- Email: [email protected]
Radical cation salt based on organic donor with close-lying highest occupied molecular orbitals
Synthetic Metals, 41--43 (1991) 2301-2304
2 3 01
RADICAL CATION SALT BASED ON ORGANIC DONORWITH CLOSE-LYING HIGHEST OCCUPIED MOLECULAR ORBITALS
T. BJORNHOLM, K. BECHGAARD, Centrefor Interdisciplinary Studies of Molecular
Interactions, University of Copenhagen, Blegdamsvej 21, DK-2100 Copenhagen (Denmark) P. SOMMER-LARSEN, Department of General Chemistry, Royal Danish School of Pharmacy,
Universitetsparken 2, DK-2100 Copenhagen (Denmark) G. RINDORF, N. THORUP, Chemistry Department B, The Technical Un&ersity of Denmark,
DK-2800 Lyngby (Denmark)
ABSTRACT Triclinic crystals with segregated stacks of donor and acceptor molecules are produced by electrocrystallization of 4,4',6'",4'"'-Tetra-Methoxy-Hexa-m-Phenylene in the presence of ~oxalato-bis[tetrachlorostanate(IV)] dianions.
Molecular orbital calculations show that the
energy splitting between the two highest occupied molecular orbitals of the organic donor is 50 meV or less. Measured physical properties of this charge transfer crystal are presented and discussed in light of the possible ferromagnetic interactions between unpaired electrons in a stack of organic molecules with close lying highest occupied molecular orbitals. INTRODUCTION Bulk ferromagnetism is one of the few intriguing properties encountered in metals that have not yet been observed experimentally in purely organic crystals. However, a number of mechanisms for obtaining ferromagnetically interacting spins in organic solids have been proposed [1]. A common feature of these theories is the presence of degenerate energy levels for the electrons of interest. Only in a system where the degenerate levels originate from the organic iron complex ferrocene, has ferromagnetism been observed experimentally [2]. In this material donor and acceptor molecules form mixed stacks. We report here the first structurally well characterized example of an organic charge transfer solid with segregated stacks of purely organic donor molecules possessing close-lying highest occupied molecular orbitals. This type of material has been suggested as a potential candidate for organic ferromagnets based on the simple idea that two interacting molecules, each with an electron in a degenerate molecular orbital, will tend to develop a ferromagnetically coupled 0379-6779/91/$3.50
© Elsevier Sequoia/Printed in The Netherlands
2302
ground state, because the aligned spins in this ground state will favor the virtual interaction with the lowest excited state, which is assumed to be a triplet according to Hunds rule [1,3]. ( -- -- ) = Degenerate molecularorbitM
/ / /
Groundstate
(--
+ ) ( +
Virtually excited states
--)/
Fig. 1. Simple representation of the presumed origin of ferromagnetic exchange interaction between spins on two degenerate organic molecules. The experimental system described below was designed as a testing ground for these simple ideas. As we shall show, it possesses important structural and molecular electronic properties that makes it less ideal for this purpose. In particular our results indicate that the dication of the organic donor molecule is a singlet and not a triplet - the prerequisite for ferromagnetic interactions (fig. 1). RESULTS The organic donor, 4,4',6'",4'"'-Tetra-Methoxy-Hexa-m-Phenylene (TMHMP) was prepared via a three step synthesis starting from o-bromo-anisole and 1,3-dibromo-benzene (11% overall yield)
[5].
Electrocrystallization
of
TMHMP
in
the
presence
of
iz-oxalato-
bis[tetrachlorostannate(IV)] or i~-oxalato-bis[tetrabromostannate(IV)] dianions (C204X8Sn22-, X = CI, Br) yields triclinic crystals with segregated stacks of the donor and acceptor molecules. The crystals are semiconducting. Shown in figure 2 is a projection along the stacking direction of the structure solved by x-ray diffraction on a single crystal of (TMHMP)3(C204C18Sn22-)l. The stack structure arises by repeating a definite molecular overlap of the TMHMP-molecules [4]. In the stack, anions surround every third donor molecule while the remaining two donor molecules in the unit cell do not have anions as nearest neighbors. The 3:1 stoichiometry results in a average charge on every T M H M P molecule of +2/3. Three different types of quantum chemical calculations (VEH, AM1, PPP) on the neutral T M H M P molecules show that irrespective of calculation method or detailed molecular geometry, the energy splitting between the two highest occupied molecular energy levels is 50 meV or less [5-6].
