Highly Deformed o-Carborane Functionalised Non-linear Polycyclic Aromatics with Exceptionally Long C?C Bonds

Article abstract of DOI:10.1002/chem.202004517

The effect of substituting o-carborane into the most sterically hindered positions of phenanthrene and benzo(k)tetraphene is reported. Synthesised via a Bull–Hutchings–Quayle benzannulation, the crystal structures of these non-linear acenes exhibited the highest aromatic deformation parameters observed for any reported carborane compound to date, and among the largest carboranyl C?C bond length of all organo-substituted o-carboranes. Photoluminescence studies of these compounds demonstrated efficient intramolecular charge-transfer, leading to aggregation induced emission properties. Additionally, an unusual low-energy excimer was observed for the phenanthryl compound. These are two new members of the family of carborane-functionalised non-linear acenes, notable for their peculiar structures and multi-luminescent properties.

Full text of DOI:10.1002/chem.202004517

Accepted Article
Accepted Article
Title: Highly deformed o-carborane-functionalised non-linear polycyclic
aromatics with exceptionally long C-C bond lengths
Authors: Martin J. Heeney, Adam V. Marsh, Mark Little, Nathan J.
Cheetham, Matthew J. Dyson, Matthew Bidwell, Andrew J. P.
White, Colin N. Warriner, Anthony C. Swain, Iain McCulloch,
and Paul N. Stavrinou
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
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to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.202004517
Link to VoR: https://doi.org/10.1002/chem.202004517
01/2020

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Highly deformed o-carborane-functionalised non-linear polycyclic
aromatics with exceptionally long C-C bond lengths
Adam V. Marsh,[a] Mark Little,[a] Nathan J. Cheetham,[b] Matthew J. Dyson,[c] Matthew Bidwell,[a] Andrew
J. P. White,[a] Colin N. Warriner,[d] Anthony C. Swain,[d] Iain McCulloch,[e] Paul N. Stavrinou,*[f] Martin
Heeney*[a]
[d]
Dr. C. N. Warriner, Dr. A. C. Swain
AWE
[a]
Dr. A. V. Marsh, Dr. M. Little, M. Bidwell, Dr. A. J. P. White, Prof. M.
Heeney
Reading RG7 4PR, UK
Prof. I. McCulloch
Department of Chemistry and Centre for Plastic Electronics
Imperial College London
[e]
Department of Chemistry, Chemistry Research Laboratory
University of Oxford
Oxford OX1 3TA, UK
London SW7 2AZ, UK
E-mail: m.heeney@imperial.ac.uk
Dr. N. J. Cheetham
[b]
[c]
Department of Physics and Centre for Plastic Electronics
Imperial College London
London SW7 2AZ, UK
[f]
Prof. P. N. Stavrinou
Department of Engineering Science
University of Oxford
Dr. M. J. Dyson
Oxford OX1 3PJ, UK
Molecular Materials and Nanosystems and Institute for Complex
Molecular Systems
Eindhoven University of Technology
5600 MB, Eindhoven, Netherlands
Supporting information for this article is given via a link at the end of
the document.
Abstract: The effect of substituting o-carborane into the most
sterically hindered positions of phenanthrene and
formation of an intramolecular charge-transfer (ICT) state, in
which the electron density is highly localised on the cage.[11–16]
Charge transfer has been shown to occur preferentially when
the carboranyl carbon-carbon (CC-CC) bond is perpendicular to
plane of the donor fluorophore and, structurally, this state is
notable for an elongated carborane CC-CC bond.[17] In good
solvents, these ICT states are very weakly emissive, as vibration
of the CC-CC bond acts as a non-radiative decay pathway for the
excited state.[18–22] Whenever molecular motion is restricted,
however, carboranyl ICT states become more radiative. This
benzo(k)tetraphene is reported. Synthesised via a Bull-Hutchings-
Quayle benzannulation, the crystal structures of these non-linear
polycyclic aromatics exhibited the highest aromatic deformation
parameters observed for any reported carborane compound to date,
with the largest carboranyl C-C bond length of all organo-substituted
o-carboranes. Photoluminescence studies of these compounds
demonstrated efficient intramolecular charge-transfer, leading to
aggregation induced emission properties. Additionally, an unusual
low-energy excimer was observed for the phenanthryl compound.
