Preparation and photochemical properties of p-phenylene oligomers encapsulated within faujasite Y

Article abstract of DOI:10.1039/b314477a

Phenylene oligomers have been prepared by ship-in-a-bottle synthesis inside palladium containing basic zeolites by homocoupling of 1,4-phenylenediboronic acid. Product analysis and "in situ" IR spectroscopy support the formation of oligomers of three to f

Full text of DOI:10.1039/b314477a

View Article Online / Journal Homepage / Table of Contents for this issue
C O M M U N I C A T I O N
Preparation and photochemical properties of p-phenylene oligomers
encapsulated within faujasite Y
a
Mercedes Alvaro, Belen Ferrer, Hermenegildo Garcıa* and Antonio Leyva
b
b
b
´
´
´
a
´
Departamento de Quımica, Universidad Politecnica de Valencia, Avda. de Los Naranjos s/n,
46022, Valencia, Spain
´
b
´
Instituto de Tecnologıa Quımica CSIC-UPV, Avda. de Los Naranjos s/n, 46022,
Valencia, Spain. E-mail: hgarcia@quim.upv.es
´
Received11th November 2003, Accepted 8th December 2003
F|rst published as an AdvanceArticle on the web16th December 2003
Phenylene oligomers have been prepared by ship-in-a-bottle
synthesis inside palladium containing basic zeolites by homo-
coupling of 1,4-phenylenediboronic acid. Product analysis and
‘‘in situ’’ IR spectroscopy support the formation of oligomers
of three to five phenylene rings. The encapsulated oligomers emit
the characteristic blue light upon excitation and laser flash
photolysis reveals the formation of long-lived transient decaying
in hundreds of ls that has been attributed to the corresponding
triplet excited state.
Pd-complexes as catalysts in basic media.4,5 Recently we have
supported palladium species on zeolites and used the resulting
solids as heterogeneous catalysts for the Suzuki–Miyaura
cross coupling.6 Moreover, by using basic zeolites we have
demonstrated that the Suzuki–Miyaura reaction catalysed by
zeolites supporting palladium salts does not require the pre-
sence of extrinsic base.6 In particular, potassium exchanged
Y zeolite (KY) was found to exhibit an adequate basic
strength for the reaction of bromobenzene and phenylboronic
acid.6 In view of the activity of these zeolites to effect the
Suzuki–Miyaura reaction it occurred to us that it should be
possible to apply this strategy to effect the ship-in-a-bottle
synthesis of phenylene oligomers entrapped inside the zeolite
cavities.
Introduction
Encapsulation of organic compounds inside the rigid pores of
zeolites has become a routine tool in supramolecular chemistry
to alter and control the photophysical properties of incarcer-
ated guest.1 Particularly challenging is when the organic guest
is too large to be adsorbed from the exterior into the internal
pores. In this case, ship-in-a-bottle methodologies have to be
devised to achieve the in situ synthesis of the target guest. This
is the case of organic polymers or oligomers encapsulated
inside zeolites. Polyphenylene oligomers can exhibit intense
electroluminescence2,3 and it would be of interest to study their
properties when encapsulated within zeolites. Concerning the
emission efficiency it had been demonstrated that the confor-
mational mobility of the phenylene rings permitting the ring
twisting is one of the major radiationless deactivation
pathways causing the decrease of emission quantum yields.
To overcome this limitation a strategy has consisted in the
chemical modification of the polymers by putting methylene
bridges between two consecutive phenylene rings (polyfluorene
derivatives) that thwart the mobility of the rings.
An alternative to the modification of the phenylene by sub-
stitution that would require more elaborated starting materials
would be its incorporation inside the rigid framework of
microporous hosts. As a general phenomenon it has been
observed that incorporation of a guest inside the zeolite micro-
pores disfavors radiationless deactivation pathways arising
from conformational mobility.1 Herein, we describe the in situ
synthesis of p-phenylene oligomers (PP) inside the cavities of
faujasite-Y and the photochemical properties of the resulting
encapsulated oligomers.
ð1Þ
Eqn. (1) shows the reaction employed for the synthesis of
p-phenylene oligomers. The reaction was carried out in toluene
at 110ꢀ and its progress was monitored by observing in the
liquid phase the formation of terphenylene and p-tetraphenyl-
ene derivatives. We expected that the same type of compounds
were also formed within the interior of the pores and that due
to the linear geometry and rigidity of these compounds, the
diffusion through the pores will be impeded. If this were the
case then a certain amount of oligomers formed in the zeolite
cavities would become imprisoned inside the pores without any
possibility to diffuse to the exterior of the particle. To support
this likely possibility, a sample of KY zeolite after being used
for the reaction of 1,4-phenylenediboronic acid was dissolved
using concentrated HF acid and the resulting liquor extracted
with dichloromethane in order to recover the organic material
adsorbed within the pores. This extract was analysed by
1
CG-MS, FAB-MS and H-NMR.
The GC-MS after dissolving the solid reveals a complex
mixture, mostly being terphenylene (80%) accompanied with
tetraphenylene (10%) and other oligomers. Deboronation cata-
lyzed by Pd occurs concomitantly with the Suzuki homocoup-
ling. In the FAB-MS of the liquor after dissolving the zeolite,
peaks corresponding to up to 5 aromatic rings (detected in the
Results and discussion
1
In order to effect the p-phenylene coupling one of the current
most versatile reactions consists in the Suzuki–Miyaura homo-
coupling of 1,4-phenylenediboronic acid in toluene using
GC-MS) were observed. H-NMR of the organic extract after
dissolving the PdCl2–KY also shows the presence of aromatic
protons appearing as unresolved peaks between 7.8 and
T h i s j o u r n a l i s Q T h e O w n e r S o c i e t i e s 2 0 0 4
P h y s . C h e m . C h e m . P h y s . , 2 0 0 4 , 6 , 2 0 1 – 2 0 4
201

