Introduction
Molecular carbons (MCs), namely molecular cutout of carbon materials, have attracted
great attention because they are not only excellent candidates for understanding accurate
structures of carbon allotropes but also promising organic optical, electronic and
magnetic materials.[1],[2] Till now, various MC systems that feature fascinating topological structures with
planar, curved, helical or cyclic conformation and interesting physical properties
have been developed.[3],[4] The replacement of the sp2-hybrized carbon atoms in MCs with heteroatoms is an effective approach to alter their
structural, electronic and physical properties by virtue of the characteristics of
the new atoms.[5]–[7] For example, nitrogen-doped polycyclic MCs are a family of high-performance electron-transporting
materials for organic electronic devices.[8] Phosphorus-doped MCs are attractive functional molecules for the fields of supramolecular
chemistry and biology.[9] Therefore, the development of heteroatom-doped MCs is of great importance for synthetic
chemistry and materials science.
Boron has an intrinsic empty p orbital, making it readily interact with an adjacent
π-electron. Thus, the introduction of boron into π-frameworks of MCs may endow the
resulting boron-doped MCs (BMCs) with Lewis acidity and electron deficiency, which
are completely different from carbon-based or other heteroatom-doped MCs.[10] However, BMCs are more challenging to design and synthesize, due to the instability
of boron toward air and moisture. There are two widely used strategies for stabilizing
the boron atom, i.e. the first one is to use a large protective group, such as 2,4,6-trimethylphenyl
(Mes) and 2,4,6-triisopropylphenyl (Tip), to sterically stabilize the boron atom and
the second one is to embed the boron atom directly into the annulated core of π-framework
to implement structural constraint on the boron atom.[11] As a result, a variety of BMCs with the boron atoms at the edge or in the center
of π-skeletons have been reported. The intensive research on BMCs is mainly focused
on the development of synthetic methods, construction of topological structures, and
exploration of new properties and useful functions.[12],[13]
In this context, the bottom-up organic syntheses, such as the Si–B exchange reaction,
intramolecular Ni-mediated Yamamoto reaction, tandem intramolecular electrophilic
arene borylation, Scholl reaction, and photocyclization reaction, have been employed
to synthesize BMCs that feature one, two and even three boron atoms. They have not
only amazing topological structures and good stability but also intriguing photophysical
and electronic properties. Moreover, they have sufficient Lewis acidity and can coordinate
with Lewis bases to form Lewis acid–base complexes, which exhibit stimuli-responsive
functions. Notably, some of these BMCs have already been utilized in the fields of
organic reaction, optical and electronic devices, as well as supramolecular chemistry
and photothermal materials. A number of reviews dedicating to boron-containing π-systems
have been published. Herein, this review aims to highlight the design and synthetic
strategies of polycyclic BMCs reported in the last decade, and their unique physical
properties and practical applications.
Polycyclic Boron-Doped Molecular Carbons
Yamaguchi′s group first proposed a design concept of structural constraint on the
B atom for stabilization of the tricoordinate B-containing π-skeleton. They successfully
synthesized the completely planarized B-centered BMC 1, starting from 9,10-dibromo-9,10-diboraanthracene as a key precursor ([Figure 1]).[14a] Despite the absence of steric protection in the vertical direction with respect
to the B atoms, this compound is quite stable against air and water, and has a high
thermal stability with the decomposition temperature above 350 °C. The single-crystal
structure of 1 proves its planar configuration. 1 maintains sufficient Lewis acidity and can react with fluoride ion to form the fluoroborate
complex 1•2 F−, leading to a plane-to-bowl conversion of configuration. Subsequently, 1•2 F− can revert to the neutral 1 upon addition of Lewis acid BF3·OEt2, achieving reversibility of the plane-to-bowl conversion. Recently, another neutral
π-radical BMC 2 that contains a B atom in a planarized triphenylmethyl radical framework was reported.[14b] The incorporation of the B atom enhances the spin delocalization, leading to its
remarkable stability. This compound exhibits ambipolar transport characteristic with
well-balanced electron and hole mobilities. Thus, it can be regarded as a representative
example for an organic Mott-insulator transistor that works at room temperature. Notably,
the BMC 2 exhibits a relatively large on-site Coulomb repulsion (~1 eV), which cannot be achieved
for conventional organic charge-transfer complexes.
