Effects of boron doping on the structural and optoelectronic properties of 9,10-diarylanthracenes

Article abstract of DOI:10.1039/c3dt51035b

Key structural and optoelectronic properties of 9,10-dihydro-9,10- diboraanthracene (DBA) derivatives carrying mesityl (2a), 2-methylnaphthyl- (2b) and 9-phenyl-2,7-di-tert-butylanthryl (2c) substituents at the boron atoms have systematically been compared with the properties of their all-carbon congeners 4a-c. The experimental investigations have been augmented by quantum-chemical calculations. Steric repulsion leads to large dihedral angles between the aryl substituents and the DBA (2a-c) or anthrylene (4a-c) cores; as a result, the B-C bonds of 2a-c are kinetically shielded from hydrolysis and oxidative degradation. Lithium metal reduces the mesityl derivative 2a to the inverse sandwich complexes [Li(OR₂)_n]₂[2a] (X-ray crystallography; OR₂ = THF, n = 2; Et₂O, n = 1). In line with the nodal structures of the LUMO of 2a/HOMO of [Li(THF)₂] ₂[2a], the C-C bond lengths of the anionic fragment [2a]^2- show characteristic differences to those of 2a and come close to the C-C bond lengths of the isoelectronic species 4a. X-ray crystallography on anti-2b × 2 C₆H₆ and anti-4b × 2 C₆H ₆ reveals an essentially identical packing of the main molecules. The benzene solvate molecules, however, interact in a very different manner with anti-2b or anti-4b, which can be traced down to subtle disparities between the electron density distributions of the two compounds. 2a-c undergo a photoinduced aryl-to-DBA charge transfer; the back electron transfer results in blue (2a), green (2b) and red (2c) emission, albeit with low quantum yields. 4a-c are characterised by a local π-π* photoexcitation of the central 9,10-anthrylene fragments and corresponding blue emission. Each of the compounds 2a-c gives rise to two reversible DBA-centred one-electron transitions in the cyclic voltammogram.

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Effects of boron doping on the structural and
optoelectronic properties of 9,10-diarylanthracenes†
Cite this: DOI: 10.1039/c3dt51035b
Claas Hoffend, Martin Diefenbach, Estera Januszewski, Michael Bolte,
Hans-Wolfram Lerner, Max C. Holthausen* and Matthias Wagner*
Key structural and optoelectronic properties of 9,10-dihydro-9,10-diboraanthracene (DBA) derivatives
carrying mesityl (2a), 2-methylnaphthyl- (2b) and 9-phenyl-2,7-di-tert-butylanthryl (2c) substituents at
the boron atoms have systematically been compared with the properties of their all-carbon congeners
4a–c. The experimental investigations have been augmented by quantum-chemical calculations. Steric
repulsion leads to large dihedral angles between the aryl substituents and the DBA (2a–c) or anthrylene
(4a–c) cores; as a result, the B–C bonds of 2a–c are kinetically shielded from hydrolysis and oxidative
degradation. Lithium metal reduces the mesityl derivative 2a to the inverse sandwich complexes
[Li(OR2)n]2[2a] (X-ray crystallography; OR2 = THF, n = 2; Et2O, n = 1). In line with the nodal structures of
the LUMO of 2a/HOMO of [Li(THF)2]2[2a], the C–C bond lengths of the anionic fragment [2a]2− show
characteristic differences to those of 2a and come close to the C–C bond lengths of the isoelectronic
species 4a. X-ray crystallography on anti-2b × 2 C6H6 and anti-4b × 2 C6H6 reveals an essentially identical
packing of the main molecules. The benzene solvate molecules, however, interact in a very different
manner with anti-2b or anti-4b, which can be traced down to subtle disparities between the electron
density distributions of the two compounds. 2a–c undergo a photoinduced aryl-to-DBA charge transfer;
the back electron transfer results in blue (2a), green (2b) and red (2c) emission, albeit with low quantum
yields. 4a–c are characterised by a local π–π* photoexcitation of the central 9,10-anthrylene fragments
and corresponding blue emission. Each of the compounds 2a–c gives rise to two reversible DBA-centred
one-electron transitions in the cyclic voltammogram.
Received 19th April 2013,
Accepted 14th May 2013
DOI: 10.1039/c3dt51035b
www.rsc.org/dalton
preparative and computational chemistry, materials science
and device physics. From a chemist’s view, particularly exciting
Introduction
Over the past two decades, organic electronic materials have perspectives are offered by the incorporation of main-group
attracted increasing attention owing to their applications as elements, like B,8 Si,9 N,10 P11 or S,12 into the carbon frame-
diverse as sensors,1 field-effect transistors,2 photovoltaics,3,4 works of functional molecular building blocks. Detailed
lasers5 and light-emitting diodes.6,7 Plastic electronics possess studies, however, in which the optoelectronic properties of a
the major advantage over silicon-semiconductor technology specific all-carbon system have been thoroughly characterised
that organic compounds are highly tuneable. Research into and compared with those of an isostructural/isoelectronic
structure–property relationships is therefore the key to the organo-element congener are lacking. The objective of this
improvement of critical features both of the underlying mole- paper is therefore to provide a quantitative evaluation of the
cular species and of the (semi)conducting bulk systems, viz. crystal-structure changes and the modifications in the opto-
optical absorption and emission characteristics or charge electronic behaviour experienced by a series of oligoacenes
generation, recombination and transport issues. The field has upon boron doping. Our aim is to systematically identify the
consequently developed into a wide academic and industrial ingredients available for the rational design of molecular
playground for many interacting research disciplines including building blocks in order to develop optimised materials for a
given purpose.
We have selected anthracene derivatives for our investi-
Institut für Anorganische Chemie, J.W. Goethe-Universität Frankfurt, Max-von-Laue-
gations as those have become benchmark materials for
organic light-emitting devices (OLEDs) due to their high oxi-
Strasse 7, D-60438 Frankfurt (Main), Germany.
E-mail: Matthias.Wagner@chemie.uni-frankfurt.de
dation potential, small π surface and good fluorescence
†Electronic supplementary information (ESI) available. CCDC 922259–922265.
quantum yield (ϕF).13 While ϕF of pristine anthracene is only
For ESI and crystallographic data in CIF or other electronic format see DOI:
moderate (0.27; C6H6),14 an introduction of phenyl rings into
10.1039/c3dt51035b
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the 9,10-position of the molecular scaffold results in a dramati-
Herein, we will be combining both approaches. On the one
cally improved performance (ϕF = 0.96; C6H6).15 In fact, 9,10- hand, DBA provides a comparatively rigid molecular scaffold.
diphenylanthracene is frequently used as a reference standard On the other hand, the 9,10-substituents that are needed
in chemiluminescence studies.16,17
anyway to break up undesired aggregation in the solid state (cf.
In 9,10-diphenylanthracene, steric repulsion between the the research on the related all-carbon systems mentioned
phenyl rings and the hydrogen atoms in the peri-positions of above) can simultaneously serve to kinetically protect the organo-
the anthracene core results in a dihedral angle Ph//Anth of borane from hydrolytic degradation. Moreover, we found indi-
67.8° (solid-state structure; Anth = 9,10-anthrylene).18 Even cations that the emission colour of DBA fluorophores is
though any significant π-conjugation between the two frag- influenced by the nature of 9,10-aryl substituents,32 which pro-
ments is consequently interrupted, the presence of the two vides a useful set-screw for the targeted design of water-resist-
phenyl rings causes the system of singlet levels to drop in ant 9,10-diaryl-DBA luminophores.
energy while the triplet levels rise such that only the T1ππ* level
For the present study, we thus selected DBA derivatives
still lies below the S1ππ* level. The resulting decrease in the bearing three different aryl groups in the 9,10-positions, i.e.,
value of the intersystem crossing rate constant Kst explains the substituted phenyl (2a), 1-naphthyl (2b) and 9-anthryl (2c;
concomitant increase in ϕF for 9,10-diphenylanthracene com- Fig. 1) and investigated the influence of these groups on the
pared to anthracene.19 Moreover, the orthogonal phenyl substi- optoelectronic properties of the materials. We then compared
tuents help to avoid π stacking in the solid state where the DBA species 2a–c with their 9,10-anthrylene analogues
intermolecular electronic interactions can lead to undesired 4a–c.
fluorescence line-broadening and quenching phenomena.13
The strong general correlation between film quality/crystal-
Further development of the field will therefore hinge on the linity and device performance suggests that the intermolecular
identification of the best-suited 9,10-substituents for a given interaction in the solid state is also a key parameter to con-
purpose, but also on an optimisation of the anthracenes’ sider in the development of novel organic optoelectronic
HOMO/LUMO energies.
