Highly extended styrylstyrylcalix[4]arene assemblies: Synthesis and optical properties

Article abstract of DOI:10.1016/S0040-4039(00)00948-5

Novel highly extended, cone-conformed tetrakis- styrylstyryltetrapropoxycalix[4]arenes and bis- styrylstyryltetrapropoxycalix[4]arenes were synthesized by stereoselective Wadsworth-Emmons reactions. Intramolecular chromophoric interaction of these assemblies resulted in changes to optical properties relative to those of the corresponding monomers with fluorescence enhancement being observed for the assemblies end-capped with methylsulfonyl acceptors. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)00948-5

Tetrahedron Letters 41 (2000) 5719±5723
Highly extended styrylstyrylcalix[4]arene assemblies: synthesis
and optical properties
Man Shing Wong,* Zhong Hui Li and Chi Chung Kwok
Department of Chemistry, Hong Kong Baptist University, Kowloon Tong, Hong Kong, China
Received 14 March 2000; revised 2 June 2000; accepted 7 June 2000
Abstract
Novel highly extended, cone-conformed tetrakis-styrylstyryltetrapropoxycalix[4]arenes and bis-styryl-
styryltetrapropoxycalix[4]arenes were synthesized by stereoselective Wadsworth±Emmons reactions.
Intramolecular chromophoric interaction of these assemblies resulted in changes to optical properties
relative to those of the corresponding monomers with ¯uorescence enhancement being observed for the
assemblies end-capped with methylsulfonyl acceptors. # 2000 Elsevier Science Ltd. All rights reserved.
Keywords: calixarenes; ¯uorescence.
There has been increased interest into the investigation of intermolecular interactions as they
play an important role in the optical and electronic properties of materials. It has been shown
that there is a strong second harmonic signal generated from centrosymmetric squaranine dyes
1
which is attributed to an intermolecular charge±transfer interaction. In addition, the lower
photoluminescence eciency of a sexithiophene thin-®lm was attributed to the close packing of
0
the sexithiophene molecules in the solid state when compared to those of b,b -alkylsubstituted
2
3,4
sexithiophenes. There have been several theoretical studies addressing the in¯uence of inter-
molecular interactions on various physical properties; however, only few experimental studies
have been carried out so far. Therefore, an investigation of the intermolecular interactions in a
pre-oriented and pre-de®ned multi-chromophoric system would be of interest.
Calixarenes are one of the most useful building blocks for constructing supramolecular systems
and have been widely used in the preparation of arti®cial receptors for neutral and ionic molecules.
Recently, there has been some interest in the exploitation of extended calix[4]arenes for cavity-
5
6
7
enlarged receptors, second-order nonlinear optical materials and liquid crystals. One of the
unique features of these calix[4]arene derivatives is that they can exist in a relatively rigid and
stable cone structure, in which four functionalized phenolic units, oriented in the same direction
are linked at their ortho-positions by methylene bridges. With chromophores built onto this
framework, the arrangement and orientation of the chromophores may well be pre-de®ned and
pre-organized and the chromophores may be close enough to interact. This assembly may be
interesting enough for an investigation of the intra-chromophoric interactions.
*
Corresponding author. Tel: 852-2339-7069; fax: 852-2339-7348; e-mail: mswong@hkbu.edu.hk
0040-4039/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)00948-5

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To enhance the chromophoric interaction, a larger interacting area would be favourable. Here
we report the ®rst synthesis and optical properties of a novel class of highly extended, cone-conformed
calix[4]arene assemblies, tetrakis-styrylstyrylcalix[4]arenes, 1a±c and bis-styrylstyrylcalix[4]arenes,
1
d±f.
Stereoselective Wadsworth±Emmons reactions were used to construct the all-trans p-styryl-
styryl units as summarized in Scheme 1. Calix[4]arene 2 was alkylated with bromopropane using
Scheme 1. Syntheses of tetrakis-styrylstyryltetrapropoxycalix[4]arenes and bis-styrylstyryltetrapropoxycalix[4]arenes.
ꢀ
ꢀ
Conditions: (a) NaH, PrBr, DMF, 50 C; (b) NBS, acetone±CH
ꢀ
2
Cl
2
re¯ux; (c) t-BuLi, THF, ^78 C; (d) n-BuLi, THF,
^
78 C; (e) N-formylmorphine, then HCl; (f) NaH, DME, room temperature

