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Symmetry breaking localisation of the unpaired electron in spiro-bis(1,9-disubstituted-phenalenyl)boron radicals
Synthetic Metals 133–134 (2003) 367–372
Symmetry breaking localisation of the unpaired electron in spiro-bis(1,9-disubstituted-phenalenyl)boron radicals$ X. Chia, F.S. Thama, A.W. Cordesb, M.E. Itkisa, R.C. Haddona,* a b
Chemistry and Engineering Department, University of California, Riverside, CA 92506, USA Chemistry and Biochemistry Department, University of Arkansas, Fayetteville, AR 72701, USA
Abstract We report the crystal structure of a spiro-bis(1,9-disubstituted-phenalenyl)boron cation (2þ) and by comparison with the structures of a series of radicals we conclude that the changes in bond lengths in the radicals compared to those of the corresponding cation can be qualitatively predicted by the sign of the partial bond order of the singly occupied molecular orbital (SOMO), which contains the additional electron. By use of this information, we can analyse the structures of the p-dimer radicals 1 and 2 and ascertain the location of the unpaired electron in these compounds. Where these radicals exist as paramagnetic p-dimers (high-temperature regime), the unpaired electrons reside mainly in the phenalenyl units not directly involved in the p-dimer interaction. At low temperatures, where the diamagnetic p-dimers exist, the electrons localise in the phenalenyl units directly involved in the p-dimer, but there still exists some degree of delocalisation of the electrons to the other halves of the molecules. We postulate that the increase in delocalisation of the unpaired electrons is related to the increase of the conductivity by about two orders of magnitude as the electrons spin pair. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Phenalenyl; Radical; Conductivity; p-Dimer; SOMO; Unpaired electron
1. Introduction The phenalenyl system has been proposed as an approach to intrinsic molecular conductors and superconductors [1,2]. We have recently reported the first phenalenyl-based neutral radical conductors (1, 2, 3) [3,4]. Although none of the distances between the solid state molecular building blocks lies within the van der Waals distances for carbon ˚ ), these compounds exhibit the highest conatoms (3.4 A ductivity of any neutral organic molecules. Moreover, radicals 1 and 2 involve face-to-face p–p interactions, with ˚ at higher temmean plane separations of around 3.33 A ˚ at lower temperature and this perature and about 3.16 A change is accompanied by a magnetic phase transition from the paramagnetic p-dimer form to the diamagnetic p-dimer form, while their conductivities increase by about two orders of magnitude when the diamagnetic state is formed [4]. These results are unprecedented and we tentatively
$
Yamada Conference LVI, the Fourth International Symposium on Crystalline Organic Metals, Superconductors and Ferromagnets, ISCOM 2001—Abstract Number P4. * Corresponding author. E-mail address: [email protected] (R.C. Haddon).
assigned the enhancement in the conductivity to a decrease in the on-site Coulombic correlation energy (U), as the dimers form a super-molecule with twice the amount of conjugation [4].
Here we report the crystal structure of the cation 2þ, BPh4 , which we previously used to prepare the corresponding radical (2) [4]. By comparing the bond lengths of 2þ with those of the radicals 1, 2 and 3, we conclude that the unpaired electrons of the radicals 1 and 2 avoid each other in the paramagnetic p-dimer state, but spin pair in the phenalenyl units where there exist p–p interactions in the diamagnetic p-dimer state (Fig. 7).
0379-6779/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 ( 0 2 ) 0 0 2 4 5 - X
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2. Experiment The salt 2þ, BPh4 and the radicals 1, 2 and 3 were prepared as described previously [3,4]. Salt 2þ, BPh4 was crystallised from acetonitrile by slowly evaporating the solvent. The X-ray crystal structure of 2þ, BPh4 was determined using a Bruker SMART 1000 platform-CCD X-ray diffractometer system and the crystal structures of the radicals were reported in the previous papers [3,4].
3. Results and discussion Table 1 provides the crystal data for the butyl salt (2þ, BPh4 ) and Fig. 1 shows an asymmetric unit in the unit cell, Fig. 1. Asymmetric unit of the butyl salt (2þ, BPh4 ) in the lattice, showing cation and anion.
