Dianion triplet of 9,9′-bianthranyl

Volume

20, number

CHEXIICAL

2

PHYSICS

15 May 1973

LETTERS

DIANION TRIPLET OF 9,9’-BIANTNRANYL M. HOSHINO,

K. KIMURA

and M. IMAMURA

The Instituteof Pj~.~~sical and Chemical Research, Wake. Saitmm, Japan

Revised

The formation

of the dianion

triplet

Received manuscript

29 January 1973 received 28 Fcbrunry

of 9,9’-bianthranyl

wx

obscrvcd

1973

at low tempcrc?turcs.

The absorption

trrun of the monoanion implies that zn electron is locnlized in one of anthranyl rkps, and in the case ofdtiion electrons ;LTe locnlizcd in both rings. The tempcrnture dependence of the ESR signal at half field suggests that triplet dianion has many configurations compared with the singlet dianion.

spcctwo the

1. Introduction

2. Experimental

Dianions such as that of decacyclene are known to show the characteristic ESR absorption in the triplet state [l-4] . These compounds may be classified into two groups: one includes molecules with molecular symmetry D,, and the other molecules having biradical nature as in the case of bis(2,2’-biphenylene) methane. The molecular symmetry of 9,9’-bianthranyl which belongs to the latter group, has been found to be D,,. All the molecules mentioned above are expected to be degenerate as to the lowest vacant orbital in view of its molecular symmetry. Concerning the molecular structure of 9,9’-bianthranyl, Lippert et al. have argued that the molecule has DZd symmetry in the ground state, but the restricted rotation around the 9-9’ axis causes the change of the symmetry (Dz) in the excited singlet state because the fluorescence loses its vibrational structure in hydrocarbons with low viscosities [S] . Then, the dianion may be expected to be ti the triplet state if the ion has the same molecular symmetry as the ground state molecule, and to be in the singlet state if the potential curve about the rotation around the 9-9’ axis resembles that of the excited singlet state.

9,9’-bianthranyl was obtained by the reduction of anthraquinone with zinc powder, gIaciaI acetic acid and hydrochloric acid. After purification by coIumn chromatography, it was recrystallized twice from glacial acetic acid. Analytical data were as follows. Found: C, 94.69%; H, 5.20%. CalcuIated for C28H18: C, 94.9%; H, 5.1%. The melting point was 306°C and a mass spectrum of the sample showed a peak of the parent molecule at 354. The opticz! absorption spectrum indicates the absence of other molecuIes such 2s anthraquinone in the sample. Tetrahydrofurane and methyl tetrahydrofurane (THF and MTHF) were distilled on potassium and then stocked CR Na-K alloy. Negative ions were prepared by the reduction of 9,9’-bianthranyl in THF or MTHF soiution by con!ac:t with sodium or potassium metais six or se,:en times. Optical absorption spectra were recorded on a Carry 14R spectrophotometer. ESR spectra were measured by a JES-336 X band spectrometer.

3. Results and discussion

3.1. Absorption spectra The absorption

spectrum

of 9,9’-bianthranyl

is

193

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CHEMICAL PHYSICS LETTERS

Fig. 2. ESR absorption spectrum of the monoanion of P,P’ubtaincd by ?-?areduction in MTHF at room temperature. biant~~~1

Fig. 1, Absorption spectra of onions of 9,9’-bianthranyl. -: spccics A; -.-: species B;- - -: sptcics C.

