Fluorescence emission enhancement of a T-shaped benzimidazole with a mechanically-interlocked ‘suit’

Article abstract of DOI:10.1039/d0cc07471c

A fluorescent T-shaped benzimidazole was successfully designed and interlocked in a bicyclic macrocycle to form a suit[1]ane through supramolecular templated-synthesis. Compared with the bare fluorophore, suit[1]ane requires nearly two times the concentration to initialize the aggregation-caused quenching effect in solution. Furthermore, an 8-fold higher solid-state fluorescence quantum yield (21.7%) is also achieved. By taking advantage of mechanical bonding and molecular packing, such fluorescence emission enhancement through formation of a suitane opens the way to new complex fluorescent materials.

Full text of DOI:10.1039/d0cc07471c

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Fluorescence emission enhancement of a
T-shaped benzimidazole with a mechanically-
interlocked ‘suit’†
Cite this: Chem. Commun., 2021,
57, 3239
Received 13th November 2020,
Accepted 18th February 2021
a
Houyang Xu,
Yanping Huo
Meng-Di Lin,a Jun Yuan,a Baiyang Zhou,a Yingxiao Mu,b
b
a
and Kelong Zhu
*
DOI: 10.1039/d0cc07471c
rsc.li/chemcomm
A fluorescent T-shaped benzimidazole was successfully designed interactions such as p–p stacking, and the aggregation-induced
and interlocked in a bicyclic macrocycle to form a suit[1]ane emission (AIE) effect,6 which leads to light emission by for-
through supramolecular templated-synthesis. Compared with the mation of aggregates, could be extremely interesting to explore.
bare fluorophore, suit[1]ane requires nearly two times the concen- Huang et al. reported enhanced AIE of a TPE group by incor-
tration to initialize the aggregation-caused quenching effect in porating the fluorophore into a pillar[5]arene-based [2]rotax-
solution. Furthermore, an 8-fold higher solid-state fluorescence ane.7 Qu et al. and Tian et al. used the macrocyclic hosts,
quantum yield (21.7%) is also achieved. By taking advantage of including cyclodextrins,8 crown ethers,9 calix[n]arenes,10 and
mechanical bonding and molecular packing, such fluorescence cucurbit[n]urils11 to form host–guest inclusion complexes or
emission enhancement through formation of a suitane opens the rotaxanes, to regulate fluorescence emission by switching
way to new complex fluorescent materials.
between various aggregation and non-aggregation states. The
formation of a [2]catenane also has been shown to greatly
The notion of mechanically-interlocked molecules (MIMs), which enhance fluorescence emission by enabling restriction of intra-
are assemblies of molecular components that cannot be split molecular rotation processes of phenyl groups in the aggre-
without breaking a covalent bond, has fascinated supramolecular gated state.12 In addition to regulating fluorescence emission in
chemists for several decades.1 Despite the commonly acknowl- solution, these supramolecular strategies to hinder nonradia-
edged potential of artificial MIMs to perform manipulated tive deactivation have also proved effective in the solid state, as
molecular-level motion,1f increasing research interest has been reported by Yang et al.13
focused on the impact of the mechanically interlocked structure
Despite the success of using a supramolecular protecting
on the properties of the independent components. It has been group (SPG) to tune the photophysical properties of fluo-
demonstrated that by threading or encapsulating vulnerable rophores, the macrocyclic component employed has been
molecules with macrocyclic or cage-like hosts, their stability can mostly limited to monocyclic molecules such as crown ethers
be greatly enhanced.2,3
and cyclodextrins. Using three dimensional or cage-like mole-
Recently, using supramolecular approaches to regulate cules to form MIMs with fluorophores is not adequately
photophysical properties including fluorescence efficiency explored yet. Moreover, to what degree the 3D mechanical
and emission wavelength of molecular components has encapsulation can influence the ACQ effect of the fluorophore
become one of the hot research topics in the field.2,4 Specifi- remains questionable.
cally, the influence of mechanical bonds on the intermolecular
Suit[n]anes are another type of MIMs which consist of a 3D
interactions of fluorophores, e.g. the aggregation-caused molecular ‘suit’ and an encapsulated molecular ‘torso’, with the
quenching (ACQ) effect,5,6 which quenches the fluorescence number ‘n’ representing the number of ‘limbs’ protruding
emission by nonradiative deactivation through intermolecular outwards.14 Stoddart et al. reported the synthesis of suit[n]anes
with 2 to 4 limbs with various templates since they defined the
a School of Chemistry, Sun Yat-sen University, Guangzhou, 510275, China.
