Enhanced intersystem crossings of S1–T1 and T1–S0 in coronene– and pyrene–galvinoxyl systems as studied by a pulsed ESR method

Terazono, Hajime; Kawai, Akio; Tsuji, Kentaro; Shibuya, Kazuhiko

doi:10.1016/j.jphotochem.2006.02.014

Cited by 6 publications

(7 citation statements)

References 33 publications

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“…Depending on the relative magnitudes of J 0 and the Zeeman energy, ω 0 , systems have been classified under strong exchange (| J 0 | ≫ ω 0 ) and weak exchange (| J 0 | ≪ ω 0 ) regimes. The observed ESP has been rationalized by postulating that the quenching takes place at very different values of r in the two regimes . Theoretical work of Shushin shows P to be independent of J 0 in the strong exchange regime but proportional to J 0 in the weak exchange regime.…”

Section: Introductionmentioning

confidence: 93%

“…The observed ESP has been rationalized by postulating that the quenching takes place at very different values of r in the two regimes. 7 Theoretical work of Shushin 8 shows P to be independent of J 0 in the strong exchange regime but proportional to J 0 in the weak exchange regime.…”

Section: ■ Introductionmentioning

confidence: 99%

“…Observation of very different magnitudes of P S and P T in the same system, in experiments conducted under identical conditions, thus becomes an important issue. Such observations have been rationalized either by invoking that the singlet and the triplet quenching take place at very different values of r and not in the neighborhood of the level crossing region 6,7 or by postulating the presence of another electronic state, often called a charge-transfer state, facilitating the quenching. 6 The distance dependence of J(r) is usually written in the following exponential manner…”

Section: ■ Introductionmentioning

confidence: 99%

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Distance Dependence of Electron Spin Polarization during Photophysical Quenching of Excited Naphthalene by TEMPO Radical

Rane

Das

2015

J. Phys. Chem. A

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Quenching of excited states by a free radical is generally studied in systems where these two are separate entities freely moving in a liquid solution. Random diffusive encounters bring them together to cause the quenching and leave the spins of the radical polarized. In the dynamics of the radical-triplet pair mechanism of the generation of electron spin polarization (ESP), the distance-dependent exchange interaction plays a crucial role. To investigate how the distance between the excited molecule and the radical influences the ESP, we have covalently linked a naphthalene moiety to a TEMPO free radical through a spacer group of three different lengths. We compared the ESP process of these linked compounds with that of the usual "unlinked system" of naphthalene and TEMPO through time-resolved EPR experiments at low temperature in n-hexane solution. The time evolution of both the linked and the "unlinked system" was treated on a similar footing. The time-dependent EPR signal was analyzed by combining photophysical kinetics and time-dependent Bloch equations incorporating spin dynamics. Sequential quenching of the singlet state and the triplet state of naphthalene was seen in all the systems, as revealed through the spin-polarized TREPR spectra of opposite phase. The magnitudes of the ESP in the linked molecules were higher than those of the "unlinked system," showing that when the two moieties are held together greater mixing of quartet-doublet states takes place. The magnitudes of ESP steadily decrease with increasing the length of the spacer group. The polarization magnitudes due to triplet quenching and singlet quenching are very similar, differing by a factor of only ∼2. These characteristics show that for all the linked molecules the quenching takes place in the "weak exchange" regime and at almost the same distance of separation between the two moieties. Our results also showed that observation of small absorptive TREPR signals does not necessarily imply that its magnitude of polarization is small.

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Section: Introductionmentioning

confidence: 93%

Section: ■ Introductionmentioning

confidence: 99%

Section: ■ Introductionmentioning

confidence: 99%

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Distance Dependence of Electron Spin Polarization during Photophysical Quenching of Excited Naphthalene by TEMPO Radical

Rane

Das

2015

J. Phys. Chem. A

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“…One of the possible mechanisms of the fluorescence quenching is enhanced intersystem crossing (EISC). A number of models, which depend on the nature of the interaction between the radical and the chromophore, have been proposed to explain EISC. ,− If the chromophore and radical species can diffuse freely or are tethered to one another by a flexible linker, the EISC is induced by the variation of the mixing of their wave functions as the distance between them varies. − In systems in which the radical (or other paramagnetic species) is attached at a fixed distance, ,,,− the exchange coupling between the unpaired electrons is constant but can nonetheless lead to EISC as first proposed by Evans and later expanded upon by Hoytink and Ake and Gouterman . In such rigidly linked complexes, the terms sing-doublet, trip-doublet, and trip-quartet introduced by Gouterman are used to describe the states of the three-spin system to indicate the localized state of the chromophore and the overall spin multiplicity.…”

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confidence: 99%

“…In strongly coupled systems, mixing of the trip-doublet and trip-quartet sublevels by the zero-field splitting (ZFS) of the chromophore is sometimes invoked to explain the observed spin polarization. , In the moderately coupled case, the trip-doublet and trip-quartet states are determined not only by the ZFS of the chromophore but also by the exchange and Zeeman interactions . These systems have attracted strong interest due to their compact, rigid structures and rich photophysical properties ,,,,,− that can be exploited for producing nuclear hyperpolarization in some cases …”

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confidence: 99%