View Article Online
7.2 ppm. The presence of a residual amount of acid protons of
the boronic acid groups was also assessed by a signal around
9.0 ppm.
Concerning the size of the oligomers product analysis sug-
gests a distribution of short oligomers of 3, 4 and 5 rings with
an approximate distribution of 70, 15 and 5%, respectively.
Taking into account the rigidity of the polyphenylene oligo-
mers, its linear geometry as well as the geometry of the fauja-
site micropores formed by cavities tetrahedrally arranged, the
most likely possibility is that the phenylene oligomers occupy
˚
two neighbour cavities (around 26 A length). Molecular mod-
elling using the optimized geometry for zeolite and phenylene
oligomers indicates that the maximum number of polyphenyl-
ene units that can be accommodated inside the zeolite cavities
corresponds to 5. This prediction would be compatible with
the product distribution observed upon dissolving the solid with
HF in which the maximum number of rings observed was 5.
Spectroscopic characterisation of the PP-loaded PdCl2–KY
zeolite is also compatible with the presence of phenylene oligo-
mers. In particular, FT-IR spectroscopy of the PP loaded zeo-
lite after outgassing at 300 ꢀC to remove co-adsorbed water
and volatile solvents also shows the presence of aromatic rings
as indicated by the characteristic vibration bands appearing
from 1700 to 1500 cmꢁ1. Fig. 1 shows the aromatic region of
the PP loaded PdCl2–KY compared to the IR of an authentic
sample of PP prepared by conventional synthetic procedures
using Pd2(dba)3 as catalyst under the reaction conditions
analogous to those used for the preparation of the zeolite
encapsulated oligomers.
Diffuse-reflectance UV-Vis spectroscopy of the zeolite after
the Suzuki–Miyaura coupling also reveals the presence of aro-
matic compounds adsorbed in the solid exhibiting as the most
intense absorption band at 260 nm together with other less
intense bands around 350 nm.
The PP loaded PdCl2–KY exhibits the characteristic PP
photoluminescence peaking at 465 nm when the sample is
excited at 380 nm. Fig. 2 shows the corresponding emission
and excitation spectra recorded for the zeolite encapsulated
PP. While the maximum at 480 nm and the corresponding
shoulders are due to the PP emission, the peak at 620 nm cor-
responds to an optical artefact due to the front-face arrange-
ment used to measure the emission in opaque powders.
Oxygen purging does not noticeably decrease the emission
intensity and, therefore, the encapsulated PP is a fluorescent
Fig. 2 Emission (lex 380 nm) and excitation (lem 465 nm) spectra of
PP incorporated within PdCl2–KY.
solid in the open air. According to the data reported in the
literature7 the emission and excitation spectra shown in Fig. 2
is very similar to that of pure PP, although the emission of
encapsulated PP is somewhat broader and less vibrationally
resolved than that observed for pure PP thin films. However,
it has to be noted that according to the literature the photo-
luminescence of pure PP becomes broader when the emission
is recorded for powders rather than for films.7
The sample of PP encapsulated in zeolite was submitted to
laser flash photolysis. This photochemical technique is comple-
mentary to photoluminescence studies, allowing the detection
of photochemical generated transient species even if they are
not emissive. Upon 266 nm laser excitation a diffuse reflectance
absorption spectrum decaying in the ms time-scale was
recorded (Fig. 3). Analysis of the signal temporal profile at dif-
ferent wavelengths (inset of Fig. 3) shows that the decays are
coincident suggesting that the transient spectrum corresponds
to a single species.
Given the experimental difficulty in performing quenching
studies in solid samples, the nature of the transient generated
upon laser flash photolysis on zeolites is generally addressed
by comparing the optical spectrum recorded in zeolites with
those obtained for the same compound in solution.1,8 Much
to our surprise a search through the chemical database reveals
that the laser flash photolysis study of PPP has never been
previously reported. Therefore, in order to address the nature
Fig. 3 Diffuse-reflectance UV-Vis transient spectra recorded for
nitrogen purged PP incorporated within KY after 266 nm laser excita-
tion. The spectra have been recorded at 35 (S), 100 (X), 160 (L) and
235 (K) ms delay. The inset shows the signal decay monitored at 460
(S), 570 (X) and 6850 (K) nm, indicating that the whole spectrum
corresponds to a singlet transient.
Fig. 1 IR spectra of the PP recorded at room temperature in KBr (ꢁa2)
and PP loaded PdCl2–KY recorded after outgassing at 300 ꢀC at 10
Pa (b).
T h i s j o u r n a l i s Q T h e O w n e r S o c i e t i e s 2 0 0 4
202
P h y s . C h e m . C h e m . P h y s . , 2 0 0 4 , 6 , 2 0 1 – 2 0 4