Hence, the organic stack of TMHMP molecules formally meets the
requirements for degenerate or close lying electron energy levels, which can result in ferromagnetic interactions [3].
2303
T ~ O-',
c
<
c
?
3
, ~ ~ ' ~
~ (~ I ~
CH30 ~ 4. 4'. 6"'. 4 ' '
OCH3
Telramelhoxy-hexa-m-phenylen
(TMHMP)
j ~t-oxalato-b~s[tet rachloroslannale(IV)]
Fig. 2. Structure of (TMHMP)3(C204CIsSn22-)l, triclinic ~' 1, a = 11.40(1)/~, b = 13,97(1)/~, c = 19,01(2)/~, tt = 110.5(1) °, 13 = 91.9(1) °, y = 93.3(1) °.
The magnetic susceptibility, corrected for diamagnetism and paramagnetic impurities, of randomly oriented crystals of (TMHMP)x(C204C18Sn22)l is shown on figure 3 (x = 2, 3 see below). The theoretically expected behavior of a paramagnetic system consistiP.g of a singlet ground state and a triplet excited state is also shown in the figure. It is clearly seen that the two
curves
are
similar,
and
we
therefore
attribute
the
magnetic
behavior
of
(TMHMP)x(C204CIsSn22")1 to a species with singlet ground state and a triplet excited state. The magnetic susceptibility shows no sign of ferromagnetism.
0,006 -- -'4"-0,005 •
~~llkl~
Xobs(emu/mol)
Xleo(emu/mol)
E o
H = 1.3 resin 0000 loo
200
300
T(K)
Fig. 3. Magnetic susceptibility of (TMHMP)x(C204CI8Sn22-)l (x = 2, 3) corrected for a diamagnetic and Curie magnetic contribution (X(T) = X m e a s u r e d - C / T - constant).
2304
A number of crystallization experiments with TMHMP as donor and C204X8Sn22" (X = Br, CI) as corresponding anion yields structually similar crystals with 2:1 as well as 3:1 stoichiometry [5]. Simplified Madelung energy calculations of the 3:1 crystal structure show that a charge distribution in which the one TMHMP molecule with anions as nearest neighbors carries all the positive charge (2+), is favored over other possible distributions by several electron volts [6]. This indicates that the magnetic molecule is TMHMP 2÷ in agreement with other physical properties reported elsewhere [5]. The singlet ground state of the TMHMP 2÷ ion is not expected according to Hunds rule and it indicates the presence of strong electron correlations which are not fully accounted for in the molecular orbital calculations on the neutral molecule. The observed physical properties of TMHMP 2÷ may be understood in terms of a Hubbard model with two non-degenerate sites in the strong coupling limit (U > t). On the molecular level, these sites may correspond to the two 4,4'-dimethoxy-biphenyl subunits of the TMHMP molecule. The interaction between these subunits is mediated through the
meta
position on
the unsubstituted benzene rings. In general this interaction is weak and this is consistent with the strong coupling assumption of the Hubbard model. These results raise questions about the validity of Hunds rule for large organic molecules, and this aspect will be discussed further in a forthcoming paper [5]. ACKNOWLEDGEMENTS This work was supported by Esprit BRA 3314, and the Danish Materials Technological Development Programme. REFERENCES
1
See for example; J. B. Torrance, P. S. Bagus, L Johannsen, A. I. Nazzal and S. S. P. Parkin, J. Apply. Phys.. 63 (1988) 2962, and references therein.
2
J.S. Miller, A. J. Epstein and W. M. Reiff, Chem. Rev.. 88 (1988) 201, and references therein.
3
J.B. Torrance, S. Oostra and A. Nazzal, Svnth. Met., 19 (1987) 709.
4
The molecular overlap was predicted by model calculations; M. Jcrgensen, T. Bj~rnholm and K. Bechgaard, Svnth. Met.. 38 (1990) 77.
5
T. Bjcrnholm, Ph.D. Thesis. University of Copenhagen, Denmark, 1990; T. Bj4arnholm, P. Sommer-Larsen, G. Rindorf, N. Thorup and K. Bechgaard, to be published.
