The formulation induced photooxidative stability of substituted 6, 13 trialkylsilylethynylpentacene derivatives is report for the first time. It is known that 6,13 trialkylsilylethynylpentacene derivatives undergo rapid oxidation through singlet oxidation and is believed to be primarily to the 6,13 endoperoxide. The effect of solvent, organic semiconductor concentration and the presence of polymeric binder on the rate of photooxidative degradation in oxygen saturated solutions on degradation kinetics have been investigated. The photochemical stability of 6,13 trialkysilylethynylpentacene has been studied in toluene, THF, dichloromethane, chloroform, decane and dodecane at concentrations ranging from 2.5×10-6 M to 5×10-2 M by measurement of the half-life by UV-visible spectroscopy. The predominant degradation products of TIPS-Pentacene are the same regardless of solvent and concentration, being predominantly an endo-peroxide resulting from O2 addition across the 6, 13 position of the pentacene. At low concentration (< 1×10 -3 M), the half- life of TIPS-Pentacene is observed to vary with solvent and the amount of dissolved oxygen, with half-life showing a strong, positive correlation between the product of the Hansen Solubility Parameter (HSP) and the mole-fraction oxygen solubility. In this range, the half-life is observed to increase markedly with increasing solution concentration, showing a linear correlation with mean intermolecular distance within the solution. From intermediate to high concentration, the half-life increase more rapidly with increasing concentration, which is attributed to aggregation. In most solvents, this behaviour shows a specific onset point, suggesting a cooperative rather than isodesmic aggregation mechanism. Photooxidative half-lives of TMTES-pentacene and fluorinated soluble pentacene derivatives were determined from 2.5 × 10-6 M to 5.0 × 10-3 M solutions in THF, toluene, chloroform and CH2Cl2. Independent of solvent and at constant concentration the relative ordering of photo-oxidative resistance was: alpha-di-FTIPS-pentacene > β-di-FTIPS-pentacene, alpha-mono -FTIPS-pentacene, β-mono-FTIPS-pentacene > TIPS-pentacene greater than or equal to TMTES-pentacene. The same rank order for a particular derivative was generally maintained over the concentration range 2.5 x 10-6 M to 5 x 10-3 M the stability to photooxidation for all derivative increased with increasing concentration. All derivatives showed a mixture of endooxidation products as shown by APCI mass spectrometry, and 1H NMR.Added polystyrene and isotactic polystyrene were found to accelerate photooxidative degradation whist isotactic polymethylmethacrylate (i-PMMA) and poly (triaylamine) (PTAA) had no effect adding weight to the argument that anything which can act as an acceptor to the solution slows down the degradation and anything which impairs pair-wise energy transfer speeds up the degradation. Understanding of these phenomena is therefore essential for the further application of solution processing of organic materials.
|Date of Award||31 Dec 2013|
- The University of Manchester
|Supervisor||Stephen Yeates (Supervisor)|
- photooxidative stability , aggregation, organic semiconductor, 6,13 trialkysilylethynylpentacene