Generally speaking, many π-electron materials are charac-
terised by low electron affinities and thus behave as hole-trans-
port materials (p-type semiconductors). However, for the
fabrication of efficient devices, also electron-transport
materials are of great significance (n-type semiconductors).20
In the present context, one seeks the perfect match between
the LUMO energy of an OLED emitter and the Fermi level of
the cathode or between the HOMO energy and the Fermi level
of the anode in order to facilitate the injection of charge car-
riers into the emissive layer.16
A widely used tool to increase the electron affinities of
acenes is (per)fluorination.21–23 As an added value, fluorina-
tion can lead to improved stability for devices operating
under ambient conditions.24 Alternatively, electron-poor het-
erocycles have been employed as building blocks of n-type
materials (e.g., benzothiadiazoles, pyridines, quinolines).6
Our group uses boron heterocycles to create electron-deficient
π systems, because a boron atom is isoelectronic with a car-
benium ion and the vacant p orbital of a three-coordinate
boron centre efficiently extends the conjugation pathway of
an adjacent π-electron cloud.25 In particular, we are develop-
ing
derivatives
of
9,10-dihydro-9,10-diboraanthracene
(DBA),26–30 which possess outstanding optoelectronic
properties.28,31–33
A challenge generally encountered with organoboranes is
their sensitivity towards air and moisture. Common solutions
of this problem are stabilisation of the molecules by (i) steri-
cally demanding substituents34 or (ii) incorporation of the
boron atom into a rigid, planar framework.35 In the first case,
the access of an H2O molecule to the vacant boron p orbital is
prevented; in the second case, structural constraint energeti-
cally disfavours pyramidalisation of the boron centre in a puta-
tive R3B–OH2 adduct.
Fig. 1 DBA derivatives 2a–c bearing substituted phenyl, 1-naphthyl and
9-anthryl groups. 2b exists as a mixture of syn/anti-isomers. The methyl groups in
2a and 2b were attached for improved kinetic stabilisation; the tBu groups and
the phenyl rings in 2c were introduced for solubility reasons and to facilitate the
synthesis. The red-coloured fragments illustrate the similar degree of shielding
exerted by o-methyl groups and annelated benzene rings.
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materials.13 Given that 2a–c offer the rare opportunity to put
boron-doped π systems side by side with their isostructural
organic congeners, we have also compared the crystal packing
of anti-2b and anti-9,10-bis(2′-methylnaphthyl)anthracene
(anti-4b). We traced the observed differences to subtle dispar-
ities between the electron density distributions. Finally, the
capacities of DBAs to reversibly accept electrons were explored
by cyclic voltammetry as well as reduction on a preparative
scale; the π-electron density distributions in 2a and the isoelec-
tronic couple [2a]2−/4a were assessed by 13C{1H} NMR spec-
troscopy and quantum-chemical calculations.
Results and discussion
Fig. 1 compiles the DBA derivatives under investigation here.
We have used 9,10-dimesityl-DBA (2a) rather than 9,10-diphe-
nyl-DBA to achieve optimal steric shielding of the boron p
orbitals. For the same reason, 9,10-di(1′-naphthyl)-DBA was
equipped with two methyl substituents (cf. 2b). The red-
coloured fragments in Fig. 1 illustrate the similar degree of
shielding that can be achieved through the introduction of
o-methyl groups and annelated benzene rings. The tert-butyl
groups in 2c are required for solubility reasons; the sole task
of the phenyl rings was to facilitate the synthesis.32,36 To
ensure maximum comparability, the all-carbon analogues 4a–c
were equipped with precisely the same substitution patterns as
2a–c (Scheme 1).
Scheme 1 Synthesis of compounds 2a–c and 4a–c. Reagents and conditions:
2a: +2 MesMgBr, toluene, 0 °C → r.t., 12 h; 2b: +2 (2-methylnaphthyl)MgBr,
Et2O–toluene, 0 °C → r.t., 12 h; 2c: +2 10-lithio-9-phenyl-2,7-di-tert-butylanthra-
cene, Et2O–toluene, −78 °C → r.t., 12 h. 4a,c: (1) Et2O–THF, −78 °C → r.t., 12 h;
(2) +SnCl2–HClaq, r.t., 12–24 h; 4b: (1) Et2O–THF, −78 °C → r.t., 12 h; (2) +H3PO2–
HI, CH3COOH, 80 °C, 2 h.
Syntheses of the 9,10-dihydro-9,10-diboraanthracenes and
their all-carbon analogues
The mesityl- (2a),37 2-methylnaphthyl- (2b) and 9-phenyl-2,7-
di-tert-butylanthryl-substituted (2c)32 DBAs were synthesised
from 9,10-dibromo-DBA (1)28 and the respective organomagne-
sium (2a, 2b) or organolithium (2c) reagents through nucleo-
philic substitution reactions (Scheme 1). The corresponding
all-carbon analogues 4a–c are accessible by treating the appro-
priate aryllithium compound with 9,10-anthraquinone (3), fol-
lowed by reduction of the resulting lithium alcoholate with
SnCl2–HCl38 or H3PO2–HI–CH3COOH39 (Scheme 1).
Addition of excess lithium to 9,10-dimesityl-DBA (2a) in
ethereal solvents yields the red-orange lithium salt Li2[2a]
(Scheme 2), its dianion [2a]2− being isoelectronic with 9,10-
dimesitylanthracene 4a.
Scheme 2 Synthesis of compounds [Li(THF)2]2[2a] and [Li(Et2O)]2[2a]. Con-
ditions: r.t., 12 h.
For all the compounds 2a, 2c, 4a, 4c and Li2[2a], the proton from solvents of significantly different polarity (e.g., C6H6 vs.
1
integral values as well as the number of resonances in the H CHCl3) we always obtained single crystals of anti-2b only;
and 13C{1H} NMR spectra are in accord with the proposed attempts at the isolation of syn-2b by column chromatography
9,10-disubstituted framework and the molecular symmetry were also not successful. In the case of syn/anti-4b, the two
resulting from it. A peculiarity is, however, observed for the stereoisomers could be separated by fractional crystallisation;
2-methylnaphthyl derivatives 2b and 4b, which give rise to two a complete assignment of 1H/13C{1H} NMR resonances was
1
complete sets of H and 13C{1H} NMR signals. The reason lies possible after the X-ray diffraction data of syn- and anti-4b
in a hindered rotation about the exocyclic B–C or C–C bonds40 had been obtained (see below). Stereoisomerically pure syn-
leading to the formation of atropisomers syn/anti-2b (1 : 1 and anti-4b show no tendency to transform into each other in
1
ratio) or syn/anti-4b (1 : 1 ratio). Isomer syn-2b is expected to solution. In line with that, the H NMR spectra of the related
possess a higher dipole moment than anti-2b (cf. a related syn- and anti-9,10-di(1′-naphthyl)anthracene do not change
study on 9,10-di(1′-naphthyl)anthracene).40 Nevertheless, even up to 100 °C (toluene-d8);40 the naphthyl rotation barrier in
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1′,9-naphthylanthracene was computed to be as high as the case for DBA) obviously does not suffice to protect the
38 kcal mol−1 41
Similar to their all-carbon congeners, solu- boron atom efficiently from Lewis-base attack through struc-
tions of syn/anti-2b show no signs of coalescence in their 1H tural constraints.
