Through-space π-delocalization in a conjugated macrocycle consisting of [2.2]paracyclophane

Article abstract of DOI:10.1039/c9cc06492c

Herein, we report the synthesis and characterization of a [2.2]paracyclophane-containing macrocycle (PCMC) as a new through-space conjugated macrocycle using only benzene groups as the skeleton. For comparison, a diphenylmethane-containing nanohoop macrocycle (DCMC) with a non-conjugated linker was also synthesized. Their structures were confirmed by NMR and HR-MS, and their photophysical properties were studied by UV-vis and fluorescence spectroscopies combined with theoretical calculations. The strain energy of PCMC was estimated to be as high as 72.58 kcal mol^-1.

Full text of DOI:10.1039/c9cc06492c

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Through-space p-delocalization in a conjugated
macrocycle consisting of [2.2]paracyclophane†
Cite this:DOI: 10.1039/c9cc06492c
a
b
a
a
a
Yayu Wu, Guilin Zhuang, * Shengsheng Cui, Yu Zhou, Jinyi Wang,
a
a
Received 21st August 2019,
Accepted 10th November 2019
Qiang Huang and Pingwu Du
*
DOI: 10.1039/c9cc06492c
rsc.li/chemcomm
Herein, we report the synthesis and characterization of a [2.2]paracyclo-
[2.2]Paracyclophane is an appealing aromatic compound
phane-containing macrocycle (PCMC) as a new through-space including face-to-face benzene rings (‘‘decks’’) fixed by two
conjugated macrocycle using only benzene groups as the skeleton. ethylene chains (‘‘bridges’’) with a deck distance of approximately
2
1,22
For comparison, a diphenylmethane-containing nanohoop macro- 3.0 Å.
The slightly bent but stable structure of [2.2]paracyclo-
cycle (DCMC) with a non-conjugated linker was also synthesized. phane enables through-space electronic communication between
23
Their structures were confirmed by NMR and HR-MS, and their the benzene decks. Moreover, [2.2]paracyclophane can be intro-
photophysical properties were studied by UV-vis and fluorescence duced as a chiral scaffold providing target compounds with
2
4,25
spectroscopies combined with theoretical calculations. The strain interesting chiral properties.
Naturally, the introduction of
ꢀ
1
energy of PCMC was estimated to be as high as 72.58 kcal mol
.
[2.2]paracyclophane as a building block into conjugated macro-
cycles can realize the construction of p-stacked systems in the
26
Conjugated macrocycles have received much attention due to cyclic structure. In 2001, Haley, Boydston, and coworkers success-
their unique cyclic structures, intrinsic photoelectric properties, fully obtained an open-circuited conjugated macrocycle containing
1
–4
27
and extensive material application prospects.
Research on [2.2]paracyclophane and dehydrobenzoannulene units, which
5
conjugated macrocycles started in the early 1960s, and great demonstrated ‘‘global’’ through-space delocalization between
progress has been made in the last two decades. To date, a the conjugated decks of the annelated cyclophanes. Casado and
series of conjugated macrocycles consisting of thiophene, ben- coworkers synthesized a series of stilbenoid and thiophenic
3
,6
7
–12
zene, pyridine, and acetylene units have been widely studied.
compounds containing [2.2]paracyclophane units, and then
Inspired by these studies, large p-extended nanographene units studied their photophysical properties regarding the effect of
2
8
(
such as hexa-peri-hexabenzocoronene, HBC) have been success- through-bond and through space p-electron delocalization.
1
3,14
fully introduced into hoop-shaped macrocycle backbones.
Recently, Ogoshi and coworkers reported through-space
Conjugated macrocycles have many interesting advantages as p-delocalization properties in a pillar[5]arene. However, studies
2
9
1
1,15
organic materials, such as altered electronic structures,
end groups to create defects as trap-sites,
cavity to host guest molecules.
macrocycles usually consist of sp carbon skeletons, and
no on through-space p-conjugated benzene-based macrocycles are
and a tunable relatively rare, and the synthesis of such macrocycles remains
However, most conjugated challenging.