These are two new members of the family of carborane-
functionalised non-linear polycyclic aromatics notable for their
peculiar structures and multi-luminescent properties.
often occurs upon aggregation, in
a process known as
aggregation induced emission (AIE).[7,17,18,20–39] O-Carborane is
therefore considered an ‘AIE-active element block’.[40]
The AIE efficiency in carborane-containing fluorophores is
enhanced when the rotation of the carborane is structurally
restricted around the ICT geometry (i.e., when the Cc-Cc bond is
approximately perpendicular to plane of the fluorophore). This
has been achieved by introducing sterically large groups at the
second carboranyl carbon atom, as well as incorporating the
carborane into sterically demanding positions on the aromatic
fluorophore.[7,41–43] The resulting steric strain causes the CC-CC
bond to lie perpendicular to the plane of the fluorophore,
accompanied by severe deformation of the aromatic backbone
and a large increase in potential energy upon carborane rotation.
In effect, the carborane becomes restricted in a potential energy
well around a geometry favouring an ICT state. Using this
design strategy, carboranyl fluorophores in an aggregated state
have reached exceptional photoluminescent quantum yields
(ФPL >0.99).[7,42,43]
O-carborane, an icosahedral compound of formula C2B10H12, is
the most widely studied of the organoborane clusters. Following
its classified origins,[1] the first reports of o-carborane in the open
literature revealed its unusual properties, which have since been
exploited for a range of materials science purposes.[2] In recent
years, attention has turned to the use of o-carborane as a
functional unit in organic semiconductors,[3] with a focus on
developing stimuli responsive luminescent materials.[4]
There are several intrinsic properties of o-carborane which
make it attractive for use in luminescence applications.
Famously, carboranes are extraordinarily chemically and
thermally stable,[2] and confer some of this stability onto systems
to which they are attached.[5] This stability enhancement has
been observed for organic electronic materials where large,
oxidatively susceptible, π-systems are prevalent.[6–9] The
bulkiness of the o-carborane cage is also known to suppress
concentration quenching in organic fluorophores.[7] Furthermore,
while o-carborane itself is not photoluminescent, its presence
can have a powerful positive effect on the solid state emissive
properties of compounds onto which it is attached.[10]
Our
recent
work
introduced
bis(phenyl-o-
carborane)chrysene,[23]
a
non-linear polycyclic aromatic
hydrocarbon (PAH) of this architecture. Carborane incorporation
severely distorted the aromatic structure, and the presence of
the bulky phenyl group yielded a rigid ground state with the CC-
CC bond perpendicular to the aromatic plane. The compound
exhibited an array of photoluminescent properties, including a
rare chrysene excimer not previously observed in solution, and
AIE with good ФPL. This demonstrated that addition of o-
When bonded directly to an aryl fluorophore, o-carborane
acts as an acceptor moiety, accommodating an excited charge
from the aryl donor in the photoexcited state. This leads to the
carborane induced both AIE and
a variety of tuneable
1
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luminescent species in non-linear acenes, suggesting the
potential of such materials in fluorescent sensing applications.
In the present study, the family of o-carborane-
functionalised non-linear PAHs are developed by attaching
phenyl-o-carborane into the most sterically demanding positions
of phenanthrene and benzo(k)tetraphene (Figure 1). The
resulting materials exhibit highly twisted aromatic cores with
significant deformation of the carborane ring and contain some
of the longest CC-CC bonds reported to date. Both materials
exhibit a range of interesting photophysical behaviours including
efficient ICT, AIE, and, in the case of the phenanthryl compound
an unusual low-energy excimer.
The synthesis of 1 proceeded by the preparation of 4-
chlorophenanthrene via the Bull-Hutchings-Quayle (BHQ)
reaction;
a
benzannulation from the trichloroacetate.[45]
Installation of the phenylacetylene group occurred through
relatively mild Sonogashira conditions, aided by the activated
nature of these aryl chlorides.[45] Finally, the formation of 1 was
achieved using an ionic-liquid catalysed decaborane reaction[46]
in reasonable yield. The synthesis of 2 proceeded similarly, but
with two extra steps; first, a Wittig reaction (step viii) to methylate
the tetralone and, second, oxidative aromatisation (step x) of the
ring system. Pure powders of the products were isolated and
fully characterised, with NMR and high-resolution mass
spectrometry data presented in Figures S1-S8 of the Supporting
Information. Recrystallisation of 1 and 2 from a biphasic solution
of DCM and hexane yielded green and red single-crystals,
respectively, which were suitable for X-ray diffraction analysis.
