Photorefractive properties of new polymer composites incorporating poly[ethynediyl-arylene-ethynediyl-silylene]s

Abstract

The photorefractive properties of low glass transition temperature, T_g, composites based on oligomeric poly[ethynediyl-arylene-ethynediyl-silylene]s as optical chromophores, poly(9-vinylcarbazole) as photoconductor, N-ethylcarbazole and phenyltrimethoxysilane as plasticizer, and fullerenes as charge generators have been investigated. The nonlinear optical effects, including four-wave mixing and two-beam coupling were studied at 633 nm. The photorefractive origin of the refractive index changes was confirmed by the achievement of a two-beam gain as high as 67 cm⁻¹ under an externally applied electric field (E₀ = 16 V µm⁻¹). Refractive index gratings affording two-beam gains up to 51 cm⁻¹ can also be written under zero-field conditions. A possible explanation for this unique property of the materials is that chromophore orientation asymmetry can be induced by the longitudinal intensity gradient.

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Introduction

The photorefractive (PR) effect in composite materials based on organic polymers was first reported about 10 years ago. Such photorefractive polymeric materials are being intensively investigated because of the possibility of using them in a variety of applications, including optical data storage, processing techniques, phase conjugation, and wave division multiplexing technology.¹ They have considerable technological potential because of their high efficiency, structural flexibility, light weight and their ability to be easily processed at low cost into large-area films. Since the first report on organic PR polymers, much research has been done to improve the materials in order to meet the necessary requirements for photorefractive applications, a variety of types of polymeric materials having been investigated.^2–4 We report here that novel compositions incorporating poly[ethynediyl-arylene-ethynediyl-silylene] (PEAES) chromophores show PR effects even in the absence of an applied electric field and without any preliminary electric poling. We also describe the preparation and characterization of two new PEAES containing penta- and hexacoordinate silicon resulting from the presence of the 8-(dimethylamino)naphthyl ligand.⁵ Few such polymers are known although the chemical and electronic behaviour of silicon is highly dependent on coordination number.⁶

Experimental

Reactions were carried out under dinitrogen using Schlenk tube techniques. Triethylamine was distilled over powdered KOH, and toluene was distilled from CaH₂. NMR spectra were run on a Bruker AVANCE DPX 200 spectrometer operating at 200 MHz (¹H), 50.327 MHz (¹³C) or 39.763 MHz (²⁹Si). IR spectra were recorded on a Perkin Elmer 1600 FTIR instrument in CCl₄ solution or as Nujol mulls. Mass spectra were obtained by use of a Jeol JMS-DX300 instrument. Elemental analyses were done at the CNRS Service Central d'Analyse. Size exclusion chromatography (SEC) was carried out in THF at a flow rate of 0.9 ml min⁻¹ by use of a Waters 510 system equipped with 100, 500, 1000 and 10 000 Å columns and a UV detector operating at 254 nm. The PEAES (Scheme 1) I⁷ and IV⁸ were prepared as previously described.

Scheme 1

Preparation of II

A mixture of bis[8-(dimethylamino-κN)-1-naphthalenyl-κC]diethynylsilicon⁹ (0.23 g, 0.55 mmol), 2,6-diiodo-4-nitro-N,N-dimethylaniline¹⁰ (0.21 g, 0.50 mmol), bis(triphenylphosphine)palladium(II) dichloride (7.3 mg, 0.010 mmol), and copper(I) iodide (3 mg, 0.016 mmol) in toluene (5 ml) and triethylamine (1 ml) was stirred at 89 °C for 24 h and then pumped to dryness. The residue was taken up in THF and filtered through diatomaceous earth. Addition of n-pentane to the filtrate precipitated II as a yellow powder isolated by centrifugation (0.28 g, 96% yield). IR [KBr, ν/cm⁻¹]: 2139 [ν(C[triple bond, length half m-dash]C)]. ¹H NMR (CDCl₃): δ 8.7–7.3 (m, 14 H, aromatic), 2.54 (s, 6 H, NCH₃), 1.35 (s, 6 H, NCH₃). ²⁹Si NMR (CDCl₃): δ −64.5. ¹³C NMR (CDCl₃): δ 155–118 (aromatic), 107.3 (SiC[triple bond, length half m-dash]C), 101.0 (SiC[triple bond, length half m-dash]C), 48.5 (NCH₃), 47.2 (NCH₃). UV/vis. (CH₂Cl₂): λ_max 298 nm, ε 12638 l mol⁻¹ cm⁻¹. SEC (THF): M_w 5020, M_n 3530, M_w/M_n 1.4.

