- Email: [email protected]
Endor of triplet fluorene (X-traps). Intermolecular hyperfine structure
.:
Volurrk 31, number 3
,’ CHEMICALPHYSICS
tETl-ERS
1.5M&ch 1975 ‘.
_:
., ENP(aR tjF TRIPiIT FLU&@NE
‘. :
IWEFWOLECULAR
.’
(X-I-FiAPS).
HYFb:FL[r;E
ST’RUCTU~
V.Z!MhJERMA+lN,H.C.WC)LF andM. SCHWdERER Physikalischer
fnstht,
Teilinstitut 3, Uthersit;it Stuttgart, S-Vnihbgen,~Stuttgart.
‘Received li November
‘.
1.974
The intermolecular hypkfiie
,.
&mu&y
(hfl interactions
oi triplet fluoren~ molecules, acting as X-traps, have been investigated Five ,sf these ENDOR,transitions could be assigned to diI-
by tie analysis of those ENDOR lines ivhich show a small shift. ,.
‘ferent;
well-defined
matrix protons-tie
position
of the fluorene molkcule form&
the X--trap wzs identified.
,
1. Introduction I.. .-
9-4 MHi and 19 MHz,dependingon the orientation
’
Flubrene‘crystals dbped with dibenzothiophene ,show, X-traps in the triplet system. X-trags are sliglltiy disturbed fluorene molecules in the vicinity of a dibenzothiopherie mokcule. In the preceding paper wk reported their ENDOR spectrum [l] : 10 different -EmOR transitipns could be assigned to the protons cf the trap molecule. : However, the origir! of several EhmOR transitions with small shift AVE was.not analyzed. These cannot be due to protons of the trap molecule, because all
of tb! crystal in the No field, which was rotated in the molecular Xy and yr planes in steps of 2.5” over the full r:mge of 180”. The temperature of the crystal was 4.2 R. The triplet state was excited by the W light of a mercury high pressure arc (HBO 200). &Tan example, fig. 1 shows the ENDOR spectrum for the external field orientation along the molecular y axis. The transitions to be analyzed in this paper are termed a, b, c, d and e. In ccntrast to the ENDOR transitions with sm.&l shifts Av, within the triplet component 10) [I], the transitions a, b, c, d and e
possible trap ENDOR
sho$
;..
lines were assigned. Therefore,
they.tiust
be due to matrix protons, a fact which qualitatively has been reported for similar systems [2,3]. But in.contrast to glassy and polykystalline systems [4] these matrix ENDOR lines have never been analyzed quantita’tively in single crystals. This quantitative analysis is the scope of the present paper. By an analysis of this matrix ENUOR one can get detailed information on the structure of the.X&p. This is of more general interest since X-traps are the most characteristic point defects in molecul$r crystals. ‘. 2. Experimental
._
an ENDOR
de1 ‘io a principal (fig. 3).
shift also if the external field is par-
axis of the fine structure
3. Model korder to calculate the ENDOR shift A+~for protons of neighbour molecules we assume,in the follow-kg that the @tropic part of their hf coupling is zero: the crucial quantity for the assipment is the anisotropy of Av,. The difficulty in this calculation is the delocalization of the in electrons in the triplet state of flu’oretie. We will replace this delocalization
spin model.. The siin. density ,jn the-triplet fiuorene w& determini1 experimentally for carbons’2,3,.4 and 5 [l]. The ipin density,of carbon 7 is z&o; &. the sum.of all spin derkties~isone; we get from *e experimknt also’
_,by a’:?oint
are the same as in the pre_ ceding paper [I 1. ENDOR transikokwere investigatedbetween S MHZ ‘and 5q MHz. The ,ESR frequency whs :’ =:.9.37 GH.+ Ttte free protbn frequency varied between The experilmental det&
406
-._ :
” ..
‘.
._
tensor
.,
: :
.:
Volume
31, number
Fig. 1. ENDOR
CHEhlICAL
3
spectrum
of the phosphorescent
fluorene
PIIYSICS
molecule
LETTERS
in a fluorene
15 March
single crystal
ENOOR-
frequencyft4Hz
(X-trap).