Table 1 Crystal data for butyl salt 2þ, BPh4 Formula fw ˚) a (A ˚) b (A ˚) c (A b (8) ˚ 3) V (A Space group Z Temperature (K) m (mm1) Theta range for data collection Parameters refined Goodness-of-fit on F2 R indices (all data) R for reflections with I > 2sðIÞ
B2O2N2C58H52 830.64 19.210(2) 14.1624(16) 18.473(2) 115.209(2) 4547.1(9) P2(1)/c 4 218(2) 0.072 1.85–26.37 580 1.013 0.0870 0.044
including a cation and an anion. The molecular structure of the cation 2þ is very similar to that of the corresponding radical (Fig. 2), but their packing in the lattice is quite different. In the salt (2þ, BPh4 ), each cation is involved in p–p interactions with two neighbouring molecules and thus these cations form chains in the solid state (Fig. 3). In contrast, in crystals of radicals 1 and 2, only half of each molecule involves p–p interactions with one neighbouring molecule and thus these radicals form p-dimers (Fig. 4). The projection of radicals 1 [4] and 2 (Fig. 5) demonstrates the almost perfect superposition of the active carbon atoms in the p-dimers (cf. [5,6]). The active carbon atoms carry most of the spin density in the singly occupied molecular orbital (SOMO, Fig. 6).
Fig. 2. Ortep drawings and atom numbering of (a) butyl cation (2þ) and (b) butyl radical (2).
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Fig. 3. Packing diagram of the cations (2þ) in the lattice, showing the overlap between four of the neighbouring cations; (a) and (b) are perpendicular views of each of the two sets of phenalenyl units.
Fig. 4. Stereoview of the butyl radical (2) in the lattice, showing the p-dimers.
Fig. 5. Overlap between a pair of enantiomeric radicals 2.
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Fig. 6. SOMO of half of the 1,9-disubstituted-phenalenyl boron radical (LUMO in the case of the cation) (left), together with the phases of the SOMO at the heteroatoms at the spiro junction (right).
The SOMO shown in Fig. 6 is largely nonbonding [7]; nevertheless, the substituents break the symmetry of the phenalenyl system and there are nonzero coefficients on some adjacent atoms and thus it might be expected that population of this orbital would lead to changes in the bond lengths of some parts of the substituted-phenalenyl nucleus. Column 8 of Table 2, shows the sign of the partial bond order of the SOMO in the 1,9-disubstituted-phenalenyl system. A positive sign signifies that the coefficients of adjacent carbons in the SOMO have the same sign and thus the bond should be strengthened and the bond length should be shorter when the SOMO is populated, whereas a negative sign indicates that the corresponding bond should be longer. The bond lengths of the butyl cation (2þ, averaged over one phenalenyl unit), can be used as a reference for the case where the LUMO (or SOMO in the case of the radical) is empty. On the other hand, the averaged bond lengths of the
hexyl radical 3 serve as reference for the situation where one electron is delocalised in a SOMO extending over the whole molecule. Column 4 lists the differences between the averaged bond lengths of hexyl radical (3) and butyl cation (2þ). A negative value means that the bond of radical 3 is shorter than that of cation 2þ, or the bond becomes stronger when the SOMO is populated. It is immediately apparent that column 4 matches column 8 perfectly. When radicals crystallise as p-dimers (1 and 2) the C2 symmetry of the molecule is lost and the two halves of the molecule become different, since one phenalenyl unit is involved in the p-dimer (top halves of columns 5–7), while the other one is not (bottom halves of columns 5–7). The broken symmetry is apparent in the calculated standard deviation of bond length differences between the two halves of the molecule (last row of Table 2). While the standard deviations for 2þ and 3 are 0.006 and 0.009, respectively, the values for 1 and 2 are significantly larger. Again, most of the signs in columns 5–7 match column 8 and the values for the top halves of columns 5 and 6 are significantly larger than those of corresponding bonds in the bottom halves of the columns, indicating that in the diamagnetic state, the electron has partially localised in the part of the SOMO involved in p-dimer formation. Column 7 lists the bond lengths of ethyl radical (2) (referenced to the butyl cation 2þ) at 293 K, where the radical is in the paramagnetic p-dimer state. It is interesting to notice that the standard deviation of the bond length differences between the two halves at 293 K (S:D: ¼ 0:018) is larger than that at 20 K (0.013): thus the distinction between the two halves of the molecule is greater in the high-temperature regime (paramagnetic state). Careful study of column
Fig. 7. Interconversion between diamagnetic p–p-dimer (low-temperature form) and paramagnetic p–p-dimer (high-temperature form) of ethyl radical (1).
