Synthesis and fluorescence enhancement of oligophenylene-substituted calix[4]arene assemblies

Article abstract of DOI:10.1039/b210493h

Tetra-oligophenylene substituted calix[4]arene assemblies containing up to three phenylene units have been synthesized by a convergent approach using Suzuki cross-coupling and their optical properties were investigated and compared with the corresponding

Full text of DOI:10.1039/b210493h

Synthesis and fluorescence enhancement of oligophenylene-substituted
calix[4]arene assemblies†
Man Shing Wong,* Xiao Ling Zhang, Dong Zhong Chen and Wai Ho Cheung
Department of Chemistry, Hong Kong Baptist University, Kowloon Tong, Hong Kong, S. A. R., China.
E-mail: mswong@hkbu.edu.hk
Received (in Cambridge, UK) 29th October 2002, Accepted 19th November 2002
First published as an Advance Article on the web 3rd December 2002
Tetra-oligophenylene substituted calix[4]arene assemblies
containing up to three phenylene units have been synthe-
sized by a convergent approach using Suzuki cross-coupling
and their optical properties were investigated and compared
with the corresponding monomer.
1-bromo-4-iodobenzene with 2 (50%) or by nucleophilic
aromatic substitution of 4,4A-dibromobiphenyl 3 with 1-hex-
anethiol in the presence of NaOH in DMF (62%). Subsequent
conversion of bromide to boronic acid functionalities afforded
4A-hexylsulfanyl-4-biphenylboronic acid 5 in a good yield.
Hexylsulfanyl-substituted terphenylboronic acid 6 was pre-
pared accordingly using the same stepwise approach as
described above starting with the two-phenyl-ring boronic acid
5.
Calixarenes1 have been widely exploited as supramolecular
building blocks for molecular encapsulation2,3 as well as ion
recognition and sensing4 in the last decade. Recently, it has been
of great interest to extend the rigid cavity at the upper rim of
calixarenes for expanded cavity receptors5 as they are beneficial
to the encapsulation and recognition properties. In addition, the
use of calix[4]arene as the pre-defined and pre-organized
framework to incorporate with extended chromophores for
intramolecular electronic transfer6 and inter-chromophoric
interaction investigations7,8 has drawn some attention. As a
result, there are considerable synthetic effort to expand the rigid
cavity at the upper rim of calix[4]arenes. We are particularly
interested in synthesizing oligophenylene substituted calixarene
assemblies for functional property investigations as the highly
extended styrylstyrylcalix[4]arene derivatives show poor pho-
tostability in solution which is presumably due to the photo-
dimerization of the proximate co-facial vinyl units.
Several methods have been developed to prepare p-phenyl-
calix[4]arene;9 however, these procedures afford very low
yields. On the other hand, p-phenylcalix[4]arene tetraalkyl
ethers are easily synthesized in good yields by various metal
catalysed cross-coupling protocols, which include Suzuki-type
coupling,10,11 and Negishi-type coupling.12 Nevertheless, there
is a lack of example or methodology of synthesizing longer
homologues of oligophenylene substituted calix[4]arenes re-
ported so far. To this end, we report herein a facile protocol to
synthesize an homologues series of tetra-oligophenylene substi-
tuted calix[4]arene assemblies containing up to three phenylene
units end-capped with alkylsulfanyl donating substituents, 9a–c
and with alkylsulfonyl withdrawing substituents, 10a–c. In