The external
1975
--
magnetic
field is oriented along the moleculary axis. The free proton frequency is 11.77 MHz. The numbers indicate: the Protons of the phosphorcsccnt trap molcculc [ 11; the latters indimte the protons of the matrix mokcuk in the sin&t pocnd state. ‘I’= 4.2 x_
the sum of the spin densities for carbons 1 and 6. These two are apportioned according to the result of the McLachlan calculation. Fig. 2 shows the ‘complete set ‘of spin densities for the triplet fluorene, obtained by this procedure [5]. For the point spin model we must localize the spin densities further, because the ti electrons have a node in the molecular plane, and therefore are not localized at the center of the carbons. Thus we divide each spin density into two equal parts and localize them above and below the molecular plane at a distance of O-77 A [6]. These points are the two centeti of the Slate: pZ orbitals.
4. Hamiltonian In the following we consider onLy those parts of ths spin h_amiltonian,which contain the nuclear spin operator 1. The operator of the magnetic dipole interaction between one’electron and one proton is [7]
r is the distance be&veer. the electron sr.& the proton, ge the elcotronic g-value, pg the i3ohr mapeton, g the protong-value, $ ‘Jle nuclear magneton and sp the electron spin operator. In order to calcul&e the actual problem, the
ENDOR shift of matrix protcns neighbourlng the fluorene molecule, we dltide the erectron spin according to the described model. I is calculated from crystallographic data [8] for each individual neighbour proton. We omit small nega. tive spin densities and consider only the eight carbons with positive spin density. Thus the dipoie operator for one proton is the sum of sixteen different compo0,(2L
0,!2L
:_
Fig. 2. Spin densities in the triplet fluorene molecule as used ‘for the calculation ir. the point spin model. .’
nents. To.these the nudear Zeeman ten-n has to be added. The operator swam replaced by the effective spin (5) in the same way as described in ref. [I]. This lead,
~oluiie
,:
31, nuinbcr .3.
..,
CHEMICAI: tw51cs~~Ex~ms
.,. tG the folloykig i&l dip& 6perator fix each matrix ‘$-O&J N [5] : : ; ‘t;;.~~‘E,I(“~~~~,_~~~‘__~~~~~~ f=I
’
is tie spin density .’
at carbon
._
._
of dikrent
a+ discussion. ,-
,.
_&@cRL s&.. 3;-“~~?~~~~~~~~~~.~~~.~~~~P. shifts AyE. Using the intensities of the Eh!DeR lines tb dkinguish betweerrthem, one js able to construct the anisotropy of APE frckn these experimental data. -.
‘I
1975
protons
was done
of neighbow
for a large: number tiolecules.
Five of
them are shown in fig..3 as solid lines. The.other ones show a shift, which is so close.to zero that they cannot be identified. The calculatiori contains no fitting parameters. Only curve a was shifted by a constant amount c f f U.Zf Mi%. ‘This shit? is equivalent to .an
i and K 7.4 ,gepegpr$.
,
S.rResi& .’
,- 15 hlnrch
This’is iflllicated for ,Jl-ueediffe&t transitions in fig. 3 by.dashed lines. Fig. 3, &shows the cakuIated angular dependence of the EPDOR shifts, using the described spin hamil_fonian. T:Ic cdcu@ion
__
.
‘.
‘. i
- . ei
,i:
I ‘.
is~~~~i~:;~~rr~~~~~~~~n; ;vsCi+ .;amT-&+ .ggm -$f. i-&-
theoretical model. Fig. 3. shows that five different matrix protons can be identified by the ENDOR spectrum. These protons are shown in fig. 4. There the position of the ircles and the position of the
hf interaction of the proton, which is the closei of
d \molacuieIt(-
Fig 3. Anisotropy~of
the ENDQR transitions
,:pvE.Fo~tsanddash-_dl~es:
with small shift
‘?XiZzimCnt3L~tiLinCS.:~~-
-?;ated by the p&t spin hod.4 TIE fiMwas ~&ted in theyz ;. plane. ‘; ; .. +)a .;
; .; ..
‘,,
1 mnd 3_ vie arrows timedirected
to tic’ positidn of
Wphos:
‘. phorescent trap molecule responsiblefoi the hf interaction: ,. - .... .. : .:< ,’ ; ..
Volume 31, ni~mbcr3
CHEMIICAL PHYSICS LETTERS
.thhettio to molecule III with the triplet 71electrons of molecule
6.
III is calculated
to be the solid line a in fig. 3.
