Time-Resolved Electron Paramagnetic Resonance Spectroscopy

Forbes, Malcolm D D. E.; Jarocha, Lauren E.; Sim, Sooyeon; Tarasov, V. F.

doi:10.1016/b978-0-12-407754-6.00001-6

Cited by 54 publications

(40 citation statements)

References 149 publications

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“…Photostability of the dye should also be considered in order to alleviate the need to flow the sample as in our current demonstration system. Quenching efficiency may be significantly enhanced by forming radical-triplet linked systems to increase the quantum yield of radical induced triplet quenching, 15,30,31 whilst avoiding an excess of radicals over triplet states, which as we have shown results in a decreased overall saturation factor. The optimal system would therefore combine a high radical polarization per quenching event with absorption characteristics that allow good light penetration to be obtained simultaneously with a high leakage factor.…”

mentioning

confidence: 70%

Optically generated hyperpolarization for sensitivity enhancement in solution-state NMR spectroscopy

Dale

Wedge

2016

Chem. Commun.

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mentioning

confidence: 70%

Optically generated hyperpolarization for sensitivity enhancement in solution-state NMR spectroscopy

Dale

Wedge

2016

Chem. Commun.

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“…[17,18] For all building blocks and the polymer with two different chain lengths, TREPR spectra have been recorded at low temperature (80 K) and the resulting spectra fitted assuming a single triplet species for each spectrum. [17,18] For all building blocks and the polymer with two different chain lengths, TREPR spectra have been recorded at low temperature (80 K) and the resulting spectra fitted assuming a single triplet species for each spectrum.…”

Section: Wwwadvelectronicmatdementioning

confidence: 99%

“…[5,6] Using organic molecules as a replacement for conventional inorganic semiconductors comes with a number of advantages, such as mechanical flexibility, low cost, and the nearly infinite possibility of tailoring molecules for each special application. Whereas for conventional inorganic semiconductors, it is rather straightforward to create either p-type or n-type materials using well-established protocols, only very few n-type conjugated polymers are known to date, with poly{[N,N′bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)} (PNDIT2), also known as P(NDI2OD-T2) or Polyera ActivInk N2200, [7] being a notable between 0.5 and 5 nm in systems that lack long-range order [16] only TREPR spectroscopy [17] with a time resolution of up to 10 ns allows for real-time observation, e.g., of short-lived radical-pair [18,19] and triplet states [20,21] generated by pulsed laser excitation. Furthermore, as EPR is exclusively sensitive to paramagnetic states, but polymers are normally diamagnetic, applying conventional EPR spectroscopy requires to insert spin probes into the material of interest.…”

Section: Introductionmentioning

confidence: 99%

Electronic Structure Trumps Planarity: Unexpected Narrow Exciton Delocalization in PNDIT2 Revealed by Time‐Resolved Electron Paramagnetic Resonance (EPR) Spectroscopy

Meyer

Matsidik

Sommer

et al. 2018

Adv Elect Materials

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exception. Furthermore, this polymer has a remarkable electron mobility of up to µ e = 0.85 cm 2 Vs −1 , [8] which is correlated with a high degree of molecular order on the micro-and nanoscale. [9,10] The degree of (de)localization of excitons in organic semiconductors is of outstanding importance, as it directly relates to efficiency. Depending on the desired application, either localized or delocalized excitons are sought-after. In a simple picture, delocalization is directly connected to planarity and conjugation, as the orbital overlap is maximized for parallel orientation of adjacent p z orbitals. [11] However, deviations in both directions are known, such as strong conjugation despite large torsional angles [12] as well as exciton delocalization confined to a rather small part of the available π system. [13] Eventually, (de)localization highly depends on structure and ordering primarily on the mole cular scale, as organic semiconductors show normally rather restricted large-scale ordering as compared to their inorganic and often highly crystalline counterparts. However, planarity within a molecular backbone resulting in high molecular order does not necessarily lead to higher carrier mobility. [14] Therefore a detailed understanding of the electronic structure of polymers and their building blocks is essential to develop efficient materials for organic electronics.To gain insight into the electronic structure of polymers, starting with their building blocks has proven to be valuable. [15] To the best of our knowledge, no systematic spectroscopic study of PNDIT2 starting from its building blocks has been performed yet. Here, we investigate four different building blocks of PNDIT2 and the polymer itself with two different chain lengths (see Figure 1 for chemical structures), using timeresolved electron paramagnetic resonance (TREPR) spectroscopy of their triplet excitons in combination with steady-state absorption spectroscopy and quantum-chemical calculations density-functional theory (DFT).Electron paramagnetic resonance (EPR) spectroscopy is intrinsically sensitive to the local environment of the electron spin, rendering it particularly well-suited to probe both electronic structure and local ordering on a molecular level. The key is its unrivalled sensitivity and selectivity for paramagnetic states not only allowing for unambiguous assignment of the spin multiplicity, but outperforming optical spectroscopy by far in terms of resolution. While conventional EPR spectroscopy can characterize structural features in the range Exciton delocalization in organic semiconductors, due to its direct relation to device efficiency, is of outstanding importance. Time-resolved electron paramagnetic resonance spectroscopy of light-induced triplet excitons gives access to the delocalization length in a unique way, connecting it to both, electronic structure and overall conformational flexibility. Systematically investigating building blocks of increasing length and comparing the results with the polymer deepens the unders...

