White metallopolyynes for optical limiting/transparency trade-off optimization

Article abstract of DOI:10.1002/anie.200601651

High-transparency optical limiters: A series of solution-processable white metallopolyynes are shown to be excellent optical power limiters (see picture) with optimized optical transparency/non-linearity trade-off. They exhibit an optical-limiting performance superior to those of the common reverse saturable absorption dyes, such as C₆₀ and metal phthalocyanine complexes. (Figure Presented).

Full text of DOI:10.1002/anie.200601651

Angewandte
Chemie
Metallopolyynes
DOI: 10.1002/anie.200601651
White Metallopolyynes for Optical Limiting/
Transparency Trade-off Optimization**
Gui-Jiang Zhou, Wai-Yeung Wong,* Zhenyang Lin, and
Cheng Ye
With the rapid development of laser technology in the
photonic era, the damage of human eyes, optical sensors,
and sensitive optical components caused by exposure to
sudden intense laser pulses has driven many researchers to
become involved in the search for effective optical limiters
that possess high solubility, fast response speeds, good linear
transparency, and high linear transmission.[1] Optical limiters
are materials that protect sensitive electro-optical sensors
against intrusive, possibly damaging, frequency-agile pulsed
laser irradiation.[2] Currently, the materials employed for
optical-power limiting (OPL) are largely small molecules
such as fullerenes (C60),[3] phthalocyanines,[2b,4] porphyrins,[5]
diacetylenes,[6] nanotubes,[7] and organometallic compounds.[8]
Among these, metallophthalocyanines and metalloporphyrins
are the most effective optical limiters as the heavy metal
effect magnifies the OPL response by enhancement of the
intersystem crossing (ISC) from S1 to T1 states.[9] However,
their poor solubility and the technical difficulties associated
with fabricating OPL devices have hindered their practical
applications so far. The deep colors of fullerenes, phthalo-
cyanines, and porphyrins also hamper their use in practical
devices for the protection of human eyes because there are
unavoidable absorption bands in the visible wavelength
region.[2a,10] While it is known that a bathochromic shift of
the ground-state absorption maximum (lmax) generally leads
to an increase in the third-order optical nonlinearity,[11]
previously reported excellent optical limiters have not yet
achieved nonlinearity/transparency trade-off optimization.
Recent work has shown that some conjugated organic
[*] Dr. G.-J. Zhou, Dr. W.-Y. Wong
Department of Chemistry
Hong Kong Baptist University
Waterloo Road, Kowloon Tong, Hong Kong (P.R. China)
Fax: (+852)3411-7348
E-mail: rwywong@hkbu.edu.hk
Dr. Z. Lin
Department of Chemistry
The Hong Kong University of Science and Technology
Clearwater Bay, Hong Kong (P.R. China)
Prof. C. Ye
Centre for Molecular Science, Institute of Chemistry
Chinese Academy of Sciences
Beijing 100080 (P.R. China)
[**] This work was supported by a CERG grant from the Research Grants
Council of the Hong Kong SAR, P.R. China (project no. HKBU 2022/
03P) and Hong Kong Baptist University.
Supporting Information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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polyynes are good OPL materials with a performance close to
porphyrin (Zn-TPP), and CuII-tetrakis(tert-butyl)phthalocya-
nine (CuPc-tBu4). We also found that the mercury atom plays
a key role in this enhanced OPL performance.
that of C60; however, their lmax values still go beyond
400 nm.[12] Although small-molecule platinum acetylides[13]
and their dendritic structures[13e,14] have been shown to be
good chromophores for OPL, less attention has been paid to
the corresponding metallopolyynes,[13g] and related studies in
mercury-derived arylene–ethynylene copolymers remain
unexplored. Here, we report the unprecedented examples of
some solution-processable white homo- and heterometallic
polyynes that reveal superior OPL/transparency trade-offs.
The synthesis of the metal polyynes and their well-defined
model compounds is shown in Scheme 1 (the use of long octyl
chains can increase the solubility of metallopolymers).
