Quenching of diphenylmethyl radical fluorescence by cyanoaromatics and phenols

Quenching of diphenylmethyl radical fluorescence by cyanoaromatics and phenols We have used double-laser nanosecond laser flash photolysis to study the kinetics of bimolecular quenching of the lowest doublet excited state of the transient diphenylmethyl radical (2Ph2CH·) in acetonitrile at 25 °C. We chose a series of cyanoaromatic compounds as acceptor-type quenchers and a series of 4-substituted phenols as donor-type quenchers. The observed bimolecular quenching rate constants (k Q) are in the range 2 × 106–2 × 1010 M −1 s−1 for cyanoaromatics and 1 × 105–3 × 109 M −1 s−1 for phenols. The Hammett plots of the k Q values for these two series have positive and negative slopes, respectively, and establish the enhanced behavior of the photo-excited diphenylmethyl radical as both a donor and an acceptor. For cyanoaromatics, the trend in the dependence of 2Ph2CH·* quenching rate constants on the free energy change of electron transfer (ΔG el) coincides well with that of Rehm–Weller data on acceptor-type and donor-type quenchers of singlet excited states. An approximate fit of the k Q data for cyanoaromatics to the Agmon–Levine model of electron transfer gave a value of 4.5 kcal mol−1 for the free energy of activation at ΔG el = 0. For phenols, the trend in the dependence of k Q on the free energy change of electron transfer deviates significantly from that of cyanoaromatics data. This deviation was traced to the use of irreversible oxidation potentials of phenols to calculate ΔG el. No isotope effect is observed from substitution of phenolic hydrogen by deuterium, i.e., k Q,H/k Q,D ≈ 1. Phenolate ions quench 2Ph2CH·* with large rate constants, 1–2 × 1010 M −1 s−1, i.e., well in the limit of diffusion control. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Research on Chemical Intermediates Springer Journals

Quenching of diphenylmethyl radical fluorescence by cyanoaromatics and phenols

Loading next page...
 
/lp/springer_journal/quenching-of-diphenylmethyl-radical-fluorescence-by-cyanoaromatics-and-EZNXARRAup
Publisher
Springer Journals
Copyright
Copyright © 2015 by Springer Science+Business Media Dordrecht
Subject
Chemistry; Catalysis; Physical Chemistry; Inorganic Chemistry
ISSN
0922-6168
eISSN
1568-5675
D.O.I.
10.1007/s11164-014-1914-0
Publisher site
See Article on Publisher Site

Abstract

We have used double-laser nanosecond laser flash photolysis to study the kinetics of bimolecular quenching of the lowest doublet excited state of the transient diphenylmethyl radical (2Ph2CH·) in acetonitrile at 25 °C. We chose a series of cyanoaromatic compounds as acceptor-type quenchers and a series of 4-substituted phenols as donor-type quenchers. The observed bimolecular quenching rate constants (k Q) are in the range 2 × 106–2 × 1010 M −1 s−1 for cyanoaromatics and 1 × 105–3 × 109 M −1 s−1 for phenols. The Hammett plots of the k Q values for these two series have positive and negative slopes, respectively, and establish the enhanced behavior of the photo-excited diphenylmethyl radical as both a donor and an acceptor. For cyanoaromatics, the trend in the dependence of 2Ph2CH·* quenching rate constants on the free energy change of electron transfer (ΔG el) coincides well with that of Rehm–Weller data on acceptor-type and donor-type quenchers of singlet excited states. An approximate fit of the k Q data for cyanoaromatics to the Agmon–Levine model of electron transfer gave a value of 4.5 kcal mol−1 for the free energy of activation at ΔG el = 0. For phenols, the trend in the dependence of k Q on the free energy change of electron transfer deviates significantly from that of cyanoaromatics data. This deviation was traced to the use of irreversible oxidation potentials of phenols to calculate ΔG el. No isotope effect is observed from substitution of phenolic hydrogen by deuterium, i.e., k Q,H/k Q,D ≈ 1. Phenolate ions quench 2Ph2CH·* with large rate constants, 1–2 × 1010 M −1 s−1, i.e., well in the limit of diffusion control.

Journal

Research on Chemical IntermediatesSpringer Journals

Published: Jan 21, 2015

References

You’re reading a free preview. Subscribe to read the entire article.


DeepDyve is your
personal research library

It’s your single place to instantly
discover and read the research
that matters to you.

Enjoy affordable access to
over 18 million articles from more than
15,000 peer-reviewed journals.

All for just $49/month

Explore the DeepDyve Library

Search

Query the DeepDyve database, plus search all of PubMed and Google Scholar seamlessly

Organize

Save any article or search result from DeepDyve, PubMed, and Google Scholar... all in one place.

Access

Get unlimited, online access to over 18 million full-text articles from more than 15,000 scientific journals.

Your journals are on DeepDyve

Read from thousands of the leading scholarly journals from SpringerNature, Elsevier, Wiley-Blackwell, Oxford University Press and more.

All the latest content is available, no embargo periods.

See the journals in your area

DeepDyve

Freelancer

DeepDyve

Pro

Price

FREE

$49/month
$360/year

Save searches from
Google Scholar,
PubMed

Create lists to
organize your research

Export lists, citations

Read DeepDyve articles

Abstract access only

Unlimited access to over
18 million full-text articles

Print

20 pages / month

PDF Discount

20% off