View Article Online
of the transient shown in Fig. 3 and considering that our pre-
vious characterisation points to phenylene oligomers as the
most likely guests inside zeolite-Y, we submitted to laser flash
photolysis p-tetraphenyl in acetonitrile solution whereby a
transient peaking at 465 nm similar to that recorded in zeolite
was observed. p-Tetraphenyl is easily obtained by Suzuki–
Miyaura coupling of 4-biphenyl boronic acid and 4-bromo-
biphenyl. Fig. 4 shows the transient spectrum recorded for
p-tetraphenyl in acetonitrile at different delays after the laser
pulse. In addition to the transient, fluorescence appearing at
negative absorption at about 340 nm was also recorded
at short times. As expected, the presence of intense emission
(lmax 380 nm) upon 266 nm excitation was observed indepen-
dently in a fluorescence measurement in acetonitrile. This emis-
sion of p-tetraphenylene is compatible with the emission
observed for the KY zeolite (shown in Fig. 2). The transient
at lmax ¼ 475 nm observed in the laser flash photolysis decays
in the microsecond time scale with a half-life of 24 ms. This
transient was safely attributed to the triplet excited state based
on the oxygen quenching. In addition, other possible transient
species and, in particular, the radical cation, must exhibit a
very different optical spectrum according to the literature for
related biphenyl.9
Experimental
The reagents and solvents were obtained from commercial
sources and used as received. The KY zeolite was prepared
through two consecutive ion exchanges starting from a 1 M
solution of potassium acetate in bidistilled water and NaY-
zeolite (CBV-100, PQ industries). A dispersion of 1 g of zeolite
in 5 mL of solution was stirred for 5 h at 80ꢀ. Chemical analy-
sis reveals that the level of Na+-to-K+ exchange was 85%. The
PdCl2–KY was obtained by the incipient wetness methodol-
ogy. In this methodology, the required PdCl2 amount to
achieve the target loading 1% (16 mg ꢂ gꢁ1 of zeolite) of palla-
dium in the support was dissolved in the volume corresponding
to the pore of the support (1 mL) in order to fill all the voids of
the solid without loss of metal.
Synthesis of PP loaded PdCl2–KY
1,4-Phenylenediboronic acid (146.4 mg, 0.6 mmol) in toluene
(20 mL) was magnetically stirred at 110 ꢀC for 48 h in the pre-
sence of 2 g of PdCl2–KY. The solution was filtered hot and
the solid was submitted to Soxhlet extraction (ethanol and
dichloromethane). The solid was dried under reduced pressure
(10ꢁ1 bar) for 6 h.
In the case of the transient recorded for PPP encapsulated in
KY zeolite, the presence of oxygen only exerts a minor influ-
ence on the kinetics, but this is not without precedent consider-
ing the impeded diffusion of oxygen through the micropore of
hydrated zeolites.1,8 Therefore, based on the similarity of the
optical spectra in zeolite and acetonitrile solution for p-tetra-
phenyl, the transient observed in the laser flash photolysis of
zeolites was attributed to the corresponding triplet excited
state of encapsulated phenylene oligomers. The minor differ-
ences between the spectrum of p-tetraphenyl and that recorded
for PP in KY can be due to the presence of a distribution of
oligophenylenes and/or the presence of residual Pd atoms on
the zeolite.
Synthesis of non-encapsulated PP
For the sake of comparison a sample of pure non-encapsulated
PP was obtained using the same conditions as those for PP
loaded PdCl2-KY, using PdCl2 (20 mol%) as palladium
reagent and K2CO3 as base. When polymer precipitation was
observed, the solution was filtered hot and the obtained solid
was washed in boiling water until no dissolved palladium
species were observed in the aqueous phase. The black solid
was dried under reduced pressure (10ꢁ1 bar) for 6 h and the
chemical combustion analysis was found C6H4 .
Synthesis of p-tetraphenyl
4-Bromobiphenyl (1 mmol) and 4-biphenyl boronic acid (1.25
mmol) were dissolved in acetonitrile (10 mL). Then, NaAcO
(1.5 mmol) and Pd2(dba)3 (0.05 mmol) were introduced in a
vessel and the solution was added. The mixture was placed
in a preheated oil bath at 60 ꢀC and stirred magnetically for
24 h. After cooling, the mixture was filtered and the solvent
was removed under vacuum. The extract was dissolved in
dichloromethane and extracted with water and brine. The
organic phase was dried, the solvent evaporated under vacuum
and the residue submitted to column chromatography.
The product was characterised by GC-MS, 1H-NMR and
13C-NMR.
Conclusion
By applying the Suzuki–Miyaura conditions using a palla-
dium-containing basic zeolite, it has been possible to produce
a zeolite that incorporates phenylene oligomers. The solid
exhibits the characteristic photoluminescence spectrum of
polyphenylene polymer although somewhat broader. Compar-
ison of the laser flash photolysis spectrum of p-phenylene
oligomers encapsulated within Y zeolite with the spectrum
recorded for p-tetraphenyl has allowed assigning the transient
to the corresponding triplet excited state.
Photophysical measurements
Fluorescence spectra were recorded on an Edinburgh Analyti-
cal Instruments FL900 spectrophotometer. Laser flash photo-
lysis experiments were carried out using the fourth (266 nm,
ꢃ20 mJ ꢂ pulseꢁ1) harmonic of a Surelite Nd-YAG laser for
excitation (pulse ꢃ10 ns). The signal from the monochroma-
tor/photomultiplier detection system was capture by a Tektro-
nix 2440 digitizer and transferred to
a computer that
controlled the experiment and provided suitable processing
and data storage capabilities.10
Acknowledgements
Financial support of the Spanish DGES (Project MAT2003-
1226) is gratefully acknowledged. The Spanish ministry of edu-
cation is also thanked for two postgraduate scholarships to
B.F. and A.L. We thank to J. F. Cabezas for his assistance
in the laser flash photolysis.
Fig. 4 Diffuse-reflectance UV-Vis transient spectra recorded for
argon purged p-tetraphenyl in acetonitrile after 266 nm laser excita-
tion. The spectra have been recorded at 2.4 (a), 6.8 (b), 24 (c) and 64
(d) ms delay.
T h i s j o u r n a l i s Q T h e O w n e r S o c i e t i e s 2 0 0 4
P h y s . C h e m . C h e m . P h y s . , 2 0 0 4 , 6 , 2 0 1 – 2 0 4
203

