Excited-State Dynamics in a Dyad Comprising Terpyridine-Platinum(II) Ethynylene Linked to Pyrrolidino-[60]Fullerene

Abstract

A hybrid [Pt((t)Bu(3)terpy)(C[triple bond]C-Ph-C(60))](+) complex (Pt-Fu) wherein a phosphorescent platinum center is linked to fullerene has been prepared using a copper(I)-promoted cross-coupling reaction. The electrochemical and spectroscopic properties were understood in light of the properties of the isolated building blocks and references compounds, Pt and Fu. In particular, in Pt-Fu, the electrochemical studies revealed that the first reduction process is fullerene-based and that the lowest-energy Pt(+)-Fu(-) charge-separated (CS) state lies in the range 2.0-2.1 eV. The luminescence properties of the investigated species have been monitored in a CH(2)Cl(2) solvent at room temperature and in a MeOH/EtOH (1:4 v/v) glassy solution at 77 K. Upon excitation at 450 nm at room temperature and in air-free solvent, Pt displays an intense luminescence of (3)MLCT nature, with lambda(max) = 605 nm (523 nm at 77 K, corresponding to 2.37 eV), phi(em) = 0.013, and tau(em) = 920 ns. Under the same conditions, Fu exhibits the typical (1)C(60) fluorescence, with lambda(max) = 708 nm (703 nm at 77 K, corresponding to 1.76 eV), phi(em) = 6.0 x 10(-4), and tau(em) = 1.2 ns. For Pt-Fu, room-temperature excitation at 450 nm yields Pt*- and Fu*-centered excited states in a 1.2:1 proportion. However, no Pt-based emission is observed, and (i) in an air-free solvent, (1)Fu fluorescence is observed, while (ii) in an air-equilibrated solvent, singlet oxygen sensitization by the (3)Fu level takes place. Very close (1)O(2)* fluorescence intensities are observed at 1278 nm for isoabsorbing solutions at 450 nm of Fu and Pt-Fu, consistent with complete Pt --> Fu energy transfer in the dyad. The room-temperature nanosecond transient absorption spectra for Pt-Fu and Fu exhibit peaks at 680 and 690 nm with tau(TA) = 14.3 and 24.8 micros, respectively; in both cases, these are attributed to absorption by the fullerene triplet. By contrast, no CS species, Pt(+)-Fu(-), are detected. The Pt --> Fu energy transfer is discussed, and the rate constant for the (3)Pt-Fu --> Pt-(1)Fu step is evaluated, k(en) > 10(7) s(-1).