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Electron spin resonance of phosphorescent dibenzothiophene
Volume
9. number
ELECTRON
CHEMICAL PHYSICS LETTERS
5
SPIN
RESONANCE
OF
1 June 1971
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 represented in fig. I, using dietbyl ether glasses. They mainly concluded that biphenyl was a better model than phenanthrene for these molecules.
dibenzothiophene (lo-2M at tbe start) were grown from the melt under nitrogen in a Bridgeman 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 isomorphic 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 obtained through microscopic examination in polarized light. A solution of dibenzothiophene in ethanol
(5x lW3M) has also been investigated.
X=O,S.Se,CH,,NH /Jf
X
t
Fig. 1. Molecules under investigation with molecular axes and atom numbering.
In order
to get further
details
5.w
on these phos-
phorescent states, we grew crystals of dlbenzofuran (diphenyl oxide) Cl2H8 - 0 doped with dibenzothiophene (diphenylene sulphidej Cl2H8 aS. 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. 1
2. EXPERIMENTAL Single crystals 482
of dibenzofuran doped with
9.02* Fig.
2.
Cryshl
structure
of dibemofuran
i (s&
ref.
[Zj).
CHEblICAL PHYSICS LETTERS
Volume 9, number 5
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 cylindrical quartz tubes which were immersed in liquid 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.
Fig. 3. Hyperfine structure of the low-fiekiy tion of dibenzothiopheoe.
0 .S+FS.A&
where S = 1, which is the sum of anisotropic electronic Zeeman, fine structure and electronnuclear interaction terms (the nuclear Zeeman term is neglected). The tensors g and D are diagonal in the molecular axes system of fig. 1. Then the first two terms of 81 may be rewritten [?I: 81’ = S(g,S,Hx - (X$
+ gySyHy + g,S,H,)
+ 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 substitutional 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
crystal
0.0406
y” ; = -z/2 E = -(X-Y)/2 & 2
(1)
0.0333(2) -0.0741(l) 0.1112(2) -0.0032(3) 2.0025(2) 2.0033(2) 2,0032(l)
glass
b)
diethyl ether gisee al
0.0401(2) 0.0356(2) -0.0759(2) 0.1130(2) 0.1138(3) -0.0022(4) -0.0022(l)
a) See ref. [l]. b) Zero-field splittings are giveu in cm-‘.
The number in parentheses is the uncertainty of the last digit quoted.
transi-
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 nuclear 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 treatment 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 influence. The observed splitting (9.0 G) must be corrected by an angular factor (8.20): indeed the C4-R bond is not parallel to the central Cl-Cl, bond 4II]. So we compute a spin density p4 = 0.255. 483
Volume 9, number 5
CHEMICAL PHYSICS LETTERS
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). Table 2
X Y 2 P2 p4 ref.
biphenyl
0.0406 0.0333 -0.0741 (0.10-0.13)
0.0400 0.0329 -0.0730 0.12 0.25
1121
It must be noted that the lineshape of the ypattern 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 comparison with biphenyl. Further studies 6n deuterium and fluorine substituted dibenzotbiophenes are programmed to complete the spin densities determination. It also appears that the comparison of biphenyi and dibenzothiophene triplets justifies a posteriori the neglect of neighbour influence in the hyperfine coupling model. 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 antisymmetrical relative to the molecular yz plane: it represents a 3B2 state (Czv symmetry), analogous to the phosphorescent state of biphenyl
484
(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,
ACKFKIWLBMjEMENTS
dibenzothiophene
0.255 this work
1 June 1971
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 Professor Ch. Sadron and of his co-workers and by the financial support granted by the Fends National de la Eecherche Scientifique and by the Minist&e de I’Education Nationale of Belgium. 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. [4] M.Kurahashi. M. Fukuyo. A. Shimada. A. Furasaki and I. Nitta, Bull. Chem. Sot. Japan 42 (1969) 2174. and P. Chardon. J. Cbim. Phys. 60 *(51* J.M.Bonnier (1969) 1506. [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.