15 Slarch 1975
give no indication that the X-trap is formed by more than one host molecule. This fact was not unambiguously clear before. Assuming that oily those two fluorene
molecules are distorted, i.e., those which are the nearest neighbours of dibenzothiophene in the ab plane
Conclusion
The model of th,e point spin densities is adequate to describe the hyperfine interaction between the unpaired TTelectrons of the isolated triplet fluorene and m&X. pr&ans af tk hast csyst& the po~&iOn af the ~nci~~ire~inns_~~the_absalllte_v culated interaction correspond with the experimental values. The isotropic hf interaction is almost zero, which means that there is no spin density at the positions of the matrix protons. Both papers, the urP-,eding and the present one,
I
(fig. S), we can decide which one Fornls the X-trap [S] : The triplet X-trap in the fluorene crystal do@ with dibenzothiophene is that fluorene molecule which is one of the nearest neighbours of the latter in the ab plane and they axis of which is directed almos r at the 2-2’ axis Csee fit 1 of ref. Q i>.of the dibenzothiog-hene molecule. The details of tiis analysis-are discussed elsewhere [5].
Acknowledgement This work has been supported by the Deutsche Forschungsgemeinschaft (SFB 67).
,
References [l] V. Zimmermann, hf. Schwoerer and KC. WoIf. Chem. Fhys. Letters 31 (1975)401. [2] CA. Hutchison and G.A. Pearson, I. Chem. Phys. 47 (1967) 520.
-
IA
Fig. 5. View to the ob plane of the fluorene crystal, doped with dibenzolhiophene @):The molecule x axis is perpendicular to the plane. The solid straight lines indicate the tibon frame. The Stunt radii of the two different atomic groups no. 7, CH2 and S respectively, ye indicated by circles., The X-trap k the fluorene molecule no. 1.
[ 31 P. Ehret and H.C. Wolf, Z. Nakrforsch.
233. (1968) 1740. [4] J-S. Hyde, G.H. Rist and L.E. Goran Eriksson, J. F’hys. Chem. 72 (1968) 4269. [S] V. Zimmermann, Dissertation, Stuttgart (1971). [6] R. McWeeny, I. Chem. Fhys. 34 (1961) 399. [7] A. Carrington and A.D. McLachkq In~oduction tu magnetic resonance (New York, 1969). 181 . _ D.M. Burns and J. Iball. Proc. Roy. Sot. A 227 (1955) 200.
. .
409
Volurrk 31, number 3
,’ CHEMICALPHYSICS
tETl-ERS
1.5M&ch 1975 ‘.
_:
., ENP(aR tjF TRIPiIT FLU&@NE
‘. :
IWEFWOLECULAR
.’
(X-I-FiAPS).
HYFb:FL[r;E
ST’RUCTU~
V.Z!MhJERMA+lN,H.C.WC)LF andM. SCHWdERER Physikalischer
fnstht,
Teilinstitut 3, Uthersit;it Stuttgart, S-Vnihbgen,~Stuttgart.
‘Received li November
‘.
1.974
The intermolecular hypkfiie
,.
&mu&y
(hfl interactions
oi triplet fluoren~ molecules, acting as X-traps, have been investigated Five ,sf these ENDOR,transitions could be assigned to diI-
by tie analysis of those ENDOR lines ivhich show a small shift. ,.
‘ferent;
well-defined
matrix protons-tie
position
of the fluorene molkcule form&
the X--trap wzs identified.
,
1. Introduction I.. .-
9-4 MHi and 19 MHz,dependingon the orientation
’
Flubrene‘crystals dbped with dibenzothiophene ,show, X-traps in the triplet system. X-trags are sliglltiy disturbed fluorene molecules in the vicinity of a dibenzothiopherie mokcule. In the preceding paper wk reported their ENDOR spectrum [l] : 10 different -EmOR transitipns could be assigned to the protons cf the trap molecule. : However, the origir! of several EhmOR transitions with small shift AVE was.not analyzed. These cannot be due to protons of the trap molecule, because all
of tb! crystal in the No field, which was rotated in the molecular Xy and yr planes in steps of 2.5” over the full r:mge of 180”. The temperature of the crystal was 4.2 R. The triplet state was excited by the W light of a mercury high pressure arc (HBO 200). &Tan example, fig. 1 shows the ENDOR spectrum for the external field orientation along the molecular y axis. The transitions to be analyzed in this paper are termed a, b, c, d and e. In ccntrast to the ENDOR transitions with sm.&l shifts Av, within the triplet component 10) [I], the transitions a, b, c, d and e
possible trap ENDOR
sho$
;..