.
NMR spectra at temperatures up to 120 °C (toluene-d8), which
NMR spectroscopy is also a useful tool to map the
means that the barrier of rotation about the exocyclic B–C π-charge density distribution in acenes. In the present
bond exceeds 21 kcal mol−1 42
Very surprisingly, though, a context we are mainly interested in finding out (i) whether
.
single crystal of anti-2b, placed in a Shigemi-type NMR tube the different dangling substituents affect the π-electron
1
and dissolved in C6D6 at r.t., gave H NMR signals of both the cloud of the DBA or anthracene cores in 2a–c or 4a–c and
syn and the anti stereoisomer. The proton integral values of (ii) to what extent the formally isoelectronic nature of [2a]2−
the CH3 resonances again indicated a stereoisomer ratio of and 4a is reflected by a similar π-electron distribution. We
1 : 1; moreover, these two signals are connected by clear-cut will use the chemical shift values of the carbon nuclei C-a,
exchange peaks in the ROESY NMR spectrum of syn/anti-2b C-b and C-c (cf. Scheme 1) as diagnostic tools, because the
(cf. the ESI† for more details). As an explanation we point out that, shielding of a specific arene carbon atom depends linearly
on the one hand, the exocyclic B–C bonds in anti-2b (1.581(2) Å) on the corresponding π-electron density at this position and
are longer by about 0.085 Å than the exocyclic C–C bonds in its 13C{1H} shift remains largely unaffected by ring current
anti-4b (1.494(5) Å) and syn-4b (1.497(2) Å; cf. the ESI† for more anisotropies as well as the solvent employed (2a was
details), which reduces unfavourable steric interactions in the measured in C6D6 and THF-d8, Li2[2a] in THF-d8 and 4a in
transition state of 2-methylnaphthyl rotation. Yet, this effect is C6D6).46
probably small as can be deduced from the lack of coalescence
Neither in the DBA derivatives 2a–c nor in the anthracene
phenomena in the high-temperature 1H NMR spectrum of syn/ molecules 4a–c do the chemical shift differences between
anti-2b. An alternative explanation is offered by the DFT-calcu- resonances of like carbon atoms exceed a value of Δδ = 2.0
lated transition state for phenyl rotation in 9-phenylanthracene (Table 1). We therefore conclude that the electronic influ-
(TS(C); Fig. 2) in which the molecule adopts a highly strained ence of the 9,10-substituents on the DBA/anthracene cores is
non-planar conformation.41,43,44 A corresponding Lewis-acidic small in the ground state,17 which points towards a lack of
boron species can relieve angle strain in the transition state conjugative interaction across the exocyclic B–C/C–C bonds
upon tetra-coordination with a σ donor Do (TS(BDo); Fig. 2). as a result of large dihedral angles between adjacent aryl
Even though the 2-methylnaphthyl substituent is bulky planes (cf. the X-ray crystal structure analyses discussed
enough to protect syn/anti-2b from hydrolysis, a short-term below).
adduct with Do = OH− or H2O may nevertheless be sufficiently
The comparison of 2a with 4a reveals a predictable
stable to catalyse naphthyl rotation. As a proof-of-principle, we deshielding in 2a of C-a (Δδ = +13), C-b (Δδ = +8) and C-c (Δδ =
treated yellow syn/anti-2b in CD3CN with excess [Me4N]F and +15), because 2a is isoelectronic with [4a]2+ (Table 1). In turn,
observed discolouration of the sample together with an two-electron reduction of 2a to Li2[2a] leads to a better shield-
11B{1H} NMR resonance of 7 ppm, which indicates the accessibil- ing in Li2[2a] of the three carbon nuclei C-a (Δδ = −2), C-b (Δδ
ity of the vacant boron p-orbital for small Lewis bases like F− = −15) and C-c (Δδ = −11; Table 1). Even though these qualitat-
and OH−.45 Since the NMR investigation of the single crystal of ive trends are in line with a priori expectations, the quantitative
anti-2b had been performed in dried C6D6, the rotation is data still show significant differences between Li2[2a] and its
obviously catalysed already by a very small amount of OH−/ isoelectronic all-carbon relative 4a of Δδ(C-a) = +11, Δδ(C-b) =
7
H2O.45 A temperature rise will shift any B–Do association/dis- −7 and Δδ(C-c) = +5 (Table 1). The Li{1H} NMR spectrum of
sociation equilibrium towards the side of the free components. Li2[2a] indicated the presence of contact ion pairs similar to
It is therefore understandable that the proposed mechanism those established in the solid state (see the solid-state struc-
of catalytic isomerisation is no longer operative at elevated ture of [Li(THF)2]2[2a] below).47 Exclusively on the basis of
temperatures and that consequently no coalescence was
observed in the VT-NMR spectra of syn/anti-2b. We further
note that linking two aryl substituents in Ar3B derivatives (as is
Table 1 13C{1H} chemical shift values of 2a–c, Li2[2a] and 4a–c (calculated DFT
values in parentheses); only computed data are available in the case of the
hypothetical free dianion [2a]2−
2a
(C6D6)
2b
(C6D6)
2c
(C6D6)
[2a]2−
(gas phase)
Li2[2a]
(THF-d8)
C-a
C-b
C-c
139.4 (139.5)
134.0 (133.0)
145.8 (146.7)
140.2
134.3
146.0
141.1
134.6
146.8
(135.0)
(108.8)
(139.0)
137.5 (136.7)
118.8 (118.4)
134.9 (135.8)
4a (C6D6)
4b (C6D6)
4c (C6D6)
C-a
C-b
C-c
C-d
126.8 (126.2)
126.0 (124.8)
130.4 (130.9)
136.0 (141.1)
127.2
126.2
131.1
135.0
127.7
126.4
132.4
135.2
Fig. 2 Calculated transition state (TS(C)) of phenyl rotation in 9-phenylanthra-
cene; suggested structure of the corresponding donor-stabilised transition state
(TS(BDo)) in 9-phenyl-DBA.
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experimental data, it is therefore difficult to decide to what all six molecules are uncharged compounds, boron atoms are
extent the differences in the 13C{1H} NMR spectra of Li2[2a] more electropositive than carbon atoms and the 2-type species
and 4a are caused by an unequally distributed charge density have two π electrons less than the 4-type derivatives. The result-
or by the vicinity of the coordinating Li+ ion.
ing delicate differences in the σ- and π-charge distributions
We have therefore performed a quantum-chemical analysis along the DBA/anthracene scaffolds might well influence inter-
of the species 2a, [Li(THF)2]2[2a], [2a]2− and 4a with special molecular interactions and, in turn, the solid-state structures
emphasis on (i) the 13C chemical shift values (Table 1), (ii) the of 2-type vs. 4-type compounds.
frontier orbital structures, and (iii) the atomic partial charges
To test this hypothesis, we chose the crystal packing of anti-
q according to natural population (q(NPA)) and atoms-in-mole- 2b and anti-4b for a closer examination (note that 9,10-di(1′-
cules analyses of computed electron densities (q(AIM)). naphthyl)anthracenes are leading candidates as electrolumi-
Density-functional theory calculations were performed at the nescence materials in OLEDs40,58). Different packing motifs
B97D/TZVP level of theory48,49 using the Gaussian pro- can, of course, be established by identical molecules as well.
gramme.50 Atomic charges were computed using NBO soft- We have therefore also investigated two polymorphs of 2a in
ware51 as implemented in Gaussian and within the atoms-in- order to carve out the effect of boron–carbon exchange and to
molecules framework52 utilising a locally modified version of distinguish it from other structure-determining factors. In
AIMPAC.53 NMR shieldings were obtained within the GIAO addition, X-ray crystallography on Li2[2a] as a benchmark
formalism54,55 (cf. the ESI† for more details).
system was used to validate the formal isoelectronic relation-
We first note pleasingly good agreement between the ship between [2a]2− and 4a. Full details including ORTEP plots
experimentally determined and the calculated 13C shift of the X-ray crystal structure analyses of 2a, anti-2b, 4a, anti-4b,
values of [Li(THF)2]2[2a]; in contrast, significant deviations syn-4b, [Li(THF)2]2[2a] and [Li(Et2O)]2[2a] are provided in the
from the experimental data are obvious in the case of the ESI.† In the following, we will only discuss selected aspects of
free dianion [2a]2− 56
our previous conclusion that the Li+ ions stay coordinated to
.