Preparation of III

A mixture of [8-(dimethylamino-κN)-1-naphthalenyl-κC]diethynylmethylsilicon¹¹ (0.40 g, 1.52 mmol), 2,6-diiodo-4-nitro-N,N-dimethylaniline¹⁰ (0.63 g, 1.51 mmol), bis(triphenylphosphine)palladium(II) dichloride (0.8 mg, 0.001 mmol), copper(I) iodide (7.6 mg, 0.040 mmol), and triphenylphosphine (19.6 mg, 0.075 mmol) in toluene (10 ml) and triethylamine (10 ml) was stirred at 89 °C for 17 h and then filtered. The filtrate was pumped to dryness and the residue taken up in THF. The resulting solution was filtered and addition of n-pentane precipitated the oligomer as a yellow solid which was washed with methanol (acidified with a few drops of conc. HCl) and dried under vacuum (0.45 g, 70% yield). IR [Nujol, ν/cm⁻¹]: 3281 w, 2147 m [ν(C[triple bond, length half m-dash]C)], 1568 s, 1506 s, 1411 s, 1377 m, 1365 w, 1321 vs, 1252 w, 1233 w, 1206 w, 1158 m, 1094 w, 1075 s, 1022 w, 1001 s, 954 m, 904 w, 881 w, 827 w, 780 s, 757 m, 744 m, 722 w, 695 w, 664 w. ¹H NMR (CDCl₃): δ 8.5–7.5 (m, 8 H, aromatic), 3.19 (s, 3 H, NCH₃), 2.75 (s, 3 H, NCH₃), 0.77 (s, 3 H, SiCH₃). ²⁹Si NMR (CDCl₃): δ −55.7. ¹³C NMR (CDCl₃): δ 161–117 (aromatic), 102.3 (SiC[triple bond, length half m-dash]C), 93.5 (SiC[triple bond, length half m-dash]C), 48.5 (NCH₃), 44.9 (NCH₃), 2.4 (SiCH₃). SEC (THF): M_w 3100, M_n 2100, M_w/M_n 1.5.

Results and discussion

Preparation and properties of oligomeric chromophores

The PEAES I–IV (Scheme 1) were prepared by Pd catalyzed coupling between the appropriate diethynylsilane and dihaloarene as previously described for I⁷ and IV.⁸ The oligomers (average length ca. 7 units) contain Si with coordination numbers ranging from 4 to 6 [I(5), II(6), III(5), IV(4)]. The room temperature ¹H and ¹³C NMR spectra for the new oligomers II and III containing hexa- and pentacoordinate silicon, respectively, show two signals for the diastereotopic NCH₃ groups (Experimental). The acetylene ¹³C NMR resonances were assigned by analogy with the results of ¹³C–¹H coupling experiments on R₂Si(C[triple bond, length half m-dash]CPh)₂ model monomers. Decrease in Si coordination number from 6 to 5 on going from II to III results in the ²⁹Si NMR resonance being displaced downfield and the acetylene ¹³C NMR signals upfield, the ν(C[triple bond, length half m-dash]C) IR absorbance being shifted to higher energy (Table 1). On going further from III to the corresponding oligomer containing tetracoordinate Ph₂Si groups,¹⁰ the ²⁹Si and acetylene ¹³C NMR resonances are all displaced downfield, and the ν(C[triple bond, length half m-dash]C) IR absorbance is shifted to slightly higher energy. Thus, the electronic nature of the conjugated backbones of the oligomers is markedly dependent on the Si coordination state.