The crystal structures of each are presented in Figure 2, with
detailed crystallography data presented in Tables S1-2 of the
Supporting Information.
Figure 1. The structure of (a) phenanthrene substituted at the 4-position, and
(b) benzo(k)tetraphene substituted at the 7-position, with polycyclic aromatic
hydrocarbon edge nomenclature highlighted.[44]
Phenanthrene is the simplest non-linear PAH, with substituents
at the C(4)-position (in the ‘bay’ region) encountering strong
steric repulsion from the C-H vertex at the C(5)-position. For
benzo(k)tetraphene, substituents at the C(7)-position sit
between both a bay, and a ‘zig-zag’ region, experiencing steric
repulsion from the C-H vertices in the C(8)- and C(6)-positions
respectively. Incidentally, the same is true for the four-ringed
tetraphene; however, the asymmetry of the molecule creates a
difficult synthetic challenge concerning C(7) substitution. Our
strategy
carborane)phenanthrene
to
the
target
(1)
materials
and
4-(phenyl-o-
7-(phenyl-o-
carborane)benzo(k)tetraphene (2) is shown in Scheme 1, and
relied upon the cycloaddition reaction with a substituted alkyne
to form the carborane cluster.
Figure 2. Asymmetric unit crystal structures of (a) 1, and (b) 2, with
independent molecules labelled (i) and (ii). Approximate aromatic planes have
been added to highlight deformation.[47]
Table 1. Aromatic deformation parameters and CC-CC bond length for 1 and 2.
CC-CC Bond
Length (Å)
Compound
α (°)
β (°)
θ (°)
1 (i)
1 (ii)
2 (i)
2 (ii)
15.65
14.77
22.82
23.37
13.15
10.88
12.93
13.95
168.55
169.27
166.34
166.30
1.759(2)
1.769(2)
1.829(3)
1.827(3)
1 and 2 crystallised with two independent molecules in the
asymmetric unit. In each case, the CC-CC bond lies
perpendicular to the aromatic plane because of the bulky phenyl
substituent and its favourable π-interactions with the aryl group.
While 1 shows no evidence of intermolecular π-π stacking, 2
exhibits some degree of dimerisation, albeit with little spatial
(and presumably orbital) overlap. Most strikingly, however, the
ring system of both 1 and 2 are extremely distorted due to the
incorporation of o-carborane. This is exemplified by the aromatic
Scheme 1. The synthesis of 1 and 2. Reaction conditions: (i) allyl bromide,
K2CO3, MeCN, rt, 12 h; (ii) neat, 210 °C, 2 h; (iii) trichloroacetylchloride,
pyridine, ether, 0 °C, 2 h; (iv) CuCl, diglyme, reflux, 2 h; (v) phenylacetylene,
Pd(PPh3)2Cl2, P(Cy)3, Cs2CO3, DMF, 110 °C, 24 h; (vi) B10H14, bmim(Cl), Tol,
110 °C, 48 h; (vii) 2-bromo-1-tetralone, K2CO3, acetone, rt, 12 h; (viii)
MePPh3Br, KtOBu, ether, 0 °C, 12 h; (ix) pyridine, 115 °C, 2 h; (x) DDQ, DCB,
150 °C, 1 h; (xi) B10H12(MeCN)2, AgNO3, Tol, 110 °C, 48 h.
2
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deformation parameters and CC-CC carborane bond lengths
presented in Table 1.
The effects of these structural constraints on the
ultraviolet-visible (UV-Vis) and photoluminescence emission
(PL) properties of 1 and 2, we examined through a series of
optical measurements, presented in Figure 3
Referring to Figure 3a, the solutions of 1 in THF, while
weakly emissive, displayed a broad emission feature (λmax = 615
nm, ФPL = 0.03) that is strongly red-shifted relative to the
absorbance edge. An even weaker and higher-energy feature,
exhibiting vibronic structure, was also detected at λmax = 426 nm.
