I. Hamblett

S1∼>T1 intersystem crossing in π-conjugated organic polymers

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 dynamics on isolated conjugated polymer chains

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 μs. 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 effect of conjugation length on triplet energies, electron delocalization and electron–electron correlation in soluble polythiophenes

Triplet energies from a family of polythiophenes in which effective conjugation length is controlled by steric torsion have been measured for the isolated polymer chain regime, i.e., benzene solution. Triplet states are stabilized by some 0.6 eV as effective conjugation length increases up to ca. 6 repeat units, then saturation occurs. This triplet stabilization must occur by electron delocalization and implies that singlet states are stabilized as conjugation length increases by a combination of electron and exciton delocalization.

The triplet state of the ladder-type methyl-poly(p-phenylene) as seen by pulse radiolysis-energy transfer

Abstract The lowest triplet state of a ladder poly(p-phenylene) (MeLPPP) has been produced by sensitised energy transfer following pulse radiolysis of benzene solution, and absorbs at 1.34 eV with a lifetime ⩾170 μs. An S0–T1 energy separation of 2.15 ± 0.07 eV was determined, in excellent agreement with recent phosphorescence measurements, confirming the validity of the technique for the determination of the triplet energies of conjugated polymers. At high triplet populations intrachain triplet–triplet annihilation (TTA), leading to delayed fluorescence (DF) occurs. The DF spectrum is in good agreement with the prompt fluorescence and the DF kinetics support a TTA mechanism.

Intra-chain triplet–triplet annihilation and delayed fluorescence in soluble conjugated polymers

Abstract Pulse radiolysis has been used to generate high triplet populations on isolated conjugated polymer chains in benzene solutions. This allows direct observation of delayed fluorescence (DF) arising from intramolecular triplet–triplet annihilation (TTA). In poly(2-methoxy,5-(2 ′ -ethyl-hexyloxy)- p -phenylenevinylene) the triplet lifetime at low triplet concentration is 90 μs which decreases to 15 μs as the number of triplets saturates at ca. 30 triplets per chain. Analysis of the DF kinetics gives an upper limit of 1.5% for the number of triplets that contribute to TTA. We also observed DF in a range of other conjugated polymers.

Triplet state spectroscopy of conjugated polymers studied by pulse radiolysis

Using the technique of pulse radiolysis we have elucidated the energies and kinetics of triplet states in soluble luminescent conjugated polymers. Using poly(2-methoxy,5-(2 0 -ethyl-hexoxy)-p-phenylenevinylene) MEH-PPV as an example we explain this technique and show how it can be used to study the triplet states in conjugated polymers. Triplet energy transfer is used to determine 1 1 Agˇ1 3 Bu energy gaps and the kinetics of triplet‐triplet absorption yields triplet lifetimes. In the case of MEH-PPV, at concentrations up to 50 mg/l, the triplet decay rate shows no change, indicating self-quenching of triplets is not significant. However, if very high electron beam doses are used, high intra chain triplet concentrations can be generated. In this high concentration regime triplet‐triplet annihilation becomes effective, as determined by the onset of delayed fluorescence. # 2001 Elsevier Science B.V. All rights reserved.

Reversible triplet energy transfer between neo-alloocimene and anthracene

Abstract The rate constants for triplet energy transfer between neo-alloocimene and anthracene have been redetermined by a combination of pulsed laser photolysi

Lifetime, triplet-triplet absorption spectrum and relaxation energy of an acyclic conjugated triene triplet: A pulse radiolysis study

The triplet state of neo-alloocimene (τ = 333 ns: λ max = 315 nm) has been produced in toluene by pulse radiolysis. The establishment of a transient stationary position for reversible triplet energy transfer between this triene and anthracene has placed the equilibrium energy, of the triene triplet at 169 ± 2 kj mol −1 28 ± 2 kJ mol −1 less than that of the Franck-Condon state.

Transient absorption spectra of triplet states and charge carriers of conjugated polymers

Abstract The use of pulse radiolysis to study transient species in conjugated polymers in solutions is discussed, with particular reference to poly(2-methoxy-5-(2′-ethylhexoxy)- p -phenylenevinylene) (MEH-PPV). The excited triplet state is specifically generated by energy transfer on radiolysis of benzene solutions in the presence of suitable sensitisers, and its spectrum is reported. The presence of vibronic structure suggests a localised triplet state. By varying the energy of the sensitiser, it is possible to determine the energy of the lowest triplet state. Results are also presented for the triplet–triplet absorptions and energies of five other common π-conjugated polymers. The MEH-PPV triplet absorption, produced on radiolysis, does not increase linearly with either polymer concentration or radiation dose. This is suggested to be due to the presence of multiple triplet excitations on the isolated polymer chains. Intrachain triplet–triplet annihilation is observed, and is seen to lead to delayed fluorescence. Spectra are also reported for the positive and negative charge carriers of MEH-PPV and the other polymers specifically produced by pulse radiolysis of the polymers in chloroform or tetrahydrofuran solutions.

Pulse radiolysis studies on charge carriers in conjugated polymers

The charged states of the conjugated polymers poly(2-methoxy,5-(2′-ethylhexyloxy)-p- phenylenevinylene) (MEH-PPV) and poly(2,5-pyridinediyl) (PPY) have been studied by pulse radiolysis. Following pulse radiolysis of argon-saturated solutions of MEH-PPV in chloroform, a new absorption is seen to grow in over a few hundred microseconds. This has a principal absorption at 1.43 eV and a weaker, low energy band (⩽0.80 eV), and is assigned to the positive one-electron charge state (positive polaron) of MEH-PPV. The slow absorption decay is unaffected by oxygen. Negative charge states (negative polarons), with absorptions around 1.4 eV, are produced upon pulse radiolysis of MEH-PPV in argon-saturated solutions in tetrahydrofuran (THF) or benzonitrile. A small solvatochromic shift is observed. In contrast to the behavior of the positive polaron, the MEH-PPV negative charge carriers decay fairly rapidly, and are readily quenched by molecular oxygen. Previous results on chemically produced positive and negative cha...