Delocalisation of photoexcited triplet states probed by transient EPR and hyperfine spectroscopy

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.

Efficient Intersystem Crossing in the Tröger's Base Derived From 4‐Amino‐1,8‐naphthalimide and Application as a Potent Photodynamic Therapy Reagent

“…[51] It has been used for studying ISC mechanisms [21,52] and for confinementoft he triplet-state wave function. [53,54] The NI moiety itself shows efficient ISC (F T = 92 %). [55] Thus, we neededt oc larify that the ISC of NI-TB is not due to this intrinsic ISC of the NI moiety.T REPR spectroscopy is able to discriminate paramagnetic triplet states produced by different ISC mechanisms.…”

Photoexcited triplet states of twisted acenes investigated by Electron Paramagnetic Resonance

“…The orientation selectivity of the ENDOR experiment for the broad triplet state EPR signal is advantageous for characterisation of the different hyperfine coupling tensors and their relative orientation with respect to the zero-field splitting tensor. 13 As in ENDOR experiments performed at the canonical field positions the strength of the hyperfine coupling is probed along the ZFS tensor axes, two separate effects can account for changes in the ENDOR spectra observed for this series of molecules with varying geometries. The first is a purely geometrical effect: as the anthracene core twists out of planarity, the relative orientation of the principal axes of the ZFS tensor and the hyperfine tensor changes.…”

“…Transient EPR triplet state spectra additionally contain information on the zero-field splitting interaction and the ISC mechanism. 13 In order to gain a more precise understanding of the influence of twisting of the acene core on the spin density distribution, proton hyperfine couplings are measured by Electron-Nuclear DOuble Resonance (ENDOR). In addition to the dependence of triplet state properties on the type of compound, the excitation wavelength dependence is also investigated.…”

On the Influence of the Bridge on Triplet State Delocalization in Linear Porphyrin Oligomers

“…If the orientation of the zero-field splitting (ZFS) tensor is known, information about delocalization in a series of oligomers can be obtained from changes in the ZFS D -value by comparison with a suitable monomer spectrum. The cube of the average electron–electron distance, r , is inversely proportional to the D -value, related to the width of the transient EPR triplet spectrum 19 where θ refers to the angle between the dipolar Z -axis and the spin–spin vector, g e is the electronic g -factor, β e is the Bohr magneton, μ 0 is the vacuum permeability, and the angular brackets indicate the expectation value. The angle θ characterizes the shape of the spin density distribution and is equal to 90° in the case of an oblate spin density distribution and equal to 0° in the case of a prolate one.…”