1
6,17
1
4,18
2
In this present study, we report the synthesis of a conjugated
through-space conjugated macrocycles consisting of p-stacked [2.2]paracyclophane-containing macrocycle (PCMC) (Fig. 1),
systems have been studied only rarely despite their importance
1
9,20
in optoelectronic applications.
a
Hefei National Laboratory of Physical Science at the Microscale, CAS Key
Laboratory of Materials for Energy Conversion, Department of Materials Science
and Engineering, iChEM (Collaborative Innovation Center of Chemistry for Energy
Materials), University of Science and Technology of China, 96 Jinzhai Road, Hefei,
Anhui Province, 230026, China. E-mail: dupingwu@ustc.edu.cn;
Fax: +86-551-63606207; Tel: +86-551-63606207
b
College of Chemical Engineering, Zhejiang University of Technology,
1
8 Chaowang Road, Hangzhou, Zhejiang Province, 310032, China.
E-mail: glzhuang@zjut.edu.cn; Fax: +86-571-88871037; Tel: +86-571-88871037
Fig. 1 DFT optimized geometries of the designed macrocycles DCMC
and PCMC.
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/c9cc06492c
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which represents the first through-space conjugated macrocycle
composed of benzene rings. PCMC should have two enantiomers
(PCMC-S0 and PCMC-S1, Fig. S1, ESI†) and changes into a mirror
image by rotating around single bonds at room temperature
3
0
without experiencing a flat achiral conformation process.
Meanwhile, a transition state (PCMC-TS) was also obtained with
ꢀ1
the rotation-energy barrier of 30.72 kJ mol above PCMC-S0.
For comparison, a diphenylmethane-containing macrocycle
(
DCMC) with similar size and twist angles was also synthesized
3
1,32
and characterized.
These macrocycles were studied by
1
13
high-resolution mass spectrometry (HR-MS), H, C and 2D
nuclear magnetic resonance (NMR) spectroscopies, steady-state
spectroscopies, time-resolved photoluminescence decay, and
theoretical calculations.
Fig. 3 (a) HR-MS (MALDI-TOF) spectrometry for PCMC. Insert: tiny crystal
1
1
1
Our strategy for the synthesis of DCMC and PCMC is to photographs of PCMC. (b) H NMR spectrum for PCMC. H– H COSY NMR
spectra of the aromatic protons (c) and alkane protons (d) for PCMC.
connect the C-shaped fragment 1, a suitably curved synthon,
with compounds 2 and 3, respectively. Then, the target product
can be obtained by subsequent aromatization reactions. As
shown in Fig. 2, the C-shaped synthon 1 was facilely synthesized with the simulated mass data (698.2973 for DCMC and 738.3286
0
0
on a gram scale starting from 4 -bromo-[1,1 -biphenyl]-4-ol over for PCMC). Through slow solvent evaporation (v/v, methanol/
two steps with high yields (450%) according to a recently CH Cl = 4 : 1), we obtained yellow needle-like crystals of PCMC,
To construct the through-space p-conjugated but failed to collect useful X-ray diffraction data so far (Fig. 3a,
2
2
17,33
reported method.
macrocycle PCMC, 4,16-diboryl [2.2]paracyclophane 3 was synthe- inset). The H NMR spectrum of PCMC in CDCl
sized (yield 4 70%) from 4,16-dibromo[2.2]paracyclophane Fig. 3b. The aromatic protons of PCMC appear in the region of
using Pd(dppf)Cl as the catalyst in DMF. Moreover, bis(4- 6.09–7.71 ppm, and the multiplets at 7.65–7.30 ppm (H ) can be
borylphenyl)methane 2 was synthesized according to a reported assigned to the protons in the C-shaped para-linked benzene
1
3
is shown in
2
8
3
1
method to construct a diphenylmethane-containing macro- fragment according to the position and integrated areas of the
1
cycle DCMC. Finally, the target nanohoop macrocycles DCMC
and PCMC were prepared by Suzuki coupling reactions between ring shielding effects, the resonances at d = 7.71 and 7.18 ppm can
the synthon 1 and the pinacol boronic ester 2 or 3 using be assigned to the H and H atoms, respectively. Since the 2D
H NMR peaks. Due to the difference between the inner and outer
7
6
1
1
Pd(PPh )
3 4
as the catalyst at 75 1C, followed by reductive aro-
H– H COSY NMR data have no obvious cross peaks between the
multiplets at 7.65–7.30 and 6.37–6.09 ppm (Fig. 3c), the signals at
matization reactions using sodium naphthalide at ꢀ78 1C.