Interestingly, Pt-Hg-P is the first rigid-rod heteronuclear
PtII/HgII s-alkynyl polymer and is soluble in organic solvents.
All compounds were isolated as stable white/off-white solids
in high purity. The model compounds were investigated by
comparing their electronic and
Based on the five-level reverse saturable absorption
(RSA) mechanism,[16] the system can be reduced to a three-
level one by assuming that there is no saturation, diffusion, or
recombination during the laser pulse. Thus, we can neglect all
the excitations higher than the first singlet excitation (S1)
provided that the ISCrate is fast (compared with the laser
pulse duration) and all of the initial excitations are populated
in the first triplet state (T1). Excitation of the T1 state leads to
the nonlinear absorption, and the strong energy absorption
that is accompanied by a T1!Tn transition serves as a major
contributor to the nanosecond OPL behavior.
Owing to the short lifetime of the S1 state (< 10 ns), the
triplet state plays a pivotal role in the nonlinear optical
process, therefore photoluminescence (PL) spectra were
structural properties with the
parent polymers and studying the
effect of chain length in the OPL
phenomenon. The high decomposi-
tion temperatures of the samples
(366–4068C) indicate their good
thermal stability. The rigid-rod
nature of the polymer is best illus-
trated by the X-ray structure of Pt-
Hg-T (see Supporting Informa-
tion),[15] which also provides valua-
ble structural information for the
computational studies. While there
is a transparency window at 532 nm
for all the materials, the OPL
response was measured as a 92%
transmitting solution at this wave-
length using the Z-scan method.
From Figure 1a and 1b, it is clear
that all of them show excellent
OPLs at a high linear transmittance
of 92%; these values are superior to
Figure 1. Normalized transmittance (T/To) of metal polyynes and their model complexes at the same
linear transmittance (To =92%) versus incidentfluence ( To =86% for C60, Zn-TPP, and CuPc-tBu4).
current state-of-the-art materials
such as C60, ZnII-tetrakis(4-phenyl)-
Scheme 1. Synthesis of the metal polyynes and model complexes. a) trans-[PtCl2(PBu3)2], CuI, iPr2NH; b) HgCl2, NaOMe; c) trans-[PtPh(Cl)-
(PBu3)2], CuI, iPr2NH; d) MeHgCl, NaOMe; e) [PtCl2(dppe)], CuI, iPr2NH. dppe=1,2-bis(diphenylphosphanyl)ethane.
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measured for the singlet and triplet states (Figure 2). These
PL spectra show an intense triplet emission at 77 K, which is
attributed to the strong spin–orbit coupling between the
desirable to reduce its dragged absorption tail behind 400 nm
by reducing the p-conjugation. Normally, if the p-conjugation
is reduced the optical nonlinearity, and hence the OPL effect,
will be lessened.[17] However, from Figure 1c
and 1d it is evident that the conjugation factor
will become less important as the chain length is
increased up to the trimer, and Hg-T (or Pt-T)
shows almost the same OPL as that of Hg-P (or
Pt-P). As the order of p-delocalization through
the metal chromophore is PtII > HgII, we expect
that coupling of HgII to the backbone of Pt-P
breaks the p-conjugation to some extent but
favorably optimizes the transparency/nonlinear-
ity trade-off. As mentioned earlier, intramolec-
ular CT states exist between the Hg ion and the
conjugated ligand that would also contribute to
the OPL action of Pt-Hg-P. So, copolymeriza-
tion of Pt-M with HgII ion in the polyyne
backbone will shorten the conjugation length of
Figure 2. PL spectra for the metal polyynes in CH2Cl2 ata) 298 K and b) 77 K.
