Time-resolved EPR studies of charge recombination and triplet-state formation within donor-bridge-acceptor molecules having wire-like oligofluorene bridges

Tomoaki Miura, Raanan Carmieli, Michael R Wasielewski

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Abstract

Spin-selective charge recombination of photogenerated radical ion pairs within a series of donor-bridge-acceptor (D-B-A) molecules, where D = phenothiazine (PTZ), B = oligo(2,7-fluorenyl), and A = perylene-3,4:9,10- bis(dicarboximide) (PDI), PTZ-FLn-PDI, where n = 1-4 (compounds 1-4), is studied using time-resolved electron paramagnetic resonance (TREPR) spectroscopy in which the microwave source is either continuous-wave or pulsed. Radical ion pair TREPR spectra are observed for 3 and 4 at 90-294 K, while the neutral triplet state of PDI (3*PDI) is observed at 90-294 K for 2-4 and at 90 K for 1. 3*PDI is produced by three mechanisms, as elucidated by its zero-field splitting parameters and spin polarization pattern. The mechanisms are spin-orbit-induced intersystem crossing (SO-ISC) in PDI aggregates, direct spin-orbit charge-transfer intersystem crossing (SOCT) from the singlet radical pair within 1, and radical pair intersystem crossing (RP-ISC) as a result of S-T0 mixing of the radical ion pair states in 2-4. The temperature dependence of the spin-spin exchange interaction (2J) shows a dramatic decrease at low temperatures, indicating that the electronic coupling between the radical ions decreases due to an increase in the average fluorene-fluorene dihedral angle at low temperatures. The charge recombination rates for 3 and 4 decrease at low temperature, but that for 2 is almost temperature-independent. These results strongly suggest that the dominant mechanism of charge recombination for n ≥ 3 is incoherent thermal hopping, which results in wire-like charge transfer.

Original languageEnglish
Pages (from-to)5769-5778
Number of pages10
JournalJournal of Physical Chemistry A
Volume114
Issue number18
DOIs
Publication statusPublished - May 13 2010

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atomic energy levels
Paramagnetic resonance
wire
Wire
Molecules
Ions
phenothiazines
molecules
Charge transfer
Orbits
Temperature
Perylene
electron paramagnetic resonance
ions
charge transfer
Spin polarization
Exchange interactions
orbits
Dihedral angle
spin exchange

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry

Cite this

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title = "Time-resolved EPR studies of charge recombination and triplet-state formation within donor-bridge-acceptor molecules having wire-like oligofluorene bridges",
abstract = "Spin-selective charge recombination of photogenerated radical ion pairs within a series of donor-bridge-acceptor (D-B-A) molecules, where D = phenothiazine (PTZ), B = oligo(2,7-fluorenyl), and A = perylene-3,4:9,10- bis(dicarboximide) (PDI), PTZ-FLn-PDI, where n = 1-4 (compounds 1-4), is studied using time-resolved electron paramagnetic resonance (TREPR) spectroscopy in which the microwave source is either continuous-wave or pulsed. Radical ion pair TREPR spectra are observed for 3 and 4 at 90-294 K, while the neutral triplet state of PDI (3*PDI) is observed at 90-294 K for 2-4 and at 90 K for 1. 3*PDI is produced by three mechanisms, as elucidated by its zero-field splitting parameters and spin polarization pattern. The mechanisms are spin-orbit-induced intersystem crossing (SO-ISC) in PDI aggregates, direct spin-orbit charge-transfer intersystem crossing (SOCT) from the singlet radical pair within 1, and radical pair intersystem crossing (RP-ISC) as a result of S-T0 mixing of the radical ion pair states in 2-4. The temperature dependence of the spin-spin exchange interaction (2J) shows a dramatic decrease at low temperatures, indicating that the electronic coupling between the radical ions decreases due to an increase in the average fluorene-fluorene dihedral angle at low temperatures. The charge recombination rates for 3 and 4 decrease at low temperature, but that for 2 is almost temperature-independent. These results strongly suggest that the dominant mechanism of charge recombination for n ≥ 3 is incoherent thermal hopping, which results in wire-like charge transfer.",
author = "Tomoaki Miura and Raanan Carmieli and Wasielewski, {Michael R}",
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T1 - Time-resolved EPR studies of charge recombination and triplet-state formation within donor-bridge-acceptor molecules having wire-like oligofluorene bridges

