Photostability and the BOPHY architecture

Abstract

Organic molecules offer the potential to harvest solar energy and underpin diverse optoelectronic applications, including fluorescent imaging techniques, OLEDs and chemical sensing. A critical factor in such technological advances concerns the stability of the dyes under continuous exposure to light. Indeed, photobleaching of fluorescence dyes is exploited in super-resolution microscopy and in measuring diffusion coefficients by way of FRAP. In other cases, such as photodynamic therapy, it is desirable that the organic sensitizer is destroyed at the conclusion of the operation. In general, however, photobleaching hinders innovative and progressive developments in the field of applied photochemistry. An important premise of this thesis is that the logical design of improved systems requires a deep understanding of the kinetics and mechanisms of photobleaching events. To begin to understand photodegradation a model compound, Erythrosine, which is known to bleach quickly is studied. It is a water-soluble, halogenated xanthene dye used in foodstuffs that has a near unity quantum yield for triplet formation. Further interest in this dye stems from the cost of approving new materials through regulatory testing, which can be prohibitive for consumer products. The solution is to use commercial reagents already on the market. Erythrosine bleaches rapidly under monochromatic illumination, allowing determination of the effects of temperature and concentration on the efficacy of the bleaching step and identification of rate-limiting conditions. It has been possible to estimate a quantum yield for bleaching under illumination at 523 nm. The mechanism appears fully consistent with singlet oxygen playing a key role in the bleaching chemistry. To be usefully photostable, logically, dyes should be resistant to intersystem crossing to the triplet manifold. We first detail the photophysical properties of such a dye, BOPHY, a new class of organic dyes derived from the ever-popular BODIPY family. Advantages of BOPHY include facile synthesis, easy derivatization and the ability to extend the conjugation length. The parent BOPHY shows poor mirror symmetry between absorption and fluorescence spectra, which was unexpected for such a simple chromophore displaying very high levels of fluorescence. Temperature dependence studies, high-resolution NMR and the use of polymeric host materials have been exploited to explain the underlying optical transitions associated with simple BOPHY derivatives. Using iodomethane as external heavy-atom spin perturber, phosphorescence could be detected at 77K. The BOPHY chromophore can be extended to produce linear molecules with widely-spaced terminals. One such derivative (BOPHY-DMA), having N,N-dimethylamino groups at both ends, has been examined to establish the level of electronic communication along the molecular backbone. A difference was found for the pKa values for successive protonation of ii the amino sites while partial oxidation of the dye allows further characterisation of the electronic properties. Other BOPHY derivatives studied here include a conjugated BOPHY equipped with polyethylene glycol solubilizing functions and a BOPHY-perylene dyad. The latter compound is an attempt to expand the conjugation pathway by fusing together BOPHY with ethynylperylene with the objective of establishing if the resultant supermolecule possesses individual or composite electronic properties. Returning to the issue of photostability, critical examination was made of the parent BOPHY dye and also of BOPHY-DMA in both plastic films and organic solvents. Broadband illumination was used to simulate exposure to sunlight. Disparate behaviour was found according to the nature of the compound and the surrounding medium. In plastic films, the bleaching mechanism involves a contribution from auto-catalysis. The extended BOPHY-DMA bleaches at a significantly faster rate than does the parent BOPHY. This situation continues in cyclohexane solution where the rate of bleaching of BOPHY-DMA is some 700-fold greater than that of the parent. Reducing the fluorescence lifetime offers a route to increasing stability of the dye. This is realized for BOPHY-DMA because of the inherent dipolar characteristics of this dye. Thus, the rate of bleaching is some 900-fold slower in acetonitrile compared to cyclohexane. Remarkably, both BOPHY and BOPHY-DMA are stabilised by toluene and NMR spectroscopy was used to aid understanding of this effect. Stacking between solvent and chromophore minimises close contact between solute molecules and thereby eliminate the auto-catalytic step. Industry makes heavy use of stabilisers and has developed a range of anti-oxidants for protecting fragile molecules against attack by free radicals. For the stabilizer to be effective with excited states, we need covalently-linked molecular dyads. This was achieved by attaching butylated hydroxyl toluene (BHT) groups to BOPHY. Rates of photobleaching of the chromophore can be related to the photophysical properties recorded in a series of solvents. Our results implicate light-induced charge transfer from BHT to BOPHY as being responsible for shortening the excited-state lifetime. There is a beneficial effect of the anti-oxidant in terms of photostability in polar environments. A complex autocatalytic mechanism is found in nonpolar solvents.