Coupled states in dinitrofluorene: relationships between ground state and excited state mixed valence.

The electronic absorption spectrum of 9,9-dimethyl-2,7-dinitrofluorene radical anion in HMPA displays both a NIR intervalence charge transfer and a visible excited state mixed valence transition. These transitions contain a similar vibronic progression resulting from molecular orbitals that are common to both transitions. Vibrational frequency and intensity data are acquired from the resonance Raman spectrum and used to calculate a best fit for the absorption spectrum. The normal coordinate distortions are analyzed in terms of the electronic changes for both transitions to explain their similarity. The Raman scattering intensity decreases at lower excitation wavelength as a result of Raman de-enhancement caused by interference between neighboring excited states.

Oxidation products of doubly trimethylene-bridged tetrabenzyl p-phenylenediamine paracyclophane.

We report synthesis and investigation of doubly trimethylene-bridged tetrabenzyl-p-phenylenediamine 1(Bz) in its singly and doubly charged redox states. The singly oxidized monoradical cation, which is a mixed-valence (MV) system with directly interacting charge-bearing units, shows broad and solvent-sensitive intervalence bands consistent with class II compounds according to the Robin-Day classification. The doubly oxidized diradical dication of 1(Bz) exists in the spin-paired singlet state with thermally accessible triplet state. It has similar conformations as the other dimeric p-phenylenediamines, such as derivatives 1(Me) and 1(Et), in both the solid-state and solution phases. The successful isolation of the single-crystalline 1(Bz)(2+) diradical dications with two different in nature counteranions, relatively highly coordinating SbF6(-) and weakly coordinating carborane [undecamethylcarborane HCB11Me11(-) (CB(-))], reveals the distinct effect of the nature of counterions on the structural features of diradical dication. Cyclic voltammetry measurements of 1(Bz) in dichloromethane reveal separation of the first and second oxidation potential by 0.12 V (2.8 kcal/mol), indicating relatively stable mixed-valence state in the dichloromethane, whereas in the acetonitrile both the first and second oxidation potentials overlap into one unresolved redox peak with minimal separation.

Monotrimethylene-bridged bis-p-phenylenediamine radical cations and dications: spin states, conformations, and dynamics.

The properties of p-phenylenediamine- (PD-) based systems substantially depend on the molecular topology. The singly bridged PD analogues HMPD and OMPD in which the PD rings are connected by a flexible linker reveal particular electronic properties in their radical cations and dications. The EPR and UV-vis spectra of HMPD(2+••) were found to be exceptionally temperature-sensitive, following a change from the extended conformation (doublet-doublet state) predominant at room temperature to the π-stacked conformation (singlet state) prevailing at dry-ice temperature. Changing the single bridge from (CH(2))(3) to dimethylated CH(2)CMe(2)CH(2) in OMPD(2+••) causes considerably less of the π-stacked conformation to be present at low temperature as a result of the steric interactions with the methyl groups of the bridge. In contrast to HMPD(2+••) and OMPD(2+••), in which the positive charges are localized separately in each PD(+•) ring, in the extended conformation, exchange of the electron ("hole hopping") between the two PD units (fast at the time scale of EPR experiments) was observed for HMPD(+•) and OMPD(+•). This process slows in a reversible manner with decreasing temperature, thus forming the radical cation with the unpaired electron spin density predominantly on one PD core, at low temperatures. Accordingly, a subtle balance between conformational changes, electron delocalization, and spin states could be established.

Dynamics of intramolecular electron transfer in dinitrodibenzodioxin radical anions.

