Volume 9. number 5 CHEMICAL PHYSICS LETTERS 1 June 1971

ELECTRON SPIN RESONANCE OF PHOSPHORESCENT DIBENZOTHIOPHENE

M. BAIWIR Laboratoire de Cn’stallographie, lnstitut de Physique, Universitc? de Likge au Savf TiZman,

B - 4000 Liege, Bel&zm

Received 13 April 1371

ESR spectroscopy Is applied to phosphorescent dibenzothiophene oriented in a dibenzofuran crystal. The dibenzothiophene triplet is shown to be very similar te the biphenyl triplet.

1. INTRODUCTION

Siegel and Judeikis [l] have studied by ESR the triplet states of the heterocyclics represent- ed in fig. I, using dietbyl ether glasses. They mainly concluded that biphenyl was a better mod- el than phenanthrene for these molecules.

X=O,S.Se,CH,,NH

/J- X f

Fig. 1. Molecules under investigation with molecular axes and atom numbering.

In order to get further details on these phos- phorescent states, we grew crystals of dlben- zofuran (diphenyl oxide) Cl2H8 - 0 doped with dibenzothiophene (diphenylene sulphidej Cl2H8 a S. Oriented triplet studies allow molecular axes assignment and give detailed information about spin distribution in the triplet moiecule.

We present here the preliminary results of this study.

2. EXPERIMENTAL

Single crystals of dibenzofuran doped with

dibenzothiophene (lo-2M at tbe start) were grown from the melt under nitrogen in a Bridge- man furnace.

Dibenzofuran (Fhka) was zone refined after a charcoal treatment. Dibenzothiophene (Fluka) was used as received.

The crystal structure of dibenzofuran, as determined by X-ray diffraction 121, is isomor- phic with that Of flUOrene Cl2H8- CH2 [3] and carbazole Cl2H@H [4]. It iS orthorhombic (Pnam) and its unit cell, centered, contains four molecules. The crystals easily cleave in (001) plates which are normal to the x axes of the two magnetically inequivalent molecules. The two sites make an angle of 550 in the (001) plane (fig. 2). Further orientation of the samples is obtain- ed through microscopic examination in polarized light.

A solution of dibenzothiophene in ethanol (5x lW3M) has also been investigated.

t

5.w

1

9.02* i

Fig. 2. Cryshl structure of dibemofuran (s& ref. [Zj).

482

Volume 9, number 5 CHEblICAL PHYSICS LETTERS I June 1971

The ESR spectrometer was a standard Varian X-band apparatus with a 100 kIIz field modulation. The samples were sealed under nitrogen in cylin- drical quartz tubes which were immersed in liq- uid nitrogen; the ESR signals rapidly disappear with increasing temperature, due to the smallness of the energy gap between host and guest first triplet states (about 230cm-I [5]). The exciting light was provided by a IIBGZOO (Osram) lamp suitably filtered.

3. RESULTS

The description of a triplet state molecule in a static magnetic field is usually made using a phenomenologlcal spin-hamiltonian [6]:

81=/ls*g *rs +s. 0 .S+FS.A&

where S = 1, which is the sum of anisotropic electronic Zeeman, fine structure and electron- nuclear interaction terms (the nuclear Zeeman term is neglected).

The tensors g and D are diagonal in the mo- lecular axes system of fig. 1. Then the first two terms of 81 may be rewritten [?I:

81’ = S(g,S,Hx + gySyHy + g,S,H,)

- (X$ + Ys; + zs$,

with X + Y+Z = 0. From the angular dependence of the resonance

fields, we may conclude that dibenzothiophene enters the dibenaofuran lattice as a true substi- tutional impurity, the fine structure axes of the guest being coincident with the molecular axes of the host.

-Table 1 shows the principal values of the g

Table 1.

dibenzofuran ethanol diethyl ether crystal glass gisee al

y” 0.0406 (1) b) 0.0401(2) 0.0333 (2) 0.0356(2)

; = -z/2 -0.0741 (l) -0.0759 (2)

0.1112(2) 0.1138(3) 0.1130(2) E = -(X-Y)/2 -0.0032(3) -0.0022(4) -0.0022(l) & 2.0025 (2)

2 2.0033 (2) 2,0032(l)

a) See ref. [l]. b) Zero-field splittings are giveu in cm-‘. The num-

ber in parentheses is the uncertainty of the last digit quoted.

Fig. 3. Hyperfine structure of the low-fiekiy transi- tion of dibenzothiopheoe.

and 0 tensors as calculated from the stationary resonance fields [6,8].