AU - Miura, Tomoaki

AU - Carmieli, Raanan

AU - Wasielewski, Michael R

PY - 2010/5/13

Y1 - 2010/5/13

N2 - Spin-selective charge recombination of photogenerated radical ion pairs within a series of donor-bridge-acceptor (D-B-A) molecules, where D = phenothiazine (PTZ), B = oligo(2,7-fluorenyl), and A = perylene-3,4:9,10- bis(dicarboximide) (PDI), PTZ-FLn-PDI, where n = 1-4 (compounds 1-4), is studied using time-resolved electron paramagnetic resonance (TREPR) spectroscopy in which the microwave source is either continuous-wave or pulsed. Radical ion pair TREPR spectra are observed for 3 and 4 at 90-294 K, while the neutral triplet state of PDI (3*PDI) is observed at 90-294 K for 2-4 and at 90 K for 1. 3*PDI is produced by three mechanisms, as elucidated by its zero-field splitting parameters and spin polarization pattern. The mechanisms are spin-orbit-induced intersystem crossing (SO-ISC) in PDI aggregates, direct spin-orbit charge-transfer intersystem crossing (SOCT) from the singlet radical pair within 1, and radical pair intersystem crossing (RP-ISC) as a result of S-T0 mixing of the radical ion pair states in 2-4. The temperature dependence of the spin-spin exchange interaction (2J) shows a dramatic decrease at low temperatures, indicating that the electronic coupling between the radical ions decreases due to an increase in the average fluorene-fluorene dihedral angle at low temperatures. The charge recombination rates for 3 and 4 decrease at low temperature, but that for 2 is almost temperature-independent. These results strongly suggest that the dominant mechanism of charge recombination for n ≥ 3 is incoherent thermal hopping, which results in wire-like charge transfer.

AB - Spin-selective charge recombination of photogenerated radical ion pairs within a series of donor-bridge-acceptor (D-B-A) molecules, where D = phenothiazine (PTZ), B = oligo(2,7-fluorenyl), and A = perylene-3,4:9,10- bis(dicarboximide) (PDI), PTZ-FLn-PDI, where n = 1-4 (compounds 1-4), is studied using time-resolved electron paramagnetic resonance (TREPR) spectroscopy in which the microwave source is either continuous-wave or pulsed. Radical ion pair TREPR spectra are observed for 3 and 4 at 90-294 K, while the neutral triplet state of PDI (3*PDI) is observed at 90-294 K for 2-4 and at 90 K for 1. 3*PDI is produced by three mechanisms, as elucidated by its zero-field splitting parameters and spin polarization pattern. The mechanisms are spin-orbit-induced intersystem crossing (SO-ISC) in PDI aggregates, direct spin-orbit charge-transfer intersystem crossing (SOCT) from the singlet radical pair within 1, and radical pair intersystem crossing (RP-ISC) as a result of S-T0 mixing of the radical ion pair states in 2-4. The temperature dependence of the spin-spin exchange interaction (2J) shows a dramatic decrease at low temperatures, indicating that the electronic coupling between the radical ions decreases due to an increase in the average fluorene-fluorene dihedral angle at low temperatures. The charge recombination rates for 3 and 4 decrease at low temperature, but that for 2 is almost temperature-independent. These results strongly suggest that the dominant mechanism of charge recombination for n ≥ 3 is incoherent thermal hopping, which results in wire-like charge transfer.

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