The activation energy for intramolecular electron transfer in radical anions of 2,7-dinitrodibenzodioxin and 2,8-dinitrodibenzodioxin, obtained by simulation of their temperature-dependent EPR spectra, are well predicted by the values calculated by the two-state Marcus-Hush model from the optical charge-transfer bands using quartic-adjusted energy surfaces. The electronic coupling is higher in the 2,8-dinitrodibenzodioxin (H(ab) = 485 cm(-1)) than in the 2,7-dinitrodibenzodioxin radical anion (H(ab) = 250 cm(-1)), but for each solvent the reorganization energy, taken as the maximum of the optical band, is only slightly higher in the latter. These values are consistent with the fact that the reaction is faster in the 2,8-dinitrodibenzodioxin radical anion isomer, as determined by EPR spectroscopy. The pre-exponential factors obtained combining the EPR-derived rate constants and the activation energies calculated from the optical bands fit well the theoretical (modified) nonadiabatic values in the less viscous solvents. However, for the more viscous solvents, the trend of the pre-exponential values with solvent can only be explained if dynamical solvent effects increasingly influence their value. The influence of solvent dynamics in the 2,8-dinitrodibenzodioxin radical anion starts in the less viscous solvents DMF and DMSO, but in the 2,7-dinitrodibenzodioxin isomer this is only fully evident for the more viscous PhCN and HMPA. The influence of solvent dynamics is higher in the radical with the lowest activation barrier.

Effect of ortho substitution on the charge localization of dinitrobenzene radical anions.

Optical and electron paramagnetic resonance spectroscopies were used to study the radical anions of several m-dinitrobenzenes and p-dinitrobenzenes with substituents on ortho positions relative to the nitro groups. 1,4-Dinitrobenzene, 1,4-dimethyl-2,5-dinitrobenzene, and 2,5-dinitrobenzene-1,4-diamine radical anions are delocalized (class III) mixed valence species, but in the dinitrodurene radical anion the nitro groups are forced out of the ring plane due to the steric hindrance, which results in localization of the charge. The radical anions m-dinitrobenzene, 2,6-dinitrotoluene, and dinitromesitylene are all localized (class II) mixed valence species, as is common for m-dinitrobenzenes, and the rate of intramolecular electron transfer reaction strongly decreases with the number of methyl substituents. The same mechanism of rotation of the nitro groups out of the ring plane due to steric hindrance caused by neighboring methyl groups is also responsible for slowing the reaction. However, 2,6-dinitroaniline radical anion and 2,6-dinitrophenoxide radical dianion are charge-delocalized because the strong electron releasing amino and oxido groups increase the conjugation between the two charge-bearing units.

Solvent effects on the coexistence of localized and delocalized 4,4'-dinitrotolane radical anion by resonance Raman spectroscopy.

The resonance Raman spectrum of the simple alkyne bridge in 4,4'-dinitrotolane radical anion shows two distinct bands, providing proof of the solvent-dependent coexistence of charge-localized and -delocalized species. The Raman spectra of normal modes primarily involving the charge-bearing -PhNO(2) units also support the coexistence of two solvent-dependent electronic species. The temperature dependence of the spectra of the bridging unit shows an inverse relationship between the solvent reorganization energy (lambda(s)) and the temperature.

EPR and ENDOR studies of dimeric paracyclophane radical cations and dications containing tri- and pentamethylene-bridged p-phenylene diamine units.

Solution EPR and ENDOR studies on the radical cations of three dimeric p-phenylene diamine (PD)-based compounds, the tetraisopropyl-substituted bis-trimethylene-bridged [5,5]paracyclophane 1(iPr)(+) and its tetramethyl- and tetraisopropyl-substituted bis-pentamethylene-bridged [7,7]paracyclophane analogues 3(Me)(+) and 3(iPr)(+), showed that charge is localized on one PD(+) unit on the EPR time scale in all three compounds and determined the nitrogen splitting constants and several of the hydrogen splitting constants for these complex spectra. Rigid glass studies of the diradical dications of 1(iPr)(2+), 3(iPr)(2+), and its tetramethylene-bridged [6,6]paracyclophane analogue 2(iPr)(2+), all of which show significant amounts of thermally excited triplet at low temperature, demonstrated that 1(iPr)(2+) has a singlet ground state but the triplet lies only 0.07 kcal/mol higher in energy, and 3(iPr)(2+) has its triplet lying 0.05 kcal/mol higher in energy than its singlet.

Solution and solid-state studies of doubly trimethylene-bridged tetraalkyl p-phenylenediamine diradical dication conformations.

