Click chemistry for photonic applications: Triazole-functionalized platinum(ii) acetylides for optical power limiting

Article abstract of DOI:10.1039/b711269f

Three different triazole-containing platinum(ii) acetylide compounds were synthesized by click chemistry and evaluated for their use in optical power limiting (OPL) applications. The triazole unit was incorporated at three different positions within, or a

Full text of DOI:10.1039/b711269f

View Article Online / Journal Homepage / Table of Contents for this issue
PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Click chemistry for photonic applications: triazole-functionalized
platinum(II) acetylides for optical power limiting
Robert Westlund,a Eirik Glimsdal,b Mikael Lindgren,b Robert Vestberg,c Craig Hawker,c Cesar Lopesd and
Eva Malmstro¨m*a
Received 23rd July 2007, Accepted 18th October 2007
First published as an Advance Article on the web 29th October 2007
DOI: 10.1039/b711269f
Three different triazole-containing platinum(II) acetylide compounds were synthesized by click
chemistry and evaluated for their use in optical power limiting (OPL) applications. The triazole
unit was incorporated at three different positions within, or at the end of, the conjugation path of
the chromophore. The aim is to explore the possibilities of using click chemistry to prepare
dendronized chromophores, and to evaluate how the triazole structure affects the photophysical
properties and the optical power limiting abilities of these acetylide compounds. It is shown that
the concept of click chemistry can be used to attach branched monomer units to ethynyl-phenyl
arms by Huisgen 1,3-dipolar cycloaddition, forming triazole units within the chromophore.
Photophysical characterization of these triazole-containing materials shows an absorption
maximum within the UV-A region and emission through both fluorescence and phosphorescence.
Bright phosphorescence was emitted from argon purged samples, and decay measurements
thereof showed triplet lifetimes of up to 100 ms. The results from the photophysical
characterization suggest that the triazole does break the conjugation path, and in order to gain
maximum optical limiting the triazole needs to be placed at the end of the conjugation. All three
investigated triazole-containing platinum(II) acetylides show good optical power limiting at
532 nm (10 ns pulse, f/5 set-up, 2 mm cells). The most efficient compound, with the triazole
positioned at the end of the conjugation, reaches a defined clamping level of 2.5 mJ for a sample
with a concentration of 50 mM in THF and a linear transmission above 80% at 532 nm. These
data can be compared to the OPL properties of Zn-based porphyrins or derivatized thiophenes,
reaching clamping levels of 6–15 mJ.
intersystem crossing (ISC),12–15 addition of donor/acceptor
Introduction
moieties,7,16 dendritic capping,17,18 or sol–gel processing
units.19,20 NLO chromophores suitable for sensor protection
The use of lasers has increased over the years and likewise
there has also been
a rapid development of powerful
require certain factors to be considered. Firstly, the chromo-
phore must possess efficient nonlinear absorption for high-
intensity light, thus limiting the transmission of high-intensity
light to a low level, while the transmission of low-intensity light
is kept high. Secondly, the nonlinear response must be fast,
within picoseconds.2,3
wavelength-agile lasers.1 Consequently, there is an increasing
interest in research and development of new, efficient optical
power limiting (OPL) materials2–4 in order to meet safety
requirements and potential laser threats. Nonlinear optical
(NLO) limiting materials5,6 can be used to reduce the trans-
mission of high-intensity light. There are several mechanisms
that may give rise to nonlinear absorption, for example two
photon absorption (TPA)7–9 or excited state absorption
(ESA).10,11 Organic chromophores consisting of p-conjugated
aromatics can possess such NLO properties and novel
synthetic routes are being developed to enhance the OPL
function and to simultaneously add additional functionalities,
for example, increase of p-conjugation length,3,12 incorporat-
ing heavy atoms to gain spin–orbit coupling for enhanced
Earlier studies pioneered by McKay and Staromlynska
et al.,21–23 and by the present authors,17,18 have shown that
platinum(II) acetylides possess good optical power limiting
properties due to efficient excited state absorption. It has also
been shown that by adding dendritic substituents to the Pt
chromophore, the OPL properties can be enhanced greatly due
to increased site isolation.18 The larger the dendritic sub-
stituents on the chromophore, the better clamping levels could
be reached. Thus, the dendrimer here provides the ability
to create a micro-environment for the core-molecule,24 and
acts as a ‘‘molecular bumper’’, minimizing quenching of
excited triplet states by collisions with other solute molecules
or oxygen.18,25
aKTH Fibre and Polymer Technology, Royal Institute of Technology,
Teknikringen 56-58, SE-100 44 Stockholm, Sweden.
E-mail: mave@polymer.kth.se; Fax: +46 8790 8283; Tel: +46 8790 8273
bDepartment of Physics, Norwegian University of Science and
Technology, NO-7491 Trondheim, Norway
Click chemistry reactions are usually defined as regiospecific
reactions resulting in easily isolated products in quantitative
yields.26 Triazole formation through 1,3-dipolar cyclo-
addition27 of azides and alkynes is a commonly used click
cMaterials Research Laboratory, University of California, Santa
Barbara, CA 93106, USA
dDepartment of Functional Materials, Swedish Defence Research
Agency, SE-581 11 Linko¨ping, Sweden
166 | J. Mater. Chem., 2008, 18, 166–175
This journal is ß The Royal Society of Chemistry 2008

View Article Online
reaction and it can be used successfully to build up
dendrimers,28,29 or dendronized polymers30 due to mild
reaction conditions and compatibility with a wide variety of
functional groups. In addition, the introduced triazole ring is
chemically and thermally stable while also possessing a large
dipole moment, features that may be beneficial in OPL
applications. The increased number of end-groups in dendri-
mers is especially useful in the preparation of solid state
materials where functionalization of the chromophore is
necessary in order to obtain well dispersed hybrid materials.19
The aim of this study has been to investigate the ability to use
‘click chemistry’ in order to synthesize platinum(II) acetylides
functionalized with first generation 2,2-bis(methylol)propionic
acid (bis-MPA) dendrons, targeted for OPL. Specifically, we
used the triazole-forming click reaction to attach branched
monomer units to ethynyl-phenyl arms to prepare a set of
model chromophores. The motive for using click chemistry
was to efficiently introduce the required molecular building
blocks for construction of reactive OPL derivatives while at
the same time introducing functional triazole units. The
advantage of using click chemistry is that the dendritic units
may be pre-synthesized and then added to the phenyl-ethynyl
ligands. Previously, dendrons were grown divergently from
the acetylide arms18 and hence, for each generation being
synthesized, precious amounts of acetylide arms were lost due
to chromatographic purification.