second stage of the reduction has a large absorption Feak located at around 725 nm which was not observed for species A, but the profile of the absorption

very similar to that of anthraccne, but a slight shift of the I& band to longer wavelengths is observed. This

spectrum

fact may indicate the absence of the electronic interaction between two anthranyl groups and the shift of the band may be interpreted by thz bathochromic effect of the substituted onthra~y! group. When 9,9’bianthranyt was in contact with metallic sodium? the spectrum changed successively, showing that the reduction proceeded stepwise. At first, the cofour of the solution turned to brilliant blue, then to deep bhx and finafiy to dark blue. Three kinds of the spectra were obtained correspondingly with the coloui of the solution as shown in fig. 1. The colour of the solution was found to be brownish after being allowed to stand in the dark for a few days. Only the brownish solution was diamagnetic while the other vim all paramagnetic. The b~i~~j~~tblue solution (species A) appearing at the first stage of the reduction showed a peak of absorption zt around 670 nm with a vibrational structure as shown in fig. 1. Although the absorption band appears at shorter wavelengths compared with that of the anthraccne monoanion [6]. the general features of the absorption are essentially the same and this species may possibly be assign& to the monoanion of. 9,9’-bianthranyi. No other absorption, as due to charge resonance, was observed.. Thcrcfore, it is reasonable to deduce that the electronic iitteraction between two anthranyi rings is not so strong as to cause a remarkabfe spectral change compared with the absorption spectrum of the anthraccnc n~ono~ion. The deep blue solution (species B) obtained at the 194

about

the vibrational

progression

wzs quite

similar. The sofution of species B gave an ESR absorption due to triple: state at liquid-nitrogen temperature as shown in fig. 2. This is the reason why we assigned B to the dianion. At the third stage of the reduction, we obtained a dark blue solution (species C) with an absorption peak at 600 nm. The coloured substance may be assigned to the triple-anion of 9,9’-bianthranyi because it shows an ESR signd. When these solutions were in contact with air, the colour disappeared rapidly and changed into the original substance. Only ‘the brownish solution gave no ESR signal and changed into a pale pink solu-

tion after introduction of air. All the results mentioned above may be represented by the following scheme. N3 An-An -+ An--An,

CA)

Na

An--An &--An-

--f An--An-

,

03

1x3

-* (An-An)3-

.

(0

Here, An shows the anthranyl group and An-An, 9,?‘-bianthranyl. As described above, species A is regarded as an ion in which an electron is localized in one anthranyl ring and species B, as an ion in which two electrons are localized in both rings. We could find no other dianion in which two electrons are localized in one of the anthranyl rings.

Volume 20, number 2

IS 51ay 1973

CHEMICAL PHYSICS LETTERS

Fig. 3. ESR spectrum of the dianion triplet of 9,9’-bianthranyl in normal field at 77°K.

3.2. ESR spectra of &anion Fig. 2 shows the ESR absorption spectrum of species A which is considered to be the monoanion of 9,9’-binnthranyl. The deep blue solution (species B) showed a characteristic ESR s,pectrum of the triplet state at 77°K and the spectrum at normal tieid reflects the Dzd symmetry of the dianion as shown in fig. 3. A separation between two electrons is evaluated from the D value (170 C) to be approximately 5 A on the basis of a point dipole-dipole model. A molecular symmetry of Dzd implies that the molecular structure of the dianion is the same one as in the case of the neutral molecule. The lineshape of the dianion at half field was gaussian. At normal field, the lineshape varied successively with an increase in temperature, especially above 1 lO”K, but the lineshape at half field did not change so much over the temperature range between 77 and 1 lOoK, and above 110°K the sipal disappeared. The signal intensities at half field were measured at various temperatures and a correction was made for the signal intensities by using Mn2+ as a standard sample. The dependence of the signal intensities on temperature has been investigated by Innes et al. on biradicals such as CT complexes which are in equilibrium between singlet and triplet states, and on paramagnetic dianions and dications by Wasserman [7, S] _ A general formula for the signal intensity can be given in the high temperature approximation as follows.

Fig. 4. Plot of I(T)T versus T. o: experimental vaIues. The solid line is calculated from eq. (1). where K is chosen to be 8.66 X IO6 and &‘, 0.2 CV and C is evaluated fo be 7.8.

I(T) = (C/kT)f(T)

(1)

.