X. Chi et al. / Synthetic Metals 133–134 (2003) 367–372
371
Table 2 Bond lengths of cation 2þ, radical 3 and the referenced bond lengths of radicals 1 and 2 and 3, together with the sign of partial bond order for SOMO Bonds
˚ ) referenced to averaged butyl cation (2þ)a Bond lengths (A
˚) Bond length (A þ
Butyl cation (2 )
þ
þ
þ
þ
Averaged 3 –2
2–2
1 (20 K)–2
1 (293 K)–2
Phenalenyl unit that involved in p-dimer (1 and 2) C1–C2 1.413 1.412 C2–C3 1.358 1.37 C3–C3a 1.414 1.413 C3a–C4 1.411 1.414 C4–C5 1.37 1.371 C5–C6 1.388 1.4 C6–C6a 1.392 1.398 C6a–C7 1.427 1.428 C7–C8 1.346 1.362 C8–C9 1.437 1.432 C9–C9a 1.439 1.44 C9a–C1 1.401 1.396 C9a–C9b 1.42 1.43 C9b–C3a 1.41 1.424 C9b–C6a 1.413 1.421 C1–O1 1.326 1.346 B–O1 1.466 1.474 C9–N9 1.336 1.352 B–N9 1.534 1.547
0.004 0.0135 0.0005 0.0035 0.003 0.002 0.011 0.0055 0.0185 0.0085 0.0025 0 0.0115 0.009 0.0045 0.0185 0.004 0.02 0.0035
0.02 0.0115 0.003 0.0045 0.01 0.0065 0.0145 0.016 0.0245 0.0225 0.009 0.008 0.0045 0.012 0.006 0.024 0.029 0.032 0.029
0.007 0.0175 0.003 0.0025 0.012 0.0005 0.0175 0.004 0.0255 0.0175 0.003 0.003 0.0055 0.015 0.008 0.016 0.019 0.028 0.019
0.001 0.0055 0.005 0.0035 0 0.0055 0.0065 0.003 0.0065 0.0045 0 0.002 0.0025 0 0 0.002 0.007 0.002 0.009
Phenalenyl unit not involved in p-dimer (1 and 2) B–N19 1.552 1.532 C19–N19 1.334 1.358 B–O11 1.476 1.46 C11–O11 1.334 1.351 C19b–C16a 1.419 1.42 C19b–C13a 1.418 1.422 C19a–C19b 1.417 1.43 C19a–C11 1.399 1.404 C19–C19a 1.437 1.431 C18–C19 1.44 1.428 C17–C18 1.345 1.366 C16a–C17 1.431 1.419 C16–C16a 1.387 1.403 C15–C16 1.389 1.381 C14–C15 1.376 1.381 C13a–C14 1.41 1.414 C13–C13a 1.42 1.42 C12–C13 1.361 1.376 C11–C12 1.409 1.402
0.0035 0.02 0.004 0.0185 0.0045 0.009 0.0115 0 0.0025 0.0085 0.0185 0.0055 0.011 0.002 0.003 0.0035 0.0005 0.0135 0.004
0.021 0.003 0.013 0.003 0.002 0.008 0.0025 0.007 0.006 0.0055 0.0025 0.005 0.0025 0.0015 0 0.0045 0.001 0.0035 0.003
0.003 0.016 0.016 0 0.008 0.007 0.0085 0.004 0.002 0.0045 0.0155 0.004 0.0135 0.0005 0.01 0.0065 0.01 0.0075 0.005
0.027 0.04 0.014 0.021 0.007 0.003 0.0065 0.008 0.013 0.0255 0.0215 0.019 0.0245 0.0115 0.006 0.0075 0.006 0.0055 0.017
0.0087
0.0135 0.0212
0.0108 0.0126
0.0131 0.0181
S.D.b S.D.c
0.0058
Hexyl radical (3)
a
0.0087
Sign of partial bond order for SOMO
þ þ 0 0 þ þ 0 0 0 0 0
0 0 0 0 0 þ þ 0 0 þ þ
a Each bond in one phenalenyl unit (for example, C1–C2) has a bond in the other phenalenyl unit (for example, C11–C12), related by the C2 symmetry of the molecule; by averaging these two bonds, an averaged C1–C2 (or C11–C12) bond length is obtained. b Tabulated values. c Between two halves of molecule.