View Article Online
6
7
8
9
A. Corma, H. Garcia and A. Leyva, Appl. Catal. A, 2002, 236,
179–185.
C. H. Lee, G. W. Kang, J. W. Jeon, W. J. Song and C. Seoul, Thin
Solid Films, 2000, 363, 306–309.
H. Garcia and H. D. Roth, Chem. Rev., 2002, 102, 3947–
4007.
References
1
2
A. B. Holmes, A. Kraft and A. C. Grimsdale, Angew. Chem. Int.
Ed., 1998, 37, 402–428.
G. Wegner, T. Pakula, T. F. McCarthy and H. Witteler, Macro-
molecules, 1995, 28, 8350–8362.
A. Moissette, H. Vezin, I. Gener, J. Patarin and C. Bremard,
Angew. Chem. Int. Ed., 2002, 41, 1241–1244.
10 L. M. Hadel, Handbook of Organic Chemistry, CRC Press, Boca
3
4
5
J. C. Scaiano and H. Garcia, Acc. Chem. Res., 1999, 32, 783–793.
J. K. Kochi, J. Organomet. Chem., 2002, 653, 11–19.
M. Kotora and T. Takahashi, in Handbook of Organopalladium
Chemistry for Organic Synthesis, 2002, vol. 1, pp 973–993.
Raton, 1989.
T h i s j o u r n a l i s Q T h e O w n e r S o c i e t i e s 2 0 0 4
204
P h y s . C h e m . C h e m . P h y s . , 2 0 0 4 , 6 , 2 0 1 – 2 0 4