Figure 1 Examples of boron-doped molecular carbons from Yamaguchiʼs group, and schematic illustration
of the formation and dissociation of a Lewis acid–base adduct based on 4.
The BMCs 3a–3c were prepared by the bottom-up organic synthesis via intramolecular oxidative cyclodehydrogenation
(Scholl reaction).[14c]
[d] The single-crystal structures depict their nonplanar configurations because of steric
overcrowding in the cove regions. The broad absorption that covers the entire visible
region is attributed to the significant contribution of the B atoms, whose p orbital
contributes to the molecular orbitals that are involved in the electronic transitions
for the absorption spectra. They can react with some Lewis bases, leading to the obviously
changed absorption and fluorescence spectra, as well as molecular conformations. For
instance, the THF solution of 3a shows a weak near-infrared (NIR) fluorescence (Φ
PL = 0.03). Upon exposure to gaseous NH3, the complex exhibits bright yellowish green fluorescence with the emission peak
at 526 nm (Φ
PL = 0.42). In addition, they exhibit reversible multiredox properties, and thus 3a and 3b can be applied as active materials for Li battery electrodes. Notably, 3b possesses a high capacity and stable charge/discharge performance in the voltage
range of 1.5 – 4.0 V.
The partially fused BMC 4 was synthesized via treatment of tris(8-bromonaphthyl)borane with (Me3Si)3SiH and 1,1′-azobis(cyclohexanecarbonitrile) as a radical initiator.[15a] This compound shows high chemical and thermal stability, and its sufficient Lewis
acidity enables it to generate Lewis adducts with pyridine derivatives. The solution
of 4 shows an orange emission with a fluorescence peak at 573 nm and Φ
PL of 0.15. Incremental addition of pyridine leads to a new emission band at 500 nm
that originates from 4•pyridine, but along with only a slight decrease in intensity for the fluorescence
of 4. This observation is in virtue of the photodissociation of partial 4•pyridine molecules in the excited state, and thus the dual fluorescence arising from
both of 4 and 4•pyridine is observed. This coordination/dissociation dynamics of the B–N Lewis adduct
is used for the fabrication of organic field-effect transistors (OFETs).[15b] The solubility of 4 is drastically enhanced by the addition of 1 wt% of Lewis basic pyridines. Spin coating
of these soluble Lewis complexes affords amorphous thin films, which can be converted
into polycrystalline films of 4 by simple thermal annealing. Therefore, an OFET device with 4 as the semiconductor is prepared, which exhibits a typical p-type characteristic
with a hole mobility of 2.5 × 10−4 cm2 · V−1 · s−1 that is higher than that of the corresponding vapor-deposited OFET device. This coordination/dissociation
strategy may inspire new device fabrication methods for organic semiconductors with
low solubility.
Another partially fused BMC 5, in which an antiaromatic borole moiety is embedded within the polycyclic π-system,
exhibits a planar structure with a distorted geometry around the B center and forms
the columnar structures with slipped face-to-face π-stacking.[13e] It exhibits remarkably high Lewis acidity, which is contributed by the antiaromatic
character and strained structure of the borole substructure. Notably, its Lewis acidity
allows an unprecedented interaction with P-containing polycyclic π-systems to form
a Lewis acid−base complex. The single-crystal structure of the thus-obtained complex
reveals the coordination between the B- and P-embedded π-systems via the formation
of a B−P dative bond. Furthermore, this complex exhibits an interesting stimuli-responsive
fluorescence behavior upon photoirradiation.
The BMC 6 bearing hydrophilic side chains was synthesized. This compound shows relatively low
Lewis acidity compared to the partially fused counterparts, such as 4 and 5.[15c] The B–N Lewis acid–base adduct based on 6 and strong, charge-neutral Lewis base N,N-dimethylaminopyridine (DMAP) can provide the photo-dissociation behavior. In particular,
6 is an amphiphilic derivative owing to its hydrophilic side chains, and thus shows
intriguing supramolecular assembly properties. Increasing water content in DMSO/H2O mixed media of 6 leads to significant spectral changes and the formation of sheet-like aggregates.
Hence, making use of the labile complexation ability of the B center, the assembly
and disassembly processes from the aggregates are facilely manipulated via the successive
addition of DMAP and trifluoroacetic acid in an aqueous system. The approach for stabilization
of the B atom by structural constraint proposed by Yamaguchi et al. dramatically promotes
the development of BMCs and their related functional materials.