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721
8
NaH as a base in DMF. Bromination of tetrapropoxycalix[4]arene with NBS in mixed solvent
acetone±CH Cl ) a€orded the bromocalixarene 3. Exhaustive lithiation of 3 using tert-butyl
(
2
2
9
lithium in THF followed by quenching with N-formylmorphine and hydrolysis a€orded tetra-
formyl-tetrapropoxycalix[4]arene 4. On the other hand, selective 1,3-formylation of 3 was
accomplished using n-butyllithium as a lithiation agent. Styrylbenzyl-phosphonates were
prepared either by a literature procedure10 or via Wadsworth±Emmons reactions of terephthal-
aldehyde monodiethylacetal and the benzyl-phosphonates followed by acidic work up which
a€orded the styrylbenzaldehydes. Reduction of styrylbenzaldehyde by NaBH , treatment of the
4
alcohol with SOCl and ®nally reaction with neat P(OEt) gave the required phosphonates. The
2
3
Wadsworth±Emmons reactions of 4 or 5 with the corresponding styrylbenzyl-phosphonates 6
gave the desirable tetrakis-styrylstyrylcalix[4]arenes, 1a±c or bis-styrylstyrylcalix[4]arenes, 1d±f in
good to moderate yields, respectively.11
It has been shown that in solution two rapidly inter-convertible and C2v symmetry related
pinched cone conformers of calixarenes are present.12 The cone conformation of the highly
1
extended tetrakis-styrylstyrylcalix[4]arenes in CDCl3 has been con®rmed by H NMR
measurements. On the other hand, the bis-styrylstyryl-calix[4]arenes exist in a pinched cone
conformation with the highly extended p-styrylstyryl units stretched out which was also found to
be the optimized geometry as calculated by the MM2 force®eld.
13
There is a consistent bathochromic shift of the absorption maxima of bis-styrylstyrylcalix[4]-
arenes (1±6 nm) and a signi®cant hypsochromic shift of the absorption maxima of tetrakis-
styrylstyrylcalix[4]arenes (6±14 nm) in various solvents when compared to those of the
corresponding monomers. (Table 1) Therefore, such shifts in absorption maxima of styrylstyryl-
calix[4]arenes are attributed to the excitonic interaction of proximate chromphores within the
calix[4]arene assembly.14 The average proximity of two co-facial p-styrylstyryl units in tetrakis-
Ê
styrylstyrylcalix[4]arenes is 3.4 A, approximated from the MM2-optimized geometry of the
pinched cone conformation, in which the absorption spectra are expected to shift hypsochromically
according to the excitonic model. On the other hand, the oblique p-styrylstyryl units adopt a
V-shaped arrangement in the pinched cone conformation of bis-styrylstyryl-calix[4]arenes
resulting in a bathochromic shift of absorption maxima. The nearest van der Waals distance of
Ê
the two stretched p-styrylstyryl units is about 5.5 A in its MM2-optimized geometry.
In contrast to the electronic absorption behaviour, the emission maxima of all styrylstyryl-
calix[4]arenes exhibit only bathochromic shifts (3±14 nm) in various solvents relative to those of
the corresponding monomers. In addition, their emission maxima show signi®cant solvato-
chromic e€ect particularly in the donor-acceptor series (Á55 nm) which suggests that the emission
states of these styrylstyrylcalix[4]arenes are relatively more polar than those of the corresponding
monomers. The relative ¯uorescence quantum yields in chloroform were determined by the
dilution method using the corresponding monomer as a reference as described by Parker et al.15
For styrylstyrylcalix[4]arenes end-capped with or without electron-donors (methysulfanyl groups)
the ¯uorescence quantum yields are generally lower than those of the corresponding monomers.
(
Table 1) The ¯uorescence quenching is likely to be due to the enhancement of the radiationless
14
triplet intersystem crossing in the coupled excitonic states. The quenching e€ect is more
detrimental to the bis-assembly than the tetrakis-assembly. However, styrylstyrylcalix[4]arenes
end-substituted with methylsulfonyl groups, electron accepting, exhibit a consistent ¯uorescence
enhancement in both the bis-assembly and tetrakis-assembly as compared to that of the monomer
(Table 1). Such an enhancement is presumably due to the retardation of the radiationless triplet
intersystem crossing in the coupled excitonic states.