Fig. 1 Experimental set-up for time-resolved and luminescence mea-
surements. The frequency doubled (SHG) beam (exiting SHG/THG) is
used for single-photon excitation, and the fundamental beam (exiting
the pulse picker) for two-photon measurements.
emission lifetimes were measured using the luminescence
spectrometer in time-correlated single photon counting (TC-
SPC) mode. The IBH TB-01 module (optical trigger) was used
as time-reference in those experiments, using a thin glass wedge
to take out a small part of the fundamental. Pseudo-steady
state emission spectra were recorded by scanning the mono-
chromator in front of the photomultiplier tube (PMT).
Fluorescence quantum efficiencies were found from
relative measurements in low concentration samples (absorp-
tion , 0.1 OD) by comparing the results to a known reference
material, in a similar way to the method of Williams et al.,31
and hence, the absolute quantum yield value could be
calculated. The chromophore coumarin 110 (C110) was used
as both quantum efficiency reference material, and as a
reference in the two-photon measurements. C110 has pre-
viously been characterized and found to have a quantum yield
of 0.60 in THF solvent, when compared to the well known
quantum yield reference quinine sulfate.32 The two-photon
absorption cross section of C110 was found to be approxi-
mately 7 GM at 720–760 nm (1 GM = 10250 cm4 s photon21).
Optical limiting measurements were conducted in a f/5 set up
with a frequency doubled Nd : YAG laser operating at 10 Hz,
delivering 5 ns pulses. Measurements were performed using
2 mm quartz cells and THF as solvent (50 mM solutions).
MALDI-TOF spectra were recorded on a Bruker UltraFlex
MALDI-TOF mass spectrometer with a SCOUT-MTP ion
source (Bruker Daltronics) equipped with a nitrogen laser
(337 nm), a gridless ion source and a reflector. A THF solution
of 9-nitroanthracene and trifluoroacetic acid sodium salt was
used as matrix.
By employing click chemistry, three different structures of
platinum(II) acetylides containing triazole units were success-
fully synthesized. Herein, we also investigate the effect of the
triazole unit on the photophysical properties and the OPL
performance of these modified platinum(II) acetylides.
Experimental
1H and 13C NMR spectra were recorded on a Bruker AM 400
using acetone-d6, DMSO-d6 or CDCl3 as solvent. The solvent
signal was used as internal standard. Fourier transformed
infrared spectra (FTIR) were recorded on a Perkin-Elmer
Spectrum 2000 FT-IR instrument using a MKII Golden Gate,
single reflection ATR system.
Steady state absorption spectra were recorded using a
Shimadzu UV-1601PC Spectrometer. Measurements were
performed with samples diluted to 10 mM in tetrahydrofuran
solvent (THF, ¢99.5% spectrophotometric grade, from
Sigma-Aldrich) using 10 mm quartz cells (Hellma Precision).
Luminescence spectra and excited state lifetimes were recorded
using
a Jobin Yvon IBH FluoroCube photon-counting
spectrometer as detection unit. A titanium : sapphire laser
(Coherent Mira 900-f) was used as radiation source in all
experiments. The fundamental output from the mode-locked
laser is pulses of approximately 200 femtosecond duration and
76 MHz pulse repetition frequency (prf). The pulse repetition
frequency was controlled by an acousto-optic modulator
(Coherent 9200 Pulse Picker) between 9 kHz and 4.75 MHz.
Single-photon excitation experiments were performed by fre-
quency doubling of the fundamental output, to below 400 nm
using a SHG crystal (Inrad Ultrafast Harmonic Generation
System, Model 5-050). A schematic view of the experimental
set-up is shown in Fig. 1. Fluorescence and phosphorescence
Materials
2,2-Bis(methylol)propionic acid (bis-MPA) was supplied by
Perstorp AB, Sweden. The acetonide protected bis-MPA and
the anhydride were synthesized according to Malkoch et al.33
Cu(PPh3)3Br,34
4-(4-iodophenyl)-2-methyl-3-butyn-2-ol,18
Pt(PBu3)2Cl2 (trans configuration),18 and 4-(dimethylamino)
pyridinium 4-toluenesulfonate35 (DPTS) were synthesized as
This journal is ß The Royal Society of Chemistry 2008
J. Mater. Chem., 2008, 18, 166–175 | 167

View Article Online
described previously. All other reagents were obtained from
Aldrich and used as received.
Pt(PBu3)2Cl2 (1.19 g; 1.79 mmol) were dissolved in 50 mL
THF–triethylamine (TEA, 50 : 50). The reaction was heated to
60 uC and a catalytic amount of CuI was added. The reaction
was stirred at 60 uC for 20 min and followed by TLC. When
complete, solvents were evaporated off and the crude reaction
mixture was redissolved in DCM and extracted twice with
NaHSO4 (10%). The organic phase was dried over MgSO4 and
purified by flash chromatography, eluting with ethyl acetate–
heptane starting from 20 : 80 and gradually increasing to
70 : 30. The product was obtained as a pale yellow solid in 79%
yield. 1H NMR (CDCl3): d 0.86 (t, 18H, J = 8.0, –CH2–CH3),
1.07 (s, 6H, –CH2–C–CH3), 1.36 (s, 6H, O–C–CH3), 1.44 (s,
6H, O–C–CH3), 1.45 (m, 12H, –CH2–CH3), 1.56 (m, 12H,
CH2–CH2–CH3), 2.08 (m, 12H, –P–CH2–), 2.28 (m, 4H,
–CH2–CH2–O–), 3.61 (d, 4H, J = 12.0 Hz, –C–CH2–O–),
4.15 (d, 4H, J = 12.0 Hz, –C–CH2–O–), 4.16 (t, 4H, J = 6.0 Hz,
–N–CH2), 4.47 (t, 4H, J = 6.0 Hz, –CH2–CH2–O–), 7.62 (d,
4H, J = 10.0 Hz, Ar–H), 7.62 (d, 4H, J = 10.0 Hz, Ar–H), and
7.79 (s, 2H, –N–CH–). 13C NMR (CDCl3): d 13.78, 18.31,
20.94, 23.91, 24.37, 26.33, 26.40, 29.42, 42.13, 46.74, 61.14,
66.18, 98.13, 109.00, 109.79, 119.65, 125.21, 126.80, 128.90,
131.10, 147.93, 174.11. FTIR: nC–H 2800–3000 cm21, nPt–CMC
2099 cm21, nCLO 1723 cm21, nCLC Ar 1608 cm21, nC–O–C–O–C
Synthesis of chloro-functional bis-MPA (1). 3-Chloro-1-pro-
panol (5.0 g; 52.9 mmol) and acetonide protected bis-MPA
(11 g; 63.5 mmol) was dissolved in DCM. DPTS (3.11 g;
10.6 mmol) and N,N9-dicyclohexylcarbodiimide (DCC, 13.1 g;
63.5 mmol) were added and the reaction was stirred at room
temperature for 24 h. DCC-urea was filtered off and the
organic solution was extracted 3 times with Na2CO3 (10%) and
once with brine. The organic phase was dried over MgSO4 and
purified by flash chromatography, starting with pure heptane
and gradually increasing polarity to 20 : 80 ethyl acetate–
1
heptane. Yield 66%. H NMR (CDCl3): d 1.03 (s, 3H, –CH2–
C–CH3), 1.27 (s, 3H, O–C–CH3), 1.31 (s, 3H, O–C–CH3), 2.03
(m, 2H, –CH2–CH2–O–), 3.54 (t, 2H, –CH2–Cl), 3.55 (d, 2H, J =
12.0 Hz, –C–CH2–O–), 4.08 (d, 2H, J = 12.0 Hz, –C–CH2–O–),
4.19 (t, 2H, J = 6.0 Hz, –CH2–CH2–O–). 13C NMR (CDCl3): d
18.43, 22.08, 25.29, 31.48, 42.29, 61.49, 66.06, 98.03, 174.07.
