆₀ in the presence of tetra-n-butylammonium fluoride (TBAF) yielded the C ₆₀-substituted Pc precursor (3). Mixed condensation of 3 and 4,5-dibutylsulfonylphthalonitrile (4) in a thermally promoted template reaction using a nickel salt successfully gave the Pc-C ₆₀ conjugate (5). Results of mass spectrometry and ¹H and ¹³C NMR spectroscopy clearly indicate the formation of the anticipated Pc-C ₆₀ conjugate. Direct coupling of C ₆₀ with the Pc analogue that contained eight peripheral trimethylsilyl (TMS) groups (8) also proceeded successfully, such that mono and bis C ₆₀-adducts were detected by their mass, although the isolation of each derivative was difficult. The absorption and magnetic circular dichroism (MCD) spectra of 5 and the reference compound (7) differ from each other in the Q-band region, thereby suggesting that the presence of the C ₆₀ moiety affects the electronic structure of the conjugate. The reduction and oxidation potentials of 5 and 7 obtained by cyclic voltammetry are comparative, except for the C ₆₀-centered reduction couple at -1.53 V versus Fc ⁺/Fc in o-dichlorobenzene (o-DCB). A one-electron reduction of 5 and 7 in tetrahydrofuran (THF) by using the sodium mirror technique results in the loss of band intensity in the Q-band region, whereas the characteristic marker bands for Pc-ring-centered reduction appear at around 430, 600, and 900 nm for both compounds. The final spectral shapes of 5 and 7 upon the reduction resemble each other, thus indicating that no significant molecular orbital (MO) interactions between the C ₆₀ and Pc units are present for the reduced species of 5. In contrast, the oxidized species of 5 and 7 generated by the addition of NOBF ₄ in CH ₂Cl ₂ show significantly different absorption spectra from each other. Whereas the broad bands at approximately 400-550 nm of 7 ⁺ are indicative of the cationic π-radical species of metallo-Pcs and can be assigned to a transition from a low-lying MO to the half-filled MO, no corresponding bands were observed for 5 ⁺. These spectral characteristics have been tentatively assigned to the delocalized occupied frontier MOs for 5 ⁺. The experimental results are broadly supported by DFT calculations.