Mat. Res. Soc. Symp. Proc. Vol. 771 © 2003 Materials Research Society
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Sol-gel Nanohybrid Materials Incorporating Functional Thiacalixarenes for Non-Linear Optical Applications C. Desroches1, S. Parola1*, D. Cornu1, P. Miele1, P. L. Baldeck2, C. Lopes3, 1
Laboratoire des Multimatériaux et Interfaces, UMR 5615 CNRS, Université Claude Bernard-Lyon 1, 69622 Villeurbanne, France. E-mail :
[email protected] 2 Laboratoire de Spectrométrie Physique, UMR 5588 CNRS, Université Joseph Fourier , Saint Martin d’Hères, France 3 Swedish Defence Research Agency, Division of Sensor Technology, 581 11, Linköping, Sweden
ABSTRACT Macrocyclic thiacalixarenes have interesting non-linear optical properties. Thiacalixarenes bearing phenylazo, imino, ethynylic groups or platinum derived acetylides on the upper rims were designed, prepared and fully characterised. They showed interesting properties for optical limiting. Moreover these macrocycles are highly soluble in organic solvents and can easily be embedded in inorganic matrix using the sol-gel routes. Thiacalixarene based class I and II solid-state materials were prepared and their optical limiting behaviour was investigated. INTRODUCTION Optical Power Limiting (OPL) materials are being developed for the protection of electro-optical sensors and eyes against laser aggressions [1-4]. One property, which plays an important role for optical limiting, is the delocalisation of π-electrons. The optical limiting can also be improved by the formation of metal complexes due to spin-orbit coupling [5]. Functionalised thiacalix[4]arenes were prepared for optical limiting on the basis that they display delocalised π-electrons, they can be selectively functionalised either on the “upper” or the “lower rims” and are interesting species for the formation of metal complexes [6-12] . Moreover, the presence of electron donating sulphur bridges can play an important role regarding non-linear optical properties. Also, functional groups can be selected in order to increase the electron delocalisation over the molecule and to form metal complexes. The thiacalixarenes have also shown high stability upon heating, with decomposition temperatures in the range 210 - 320°C depending on the functionalisation. These macrocycles possess interesting non-linear optical properties, are quite soluble in organic solvents and are thermally stable. They are also an interesting class of molecules for the preparation of sol-gel hybrid materials for solid-state optical limiting devices. EXPERIMENTAL The synthesis of the functionalised thiacalixarenes, metal complexes and sol-gel precursors were previously described in the literature [12].
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For the preparation of class I materials, 20.6 ml of a solution of thiacalixarene in THF (1.8 mM) was added to a solution of tetramethylorthosilicate TMOS (3.7 ml) in ethanol (10.3 ml) and formamide (1 ml). Hydrolysis of the tetraalkoxysilane was performed by adding water (2.3 ml) in acidic conditions (HNO3, 0.063 ml). The solution was then poured in polypropylene moulds. Gelation and careful drying of the gels were then performed at room temperature during several weeks. Homogeneous and transparent hybrid monoliths with concentrations of about 15 mM in thiacalixarene were obtained. The xerogels could be cut and polish for optical characterisations. For the preparation of class II hybrids, the functionalised sol-gel precursor could either be used as single source precursor of hybrid organic-inorganic material or together with TMOS using the same procedure as for the elaboration of class I xerogels. Optical limiting experiments were performed in a f/5 set-up, using 2 mm quartz cells for the solutions or 2 mm solid-state glass materials. A frequency doubled Nd-YAG laser operating at 10 Hz, delivering 5 ns pulses at 532 nm was used in the experiments.. A Nd:YAG-pumped optical pametric oscillator which supplies a collimated beam (140 µm HW 1/e²) of 2.6 ns (FWHM) pulses in the 450-650 nm spectral range was used to characterize nonlinear absorption spectra. DISCUSSION Regarding the literature, the chemistry of thiacalixarenes is much less developed than for similar calixarenes. This is probably due to the difference in reactivity between these two families of macrocycles. Most of the substitution reactions, which were reported for the thiacalixarenes, concerned the “lower rims”. We were interested in developing the “upper rims,” since this gives the possibility of increasing the electron delocalisation along the four aromatic rings. Reaction pathways were thus investigated for the preparation of thiacalixarenes functionalised through C=N and C≡C bonds [6, 9-12]. Alkynylthiacalixarenes are prepared from the tetrahalothiacalixarenes and substituted acetylenes (Scheme 1) by using the well known Sonogashira catalysis. Several functionalised macrocycles with phenyl, phenylethynylthiophene or platinum acetylides groups, presenting various chain lengths for the electron delocalisation, could be prepared using similar reaction pathways (figure 1). These macrocycles were purified and fully characterized prior to optical investigations. The structures are all 1,3 alternate without any rotation possibility because of the steric effect of the propyl groups grafted on the lower rims [12]. R
X Sonogashira catalysis
S OPr X = Br, I.
S OPr
4
4
R = H, phenyl, Pt acetylides....