Azide-functional bis-MPA (2). Chloro-functional bis-MPA 1
(8.7 g; 34.7 mmol) was dissolved in 60 mL DMSO. Sodium
azide (5.64 g; 86.9 mmol) was added and the reaction was
stirred at 80 uC for 24 h. 10 mL of H2O and 20 mL of ether
were added. The water phase was removed and the organic
phase was further extracted twice with water. The organic
phase was dried over MgSO4 and concentrated, yielding 98%
1079 cm21, n1,4-disubst. benzene 842 cm21, nC–O–C–O–C 828 cm21
MALDI-TOF: calculated Mw + Na+ = 1386.61 g mol21, found
Mw + Na+ = 1386.77 g mol21
.
.
1
of a clear liquid. H NMR (CDCl3): d 1.11 (s, 3H, –CH2–C–
4-Azidobenzyl alcohol (4). 4-Bromobenzyl alcohol (1.0 g;
5.34 mmol), sodium azide (0.87 g; 13.36 mmol), CuI (0.10 g;
0.53 mmol), L-proline (0.184 g; 1.60 mmol) and sodium
hydroxide (64 mg; 1.60 mmol) were dissolved in 10 mL of
ethanol–water (70 : 30). The reaction was run at 95 uC for 24 h.
Additional water (5 mL) was added and the reaction was
stirred for another 30 min. The water solution was extracted
3 times with ether. The collected organic phases were then
extracted twice with water and twice with NaHSO4 (10%).
Finally the organic phase was dried with MgSO4 and solvents
were evaporated off. Yield 78%. 1H NMR (CDCl3): d 2.79
(s, 1H, –OH), 4.56 (s, 2H, –CH2–), 6.97 (d, 2H, J = 8.8 Hz,
Ar–H), 7.28 (d, 2H, J = 8.8 Hz, Ar–H). 13C NMR (CDCl3): d
64.31, 118.93, 128.38, 137.40, 139.14.
CH3), 1.33 (s, 3H, O–C–CH3), 1.36 (s, 3H, O–C–CH3), 1.88 (m,
2H, –CH2–CH2–O–), 3.35 (t, 2H, –CH2–N3), 3.58 (d, 2H, J =
12.0 Hz, –C–CH2–O–), 4.13 (d, 2H, J = 12.0 Hz, –C–CH2–O–),
4.19 (t, 2H, J = 6.0 Hz, –CH2–CH2–O–). 13C NMR (CDCl3): d
18.43, 21.98, 25.16, 28.10, 41.89, 48.00, 61.59, 66.00, 98.00,
174.02.
General procedure for triazole formation, exemplified with the
synthesis of compound 3. Azide-functional bis-MPA 2 (2.04 g;
7.92 mmol) and 1,4-diethynyl benzene (1.0 g; 7.92 mmol) were
dissolved in 20 mL THF. Cu(PPh3)3Br (0.74 g; 0.792 mmol)
and diisopropyl ethylamine (DIPEA, 1.02 g; 7.92 mmol) were
added and the reaction was stirred at room temperature for
24 h. Solvent was evaporated off and the crude reaction
mixture was redissolved in DCM and extracted twice with
NaHSO4 (10%). The organic phase was dried with MgSO4 and
then purified with flash chromatography, eluting with hexane,
gradually increasing to pure ethyl acetate. The product was
obtained as a pale brown solid in 52% yield. 1H NMR
(CDCl3): d 1.11 (s, 3H, –CH2–C–CH3), 1.36 (s, 3H, O–C–
CH3), 1.44 (s, 3H, O–C–CH3), 2.34 (m, 2H, –CH2–CH2–O–),
3.28 (s, 1H, –CMC–H), 3.66 (d, 2H, J = 12.0 Hz, –C–CH2–O–),
4.21 (d, 2H, J = 12.0 Hz, –C–CH2–O–), 4.31 (t, 2H, J = 6.0 Hz,
–CH2–N), 4.61 (t, 2H, J = 6.0 Hz, –CH2–CH2–O–), 7.62 (d,
2H, J = 10.0 Hz, Ar–H), 7.82 (d, 2H, J = 10.0 Hz, Ar–H), and
8.09 (s, 1H, –N–CH–). 13C NMR (CDCl3): d 18.31, 20.72,
26.71, 29.42, 42.20, 46.90, 61.05, 66.24, 83.44, 98.20, 99.97,
120.60, 121.76, 125.55, 130.79, 132.62, 146.94, 174.18.
Triazole formation (5). The synthesis was conducted
according to the general procedure for triazole formation as
described above. Yield 22%. 1H NMR (DMSO): d 4.29 (s, 1H,
–CMC–H), 4.59 (d, 2H, J = 4.0 Hz, –CH2–), 5.38 (t, 1H, J =
4 Hz, –OH), 7.56 (d, 2H, J = 8.0 Hz, Ar–H), 7.61 (d, 2H,
J = 8.0 Hz, Ar–H), 7.90 (d, 2H, J = 8.0 Hz, Ar–H), 7.96 (d, 2H,
J = 8.0 Hz, Ar–H), 9.35 (s, 1H, –N–CH–). 13C NMR (DMSO):
d 62.13, 81.58, 83.26, 119.72, 120.13, 125.35, 127.59, 129.79,
130.61, 132.34, 135.08, 143.34, 146.37.
