Exponential distance dependence of photoinitiated stepwise electron transfer in donor-bridge-acceptor molecules

Implications for wirelike behavior

Annie Butler Ricks, Kristen E. Brown, Matthias Wenninger, Steven D. Karlen, Yuri A. Berlin, Dick T Co, Michael R Wasielewski

Research output: Contribution to journalArticle

40 Citations (Scopus)

Abstract

Donor-bridge-acceptor (D-B-A) systems in which a 3,5-dimethyl-4-(9- anthracenyl)julolidine (DMJ-An) chromophore and a naphthalene-1,8:4,5- bis(dicarboximide) (NI) acceptor are linked by oligomeric 2,7-fluorenone (FN n) bridges (n = 1-3) have been synthesized. Selective photoexcitation of DMJ-An quantitatively produces DMJ -An , and An acts as a high-potential electron donor. Femtosecond transient absorption spectroscopy in the visible and mid-IR regions showed that electron transfer occurs quantitatively in the sequence: DMJ -An -FN n-NI → DMJ -An-FN n -NI → DMJ -An-FN n-NI . The charge-shift reaction from An to NI exhibits an exponential distance dependence in the nonpolar solvent toluene with an attenuation factor (β) of 0.34 Å -1, which would normally be attributed to electron tunneling by the superexchange mechanism. However, the FN n radical anion was directly observed spectroscopically as an intermediate in the charge-separation mechanism, thereby demonstrating conclusively that the overall charge separation involves the incoherent hopping (stepwise) mechanism. Kinetic modeling of the data showed that the observed exponential distance dependence is largely due to electron injection onto the first FN unit followed by charge hopping between the FN units of the bridge biased by the distance-dependent electrostatic attraction of the two charges in D -B -A. This work shows that wirelike behavior does not necessarily result from building a stepwise, energetically downhill redox gradient into a D-B-A molecule.

Original languageEnglish
Pages (from-to)4581-4588
Number of pages8
JournalJournal of the American Chemical Society
Volume134
Issue number10
DOIs
Publication statusPublished - Mar 14 2012

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Electrons
Molecules
Electron injection
Electron tunneling
Photoexcitation
Naphthalene
Chromophores
Absorption spectroscopy
Toluene
Electrostatics
Negative ions
Kinetics
3,5-dimethyl-4-(9-anthracenyl)julolidine
Static Electricity
Oxidation-Reduction
Anions
Spectrum Analysis
Injections

ASJC Scopus subject areas

  • Chemistry(all)
  • Catalysis
  • Biochemistry
  • Colloid and Surface Chemistry

Cite this

Exponential distance dependence of photoinitiated stepwise electron transfer in donor-bridge-acceptor molecules : Implications for wirelike behavior. / Ricks, Annie Butler; Brown, Kristen E.; Wenninger, Matthias; Karlen, Steven D.; Berlin, Yuri A.; Co, Dick T; Wasielewski, Michael R.

In: Journal of the American Chemical Society, Vol. 134, No. 10, 14.03.2012, p. 4581-4588.

Research output: Contribution to journalArticle

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title = "Exponential distance dependence of photoinitiated stepwise electron transfer in donor-bridge-acceptor molecules: Implications for wirelike behavior",
abstract = "Donor-bridge-acceptor (D-B-A) systems in which a 3,5-dimethyl-4-(9- anthracenyl)julolidine (DMJ-An) chromophore and a naphthalene-1,8:4,5- bis(dicarboximide) (NI) acceptor are linked by oligomeric 2,7-fluorenone (FN n) bridges (n = 1-3) have been synthesized. Selective photoexcitation of DMJ-An quantitatively produces DMJ +̇-An -̇, and An -̇ acts as a high-potential electron donor. Femtosecond transient absorption spectroscopy in the visible and mid-IR regions showed that electron transfer occurs quantitatively in the sequence: DMJ +̇-An -̇-FN n-NI → DMJ +̇-An-FN n -̇-NI → DMJ +̇-An-FN n-NI -̇. The charge-shift reaction from An -̇ to NI -̇ exhibits an exponential distance dependence in the nonpolar solvent toluene with an attenuation factor (β) of 0.34 {\AA} -1, which would normally be attributed to electron tunneling by the superexchange mechanism. However, the FN n -̇ radical anion was directly observed spectroscopically as an intermediate in the charge-separation mechanism, thereby demonstrating conclusively that the overall charge separation involves the incoherent hopping (stepwise) mechanism. Kinetic modeling of the data showed that the observed exponential distance dependence is largely due to electron injection onto the first FN unit followed by charge hopping between the FN units of the bridge biased by the distance-dependent electrostatic attraction of the two charges in D +̇-B -̇-A. This work shows that wirelike behavior does not necessarily result from building a stepwise, energetically downhill redox gradient into a D-B-A molecule.",
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AU - Ricks, Annie Butler

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N2 - Donor-bridge-acceptor (D-B-A) systems in which a 3,5-dimethyl-4-(9- anthracenyl)julolidine (DMJ-An) chromophore and a naphthalene-1,8:4,5- bis(dicarboximide) (NI) acceptor are linked by oligomeric 2,7-fluorenone (FN n) bridges (n = 1-3) have been synthesized. Selective photoexcitation of DMJ-An quantitatively produces DMJ +̇-An -̇, and An -̇ acts as a high-potential electron donor. Femtosecond transient absorption spectroscopy in the visible and mid-IR regions showed that electron transfer occurs quantitatively in the sequence: DMJ +̇-An -̇-FN n-NI → DMJ +̇-An-FN n -̇-NI → DMJ +̇-An-FN n-NI -̇. The charge-shift reaction from An -̇ to NI -̇ exhibits an exponential distance dependence in the nonpolar solvent toluene with an attenuation factor (β) of 0.34 Å -1, which would normally be attributed to electron tunneling by the superexchange mechanism. However, the FN n -̇ radical anion was directly observed spectroscopically as an intermediate in the charge-separation mechanism, thereby demonstrating conclusively that the overall charge separation involves the incoherent hopping (stepwise) mechanism. Kinetic modeling of the data showed that the observed exponential distance dependence is largely due to electron injection onto the first FN unit followed by charge hopping between the FN units of the bridge biased by the distance-dependent electrostatic attraction of the two charges in D +̇-B -̇-A. This work shows that wirelike behavior does not necessarily result from building a stepwise, energetically downhill redox gradient into a D-B-A molecule.

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