9. number
ELECTRON
CHEMICAL PHYSICS LETTERS
5
SPIN
RESONANCE
OF
1 June 1971
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 represented in fig. I, using dietbyl ether glasses. They mainly concluded that biphenyl was a better model than phenanthrene for these molecules.
dibenzothiophene (lo-2M at tbe start) were grown from the melt under nitrogen in a Bridgeman 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 isomorphic 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 obtained through microscopic examination in polarized light. A solution of dibenzothiophene in ethanol
(5x lW3M) has also been investigated.
X=O,S.Se,CH,,NH /Jf
X
t
Fig. 1. Molecules under investigation with molecular axes and atom numbering.
In order
to get further
details
5.w
on these phos-
phorescent states, we grew crystals of dlbenzofuran (diphenyl oxide) Cl2H8 - 0 doped with dibenzothiophene (diphenylene sulphidej Cl2H8 aS. 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. 1
2. EXPERIMENTAL Single crystals 482
of dibenzofuran doped with
9.02* Fig.
2.
Cryshl
structure
of dibemofuran
i (s&
ref.
[Zj).
CHEblICAL PHYSICS LETTERS
Volume 9, number 5
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 cylindrical quartz tubes which were immersed in liquid 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.
Fig. 3. Hyperfine structure of the low-fiekiy tion of dibenzothiopheoe.
0 .S+FS.A&
where S = 1, which is the sum of anisotropic electronic Zeeman, fine structure and electronnuclear interaction terms (the nuclear Zeeman term is neglected). The tensors g and D are diagonal in the molecular axes system of fig. 1. Then the first two terms of 81 may be rewritten [?I: 81’ = S(g,S,Hx - (X$
+ gySyHy + g,S,H,)
+ 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 substitutional 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
crystal
0.0406
y” ; = -z/2 E = -(X-Y)/2 & 2
(1)
0.0333(2) -0.0741(l) 0.1112(2) -0.0032(3) 2.0025(2) 2.0033(2) 2,0032(l)
glass
b)
diethyl ether gisee al
0.0401(2) 0.0356(2) -0.0759(2) 0.1130(2) 0.1138(3) -0.0022(4) -0.0022(l)
a) See ref. [l]. b) Zero-field splittings are giveu in cm-‘.
The number in parentheses is the uncertainty of the last digit quoted.
transi-
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 nuclear 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 treatment 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 influence. The observed splitting (9.0 G) must be corrected by an angular factor (8.20): indeed the C4-R bond is not parallel to the central Cl-Cl, bond 4II]. So we compute a spin density p4 = 0.255. 483
Volume 9, number 5
CHEMICAL PHYSICS LETTERS
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). Table 2
X Y 2 P2 p4 ref.
biphenyl
0.0406 0.0333 -0.0741 (0.10-0.13)
0.0400 0.0329 -0.0730 0.12 0.25
1121
It must be noted that the lineshape of the ypattern 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 comparison with biphenyl. Further studies 6n deuterium and fluorine substituted dibenzotbiophenes are programmed to complete the spin densities determination. It also appears that the comparison of biphenyi and dibenzothiophene triplets justifies a posteriori the neglect of neighbour influence in the hyperfine coupling model. 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 antisymmetrical relative to the molecular yz plane: it represents a 3B2 state (Czv symmetry), analogous to the phosphorescent state of biphenyl
484
(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,
ACKFKIWLBMjEMENTS
dibenzothiophene
0.255 this work
1 June 1971
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 Professor Ch. Sadron and of his co-workers and by the financial support granted by the Fends National de la Eecherche Scientifique and by the Minist&e de I’Education Nationale of Belgium. 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. [4] M.Kurahashi. M. Fukuyo. A. Shimada. A. Furasaki and I. Nitta, Bull. Chem. Sot. Japan 42 (1969) 2174. and P. Chardon. J. Cbim. Phys. 60 *(51* J.M.Bonnier (1969) 1506. [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.