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“…As reviewed elsewhere there are numerous CIDEP mechanisms, 27,28 with the RTPM and related Electron Spin Polarization Transfer (ESPT) mechanisms unique in that they can polarize persistent radicals, as opposed to the triplet mechanism and the radical pair mechanism which polarize transient radicals. For an optical DNP scheme it is essential that the radical persists for sufficient time to permit transfer of polarization to the nuclei, hence the RTPM and ESPT are ideal candidates as there is no radical decay process in competition with polarization transfer.…”

Section: B Radical-triplet Pair Mechanismmentioning

confidence: 99%

Optically-generated Overhauser dynamic nuclear polarization: A numerical analysis

Cheney

Wedge

2020

The Journal of Chemical Physics

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Recently, an alternative approach to dynamic nuclear polarization (DNP) in the liquid state was introduced, using optical illumination instead of microwave pumping. By exciting a suitable dye to the triplet state which undergoes diffusive encounter with a persistent radical forming a quartet-doublet pair in the encounter complex, dynamic electron polarization (DEP) is generated via the radical-triplet pair mechanism (RTPM). Subsequent cross-relaxation generates nuclear polarization without the need for microwave saturation of the electronic transitions. Here, we present a theoretical justification for the initial experimental results, by means of numerical simulations. These allow investigation of the effects of various experimental parameters, such as radical and dye concentrations, sample geometry, and laser power, on the DNP enhancement factors, providing targets for experimental optimization. It is predicted that reducing the sample volume will result in larger enhancements, by permitting a higher concentration of triplets in a sample of increased optical density. We also explore the effects of pulsed laser rather than continuous-wave illumination, rationalising the failure to observe the optical DNP effect under illumination conditions common to DEP experiments. Examining the influence of illumination duty cycle the conditions necessary to permit use of pulsed illumination without compromising signal enhancement are determined, which may reduce undesirable laser heating effects. This first simulation of the optical DNP method therefore underpins the further development of the technology. I. INTRODUCTION Among all of the hyperpolarization techniques for overcoming the intrinsically low sensitivity of Nuclear Magnetic Resonance (NMR) spectroscopy, Dynamic Nuclear Polarization (DNP) is the most studied and widely applicable. 1 It was first proposed by Overhauser in 1953, 2 and demonstrated experimentally by Carver and Slichter, 3 that saturating Electron Paramagnetic Resonance (EPR) transitions results in an increase in NMR sensitivity. This effect, due to cross-relaxation between the electron and nuclear spins, was later extended from lithium to free radicals in solution, 4 but has never been widely applied due to technical challenges and problems with reducing Overhauser efficiency at the high magnetic fields necessary for high resolution NMR spectroscopy.

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Time-Resolved Electron Paramagnetic Resonance Spectroscopy

Cited by 54 publications

References 149 publications

Optically generated hyperpolarization for sensitivity enhancement in solution-state NMR spectroscopy

Optically generated hyperpolarization for sensitivity enhancement in solution-state NMR spectroscopy

Electronic Structure Trumps Planarity: Unexpected Narrow Exciton Delocalization in PNDIT2 Revealed by Time‐Resolved Electron Paramagnetic Resonance (EPR) Spectroscopy

Optically-generated Overhauser dynamic nuclear polarization: A numerical analysis

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