Competitive electron transfer and enhanced intersystem crossing in photoexcited covalent TEMPO-Perylene-3,4: 9,10-bis(dicarboximide) dyads: unusual spin polarization resulting from the radical-triplet interaction

Michael T. Colvin, Emilie M. Giacobbe, Boiko Cohen, Tomoaki Miura, Amy M Scott, Michael R. Wasielewski

Research output: Contribution to journalArticle

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Abstract

A stable 2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPO) radical was covalently attached at its 4-position to the imide nitrogen atom of a perylene-3,4:9,10-bis(dicarboximide) (PDI) to produce TEMPO-PDI, 1, having a well-defined distance and orientation between TEMPO and PDI. Transient optical absorption experiments in toluene following selective photoexcitation of the PDI chromophore in TEMPO-PDI show that enhanced intersystem crossing occurs with τ = 45 ± 1 ps, resulting in formation of TEMPO-3* PDI, while the same experiment in THF shows that the electron-transfer reaction TEMPO-1*PDI → TEMPO+̇-PDI-̇ occurs with τ = 1.2 ± 0.2 ps and thus competes effectively with enhanced intersystem crossing. Time-resolved EPR (TREPR) spectroscopy on the photogenerated three-spin system TEMPO-3*PDI in toluene at 295 K initially shows a broad signal assigned to spin-polarized 3* PDI, which thermalizes at longer times and is accompanied by formation of an emissively polarized TEMPO radical. No signals are observed in THF at 295 K. The TREPR spectrum of TEMPO-3*PDI at 85 K in toluene shows an emissive/absorptive signal due to TEMPO and a broad triplet signal due to 3*PDI having a spin polarization pattern characteristic of overpopulation of its T0 sublevel. This unusual spin polarization pattern does not result from radical pair intersystem crossing because electron transfer does not occur at 85 K. The observed spin polarization of 3*PDI cannot be readily explained by mechanisms discussed previously, leading us to propose a new spin polarization mechanism, which requires that the radical and attached triplet are in the weak exchange regime.

Original languageEnglish (US)
Pages (from-to)1741-1748
Number of pages8
JournalJournal of Physical Chemistry A
Volume114
Issue number4
DOIs
StatePublished - 2010
Externally publishedYes

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Perylene
Spin polarization
electron transfer
Toluene
Electrons
polarization
Paramagnetic resonance
toluene
interactions
Imides
Photoexcitation
Chromophores
Light absorption
Nitrogen
Experiments
imides
Spectroscopy
photoexcitation
Atoms
nitrogen atoms

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry

Cite this

Competitive electron transfer and enhanced intersystem crossing in photoexcited covalent TEMPO-Perylene-3,4 : 9,10-bis(dicarboximide) dyads: unusual spin polarization resulting from the radical-triplet interaction. / Colvin, Michael T.; Giacobbe, Emilie M.; Cohen, Boiko; Miura, Tomoaki; Scott, Amy M; Wasielewski, Michael R.

In: Journal of Physical Chemistry A, Vol. 114, No. 4, 2010, p. 1741-1748.

Research output: Contribution to journalArticle

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abstract = "A stable 2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPO) radical was covalently attached at its 4-position to the imide nitrogen atom of a perylene-3,4:9,10-bis(dicarboximide) (PDI) to produce TEMPO-PDI, 1, having a well-defined distance and orientation between TEMPO and PDI. Transient optical absorption experiments in toluene following selective photoexcitation of the PDI chromophore in TEMPO-PDI show that enhanced intersystem crossing occurs with τ = 45 ± 1 ps, resulting in formation of TEMPO-3* PDI, while the same experiment in THF shows that the electron-transfer reaction TEMPO-1*PDI → TEMPO+̇-PDI-̇ occurs with τ = 1.2 ± 0.2 ps and thus competes effectively with enhanced intersystem crossing. Time-resolved EPR (TREPR) spectroscopy on the photogenerated three-spin system TEMPO-3*PDI in toluene at 295 K initially shows a broad signal assigned to spin-polarized 3* PDI, which thermalizes at longer times and is accompanied by formation of an emissively polarized TEMPO radical. No signals are observed in THF at 295 K. The TREPR spectrum of TEMPO-3*PDI at 85 K in toluene shows an emissive/absorptive signal due to TEMPO and a broad triplet signal due to 3*PDI having a spin polarization pattern characteristic of overpopulation of its T0 sublevel. This unusual spin polarization pattern does not result from radical pair intersystem crossing because electron transfer does not occur at 85 K. The observed spin polarization of 3*PDI cannot be readily explained by mechanisms discussed previously, leading us to propose a new spin polarization mechanism, which requires that the radical and attached triplet are in the weak exchange regime.",
author = "Colvin, {Michael T.} and Giacobbe, {Emilie M.} and Boiko Cohen and Tomoaki Miura and Scott, {Amy M} and Wasielewski, {Michael R.}",
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T1 - Competitive electron transfer and enhanced intersystem crossing in photoexcited covalent TEMPO-Perylene-3,4

T2 - 9,10-bis(dicarboximide) dyads: unusual spin polarization resulting from the radical-triplet interaction

AU - Colvin, Michael T.

AU - Giacobbe, Emilie M.

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AU - Scott, Amy M

AU - Wasielewski, Michael R.

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AB - A stable 2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPO) radical was covalently attached at its 4-position to the imide nitrogen atom of a perylene-3,4:9,10-bis(dicarboximide) (PDI) to produce TEMPO-PDI, 1, having a well-defined distance and orientation between TEMPO and PDI. Transient optical absorption experiments in toluene following selective photoexcitation of the PDI chromophore in TEMPO-PDI show that enhanced intersystem crossing occurs with τ = 45 ± 1 ps, resulting in formation of TEMPO-3* PDI, while the same experiment in THF shows that the electron-transfer reaction TEMPO-1*PDI → TEMPO+̇-PDI-̇ occurs with τ = 1.2 ± 0.2 ps and thus competes effectively with enhanced intersystem crossing. Time-resolved EPR (TREPR) spectroscopy on the photogenerated three-spin system TEMPO-3*PDI in toluene at 295 K initially shows a broad signal assigned to spin-polarized 3* PDI, which thermalizes at longer times and is accompanied by formation of an emissively polarized TEMPO radical. No signals are observed in THF at 295 K. The TREPR spectrum of TEMPO-3*PDI at 85 K in toluene shows an emissive/absorptive signal due to TEMPO and a broad triplet signal due to 3*PDI having a spin polarization pattern characteristic of overpopulation of its T0 sublevel. This unusual spin polarization pattern does not result from radical pair intersystem crossing because electron transfer does not occur at 85 K. The observed spin polarization of 3*PDI cannot be readily explained by mechanisms discussed previously, leading us to propose a new spin polarization mechanism, which requires that the radical and attached triplet are in the weak exchange regime.

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