General procedure for anhydride coupling, exemplified with
the synthesis of compound 6. Alcohol 5 (200 mg; 0.73 mmol)
and 4-dimethylaminopyridine (DMAP, 20 mg; 0.16 mmol)
were dissolved in 10 mL of pyridine. Acetonide protected bis-
MPA anhydride (0.72 g; 2.2 mmol) was dissolved in 3 mL
DCM and added to the reaction mixture. The reaction was
stirred at room temperature overnight. The residual anhydride
General procedure for platinum coupling reaction, exemplified
with the synthesis of Z1. Alkyne 3 (1.37 g; 3.57 mmol) and
168 | J. Mater. Chem., 2008, 18, 166–175
This journal is ß The Royal Society of Chemistry 2008

View Article Online
was quenched with 5 mL of water under rigorous stirring for
a couple of hours. Solvents were evaporated and the crude
reaction mixture was redissolved in DCM. The organic phase
was extracted 3 times with NaHSO4 (10%), 3 times with
Na2CO3 (10%), and finally once with brine. The organic phase
was dried with MgSO4 and purified by flash chromatography,
eluting with heptane, gradually increasing to 50 : 50 ethyl
acetate–heptane. Yield 95%. 1H NMR (CDCl3): d 1.16
(s, 3H, –CH2–C–CH3), 1.36 (s, 3H, O–C–CH3), 1.41 (s, 3H,
O–C–CH3), 3.16 (s, 1H, –CMC–H), 3.67 (d, 2H, J = 12.0 Hz,
–C–CH2–O–), 4.22 (d, 2H, J = 12.0 Hz, –C–CH2–O–), 5.24 (s,
2H, Ar–CH2–), 7.34 (d, 2H, J = 12.0 Hz, Ar–H), 7.51 (d, 2H, J =
12.0 Hz, Ar–H), 7.76 (m, 4H, Ar–H), 8.15 (s, 1H, –N–CH–).
13C NMR (CDCl3): d 18.29, 21.64, 25.41, 41.90, 65.90, 78.09,
83.36, 98.07, 117.79, 120.54, 122.09, 125.61, 129.09, 130.36,
132.66, 136.52, 136.98, 147.67, 173.98.
Deprotection of TMS (8). Compound 7 (1.89 g; 3.40 mmol)
was dissolved in 100 mL THF, whereafter tetrabutylammo-
nium fluoride (TBAF, 1 M solution in THF, 4.1 mL;
4.09 mmol) was added dropwise. The deprotection was
allowed to proceed at room temperature for 1 h, followed by
TLC. When complete, THF was evaporated off and the crude
mixture redissolved in ethyl acetate and extracted three times
with NaHCO3. The organic phase was dried with MgSO4 and
concentrated. The product was further purified by flash
chromatography, eluting with ethyl acetate–heptane starting
from 20 : 80 and gradually increasing to 40 : 60. Yield: 72%.
1H NMR (CDCl3): d 1.12 (s, 3H, –CH2–C–CH3), 1.38 (s, 3H,
O–C–CH3), 1.45 (s, 3H, O–C–CH3), 2.35 (m, 2H, –CH2–CH2–
O–), 3.17 (s, 1H, –CMC–H), 3.66 (d, 2H, J = 12.0 Hz, –C–CH2–
O–), 4.21 (d, 2H, J = 12.0 Hz, –C–CH2–O–), 4.22 (t, 2H, J =
6.0 Hz, –N–CH2–), 4.53 (t, 2H, J = 6.0 Hz, –CH2–CH2–O–),
7.47 (s, 4H, Ar–H), 7.57 (d, 2H, J = 8.0 Hz, Ar–H), 7.82 (d, 2H,
J = 8.0 Hz, Ar–H), and 7.86 (s, 1H, –N–CH–). 13C NMR
(CDCl3): d 18.30, 20.75, 26.65, 29.42, 42.18, 46.85, 61.06,
66.22, 78.94, 83.23, 89.62, 91.24, 98.18, 120.50, 121.89, 122.51,
123.65, 125.59, 130.61, 131.43, 132.04, 132.10, 147.05, 174.16.
Platinum coupling (Z2). The synthesis was conducted
according to the general procedure for platinum coupling
as described above. Yield 90%. 1H NMR (CDCl3): d 0.93
(t, 18H, J = 8.0, –CH2–CH3), 1.16 (s, 3H, –CH2–C–CH3), 1.36
(s, 3H, O–C–CH3), 1.41 (s, 3H, O–C–CH3), 1.43 (m, 12H,
–CH2–CH3), 1.62 (m, 12H, CH2–CH2–CH3), 2.15 (m, 12H,
–P–CH2–), 3.67 (d, 4H, J = 12.0 Hz, –C–CH2–O–), 4.22 (d,
4H, J = 12.0 Hz, –C–CH2–O–), 5.24 (s, 4H, Ar–CH2–), 7.34 (d,
4H, J = 12.0 Hz, Ar–H), 7.51 (d, 4H, J = 12.0 Hz, Ar–H), 7.76
(m, 8H, Ar–H), 8.15 (s, 2H, –N–CH–). 13C NMR (CDCl3): d
13.73, 18.34, 21.68, 23.87, 24.30, 25.40, 26.27, 30.19, 41.94,
65.40, 65.96, 98.01, 116.95, 120.35, 125.28, 125.30, 126.30,
128.09, 128.98, 131.08, 131.10, 135.64, 136.62, 148.49, 173.89.
FTIR: nC–H 2800–3000 cm21, nPt–CMC 2095 cm21, nCLO
Platinum coupling (Z3). The synthesis was conducted
according to the general procedure for platinum coupling as
described above. Yield 75%. 1H NMR (CDCl3): d 0.91 (t, 18H,
J = 8.0, –CH2–CH3), 1.10 (s, 6H, –CH2–C–CH3), 1.36
(s, 6H, O–C–CH3), 1.44 (s, 6H, O–C–CH3), 1.45 (m, 12H,
–CH2–CH3), 1.60 (m, 12H, CH2–CH2–CH3), 2.12 (m, 12H,
–P–CH2–), 2.33 (m, 4H, –CH2–CH2–O–), 3.66 (d, 4H, J =
12.0 Hz, –C–CH2–O–), 4.21 (d, 4H, J = 12.0 Hz, –C–CH2–O–),
4.22 (t, 4H, J = 6.0 Hz, –N–CH2–), 4.51 (t, 4H, J = 6.0 Hz,
–CH2–CH2–O–), 7.22 (d, 4H, J = 8.0 Hz, Ar–H), 7.37 (d, 4H,
J = 8.0 Hz, Ar–H), 7.55 (d, 4H, J = 8.0 Hz, Ar–H), 7.79 (d, 4H,
J = 8.0 Hz, Ar–H), and 7.84 (s, 2H, –N–CH–). 13C NMR
(CDCl3): d 13.71, 18.21, 20.73, 23.85, 24.29, 26.25, 26.52,
29.32, 42.08, 46.76, 61.00, 66.12, 77.20, 89.64, 90.84, 98.07,
109.27, 119.01, 120.35, 123.09, 125.46, 129.06, 130.02, 130.58,
1740 cm21, nCLC
1608 cm21, nC–O–C–O–C 1074 cm21
,
Ar
n1,4-disubst.
