Interpretation of Mixed-Valence Compound Optical Spectra Near the Class II/III Border: Dinitrobiphenyl and Dinitrophenanthrene Radical Anions
The optical spectra of 4,4′-dinitrobiphenyl and other similar 9-bond bridged radical anions show that these mixed-valence compounds are predominantly charge-localized in the high λS solvent MeCN, charge-delocalized in the low λS solvent HMPA, and show intermediate behavior in DMF. Hush analysis of t...
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Published in | The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Vol. 112; no. 49; pp. 12622 - 12628 |
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Main Authors | , , |
Format | Journal Article |
Language | English |
Published |
United States
American Chemical Society
11.12.2008
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Subjects | |
Online Access | Get full text |
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Summary: | The optical spectra of 4,4′-dinitrobiphenyl and other similar 9-bond bridged radical anions show that these mixed-valence compounds are predominantly charge-localized in the high λS solvent MeCN, charge-delocalized in the low λS solvent HMPA, and show intermediate behavior in DMF. Hush analysis of the localized charge-transfer band in MeCN allowed the calculation of the electronic coupling between nitro groups (H ab). H ab changes with bridge structure, depending mainly on the twist angle between the two aromatic rings: H ab is higher for the planar 9,9-dimethyl-2,7-dinitrofluorene radical anion (1100 cm−1) and about one-half of this value for the more twisted 2,2′-dimethyl-4,4′-dinitrobiphenyl radical anion (540 cm−1). The reorganization energy λ decreases as H ab increases. We suggest that this is due to a decrease of the internal reorganization energy λv as the Class II/Class III borderline is approached, and that λv should be zero at the borderline. Subtracting from the experimental spectra the fraction corresponding to the delocalized part (taken as the spectrum in HMPA or THF), we get localized charge-transfer bands that show a significant cutoff effect at the low energy side, as predicted by classical Marcus−Hush theory. |
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Bibliography: | istex:C1A2CFA5F6CD85C6A6F677B6374B45E8F2A42CB7 ark:/67375/TPS-NQR4MB72-V Comparison of the observed spectrum of 4- with those having weighted and shifted THF and HMPA spectra subtracted in MeCN, DMSO, and DMF, and similar information for 5- with HMPA spectra subtracted in MeCN and DMF. Comparison of observed and Koopmans-based calculated spectra for 5- and 9-. This material is available free of charge via the Internet at http://pubs.acs.org. ObjectType-Article-1 SourceType-Scholarly Journals-1 ObjectType-Feature-2 content type line 23 |
ISSN: | 1089-5639 1520-5215 |
DOI: | 10.1021/jp807602k |