These findings lend further support to the crystallographic work.
We have determined the crystal structure of 2a for compari-
the anionic framework in THF solution. In the case of 4a, son with the data of a polymorph (i.e. 2a*) obtained by Kawa-
C-a and C-b possess essentially the same chemical shift shima et al.37 The respective single crystals (monoclinic, P21/
values. In comparison, C-a of Li2[2a] is deshielded by c)60 were grown by slow evaporation of saturated solutions of
+11 ppm, whereas the signal of C-b experiences an upfield the compound in toluene (2a) and Et2O (2a*). A decisive differ-
shift of −7 ppm. These effects are obviously not artefacts of ence between the two polymorphs lies in the fact that the
Li+ coordination, because an even larger shielding of C-b is molecules of 2a (Z = 4) contain no symmetry element whereas
calculated for the free dianion [2a]2− (Δδ = −16 ppm; DFT
results, Table 1).
Finally, calculations of nucleus-independent chemical
shifts (NICS(0), NICS(1))57 contribute a quantum-chemical cri-
terion to compare the aromatic characters of the individual
six-membered rings in [Li(THF)2]2[2a] and 4a. As can be
expected, the NICS(0)/NICS(1) values of the mesityl substitu-
ents are the same for both compounds; larger differences are
found for the NICS(0)/NICS(1) data of the peripheral C6H4
rings (cf. ESI†). The NICS(0)/NICS(1) values of the central B2C4
ring of [Li(THF)2]2[2a] also deviate somewhat from the corres-
ponding parameters of the central C6 ring of 4a (−9.0 ppm/
−13.1 ppm vs. −9.9 ppm/−11.1 ppm, respectively). However,
there remains no question that two-electron reduction trans-
forms the anti-aromatic B2C4 ring of 2a (NICS(0)/NICS(1) =
10.5 ppm/4.4 ppm) into an aromatic π system (cf. the ESI† for
more details).
Key structural features of the 9,10-dihydro-9,10-
diboraanthracenes and their all-carbon analogues
As alluded to above, the relative arrangement of the individual
molecules in the solid state is of crucial importance for all
organic electronic devices.13,58 A three-coordinate boron atom
has a slightly larger atomic radius than a three-coordinate
carbon atom.59 Apart from that, 2a/4a, 2b/4b and 2c/4c should
Fig. 3 Overlays of the two molecules of the polymorphic structures 2a (green)
possess very similar molecular structures and even adopt
and 2a* (black). The position of the DBA centroids (COG) has been marked in
closely related conformations in the crystal lattice. Although red; for 2a* the COG is identical with the inversion centre.
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those of 2a* (Z = 2) possess an inversion centre. Fig. 3 shows
an overlay of the molecular structures of 2a (green) and 2a*
(black) in two different orientations.
In Fig. 3 (top) the position of the DBA centroids (COG) has
been marked in red; for 2a* the COG is identical with the
inversion centre. Both 2a and 2a* reveal an orthogonal
arrangement between the mesityl planes and the DBA core. In
contrast to the planar DBA framework of compound 2a*, the
B2C4 heterocycle in 2a adopts a shallow boat conformation
(Fig. 3, bottom) resulting in an angle of 169.2° between the
two Mes-i carbon atoms and the COG. All other key structural
parameters of 2a and 2a* are essentially the same. The mutual
orientation of the molecules in the crystal lattices of the two
polymorphs is strikingly dissimilar. Only in the case of 2a we
observe an interpretable short contact, i.e. a parallel-displaced
π-stacking interaction between two o-phenylene rings of adja-
cent DBA moieties (3.421 Å).
Crystals grown from a syn/anti-mixture of 2b in C6H6 exclu-
sively contained the anti rotamer together with two equivalents
of solvent molecules (monoclinic, P21/c; Z = 2). The all-carbon
congener anti-4b also co-crystallises with two equivalents of
Fig. 4 Overlay of anti-2b (green) and anti-4b (black) with the shortest contacts
between neighbouring molecules indicated.
ˉ
C6H6 (triclinic, P1; Z = 1) thereby allowing for a direct compari-
son of the two crystal structures. We first note that the density
of anti-2b (1.169 g cm−3) is 3% lower than that of anti-4b
(1.204 g cm−3). Both anti-2b and anti-4b are Ci-symmetric; the
dihedral angles between the 2-methylnaphthyl substituents
and the DBA or 9,10-anthrylene cores amount to 85.5(1)° and
82.8(3)°, respectively. As can be expected, all three B–C bonds
in anti-2b are longer than the corresponding C–C bonds in
anti-4b. Moreover, the difference between the lengths of the B–
Cexo single bond (1.581(2) Å) and the average B–Cendo bond
(1.561(2) Å) is considerably smaller (Δ = 0.020(2) Å) than the
difference between C–Cexo (1.494(5) Å) and C–Cendo (1.408(5) Å)
in anti-4b (Δ = 0.086(5) Å). This feature reflects a low degree of
BvCendo π bonding in the ground state of anti-2b as opposed
to significant CvCendo double-bond character in anti-4b. Apart
from these subtle variations, the general molecular frame-
works of anti-2b and anti-4b are very much alike as illustrated
by the overlay of both structures in Fig. 4 (anti-2b: green; anti-
4b: black).
Fig. 4 also shows the short contacts between adjacent mole-
cules, which are basically the same in both species (anti-2b/
anti-4b: H(33)–C(21) = 2.81/2.87 Å; C(33)–H(33)–C(21) = 135/
131°; Ar(C(21))//Ar(C(33)) = 84.8(1)/81.3(1)°). A most interesting
difference between anti-2b and anti-4b is visible in their inter-
action with the C6H6 solvate molecules (Fig. 5): in the case of
the organoborane, C6H6 acts as a hydrogen donor in a
T-shaped arrangement and the C–H vector points to a boron-
bonded carbon atom (H(45)–C(31) = 2.90 Å; C(45)–H(45)–C(31)
= 170°; C6H6//B2C4 = 76.1(1)°). In the all-carbon species,
Fig. 5 Different interactions between the C6H6 solvate molecules and the main
molecules in the crystal lattices of anti-2b (green) and anti-4b (black).
however, the π-electron system of C6H6 acts as a hydrogen between the main molecule and the benzene centroids as
acceptor and the 2-methylnaphthyl substituent as the donor anti-4b.
(H(4)–COG(C6H6) = 2.69 Å; C(4)–H(4)–COG(C6H6) = 148°;
Since the steric boundary conditions are the same in anti-
C6H6//Ar(C(4)) = 64.6(2)°). We wish to emphasise in this 2b and anti-4b, the different modes of C6H6 interaction are
context that syn-4b, which also crystallises as solvate with two probably of electronic origin. A comparison of the relevant
equivalents of C6H6, establishes the same short contacts 13C{1H} chemical shift values of anti-2b and anti-4b gives no
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indication of an accumulation of π-charge density at the endo- (THF)2]2[2a], thereby suggesting that the bulky mesityl substi-
cyclic boron-bonded carbon atoms in anti-2b (δ(C-c) = 146.0 tuents have no negative impact on the Li+⋯(B2C4) coordi-
(anti-2b), 131.1 (anti-4b)). We therefore assume that it is the +I nation. We further note that X-ray crystallography on [Li-
effect of the electropositive boron atom that leads to a negative (THF)2]2[2a] leads to results that are fully consistent with the
7
partial charge at the neighbouring carbon atoms and, in turn, conclusions drawn from our Li{1H} NMR spectroscopic inves-
to an attraction of the C–H dipole of C6H6. This interpretation tigation of Li2[2a] in THF-d8 (see above).
is fully supported by the calculated atomic charges on C-c
A comparison of the DBA bond lengths of 2a with those of
(equiv. to C(31) in the X-ray structures shown in Fig. 5) of anti- [Li(OR2)n]2[2a] revealed a major perturbation of the DBA
2b and 4b, which amount to q(NPA) = −0.38 or q(AIM) = −0.63 scaffold upon reduction:62 the endocyclic B–C distances
in the organoborane, but only to q(NPA) = −0.05 or q(AIM) = become shorter (−0.032 Å), thereby testifying to an increased
−0.01 in the all-carbon compound.