addition, their optical properties were characterized and com-
pared with the corresponding monomer.
We found that Suzuki cross coupling was particularly
versatile for the convergent synthesis of the homologues series
of oligophenylene-substituted calix[4]arene assemblies as good
reactivity and regioselectivity could be obtained by various
catalytic palladium compounds/complexes. Cross coupling of
oligoarylboronic acid and tetrahalocalix[4]arene tetraalkyl ether
was used as a key step to construct the highly extended cone-
conformed assemblies. The syntheses of (oligo-)arylboronic
acids are summarized in Scheme 1. Alkylation of 4-bromothio-
phenol 1 with 1-bromohexane in basic medium afforded
1-bromo-4-hexylsulfanylbenzene in excellent yield (97%).
Lithium bromide exchange of the bromine at 278 °C followed
by quenching with trimethyl borate at room temperature and
subsequent acid hydrolysis afforded 4-hexylsulfanylbenzene-
boronic acid 2. 4-Bromo-4A-hexylsulfanylbiphenyl 4 was pre-
pared either by regioselective Suzuki cross-coupling of
Tetraiodocalix[4]arene tetradecyl ether 7 was prepared
according to the procedures published in the literature.13 Cross-
coupling of 7 with boronic acid 2 or 5 in the presence of a
catalytic amount of Pd(OAc)2–2P(o-tol)3 complex afforded the
corresponding tetraoligophenylene substituted calix[4]arene
assemblies 9a or 9b, respectively, in good yields (Scheme 2). It
should be stated that the Pd(OAc)2–2P(o-tol)3 complex is found
to be more efficient than the classical Pd(PPh3)4 as a catalyst for
the cross-coupling and the coupling reaction also worked well
with tetrabromocalix[4]arene tetradecyl ether but required more
drastic conditions. However, the reaction of tetraiodocalix[4]ar-
ene 7 with terphenylboronic acid 6 could not be driven to
completion which is presumably due to the increased steric
crowdedness of the calix[4]arene reaction sites created by the
assembled oligophenylene units particularly in the construction
of the higher homologues of an assembly. To alleviate the
crowdedness of the calix[4]arene reaction sites, the extended
tetrabromophenylcalix[4]arene tetradecyl ether 8 was pursued
and prepared by selective cross-coupling of commercially
available bromophenylboronic acid and 7 using Pd(PPh3)4 as a
catalyst. It is important to note that the use of Pd(OAc)2–2P(o-
tol)3 the complex as a catalyst in this reaction afforded no
desired product. Cross coupling of 8 and biphenylboronic acid
5 under typical conditions afforded 9c in a moderate yield.
(Scheme 2). Besides, assembly 9b can be prepared by coupling
of 8 with boronic acid 2. The MALDI-TOF mass spectra of 9b
and 9c showed a major peak at m/z 2081 and 2385, respectively,
corresponding to [M + H + Na]+. MCPBA-oxidation of alkyl-
sulfanyl-substituted calix[4]arene assemblies, 9a–c in CH2Cl2
afforded the corresponding alkylsulfonyl-substituted calix[4]ar-
Scheme 1 Reagents and conditions: (i) C6H13Br, NaH, DMF, 70 °C,
overnight; (ii) a, BunLi, THF, 278 °C, 1 h. b, B(OMe)3, 278 °C to rt, 1 h.
c, H+; (iii) IC6H4Br, 5 mol% Pd(OAc)2–2P(o-tol)3, K2CO3, toluene, 50 °C,
6 h; (iv) C6H13SH, NaOH, DMF, 150 °C.
† Electronic supplementary information (ESI) available: experimental
procedures and spectral data of all new compounds. See http://www.rsc.org/
suppdata/cc/b2/b210493h/
138
CHEM. COMMUN., 2003, 138–139
This journal is © The Royal Society of Chemistry 2003

