Practical designed syntheses of all stereoisomeric bis(2,2')- and tris(2,2',2' ')-tetrahydrofurans

A convenient synthetic entry to the entire group of bis(2,2')- and tris(2,2',2' ')-tetrahydrofurans has been developed. The method is concise and relies on chromatographic separation, decarbonylation, and annulation to arrive at a specific product of defined relative configuration. [reaction: see text]     

Intramolecular energy-transfer processes in a bis(porphyrin)-ruthenium(II) bis(2,2':6',2'-terpyridine) molecular array

A molecular triad has been synthesized comprising two free-base porphyrin terminals linked to a central ruthenium(II) bis(2,2':6',2'-terpyridine) subunit via meso-phenylene groups. Illumination into the ruthenium(II) complex is accompanied by rapid intramolecular energy transfer from the metal-to-ligand, charge-transfer (MLCT) triplet to the lowest-energy pi-pi* triplet state localized on one of the porphyrin subunits. Transfer takes place from a vibrationally excited level which lowers the activation energy. The electronic coupling matrix element for this process is 73 cm(-1). Selective illumination into the lowest-energy singlet excited state (S1) localized on the porphyrin leads to fast singlet-triplet energy transfer that populates the MLCT triplet state with high efficiency. This latter process occurs via Dexter-type electron exchange at room temperature, but the activation energy is high and the reaction is prohibited at low temperature. For this latter process, the electronic coupling matrix element is only 8 cm(-1).     

Photophysical properties of closely-coupled, binuclear ruthenium(II) bis(2,2':6',2''-terpyridine) complexes

The photophysical properties of closely-coupled, binuclear complexes formed by connecting two ruthenium(II) bis(2,2':6',2'-terpyridine) complexes via an alkynylene group are compared to those of the parent complex. The dimers exhibit red-shifted emission maxima and prolonged triplet lifetimes in deoxygenated solution. Triplet quantum yields are much less than unity and the dimers generate singlet molecular oxygen with low quantum efficiency. Temperature dependence emission studies indicate coupling to higher-energy triplet states while cyclic voltammetry shows that the metal centres are only very weakly coupled but that extensive electron delocalization occurs upon one-electron reduction. The radiative rate constants derived for these dimers are relatively low, because the lowest-energy metal-to-ligand, charge-transfer states possess increased triplet character. In contrast, the rate constants for nonradiative decay of the lowest-energy triplet states are kept low by extended electron delocalization over the polytopic ligand. The poor triplet yields are a consequence of partitioning at the second triplet level.     

Reversible luminescence switching in a ruthenium(II) bis(2,2':6',2'-terpyridine)-benzoquinone dyad

An electroactive luminescent switch has been synthesized that comprises a hydroquinone-functionalized 2,2':6',2'-terpyridine ligand coordinated to a ruthenium(II) (4'-phenylethynyl-2,2':6',2'-terpyridine) fragment. The assembly is sufficiently rigid that the hydroquinone-chromophore distance is fixed. Excitation of the complex via the characteristic metal-to-ligand charge-transfer (MLCT) absorption band produces an excited triplet state in which the promoted electron is localized on the terpyridine ligand bearing the acetylenic group. The triplet lifetime in butyronitrile solution at room temperature is 46 +/- 3 ns but increases markedly at lower temperature. Oxidation of the hydroquinone to the corresponding benzoquinone switches on an electron-transfer process whereby the MLCT triplet donates an electron to the quinone. This reaction reduces the triplet lifetime to 190 +/- 12 ps and essentially extinguishes emission. The rate of electron transfer depends on temperature in line with classical Marcus theory, allowing calculation of the electronic coupling matrix element and the reorganization energy as being 22 cm(-1) and 0.84 eV, respectively. The switching behavior can be monitored using luminescence spectroelectrochemistry. The on/off level is set by temperature and increases as the temperature is lowered.     

Competition between energy transfer and interligand electron transfer in porphyrin-osmium(II) bis(2,2':6',2' '-terpyridine) dyads

Rapid intramolecular energy transfer occurs from a free-base porphyrin to an attached osmium(II) bis(2,2':6',2' '-terpyridine) complex, most likely by way of the F?rster dipole-dipole mechanism. The initially formed metal-to-ligand, charge-transfer (MLCT) excited-singlet state localized on the metal complex undergoes very fast intersystem crossing to form the corresponding triplet excited state ((3)MLCT). This latter species transfers excitation energy to the (3)pi,pi* triplet state associated with the porphyrin moiety, such that the overall effect is to catalyze intersystem crossing for the porphyrin. Interligand electron transfer (ILET) to the distal terpyridine ligand, for which there is no driving force, competes poorly with triplet energy transfer from the proximal (3)MLCT to the porphyrin. Equipping the distal ligand with an ethynylene residue provides the necessary driving force for ILET and this process now competes effectively with triplet energy transfer to the porphyrin. The rate constants for all the relevant processes have been derived from laser flash photolysis studies.     

Comparison of the photophysical properties of osmium(II) bis(2,2':6',2' '-terpyridine) and the corresponding ethynylated derivative

The photophysical properties of osmium(II) bis(2,2':6',2' '-terpyridine) have been recorded over a wide temperature range. An emission band is observed and attributed to radiative decay of the lowest-energy metal-to-ligand, charge-transfer (MLCT) triplet state. This triplet is coupled to two other triplet states that lie at higher energy. The second triplet, believed to be of MLCT character, is reached by crossing a barrier of only 640 cm(-1), but the highest-energy triplet, considered to be of metal-centered (MC) character, is separated from the lowest-energy MLCT triplet by a barrier of 3500 cm(-1). Analysis of the emission spectrum shows that both low- and high-frequency modes are involved in the decay process, while weak emission is seen from the second excited triplet state. The magnitude of the low- and high-frequency modes depends on temperature in fluid solution but not in a KBr disk. Apart from a substantial lowering of the triplet energy, the photophysical properties are relatively insensitive to the presence of an ethynylene substituent at the 4' position of each terpyridine ligand. However, the barrier to reaching the MC triplet is markedly reduced, and the vibrational modes become less sensitive to changes in temperature.