lines were assigned. Therefore,
they.tiust
be due to matrix protons, a fact which qualitatively has been reported for similar systems [2,3]. But in.contrast to glassy and polykystalline systems [4] these matrix ENDOR lines have never been analyzed quantita’tively in single crystals. This quantitative analysis is the scope of the present paper. By an analysis of this matrix ENUOR one can get detailed information on the structure of the.X&p. This is of more general interest since X-traps are the most characteristic point defects in molecul$r crystals. ‘. 2. Experimental
._
an ENDOR
de1 ‘io a principal (fig. 3).
shift also if the external field is par-
axis of the fine structure
3. Model korder to calculate the ENDOR shift A+~for protons of neighbour molecules we assume,in the follow-kg that the @tropic part of their hf coupling is zero: the crucial quantity for the assipment is the anisotropy of Av,. The difficulty in this calculation is the delocalization of the in electrons in the triplet state of flu’oretie. We will replace this delocalization
spin model.. The siin. density ,jn the-triplet fiuorene w& determini1 experimentally for carbons’2,3,.4 and 5 [l]. The ipin density,of carbon 7 is z&o; &. the sum.of all spin derkties~isone; we get from *e experimknt also’
_,by a’:?oint
are the same as in the pre_ ceding paper [I 1. ENDOR transikokwere investigatedbetween S MHZ ‘and 5q MHz. The ,ESR frequency whs :’ =:.9.37 GH.+ Ttte free protbn frequency varied between The experilmental det&
406
-._ :
” ..
‘.
._
tensor
.,
: :
.:
Volume
31, number
Fig. 1. ENDOR
CHEhlICAL
3
spectrum
of the phosphorescent
fluorene
PIIYSICS
molecule
LETTERS
in a fluorene
15 March
single crystal
ENOOR-
frequencyft4Hz
(X-trap).
The external
1975
--
magnetic
field is oriented along the moleculary axis. The free proton frequency is 11.77 MHz. The numbers indicate: the Protons of the phosphorcsccnt trap molcculc [ 11; the latters indimte the protons of the matrix mokcuk in the sin&t pocnd state. ‘I’= 4.2 x_
the sum of the spin densities for carbons 1 and 6. These two are apportioned according to the result of the McLachlan calculation. Fig. 2 shows the ‘complete set ‘of spin densities for the triplet fluorene, obtained by this procedure [5]. For the point spin model we must localize the spin densities further, because the ti electrons have a node in the molecular plane, and therefore are not localized at the center of the carbons. Thus we divide each spin density into two equal parts and localize them above and below the molecular plane at a distance of O-77 A [6]. These points are the two centeti of the Slate: pZ orbitals.
4. Hamiltonian In the following we consider onLy those parts of ths spin h_amiltonian,which contain the nuclear spin operator 1. The operator of the magnetic dipole interaction between one’electron and one proton is [7]
r is the distance be&veer. the electron sr.& the proton, ge the elcotronic g-value, pg the i3ohr mapeton, g the protong-value, $ ‘Jle nuclear magneton and sp the electron spin operator. In order to calcul&e the actual problem, the
ENDOR shift of matrix protcns neighbourlng the fluorene molecule, we dltide the erectron spin according to the described model. I is calculated from crystallographic data [8] for each individual neighbour proton. We omit small nega. tive spin densities and consider only the eight carbons with positive spin density. Thus the dipoie operator for one proton is the sum of sixteen different compo0,(2L
0,!2L
:_
Fig. 2. Spin densities in the triplet fluorene molecule as used ‘for the calculation ir. the point spin model. .’
nents. To.these the nudear Zeeman ten-n has to be added. The operator swam replaced by the effective spin (5) in the same way as described in ref. [I]. This lead,
~oluiie
,:
31, nuinbcr .3.
..,
CHEMICAI: tw51cs~~Ex~ms
.,. tG the folloykig i&l dip& 6perator fix each matrix ‘$-O&J N [5] : : ; ‘t;;.~~‘E,I(“~~~~,_~~~‘__~~~~~~ f=I
’
is tie spin density .’
at carbon
._
._
of dikrent
a+ discussion. ,-
,.
_&@cRL s&.. 3;-“~~?~~~~~~~~~~.~~~.~~~~P. shifts AyE. Using the intensities of the Eh!DeR lines tb dkinguish betweerrthem, one js able to construct the anisotropy of APE frckn these experimental data. -.