7 reveals that the values in the bottom half are significantly larger than those in the top half of the column: thus the SOMO is now localised more in the half of the molecule where p–p overlap is absent. The results of this structural analysis are summarised in Fig. 7. In the high-temperature regime, the unpaired electrons avoid each other and localise mainly in the left (top) and right (bottom) parts of the molecule, respectively. Below the magnetic transition temperature (150 K for radical 1 and 350 K for radical 2),
the electrons come together, spin pair and form a p-bond, while the interplanar distance decreases from 3.33 to ˚ . It is important to note that the bond length differ3.16 A ences between the two halves of radical 1 at 20 K are smaller than those at 293 K, indicating that the electron is more delocalised in the diamagnetic state. We suggest that the enhanced delocalisation is related to the increase in conductivity by two orders of magnitude when the diamagnetic state is formed [4].
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Acknowledgements This work was supported by the Office of Basic Energy Sciences, Department of Energy under Grant No. DE-FG0297ER45668.
References [1] R.C. Haddon, Nature 256 (1975) 394. [2] R.C. Haddon, Aust. J. Chem. 28 (1975) 2343.
[3] X. Chi, M.E. Itkis, B.O. Patrick, T.M. Barclay, R.W. Reed, R.T. Oakley, A.W. Cordes, R.C. Haddon, J. Am. Chem. Soc. 121 (1999) 10395. [4] X. Chi, M.E. Itkis, K. Kirschbaum, A.A. Pinkerton, R.T. Oakley, A.W. Cordes, R.C. Haddon, J. Am. Chem. Soc. 123 (2001) 4041. [5] K. Goto, T. Kubo, K. Yamamoto, K. Nakasuji, K. Sato, D. Shiomi, T. Takui, M. Kubota, T. Kobayashi, K. Yakusi, J. Ouyang, J. Am. Chem. Soc. 121 (1999) 1619. [6] P.A. Koutentis, Y. Chen, Y. Cao, T.P. Best, M.E. Itkis, L. Beer, R.T. Oakley, C.P. Brock, R.C. Haddon, J. Am. Chem. Soc. 123 (2001) 3864. [7] R.C. Haddon, S.V. Chichester, J.H. Marshall, Tetrahedron 42 (1986) 6293.
Symmetry breaking localisation of the unpaired electron in spiro-bis(1,9-disubstituted-phenalenyl)boron radicals$ X. Chia, F.S. Thama, A.W. Cordesb, M.E. Itkisa, R.C. Haddona,* a b
Chemistry and Engineering Department, University of California, Riverside, CA 92506, USA Chemistry and Biochemistry Department, University of Arkansas, Fayetteville, AR 72701, USA
Abstract We report the crystal structure of a spiro-bis(1,9-disubstituted-phenalenyl)boron cation (2þ) and by comparison with the structures of a series of radicals we conclude that the changes in bond lengths in the radicals compared to those of the corresponding cation can be qualitatively predicted by the sign of the partial bond order of the singly occupied molecular orbital (SOMO), which contains the additional electron. By use of this information, we can analyse the structures of the p-dimer radicals 1 and 2 and ascertain the location of the unpaired electron in these compounds. Where these radicals exist as paramagnetic p-dimers (high-temperature regime), the unpaired electrons reside mainly in the phenalenyl units not directly involved in the p-dimer interaction. At low temperatures, where the diamagnetic p-dimers exist, the electrons localise in the phenalenyl units directly involved in the p-dimer, but there still exists some degree of delocalisation of the electrons to the other halves of the molecules. We postulate that the increase in delocalisation of the unpaired electrons is related to the increase of the conductivity by about two orders of magnitude as the electrons spin pair. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Phenalenyl; Radical; Conductivity; p-Dimer; SOMO; Unpaired electron
