Influence of structural dynamics on charge recombination rates in photogenerated radical ion pairs: Evidence from EPR spectroscopy and computation

Research output: Contribution to journalArticle

4 Citations (Scopus)

Abstract

In order to better understand the dependence of charge recombination rate vs. temperature kCR(T) within a linear donor-chromophore-acceptor (D-C-A) molecular triad, the structural dynamics of the cation radical D -C is studied individually using variable-temperature electron paramagnetic resonance (EPR) spectroscopy and electronic structure calculations. Here, the donor D is p-methoxyaniline, the chromophore C is 4-(N-piperidinyl)-naphthalene-1,8-dicarboximide, and the acceptor A is naphthalene-l,8:4,5-bis(dicarboximide). The EPR spectra of D-C exhibit marked changes in their overall shape throughout the 190-295 K temperature range. These spectra have hyperfine splittings that are strikingly well simulated with a model that includes methoxy group rotation, which occurs at a rate of 2.6 · 104 s-1 at 210 K and speeds up to 1.25 · 107 s-1 at 295 K, corresponding to an energy barrier of 38 kJ/mol. This considerable barrier reflects the partial conjugation between MeO and the aromatic ring and is confirmed by the calculated energy of a series of D-C rotamers. The simulations also reveal that inversion of the anilino N center emerges at T > 250 K and can be represented by a planar and a pyramidal conformation with the equilibrium constant K = [pyramidal]/[planar] increasing from 0.029 at 250 K to 0.56 at 295 K. In the same temperature range, the charge recombination rate of D -C-A accelerates abruptly and can be separated into two components, according to the above planar/pyramidal equilibrium. The kCR(T) of the pyramidal conformation has an activation energy of 41 kJ/mol, virtually the same as the barrier of MeO rotation. These results show that the intramolecular structural dynamics of the radical cation within D -C-A control the overall charge recombination reaction with this radical ion pair.

Original languageEnglish
Pages (from-to)253-270
Number of pages18
JournalApplied Magnetic Resonance
Volume31
Issue number1-2
Publication statusPublished - 2007

Fingerprint

dynamic structural analysis
Structural dynamics
Paramagnetic resonance
electron paramagnetic resonance
Spectroscopy
Ions
Chromophores
Conformations
Cations
naphthalene
spectroscopy
chromophores
ions
Energy barriers
Equilibrium constants
cations
recombination reactions
Temperature
Electronic structure
temperature

ASJC Scopus subject areas

  • Spectroscopy
  • Atomic and Molecular Physics, and Optics

Cite this

@article{576cb6d356174b72bb68c7c498e7513e,
title = "Influence of structural dynamics on charge recombination rates in photogenerated radical ion pairs: Evidence from EPR spectroscopy and computation",
abstract = "In order to better understand the dependence of charge recombination rate vs. temperature kCR(T) within a linear donor-chromophore-acceptor (D-C-A) molecular triad, the structural dynamics of the cation radical D +̇-C is studied individually using variable-temperature electron paramagnetic resonance (EPR) spectroscopy and electronic structure calculations. Here, the donor D is p-methoxyaniline, the chromophore C is 4-(N-piperidinyl)-naphthalene-1,8-dicarboximide, and the acceptor A is naphthalene-l,8:4,5-bis(dicarboximide). The EPR spectra of D+̇-C exhibit marked changes in their overall shape throughout the 190-295 K temperature range. These spectra have hyperfine splittings that are strikingly well simulated with a model that includes methoxy group rotation, which occurs at a rate of 2.6 · 104 s-1 at 210 K and speeds up to 1.25 · 107 s-1 at 295 K, corresponding to an energy barrier of 38 kJ/mol. This considerable barrier reflects the partial conjugation between MeO and the aromatic ring and is confirmed by the calculated energy of a series of D+̇-C rotamers. The simulations also reveal that inversion of the anilino N center emerges at T > 250 K and can be represented by a planar and a pyramidal conformation with the equilibrium constant K = [pyramidal]/[planar] increasing from 0.029 at 250 K to 0.56 at 295 K. In the same temperature range, the charge recombination rate of D +̇-C-A-̇ accelerates abruptly and can be separated into two components, according to the above planar/pyramidal equilibrium. The kCR(T) of the pyramidal conformation has an activation energy of 41 kJ/mol, virtually the same as the barrier of MeO rotation. These results show that the intramolecular structural dynamics of the radical cation within D +̇-C-A control the overall charge recombination reaction with this radical ion pair.",
author = "Q. Mi and Weiss, {Emily A} and Ratner, {Mark A} and Wasielewski, {Michael R}",
year = "2007",
language = "English",
volume = "31",
pages = "253--270",
journal = "Applied Magnetic Resonance",
issn = "0937-9347",
publisher = "Springer Wien",
number = "1-2",

}

TY - JOUR

T1 - Influence of structural dynamics on charge recombination rates in photogenerated radical ion pairs

T2 - Evidence from EPR spectroscopy and computation

AU - Mi, Q.