When the static magnetic field is parallel to the molecu’ar y axis, hyperfine structure from two equivalent protons can be observed (fig. 3). In the x- and z- spectra we did not observe any resolved structure. Furthermore, the width of the x-lines is not compatible with a high spin density in the 2, 3 or 5 positions. Then we conclude that the observedy-pattern results from the coupling of the electronic spin with the nu- clear moments of the two equivalent protons in 4 and 4’ positions.

The derivation of the spin den&y from the observed splitting requires a numerical model of the proton hyperfine interaction. We have chosen the one currently used for the analysis of triplet ESR spectra [9]. It is derived from the work of MCCOMeU et al. [16] on the malonic acid radical, and its has been confirmed by the treat- ment of a conjugated :C-A fragment in tripIet naphthalene by Hirota et al. [9]. The principal values of the proton hyperfine tensor are in gauss:

AH = -12.0 (along the C-E bond) BR = -24.6 (normal to the molecular plane) CR = -36.0 (normal to bothA a.ndB)

if one neglects any neighhour spin density in- fluence.

The observed splitting (9.0 G) must be cor- rected by an angular factor (8.20): indeed the C4-R bond is not parallel to the central Cl-Cl, bond 4 II]. So we compute a spin density p4 = 0.255.

483

Volume 9, number 5 CHEMICAL PHYSICS LETTERS 1 June 1971

4. CONCLUSIONS

Both the file and hyperfine structures of the triplet ESR spectra of clibensothiophene show a very strong likeness with the related spectra of triplet biphenyl, as described by M&pelter et al. 1121 (table 2).

(D2h symmetry). This conclusion supports the assumption of the lack of conjugation through the heteroatom.

A mixed crystal of fluorene in a dibenzofuran matrix is being studied to improve these con- clusions,

Table 2 ACKFKIWLBMjEMENTS

X Y 2 P2 p4 ref.

dibenzothiophene biphenyl

0.0406 0.0400 0.0333 0.0329

-0.0741 -0.0730 (0.10-0.13) 0.12 0.255 0.25 this work 1121

This work has been performed during a stay in the Laboratoire de Biophysique du Museum National d’Histoire NatureHe (Paris). It has been made possible by the hospitality of Profes- sor Ch. Sadron and of his co-workers and by the financial support granted by the Fends National de la Eecherche Scientifique and by the Minis- t&e de I’Education Nationale of Belgium.

It must be noted that the lineshape of the y- pattern of triplet dibenzothiophene ESR spectra is quite compatible with a spin of 0.10 - 0.13 on the (2,2’), (3,3’) or (5,5’) carbon atoms, the first position being much more probable by compari- son with biphenyl. Further studies 6n deuterium and fluorine substituted dibenzotbiophenes are programmed to complete the spin densities de- termination.

REFERENCES

[l] S. Siegel anti H. S. Judeikis, J. Phys. Chem. 70 (l966) 2201.

[2] J. M. Andr6, 0. Dideberg and L. Dupont. private co~municatioo.

[3] D. IrI.Burns and J.Iball, Proc. Roy. 5x. A227 (1955) 200.

It also appears that the comparison of biphenyi and dibenzothiophene triplets justifies a poste- riori the neglect of neighbour influence in the hyperfine coupling model.

[4] M.Kurahashi. M. Fukuyo. A. Shimada. A. Furasaki and I. Nitta, Bull. Chem. Sot. Japan 42 (1969) 2174.

(51 J.M.Bonnier and P. Chardon. J. Cbim. Phys. 60 * * (1969) 1506.

Ox conclusions are as follows: (i) the results presented in this letter are a

further argument in favour of the planarity of the triplet biphenyl proposed by Mispelter et al. [12].

(ii) It is confirmed that biphenyl is a good model for the triplet state of the bridged similar compounds, as proposed Siegel and Judeikis [l].

(iii) Then the triplet state wavefunction is an- tisymmetrical relative to the molecular yz plane: it represents a 3B2 state (Czv symmetry), anal- ogous to the phosphorescent state of biphenyl

[S] C.A. Hutchison Jr. and B. W.Mangum, J. Chem. Pbys. 34 (1961) 980.

[7] J, H. van der Lk~als ad G. ter Maten. Mol. Phys. 8 (1964) 301.

IS] P. Knttis and R. Lefebvre. J. Cbem. Phya. 39 (1963) 393; 41 (1964) 379.

[9] N.Hirota, C.A.Hutcb.ison Jr. and P.Palmer, J. Chen. Phys. 40 (1964) 3717.

[lo] H. M. McConnell, C. Heller, T. Cole and R. Fessenden, J. Am. Chem. Sot. 82 (1960) 766.

[ll] R.M.Schaffrin and J.Trotter, J. Chem. Sot. A (1970) 1561.

[I21 J. Mispelter, J. P. Grivet and J. M. Lhoste, Mol. Phys . , to be published.

484