BvC double bond character in [Li(OR2)n]2[2a]. All C–C bond
In order to gain deeper insight into the electron-accepting lengths change within the range −0.035 Å to +0.052 Å. We
properties of 9,10-diaryl-DBAs and to explore the scope and interpret these changes in the light of a frontier orbital analy-
limitations of the isoelectronic principle in this particular sis, which reveals essentially identical isosurfaces of the
case, we compared the solid-state structures of 2a, Li2[2a] and LUMO of 2a and the HOMO of the reduced species [Li-
4a. Crystals of the lithium compound were obtained as ether (THF)2]2[2a] (Fig. 7 and ESI†). The individual bond-length vari-
adducts [Li(THF)2]2[2a] and [Li(Et2O)]2[2a]. Fig. 6 shows a plot ations quantified in Fig. 7 appear as a direct consequence of
of the molecular structure of [Li(THF)2]2[2a]; details of the the occupation of the LUMO of 2a with its specific nodal struc-
crystal structure analysis of [Li(Et2O)]2[2a] are provided in the ture.63 The C–C bond lengths of 4a, the isoelectronic congener
ESI.†
of [Li(OR2)n]2[2a], deviate in the same qualitative fashion from
The molecules of [Li(THF)2]2[2a] exist as discrete centro- those of 2a; the quantitative differences, however, are smaller
symmetric entities in the crystal lattice. In each of these mole- (Fig. 7). Also this finding is reflected in the particular structure
cules, two [Li(THF)2]+ ions are η6-coordinated to both sides of of the HOMO of 4a, to which the C-c atoms do not contribute
the B2C4 ring to form an inverse sandwich complex. The dis- to a significant extent (Fig. 7).
tance Li(1)⋯COG(B2C4) is 1.937 Å. The general structural
A further interesting analogy to the couple 2a (Hückel
motif of the [Li(Et2O)]2[2a] complex is largely identical, apart 4π-antiaromatic central ring)/[2a]2− (Hückel 6π-aromatic
from the fact that the distance Li(1)⋯COG(B2C4) = 1.740 Å is central ring) is provided by cyclobutadiene and its lithium di-
shorter by 0.197 Å, obviously as a result of the reduced steric anions, which also crystallise as contact ion pairs with two Li+
demand of the complex cation. Remarkably, the Li⋯COG ions located above and below the plane of the four-membered
(B2C4) distance in the lithium salt [Li(THF)2]2[HB(C6H4)2BH] ring.64
of pristine DBA (1.960 Å)61 is longer than that in [Li-
Electrochemical properties of the 9,10-dihydro-9,10-
diboraanthracenes and their all-carbon analogues
Cyclic voltammograms of 2a–c and 4a–c were recorded on THF
solutions using [nBu4N][PF6] (0.1 M) as the supporting electro-
lyte; all potential values (Table 2) are referenced against the
ferrocene/ferrocenium couple (FcH/FcH+ = 0 V).
The assignment of redox waves was achieved by means of
spectroelectrochemistry using the UV/vis spectra of the mono-
and dianion of pristine 9,10-dihydro-9,10-diboraanthracene,
[HB(C6H4)2BH]− and [HB(C6H4)2BH]2−, as references.32,61 In
situ UV/vis monitoring of the reduction of 2a (with Li) and 2b
(with K, which gave better results than Li) in THF first showed
the signature of the DBA monoanion develop in the spectrum.
After some time, the corresponding bands vanished and
absorptions characteristic of the DBA dianion appeared
instead (cf. the ESI† for plots of the spectra and ref. 32 for a
more detailed discussion of the NIR absorptions that occur
upon reduction). We therefore assign the redox events at
Fig. 6 Molecular structure of [Li(THF)2]2[2a] in the solid state; displacement
−1.84/−2.73 V (2a) and −1.72/−2.58 V (2b) to DBA-centred elec-
ellipsoids are drawn at the 50% probability level; H atoms have been omitted
for clarity. Selected bond lengths (Å), bond angles (°), and dihedral angle (°): tron transitions. Similar spectroelectrochemical experiments
B(1)–C(1) 1.624(3), B(1)–C(31) 1.509(2), B(1)–C(36A) 1.545(3), C(31)–C(36) 1.480
have in previous studies led to the conclusion that also the
redox transitions of 2c with E1/2 = −1.68 and −2.46 V occur at
the DBA π-system whereas the irreversible electron transition
(3), C(31)–C(32) 1.449(3), C(32)–C(33) 1.347(2), C(33)–C(34) 1.425(3), C(34)–
C(35) 1.366(3), C(35)–C(36) 1.420(2), Li(1)⋯COG(B2C4) 1.937; C(1)–B(1)–C(31)
123.6(2), C(1)–B(1)–C(36A) 119.5(1), C(31)–B(1)–C(36A) 116.9(2); Ar(C1)//C(31)
at Epc = −3.32 V is likely associated with the pendant anthryl
B(1)C(36A) 65.8(1); Ar = phenyl. The symmetry transformation used to generate
equivalent atoms: A: −x, −y + 1, −z + 1.
substituents.32 In line with these experimental observations we
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note that the calculated LUMOs of 2a–c are exclusively located
on the respective DBA fragments (cf. the ESI† for corres-
ponding orbital plots).65
Comparing the DBA derivatives, we find a small but signifi-
cant anodic shift along the sequence 2a → 2b → 2c for both
redox waves. More importantly, the potential values required
for the one-electron reduction of 2a–c are lower by 0.75, 0.78
and 0.82 V, respectively, than those necessary for the reversible
mono-reduction of 4a–c; all these potential differences are
larger than in the case of 1,2,3,4,5,6,7,8-octafluoro-9,10-diphen-
ylanthracene vs. 9,10-diphenylanthracene (Δ(E1/2) = 0.52 V;22
recorded in CH3CN). Both one-electron reductions of 2a–c are
fully reversible and can even be achieved on a preparative scale
as evidenced by the successful synthesis of Li2[2a]. In contrast,
the more cathodically shifted second redox waves of 4a–c show
features of chemical irreversibility. As has already been docu-
mented in the literature for other anthracene derivatives, the
highly basic dianions tend to undergo protonation through
Hofmann elimination of the supporting tetraalkylammonium
electrolyte.66,67 In summary, the comparatively anodic
reduction potentials and the stability of the mono- and di-
anions obtained lead to the conclusion that boron doping
renders 2a–c excellent candidates for n-type organic materials.
We recorded cyclic voltammograms of 4a–c also in the posi-
tive potential regime but found no electron transitions up to
the limit of the solvent window (THF). According to the litera-
ture, 9,10-di(1′-naphthyl)anthracene undergoes electrochemi-
cal oxidation in CH2Cl2 to a cation radical, which, however,
slowly decomposes to another electroactive species.68 It can
therefore be concluded that DBA derivatives are potentially
useful substitutes for the isoelectronic, yet elusive, stable
anthracene dications.