the ligands and result in a higher energy triplet
state accompanied by an increase in the triplet
quantum yield (FP), in accordance with the
ligand p and the metal d orbitals in the polyynes.[13c,d] To
confirm this, molecular orbital calculations on Pt-M, Hg-M,
and Pt-Hg-T as models for Pt-P, Hg-P, and Pt-Hg-P,
respectively, were carried out at the B3LYP level of density
functional theory based on their experimental geometries. In
general, the HOMO and LUMO are mainly confined to the p
and p* orbitals of the conjugated organic ligands, although
mixing of the metal dp orbitals with the ligand p orbitals in the
HOMOs can also be seen. The contribution of the dp orbitals
of PtII in both Pt-M and Pt-Hg-T and HgII in Hg-M to the
HOMOs of their corresponding complexes is 13.2%, 10.2%,
and 0.5%, respectively, according to the Mulliken population
analysis (see Supporting Information). It is known that spin–
orbit coupling increases with the extent of metal–ligand
orbital mixing, therefore Pt-P and Pt-Hg-T, which show a
stronger spin–orbit coupling effect, have a more effective ISC
that gives rise to a larger OPL response according to the RSA
mechanism. These theoretical results were confirmed exper-
imentally by the observation of weak triplet emission at room
temperature in Pt-P and Pt-Hg-P, which is almost absent in
Hg-P, and their slightly stronger OPL compared with that of
Hg-P. In spite of this, the OPL performance of Hg-P is still
attractive, and indicates that the OPL effect is not only
induced by the triplet absorption. To look for a possible
mechanism, we can see from the DFT results that the
contribution from the dp orbitals of HgII to the HOMO of
Hg-M is only 0.5%, while the metal pp orbitals contribute
energy-gap law for the triplet states in metal polyynes.[18] This
is also reflected by the lower singlet quantum yield (FF) of Pt-
Hg-P compared with that of Pt-P (see Supporting Informa-
tion). Pertaining to the RSA mechanism, we note that the
contribution of an increased FP of Pt-Hg-P to OPL can
compensate for the loss in OPL due to interruption of the
conjugation by Hg, hence Pt-Hg-P still shows a similar OPL
to that of Pt-P (Figure 1a). In addition, Pt-Hg-P has an
improved transparency window for optimizing the properties
of OPL materials of this kind (lmax = 386 nm for Pt-Hg-P vs.
399 nm for Pt-P, with the former being whiter than the latter).
By following the same approach to increase FP through such a
conjugation-breaking strategy, cis-Pt-P was designed and
synthesized.[19] By changing from a trans- to a cis-configured
PtII unit, the conjugation chain in cis-Pt-P remains disturbed
(lmax = 364 nm). What is remarkable here is that cis-Pt-P
shows almost the same OPL as Pt-P (Figure 1b), thus
indicating that the negative effect on OPL caused by
conjugation-interruption can be overcome effectively by the
positive contribution of increased FP. This is supported by the
much stronger triplet emission at 298 K (Figure 2a) and much
lower FF for cis-Pt-P compared with that of Pt-P.
Comparing the UV/Vis spectra of the polyynes and
common optical-limiting materials currently in use
(Figure 3), the merits of our polyynes as RSA optical limiters
for protecting eyes and optical sensors are obvious. They are
all white powders with very good transparencies in the visible
region, and cis-Pt-P in particular has a high triplet yield, all of
which makes these metallopolyynes excellent for OPL
applications. Some of the best ever materials, including
CuPc-tBu4, Zn-TPP, and C60, are all deeply colored with
strong visible absorption bands, therefore their poor optical
transparencies will make them less competitive with our
metallopolyynes as optical limiters for the protection of
human eyes.
19.6% to its LUMO. This reveals that the S0!S1 electronic
*
ꢀ
ꢀ
transition of Hg-P due to p(C CR)!p (C CR) orbitals also
possesses charge-transfer (CT) character.[9b] Its OPL effects
may be enhanced by the strong absorption of CT states, which
is known to be beneficial to OPL in other organometallic
systems.[13b]
We have recognized the salient problem of how to
increase the optical transparency range while maintaining
the OPL response of the polymers constant. In order to
improve the transparency window of Pt-P further, it is highly
The performance of excited-state absorbers can be
quantitatively characterized by the Figure of Merit factor
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Communications
conjugation by copolymerizing with Hg and tuning the Pt
geometry. These results are an encouraging milestone in the
progress towards commercially viable optical-limiting poly-
mers with optimized transparency/OPL trade-offs.
