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

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 C-60 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 H-1 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 x 10(10) s(-1)), tentatively attributed to an energy transfer mechanism. The bimolecular quenching constant of the lowest tripler excited state of ZnTPP by PC60 was determined aia a Stern-Volmer analysis (k(q) = 6.7 x 10(9) s(-1)).