In H2O/THF (99/1, v/v) and the solid-state, the molecules
of 1 aggregate and the spectra become dominated by, what
appears to be a second emissive species, occurring at higher
energy (λmax = 548 nm) and accompanied by a significant
As indicated by the large α and β angles (outlined in Figure S9),
both independent molecules of 1 in the asymmetric unit contain
o-carborane moieties that are significantly raised above the
aromatic plane. Similarly, the value of θ, which is the degree to
which the carborane is tilted away from an ideal 180° rotation
axis, demonstrates the carboranes are tilted toward the
phenanthrene ring system, reflecting the favourable phenyl π-
stacking interactions.[42] The CC-CC bond lengths of both 1
molecules are also extremely elongated relative to that of o-
carborane (1.626 Å).[48] These observations are also true for the
independent molecules of 2, but greater in magnitude. In fact, to
the best of our knowledge, the aromatic deformation of 2 is the
largest of any reported carboranyl compound (Table S3), and
almost to the degree of 9,10-bridged anthracenes.[49] Similarly,
the CC-CC bond lengths of 2 are larger than all neutral
carboranes substituted with aromatic groups and, more
generally, exceeded only by carboranes disubstituted with
exceptionally strong amino donating groups.[50]
increase in emission efficiency (ФPL
= 0.50 and 0.51,
respectively). In contrast, and under the same conditions, the
equivalent spectra for 2 show only broad emission features,
centred around λmax ≈ 640 nm (Figure 3b). Solutions of 2 in THF
were again found to exhibit inferior PL quantum yields (ФPL <<
0.01). However, in an aggregated state, from either the H2O/THF
solutions or in the solid-state, the emission efficiency once again
dramatically increased (ФPL = 0.11 and 0.38, respectively). Both
1 and 2 clearly demonstrate AIE properties.
The effects of the aromatic distortion on o-carborane
rotation, and the overall rigidity of the molecules, was
investigated with
a
density functional theory (DFT)
A solvatochromic study was employed to probe the nature
of the emissive species and conveniently presented in the form
of a Lippert-Mataga plot in Figure S11. The Stokes shift of each
computational study, Figure S10. Rotation of the carboranes
away from the ground state geometry leads to a significant
increase in potential energy, particularly when rotation is
directed towards the phenanthryl protons. Molecules will
therefore demonstrate an especially strong tendency to adopt
the perpendicular Cc-Cc ground state geometry, thus aiding ICT
state formation following excitation.
emissive species is compared, as
a function of solvent
orientation polarizability, with positive linear correlation
a
indicative of an excited state with a significant dipole moment.[51]
The Stokes shift of the short wavelength emission (λmax ≈ 426
nm) of 1 was found to be independent of solvent and,
suggesting a localised emissive species (LE), is attributed to the
phenanthrene ring system. The long-wavelength emissive
features of 1 and 2 are, however, solvent-dependent, implying
that these states have
a
delocalised character.. Such
delocalised states can be intramolecular (e.g. ICT) or
intermolecular (e.g. excimeric) in nature. Further concentration
and H2O/THF v/v solvent dependent optical studies were
undertaken to confirm their origin and presented in Figures 4
and S12.
Figure 4a shows that the λmax long-wavelength emission of
1 remains constant with increasing concentration, but its
intensity rises relative to the LE. The observation is suggestive
of a non-unimolecular excited state, in which the probability of
formation increases with concentration. Furthermore, the
emission peak of 1 initially red-shifts with lower H2O ratios,
before blue-shifting with higher H2O ratios to match that of the
solid state (Figure 4b). This behaviour strongly resembles our
previous reports from a related chrysene compound, which
exhibited weak excimer emission in THF solution, and ICT in
aggregated states.[23] Accordingly, we propose that the two
delocalised species of 1 appearing at λmax = 548 nm and 615 nm
are attributable to ICT and excimer emission, respectively.
Figure 4c and 4d demonstrate that 2 is very weakly emissive in
THF and in non-aggregated mixed H2O/THF,and exhibits a
single emissive species. This species is also responsible for the
AIE property of 2 observed in higher H2O ratios which, for o-
carborane containing compounds of this type, are attributable to
an ICT state.[17,20,27]
Figure 3. UV-Vis (THF) and peak-normalised PL spectra measured in THF,
H2O/THF (99/1, v/v) and the solid state for (a) 1, and (b) 2. Pictures of the UV
(365 nm) irradiated samples measured in THF, H2O/THF (99/1, v/v) and the
solid state are presented for 1 in (i), (ii), and (iii), respectively, and for 2 in (iv),
(v), and (vi), respectively. Concentrations ≈ 10-4 M, except PL of the THF
solution of 2 where the concentration ≈ 10-3 M owing to its weak luminescence.