Optical spectra of the 9,10-dihydro-9,10-diboraanthracenes
and their all-carbon analogues
Unless noted otherwise, all absorption/emission wavelengths
refer to solutions in C6H6 (Table 3). The all-carbon species 4a
and anti-4b give rise to essentially identical UV/vis absorption
and emission spectra (Fig. 8). All bands show the vibrational
fine structure usually observed for anthracene derivatives and
the Stokes shifts are very small.69 The trianthrylene 4c pos-
sesses an only marginally red-shifted absorption maximum;
the emission, however, appears at lower energy than in the
Fig. 7 Differences between average endocyclic bond lengths (10−3 Å) of [Li-
(OR2)n]2[2a] vs. 2a (top; OR2 = THF, n = 2; Et2O, n = 1) and of 4a vs. 2a (bottom),
which are in agreement with the nodal structures of the calculated HOMOs of
[Li(THF)2]2[2a] and 4a (surfaces drawn at an isovalue of 0.05 a0−3/2; the Li+ ions
are marked in magenta; the THF ligands have been omitted for clarity).
Table 3 Photophysical data of 2a–c and 4a–c in C6H6
Table 2 Electrochemical data of 2a–c and 4a–ca
E1/2 in [V] vs. FcH/FcH+
λmax(abs) [nm]
λmax(em) [nm]
(λex [nm])
Stokes shift
(ε [mol−1 dm3 cm−1])
[cm−1
]
ϕF
a
2a
2b
2c
4a
4b
4c
−1.84, −2.73b
2a
2b
2c
4a
4b
349 (10 400)
460 (350)
513 (420)
635 (530)
403 (358)
404 (358)
442 (380)
6900
4200
3100
250
300
2000
0.03
0.07
0.05
0.80
0.77
0.71
−1.72, −2.58
357 (8600), 422 (1300)
374 (28 200), 531 (1500)
399 (22 300)
399 (17 900)
406 (47 700)
−1.68, −2.46, −3.32c,d
−2.59, −3.40c
−2.50, −3.30c
−2.50, −2.81, −2.96, −3.54c 4c
a THF, supporting electrolyte: [nBu4N][PF6] (0.1 M), scan rate: 200 mV s−1
b Literature values: −1.82, −2.78 V, cf. ref. 37. c Epc value. d Ref. 32.
.
a Quantum yields were determined using a calibrated integrating
sphere.
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Fig. 8 UV/vis absorption (left) and emission (right) spectra of 2a–c and 4a–c in C6H6; 2a (blue), syn/anti-2b (green), 2c (red), 4a (black), anti-4b (orange) and 4c
(purple).
cases of 4a and anti-4b (Δ(λmax(em)) ∼ 40 nm) and the corres- π–π* interactions within the individual anthracene
ponding band is devoid of any fine structure.70
fragments.17
The absorption spectra of 2a and 2b contain intense bands
Correspondingly, the calculated HOMOs of 2a–c are located
at λmax(abs) = 349 and 357 nm, respectively, which are tenta- on the aryl substituents but the LUMOs are all DBA-centred,
tively assigned to π–π* excitations of the DBA core (Fig. 8; whereas in the cases of 4a–c both the HOMOs and the LUMOs
Table 3).32 In the spectrum of 2c, this band is likely hidden are 9,10-anthrylene-centred. We further find a satisfactory cor-
underneath the broad, structured absorption of the 9-anthryl relation between the calculated HOMO–LUMO gaps of 2a–c
substituents. As additional features, we observe a shoulder at and the corresponding values derived from the UV-vis
∼400 nm in the spectrum of 2a and well-resolved bands at spectra71,72 (cf. the ESI† for more details).
422 nm (ε = 1300 mol−1 dm3 cm−1) and 531 nm (ε =
In the cases of 4a–c, the quantum yields ϕF are high and
1500 mol−1 dm3 cm−1) for 2b and 2c, respectively, which are vary in the range between 0.71 and 0.80 (C6H6; Table 3). In
responsible for the yellow (2a, 2b) and deep red (2c) colours of contrast to that, 2a–c show lower quantum yields of 0.03–0.07
the compounds. Especially the extremely broad line width of (C6H6; Table 3), likely because intramolecular charge transfer
the 531 nm-band of 2c suggests that these features originate acts as an efficient non-radiative decay channel.
from a charge-transfer interaction between the respective elec-
tron-rich 9,10-aryl donors and the electron-poor DBA
acceptors.32
Conclusions
In contrast to the anthracene compounds 4a–c, which are
all blue-light emitters, the emission wavelengths of 2a 9,10-Dihydro-9,10-diboraanthracene (DBA) derivatives carrying
(460 nm), 2b (513 nm) and 2c (635 nm) cover the full range mesityl (2a), 2-methylnaphthyl- (2b) and 9-phenyl-2,7-di-tert-
from blue to green to red. Pronounced differences are also butylanthryl (2c) substituents at the boron atoms are stable
obvious in the solvatochromic behaviour of 4a–c, on the one towards air and moisture even in solution over several hours
hand, and 2a–c, on the other. Spectra have been recorded in (2c) or even days (2a, 2b). According to NICS(0)/NICS(1) calcu-
C6H12, C6H6, THF and CH2Cl2 (cf. the ESI† for more details). lations, the compounds contain Hückel-anti-aromatic central
4a and anti-4b reveal no noteworthy solvatochromism at all; B2C4 rings; upon two-electron reduction, they are transformed
the emission band of 4c shifts continuously to the red by an into the Hückel-aromatic dianions [2a–c]2−
overall Δ(λmax(em)) of 62 nm. The DBA derivatives, in contrast, As exemplified by the crystal structure analyses of anti-2b
.
show marked positive solvatochromism, i.e., 2a (Δ(λmax(em)) = and anti-9,10-bis(2′-methylnaphthyl)anthracene (anti-4b), the
74 nm), 2b (Δ(λmax(em)) = 97 nm) and 2c (Δ(λmax(em)) = DBA derivative and its all-carbon analogue possess very similar
165 nm). These data provide further evidence for the assump- molecular structures and also show similar crystal packing
tion that charge-transfer transitions play a major role in the motifs. Subtle differences between the crystal lattices of anti-
optical properties of 2a–c, whereas the low-energy regime of 2b and anti-4b are, however, evident in the kinds of intermole-
the electronic spectra of 4a and anti-4b is largely governed by cular C–H⋯π(arene) interactions (T-stacking).
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2a–c behave as blue-, green- and red-emitting materials, 13 J. E. Anthony, Chem. Rev., 2006, 106, 5028–5048.
respectively, and consequently provide the three primary 14 W. R. Dawson and M. W. Windsor, J. Phys. Chem., 1968, 72,
colours required for full-colour displays. In marked contrast, 3251–3260.
each of the three corresponding all-carbon systems 4a–c is a 15 J. V. Morris, M. A. Mahaney and J. R. Huber, J. Phys. Chem.,
blue-light emitter. According to the frontier orbital structures, 1976, 80, 969–974.
the underlying transitions are (i) a charge transfer from the 16 K. Danel, T.-H. Huang, J. T. Lin, Y.-T. Tao and C.-H. Chuen,
electron-rich aryl substituents to the electron-poor DBA cores Chem. Mater., 2002, 14, 3860–3865.
in the cases of 2a–c and (ii) a local π–π* excitation of the 17 H. Park, J. Lee, I. Kang, H. Y. Chu, J.-I. Lee, S.-K. Kwon and
central 9,10-anthrylene fragments in the cases of 4a–c. Y.-H. Kim, J. Mater. Chem., 2012, 22, 2695–2700.
The boron species 2a–c undergo sequential and reversible 18 V. Langer and H.-D. Becker, Z. Kristallogr., 1992, 199, 313–
two-electron reductions at their DBA cores. The most cathodic 315.
reduction potentials of 2a–c are lower by 0.75–0.82 V than 19 N. I. Nijegorodov and W. S. Downey, J. Phys. Chem., 1994,
those necessary for the reversible mono-reductions of 4a–c, 98, 5639–5643.
which leads to the conclusion that boron doping increases the 20 H. Usta, A. Facchetti and T. J. Marks, Acc. Chem. Res., 2011,
electron affinity of anthracene derivatives to a higher degree
than 1,2,3,4,5,6,7,8-octafluorination.
44, 501–510.
21 M.-H. Yoon, A. Facchetti, C. E. Stern and T. J. Marks, J. Am.
Chem. Soc., 2006, 128, 5792–5801.