Wagner′s group developed a rational synthetic strategy based on Peterson olefination,
stilbene-type photocyclization, and Si–B exchange reaction to obtain a series of BMCs,
such as 7 and 8 ([Figure 2]).[16a] The single-crystal structure of 8 shows a fully planar Ci
-symmetric configuration. It emits the bright blue fluorescence with a high Φ
PL of 78%, compared to the planarized double silicon-bridged congener and the parent
molecule dibenzo-[g,p]chrysene. This is mainly attributed to the specific electronic effect of the B atoms
with their π-conjugated p orbitals. Therefore, replacing two C atoms in bisanthene
by the B atoms affords an efficient blue luminophore. Then, they disclose the Ru-catalyzed
cyclization of aryl eneynes as a useful approach to annulate benzene rings onto pre-existing
aryl scaffolds. The two BMCs 9a and 9b were steadily synthesized.[16b] As the dangling phenyl ring in 9b is a part of a stilbene-type substructure, 9b can undergo a photocyclization reaction to obtain a fully fused BMC. All of them
are inert toward air and moisture and highly luminescent in the short-wavelength region.
By combining a Peterson olefination with Ru-catalyzed and/or photoinduced cyclization
reactions, several related B/N- and B/S-containing MCs were further developed.[16c]
Figure 2 Examples of boron-doped molecular carbons from Wagnerʼs group, and two different
cyclization modes of 10.
This group adopted a two-step synthesis and furnished quadruply annulated borepins
in high yields.[16d] A nucleophilic substitution reaction affords 10, and then an intramolecular Ni-mediated Yamamoto reaction on 10 produces the borepin 11 via the formation of the seven-membered ring. The formation of 11 is accompanied by a C−H activation reaction, simultaneously affording compound 12 that contains a six-membered boracycle. The product ratio 11/12 is dominated by the local Ni(0) concentration. The long-wavelength absorption peaks
of 11 and 12 are observed at 444 and 415 nm, respectively, whereas their emission maxima are similar
(λ
em = 462 nm for 11 and λ
em = 468 nm for 12). Thus, a larger Stokes shift of 53 nm is observed for 12. In addition, while 11 undergoes a reversible one-electron reduction, the electrochemical reduction of 12 is fully irreversible. Notably, this mesityl-substituted borepin 11 is stable enough toward air and moisture, which is different from most of the reported
borepins that are sterically protected by more demanding supermesityl groups, such
as 2,4,6-tri-tert-butylphenyl.
The intramolecular Ni-mediated Yamamoto reaction was further employed for the synthesis
of the BMC 13 from the tetrabrominated 9,10-di(naphth-1-yl)-9,10-dihydro-9,10-diboraanthracene.[16e] It is notable that this intramolecular C–C heterocoupling reaction exhibits an obvious
solvent dependence: 13 was synthesized only in pyridine with a yield of 79%, whereas an oxadiborepin was
formed in THF with a yield of 81%. The single-crystal structure of 13 has a C
2 symmetry and two different kinds of axial chiral units, thus leading to its good
solubility even in c-hexane. Compared to its parent compound tetrabenzo[de,hi,op,st]pentacene, the emission color of 13 shifts from red to blue. The titration experiment proves that 13 behaves as a Lewis acid, but only monoadduct is possibly formed, even with the strong
Lewis base DMAP. Additionally, 13 is an electron acceptor and undergoes reversible reduction at half-wave potential
of E
1/2 = − 1.73 V (vs. Fc/Fc+) in the cyclic voltammogram curve. Thus, the synthetic strategies developed by Wagner
et al. are very desirable for the construction of BMCs with fascinating structures
and properties.
Inglesonʼs group developed a facile yet versatile strategy to synthesize BMCs in a
controlled manner.[17a] They adopted one-pot borylative cyclization/intramolecular electrophilic C–H borylation
of naphthyl-alkynes, oxidation using [Ph3C][BF4]/2,4,6-tri-tert-butyl pyridine and final reaction with MesMgBr to generate the polycyclic BMC 14 ([Figure 3]). For the doubly B-doped MC 15, the corresponding diyne precursor was employed and the similar reactions were performed.
In addition, the B–Cl-containing intermediates could also react with (2,6-di(prop-1-en-2-yl)phenyl)lithium,
and then treatment of the resulting compounds with Sc(OTf)3 may afford the fully planarized BMCs via intramolecular Friedel−Crafts cyclization.