Scheme 1. Synthesis of the C≡C substituted thiacalixarenes
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C5H11
C5H11 H
S OPr
(Et)3P
S
S OPr
4
(1)
Pt
S
S OPr
4
(2)
(3)
P(Et)3
4
OPr
4
(4)
Figure 1. Examples of functionalised thiacalixarenes for optical limiting applications Optical investigations on the alkynylthiacalix[4]arenes in solution showed very high linear transmissions in the visible wavelengths, nearly 100%. Optical limiting measurements on a solution of the tetra-phenylethynylthiacalix[4]arene (2) in THF (0.15M) with a f/5 set-up at 532 nm showed a clamping level below 4 µJ for input energies up to 140 µJ (figure 2). Transmission measurements using the OPO set-up showed a strong non-linear absorption in the blue area between 450 nm and 500 nm (figure 3a) and no absorption over 550 nm. Figure 3b is the corresponding calculated absorption spectra using a three-photon absorption model, i.e. two-photon absorption followed by an excited-state absorption during the pulse duration. These transmission spectra are confirmed by measurements at fixed wavelength versus the input energy. The transmitted energy decreases for the wavelengths 450 and 550 nm while it remains almost constant for 650 nm (Figure 3c). A similar behavior was observed for the other macrocycles (3-4) for which investigations are still in progress. The Pt complexes showed low clamping values similar to the thiacalixarene (2) (3-4 µJ) but with much lower concentrations (0.03M). This is important since the optical limiting is increases with the concentration of absorbing molecules. The non-linear absorption of phenylazothiacalixarenes was previously evidenced to be a Reverse Saturable Absorption in the visible spectral range, and a. a Two Photon Absorption at 796 nm (Ti:saphirre laser with 170 fs pulses) [10].
Transmitted energy [µJ]
4
3
2
1
0 0
20
40
60
80
100
120
140
Input energy [µJ]
Figure 2. OPL measurement in a f/5 set-up at 532 nm with 5 ns pulses (98% linear transmission @ 532 nm) for the tetra-(phenylethynyl)-tetrapropoxythiacalix[4]arene.
1,2
12000
1,0
10000
1,0
0,8
8000
0,8
λ=450 nm λ=550 nm λ=650 nm
0,6
0,4
6000
3
E=20 µJ E=200 µJ E=300 µJ
Transmission
2
α3 (cm /GW )
Transmission
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0,2
4000
0,6
0,4
2000
0,0
0,2
0 450
500
550
600
650
0,0
450
500
550
Wavelength (nm)
600
650
0,01
Wavelength (nm)
(a)
0,1
Input Energy (mJ)
(b)
(c)
Figure 3. Non-linear absorption measurements showing transmitted energy versus wavelength with fixed energy (a), the spectrum of the 3 photon non linear absorption coefficient (b) and transmitted energy versus laser energy with fixed wavelength (c). Incorporation of the optical limiting compounds into a solid state inorganic matrix (a solid host) can improve the nonlinear properties. Site-isolation minimises the possibilities of interactions between the macrocycles. This can lead to a lengthening of the excited state lifetimes and stronger non-linear absorption. Solid state optical limiters are also more attractive than liquid based limiters. It is well known that the sol-gel route allows easy inclusion of organic dyes into an oxide network because of the low temperature of the process [12-15] . Two major classes of the so-called organic-inorganic hybrids can be considered. In class I hybrids the organic molecules have weak chemical interactions with the inorganic network (Van der Waals, hydrogen bonds). In class II hybrids the organics are strongly bonded to the inorganic network (covalent). Class I materials were prepared with thiacalixarenes concentration of about 0.015 M in the final material (figure 4). This was not sufficient to observe a non-linear absorption. Higher doping concentration is needed for obtaining interesting clamping values. Investigation are going on to improve the stability of highly concentrated class I hybrids. This can be achieved through functionalisation of the lower rims of the aromatic rings with other alkyl groups to increase their solubility. R
R
R
Br Br
1-iodo-2-tertiobutyldimethylsiloxyethane
S OH
O 4
O Si
S
4
3-(isocyanatopropyl) -triethoxysilane
NBu4F
R-acetylene
O
S
4
O Si
O
S
S O
4
4
O
OH O
NH (CH2)3 Si(OEt)3
Scheme 2. Reaction pathway to the class II precursors
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Si(OR)3 Si(OR)3 C5H11
C5H11 O
S O
NHHN O O O
S
O O S S O
O O
O O
NH
NH
C5H11 C5H11
Si(OR)3
Si(OR)3
Figure 4. Example of designed sol-gel precursor for nanohybrid optical materials and examples of hybrid lenses with embedded thiacalixarenes. Class II hybrids were also considered in order to permit the elaboration of more concentrated and stable hybrids. A second functionalisation (lower rims) with metal alkoxide groups had to be investigated in order to prepare the class II sol-gel hybrid materials incorporating the optically active molecule. Acylation reaction was used for the functionalisation with the siloxane group. The hydroxyl group of the thiacalixarene was not reactive towards the isocyanato derivative probably because of steric effects. A spacer bearing a primary alcohol function was then grafted on the macrocycles. The well known acylation reaction could then be realised on the thiacalixarene giving the functionalised triethoxysilane, which can be used as starting precursor of the final hybrids (scheme 2). CONCLUSION Thiacalixarenes are very interesting macrocycles for non-linear optical applications because of the possibility of selective functionalisations. This was confirmed by the nonlinear absorption and optical limiting measurements. The same macrocycle core was thus bifunctionalised, and used for its optical properties on one side and as a precursor for the solgel process on the other side. Preparation of nanoscaled homogeneous hybrid materials can thus be achieved by controlling the sol-gel process at the molecular level from the functionalised alkoxide.
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