845 cm21, nC–O–C–O–C 828 cm21. MALDI-
benzene
TOF: calculated Mw + Na+ = 1482.70 g mol21, found
Mw + Na+ = 1482.87 g mol21
.
Synthesis of compound 7. Alkyne 3 (1.62 g, 4.23 mmol) was
dissolved in 30 mL of TEA–pyridine (50 : 50). PPh3 (66.5 mg;
0.25 mmol), CuI (20.1 mg; 0.106 mmol), Pd(PPh3)2Cl2 (59.4 mg;
84.6 mmol) was added and the solution was heated to 120 uC.
(4-Iodophenylethynyl)trimethylsilane (1.27 g; 4.23 mmol) was
dissolved in 3 mL of TEA–pyridine and added dropwise to the
heated solution. The reaction was refluxed for 2 h and followed
by TLC. When complete, solvents were evaporated off and the
crude mixture redissolved in DCM. The organic solution was
extracted three times with NaHSO4 (10%), dried with MgSO4,
and concentrated. The product was further purified by flash
chromatography, eluting with heptane, gradually increasing to
40 : 60 ethyl acetate–heptane. Yield: 80%. 1H NMR (CDCl3): d
0.25 (s, 9H, –Si–CH3), 1.11 (s, 3H, –CH2–C–CH3), 1.38 (s, 3H,
O–C–CH3), 1.45 (s, 3H, O–C–CH3), 2.35 (m, 2H, –CH2–CH3),
3.69 (d, 2H, J = 12.0 Hz, –C–CH2–O–), 4.22 (d, 2H, J = 12.0 Hz,
–C–CH2–O–), 4.23 (t, 2H, J = 6.0 Hz, –CH2–CH2–O–), 4.53 (t,
2H, J = 6.0 Hz, –N–CH2–), 7.45 (m, 4H, Ar–H), 7.57 (d, 2H,
J = 8.0 Hz, Ar–H), 7.82 (d, 2H, J = 8.0 Hz, Ar–H), 7.88 (s, 1H,
–N–CH–). 13C NMR (CDCl3): d 20.10, 18.28, 20.67, 26.73,
29.00, 29.40, 42.18, 46.99, 61.06, 66.22, 89.86, 91.10, 96.33,
98.12, 104.45, 120.52, 122.58, 122.88, 123.19, 125.57, 130.53,
131.34, 131,87, 132.08, 174.16.
131.11, 131.81, 147.05, 174.05. FTIR: nC–H 2800–3000 cm21
nPt–CMC 2094 cm21, nCLO 1736 cm21, nCLC 1595 cm21
,
,
Ar
n
C–O–C–O–C 1077 cm21, n1,4-disubst. benzene 846 cm21, nC–O–C–O–C
832 cm21
.
MALDI-TOF: calculated Mw
+
Na+
=
1586.85 g mol21, found Mw + Na+ = 1586.89 g mol21
.
Results and discussion
Various structures of platinum(II) acetylides based on aromatic
alkynes have been synthesized and thoroughly characterized
previously.23,36,37 However, reports on platinum(II) acetylides
prepared by click chemistry are rare. In this study triazole units
were incorporated at three different positions in an NLO
chromophore in order to evaluate how the position of the
triazole affects the OPL properties of these chromophores.
The three molecules are illustrated in Scheme 1. The smallest
molecule in the study, Z1, contains a short conjugation
path length, with the triazole unit at the end of the
conjugation. This molecule resembles the previously investi-
gated platinum(II) acetylide20 (Pt1-G1) with the difference of a
triazole instead of an ethynyl-phenyl group. The Z2 molecule
has the triazole unit in between two aromatic rings. The Z3
This journal is ß The Royal Society of Chemistry 2008
J. Mater. Chem., 2008, 18, 166–175 | 169

View Article Online
Scheme 1 Chemical structures of the triazole-containing platinum(II) acetylides, Z1, Z2, and Z3, and the previously studied Pt1-G1.
molecule has a more extended conjugation than the previous
two, and the triazole unit is located at the end of the
conjugation. These molecules were chosen to evaluate whether
the triazole contributes to the conjugation and participates
in the electron transfer. The Z2 molecule was examined to
investigate whether the triazole disrupts the conjugation over
the aromatics, or if efficient electron transfer can be achieved
through the triazole.
procedure of deprotection and subsequent addition of the
acetonide protected bis-MPA anhydride according to the
literature.33 The azide-functional acetonide protected bis-MPA
was then coupled to di-ethynyl benzene through click
chemistry, creating a triazole-containing phenyl-ethynyl arm.
Finally, the triazole-modified phenyl-ethynyl arms were
coupled to a platinum salt by Sonogashira coupling to obtain
the final Z1 molecule (Scheme 2).
The synthesis of Z2 is somewhat different from the
approach to Z1. The Z2 molecule was prepared by adding
sodium azide38 to bromobenzyl alcohol39 and subsequent
cyclo-addition of the azide with diethynyl benzene. The
hydroxy functionality was then reacted with the acetonide
protected bis-MPA anhydride to accomplish a generation 1
alkyne arm. As for Z1, the final Z2 chromophore was
Synthesis of molecules
The Z1 molecule was synthesized by coupling a chloro-
functional spacer to the acetonide protected bis-MPA followed
by substitution of the chloride to an azide. At this point
larger dendrons can be grown divergently using the iterative
Scheme 2 Synthesis of chromophore Z1.
170 | J. Mater. Chem., 2008, 18, 166–175
This journal is ß The Royal Society of Chemistry 2008

View Article Online
Scheme 3 Synthesis of chromophore Z2.
Scheme 4 Synthesis of chromophore Z3.
obtained by Sonogashira coupling to the platinum salt
(Scheme 3).
The Z3 molecule was synthesized in a similar manner to Z1
and Z2 (Scheme 4). All click reactions were conducted using an
alkyl azide and a difunctional ethynyl compound in a 1 : 1
equivalents ratio. Hence, the aim was only for the monosub-
stituted product; however, various amounts of the disubstituted
product were also formed. The yields for the monosubstituted
triazole formation products were up to 90%. As mentioned
in the introduction, click reactions are associated with high
yields, and highly pure products where column chromatography
purification is not necessary. However, considering the circum-
stances with the difunctional ethynyl reagent, formation of
disubstituted product will be inevitable and column chromato-
graphy is needed to isolate the monosubstituted product.
