A new pyridyl-substituted methanofullerene derivative. Photophysics, electrochemistry and self-assembly with zinc(II) meso-tetraphenylporphyrin (ZnTPP)

Abstract

A new methanofullerene derivative with a pyridine residue at the methano bridge (PC₆₀) has been synthesized. Its electrochemical and photophysical properties have been investigated in toluene solution along with those of the analogous fullerene mono-malonate adduct RC₆₀, which does not contain the pyridyl moiety. PC₆₀ and RC₆₀ display almost identical steady state absorption and luminescence properties and transient absorption spectra, as shown by means of nano- and picosecond laser flash photolysis. Their markedly different spectroscopic features with respect to plain C₆₀ are discussed, with the aid of semi-empirical calculations. PC₆₀ binds zinc(II) meso-tetraphenylporphyrin (ZnTPP) through coordination of the pyridyl moiety to the zinc ion giving, to the best of our knowledge, the first non-covalent assembly of a porphyrin–fullerene diad. The association constant of the 1:1 complex between PC₆₀ and ZnTPP has been determined at 300 K by ¹H NMR (K_a=3600±150 L mol^-1, C₆D₆ solution) and fluorescence titrations (K_a=3000±400 L mol^-1, toluene solution). Mixtures of RC₆₀ and ZnTPP have also been investigated, giving no evidence of association. Extremely fast ZnTPP luminescence quenching in the PC₆₀–ZnTPP complex was observed by time-resolved luminescence spectroscopy (k>5×10¹⁰ s^-1), tentatively attributed to an energy transfer mechanism. The bimolecular quenching constant of the lowest triplet excited state of ZnTPP by PC₆₀ was determined via a Stern–Volmer analysis (k_q=6.7×10⁹ s^-1).