In summary, our results demonstrate that the performance
of organic materials can indeed be significantly modified if 22 J. F. Tannaci, M. Noji, J. McBee and T. D. Tilley, J. Org.
selected carbon atoms are replaced by boron atoms.
Chem., 2007, 72, 5567–5573.
23 J. F. Tannaci, M. Noji, J. L. McBee and T. D. Tilley, J. Org.
Chem., 2008, 73, 7895–7900.
24 H. E. Katz, J. Johnson, A. J. Lovinger and W. Li, J. Am.
Chem. Soc., 2000, 122, 7787–7792.
Acknowledgements
M.C.H and M.W. acknowledge financial support from the Beil- 25 The following reviews provide more detailed insight into
stein Institute, Frankfurt/Main, Germany, within the research
collaboration NanoBiC.
optical, electronic andsensory applications of organobor-
anes. (a) C. D. Entwistle and T. B. Marder, Angew. Chem.,
Int. Ed., 2002, 41, 2927–2931; (b) C. D. Entwistle and
T. B. Marder, Chem. Mater., 2004, 16, 4574–4585;
(c) S. Yamaguchi and A. Wakamiya, Pure Appl. Chem., 2006,
78, 1413–1424; (d) F. Jäkle, Coord. Chem. Rev., 2006, 250,
1107–1121; (e) M. Elbing and G. C. Bazan, Angew. Chem.,
Int. Ed., 2008, 47, 834–838; (f) Ref. 8 (g) A. Lorbach,
A. Hübner and M. Wagner, Dalton Trans., 2012, 41, 6048–
6063.
References
1 S. W. Thomas III, G. D. Joly and T. M. Swager, Chem. Rev.,
2007, 107, 1339–1386.
2 J. Zaumseil and H. Sirringhaus, Chem. Rev., 2007, 107,
1296–1323.
3 S. Günes, H. Neugebauer and N. S. Sariciftci, Chem. Rev., 26 A. Lorbach, C. Reus, M. Bolte, H.-W. Lerner and
2007, 107, 1324–1338. M. Wagner, Adv. Synth. Catal., 2010, 352, 3443–3449.
4 B. C. Thompson and J. M. J. Fréchet, Angew. Chem., Int. Ed., 27 A. Lorbach, M. Bolte, H.-W. Lerner and M. Wagner, Chem.
2008, 47, 58–77. Commun., 2010, 46, 3592–3594.
5 D. Pisignano, M. Anni, G. Gigli, R. Cingolani, M. Zavelani- 28 E. Januszewski, A. Lorbach, R. Grewal, M. Bolte, J. W. Bats,
Rossi, G. Lanzani, G. Barbarella and L. Favaretto, Appl.
Phys. Lett., 2002, 81, 3534–3536.
H.-W. Lerner and M. Wagner, Chem.–Eur. J., 2011, 17,
12696–12705.
6 A. P. Kulkarni, C. J. Tonzola, A. Babel and S. A. Jenekhe, 29 C. Reus, N.-W. Liu, M. Bolte, H.-W. Lerner and M. Wagner,
Chem. Mater., 2004, 16, 4556–4573. J. Org. Chem., 2012, 77, 3518–3523.
7 S. Chen, L. Deng, J. Xie, L. Peng, L. Xie, Q. Fan and 30 Ö. Seven, Z.-W. Qu, H. Zhu, M. Bolte, H.-W. Lerner,
W. Huang, Adv. Mater., 2010, 22, 5227–5239.
8 F. Jäkle, Chem. Rev., 2010, 110, 3985–4022.
M. C. Holthausen and M. Wagner, Chem.–Eur. J., 2012, 18,
11284–11295.
9 G. Yu, S. Yin, Y. Liu, J. Chen, X. Xu, X. Sun, D. Ma, X. Zhan, 31 A. Lorbach, M. Bolte, H. Li, H.-W. Lerner,
Q. Peng, Z. Shuai, B. Tang, D. Zhu, W. Fang and Y. Luo,
J. Am. Chem. Soc., 2005, 127, 6335–6346 and references
cited herein.
M. C. Holthausen, F. Jäkle and M. Wagner, Angew. Chem.,
Int. Ed., 2009, 48, 4584–4588.
32 C. Hoffend, F. Schödel, M. Bolte, H.-W. Lerner and
M. Wagner, Chem.–Eur. J., 2012, 18, 15394–15405.
33 E. Januszewski, M. Bolte, H.-W. Lerner and M. Wagner,
Organometallics, 2012, 31, 8420–8425.
34 Selected references that have not been covered by the
earlier reviews cited above: (a) A. Wakamiya, T. Taniguchi
and S. Yamaguchi, Angew. Chem., Int. Ed., 2006, 45, 3170–
10 Y. Shirota and H. Kageyama, Chem. Rev., 2007, 107, 953–
1010.
11 T. Baumgartner and R. Réau, Chem. Rev., 2006, 106, 4681–
4727.
12 Handbook of Oligo- and Polythiophenes, ed. D. Fichou, Wein-
heim, Germany, 1999.
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Paper
3173; (b) H. Li and F. Jäkle, Macromol. Rapid Commun.,
2010, 31, 915–920; (c) H. Li, A. Sundararaman,
T. Pakkirisamy, K. Venkatasubbaiah, F. Schödel and
in the corresponding B–F adduct: E. M. L. Fink, Bachelor
thesis, Goethe Universität Frankfurt, Frankfurt (Main),
Germany, 2010; (b) The syn/anti-re-equilibration of single-
crystalline anti-2b in dried C6D6 is fast and already com-
plete immediately after preparation of the NMR sample.
We therefore found it impossible to deliberately dope the
sample with varying amounts of H2O at various pH and to
systematically examine the resulting effect on the equili-
bration rate.
F.
Jäkle,
Macromolecules,
2011,
44,
95–103;
(d) Z. M. Hudson, X.-Y. Liu and S. Wang, Org. Lett., 2011,
13, 300–303; (e) Z. M. Hudson, M. G. Helander, Z.-H. Lu
and S. Wang, Chem. Commun., 2011, 47, 755–757;
(f) C. Sun, J. Lu and S. Wang, Org. Lett., 2011, 13, 1226–
1229; (g) E. Sakuda, Y. Ando, A. Ito and N. Kitamura, Inorg.
Chem., 2011, 50, 1603–1613; (h) Y. Kim, H.-S. Huh, 46 (a) M. Karplus and J. A. Pople, J. Chem. Phys., 1963, 38,
M. H. Lee, I. L. Lenov, H. Zhao and F. P. Gabbaï, Chem.–
Eur. J., 2011, 17, 2057–2062; (i) C. Bresner, C. J. E. Haynes,
D. A. Addy, A. E. J. Broomsgrove, P. Fitzpatrick, D. Vidovic,
A. L. Thompson, I. A. Fallis and S. Aldridge, New J. Chem.,
2010, 34, 1652–1659.
2803–2807; (b) H. Günther, H. Schmickler, H. Königshofen,
K. Recker and E. Vogel, Angew. Chem., Int. Ed., 1973, 12,
243–245; (c) R. H. Levin and J. D. Roberts, Tetrahedron Lett.,
1973, 14, 135–138; (d) D. G. Farnum, Adv. Phys. Org. Chem.,
1975, 11, 123–175.