On the other hand, the electrophilic bromination on 14, Sonogashira coupling, and then the similar electrophilic borylation and oxidation
as described above may produce the triply B-doped MC 16.[17b] From 14 to 15 to 16, the π-electron delocalization is extended and the LUMO energy level gradually decreases,
leading to the decreased optical band gap and red-shifted lowest energy adsorption/emission
maxima.
Figure 3 Examples of boron-doped molecular carbons from Inglesonʼs group, and two different
cyclization modes of 17.
This group reported using the borylative cyclization to achieve pristine B-doped phenalene
(1-boraphenalene), which can be regarded as the smallest-size polycyclic BMCs.[17c] Phenalenyl is an open-shell MC molecule that contains 13 C atoms and 13 π electrons,
whereas the phenalenyl cation is isoelectronic to 1-boraphenalene (C12B). 18 and 19 containing no additional annulation were steadily synthesized starting from 17 (1-ethynylnaphthalene for 18 and 1-(p-tolylethynyl)naphthalene for 19) in the presence of BBr3. Both of them have planar structures and good bench stability. The density functional
theory calculation results indicate that the nature of the LUMOs in 18 and 19 is closely comparable to that in carbon-based phenalenyl cation analogues, but 18 and 19 have obviously lower aromatic stabilization of the C5B ring than observed in each ring in the D3h phenalenyl cations, owing to the less delocalized nature of the occupied orbitals
of π symmetry in the 1-boraphenalenes. Based on this facile synthesis strategy, the
BMC 20 was synthesized starting from 1,6-bis((4-(tert-butyl)phenyl)ethynyl)pyrene.[17d] Cyclic voltammetry study reveals that it has a low LUMO energy level of −4.12 eV,
indicative of its potential application as an electron acceptor unit in organic electronic
devices. Subsequently, the C–Br units in 20 enable the construction of an electron donor–acceptor–donor (D-A-D) molecule using
Negishi cross-coupling reaction. The thus-synthesized compound bearing two triphenylamine
moieties displays a broad absorption band stretching up to 750 nm and a narrow optical
gap of 1.65 eV. Thus, the electron-accepting character of 20 along with the electron-donating effect of the triphenylamine group contributes to
the small energy gap. The simplicities of these synthetic approaches make them be
a powerful tool for rapidly generating stable BMCs, which are of importance for facilitating
structure–property relationship studies and optoelectronic applications.
Würthnerʼs group reported an alternative one-pot synthetic strategy to develop novel
BMCs. The alkene hydroboration and C–H borylation with a N-heterocyclic carbene (NHC)-borenium
ion 21 followed by dehydrogenation and hydrolysis enabled the successful synthesis of the
BMC 22 ([Figure 4]).[18a] The single-crystal structure of 22 exhibits infinite 1D π-stacking along with significant structural overlap. It exhibits
the absorption maximum at 561 nm and a fluorescence peak at 603 nm with a Φ
PL of 0.63. It can undergo two-step reversible one-electron reductions at −1.30 and
−1.64 eV vs. Fc/Fc+ in DMSO. Despite the absence of steric protection or planar constraint on the B center,
it also achieves good stability and relatively facile electrochemical reductions.
Furthermore, the reaction of 22 with BBr3 and subsequent MesMgBr in toluene generates the BMC 23.[18b] Compared to 22, the absorption and emission spectra of 23 are significantly red-shifted, and notably, the fluorescence band of 23 appears in the NIR region with the main peak at 668 nm. Two-step reversible one-electron
reductions of 23 occur at −1.07 V and −1.47 V, along with the first reduction at a more positive potential,
suggesting a lower LUMO energy level of 23. Furthermore, the application of 23 as an active material in organic electronic devices is demonstrated. Notably, the
OTFT device using it as n-type semiconductor exhibits the electron mobility of 3 × 10−3 cm2 · V−1 · s−1, and organic solar cells using it as electron acceptor achieve the power conversion
efficiency up to 3%. This latter application represents the first use of BMC as an
acceptor material in organic photovoltaics.
Figure 4 Examples of boron-doped molecular carbons from Würthnerʼs group, and one representative
synthetic route for 23.
