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

Two photoactive dyads bearing pyrene and 2,2'-bipyridine terminals bridged by an ethynylene or a Pt(ii) bis(sigma-acetylide) moiety have been studied by fluorescence and transient absorption spectroscopy following excitation into the pyrene unit. Pyrene-like fluorescence and phosphorescence are obse rved for the Pt(ii) bis(sigma-acetylide)-bridged dyad but emission lifetime s are shortened by the internal heavy-atom effect. There is a modest thermo dynamic driving force for electron transfer from the pyrene-like excited si nglet state to the appended 2,2'-bipyridine but this process does not compe te effectively with intersystem crossing to the triplet manifold. Connectin g the terminals by a single ethynylene group promotes intramolecular charge transfer from pyrene to 2,2'-bipyridine and perturbs both absorption and f luorescence spectra. In polar solvents pyrene-like fluorescence is not obse rved and the lowest-energy excited singlet state contains appreciable charg e-transfer character, whereas in nonpolar solvents pyrene-like pi,pi* fluor escence is seen. The intramolecular charge-transfer state is relatively lon g-lived in acetonitrile and decays to form the pyrene-like pi,pi* triplet s tate in quantitative yield. In fact, direct charge recombination to the gro und 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 a re discussed in terms of the ability of the Pt(ii) bis(sigma-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 excite d singlet state in both dyads.