X-ray crystallographic structures are reported for 1(Me)(2+)(SbCl(6)(-))(2) x 2 CH(3)CN, 2(Et)(2+)(SbF(6)(-))(2) x 2 CH(3)CN x 2 CH(2)Cl(2), and 1(iPr)(2+)(SbF(6)(-))(2), which also contained unresolved solvent and is in a completely different conformation than the methyl- and ethyl-substituted compounds. A quite different structure of 1(Me)(2+)(SbF(6)(-))(2) than that previously published was obtained upon crystallizing it from a mixture rich in monocation. It does not contain close intramolecular PD(+),PD(+) contacts but has close intermolecular ones. Low temperature NMR spectra of 1(Me)(2+) and 1(Et)(2+) in 2:1 CD(3)OD/CD(3)CN showed that both contain three conformations of all-gauche NCCC unit material with close intramolecular PD(+),PD(+) contacts. In addition to the both PD(+) ring syn and anti material that had been seen in the crystal structure of 1(Me)(2+)(SbF(6)(-))(2) x 2 CH(3)CN published previously, an unsymmetrical conformation having one PD(+) ring syn and the other anti (abbreviated uns) was seen, and the relative amounts of these conformations were significantly different for 1(Me)(2+) and 1(Et)(2+). Calculations that correctly obtain the relative amounts of both the methyl- and ethyl-substituted material as well as changes in the optical spectra between 1(Me)(2+) and 1(Et)(2+), which contains much less of the uns conformation, are reported.

O-capped heteroadamantyl-substituted hydrazines and their oxidation products.

The mixed valence bishydrazine radical cation 6(+), obtained by oxidation of 2,6-bi-(2'-oxa-6'-azaadamantane-6'-yl)-2,6-diazaadamantane-2,6-diyl (6) with silver or nitrosonium salts, has been prepared and studied. 6 is obtained along with lesser amounts of the trishydrazine, some of the tetrahydrazine, and apparently traces of the pentahydrazine upon reaction of deprotonated 2-oxa-6-azaadamantane with 2,6-dichloro-2,6-diazaadamantane. The EPR spectrum of 6(+) shows that its charge is localized on one hydrazine unit on the EPR time scale. It shows a Hush-type Robin-Day class II mixed valence band in its optical spectrum despite the fact that the nitrogen lone pairs are held in a perpendicular geometry that would lead to no electronic interaction between the nitrogen atoms that are separated by only four sigma bonds if its nitrogens were planar. The electron transfer distance that is estimated from the calculated dipole moment of 6(+) is the same as that obtained using the average distance between the electrons of the triplet state of the dication 6(2+), calculated from its dipolar EPR splitting, as a model for the electron transfer distance of 6(+), 3.7 A. Using Hush's Gaussian approximation for the optical spectrum with this electron transfer distance produces an estimate of the electronic coupling V(ab) through the saturated bridge of about 400 cm(-1), which is consistent with the observed EPR spectrum of 6(+). From the observed dipolar splitting, the trishydrazine diradical dication has its remote hydrazine units oxidized, although the monocation presumably forms at the central hydrazine unit, which lacks substitution by the more electron-withdrawing oxygen atoms.

Electron transfer within charge-localized dinitroaromatic radical anions.

Rate constants for the intramolecular electron-transfer reaction in the 2,7-dinitronaphthalene (2(-)), 4,4'-dinitrotolane (3(-)), and 2,2'-dimethyl-4,4'-dinitrobiphenyl (4(-)) radical anions in several polar aprotic solvents were estimated by simulating their ESR spectra at different temperatures. At 298 K, the rate constants are in the 2.0-8.0 x 10(9) s(-1) range for 2(-) and 3(-) and in the 0.4-2.6 x 10(9) s(-1) range for 4(-). The rate constants of 3(-) and 4(-), when corrected for changes in the activation energy (taken as the changes in lambda, the transition energy of the mixed valence band), correlate with the inverse of the solvent relaxation time, showing that the reaction is controlled by solvent dynamics. Solvent effects are only found for 2(-) in benzonitrile (PhCN), the most viscous solvent studied. Calculations of the rate constants using the Kramers-based theory adapted to the adiabatic limit fit the Eyring plots of 2(-) in PhCN and of 3(-) and 4(-) both in MeCN and PhCN rather well.

Electron transfer within charge-localized arylhydrazine-centered mixed valence radical cations having larger bridges.