The successful synthesis of DCMC and PCMC can be confirmed 6.37–6.09 ppm are tentatively assigned to the protons in the
by HR-MS and NMR spectroscopies (Fig. 3 and Fig. S2–S12, ESI†). [2.2]paracyclophane unit. To further assign the NMR resonances
The exact mass spectrometries show the main peaks at m/z for PCMC, we carried out DEPT, HMBC, and HSQC measurements
6
98.2980 for DCMC and 738.3257 for PCMC, which match well (Fig. S6–S10, ESI†). Based on these 2D-NMR spectra, most of the
1
13
observed signals in H- and C-NMR for PCMC (Fig. S11 and S12,
ESI†) can be assigned. Notably, the bridged methylenes of the
cyclophane unit in PCMC have four separated resonances in the
region of 1.80–3.40 ppm which shifted to a higher field than
the proton signals of the methylene unit in DCMC and show
significant cross peaks between each other (Fig. 3d), which can
be ascribed to the through-space communication and cyclic
structure of PCMC. This through-space p-delocalization was
further confirmed by the 2D NOESY and ROESY NMR spectra
(Fig. S13, ESI†), which revealed the close spatial relationship of
protons on four methylene groups.
The photophysical properties of DCMC and PCMC were
further studied by UV-vis absorption and fluorescence spectro-
scopies at room temperature in the air atmosphere (Fig. 4a). The
ꢀ
5
diluted solution of DCMC in CH Cl (c = 1.0 ꢁ 10 M) displayed
2
2
obvious absorption bands in the range of 250–450 nm, and
4
ꢀ1
ꢀ1
ꢀ1
maximized at 323 nm (e = 4.32 ꢁ 10 M cm ) along with a
Fig. 2 Synthetic procedure for the macrocycles DCMC and PCMC. Reaction
conditions: (a) DAIB, MeOH, RT, 48 h; (b) (1) N-BuLi, THF, ꢀ78 1C, 2 h; (2) NaH,
4
ꢀ1
small shoulder at 380 nm (e = 0.89 ꢁ 10 M cm ). In contrast,
the absorption band of PCMC is redshifted, demonstrating an
absorption maximum at 326 nm along with one shoulder peak
3 4 2
MeI, THF, 0 1C, 8 h; (c) (1) KOH, Pd(PPh ) , THF/H O, 75 1C, 48 h; (2) Sodium
naphthalide/THF, ꢀ78 1C, 2 h.
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Fig. 4 (a) UV-vis absorption (dashed lines) and fluorescence spectra (solid
ꢀ5
lines) for DCMC (blue) and PCMC (red) in CH
Fluorescence photographs of DCMC (left) and PCMC (right) in CH
UV irradiation at l = 365 nm. (b) Fluorescence decay lifetime for PCMC in
CH Cl measured at l = 488 nm. Insert: Curve-fitting residuals for PCMC.
2
Cl
2
(1.0 ꢁ 10 M). Inset:
2
Cl under
2
Fig. 5 (left) UV-vis absorption (dashed lines) and fluorescence spectra
(
cence photographs of PCMC in different solvents (i: toluene; ii: DCM; iii:
DMSO.) under UV irradiation at l = 365 nm.
solid lines) for PCMC in different solvents (1.0 ꢁ 10ꢀ M). (Right) Fluores-
5
2
2
at 355 nm and one small shoulder peak at 398 nm. The molecular
4
absorption coefficients for these absorption peaks are 2.89 ꢁ 10 , solvents, as evidenced by a color change from green to yellow with
4
4
ꢀ1
ꢀ1
2
.10 ꢁ 10 , and 0.54 ꢁ 10 M cm , respectively. Compared with increasing solvent polarities (Fig. 5, inset). The PL maximum peak
the absorption feature of DCMC, the redshift and extended absorp- of DCMC in DMSO is only slightly red shifted by B7 nm (0.04 eV)
tion of PCMC can also be ascribed to the p-stacked structure of the compared to that in toluene. In contrast, the PL maximum peak of
cyclophane moiety undergoing through-space interaction.
34,35
41,42
PCMC in DMSO is redshifted up to B38 nm (0.21 eV).