AU - Weiss, Emily A

AU - Ratner, Mark A

AU - Wasielewski, Michael R

PY - 2007

Y1 - 2007

N2 - In order to better understand the dependence of charge recombination rate vs. temperature kCR(T) within a linear donor-chromophore-acceptor (D-C-A) molecular triad, the structural dynamics of the cation radical D +̇-C is studied individually using variable-temperature electron paramagnetic resonance (EPR) spectroscopy and electronic structure calculations. Here, the donor D is p-methoxyaniline, the chromophore C is 4-(N-piperidinyl)-naphthalene-1,8-dicarboximide, and the acceptor A is naphthalene-l,8:4,5-bis(dicarboximide). The EPR spectra of D+̇-C exhibit marked changes in their overall shape throughout the 190-295 K temperature range. These spectra have hyperfine splittings that are strikingly well simulated with a model that includes methoxy group rotation, which occurs at a rate of 2.6 · 104 s-1 at 210 K and speeds up to 1.25 · 107 s-1 at 295 K, corresponding to an energy barrier of 38 kJ/mol. This considerable barrier reflects the partial conjugation between MeO and the aromatic ring and is confirmed by the calculated energy of a series of D+̇-C rotamers. The simulations also reveal that inversion of the anilino N center emerges at T > 250 K and can be represented by a planar and a pyramidal conformation with the equilibrium constant K = [pyramidal]/[planar] increasing from 0.029 at 250 K to 0.56 at 295 K. In the same temperature range, the charge recombination rate of D +̇-C-A-̇ accelerates abruptly and can be separated into two components, according to the above planar/pyramidal equilibrium. The kCR(T) of the pyramidal conformation has an activation energy of 41 kJ/mol, virtually the same as the barrier of MeO rotation. These results show that the intramolecular structural dynamics of the radical cation within D +̇-C-A control the overall charge recombination reaction with this radical ion pair.

AB - In order to better understand the dependence of charge recombination rate vs. temperature kCR(T) within a linear donor-chromophore-acceptor (D-C-A) molecular triad, the structural dynamics of the cation radical D +̇-C is studied individually using variable-temperature electron paramagnetic resonance (EPR) spectroscopy and electronic structure calculations. Here, the donor D is p-methoxyaniline, the chromophore C is 4-(N-piperidinyl)-naphthalene-1,8-dicarboximide, and the acceptor A is naphthalene-l,8:4,5-bis(dicarboximide). The EPR spectra of D+̇-C exhibit marked changes in their overall shape throughout the 190-295 K temperature range. These spectra have hyperfine splittings that are strikingly well simulated with a model that includes methoxy group rotation, which occurs at a rate of 2.6 · 104 s-1 at 210 K and speeds up to 1.25 · 107 s-1 at 295 K, corresponding to an energy barrier of 38 kJ/mol. This considerable barrier reflects the partial conjugation between MeO and the aromatic ring and is confirmed by the calculated energy of a series of D+̇-C rotamers. The simulations also reveal that inversion of the anilino N center emerges at T > 250 K and can be represented by a planar and a pyramidal conformation with the equilibrium constant K = [pyramidal]/[planar] increasing from 0.029 at 250 K to 0.56 at 295 K. In the same temperature range, the charge recombination rate of D +̇-C-A-̇ accelerates abruptly and can be separated into two components, according to the above planar/pyramidal equilibrium. The kCR(T) of the pyramidal conformation has an activation energy of 41 kJ/mol, virtually the same as the barrier of MeO rotation. These results show that the intramolecular structural dynamics of the radical cation within D +̇-C-A control the overall charge recombination reaction with this radical ion pair.

UR - http://www.scopus.com/inward/record.url?scp=34249111115&partnerID=8YFLogxK

UR - http://www.scopus.com/inward/citedby.url?scp=34249111115&partnerID=8YFLogxK

M3 - Article

AN - SCOPUS:34249111115

VL - 31

SP - 253

EP - 270

JO - Applied Magnetic Resonance

JF - Applied Magnetic Resonance

SN - 0937-9347

IS - 1-2

ER -