This group further devised and executed a new synthesis for BMCs based on the above
strategy.[18c] The consecutive hydroboration/electrophilic borylation/dehydrogenation and BBr3/AlCl3/2,6-dichloropyridine-mediated C–H borylation steps afforded the BMC 24. The single-crystal structure of 24 shows a moderately planar π-surface and thus exhibits 1D columnar π-stacking with
an intermolecular packing distance of 3.4 Å. This strong intermolecular interaction
decreases the solubility of 24 in most organic solvents. This compound absorbs strongly in the visible region and
emits in the NIR region up to 1150 nm with the fluorescence peak at 757 nm. In addition,
24 exhibits two reversible reductions at −1.00 V and −1.17 V vs. Fc/Fc+, indicating its facile reduction ability and low-lying LUMO level. Following this
synthetic strategy, they further developed two BMCs 25 and 26 using the corresponding alkenes.[18d]
[e] Interestingly, Pd-catalyzed [5 + 2] annulation with suitable aryl dihalides toward
25 or 26 generates carbon-based MCs that feature seven-membered rings. This method is very
useful to expand the scope of available nonplanar MCs with both positive and negative
curvatures.
Hatakeyamaʼs group developed the tandem intramolecular electrophilic arene borylation
to facilitate access of BMCs.[19] Precursor 27 is subjected to lithium−halogen exchange, followed by trapping of the resulting aryllithium
with BBr3 to afford the key intermediate ([Figure 5]). Then, 2 equiv of EtNiPr2 promotes the cyclization to afford 28 with the yield of 55%. Notably, Wagner et al. also reported the synthesis of 28 but using a different method, namely through a Ni-mediated Yamamoto C–C-coupling
reaction. The single-crystal structure of 28 adopts a helical configuration due to the steric repulsion of the cove region. Although
28 owns a nonplanar structure, it is stable enough toward oxygen, 1 N HCl, and 1 N NaOH.
Its toluene solution exhibits strong visible absorption and fluorescence with a Φ
PL of 0.90. It undergoes one reversible reduction with a peak potential at −1.76 V vs.
Fc/Fc+ in CH3CN. Thus, the use of 28 as an active material in organic light-emitting diode (OLED) and OFET devices is
reported.
Figure 5 Examples of boron-doped molecular carbons from Hatakeyamaʼs group, Fengʼs group,
Wagnerʼs group and Wangʼs group, and their representative synthetic routes.
Fengʼs group disclosed a novel one-pot synthetic strategy for the synthesis of BMCs,
from the easily accessed alkyne precursors.[20] The reaction mechanism involves a sequence of borylative cyclization, 1,4-boron
migration, and electrophilic C–H borylation, determined by experimental and theoretical
investigations. For instance, BBr3 is added into a solution of 29 and 2,4,6-tri-tert-butylpyridine (TBP) in 1,2,4-trichlorobenzene, and the mixture is heated at 200 °C,
leading to the successive formation of 29a, 29b and 29c ([Figure 5]). Subsequently, treatment of the solution with MesMgBr directly affords the BMC
30. Using the corresponding dialkyne precursor, doubly B-doped MC 31 was also obtained. From 30 to 31, the longest-wavelength absorption peak and emission maximum are red-shifted owing
to the increase of the conjugation length. 31 displays a high photoluminescence quantum yield (Φ = 0.91) and thus can be used to fabricate the OLED device, further demonstrating
the promising application of BMCs in organic optoelectronics.
Two examples of B-doped acenes as linearly fused BMCs are also shown and discussed.
Wagner′s group reported using a vicinal electrophilic diborylation reaction to synthesize
a variety of BMCs.[21] For example, 4,5-dichloro-1,2-bis(trimethylsilyl)benzene (32), BBr3 and anthracene (33) in hexane were heated at 120 °C for 2.5 days. After removing the residual BBr3, MesMgBr was added to afford 34. Its cyclohexane solution shows a bright fluorescence (λ
em = 470 nm) with a small Stokes shift (5 nm) and a quantum efficiency of 90%. The dechlorinated
derivative was thus employed as the emissive material in the OLED device.
Wangʼs group synthesized the longest doubly B-doped acenes.[22] The Si/B exchange reaction on 35 with BBr3 affords the linear dihydrodiboraheptacene backbone, and subsequently treatment of
this intermediate with TipMgBr leads to 36. Single crystals prove that 36 adopts a nearly coplanar π-conjugated framework. In particular, 36 can undergo chemical reductions in THF to generate radical anion and dianion derivatives
by adding 1 or 2 equiv potassium, respectively. Furthermore, the THF solution of 36 exhibits an ultrahigh photoluminescence quantum yield (Φ
PL = 98 ± 2%), indicating its promising application as luminescent materials in organic
optoelectronics.