Photo-physical characterization
Fig. 2 shows linear absorption spectra for the three final
Fig. 2 Linear absorption spectra for Z1, Z2 and Z3 (10 mM solutions
in THF).
chromophores. The spectra of Z1 and Z2 are almost identical,
This journal is ß The Royal Society of Chemistry 2008
J. Mater. Chem., 2008, 18, 166–175 | 171

View Article Online
Table 1 Absorption peaks, emission peaks, extinction coefficient, quantum yield and excited state lifetimes of the three investigated compounds
Absorption
peaks/nm
Emission
peaks/nm
Extinction coeff./104 M21 cm21
and quantum yield
,
Fluorescence and
phosphorescence lifetimesa
Molecule
Z1
Z2
Z3
a
(296), 342
(298), 342
(308), 365
388, 486
388, 488
396, 535
7.1, ,0.001b
8.5, ,0.001b
14.3, ¡0.001
1–10 ps, 50 ns
1–10 ps, 76 ns
1–10 ps, 260 ns
Phosphorescence lifetimes measured in oxygen saturated samples. Longer lifetimes are obtained for oxygen evacuated samples (see text).
Very weak fluorescence signal; the quantum yield values were not possible to determine more accurately.
b
exhibiting two absorption bands, one weak band at 296 nm
coupling for these chromophores and thus allows long-lived
phosphorescence. Interestingly, Z1 and Z2 show fairly weak
fluorescence bands, whereas Z3 emit more efficiently at a
slightly higher wavelength band around 400 nm. The phos-
phorescence bands are equally weak for all chromophores in
oxygen saturated solutions. However, the phosphorescence
peak for Z3 is red-shifted by almost 50 nm.
and a stronger band at 342 nm. The minor peak for the Z3
molecule is red-shifted by approximately 10 nm, giving rise to
a weak band at 308 nm, whereas the major peak is red-shifted
by 23 nm and peaks at 365 nm (very similar to the spectrum
of the previously investigated platinum(II) acetylides without
triazole units18,20). The red-shift is most likely due to the
extended conjugation in the Z3 molecule. The large peak for
all compounds corresponds to the singlet–singlet transition
(S0 A S1). Absorption peak wavelengths and extinction
coefficients are summarized in Table 1.
Fluorescence decay times were found to be very short. For
all three compounds they were found to be below 10 ps. The
phosphorescence decay times (single component) were mea-
sured to be between 50–260 ns in oxygen saturated samples
(Table 1) with representative decay curves as shown in Fig. 4.
The fluorescence quantum efficiency for all compounds,
Z1, Z2 and Z3, was found to be very small. They were all
estimated to be below 1023, which is in the same order of
magnitude as the previously investigated Pt1-G1.17 With our
experimental set-up it was not possible to determine the values
more accurately. However, we could see that the values for Z1
and Z2 were comparable and somewhat lower than those
of Z3 and Pt1-G1 (data not shown). The low fluorescence
emission is explained by the very efficient ISC to the triplet
manifold for such heavy-atom containing chromophores.37,40
The triplet state is almost totally quenched by the presence
of oxygen in the solution, therefore the total amount of
‘‘visible’’ triplet luminescence from these samples is very low. It
was shown by Lindgren et al.17 that bright phosphorescence
from, e.g., Pt1-G1 can be obtained by removing the dissolved
oxygen in the sample using a series of freeze–thaw cycles under
The similarity of the Z1 and Z2 molecules indicates that the
electron transfer over the triazole unit in the Z2 molecule is in
fact interrupted by the triazole unit. Otherwise, the absorption
bands for Z2 would most likely also be red-shifted as for
the Z3 molecule. Transmission spectra show that all three
molecules possess high linear transmission above 400 nm.
Luminescence spectra and excited state decay lifetimes were
also recorded for all three molecules (Fig. 3). Two distinct
bands were observed in the luminescence spectrum for all
chromophores. The band that peaks at around 390 nm results
from radiative relaxation from S1 A S0 and is denoted
fluorescence due to its short life-time. The second band that
peaks around 490 nm for Z1 and Z2, and around 535 nm
for Z3, is associated with phosphorescence from T1 A S0
(through intersystem crossing (ISC) from S1 A Tn) due
to its long life-time. The intersystem crossing is greatly
increased by the platinum atom which enhances the spin–orbit
Fig. 4 Phosphorescence lifetime decay curves for Z1, Z2 and Z3 in
oxygen saturated samples. Decay of the previously studied Pt1-G1 is
shown for comparison. Excitation wavelength at 360 nm, and 500 kHz
prf. All samples in approximately 10 mM THF solutions.
Fig. 3 Luminescence spectra for Z1, Z2 and Z3. Excitation wave-
length at 350 nm at 4.75 MHz prf. Samples diluted to 10 mM in THF
solvent. The inset shows the right wing of the spectrum on a different
scale in order to show the phosphorescence bands more clearly.
172 | J. Mater. Chem., 2008, 18, 166–175
This journal is ß The Royal Society of Chemistry 2008

View Article Online
oxygen, even for Z3 and Pt1-G1) the ISC channel is verified as
the main output channel from the first excited singlet state.
This is in agreement with the de-excitation process previously
proposed by Lindgren et al.17
Multiphoton absorption
The emission upon two-photon absorption (TPA) was also
recorded as shown in Fig. 6. The molecules were excited at
approximately twice the wavelength of their linear absorption
maximum, and consequently two photons are needed to excite
the electrons to higher excited states, whereafter the radiative
relaxation of these transitions was analyzed. All spectra show
the same trends as in the one-photon excitation case. The
emission measurements from TPA display very weak fluores-
cence and phosphorescence for Z1 and Z2 (just as in the one-
photon case). In contrast, Z3 shows greater fluorescence and
phosphorescence in the TPA luminescence measurements,
compared to the other two compounds. The two-photon
absorption process will have a quadratic dependence on the
incoming intensity. Therefore the amount of two-photon
excited fluorescence will also have this intensity dependence
on excitation. A logarithmic plot of the fluorescence emission
vs. the logarithm of the peak laser intensity will theoretically
give a straight line with a gradient of two. Such a plot of the
measured fluorescence emission for compound Z3 is shown in
Fig. 7. With a linear gradient of 1.96 it is very close to the
expected value. This shows that we indeed have two-photon
absorption. There are also some uncertainties due to random
fluctuations because of the low fluorescence signal in this case.
The pulse repetition frequency was 4.75 MHz. Even though
this is high, it does not seem to interfere with the triplet-energy
levels and ESA from the triplet states since we are monitoring
the fluorescence directly from the singlet manifold.