1. Introduction The phenalenyl system has been proposed as an approach to intrinsic molecular conductors and superconductors [1,2]. We have recently reported the first phenalenyl-based neutral radical conductors (1, 2, 3) [3,4]. Although none of the distances between the solid state molecular building blocks lies within the van der Waals distances for carbon ˚ ), these compounds exhibit the highest conatoms (3.4 A ductivity of any neutral organic molecules. Moreover, radicals 1 and 2 involve face-to-face p–p interactions, with ˚ at higher temmean plane separations of around 3.33 A ˚ at lower temperature and this perature and about 3.16 A change is accompanied by a magnetic phase transition from the paramagnetic p-dimer form to the diamagnetic p-dimer form, while their conductivities increase by about two orders of magnitude when the diamagnetic state is formed [4]. These results are unprecedented and we tentatively
$
Yamada Conference LVI, the Fourth International Symposium on Crystalline Organic Metals, Superconductors and Ferromagnets, ISCOM 2001—Abstract Number P4. * Corresponding author. E-mail address: [email protected] (R.C. Haddon).
assigned the enhancement in the conductivity to a decrease in the on-site Coulombic correlation energy (U), as the dimers form a super-molecule with twice the amount of conjugation [4].
Here we report the crystal structure of the cation 2þ, BPh4 , which we previously used to prepare the corresponding radical (2) [4]. By comparing the bond lengths of 2þ with those of the radicals 1, 2 and 3, we conclude that the unpaired electrons of the radicals 1 and 2 avoid each other in the paramagnetic p-dimer state, but spin pair in the phenalenyl units where there exist p–p interactions in the diamagnetic p-dimer state (Fig. 7).
0379-6779/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 ( 0 2 ) 0 0 2 4 5 - X
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2. Experiment The salt 2þ, BPh4 and the radicals 1, 2 and 3 were prepared as described previously [3,4]. Salt 2þ, BPh4 was crystallised from acetonitrile by slowly evaporating the solvent. The X-ray crystal structure of 2þ, BPh4 was determined using a Bruker SMART 1000 platform-CCD X-ray diffractometer system and the crystal structures of the radicals were reported in the previous papers [3,4].
3. Results and discussion Table 1 provides the crystal data for the butyl salt (2þ, BPh4 ) and Fig. 1 shows an asymmetric unit in the unit cell, Fig. 1. Asymmetric unit of the butyl salt (2þ, BPh4 ) in the lattice, showing cation and anion.
Table 1 Crystal data for butyl salt 2þ, BPh4 Formula fw ˚) a (A ˚) b (A ˚) c (A b (8) ˚ 3) V (A Space group Z Temperature (K) m (mm1) Theta range for data collection Parameters refined Goodness-of-fit on F2 R indices (all data) R for reflections with I > 2sðIÞ
B2O2N2C58H52 830.64 19.210(2) 14.1624(16) 18.473(2) 115.209(2) 4547.1(9) P2(1)/c 4 218(2) 0.072 1.85–26.37 580 1.013 0.0870 0.044
including a cation and an anion. The molecular structure of the cation 2þ is very similar to that of the corresponding radical (Fig. 2), but their packing in the lattice is quite different. In the salt (2þ, BPh4 ), each cation is involved in p–p interactions with two neighbouring molecules and thus these cations form chains in the solid state (Fig. 3). In contrast, in crystals of radicals 1 and 2, only half of each molecule involves p–p interactions with one neighbouring molecule and thus these radicals form p-dimers (Fig. 4). The projection of radicals 1 [4] and 2 (Fig. 5) demonstrates the almost perfect superposition of the active carbon atoms in the p-dimers (cf. [5,6]). The active carbon atoms carry most of the spin density in the singly occupied molecular orbital (SOMO, Fig. 6).
Fig. 2. Ortep drawings and atom numbering of (a) butyl cation (2þ) and (b) butyl radical (2).
