A NEW CLASS OF LUMINESCENT ORGANOMETALLICS -DIETHYNYLRHODACYCLOPENTADIENES

Abstract

The main objective in this project is to develop a greater understanding of the unusual photophysical properties of 2,5-bis(arylethynyl)rhodacyclopentadienes. Three distinct and unusual photophysical properties were found in the 2,5-bis(arylethynyl)-rhodacyclopentadienes: (i) long-lived singlet excited states, from which some of them exhibit high-intensity fluorescence with nanosecond lifetimes; (ii) slow intersystem crossing rates (k values ≈ 108 s-1) compared to typical luminescent organometallic complexes (with k values ≈ 1012 s-1); and (iii) no phosphorescence was observed even at 77 K in a rigid glass. Many photophysical experiments such as e.g. low-temperature lifetime measurments, singlet oxygen sensitisation and time-resolved infrared (TRIR) have been carried out in order to investigate further and explain the unusual photophysical properties of this class of organometallic complexes.
Five different types of ligand X on 2,5-bis(p-R-arylethynyl)-X-rhodacyclopentadienes [X = 4-[4-(N,N-di-n-hexylamino)phenylethynyl]phenylethynyl- (DHAPEPE-), trimethyl silylethynyl- (TMSE-), methyl- (Me-), η2-benzoato- and acetylacetonato- (acac-)] have been synthesised and the photophysical properties of the complexes were investigated. The TMSE-rhodacyclopentadienes gave the highest fluorescence quantum yields compared to the other series of rhodacyclopentadienes. Extended phenylene-ethynylene ligands (i.e. DHAPEPE-) did not impart any effects on the max values in absorption and emission but the quantum yields were lower than those for the TMSE-rhodacyclopentadienes. η2-Benzoato- and acac- ligands shifted the max values in absorption and emission to lower energy, which implies that they induce smaller energy gaps between the excited and ground states. The emissions from the η2-benzoato-rhodacyclopentadienes were quenched (especially for those with R = H and SMe substituents, which have quantum yields of less than 0.01).
The first example of isomeric biphenyl-rhodacyclopentadiene by-product formation was found in the synthesis of acac-rhodacyclopentadienes. The isomeric biphenyl-rhodacyclopentadiene by-product with R = CO2Me was isolated and its molecular structure was confirmed by X-ray analysis. Its emission spectrum shows two emission bands with max values of 394 and 544 nm in degassed toluene solution. The fluorescent emission at 394 nm has a quantum yield of 0.03, whereas the phosphorescent emission at 544 nm has a quantum yield of 0.05. The unusual long lifetime (237.6 s) of the phosphorescence at room temperature indicates that the transition is from a ligand-centred (LC)   * transition.
In addition, the syntheses of 1,4-bis(p-R-phenyl)buta-1,3-diynes and novel 1,12-bis(p-R-phenyl)dodaca-1,3,9,11-tetraynes, which serve as the starting materials for the synthesis of the rhodacyclopentadienes, are also reported. Four novel 1,12-bis(p-R-phenyl)dodaca-1,3,9,11-tetraynes (where R = H, SMe, CO2Me and BMes2) have been synthesised and characterised. The formation of homo-coupling products was a major problem which reduced the yields of the 1,3,9,11-dodacatetraynes. The 1,3,9,11-dodacatetraynes were separated from their respective homo-coupling products using column chromatography, and the yields obtained were 30 – 46%.