Photophysical properties of pyrene-(2,2′-bipyridine) dyads

Abstract

Two photoactive dyads bearing pyrene and 2,2′-bipyridine terminals bridged by an ethynylene or a Pt(II) bis(σ-acetylide) moiety have been studied by fluorescence and transient absorption spectroscopy following excitation into the pyrene unit. Pyrene-like fluorescence and phosphorescence are observed for the Pt(II) bis(σ-acetylide)-bridged dyad but emission lifetimes are shortened by the internal heavy-atom effect. There is a modest thermodynamic driving force for electron transfer from the pyrene-like excited singlet state to the appended 2,2′-bipyridine but this process does not compete effectively with intersystem crossing to the triplet manifold. Connecting the terminals by a single ethynylene group promotes intramolecular charge transfer from pyrene to 2,2′-bipyridine and perturbs both absorption and fluorescence spectra. In polar solvents pyrene-like fluorescence is not observed and the lowest-energy excited singlet state contains appreciable charge-transfer character, whereas in nonpolar solvents pyrene-like π,π* fluorescence is seen. The intramolecular charge-transfer state is relatively long-lived in acetonitrile and decays to form the pyrene-like π,π* triplet state in quantitative yield. In fact, direct charge recombination to the ground state is extremely slow in this system, despite the close proximity of the reactants and the fact that they are connected by a highly-conductive “spacer’' group, because of a negligible Franck–Condon factor. The results are discussed in terms of the ability of the Pt(II) bis(σ-acetylide) to function as an insulator. Coordination of a metal centre to the vacant 2,2′-bipyridine opens up a new route for deactivation of the pyrene-like excited singlet state in both dyads.