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Fig. 3. Packing diagram of the cations (2þ) in the lattice, showing the overlap between four of the neighbouring cations; (a) and (b) are perpendicular views of each of the two sets of phenalenyl units.
Fig. 4. Stereoview of the butyl radical (2) in the lattice, showing the p-dimers.
Fig. 5. Overlap between a pair of enantiomeric radicals 2.
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Fig. 6. SOMO of half of the 1,9-disubstituted-phenalenyl boron radical (LUMO in the case of the cation) (left), together with the phases of the SOMO at the heteroatoms at the spiro junction (right).
The SOMO shown in Fig. 6 is largely nonbonding [7]; nevertheless, the substituents break the symmetry of the phenalenyl system and there are nonzero coefficients on some adjacent atoms and thus it might be expected that population of this orbital would lead to changes in the bond lengths of some parts of the substituted-phenalenyl nucleus. Column 8 of Table 2, shows the sign of the partial bond order of the SOMO in the 1,9-disubstituted-phenalenyl system. A positive sign signifies that the coefficients of adjacent carbons in the SOMO have the same sign and thus the bond should be strengthened and the bond length should be shorter when the SOMO is populated, whereas a negative sign indicates that the corresponding bond should be longer. The bond lengths of the butyl cation (2þ, averaged over one phenalenyl unit), can be used as a reference for the case where the LUMO (or SOMO in the case of the radical) is empty. On the other hand, the averaged bond lengths of the
hexyl radical 3 serve as reference for the situation where one electron is delocalised in a SOMO extending over the whole molecule. Column 4 lists the differences between the averaged bond lengths of hexyl radical (3) and butyl cation (2þ). A negative value means that the bond of radical 3 is shorter than that of cation 2þ, or the bond becomes stronger when the SOMO is populated. It is immediately apparent that column 4 matches column 8 perfectly. When radicals crystallise as p-dimers (1 and 2) the C2 symmetry of the molecule is lost and the two halves of the molecule become different, since one phenalenyl unit is involved in the p-dimer (top halves of columns 5–7), while the other one is not (bottom halves of columns 5–7). The broken symmetry is apparent in the calculated standard deviation of bond length differences between the two halves of the molecule (last row of Table 2). While the standard deviations for 2þ and 3 are 0.006 and 0.009, respectively, the values for 1 and 2 are significantly larger. Again, most of the signs in columns 5–7 match column 8 and the values for the top halves of columns 5 and 6 are significantly larger than those of corresponding bonds in the bottom halves of the columns, indicating that in the diamagnetic state, the electron has partially localised in the part of the SOMO involved in p-dimer formation. Column 7 lists the bond lengths of ethyl radical (2) (referenced to the butyl cation 2þ) at 293 K, where the radical is in the paramagnetic p-dimer state. It is interesting to notice that the standard deviation of the bond length differences between the two halves at 293 K (S:D: ¼ 0:018) is larger than that at 20 K (0.013): thus the distinction between the two halves of the molecule is greater in the high-temperature regime (paramagnetic state). Careful study of column
Fig. 7. Interconversion between diamagnetic p–p-dimer (low-temperature form) and paramagnetic p–p-dimer (high-temperature form) of ethyl radical (1).