Kinetics for intramolecular charge transfer between two diarylhydrazine units, measured by ESR, are reported for six charge-localized mixed valence compounds having 9, 11, 13, and 16 bonds between the nitrogen atoms. A 17-bond bridged compound had too slow electron transfer to measure the rate constant by ESR. The optical spectra of these radical cations are compared with tert-butyl,aryl-substituted hydrazines, and rate constants calculated using parameters derived from the optical spectra are compared with the experimental values where possible. The charge-transfer band overlapped too badly with bridge-centered absorption for the 16-bond bridged compound to allow the comparison to be made. The 13-bond bridged compound gave worse agreement than the other compounds. Its optical rate constant was about 5.4 times the ESR rate constant at a temperature between the ranges in which the data were collected.

Charge distribution in arylhydrazine-centered mixed valence compounds with smaller bridges (five to nine bonds between closest nitrogens).

Charge distribution in six aromatic-bridged, aryldialkylhydrazine-centered mixed valence radical cations is discussed through consideration of their optical spectra. The compounds considered have two 2-phenyl-2,3-diazabicyclo-[2.2.2]octane-3-yl (HyPh) charge-bearing units linked by a 1,4-phenylene bridge and its p-methoxyphenyl (HyAn) analogue, as well as the (HyPh)(2)-substituted 1,4-naphthalene, 2,6-naphthalene, 9,10-anthracene, and 4,4'-biphenyl compounds in methylene chloride and acetonitrile. Consideration of band shape and position leads us to assign the 1,4-phenylene- and 2,6-naphthalene-bridged compounds as charge-delocalized (class III) in both solvents, but the 1,4-naphthalene-bridged one lies closer to the borderline, and appears to be charge-localized (class II) in acetonitrile. The 4,4'-biphenyl-bridged compound is clearly class II in acetonitrile, and possibly also in methylene chloride. The lowest energy absorption band for the 9,10-anthracene-bridged compound is assigned as a bridge-to-HyPh band, and its charge distribution is not clear. Problems with the often-used relationship that the electronic coupling is half the transition energy for the lowest energy band of class III mixed valence compounds are discussed, as is interpretation of the vertical reorganization energy near the class II, class III borderline.

Three-chromophore excited-state mixed valence.

The lowest energy optical electronic absorption band of the three-chromophore system tris(4-bromophenyl)amine radical cation is analyzed. The lowest energy electronic transition corresponds to a p-bromophenyl orbital to nitrogen p orbital transition that places the positive charge on three equivalent p-bromophenyl chromophores. The excited electronic state is an example of excited-state mixed valence (ESMV), and the spectrum is interpreted using two ESMV models. The simplest model invokes the concept of an "effective coupling" between the three identical chromophores with an excited-state energy splitting equal to three times the coupling. A more accurate model, the "neighboring orbital model", utilizes the coupling between the bridge's and charge-bearing unit's orbitals closest in energy. The three-chromophore system provides a striking illustration of the failure of an effective coupling term to account for ESMV splitting. The calculated relative energies of the diabatic and adiabatic states are different, but the calculated absorption spectra of the two models show nearly identical vibrational fine structure. Resonance Raman data and the time-dependent theory of electronic and resonance Raman spectroscopies are used to calculate the spectra.

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 lambda(S) solvent MeCN, charge-delocalized in the low lambda(S) 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 (Hab). Hab changes with bridge structure, depending mainly on the twist angle between the two aromatic rings: Hab 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 lambda decreases as Hab increases. We suggest that this is due to a decrease of the internal reorganization energy lambda(v) as the Class II/Class III borderline is approached, and that lambda(v) 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.

Alkylated trimethylene-bridged bis(p-phenylenediamines).

