TAY, MENG,GUAN (2010) A NEW CLASS OF LUMINESCENT ORGANOMETALLICS -DIETHYNYLRHODACYCLOPENTADIENES. Doctoral thesis, Durham University.
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%.
|Item Type:||Thesis (Doctoral)|
|Award:||Doctor of Philosophy|
|Keywords:||Rhodacyclopentadienes, fluorescence, phosphorescence, organometallics, photophysical properties|
|Faculty and Department:||Faculty of Science > Chemistry, Department of|
|Copyright:||Copyright of this thesis is held by the author|
|Deposited On:||12 May 2010 14:44|