Abstract
Photoexcited triplet states play a crucial role in photochemical mechanisms: long known to be of paramount importance in the study of photosynthetic reaction centres, they have more recently also been shown to play a major role in a number of applications in the field of molecular electronics. Their characterisation is crucial for an improved understanding of these processes with a particular focus on the determination of the spatial distribution of the triplet state wavefunction providing information on charge and energy transfer efficiencies. Currently, active research in this field is mostly focussed on the investigation of materials for organic photovoltaics (OPVs) and organic light emitting diodes (OLEDs). As the properties of triplet states and their spatial extent are known to have a major impact on device performance, a detailed understanding of the factors governing triplet state delocalisation is at the basis of the further development and improvement of these devices. Electron Paramagnetic Resonance (EPR) has proven a valuable tool in the study of triplet state properties and both experimental methods as well as data analysis and interpretation techniques have continuously improved over the last few decades. In this review, we discuss the theoretical and practical aspects of the investigation of triplet states and triplet state delocalisation by transient continuous wave and pulse EPR and highlight the advantages and limitations of the presently available techniques and the current trends in the field. Application of EPR in the study of triplet state delocalisation is illustrated on the example of linear multi-porphyrin chains designed as molecular wires.
Highlights
The photoexcited triplet states of p-conjugated organic molecules designed for applications in solar energy conversion [1,2], molecular electronics [3] and spintronics [4] have received increased attention over the last two decades after initial focus on the properties of the corresponding singlet excitons
In systems designed as molecular wires, the extent of triplet state delocalisation provides information on the efficiency of electronic communication between the monomeric building blocks and understanding of the factors limiting triplet state delocalisation can guide the design of systems with improved charge transport properties
We review the currently available methods for the analysis of triplet state delocalisation by EPR and discuss their advantages and limitations
Summary
The photoexcited triplet states of p-conjugated organic molecules designed for applications in solar energy conversion [1,2], molecular electronics [3] and spintronics [4] have received increased attention over the last two decades after initial focus on the properties of the corresponding singlet excitons. Triplet state delocalisation in p-conjugated oligomeric structures designed for wire-like charge transport has been investigated by EPR [18,19,20,35,36,37,38] In these studies the determination of the extent of the triplet state wavefunction was based mainly on the interpretation of the zero-field splitting D value. More recent investigations on meso-to-meso butadiyne-bridged zinc porphyrin oligomers (Fig. 1e) combined time-resolved EPR for the determination of the zero-field splitting parameters and triplet state ENDOR and ESEEM for the characterisation of proton and nitrogen hyperfine couplings with DFT calculations to investigate triplet state delocalisation.
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