Experimental Section
Pt-Hg-T: Pt-M (0.100 g, 0.068 mmol) and MeHgCl (0.043 g,
0.169 mmol) were mixed in MeOH/CH2Cl2 (40 mL, 1:40, v/v) to
form a clear solution. A basic NaOMe solution (1.0 mL, 0.2 mmolLꢁ1
in MeOH) was added to the mixture at room temperature. After
stirring the mixture overnight at room temperature, it was filtered and
the filtrate was allowed to evaporate slowly to form pale-yellow
crystals of Pt-Hg-T. These were collected and recrystallized from a
CH2Cl2/MeOH (3:1, v/v) solution (yield: 0.085 g, 66%).
cis-Pt-P:
2,7-Diethynyl-9,9-dioctylfluorene
(0.055 g,
Figure 3. Comparison of the optical transparency of our metal polyynes
with some good optical limiters.
0.126 mmol), [PtCl2(dppe)] (0.083 g, 0.126 mmol), and CuI (3 mg)
were mixed in NEt3/CH2Cl2 (40 mL, 1:10, v/v) at room temperature.
After stirring overnight at room temperature, the reaction mixture
was filtered through a short pad of silica gel. The filtrate was
concentrated and the solution was poured into MeOH (50 mL). The
polymer powder was collected, washed with hexane, and dried under
vacuum (yield: 0.097 g, 75%).
seff/so, where seff and so are the effective excited-state and
ground-state absorption cross-sections, respectively. From
Table 1, it is clear that our metallopolymers are efficient
Pt-Hg-P: Pt-M (0.150 g, 0.102 mmol) and HgCl2 (0.028 g,
0.102 mmol) were dissolved in MeOH/CH2Cl2 (40 mL, 1:50, v/v) at
room temperature. A basic NaOMe solution (1.2 mL, 0.2 mmolLꢁ1 in
MeOH) was added. After stirring the reaction mixture overnight at
room temperature, it was filtered through a short pad of silica gel to
remove the insoluble solid. The solvent was removed under reduced
pressure. The residue was dissolved in a minimum volume of CH2Cl2
(6 mL) and the solution poured into MeOH (50 mL). The white
precipitate was collected by filtration and washed with hexane to
afford the copolymer (yield: 0.125 g, 73%). All the spectroscopic data
can be found in the Supporting Information.
Table 1: Comparison of the optical limiting parameters of metal polyynes
with some state-of-the-art materials.
[d]
Compound
F ]
th [Jcmꢁ2 [a] To [%][b] L [mm][c] seff/so
[{tri(n-hexyl)siloxy}InPc][4a] 0.070
[{tri(n-hexyl)siloxy}AlPc][4a] 0.26
84
84
84
62
[e]
86
55
84
92
92
92
92
10
10
10
[e]
[e]
1
2
1
1
1
16[4a]
10[4a]
14[4a]
[e]
[e]
6.20
[e]
3.89
20.81
19.07
18.32
18.62
[{tri(n-hexyl)siloxy}GaPc][4a] 0.12
[20a]
PbPc(b-CP)4
0.070
0.10
0.13
[20b]
CuPc-tBu4
CuPc-tBu4
[20c]
C60
0.18
Received: April 26, 2006
Revised: June 21, 2006
Published online: August 17, 2006
C60
0.19
Hg-P
Pt-P
Pt-Hg-P
cis-Pt-P
0.11
0.070
0.083
0.080
1
1
Keywords: alkynes · mercury · platinum · polymers · polyynes
.
[a] Optical-limiting threshold, Fth, defined as the input light fluence at
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cross-section, so is the ground-state absorption cross-section, and Tsat is
the transmittance at the saturation fluence. [d] Not reported.
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