‘I
1975
protons
was done
of neighbow
for a large: number tiolecules.
Five of
them are shown in fig..3 as solid lines. The.other ones show a shift, which is so close.to zero that they cannot be identified. The calculatiori contains no fitting parameters. Only curve a was shifted by a constant amount c f f U.Zf Mi%. ‘This shit? is equivalent to .an
i and K 7.4 ,gepegpr$.
,
S.rResi& .’
,- 15 hlnrch
This’is iflllicated for ,Jl-ueediffe&t transitions in fig. 3 by.dashed lines. Fig. 3, &shows the cakuIated angular dependence of the EPDOR shifts, using the described spin hamil_fonian. T:Ic cdcu@ion
__
.
‘.
‘. i
- . ei
,i:
I ‘.
is~~~~i~:;~~rr~~~~~~~~n; ;vsCi+ .;amT-&+ .ggm -$f. i-&-
theoretical model. Fig. 3. shows that five different matrix protons can be identified by the ENDOR spectrum. These protons are shown in fig. 4. There the position of the ircles and the position of the
hf interaction of the proton, which is the closei of
d \molacuieIt(-
Fig 3. Anisotropy~of
the ENDQR transitions
,:pvE.Fo~tsanddash-_dl~es:
with small shift
‘?XiZzimCnt3L~tiLinCS.:~~-
-?;ated by the p&t spin hod.4 TIE fiMwas ~&ted in theyz ;. plane. ‘; ; .. +)a .;
; .; ..
‘,,
1 mnd 3_ vie arrows timedirected
to tic’ positidn of
Wphos:
‘. phorescent trap molecule responsiblefoi the hf interaction: ,. - .... .. : .:< ,’ ; ..
Volume 31, ni~mbcr3
CHEMIICAL PHYSICS LETTERS
.thhettio to molecule III with the triplet 71electrons of molecule
6.
III is calculated
to be the solid line a in fig. 3.
15 Slarch 1975
give no indication that the X-trap is formed by more than one host molecule. This fact was not unambiguously clear before. Assuming that oily those two fluorene
molecules are distorted, i.e., those which are the nearest neighbours of dibenzothiophene in the ab plane
Conclusion
The model of th,e point spin densities is adequate to describe the hyperfine interaction between the unpaired TTelectrons of the isolated triplet fluorene and m&X. pr&ans af tk hast csyst& the po~&iOn af the ~nci~~ire~inns_~~the_absalllte_v culated interaction correspond with the experimental values. The isotropic hf interaction is almost zero, which means that there is no spin density at the positions of the matrix protons. Both papers, the urP-,eding and the present one,
I
(fig. S), we can decide which one Fornls the X-trap [S] : The triplet X-trap in the fluorene crystal do@ with dibenzothiophene is that fluorene molecule which is one of the nearest neighbours of the latter in the ab plane and they axis of which is directed almos r at the 2-2’ axis Csee fit 1 of ref. Q i>.of the dibenzothiog-hene molecule. The details of tiis analysis-are discussed elsewhere [5].
Acknowledgement This work has been supported by the Deutsche Forschungsgemeinschaft (SFB 67).
,
References [l] V. Zimmermann, hf. Schwoerer and KC. WoIf. Chem. Fhys. Letters 31 (1975)401. [2] CA. Hutchison and G.A. Pearson, I. Chem. Phys. 47 (1967) 520.
-
IA
Fig. 5. View to the ob plane of the fluorene crystal, doped with dibenzolhiophene @):The molecule x axis is perpendicular to the plane. The solid straight lines indicate the tibon frame. The Stunt radii of the two different atomic groups no. 7, CH2 and S respectively, ye indicated by circles., The X-trap k the fluorene molecule no. 1.
[ 31 P. Ehret and H.C. Wolf, Z. Nakrforsch.
233. (1968) 1740. [4] J-S. Hyde, G.H. Rist and L.E. Goran Eriksson, J. F’hys. Chem. 72 (1968) 4269. [S] V. Zimmermann, Dissertation, Stuttgart (1971). [6] R. McWeeny, I. Chem. Fhys. 34 (1961) 399. [7] A. Carrington and A.D. McLachkq In~oduction tu magnetic resonance (New York, 1969). 181 . _ D.M. Burns and J. Iball. Proc. Roy. Sot. A 227 (1955) 200.
. .
409