notion of suit[n]anes.14 We have recently synthesized a set of
suit[1]anes (Scheme 1b) constructed by permanently embodying a
2,4,7-trisubstituted benzimidazolium in a cryptand using host–
guest chemistry and ring closing metathesis (RCM).15–17 The
encapsulated structure exhibits resistance to strong bases for the
deprotonation of NH when compared to the bare benzimidazole.15
E-mail: zhukelong@mail.sysu.edu.cn
b School of Chemical Engineering and Light Industry, Guangdong University of
Technology, Guangzhou, 510006, China
† Electronic supplementary information (ESI) available. CCDC 2043995 and
2043996. For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/d0cc07471c
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Scheme 2 Preparation of benzimidazolium axle [1b-H][BF4] (a) and
macrocyclic wheel 2c (b); template-directed synthesis of fluorescent
suit[1]ane 3 (c).
and thus exhibited greater difficulty to be neutralized by various
bases including NaOH, NaH, and 1,8-bis(dimethylamino)
naphthalene. The deprotonation of [3-H][BF4] to 3 can finally be
achieved in situ by addition of 20 equivalents of t-BuOK in solvents
including methylene chloride, chloroform, or dimethyl sulfoxide as
solvent. The structures of the obtained suit[1]anes have been con-
firmed by 1H NMR and single-crystal X-ray analysis.
Scheme 1 (a) Mechanically interlocked molecules (MIMs) with fluoro-
phores. (b) Conversion of a [2]pseudorotaxane to a suit[1]ane based on the
recognition motif of a T-shaped benzimidazolium axle with crown ethers.
(c) The design of a new fluorescent suit[1]ane from a DB24C8 derived
macrocycle.
This alternation of reactivity after forming the interlocked structure
encouraged us to further probe the influence on the fluorescence
emission of the suit[1]ane. However, the use of the triphenyl
benzimidazolium (1a) and a BMP26C8 (2b) derived macrocycle in
the synthesis gave a relatively low yield. Moreover, the resulting
isomeric product has further limited the study and application of
the suit[1]ane.16,17 Therefore, in the present design a dibenzo-
[24]crown-8 ether (DB24C8) (2a) derived macrocycle (2c) and a
tert-butyl substituted triphenyl benzimidazole axle (1b) are
employed to improve the synthesis. Furthermore, the installation
of the electron-withdrawing aldehyde group could make 1b a
potential push–pull type chromophore and functionalized fluoro-
phore. Herein, we report the synthesis of a novel T-shaped benzi-
midazole suit[1]ane and its fluorescence properties in both solution
and the solid state.
The template-directed synthesis of suit[1]ane 3 was outlined in
Scheme 2. The axle 1b and its protonated form [1b-H][BF4] were
obtained via a mature methodology reported before.17 The DB24C8-
derived macrocycle 2c was synthesized by bromination and subse-
quent alkylation from a known precursor diol 518 with a moderate
yield of 63% for two steps. 2c exhibits a much stronger affinity to
the protonated axle [1b-H][BF4] with an association constant of
4073 Mꢀ1 in CDCl3 compared to the BMP26C8-based macrocycle
with a 2,4,7-triphenylbenzoimidazolium cation.15,16 After the for-
mation of the [2]pseudorotaxane intermediate in dichloromethane,
the protonated suit[1]ane [3-H][BF4] was obtained by ring-closing
metathesis (RCM) and subsequent hydrogenation with a combined
yield of 44%. This novel suit[1]ane [3-H][BF4] possesses greater
The comparison of 1H NMR spectra of free axles 1b and the
neutral suit[1]ane 3 is shown in Fig. 1. After 1b was encapsu-
lated with the ‘suit’, downfield chemical shifts of the NH proton
(+1.7 ppm) and proton c (+0.93 ppm) are observed, which
indicates the presence of hydrogen-bonding between the
axle and polyether chains. In contrast, the large upfield
chemical shifts of the aromatic protons f and f0 (ꢀ0.68 ppm)
might be attributed to the shielding effect of the benzo groups from
the clamped macrocycle which further confirmed its similar struc-
ture to those previously reported for suit[1]ane and [2]pseudo-
rotaxanes.15,16,19
The single crystal structures of axle 1b and suit[1]ane [3-H]
[BF4] are shown in Fig. 2. 1b forms a close packing structure in
the manner of p-stacking the aldehyde group substituted
phenyl ring with the benzimidazole moiety with a face-to-face
distance of 3.42 Å. This type of aggregation (Fig. 2b) may cause
Fig. 1 Comparison of the partial 1H NMR spectra (298 K, 400 MHz,
intimacy and stronger binding between the axle and the macrocycle, CD2Cl2) of 1b (a) and 3 (b).