We
The fluorescence emission spectrum of DCMC exhibited one propose that through-space p-delocalization across the [2.2]para-
vibronic emission band maximized at 458 nm upon excitation cyclophane core is more polarizable than through-bond interaction
43,44
at 320 nm. Interestingly, the emission spectrum of PCMC showed in the excited state.
an apparent redshift feature with one maximum emission peak at To investigate the dynamic racemization process of PCMC, we
88 nm. Compared to the Stokes shift of DCMC, PCMC has a performed VT-NMR experiments in CD Cl . When the temperature
4
2
2
larger Stokes shift of 27 nm, probably due to the p-stacked was decreased from 273 K to 198 K, the methylene proton
through-space conjugation and/or the increased flexibility of resonances in the cyclophane unit showed appreciable changes
2
9
PCMC. Using anthracene as the reference (F
ethanol), the photoluminescence quantum yields of DCMC behavior (Fig. S16, ESI†). At 273 K, the methylenes have four
and PCMC were determined to be F = 37% and F = 31%, singlets in the range of 1.4–4.1 ppm, demonstrating that the
respectively. Under a hand-held UV lamp excited at l = 365 nm, ring rotation process is rapid on the NMR time scale. As
the photoluminescence of PCMC in a solid and in CH Cl the temperature decreases, the rotation process should slow
solution (c = 1.0 ꢁ 10 M) presents an intense green color, down, and the spectrum will have more resonances if PCMC is
whereas DCMC is blue (Fig. 4a, inset and Fig. S15a, ESI†). conformationally rigid. At B198 K, the proton resonances in
F
= 30% in of the chemical shifts, indicating its interesting racemization
F
F
4
5
2
2
ꢀ
5
A time-resolved photoluminescence (TRPL) technique was 1.4–3.0 ppm show obvious line broadening and were split into
carried out to investigate the excited-state lifetimes of DCMC small peaks with partially overlapping signals (Fig. S16, ESI†),
and PCMC in CH
2 2
Cl at room temperature. The PL decay results confirming the dynamic racemization process of PCMC using
2
5
of DCMC and PCMC are shown in Fig. S15b (ESI†) and Fig. 4b. the rubber glove mechanism. The activation barrier for the
Upon excitation at 320 nm, the luminescence lifetime of PCMC racemization of PCMC was estimated to be 36 kJ mol based
was determined to be approximately 3.75 ns at 488 nm by single- on the shift difference of 198 Hz for H /H1’ and the coalescence
exponential decay fitting. The curve fitting residuals confirm the temperature (198 K).
accuracy of the TRPL measurement results (Fig. S15b (ESI†) and Variable-temperature UV-vis and fluorescence spectra were
ꢀ
1
1
4
6
Fig. 4b, inset). As for DCMC, a single-exponential decay was further recorded. By increasing the temperature (Fig. S17a,
observed with one lifetime of 2.95 ns at 458 nm when excited at ESI†), the absorption intensity of PCMC in THF decreased,
3
20 nm. These different photophysical data also indicate the which is probably due to the faster rotational motions at a
4
7
structural discrepancy between DCMC and PCMC.
higher temperature to reduce effective electronic transitions.
Through-space p-delocalization in a conjugated system is Notably, the fluorescence spectra of PCMC in the solid state
36
sensitive to the polarity of solvents. Table S1 (ESI†) summarizes exhibited an interesting change when the temperature was
the spectral data for PCMC and DCMC in different solvents from increased from 50 K to 300 K (Fig. S17b, ESI†). The single broad
hexane (a nonpolar solvent) to dimethyl sulfoxide (a polar solvent). emission band was split into two peaks with a substantial increase
As shown in Fig. 5 and Table S1 (ESI†), the UV-vis absorption of of the emission intensity when decreasing the temperature below
37
PCMC is only slightly affected by the solvent polarity, indicating 250 K. Furthermore, the results showed that PCMC gradually
that the ground state of PCMC has no significant intramolecular becomes conformationally rigid at temperatures below 250 K
38–40
charge transfer (ICT) character compared to DCMC.
However, (Fig. S16, ESI†). The increase of molecular rigidity can reduce
the emission features of PCMC show obvious changes in different the quenching rate of the excited state and increase the coplanarity
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1
of the macrocycle and facilitate the through-space p-delocalization,
resulting in enhanced absorption and emission intensities.
In order to further reveal the molecular geometry and
electronic nature of these two macrocycles, we performed
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49
1
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4
ꢀ1
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DCMC. All the results can be found in Fig. S18–S22 (ESI†).
In conclusion, we have successfully synthesized a through-
1
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