6
P. Sommer-Larsen, Ph, D. Thesis, University of Copenhagen, Denmark, 1989.
2 3 01
RADICAL CATION SALT BASED ON ORGANIC DONORWITH CLOSE-LYING HIGHEST OCCUPIED MOLECULAR ORBITALS
T. BJORNHOLM, K. BECHGAARD, Centrefor Interdisciplinary Studies of Molecular
Interactions, University of Copenhagen, Blegdamsvej 21, DK-2100 Copenhagen (Denmark) P. SOMMER-LARSEN, Department of General Chemistry, Royal Danish School of Pharmacy,
Universitetsparken 2, DK-2100 Copenhagen (Denmark) G. RINDORF, N. THORUP, Chemistry Department B, The Technical Un&ersity of Denmark,
DK-2800 Lyngby (Denmark)
ABSTRACT Triclinic crystals with segregated stacks of donor and acceptor molecules are produced by electrocrystallization of 4,4',6'",4'"'-Tetra-Methoxy-Hexa-m-Phenylene in the presence of ~oxalato-bis[tetrachlorostanate(IV)] dianions.
Molecular orbital calculations show that the
energy splitting between the two highest occupied molecular orbitals of the organic donor is 50 meV or less. Measured physical properties of this charge transfer crystal are presented and discussed in light of the possible ferromagnetic interactions between unpaired electrons in a stack of organic molecules with close lying highest occupied molecular orbitals. INTRODUCTION Bulk ferromagnetism is one of the few intriguing properties encountered in metals that have not yet been observed experimentally in purely organic crystals. However, a number of mechanisms for obtaining ferromagnetically interacting spins in organic solids have been proposed [1]. A common feature of these theories is the presence of degenerate energy levels for the electrons of interest. Only in a system where the degenerate levels originate from the organic iron complex ferrocene, has ferromagnetism been observed experimentally [2]. In this material donor and acceptor molecules form mixed stacks. We report here the first structurally well characterized example of an organic charge transfer solid with segregated stacks of purely organic donor molecules possessing close-lying highest occupied molecular orbitals. This type of material has been suggested as a potential candidate for organic ferromagnets based on the simple idea that two interacting molecules, each with an electron in a degenerate molecular orbital, will tend to develop a ferromagnetically coupled 0379-6779/91/$3.50
© Elsevier Sequoia/Printed in The Netherlands
2302
ground state, because the aligned spins in this ground state will favor the virtual interaction with the lowest excited state, which is assumed to be a triplet according to Hunds rule [1,3]. ( -- -- ) = Degenerate molecularorbitM
/ / /
Groundstate
(--
+ ) ( +
Virtually excited states
--)/
Fig. 1. Simple representation of the presumed origin of ferromagnetic exchange interaction between spins on two degenerate organic molecules. The experimental system described below was designed as a testing ground for these simple ideas. As we shall show, it possesses important structural and molecular electronic properties that makes it less ideal for this purpose. In particular our results indicate that the dication of the organic donor molecule is a singlet and not a triplet - the prerequisite for ferromagnetic interactions (fig. 1). RESULTS The organic donor, 4,4',6'",4'"'-Tetra-Methoxy-Hexa-m-Phenylene (TMHMP) was prepared via a three step synthesis starting from o-bromo-anisole and 1,3-dibromo-benzene (11% overall yield)
[5].
Electrocrystallization
of
TMHMP
in
the
presence
of
iz-oxalato-
bis[tetrachlorostannate(IV)] or i~-oxalato-bis[tetrabromostannate(IV)] dianions (C204X8Sn22-, X = CI, Br) yields triclinic crystals with segregated stacks of the donor and acceptor molecules. The crystals are semiconducting. Shown in figure 2 is a projection along the stacking direction of the structure solved by x-ray diffraction on a single crystal of (TMHMP)3(C204C18Sn22-)l. The stack structure arises by repeating a definite molecular overlap of the TMHMP-molecules [4]. In the stack, anions surround every third donor molecule while the remaining two donor molecules in the unit cell do not have anions as nearest neighbors. The 3:1 stoichiometry results in a average charge on every T M H M P molecule of +2/3. Three different types of quantum chemical calculations (VEH, AM1, PPP) on the neutral T M H M P molecules show that irrespective of calculation method or detailed molecular geometry, the energy splitting between the two highest occupied molecular energy levels is 50 meV or less [5-6].