Douʼs group proposed controlled cyclization of conjugated organoboranes as an efficient
methodology to construct BMCs. This approach includes two key advantages: (1) rapid
π-extension of the framework can be achieved, which is very desirable to design large-size
and even nanoscale BMCs, and (2) the edge structures and B-doping modes of BMCs can
be precisely controlled, which are of importance for manipulating electronic structures
and properties and further exploring new functions.
They designed and synthesized a new B-containing building block, namely doubly B-doped
heptazethrene 37, via double Si–B exchange reaction ([Figure 6]).[23] Treatment of 37 with MesMgBr generates the BMC 38, which is stable enough for purification and characterization. Compared with the
P=O- and Si-containing analogues, 38 has a smaller energy gap and thus remarkably red-shifted absorption and fluorescence
bands by over 80 nm, which are attributed to the presence of unique p–π conjugation
of the B atoms. Treatment of 37 with 9-lithium-bis(mesityloxy)anthracene followed by Scholl reaction on 39 affords the fully fused BMC 40. As determined by the single-crystal structure, 40 has 19 hexagonal rings that are fused together to construct its planar graphene nanoflake
with a C54B2 skeleton. Despite the presence of the large steric hindrance at the periphery of
40, one molecule is rotated by about 60° to stack on the other molecule with the formation
of a π–π stacking dimer. Thus, this unprecedented bilayer offset-assembled dimer may
be regarded as a molecular cutout of layered B-doped graphene, which is useful for
understanding the structure and properties of B-doped graphene at the molecular level.
40 possesses the very broad light absorption that covers the entire visible range of
350 – 750 nm with absorption peaks at 725, 603 and 556 nm, and sharp NIR fluorescence
with a peak at 729 nm and an FWHM of only 26 nm. Such unique photophysical properties
contributed by the B atom are very rarely achieved for other reported BMC systems.
On the other hand, 40 displays five-step reversible redox ability, and in situ vis–NIR spectroelectrochemistry
measurements prove the formation of three reduced species of 40. The theoretical calculations show that 40 has obvious aromatic character, whereas the dicationic species of its all-carbon
analogue has dominant global aromaticity. This result reveals that B-doping of polycyclic
π-systems may alter their electronic structures, which cannot be easily obtained by
oxidative manipulation of their all-carbon analogues.
Figure 6 Examples of boron-doped molecular carbons from Douʼs group, and the synthetic routes
for 38 and 40.
This group then reported two organoborane cyclophanes including 41, which are composed of B-doped π-skeletons and flexible alkyl chain linkers, thus
representing a new kind of non-conjugated organoborane macrocycles.[24a] They selected the BMC 28 as the key building block and employed the ruthenium-catalyzed olefin metathesis
reaction for the synthesis. The photophysical properties and Lewis acidity of these
two molecules are fully investigated, and notably, their Lewis acid–base adducts with
DMAP may dissociate in the excited state and thus display intriguing photo-responsive
fluorescence properties, which can be further modulated by temperature.
Molecular ribbons (MRs), namely graphene nanoribbons with accurate and monodisperse
structures, have wonderful topologies and properties. Herein, they disclosed that
the modular synthetic method that involves mild photochemical cyclization methodology
is desirable for the synthesis of B-doped MRs.[24b] The BMCs 42–44 with multiple cove edges were prepared through Mallory photochemical cyclization
on conjugated organoboranes in solution. The single-crystal structures of 43 and 44 feature the isomeric C68B2 π-skeletons with 2.2 nm in length, thus representing a new kind of B-doped MRs. From
42 to 43 and 44, the absorption and emission wavelengths are comparable, but the molar absorption
coefficients are enhanced by more than two times. The B atoms endow them with sufficient
Lewis acidity, and notably, the formed Lewis acid–base adducts based on 43 and 44 containing two B atoms display the unprecedented photo-induced dual-dissociation
progress, further inducing stimuli-responsive fluorescence properties. In addition,
despite the highly contorted topological conformations, their potential utility as
organic semiconductors is demonstrated by fabrication of the single-crystal OFET devices.