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Table 1
Summaries of physical measurements of tetrakis-styrylstyrylcalix[4]arenes, 1a±c, bis-styrylstyrylcalix[4]arenes, 1d±f and
a�c
the corresponding monomers, 3,5-dimethyl-4-propoxydistyrylbenzene derivatives, 1g±i
In summary, a novel series of highly extended, cone-conformed tetrakis-styrylstyrylcalix[4]-
arenes and bis-styrylstyrylcalix[4]arenes has been synthesized for the investigation of intramole-
cular chromophoric interactions. Excitonic coupling has been evidenced in such assemblies
resulting in a spectral shift and a change of ¯uorescence quantum yield. Those shifts and changes
are greatly dependent upon the nature, arrangement and proximity of chromophores in these
assemblies. Importantly, we have found that there is a ¯uorescence enhancement in the tetra-
propoxy-styrylstyrylcalix[4]arene assemblies that are end-substituted with electron-accepting
methylsulfonyl groups.
Acknowledgements
This work was supported by Faculty Research Grants (FRG/98-99/II-10 and FRG/99-00/I-04)
from Hong Kong Baptist University.

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723
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1
1. Selected spectroscopic data: compound 1c: H NMR (270 MHz, CDCl
Hz, 8H), 7.29 (s, 16H), 7.13 (d, J=16 Hz, 4H), 7.00 (d, J=16 Hz, 4H), 6.85 (s, 8H), 6.87 (d, J=16 Hz, 4H), 6.74
3
) ꢀ 7.77 (d, J=8.5 Hz, 8H), 7.50 (d, J=8.5
(
(
d, J=16 Hz, 4H), 4.49 (d, J=13 Hz, 4H), 3.90 (t, J=7 Hz, 8H), 3.22 (d, J=13 Hz, 4H), 3.07 (s, 12H), 1.95±1.92
ꢀ
+
m, 8H) 1.01 (t, J=7 Hz, 12H), MS (FAB) m/z 1722 (M ), mp 215±220 C. Anal. calcd: C, 67.57; H, 5.70. Found:
1
C, 67.45; H, 5.70; compound 1f: H NMR (270 MHz, CDCl ) ꢀ 7.78 (d, J=8.5 Hz, 4H), 7.52 (d, J=8 Hz, 4H)
3
7
6
.31 (s, 8H), 7.14 (d, J=16 Hz, 2H), 7.02 (d, J=16 Hz, 2H), 6.86 (s, 4H), 6.82 (d, J=16.5 Hz, 2H), 6.77 (s, 4H),
.70 (d, J=16 Hz, 2H), 4.41(d, J=13 Hz, 4H), 3.87±3.81 (m, 8H), 3.14 (d, J=13.5 Hz, 4H), 3.08 (s, 6H), 1.94±1.86
+
ꢀ
(
m, 8H), 1.01±0.91 (m, 12H), MS (FAB) m/z 1314 (M ), mp 185±190 C.
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1
1
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