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Title: Probing Intermolecular Electron Delocalization in Dimer Radical Anions by Vibrational Spectroscopy

Abstract

Delocalization of charges is one of the factors controlling charge transport in conjugated molecules. It is considered to play an important role in the performance of a wide range of molecular technologies, including organic solar cells and organic electronics. Dimerization reactions are well-suited as a model to investigate intermolecular spatial delocalization of charges. And while dimerization reactions of radical cations are well investigated, studies on radical anions are still scarce. Upon dimerization of radical anions with neutral counterparts, an electron is considered to delocalize over the two molecules. By using time-resolved infrared (TRIR) detection coupled with pulse radiolysis, we show that radical anions of 4-n-hexyl-4'-cyanobiphenyl (6CB) undergo such dimerization reactions, with an electron equally delocalized over the two molecules. We have recently demonstrated that nitrile ν(C≡N) vibrations respond to the degree of electron localization of nitrile-substituted anions: we can quantify the changes in the electronic charges from the neutral to the anion states in the nitriles by monitoring the ν(C≡N) IR shifts. In the first part of this article, we show that the sensitivity of the ν(C≡N) IR shifts does not depend on solvent polarity. In the second part, we describe how probing the shifts of the nitrile IR vibrationalmore » band unambiguously confirms the formation of dimer radical anions, with K dim = 3 × 10 4 M –1. IR findings are corroborated by electronic absorption spectroscopy and electronic structure calculations. We find that the presence of a hexyl chain and the formation of π–π interactions are both crucial for dimerization of radical anions of 6CB with neutral 6CB. Our study provides clear evidence of spatial delocalization of electrons over two molecular fragments.« less

Authors:
ORCiD logo [1]; ORCiD logo [2]
  1. Univ. of Connecticut, Storrs, CT (United States). Dept. of Chemistry; Brookhaven National Lab. (BNL), Upton, NY (United States). Chemistry Division
  2. Brookhaven National Lab. (BNL), Upton, NY (United States). Chemistry Division
Publication Date:
Research Org.:
Brookhaven National Laboratory (BNL), Upton, NY (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
OSTI Identifier:
1399684
Report Number(s):
BNL-114408-2017-JA
Journal ID: ISSN 1520-6106; R&D Project: CO026; KC0304030; TRN: US1703088
Grant/Contract Number:
SC0012704
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Journal of Physical Chemistry. B, Condensed Matter, Materials, Surfaces, Interfaces and Biophysical Chemistry
Additional Journal Information:
Journal Volume: 121; Journal Issue: 30; Journal ID: ISSN 1520-6106
Publisher:
American Chemical Society
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY

Citation Formats

Mani, Tomoyasu, and Grills, David C. Probing Intermolecular Electron Delocalization in Dimer Radical Anions by Vibrational Spectroscopy. United States: N. p., 2017. Web. doi:10.1021/acs.jpcb.7b02559.
Mani, Tomoyasu, & Grills, David C. Probing Intermolecular Electron Delocalization in Dimer Radical Anions by Vibrational Spectroscopy. United States. doi:10.1021/acs.jpcb.7b02559.
Mani, Tomoyasu, and Grills, David C. Wed . "Probing Intermolecular Electron Delocalization in Dimer Radical Anions by Vibrational Spectroscopy". United States. doi:10.1021/acs.jpcb.7b02559. https://www.osti.gov/servlets/purl/1399684.
@article{osti_1399684,
title = {Probing Intermolecular Electron Delocalization in Dimer Radical Anions by Vibrational Spectroscopy},
author = {Mani, Tomoyasu and Grills, David C.},
abstractNote = {Delocalization of charges is one of the factors controlling charge transport in conjugated molecules. It is considered to play an important role in the performance of a wide range of molecular technologies, including organic solar cells and organic electronics. Dimerization reactions are well-suited as a model to investigate intermolecular spatial delocalization of charges. And while dimerization reactions of radical cations are well investigated, studies on radical anions are still scarce. Upon dimerization of radical anions with neutral counterparts, an electron is considered to delocalize over the two molecules. By using time-resolved infrared (TRIR) detection coupled with pulse radiolysis, we show that radical anions of 4-n-hexyl-4'-cyanobiphenyl (6CB) undergo such dimerization reactions, with an electron equally delocalized over the two molecules. We have recently demonstrated that nitrile ν(C≡N) vibrations respond to the degree of electron localization of nitrile-substituted anions: we can quantify the changes in the electronic charges from the neutral to the anion states in the nitriles by monitoring the ν(C≡N) IR shifts. In the first part of this article, we show that the sensitivity of the ν(C≡N) IR shifts does not depend on solvent polarity. In the second part, we describe how probing the shifts of the nitrile IR vibrational band unambiguously confirms the formation of dimer radical anions, with Kdim = 3 × 104 M–1. IR findings are corroborated by electronic absorption spectroscopy and electronic structure calculations. We find that the presence of a hexyl chain and the formation of π–π interactions are both crucial for dimerization of radical anions of 6CB with neutral 6CB. Our study provides clear evidence of spatial delocalization of electrons over two molecular fragments.},
doi = {10.1021/acs.jpcb.7b02559},
journal = {Journal of Physical Chemistry. B, Condensed Matter, Materials, Surfaces, Interfaces and Biophysical Chemistry},
number = 30,
volume = 121,
place = {United States},
year = {Wed Jul 05 00:00:00 EDT 2017},
month = {Wed Jul 05 00:00:00 EDT 2017}
}

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