Hence, the organic stack of TMHMP molecules formally meets the
requirements for degenerate or close lying electron energy levels, which can result in ferromagnetic interactions [3].
2303
T ~ O-',
c
<
c
?
3
, ~ ~ ' ~
~ (~ I ~
CH30 ~ 4. 4'. 6"'. 4 ' '
OCH3
Telramelhoxy-hexa-m-phenylen
(TMHMP)
j ~t-oxalato-b~s[tet rachloroslannale(IV)]
Fig. 2. Structure of (TMHMP)3(C204CIsSn22-)l, triclinic ~' 1, a = 11.40(1)/~, b = 13,97(1)/~, c = 19,01(2)/~, tt = 110.5(1) °, 13 = 91.9(1) °, y = 93.3(1) °.
The magnetic susceptibility, corrected for diamagnetism and paramagnetic impurities, of randomly oriented crystals of (TMHMP)x(C204C18Sn22)l is shown on figure 3 (x = 2, 3 see below). The theoretically expected behavior of a paramagnetic system consistiP.g of a singlet ground state and a triplet excited state is also shown in the figure. It is clearly seen that the two
curves
are
similar,
and
we
therefore
attribute
the
magnetic
behavior
of
(TMHMP)x(C204CIsSn22")1 to a species with singlet ground state and a triplet excited state. The magnetic susceptibility shows no sign of ferromagnetism.
0,006 -- -'4"-0,005 •
~~llkl~
Xobs(emu/mol)
Xleo(emu/mol)
E o
H = 1.3 resin 0000 loo
200
300
T(K)
Fig. 3. Magnetic susceptibility of (TMHMP)x(C204CI8Sn22-)l (x = 2, 3) corrected for a diamagnetic and Curie magnetic contribution (X(T) = X m e a s u r e d - C / T - constant).
2304
A number of crystallization experiments with TMHMP as donor and C204X8Sn22" (X = Br, CI) as corresponding anion yields structually similar crystals with 2:1 as well as 3:1 stoichiometry [5]. Simplified Madelung energy calculations of the 3:1 crystal structure show that a charge distribution in which the one TMHMP molecule with anions as nearest neighbors carries all the positive charge (2+), is favored over other possible distributions by several electron volts [6]. This indicates that the magnetic molecule is TMHMP 2÷ in agreement with other physical properties reported elsewhere [5]. The singlet ground state of the TMHMP 2÷ ion is not expected according to Hunds rule and it indicates the presence of strong electron correlations which are not fully accounted for in the molecular orbital calculations on the neutral molecule. The observed physical properties of TMHMP 2÷ may be understood in terms of a Hubbard model with two non-degenerate sites in the strong coupling limit (U > t). On the molecular level, these sites may correspond to the two 4,4'-dimethoxy-biphenyl subunits of the TMHMP molecule. The interaction between these subunits is mediated through the
meta
position on
the unsubstituted benzene rings. In general this interaction is weak and this is consistent with the strong coupling assumption of the Hubbard model. These results raise questions about the validity of Hunds rule for large organic molecules, and this aspect will be discussed further in a forthcoming paper [5]. ACKNOWLEDGEMENTS This work was supported by Esprit BRA 3314, and the Danish Materials Technological Development Programme. REFERENCES
1
See for example; J. B. Torrance, P. S. Bagus, L Johannsen, A. I. Nazzal and S. S. P. Parkin, J. Apply. Phys.. 63 (1988) 2962, and references therein.
2
J.S. Miller, A. J. Epstein and W. M. Reiff, Chem. Rev.. 88 (1988) 201, and references therein.
3
J.B. Torrance, S. Oostra and A. Nazzal, Svnth. Met., 19 (1987) 709.
4
The molecular overlap was predicted by model calculations; M. Jcrgensen, T. Bj~rnholm and K. Bechgaard, Svnth. Met.. 38 (1990) 77.
5
T. Bjcrnholm, Ph.D. Thesis. University of Copenhagen, Denmark, 1990; T. Bj4arnholm, P. Sommer-Larsen, G. Rindorf, N. Thorup and K. Bechgaard, to be published.
6
P. Sommer-Larsen, Ph, D. Thesis, University of Copenhagen, Denmark, 1989.