3
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Figure 4. Concentration (THF) and H2O/THF v/v solvent dependent PL studies for compound 1 (a) and (b), respectively and compound 2 (c) and (d), respectively.
In (a) and (c) the PL spectra are normalised at λ = 427 nm, and λ = 537 nm, respectively, whereas for (b) and (d) peak normalisation is used. Concentration ≈ 10-4
M for (b) and (d), except the 0/100 condition for 2 where the concentration ≈ 10-3 M owing to its weak luminescence.
The excimer emission of 1 is unusual, as phenanthrene is not
known to form excimers except under exceptional
environments.[52] The structure of the excimer has not been
elucidated, and we do not discount that the phenanthrene
moiety is not electronically involved in this species; however, it is
clear that incorporation of o-carborane into this sterically
hindered region does induce the excimeric behaviour.
Furthermore, the low-energy nature, particularly when compared
to reports of phenanthrene excimers[53] suggests that the
the CC-CC bond to be perpendicular to the aromatic plane, in a
geometry aiding ICT. Both the compounds investigated
displayed significant distortion in the aromatic structure, with 2
exhibiting the most considerable aromatic deformation
parameters of any carborane compound reported to date and
the largest neutral state CC-CC bond length for any organo-
substituted carborane. Photoluminescence studies of 1 revealed
three distinct emissive peaks which were assigned to LE, ICT,
and an unusual excimer species, whilst similar studies of 2
carborane is likely to be electronically involved. In good solutions, revealed a single ICT emissive peak under all conditions. AIE
such as THF, the LE and ICT for both 1 and 2 are either not
observed or extremely weak. We believe this is a result of the
rigid and perpendicular CC-CC geometry favouring ICT formation
from the locally excited phenanthrene, and the extremely
elongated CC-CC bond that enables efficient non-radiative decay
through its vibration. In aggregated states of 1 and 2, however,
this vibration is mechanically inhibited and leads to AIE. Notably,
the ФPL values in these aggregate states appear lower when
compared with values from other similarly hindered o-carboranyl
compounds bearing non-aromatic substituents in the carboranyl
C(2) position.[42,43] As reported elsewhere, this is likely due to
intramolecular π-interactions between the phenyl group and the
aromatic donor, which thereby introduce a non-radiative decay
pathway.[42] It follows that replacement of the phenyl group in 1
and 2, with a similarly sized non-aromatic group, may be
expected to increase the AIE efficiency of these stimuli-
responsive materials.
from the ICT state was demonstrated for both 1 and 2, aided by
the perpendicular orientation of the CC-CC bond in the ground
state, and their high structural rigidity. These are two new
members of the growing family of o-carborane functionalised
non-linear PAHs, notable for their highly distorted structures,
multiple emissive species, and stimuli-responsive luminescence.
Acknowledgements
We thank the Engineering and Physical Sciences Research
Council (EPSRC EP/L016702/1), the Royal Society and the
Wolfson Foundation (Royal Society Wolfson Fellowship for
funding. We also thank AWE for their input and financial support
of A. V. M. We thank the EPSRC UK National Mass
Spectrometry Facility at Swansea University, and the Marie
Skłodowska-Curie Actions Innovative Training Networks
“H2020-MSCA-ITN-2014 INFORM -675867” for funding. This
article is UK Ministry of Defence © Crown Owned Copyright
2020/AWE.
In conclusion, we have successfully substituted phenyl-o-
carborane into the most sterically hindered positions of
phenanthrene and benzo(k)tetraphene via a BHQ reaction
pathway. X-ray crystallography of the compounds has revealed
4
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Keywords: AIE • carboranes • charge transfer • fluorescence •
polycyclic aromatic hydrocarbons
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10.1002/chem.202004517
Chemistry - A European Journal
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We report o-carborane-functionalised phenanthrene and benzo(k)tetraphene; two highly sterically hindered structures, with the latter
displaying the largest aromatic deformation parameters, and largest carboranyl C-C bond, of any organo-substituted o-carborane
compound. Functionalisation of these polycyclic aromatic hydrocarbons induces stimuli-responsive luminescence properties,
including aggregation-induced emission.
Institute and/or researcher Twitter usernames:
@GroupHeeney @CPE_Imperial
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This article is protected by copyright. All rights reserved.