CHARGE-LOCALIZED P-PHENYLENEDIHYDRAZINE RADICAL CATIONS - ESR AND OPTICAL STUDIES OF INTRAMOLECULAR ELECTRON-TRANSFER RATES

l-2,3-diazabicyclo[2.2.2]oct-3-yl)benzene-1,4-diyl (2) its 2,5-dimethy l and 2,3,5,6-tetramethyl derivatives (3 and 4), their radical cations , and bis-radical dications are studied. Crystal structures including those of 2(+)BPh(4)(-), 3(2+)(BPh4-)(2), 4(+)BPh(4)(-), and 4(2+)(BPh4 -)(2) establish that ring methylation causes more N-lone pair, aryl pi twist without changing the N-Ar,N-Ar distance significantly and that both 2(+) and 4(+) have the charge localized in one hydrazine unit. NM R measurements show that 3(+) has about 6% of its spin at the four ary l CH and CMe carbons, while 4(+) has about 1.5% of its spin at the fou r CMe carbons. The average distance between the unpaired electrons of 3(2+) and 4(2+) was obtained from the dipolar splittings of their ther mally excited triplet states and, as expected, is significantly smalle r for 3(2+) (5.25 Angstrom) than for 4(2+) (5.63 Angstrom). Rate const ants for electron transfer between the hydrazine units of 3(+) and 4() in CH2Cl2 and CH3CN were determined by dynamic ESR. The intervalence radical cations show charge transfer bands corresponding to vertical electron transfer between the ground state and the highly vibrationall y excited electron-shifted state, allowing calculation of the paramete rs controlling electron transfer. Electron transfer parameters obtaine d from the CT bands using adiabatic energy surfaces which approximate the CT band shapes observed produce rate constants within experimental error of those extrapolated to room temperature from the ESR data for both 3(+) and 4(+) in both solvents, without using tunneling correcti ons. The effects of mixing of the electronic wave functions of the red uced and oxidized hydrazine units of 2(+) on d(NN), the C(t-Bu)N,NA(Ar ) twist angle, and the aryl nitrogen lone pair, aryl pi twist angle wh ich are observed by X-ray are close to those predicted from the positi on of the minima on the ET coordinate X of the adiabatic energy surfac e calculated from the CT band.