Fig. 5 Bright phosphorescence from 10 mM samples purged with
argon gas. Large double peaks are shown for both Z3 and Pt1-G1
(527, 556 nm, and 538, 573 nm, respectively). Also some enhancement
of the phosphorescence peaks for Z1 and Z2 is shown, compared to
Fig. 3.
vacuum. The bright phosphorescence that then appears
showed a decay time in the order of 100 ms, a value three
orders of magnitude larger than the oxygen saturated decay
time. In this study it was found that the same result could be
achieved by bubbling argon gas through the sample for
10–15 min, hence displacing most of the oxygen dissolved
in the solution. Thus, when the samples of Z1, Z2, Z3,
and Pt1-G1 were purged with argon gas, bright green visible
phosphorescence appeared (Fig. 5). At the same time the
phosphorescence decay times for both Pt1-G1 and Z3 were
increased to approximately 100 ms (data not shown), in
accordance with the previously obtained results for the
Pt-capped dendrimers.17 Interestingly, for Z1 and Z2 the
procedure with argon purging was not as successful as for Z3
and Pt1-G1 even though the experiments were carried out
under identical conditions. Thus, the detailed molecular
structure must also be of importance for the quenching
of triplet states. Even if the observed effect was not as large
for Z1 and Z2, still an enhancement of the amount of
phosphorescence was observed together with a significant
increase in the phosphorescence lifetime to approximately 6 ms
for both Z1 and Z2 (data not shown).
An estimation of the TPA cross section can be made from
the luminescence spectrum obtained for Z3 compared to the
spectrum for coumarin 110 (data not shown). Also, since the
quantum efficiency was not found very accurately, it is
convenient to give the value for the product of fluorescence
quantum efficiency Q and TPA cross section s2. This is found
By the same method as above, an estimation of the
phosphorescence emission quantum efficiency was performed.
These measurements were performed on samples where a
significant part of the dissolved oxygen was removed by
argon-gas purging (Fig. 5). The phosphorescence quantum
efficiency, as defined here, contains both the ISC factor and
the phosphorescence emission yield; hence, it is a product of
the two factors. The measurement gave approximate values of
0.04 and 0.05 for Z3 and Pt1-G1, respectively. These values are
significantly higher than the fluorescence quantum yields,
showing that the ISC process must be a very efficient process.
For Z1 and Z2 the values are one order of magnitude lower, in
the order of 1023. Taking into account that in all samples some
triplet states may still be oxygen quenched (since the argon-gas
purging is not assumed to completely remove the dissolved
Fig. 6 Two-photon emission spectra for Z1, Z2 and Z3 (and
Pt1-G1). Excitation wavelength is 720 nm. 200 kHz pulse repetition
frequency. Samples in 50 mM solutions dissolved in THF.
This journal is ß The Royal Society of Chemistry 2008
J. Mater. Chem., 2008, 18, 166–175 | 173

View Article Online
(80% linear transmission at 532 nm). It is believed that the
extended conjugation combined with the triazole end-func-
tionalities of the Z3 chromophore are responsible for the
improved OPL response due to efficient electron transfer and
strong excited state absorption. In comparison, these clamping
levels were similar to or better than the optical power limiting
results of previously investigated chromophores, such as zinc
based porphyrins reaching a clamping level of 6 mJ, and
dendronized bis(phenylethynyl)thiophenes reaching clamping
levels in the range of 8–15 mJ (for samples with three times
higher concentration).5,6 The triazole-functionalized chromo-
phore, Z3 (with a clamping level of 2.5 mJ), also performed
better than the predecessor Pt1-G1 chromophore which
reached a clamping level of 3.3 mJ.
Conclusions
Fig. 7 Graph showing two-photon excited fluorescence vs. laser
intensity (log–log plot) for the Z3 compound (sample concentration is
approx. 10 mM in THF). The excitation wavelength was set to 750 nm
at 4.75 MHz prf.
It has been shown that triazole-functionalized platinum(II)
acetylides, targeted for OPL applications, can be synthesized
by click chemistry. Furthermore, utilizing click chemistry does
offer an alternative route to the preparation of dendronized
(bis-MPA) platinum(II) acetylide chromophores. The bis-MPA
end-group on NLO chromophores has several benefits. Firstly,
it acts as model compound proving that similar procedures
can be used to obtain chromophores with larger dendritic
substituents. Hence, adding larger dendritic substituents will
increase the site isolation and thus improve the optical
power limiting. Secondly, the bis-MPA unit facilitates the
solubility of the platinum(II) acetylide chromophore in many
organic solvents, and prevents p-stacking of chromophores.
Aggregation of chromophores is unwanted due to the decrease
in linear transmission that clusters may cause. Moreover, the
bis-MPA unit may be beneficial for further functionalization
of the chromophore,20 offering multiple reaction sites available
for further modification.
to be in the order of 0.020 GM, that is, a cross section of
minimum 20 GM if the upper limit of the quantum efficiency is
taken to be 0.001. This result gives a value for the TPA cross
section for Z3 that is a little larger, but still on the same order
of magnitude as the value for Pt1-G1 obtained previously.17
Therefore, this indicates that the extended conjugation, along
with the triazole, may have a small contribution to the ability
for two photon absorption of these molecules.
Optical power limiting
Optical power limiting measurements at 532 nm show that all
three molecules possess good optical limiting ability and reach
fairly low clamping levels (Fig. 8). Z1 is the least efficient of
the three, with an output of 8 mJ for 200 mJ input energy
(91% linear absorption at 532 nm). Z2 reaches down to
5.5 mJ output energy for 200 mJ input energy (69% linear
transmission at 532 nm). Z3 gives the best OPL response
reaching down to a clearly defined clamping level of 2.5 mJ
Photophysical characterization show that the triazole units
do contribute to the nonlinear optical properties of these
molecules; however, the most important factor for efficient
power limiting is due to extended conjugation together with
spin–orbit coupling offered by the platinum atom. Optical
power limiting measurements show that all compounds
(Z1, Z2 and Z3) possess good limiting abilities. The best
clamping level was reached for the Z3 molecule due to its
extended conjugation and placement of the triazole units at the
end of the conjugation.
Acknowledgements
This work is supported by the programs ‘‘Photonics in Defence
Applications’’ and ‘‘Swedish Defence Nano Technology
Programme’’ run jointly by the Swedish Defence research
agency (FOI) and the Defence Material Administration
(FMV). Anders Eriksson at FOI is thanked for the OPL
measurements. ML and EM also acknowledge a grant from
the Norwegian Research Council within the NanoMat
program (contract #163529/s10). RV and CH acknowledge
the National Science Foundation through the UCSB
Materials Research Laboratory (NSF DMR-0520415),
Chemistry Division (CHE-0514031).
Fig. 8 Optical power limiting measurements showing input energy
versus transmitted (output) energy at 532 nm for Z1, Z2 and Z3
(50 mM solutions in THF).
174 | J. Mater. Chem., 2008, 18, 166–175
This journal is ß The Royal Society of Chemistry 2008

View Article Online
and C. Lopes, Proc. SPIE—Int. Soc. Opt. Eng., 2006, 6401,
64010H.
References
21 J. Staromlynska, P. B. Chapple, J. R. Davy and T. J. McKay, Proc.
SPIE—Int. Soc. Opt. Eng., 1994, 2229, 59–66.
22 J. Staromlynska, T. J. McKay and P. Wilson, J. Appl. Phys., 2000,
88, 1726–1732.