35 (a) Z. Zhou, A. Wakamiya, T. Kushida and S. Yamaguchi, 47 The 7Li{1H} NMR spectrum of Li2[2a] in THF-d8 gave one
J. Am. Chem. Soc., 2012, 134, 4529–4532; (b) T. Kushida,
Z. Zhou, A. Wakamiya and S. Yamaguchi, Chem. Commun.,
2012, 48, 10715–10717; (c) S. Saito, K. Matsuo and
S. Yamaguchi, J. Am. Chem. Soc., 2012, 134, 9130–9133;
(d) C. Dou, S. Saito, K. Matsuo, I. Hisaki and S. Yamaguchi,
Angew. Chem., Int. Ed., 2012, 51, 12206–12210.
signal at 0.4 ppm and a second signal at −7.2 ppm with an
integral ratio of approximately 1 : 8. The smaller, low-field
resonance falls in the range of solvent-separated ion pairs,
whereas the larger, high-field signal is indicative of contact
ion pairs in which the Li+ ion resides above the plane of an
aromatic π system and thus in the shielding region of the
ring current. For comparison, we note that the 7Li reson-
ance of the contact ion pair η5-LiCp is detected at
−8.37 ppm (THF). (a) R. H. Cox and H. W. Terry Jr.,
J. Magn. Reson., 1974, 14, 317–322; (b) L. A. Paquette,
W. Bauer, M. R. Sivik, M. Bühl, M. Feigel and P. v. Ragué
Schleyer, J. Am. Chem. Soc., 1990, 112, 8776–8789.
36 We have prepared the unsubstituted species 9,10-diphenyl-
DBA, 9,10-bis(1′-naphthyl)-DBA and 9,10-bis(9′-anthryl)-
DBA and found only negligible differences in their opto-
electronic properties compared to 2a–c. However, because
of higher sensitivity to air and moisture or pure solubility
in all common solvents, analytically pure samples could not
be obtained. We therefore restrict our discussion to 2a–c.
48 S. Grimme, J. Comput. Chem., 2006, 27, 1787–1799.
37 A six-step synthesis protocol together with an X-ray crystal 49 A. Schäfer, C. Huber and R. Ahlrichs, J. Chem. Phys., 1994,
structure analysis of 2a has already been described (i.e. 100, 5829–5835.
2a*): T. Agou, M. Sekine and T. Kawashima, Tetrahedron 50 M. J. Frisch, et al., Gaussian 09, Revision C.01, Gaussian,
Lett., 2010, 51, 5013–5015. Herein, we report an alternative Inc., Wallingford, CT, USA, 2010.
synthesis route to 2a, its NMR data in C6D6 and THF-d8, 51 For an overview, see: E. D. Glendening, C. R. Landis and
which we require for comparison, and the X-ray crystal
structure analysis of a polymorph.
38 J. E. Anthony, D. L. Eaton and S. R. Parkin, Org. Lett., 2002,
4, 15–18.
39 U. Müller, M. Adam and K. Müllen, Chem. Ber., 1994, 127,
437–444.
F. Weinhold, WIREs Comput. Mol. Sci., 2012, 2, 1–42.
52 The Quantum Theory of Atoms in Molecules, ed. C. F. Matta
and R. J. Boyd, Weinheim, Germany, 2007.
53 R. F. W. Bader, Atoms in Molecules: A Quantum Theory, Clar-
endon Press, Oxford, UK, 1994; see http://www.chemistry.
mcmaster.ca/aimpac/
40 C. H. Lee, M. J. Kim, S.-P. Han, Y.-S. Lee, S. K. Kang, 54 R. Ditchfield, Mol. Phys., 1974, 27, 789–807.
J. H. Song, J. T. Je, H.-Y. Oh and Y.-J. Kim, Tetrahedron, 55 K. Wolinski, J. F. Hinton and P. Pulay, J. Am. Chem. Soc.,
2010, 66, 3360–3364.
1990, 112, 8251–8260.
41 D. Nori-shargh, S. Asadzadeh, F.-R. Ghanizadeh, 56 The gas-phase calculations for the dianionic species are of
F. Deyhimi, M. M. Amini and S. Jameh-Bozorghi, J. Mol.
Struct. (THEOCHEM), 2005, 717, 41–51.
42 A. R. Katritzky, C. H. Marson, G. Palenik, A. E. Koziol,
H. Luce, M. Karelson, B.-C. Chen and W. Brey, Tetrahedron,
1988, 44, 3209–3214.
approximate value only: due to the lack of diffuse functions
(and the lack of solvation) the electrons in the free dianio-
nic species are formally unbound (the orbital energies are
positive).
57 P. v. Ragué Schleyer, C. Maerker, A. Dransfeld, H. Jiao and
N. J. R. v. Eikema Hommes, J. Am. Chem. Soc., 1996, 118,
6317–6318.
43 W. Nowak and M. Wierzbowska, J. Mol. Struct. (THEO-
CHEM), 1996, 368, 223–234.
44 K. Nikitin, H. Müller-Bunz, Y. Ortin, J. Muldoon and 58 T. Hinoue, Y. Shigenoi, M. Sugino, Y. Mizobe, I. Hisaki,
M. J. McGlinchey, Org. Lett., 2011, 13, 256–259.
M. Miyata and N. Tohnai, Chem.–Eur. J., 2012, 18, 4634–
45 (a) This interpretation is in line with preliminary compu-
4643.
tational results, clearly indicating a substantial barrier for 59 P. Pyykkö and M. Atsumi, Chem.–Eur. J., 2009, 15, 12770–
2-methylnaphthyl rotation in 2b, which drops significantly
12779.
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60 T. Kawashima et al. have used the equivalent setting P21/a
for 2a*.
calculations in order to save computer time (cf. [2c]model in
the ESI†).
61 A. Lorbach, M. Bolte, H.-W. Lerner and M. Wagner, Organo- 66 B. S. Jensen and V. D. Parker, J. Am. Chem. Soc., 1975, 97,
metallics, 2010, 29, 5762–5765. 5211–5217.
62 For this comparison we used averaged bond lengths of 67 K. Meerholz and J. Heinze, J. Am. Chem. Soc., 1989, 111,
(i) our structure 2a and Kawashima’s structure 2a*, (ii) [Li-
(THF)2]2[2a] and [Li(Et2O)]2[2a].
63 Related arguments have been used by Bogdanović et al. and
2325–2326.
68 L. S. Marcoux, A. Lomax and A. J. Bard, J. Am. Chem. Soc.,
1970, 92, 243–250.
Bock et al. to explain C–C bond length changes occurring 69 At higher optical densities, the shortest-wavelength emis-
upon the one-electron reduction of anthracene to [anthra-
cene]˙−. (a) B. Bogdanović, N. Janke, C. Krüger, R. Mynott,
K. Schlichte and U. Westeppe, Angew. Chem., Int. Ed. Engl.,
1985, 24, 960–961; (b) H. Bock, C. Arad, C. Näther and
Z. Havlas, J. Chem. Soc., Chem. Commun., 1995, 2393–2394.
sion of anthracene is selectively attenuated by self-absorp-
tion as a result of the small Stokes shift (in our case, the
bands at λmax(em) = 403 nm (4a) or 404 nm (anti-4b) are
affected). S. Kao, A. N. Asanov and P. B. Oldham, Instrum.
Sci. Techol., 1998, 26, 375–387.
64 (a) G. Boche, H. Etzrodt, W. Massa and G. Baum, Angew. 70 For a detailed study of the optical properties of a series of
Chem., Int. Ed. Engl., 1985, 24, 863–864; (b) A. Sekiguchi,
T. Matsuo and H. Watanabe, J. Am. Chem. Soc., 2000, 122,
5652–5653; (c) A. Sekiguchi, T. Matsuo and M. Tanaka,
oligomeric 9,10-anthrylenes see: R. Fritz, W. Rettig,
K. Nishiyama, T. Okada, U. Müller and K. Müllen, J. Phys.
Chem. A, 1997, 101, 2796–2802.
Organometallics, 2002, 21, 1072–1076. See also the pioneer- 71 D. A. M. Egbe, C. Bader, J. Nowotny, W. Günther and
ing work of Katz on the cyclooctatetraenyl dianion:
T. J. Katz, J. Am. Chem. Soc., 1960, 82, 3784–3785.
65 In the case of 2c we have not included the tert-butyl groups
and the phenyl rings on the anthryl substituents in our
E. Klemm, Macromolecules, 2003, 36, 5459–5469.
72 D. A. M. Egbe, T. Kietzke, B. Carbonnier, D. Mühlbacher,
H.-H. Hörhold, D. Neher and T. Pakula, Macromolecules,
2004, 37, 8863–8873.
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