Innes et al. gave the function f(T) as [ 1 + f exp (-U/W)] -1, which implies the equilibrium between a singlet and a triplet state which have the same configuration, different spin multipIicity and energy separation L/Z (= 3 [J]). However, if the two states have different configurations, the situation may be somewhat different. Fig. 4 shows a plot of I(T)T [= Cf(T)I against T. Here, I(T) was taken as the peak to peak height of the signal at half field because we found that the Iineshape did not vary in our experiments. The experimental results fit on a curve;ff(l‘) is taken as [l +f(f~p(-ti/kT)j-1, where k’ is 8.66 X 106 and ilE is 0.2 eV. This result may be interpreted as follows. If we assume a change of configurations between the singlet and triplet states, a term of entropy change should be added to eq. (1). Therefore,f(T) must be represented by f(T)

=

3gT exp(-ET/kT) -gS exp(-ES/kT) + 3gTexp(-ET/kT)

(2)

.

Here, gs and gT stand for the number oFconfigurations and ES and ET are the energies in the singIet and triplet states, respectively. Eq. (2) is then reduced to f(T)

= [b + j exp(A&Y/k-fU-/W)~-l

.

(3)

A similar treatment was made by Iizuka and Kotani for interpreting the equilibrium between the high195

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CHEMICAL PHYSICS LETTERS

and low-spin states of oxidized hem proteins [9]. The entropy change, aS, was calculated from the

value of K to be 34.4 cal/deg. A large ds may imply that the singlet dianion has a looser structure compared with that of the triplet dianion. In other words, the former has possibly more configurations than the latter. After disappearance of the ESR signal at half field above 1 lOoK, ESR spectra due to triplet species were observed in normal field. These spectra may be ascribed to a thermally excited triplet as in the case of the coronene dianion. In the coronene molecule, the lowest vacant orbital is likely to be degenerate in view of its molecular symmetry. However, upon coupling of some distortions, the degeneracy wouId be removed and the dianion triplet would be expected to be formed thermally. Strong evidence for the formation of the thermally excited triplet of the coronene dianion, which was proposed by Hoytink, was obtained from the observation of an increase in the signal intensities of the triplet in normal field with increasing temperatiire [4]. For the 9,9’-bianthranyl dianion, the remaining signal of the triplet in normal field above 110°K showed little participation of different species which may be assigned to a thermally excited triplet. This

1.5 May 1973

may have resulted from the consideration that the degeneracy of the lowest vacant orbit& in 9,9’-bianthranyl should be removed by internal motions, such as a restricted rotation around the 9-9’ axis, which is caused by the softening of the MTHF mat1:iu with increasing temperature. Further studies are in progress.

References 111 R.%. Jesse, P. Bilocn, R. Pcrins, J.D.W. van Voorst and C.J. Hoytink, x10:. Phys. 6 (1963) 633.

121 R.D. Cowell, C. Urry

and S.I. Weissman, J. Chem. Phys. 38 (1963) 2028. 131 R.D. Cowcll, G. Urry and S.I. Weissman, J. Am. Chcm. SOL’. 85 (1963) 822. J. 141 hi. Clasbeck, J.D.W. van Voorst and G.J. Hoytink, Chcm. Phys. 45 (1966) 1852. 151 F. Schneider and E. Lippert, Ber. Bunsenges. Physik

Chem. 72 (1968) 1355.

161P. rjalk, G.J. Hoytink and J.W.H. Schreurs, Rec. Trav. Chim. 76 (1957) 513. 171 D. Sijl, H. Weiner and A.C. Rose

Inncs,

J. Chcrn.

Phys.

30 (1959) 765. 181R. 3reslow, H.W. Chnng, R. Hill and E. Wasserman, J. Am. Chem. Sot. 89 (1967) 1112. 191T. Iizukn, hf. Kotani and T. Yonetani, Biochim. Biophys. Acta

167 (1968)

257.