1 S. Sanders, Opt. Photonics News, 2005, 16, 36–41.
2 C. W. Spangler, J. Mater. Chem., 1999, 9, 2013–2020.
3 C. Li, C. Liu, Q. Li and Q. Gong, Chem. Phys. Lett., 2004, 400,
569–572.
4 R. C. Hollins, Curr. Opin. Solid State Mater. Sci., 1999, 4, 189–196.
5 R. Vestberg, A. Nystro¨m, M. Lindgren, E. Malmstro¨m and
A. Hult, Chem. Mater., 2004, 16, 2794–2804.
6 R. Vestberg, C. Nilsson, C. Lopes, P. Lind, B. Eliasson and
E. Malmstro¨m, J. Polym. Sci., Part A: Polym. Chem., 2005, 43,
1177–1187.
7 J. W. Baur, M. D. Alexander, Jr., M. Banach, L. R. Denny,
B. A. Reinhardt, R. A. Vaia, P. A. Fleitz and S. M. Kirkpatrick,
Chem. Mater., 1999, 11, 2899–2906.
8 J. E. Ehrlich, X. L. Wu, I. Y. S. Lee, Z. Y. Hu, H. Rockel,
S. R. Marder and J. W. Perry, Opt. Lett., 1997, 22, 1843–1845.
9 G. S. He, G. C. Xu, P. N. Prasad, B. A. Reinhardt, J. C. Bhatt and
A. G. Dillard, Opt. Lett., 1995, 20, 435–437.
10 T. J. McKay, J. Staromlynska, P. Wilson and J. Davy, J. Appl.
Phys., 1999, 85, 1337–1341.
11 R. Kannan, G. S. He, T.-C. Lin, P. N. Prasad, R. A. Vaia and
L.-S. Tan, Chem. Mater., 2004, 16, 185–194.
12 C. E. Powell and M. G. Humphrey, Coord. Chem. Rev., 2004, 248,
725–756.
13 C.-H. Tao, N. Zhu and V. W.-W. Yam, Chem.–Eur. J., 2005, 11,
1647–1657.
14 N. Chawdhury, A. Koehler, R. H. Friend, M. Younus, N. J. Long,
P. R. Raithby and J. Lewis, Macromolecules, 1998, 31, 722–727.
15 J. Lewis, N. J. Long, P. R. Raithby, G. P. Shields, W.-Y. Wong
and M. Younus, J. Chem. Soc., Dalton Trans., 1997, 4283–4288.
16 R. Kannan, G. S. He, L. Yuan, F. Xu, P. N. Prasad,
A. G. Dombroskie, B. A. Reinhardt, J. W. Baur, R. A. Vaia and
L.-S. Tan, Chem. Mater., 2001, 13, 1896–1904.
17 M. Lindgren, B. Minaev, E. Glimsdal, R. Vestberg, R. Westlund
and E. Malmstro¨m, J. Lumin., 2007, 124, 302–310.
18 R. Vestberg, R. Westlund, A. Eriksson, C. Lopes, M. Carlsson,
B. Eliasson, E. Glimsdal, M. Lindgren and E. Malmstro¨m,
Macromolecules, 2006, 39, 2238–2246.
19 S. Parola, M. Oeertenblad, F. Chaput, C. Desroches, P. Miele,
P. L. Baldeck, E. Malmstro¨m, M. Lindgren, B. Eliasson,
A. Eriksson and C. Lopes, Proc. SPIE—Int. Soc. Opt. Eng.,
2005, 5934, 593404.
20 R. Westlund, E. Malmstro¨m, M. Hoffmann, R. Vestberg,
C. Hawker, E. Glimsdal, M. Lindgren, P. Norman, A. Eriksson
23 T. J. McKay, J. A. Bolger, J. Staromlynska and J. R. Davy,
J. Chem. Phys., 1998, 108, 5537–5541.
24 P. Furuta and J. M. J. Fre´chet, J. Am. Chem. Soc., 2003, 125,
13173–13181.
25 H. Ma and A. K. Y. Jen, Adv. Mater., 2001, 13, 1201–1205.
26 H. C. Kolb, M. G. Finn and K. B. Sharpless, Angew. Chem., Int.
Ed., 2001, 40, 2004–2021.
27 P. Wu, A. K. Feldman, A. K. Nugent, C. J. Hawker, A. Scheel,
B. Voit, J. Pyun, J. M. J. Frechet, K. B. Sharpless and V. V. Fokin,
Angew. Chem., Int. Ed., 2004, 43, 3928–3932.
28 M. Malkoch, K. Schleicher, E. Drockenmuller, C. J. Hawker,
T. P. Russell, P. Wu and V. V. Fokin, Macromolecules, 2005, 38,
3663–3678.
29 M. J. Joralemon, R. K. O’Reilly, J. B. Matson, A. K. Nugent,
C. J. Hawker and K. L. Wooley, Macromolecules, 2005, 38,
5436–5443.
30 B. Helms, J. Mynar, C. Hawker and J. M. Fre´chet, J. Am. Chem.
Soc., 2005, 126, 15020–15021.
31 A. T. R. Williams, S. A. Winfield and J. N. Miller, Analyst, 1983,
108, 1067–1071.
32 E. Glimsdal, M. Carlsson, B. Eliasson, B. Minaev and
M. Lindgren, J. Phys. Chem. A, 2007, 111, 244–250.
33 M. Malkoch, E. Malmstro¨m and A. Hult, Macromolecules, 2002,
35, 8307–8314.
34 R. Gujadhur, D. Venkataraman and J. T. Kintigh, Tetrahedron
Lett., 2001, 42, 4791–4793.
35 J. S. Moore and S. I. Stupp, Macromolecules, 1990, 23, 65–70.
36 M. I. Bruce, J. Davy, B. C. Hall, Y. J. Van Galen, B. W. Skelton
and A. H. White, Appl. Organomet. Chem., 2002, 16, 559–568.
37 J. E. Rogers, T. M. Cooper, P. A. Fleitz, D. J. Glass and
D. G. McLean, J. Phys. Chem. A, 2002, 106, 10108–10115.
38 M. Peer, Spec. Chem. Mag., 1998, 18, 256–257; M. Peer, Spec.
Chem. Mag., 1998, 18, 259; M. Peer, Spec. Chem. Mag., 1998, 18,
261; M. Peer, Spec. Chem. Mag., 1998, 18, 263.
39 W. Zhu and D. Ma, Chem. Commun., 2004, 888–889.
40 J. E. Rogers, B. C. Hall, D. C. Hufnagle, J. E. Slagle, A. P. Ault,
D. G. McLean, P. A. Fleitz and T. M. Cooper, J. Chem. Phys.,
2005, 122, 214708.
This journal is ß The Royal Society of Chemistry 2008
J. Mater. Chem., 2008, 18, 166–175 | 175