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Quantum yields for triplet formation have been determined for seven common π-conjugated polymers in benzene solution using time-resolved photoacoustic calorimetry (PAC) in conjunction with fluorescence quantum yields, singlet and triplet energies. The polymers studied include three poly(thiophenes), poly(2-methoxy,5-(2′-ethylhexyloxy)-p-phenylenevinylene) (MEH-PPV), a cyano derivative of MEH-PPV, a ladder type poly(p-phenylene) (MeLPPP), and a poly(fluorene). Yields of singlet oxygen formation have also been determined for these polymers in benzene by time-resolved phosphorimetry, and are in reasonable agreement with triplet yields obtained by PAC. Polythiophenes show the highest intersystem crossing yields, which are suggested to result from extensive spin-orbit coupling. Where singlet oxygen yields are less than triplet yields, it is suggested that interaction of molecular oxygen with the ground state of the polymers may be involved.

Triplet state behaviour has been studied with several conjugated polymers in dilute benzene solutions by flash photolysis, photoacoustic calorimetry (PAC) and pulse radiolysis/energy transfer. With polythiophenes and the ladder poly(p-phenylene) MeLPPP, singlet-triplet intersystem crossing (ISC) is relatively efficient. In contrast, it is inefficient with poly(p-phenylenevinylene)s (PPVs) and polyfluorene, while with cyano-substituted PPV, there is no evidence for any long-lived triplet state. Energy transfer from triplet biphenyl to MEH-PPV is diffusion controlled and triplet state lifetimes are typically tens or hundreds of ls. All the triplet states are quenched by molecular oxygen, leading to formation of singlet oxygen with yields which are generally close to those for triplet formation. With pulse radiolysis at high doses, it is possible to have more than one triplet state per polymer chain. This can lead to delayed fluorescence via intrachain triplet-triplet annihilation. Kinetic analysis of this shows slow movement of triplets by hopping along the chain.

The pH dependence of singlet oxygen quenching by histidine, N-acetyltyrosine ethyl ester (ATEE), ascorbic acid, Trolox C, and tryptophan has been observed using time-resolved infrared luminescence measurements in a D2O/acetonitrile (50:50 v/v) solvent. Deprotonation of ascorbic acid, the protonated imidazole ring of histidine and the phenolic group of ATEE leads to an increase in the quenching rate constants by between 2 and 3 orders of magnitude. Such changes appear to be the basis for wide variations in quoted literature values of singlet oxygen quenching constants for these and related compounds. It is estimated that these pH-dependent quenching rate constants predict a modest (approximately 2- to 3-fold) change in singlet oxygen lifetime between the extremes of cellular pH. Activation data for singlet oxygen quenching show that the enthalpies of activation are low in all cases (between 0 and 11 kJ mol-1) and that substantially negative entropies of activation (between −49 and −116 J K-1 mol-1) result in rate constants being much lower than the diffusion-controlled limit. In all cases the data are consistent with quenching via reversible formation of an exciplex, all reactions being at the preequilibrium limit over the available temperature range.

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We investigated the effect of oxygen on the photostability of the laser dyes Pyrromethene 567, Perylene Orange, and Rhodamine 590 by determining their longevity of laser operation when pumped by the second harmonic of aQ -switched Nd:YAG laser. In solution, dissolved oxygen accelerated the photodegradation of Pyrromethene 567 and Perylene Orange but not Rhodamine 590. The photostability of Pyrromethene 567 was also found to be dependent on the solvent and on the lifetime of singlet oxygen. Deoxygenated Pyrromethene 567 doped polycom glass and modified poly(methyl methacrylate) (MPMMA) samples were tested for longevity of laser operation. A factor of 6 improvement in photostability was found for Pyrromethene 567 in MPMMA upon deoxygenation, and the total absorbed energy per mole of dye molecules to one-half output pulse energy was 36 GJ mol-1 .

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