Tuning Aryl, Hydrazine Radical Cation Electronic Interactions Using Substitutent Effects Guadalupe Valverde-Aguilar, Xianghuai Wang, Edward Plummer, Jenny V. Lockard, and Jeffrey I. Zink* Department of Chemistry and Biochemistry, UniVersity of California Los Angeles, California 90095 Yun Luo, Michael N. Weaver, and Stephen F. Nelsen* Department of Chemistry, UniVersity of Wisconsin, 1101 UniVersity AVenue, Madison, Wisconsin 53706-1396 ReceiVed: December 21, 2007; ReVised Manuscript ReceiVed: May 27, 2008 Absorption spectra for 2,3-diaryl-2,3-diazabicyclo[2.2.2]octane radical cations (2(X) •+ ) and for their monoaryl analogues 2-tert-butyl-3-aryl-2,3-diazabicyclo[2.2.2]octane radical cations (1(X) •+ ) having para chloro, bromo, iodo, cyano, phenyl, and nitro substituents are reported and compared with those for the previously reported 1- and 2(H) •+ and 1- and 2(OMe) •+ . The calculated geometries and optical absorption spectra for 2(Cl) •+ demonstrate that p-C 6 H 4 Cl lies between p-C 6 H 4 OMe and C 6 H 5 in its ability to stabilize the lowest energy optical transition of the radical cation, which involves electron donation from the aryl groups toward the π*(NN) + -centered singly occupied molecular orbital of 2(X) •+ . Resonance Raman spectral determination of the reorganization energy for their lowest energy transitions (λ v sym ) increase in the same order, having values of 1420, 5300, and 6000 cm -1 for X ) H, Cl, and OMe, respectively. A neighboring orbital analysis using Koopmans-based calculations of relative orbital energies indicates that the diabatic aryl π-centered molecular orbital that interacts with the dinitrogen π system lies closest in energy to the bonding π(NN)-centered orbital and has an electronic coupling with it of about 9200 ( 600 cm -1 , which does not vary regularly with electron donating power of the X substituent. Introduction This work principally concerns the optical spectra of the radical cations of monoaryl and diaryl 2,3-diaza- bicyclo[2.2.2]octanes, 1(X) •+ and 2(X) •+ . For convenience their designation includes their aryl substituents. All the compounds considered have p-substituted phenyl substituents, except for the 2,4-dinitrocompound shown as 2(NO 2 ) 2 . These compounds show lowest energy optical spectrum absorption bands that correspond to Ar-to-(NN) •+ electron transfer, and their transition energies are sensitive to the substituents X. Hydrazine radical cations are unusual delocalized (Robin- Day Class III) 1 mixed valence (MV) compounds that lack a bridge. They have their disubstituted amino charge-bearing units directly attached by the NN σ bond, and an NN π system having a pair of π electrons, and one π* electron. 2 In the case of 2(X) •+ some charge and spin leaks onto the aryl groups but most resides on the hydrazine 3e-π system. We believe that Creutz first pointed out that the two-state model requires that the transition energy for a Class III MV compound is 2H ab , 3 where H ab is the electronic coupling between the M groups. Many theoretical investigations of H ab have relied upon this prediction of the two-state model, usually with reference to the pioneering work of Larsson and co-workers. 4 Three reviews of IV compounds that have H ab near λ/2, so they are near the Class II/Class III borderline and have very fast electron transfer, have appeared since 2000, 5–7 all employing the two-state model. A principal theme of the present work is that the two-state model should not be applied without alteration to 2(X) •+ or other delocalized MV compounds. In a previous absorption, fluorescence, and resonance Raman study of a 1,4-dihydrazine-substituted durene diradical dication, 8 we pointed out that it does not have a single excited state centered at 0.5 in a Marcus-Hush diagram that the two-state model predicts, because the excited state is split by the fact that either hydrazine group could participate in superexchange electron transfer, so this com- pound has a Marcus-Hush two-state diagram for its lowest excited state. It is not at an MV oxidation level because each hydrazine is at the +1 oxidation level. A somewhat analogous split excited state was found to be exhibited by the MV monocation 2(H) •+ , 9 although the weaker higher energy component of its excited state absorption was not well resolved from other, higher energy, absorptions. The higher energy component of the split excited state was larger and its resolution better for the anisyl-substituted compound 2(OMe) •+ , 10 which allowed a resonance Raman spectroscopy study showing that the excited state coupling affects the absorption band widths for the two components in a manner that may be successfully calculated from the observed Raman intensities and excitation profiles. In this work we consider the trends in excited state splitting, reorganization energy, and electronic coupling that are caused by tuning the energy of the aryl group molecular orbital (MO) relative to those of * Corresponding authors. J. Phys. Chem. A 2008, 112, 7332–7341 7332 10.1021/jp7120005 CCC: $40.75  2008 American Chemical Society Published on Web 07/23/2008