Spin-Lattice and Spin-Spin Relaxation in Photo-Excited Triplet States in Molecular Crystals

Schmidt, Jan

doi:10.1007/978-94-009-0863-5_2

Cited by 10 publications

(6 citation statements)

References 54 publications

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“…In this regard, we have determined the spin-spin and spin-lattice relaxation times for the triplet exciton in compounds 5 and 7 over the 4-30 K temperature range. As relaxation of the triplet exciton is governed primarily by modulation of the ZFS interaction, it is hypothesized that triplet exciton migration should result in enhanced spin-lattice relaxation where there is monomeric unit-to-unit disorder, 54 resulting in a decrease in the spin-lattice relaxation time with increasing conjugation length. A corresponding absence of spin-lattice relaxation scaling with conjugation length would suggest that the triplet exciton is confined to one monomeric unit.…”

Section: Resultsmentioning

confidence: 99%

Impact of Electronic Asymmetry on Photoexcited Triplet-State Spin Distributions in Conjugated Porphyrin Oligomers Probed via EPR Spectroscopy

et al. 2004

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The photophysics of triplet excitons in a series of electronically asymmetric “push−pull” π-conjugated meso-to-meso ethyne-bridged (porphinato)metal oligomers, along with electronically symmetric analogues, were studied by X-band electron paramagnetic resonance (EPR) spectroscopy under continuous-wave (CW) optical pumping conditions in the 4−100 K temperature range. In all of the systems studied, the spatial extent of the triplet wave function, as inferred from the |D| zero-field splitting (ZFS) parameter, never exceeds the dimensions of a single porphyryl moiety and its meso-pendant ethynyl groups. The |D| values determined for an oligomeric series of these electronically asymmetric species that span one through four porphyryl units are respectively 0.0301, 0.0303, 0.0300, and 0.0301 cm-1, indicating a common triplet wave function spatial delocalization of approximately 0.4−0.45 nm. Electron spin−spin and spin−lattice relaxation times were determined over the 4−30 K temperature range using progressive microwave power saturation for benchmark, structurally related electronically symmetric conjugated porphyrin species which possessed either terminal electron-rich [4-dimethylamino(phenyl)]ethynyl or electron-poor [4-nitro(phenyl)]ethynyl substituents. The spin−lattice relaxation times obtained from these experiments reveal no significant scaling of this parameter with conjugation length, consistent with a S = 1 spin system that is confined to a single monomeric porphyrin unit and its pendent ethynyl substituents. These results are discussed within the global context of a broader body of experiments that have probed the extent of triplet exciton delocalization within a number of families of highly π-conjugated organic oligomers and polymers.

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Section: Resultsmentioning

confidence: 99%

Impact of Electronic Asymmetry on Photoexcited Triplet-State Spin Distributions in Conjugated Porphyrin Oligomers Probed via EPR Spectroscopy

et al. 2004

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“…There is a large literature on the sub-field of photo-excited triplet states (Schmidt, 1989). These systems have the additional complexity that lifetimes of states can be limited by intersystem crossing.…”

Section: Tripletsmentioning

confidence: 99%

Relaxation Times of Organic Radicals and Transition Metal Ions

Eaton

2002

Biological Magnetic Resonance

155

165

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Abstract:Review of electron spin relaxation times for organic radicals and transition metal ions in magnetically dilute samples. Emphasis is placed on studies that have been performed as a function of temperature and that provide insight into the relaxation processes. INTRODUCTION Scope of this ChapterMost of the methods for determining distances between spins (ch.l) are either experimentally feasible or theoretically applicable for a limited range of electron spin relaxation times. For example, analysis of the dipolar contribution to the CW line shape to determine interspin distance requires that the relaxation rate for the interacting partner is slow enough that the dipolar splitting is not collapsed by electron spin relaxation. In addition, analysis of the line shapes assumes that spectra are obtained under nonsaturating conditions and are free from passage effects. Application of methods based on changes in relaxation times requires that the relaxation rate of the interacting partner fall within certain limits. The longest distance that can be obtained by pulsed techniques is limited, in principle, by the relaxation-determined width of an individual spin packet. Thus, knowledge of electron spin relaxation times is foundational to the methodologies presented in this book.

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“…To study the connection between SLR and orientational relaxation (OR) in highly viscous solution, we previously investigated the time-dependent phosphorescence of phenazine (PZ) in the glass-transition range of two alkane solvents . SLR in the triplet state of PZ leads to an initial change in the phosphorescence intensity from which an effective rate coefficient, k s , for SLR can be deduced. − The rate coefficient k r for OR can be obtained from the dynamics of the phosphorescence depolarization. − The original expectation at the start of the work in ref was that, in a sufficiently viscous solvent at low temperatures, SLR and OR could be observed simultaneously. To find a relationship between SLR and OR, two similar glass-forming solvents (sol) with significantly different glass-transition temperatures, T g , were used.…”

Section: Introductionmentioning

confidence: 99%

“…2 SLR in the triplet state of PZ leads to an initial change in the phosphorescence intensity from which an effective rate coefficient, k s , for SLR can be deduced. [2][3][4][5][6][7] The rate coefficient k r for OR can be obtained from the dynamics of the phosphorescence depolarization. [8][9][10] The original expectation at the start of the work in ref 2 was that, in a sufficiently viscous solvent at low temperatures, SLR and OR could be observed simultaneously.…”

Section: Introductionmentioning

confidence: 99%

Spin-Lattice Relaxation and Rotational Motion of Aromatic Triplet-State Molecules in Supercooled Alkane Solvents (Part 1)

Ruth

Nickel

2005

J. Phys. Chem. A

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The phosphorescence of phenazine (PZ) and quinoxaline (QX) was investigated after pulsed laser excitation in the glass-transition range of several alkane solvents. Three relaxation processes of PZ and QX in the metastable triplet state, T1, were studied as a function of temperature: (1) the decay of the selective population of the strongly phosphorescent triplet substate T1x due to spin-lattice relaxation (SLR), (2) the time-dependent red shift of the phosphorescence spectrum due to the solvation of triplet-state molecules, and (3) the decay of the phosphorescence polarization due to orientational relaxation (OR). Various aspects and connections of the mechanisms governing the three relaxation phenomena are discussed. The relaxation dynamics were characterized at temperatures above the glass-transition temperature of the respective solvent, where the fundamental processes involved are strongly dependent upon the solvent viscosities. For the systems treated here, OR and solvation were satisfactorily described by a Vogel-Fulcher-Tammann temperature behavior. SLR also depends on properties of the alkane solvent above the glass transition. Upon cooling, SLR becomes independent of the specific solvent properties and is based on mechanisms that are typical for amorphous glasses or solids. (This particular aspect will be the subject of a subsequent publication, part 2).

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Spin-Lattice and Spin-Spin Relaxation in Photo-Excited Triplet States in Molecular Crystals

Cited by 10 publications

References 54 publications

Impact of Electronic Asymmetry on Photoexcited Triplet-State Spin Distributions in Conjugated Porphyrin Oligomers Probed via EPR Spectroscopy

Impact of Electronic Asymmetry on Photoexcited Triplet-State Spin Distributions in Conjugated Porphyrin Oligomers Probed via EPR Spectroscopy

Relaxation Times of Organic Radicals and Transition Metal Ions

Spin-Lattice Relaxation and Rotational Motion of Aromatic Triplet-State Molecules in Supercooled Alkane Solvents (Part 1)

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