framework show little interaction in the ground-state. In
contrast to the absorption behaviour, the red shift of the
emission maxima of the assemblies are substantial in both series
as compared to those of the corresponding monomer. This
implies that the excited state chromophoric interaction within an
assembly is significant. On the other hand, the excitonic
interaction only affects the fluorescence lifetimes (t) of the
assemblies in a lesser extent relative to the corresponding
monomer. For the hexylsulfanyl end-functionalized assemblies
9b,c, the fluorescence quantum yields (FFL) are found to be
substantially lower than that of the corresponding monomer
11b,c. It is consistent to the general observations that ordered
packing/arrangement of the emissive molecules or polymers
often leads to excimers formation and then fluorescence
quenching. However, the hexylsulfonyl end-functionalized
assemblies 10b,c consistently exhibit larger fluorescence quan-
tum yields than that of the corresponding monomer 12b,c. This
result implies that acentric arrangements of donor–acceptor
fluorophores will lead to fluorescence enhancement.
Scheme 2 Reagents and conditions: (i) C6H13S(C6H4)nB(OH)2, 40 mol%
Pd(OAc)2–2P(o-tol)3, K2CO3, toluene, 50 °C, 6 h; (ii) MCPBA, CH2Cl2, 0
°C, 1 h; (iii) BrC6H4B(OH)2, 40 mol% Pd(PPh3)4, K2CO3, toluene, 45 °C,
overnight; (iv) C6H13S(C6H4)n21B(OH)2, 40 mol% Pd(OAc)2–2P(o-tol)3,
K2CO3, toluene, 50 °C, 6 h.
In summary, we have developed a convergent approach using
Suzuki cross-coupling reaction to synthesize novel series of
highly extended tetra-oligophenylene substituted calix[4]arene
assemblies containing up to three phenylene units end-capped
with hexylsulfanyl donating groups or hexylsulfonyl with-
drawing groups. We have also found that, in contrast to the
electron-donor substituted assemblies, the hexylsulfonyl end-
functionalized assemblies exhibit fluorescence enhancement as
compared to the corresponding monomers. These findings may
provide a new avenue to design fluorophores and emissive
materials for luminescence applications.
This work was supported by Faculty Research Grant (FRG/
01-02/II-43) from Hong Kong Baptist University and Ear-
marked Research Grant (HKBU2019/02P) from Research
Grants Council, Hong Kong. We gratefully acknowledge Dr Y.
Tao at NRC, Canada for the MALDI-TOF mass spectrometry
measurements.
ene assemblies, 10a–c, respectively in moderate to good yields.
The MALDI-TOF mass spectra of 10b and 10c showed a major
peak at 2210 corresponding to [M + 2H + Na]+ and 2512
corresponding to [M + Na]+, respectively. In addition to low/
high resolution mass spectroscopy, all the newly synthesized
assemblies were characterized with 1H NMR and 13C NMR and
found to be in good agreement with expected structures.†
For comparison, the corresponding monomers 11a–c and
12a–c were also synthesized using the same convergent
approach. Dependent on the nature of substituents end-capped
on the oligophenylene skeletons, the optical properties of the
assemblies vary differently with respect to the corresponding
monomer. Even though the spectral features of the assemblies
and the corresponding monomer are similar, the assemblies 9a–
c bearing hexylsulfanyl donating groups exhibit a slightly blue
shift in the absorption maxima, lmax and the assemblies bearing
hexylsulfonyl withdrawing groups 10a–c consistently exhibit
red shift relative to the corresponding monomer (Table 1) This
suggest that oligophenylenes built onto the calix[4]arene
Notes and references
Table 1 Summaries of physical measurements of 9a–c, 10a–c, 11a–c and
12a–c
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a
a
labs
nm
/
104emax
/
max
a,b
Compound
M21 cm21
lem a/nm FFL
t/nsa,c
3 J. Rebek, Chem. Commun., 2000, 637.
max
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9a
9b
9c
283
301
312
284
303
313
287
307
315
282
302
312
10.12
14.89
22.65
6.99
11.04
15.26
2.37
3.47
5.07
1.91
3.22
379
384
386/398
373
412
427
372
375
379/390
366
0.82
0.84
0.85
1.02
1.03
1.01
0.76
0.78
0.80
1.34
1.38
1.37
0.12
0.31
10a
10b
10c
11a
11b
11c
12a
12b
12c
0.77
0.74
0.28
0.51
405
414
0.70
0.60
4.23
a Measured in CHCl3. b Average of two independent measurements and
using quinine in 1 M H2SO4 (F313 = 0.48 at 313 nm) as a standard. c Using
nitrogen laser as an excitation source.
CHEM. COMMUN., 2003, 138–139
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