A series of photoactive dyads bearing pyrene and metal (M Ru II ) or Os II ) tris(2,2'-bipyridine) terminals bridged by an ethynylene or Pt II bis(sacetylide) moiety has been synthesized and investigated by transient spectroscopy. Selective excitation into the terminal metal complex is possible in each case and generates the lowest energy, excited triplet state localized on that molecular fragment. For both Os II -based dyads, the triplet state is unperturbed by the appended pyrene unit and the observed photophysical properties can be understood within the framework of the energy-gap law. The triplet state localized on the metal complex in the two Ru II -based dyads is involved in reversible energy transfer with the triplet associated with the pyrene unit, which is situated at slightly lower energy. When the terminal metal complex is a Ru II bis(2,2':6',2''-terpyridyl) fragment, however, the triplet levels are inverted such that the pyrene-like triplet state lies slightly above that of the metal complex. Kinetic spectrophotometry has allowed determination of the various rate constants and energy gaps and, on the basis of nonadiabatic electron-transfer theory, it appears that the central Pt bis(sacetylide) unit is a much inferior electronic conductor than is a simple ethynylene group. Reversible energy transfer of this type greatly prolongs the triplet lifetime of the Ru II tris(2,2'-bipyridyl) fragment. For example, equilibration between the triplet states is achieved within 10 ps for the ethynylenebridged dyad while the equilibrium mixture decays with a lifetime of about 40 ms in deoxygenated acetonitrile at room temperature.

Grafting an ethynylated-pyrene moiety to a Ru(ii) or Os(ii) polypyridine complex perturbs the photophysical properties of the metal fragment and, when the relevant energy levels are properly balanced, provides a 115-fold prolongation of the triplet lifetime.

Long‐range interactions over 50 Å allow electron exchange to proceed in a series of conformationally constrained Ru/Os polypyridine complexes (see picture). The dynamics of the intramolecular electron exchange indicate that the connecting 1,4‐diethynylated‐2,5‐dialkoxybenzene units act independently as superexchange mediators. Even with five such units linked together there is no indication for the population of a connector‐based triplet state.

In this paper we report a simple and effective method to build up self-assembled and wellcalibrated layers of plasmid DNA • 1,3-diaminopropane complexes onto Highly Oriented Pyrolitic Graphite (HOPG). The method is based on the self-assembly of the poly-electrolytes onto HOPG in an excess of positively-charged protonated di-amines (Dap 2+) in comparison to the negatively-charged phosphate moieties of the DNA backbone in solution. Although short distortions in the helical parameters of DNA (maximum 12% hypochromicity) are revealed by UV-Vis absorption spectrometry, the native B form of the plasmids is conserved. By fixing the excess of positive charges arising from Dap 2+ cations, it is possible to construct assemblies of a well-defined thickness ranging typically from 1 monolayer (ML) of DNA to 10 ML; 1 ML has a thickness of 2.2 ± 0.5 nm. Adding TRIS-EDTA (TE) buffer lowers considerably the damage rate observed when plasmids are mixed to Dap 2+ in pure water. The thickness of the first dense mono-layer matches well the DNA cross-sectional dimensions indicating that this layer is strongly anchored to the surface; it is insoluble in water. Conversely, thicker layers can be released in aqueous media and the plasmids do not undergo dramatic damage. In presence of TE buffer condensation of the plasmids on the HOPG surface and a further release of the deposits in water yields a loss of supercoiling that ranges typically from 10 to 20% when the layer thickness varies from 22 to 12 nanometers. Such densely-packed and releasable DNA plasmid layers with a very well characterized and constant thickness constitute a substantial progress for biochemical and radiochemical experiments.