Both 42 and 43 exhibit unipolar p-type charge carrier transport in air, along with a higher hole
mobility of 0.06 cm2 · V−1 · s−1 for 42. Although this value is modest, it remains among the best for pristine tricoordinate
B-doped MCs. Different from the use of on-surface synthesis to produce B-doped graphene
nanoribbons, this study reports the first example for in-solution synthesis of B-doped
MRs.[25]
Through in-depth combination of cyclization methodologies, BMCs with more sophisticated
polycyclic structures were constructed.[26] A class of BMCs (45–47) that feature the C56B2 or C84B2 polycyclic π-skeletons with selective cove/fjord or cove/bay edges, respectively,
were achieved by sequential cyclization via rational combination of Mallory photoreaction
and Scholl reaction. Among them, 47 has a molecular length of 2.2 nm, thus representing not only the largest B-doped
π-system but also a new kind of B-doped MRs that feature the angular (Z-shaped) one-dimensional
topology. From 45 to 46 and 47, the energy gaps determined by the onset absorptions are significantly decreased
from 2.47 eV to 2.01 eV and 1.69 eV, and the main fluorescence peaks are red-shifted
from 504 nm to 612 nm and 721 nm, respectively. These obvious spectral changes are
ascribed to the different delocalization extents of π electrons in BMCs, which are
closely related to the variation of fusion mode and π-extension of the frameworks.
On the other hand, in the femtosecond transient absorption (fs-TA) spectra, 46 displays the characteristic excited-state absorption and ground-state bleach bands,
as well as an unexpected negative bump centered at 667 nm. This latter signal is assigned
to the stimulated emission (SE) of 46, suggesting that it may display the optical-gain property. The detailed amplified
spontaneous emission (ASE) measurements were performed on the 46/polystyrene blend film. The ASE function is undoubtedly realized for 46, along with the narrow emission at 669 nm and the final FWHM of 6.4 nm. The threshold
value, a pump laser energy that can trigger ASE, is estimated to be 120 kW · cm−2. For 45 and 47, the doped films do not show any ASE behaviors under various conditions. Thus, precise
edge control leads to the first example of ASE-active BMC, which may promote the creation
of new ASE and even lasing materials and will open new avenue for polycyclic MCs for
organic photonics.
Organic diradicaloids, possessing open-shell electronic structures and properties,
have drawn great attention in organic electronics and spintronics. Recently, Douʼs
group reported the borylation of antiaromatic π-skeletons to design B-containing organic
diradicaloids.[27] Based on this strategy, they developed B/O-type and B/N-type diradicaloid compounds.
These molecules have excellent ambient stability and open-shell singlet diradical
structures, as well as intriguing magnetic and optoelectronic properties, such as
thermally accessible triplet species, reversible multiredox ability, narrow energy
gaps and NIR absorptions. Moreover, these diradicaloids possess Lewis acidity and
thus can coordinate with Lewis bases to form supramolecular diradicaloids, achieving
dynamic modulation of diradical character and properties.
Very recently, this group proposed a new strategy to design open-shell BMCs, namely
pentagon-fusion of the organoborane π-system ([Figure 7]).[28] Starting from the B-doped precursors 48 and 51, they successfully synthesized 53 and 54 that feature the C24B and C38B π-skeletons containing a pentagonal ring, respectively, in four steps based on the
Suzuki–Miyaura cross-coupling, nucleophilic addition, intramolecular Friedel–Crafts
alkylation and oxidative dehydrogenation reactions. Such pentagon fusion not only
gives rise to their local antiaromaticity, but also incorporates an internal quinoidal
substructure and thereby induces open-shell singlet diradical states, as determined
by variable-temperature 1H NMR and ESR measurements and theoretical calculations. It is notable that their
solutions display very broad absorption bands that cover the visible and NIR regions
with the range of 300 – 920 nm for 53 and 300 – 1030 nm for 54. Furthermore, the solid powders of 53 and 54 display further red-shifted absorptions with the range of 300 – 1050 nm and 300 – 1200 nm,
respectively. As a result, they can be used as organic photothermal materials, and
54 exhibits a photo-thermal power conversion efficiency of 21.2% and a solar-driven
water evaporation efficiency of 93.5%. The solar-driven water evaporation performance
of 54 is among the highest for organic photothermal materials and comparable to inorganic
solar-thermal materials.
Figure 7 Example of diradicaloid boron-doped molecular carbons from Douʼs group, and the synthetic
routes for 53 and 54.