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371
Table 2 Bond lengths of cation 2þ, radical 3 and the referenced bond lengths of radicals 1 and 2 and 3, together with the sign of partial bond order for SOMO Bonds
˚ ) referenced to averaged butyl cation (2þ)a Bond lengths (A
˚) Bond length (A þ
Butyl cation (2 )
þ
þ
þ
þ
Averaged 3 –2
2–2
1 (20 K)–2
1 (293 K)–2
Phenalenyl unit that involved in p-dimer (1 and 2) C1–C2 1.413 1.412 C2–C3 1.358 1.37 C3–C3a 1.414 1.413 C3a–C4 1.411 1.414 C4–C5 1.37 1.371 C5–C6 1.388 1.4 C6–C6a 1.392 1.398 C6a–C7 1.427 1.428 C7–C8 1.346 1.362 C8–C9 1.437 1.432 C9–C9a 1.439 1.44 C9a–C1 1.401 1.396 C9a–C9b 1.42 1.43 C9b–C3a 1.41 1.424 C9b–C6a 1.413 1.421 C1–O1 1.326 1.346 B–O1 1.466 1.474 C9–N9 1.336 1.352 B–N9 1.534 1.547
0.004 0.0135 0.0005 0.0035 0.003 0.002 0.011 0.0055 0.0185 0.0085 0.0025 0 0.0115 0.009 0.0045 0.0185 0.004 0.02 0.0035
0.02 0.0115 0.003 0.0045 0.01 0.0065 0.0145 0.016 0.0245 0.0225 0.009 0.008 0.0045 0.012 0.006 0.024 0.029 0.032 0.029
0.007 0.0175 0.003 0.0025 0.012 0.0005 0.0175 0.004 0.0255 0.0175 0.003 0.003 0.0055 0.015 0.008 0.016 0.019 0.028 0.019
0.001 0.0055 0.005 0.0035 0 0.0055 0.0065 0.003 0.0065 0.0045 0 0.002 0.0025 0 0 0.002 0.007 0.002 0.009
Phenalenyl unit not involved in p-dimer (1 and 2) B–N19 1.552 1.532 C19–N19 1.334 1.358 B–O11 1.476 1.46 C11–O11 1.334 1.351 C19b–C16a 1.419 1.42 C19b–C13a 1.418 1.422 C19a–C19b 1.417 1.43 C19a–C11 1.399 1.404 C19–C19a 1.437 1.431 C18–C19 1.44 1.428 C17–C18 1.345 1.366 C16a–C17 1.431 1.419 C16–C16a 1.387 1.403 C15–C16 1.389 1.381 C14–C15 1.376 1.381 C13a–C14 1.41 1.414 C13–C13a 1.42 1.42 C12–C13 1.361 1.376 C11–C12 1.409 1.402
0.0035 0.02 0.004 0.0185 0.0045 0.009 0.0115 0 0.0025 0.0085 0.0185 0.0055 0.011 0.002 0.003 0.0035 0.0005 0.0135 0.004
0.021 0.003 0.013 0.003 0.002 0.008 0.0025 0.007 0.006 0.0055 0.0025 0.005 0.0025 0.0015 0 0.0045 0.001 0.0035 0.003
0.003 0.016 0.016 0 0.008 0.007 0.0085 0.004 0.002 0.0045 0.0155 0.004 0.0135 0.0005 0.01 0.0065 0.01 0.0075 0.005
0.027 0.04 0.014 0.021 0.007 0.003 0.0065 0.008 0.013 0.0255 0.0215 0.019 0.0245 0.0115 0.006 0.0075 0.006 0.0055 0.017
0.0087
0.0135 0.0212
0.0108 0.0126
0.0131 0.0181
S.D.b S.D.c
0.0058
Hexyl radical (3)
a
0.0087
Sign of partial bond order for SOMO
þ þ 0 0 þ þ 0 0 0 0 0
0 0 0 0 0 þ þ 0 0 þ þ
a Each bond in one phenalenyl unit (for example, C1–C2) has a bond in the other phenalenyl unit (for example, C11–C12), related by the C2 symmetry of the molecule; by averaging these two bonds, an averaged C1–C2 (or C11–C12) bond length is obtained. b Tabulated values. c Between two halves of molecule.
7 reveals that the values in the bottom half are significantly larger than those in the top half of the column: thus the SOMO is now localised more in the half of the molecule where p–p overlap is absent. The results of this structural analysis are summarised in Fig. 7. In the high-temperature regime, the unpaired electrons avoid each other and localise mainly in the left (top) and right (bottom) parts of the molecule, respectively. Below the magnetic transition temperature (150 K for radical 1 and 350 K for radical 2),
the electrons come together, spin pair and form a p-bond, while the interplanar distance decreases from 3.33 to ˚ . It is important to note that the bond length differ3.16 A ences between the two halves of radical 1 at 20 K are smaller than those at 293 K, indicating that the electron is more delocalised in the diamagnetic state. We suggest that the enhanced delocalisation is related to the increase in conductivity by two orders of magnitude when the diamagnetic state is formed [4].
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Acknowledgements This work was supported by the Office of Basic Energy Sciences, Department of Energy under Grant No. DE-FG0297ER45668.
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