One and two electron oxidation of N,N',N'',N'''-tetramethyl-1,5,12,16-tetraaza[5,5]paracyclophane (Me3C), a bis-trimethylene bridged bis-p-phenylene diamine (PD), and its ethyl and isopropyl analogues are discussed. The monocation and dication are both stable, as demonstrated by optical studies that show they are in equilibrium in solution, with an especially small difference in first and second oxidation potentials for Me3C in MeCN (+23 to -20 mV, measured by simulation of the optical spectrum and of the cyclic voltammogram, respectively). The monocations have charge localized in one PD unit and show a Hush-type mixed valence transition between their PD0 and PD.+ groups. The dications Me3C2+ and Et3C2+ have optical spectra that appear to show large splittings between their PD.+ groups and have a weak ESR spectrum, and 1H NMR data show that the former is a ground-state singlet. iPr3C2+ has a very different optical spectrum and exhibits a triplet ESR spectrum at 120 K. X-ray crystal structures show that for Me3C0 the N(CH2)3N units on each side are in doubly anti (aa) conformations that put the aryl rings as far apart as possible, but Me3C2+ has doubly gg N(CH2)3N trimethylene bridges and both N,N and C,C distances between the PD.+ groups that are significantly below van der Walls contact. In contrast, iPr3C0 is in a doubly ag conformation, and its diradical dication is suggested to be a triplet because it does not attain the doubly gg conformation.

Ab initio calculations on the intramolecular electron transfer rates of a bis(hydrazine) radical cation.

Electron transfer (ET) rates of a charge localized (Class II) intervalence radical cation of a bis(hydrazine) are investigated theoretically. First, the intramolecular ET parameters, i.e., reorganization energy, electronic coupling, and effective frequency, are calculated using several ab initio approaches. And then, the extended Sumi-Marcus theory is employed to predict ET rates by using the parameters obtained. The results reveal that the rates of three isomers of [22/hex/22]+, oo+[22/hex/22]+, io +[22/hex/22]+, and oi+[22/hex/22]+, are agreement with the experiment quite well while the rate of isomer ii+[22/hex/22]+ is about 1000 times larger than those of the others. The validity of different ab initio approaches for this system is discussed.

Tuning aryl, hydrazine radical cation electronic interactions using substituent effects.

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-C6H4Cl lies between p-C6H4OMe and C6H5 in its ability to stabilize the lowest energy optical transition of the radical cation, which involves electron donation from the aryl groups toward the pi*(NN)(+)-centered singly occupied molecular orbital of 2(X)(*+). Resonance Raman spectral determination of the reorganization energy for their lowest energy transitions (lambda(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 pi-centered molecular orbital that interacts with the dinitrogen pi system lies closest in energy to the bonding pi(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.

Mixed valence compounds as probes to determine the polarity of 1-butyl-3-methylimadazolium ionic liquids.

Radical cations and dications of three bishydrazines belonging to the Class II mixed valence compounds have been generated, either spontaneously or by oxidation with AgSbF6, in two 1-butyl-3-methylimidazolium (4+) ionic liquids having BF4(-) and PF6(-) as counterions. The optical spectra of these intermediates have allowed evaluation of Marcus' reorganization energy lambda(s), a parameter that is directly proportional to the solvent polarity. Remarkable differences in lambda(s), as large as 600 cm(-1), have been observed as a function of the counterion, with these data providing support for the observed differences between both ionic liquids (4(+)BF4(-) and 4(+)PF6(-)) in catalysis. However, in terms of polarity, the lambda(s) values rank the hydrophilic 4(+)BF4(-) as being similar to dimethyl sulfoxide and dimethylformamide, while the polarity of hydrophobic 4(+)PF6(-) is analogous to acetonitrile. Overall, our results indicate that ionic liquids are not exceptional liquids in terms of polarity.

Charge localization in a 17-bond mixed-valence quinone radical anion.

Optical spectra in dimethylformamide are reported for the radical anions of benzoquinone, its tetramethyl and tetrachloro analogues, and tetra-ortho-alkyl derivatives of biphenyl, stilbene, terphenyl, quadriphenyl, and 1,4-bis(2-phenylethenyl)benzene quinones. The first absorption bands for all but the quadriphenyl quinone show vibrational fine structure, demonstrating that they are delocalized (Class III) mixed-valence compounds. The quadriphenyl quinone radical anion shows a wide Gaussian-shaped band having a band maximum that is strongly dependent on solvent, typical of localized (Class II) mixed-valence compounds. The simple O charge-bearing unit of these compounds maintains charge delocalization in examples with unusually large bridges.