Solvent Effects on Electronic Coupling between Dialkylamino Groups in Aromatic Diamine Radical Cations

Intervalence Transitions in the Mixed-Valence Monocations of Bis(triarylamines) Linked with Vinylene and Phenylene−Vinylene Bridges

(E)-4,4'-Bis[bis(4-methoxyphenyl)amino]stilbene, 1, (E,E)-1,4-bis[4-[bis(4-methoxyphenyl)amino]styryl]benzene, 2, and two longer homologues, (E,E,E)-4,4'-bis[4-[bis(4-methoxyphenyl)amino]styryl]stilbene, 3, and (E,E,E,E)-1,4-bis(4-[4-[bis(4-methoxyphenyl)amino]styryl]styryl)benzene, 4, have been oxidized to their mono- and dications using tris(4-bromophenyl)aminium hexachloroantimonate. The intervalence charge-transfer (IVCT) band of 1(+) is narrow and asymmetric and exhibits only weak solvatochromism. Analysis of this band indicates that 1(+) is a class-III or class-II/III borderline mixed-valence species. In contrast, a broad, strongly solvatochromic IVCT band is observed for 2(+), indicating that this species is a class-II mixed-valence species. The assignment of 1(+ ) and 2(+) as symmetric class-III and unsymmetric class-II species, respectively, is also supported by AM1 calculations. Hush analysis of the IVCT bands of both 1(+) and 2(+) gives larger electronic couplings, V, than for their analogues in which the double bonds are replaced with triple bonds. The diabatic electron-transfer distance, R, in 1(+) can be estimated by comparison of the V estimated by Hush analysis and from the IVCT maximum; it is considerably less than the geometric N-N separation, a result supported by quantum-chemical estimates of R for 1(+)-4(+). In 3(+) and 4(+), the IVCT is largely obscured by an intense absorption similar to a band seen in the corresponding dications and to that observed in the monocation of a model compound, (E,E,E)-1-[bis(4-methoxyphenyl)amino]-4-[4-[4-(4-tert-butylstyryl)styryl]styryl]benzene, 5, containing only one nitrogen redox center; we attribute this band to a bridge-to-N(+) transition. The corresponding dications 1(2+)-4(2+) show a complementary trend in the coupling between redox centers: the shortest species is diamagnetic, while the dication with the longest bridge behaves as two essentially noninteracting radical centers.

“Separated” versus “Contact” Ion-Pair Structures in Solution from Their Crystalline States: Dynamic Effects on Dinitrobenzenide as a Mixed-Valence Anion

Qualitative structural concepts about dynamic ion pairs, historically deduced in solution as labile solvent-separated and contact species, are now quantified by the low-temperature isolation of crystalline (reactive) salts suitable for direct X-ray analysis. Thus, dinitrobenzenide anion (DNB(-)) can be prepared in the two basic ion-paired forms by potassium-mirror reduction of p-dinitrobenzene in the presence of macrocyclic polyether ligands: L(C) (cryptand) and L(E) (crown-ethers). The crystalline "separated" ion-pair salt isolated as K(L(C))(+)//DNB(-) is crystallographically differentiated from the "contact" ion-pair salt isolated as K(L(E))(+)DNB(-) by their distinctive interionic separations. Spectral analysis reveals pronounced near-IR absorptions arising from intervalence transitions that characterize dinitrobenzenide to be a prototypical mixed-valence anion. Most importantly, the unique patterns of vibronic (fine-structure) progressions that also distinguish the "separated" from the "contact" ion pair in the crystalline solid state are the same as those dissolved into THF solvent and ensure that the same X-ray structures persist in solution. Moreover, these distinctive NIR patterns are assigned with the aid of Marcus-Hush (two-state) theory to the "separated"ion pair in which the unpaired electron is equally delocalized between both NO(2)-centers in the symmetric ground state of dinitrobenzenide, and by contrast, the asymmetric electron distribution inherent to "contact"ion pairs favors only that single NO(2)-center intimately paired to the counterion. The labilities of these dynamic ion pairs in solution are thoroughly elucidated by temperature-dependent ESR spectral changes that provide intimate details of facile isomerizations, ionic separations, and counterion-mediated exchanges.

Intervalence Near-IR Spectra of Delocalized Dinitroaromatic Radical Anions

The Class III (delocalized) intervalence radical anions of 1,4-dinitrobenzene, 2,6-dinitronaphthalene, 2,6-dinitroanthracene, 9,9-dimethyl-2,7-dinitrofluorene, 4,4'-dinitrobiphenyl, and 1,5-dinitronaphthalene show charge-transfer bands in their near-IR spectra. The dinitroaromatic radical anions have comparable but slightly larger electronic interactions (H(ab) values) through the same aromatic bridges as do the corresponding dianisylamino-substituted radical cations. H(ab) values range from 5410 cm(-)(1) (1,4- dinitrobenzene) to 3400 cm(-)(1) (9,9-dimethyl-2,7-dinitrofluorene), decreasing as the number of bonds between the nitro groups increases, except for the 1,5-dinitronaphthalene radical-anion, which has a coupling similar to that of 9,9-dimethyl-2,7-dinitrofluorene. All charge-transfer bands show vibrational fine structure. The vertical excitation energies (lambda(v)) were estimated from the vibrational components, obtained by simulation of the entire band. The large 2H(ab)/lambda(v) values confirm these radicals to be Class III delocalized mixed-valence species. Analysis using Cave and Newton's generalized Mulliken-Hush theory relating the transition dipole moment to the distance on the diabatic surfaces suggests that the electron-transfer distance on the diabatic surfaces, d(ab), is only 26-40% of the nitrogen-to-nitrogen distance, which implies that something may be wrong with our analysis.

Contributions of symmetric and asymmetric normal coordinates to the intervalence electronic absorption and resonance Raman spectra of a strongly coupled p-phenylenediamine radical cation

Resonance Raman spectroscopy, electronic absorption spectroscopy, and the time-dependent theory of spectroscopy are used to analyze the intervalence electron transfer properties of a strongly delocalized class III molecule, the tetraalkyl-p-phenylene diamine radical cation bis(3-oxo-9-azabicyclo[3.3.1]non-9-yl)benzene ((k33)(2)PD(+)). This molecule is a prototypical system for strongly coupled organic intervalence electron transfer spectroscopy. Resonance Raman excitation profiles in resonance with the lowest energy absorption band are measured. The normal modes of vibration that are most strongly coupled to the intervalence transition are identified and assigned by using UB3LYP/6-31G(d) calculations. Excited state distortions are obtained, and the resonance Raman intensities and excitation profiles are calculated by using the time-dependent theory of Raman spectroscopy. The most highly distorted normal modes are all totally symmetric, but intervalence electron transfer absorption spectra are usually interpreted in terms of a model based on coupling between potential surfaces that are displaced along an asymmetric normal coordinate. This model provides a convenient physical picture for the intervalence compound, but it is inadequate for explaining the spectra. The absorption spectrum arising from only the strongly coupled surfaces consists of a single narrow band, in contrast to the broad, vibronically structured experimental spectrum. The electronic absorption spectrum of (k33)(2)PD(+) is calculated by using exactly the same potential surfaces as those used for the Raman calculations. The importance of symmetric normal coordinates, in addition to the asymmetric coordinate, is discussed. The observed vibronic structure is an example of the missing mode effect; the spacing is interpreted in terms of the time-dependent overlaps in the time domain.