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the quenching of the fluorescence of the benzimidazole axle. In
the crystal structure of [3-H][BF4], two N–Hꢁ ꢁ ꢁO hydrogen bonds
are identified between the macrocycle and the axle. The two
benzimidazolium protons N–H form two hydrogen bonds with
an O-atom on the polyether chain (dNꢁꢁꢁO = 2.83 Å, +N–HꢁꢁꢁO = 1631
and dNꢁꢁꢁO = 2.88 Å, +N–HꢁꢁꢁO = 1571). Furthermore, the aromatic
group of the benzimidazole moiety is roughly parallel with both
phenyl rings from the cryptand unit, with a large proportion of
area overlapping, which attests the presence of p-stacking inter-
actions, which strengthens the pseudorotaxane association. Com-
pared to free axle 1b, the packing of [3-H][BF4] allows the good
separation of cationic [1b-H]+ since the bulky cryptand creates
large hinderance between the two axles (Fig. 2d). The resulting
new arrangements of the chromophore in the solid state may
further affect their optical properties.
Fig. 3 (a and b) UV/Vis spectra of 1b, [1b-H][BF4], 3, and [3-H][BF4] in THF.
(c and d) Fluorescence spectra of 1b, 3, [1b-H][BF4], and [3-H][BF4] in THF
when excited at 342 nm. The concentration of all samples is 10ꢀ5 mol Lꢀ1
.
UV-Vis and fluorescence spectra were further conducted on
both the neutral and protonated species. The maximum UV
absorption bands of axles 1b and [1b-H][BF4] are both at
Since the formation of suit[1]ane from the T-shaped benzi-
342 nm in tetrahydrofuran (Fig. 3a). The suit[1]ane 3 has a midazole doesn’t affect the emission to a large extent, we would
blue-shifted maximum absorption band at 322 nm compared to like to know how this mechanical protection strategy can affect
axle 1b while the protonated suit[1]ane [3-H][BF4] has the its fluorescence ACQ effect in both solution and the solid state.
maximum absorption bands at 283 nm (Fig. 3b). Upon excita- In tetrahydrofuran, the fluorescence emission intensity of 1b at
tion at 342 nm in tetrahydrofuran, the free axles 1b and [1b-H] 483 nm starts decreasing when the concentration exceeds
[BF4] show fluorescence emission at 483 nm (Fig. 3c) and 4.6 ꢂ 10ꢀ5 mol Lꢀ1. Nearly full quenching of the fluorescence
481 nm (Fig. 3d) with the fluorescence quantum yields (FF) of can be reached above the concentration of 10ꢀ3 mol Lꢀ1
46% and 44% respectively. A slight red-shift of the emission (Fig. 4a). In comparison with the free axle 1b, the fluorescence
band (490 nm) and lower FF (33%) were observed for suit[1]ane intensity of suit[1]ane 3 starts decreasing at the concentration
3 under the same conditions (Fig. 3c). By contrast, the proto- of 1 ꢂ 10ꢀ4 mol Lꢀ1 (Fig. 4b) which is two times the concen-
nated suit[1]ane [3-H][BF4] gives a lower FF (16%) compared to tration required for 1b. Fig. 4c reveals the dependence of the
those of either the free axle 1b or neutral 3 under the same emission intensities of 1b and 3 on their concentrations,
conditions. This fluorescence quenching effect might be attributed indicating that the suit[1]ane has a reduced ACQ effect. Sur-
to the p–p stacking charge transfer between the electron-rich prisingly, the emission intensities observed for 1b and 3 in the
cryptand and the benzimidazolium moiety in the protonated form.
Fig. 2 Single-crystal X-ray structure of (a) axle 1b; (b) crystal packing of 1b
with intermolecular p–p stacking highlighted with a dashed line; (c)
suit[1]ane [3-H][BF4]; (d) crystal packing of suit[1]ane [3-H][BF4]. Colour
codes: O = red, N = dark blue, C of axle = blue, C of cryptand = orange,
F = lime, B = apricot. Hydrogens not involved in H-bonding are omitted for
clarity; the light-blue dotted sticks denote H-bonds.
Fig. 4 Fluorescence emission spectra of 1b (a) and 3 (b) with varied
concentrations in THF. (c) The fluorescence intensity of 1b and 3 as a
function of concentration. (d) Fluorescence emission spectra of 1b and 3
in the solid state when excited at 342 nm. (e) Views of 1b and 3 powder
under irradiation at 365 nm using a UV lamp.
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K. Z. thanks the National Natural Science Foundation of China
(21701192, 21971268), the Program for Guangdong Introducing
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Conflicts of interest
There are no conflicts to declare.
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