Table 1 Some spectroscopic properties of oligomers of structure

					δ(^13C)/ppm
Oligomer	(R^1R^2)^a	Si coord. no.	ν(C[triple bond, length half m-dash]C)/cm^−1	δ(^29Si)/ppm	Si–C[triple bond, length half m-dash]C^b	Si–C[triple bond, length half m-dash]C^b
a NpN = 1-(8-dimethylamino)naphthyl.b Assigned by analogy with results of ^13C–^1H coupling experiments on (1-Np)2Si(C[triple bond, length half m-dash]CPh)2.
II	(NpN)2	6	2139	−64.5	101.0	107.3
III	Me(NpN)	5	2147	−55.7	93.5	102.3
Ref. 10	Ph2	4	2150	−47.8	94.9	106.5

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Photorefractive investigations

For a material to be PR-active it is necessary to have (i) a photoionizable charge generator, (ii) a transporting medium, (iii) trapping sites, and (iv) a dependence of the refractive index on the space-charge field. In order to provide the dependence of refractive index on the electric field nonlinear chromophores with large hyperpolarizabilities are used, such as DEANST,⁴ DMNPAA,¹² or PDCST.¹³ However, despite the high efficiencies of existing PR polymers, in order to make devices suitable for technological applications much effort is still needed to (a) make faster materials, (b) reduce the values of the applied electric fields, and (c) eliminate phase separation in the composites.

We studied novel PR polymer nanocomposites containing as optical chromophores the Si-containing conjugated PEAES oligomers I–IV. C₆₀ or the more readily soluble C₇₀ fullerenes were used as charge generators, fullerenes being frequently employed for this purpose.^2–4 We used the well known charge transport agent poly(9-vinylcarbazole) (PVK),^4,12–16 and the plasticizer component was a mixture of N-ethylcarbazole (EK) and phenyltrimethoxysilane (PTMS). EK is a commonly-used efficient plasticizer in PR compositions giving rise to a large increase in the gain coefficient and the diffraction efficiency.^17,18 PTMS is a reactive plasticizer that undergoes hydrolysis in the presence of moisture affording a polyorganosiloxane matrix for the photorefractive composition.

As required for acitivity as optical chromophores, the oligomers contain both donor (dimethylamino) and acceptor groups (I amide; II–IV nitro). We have previously reported a high hyperpolarizability χ⁽³⁾ (giving rise to the electro-optic effect in isotropic materials) for I, both in solution as determined by the degenerate four-wave mixing technique,⁷ and in thin organic–inorganic hybrid films as measured by the Z-scan technique.^19,20

Samples for the present studies were prepared by sandwiching a layer of composite material 50–250 µm in thickness between glass slides coated with transparent indium tin oxide electrodes. The glass transition temperature of the compositions was low (ca. 8 °C as determined by differential scanning calorimetry) thus facilitating molecular reorientation. The electrical conductivity of the films decreased with drying time (Fig. 1) as the toluene solvent evaporated at ambient temperature in air and the sol–gel crosslinking reaction involving phenyltrimethoxysilane took place (for compositions see captions to Fig. 3–5 below). It should be noted that the samples exhibited low-voltage breakdown (at fields less than 10 V µm⁻¹) after short drying times (less than 30 min). Therefore, the experiments were carried out on samples of low conductivity (less than 7 pS m⁻¹ at E₀ = 16 V µm⁻¹). Oligomer I contains amide groups known to give rise to hybrid films of good optical quality.²¹ Films made from II–IV were also of satisfactory optical quality.

Fig. 1 The dependence of sample conductivity on the drying time, (•) measured at bias dc voltage 40 V; (■) measured at bias dc voltage 1.6 kV. Composite : PEAES(II) 5%, PVK 45%, C₇₀ 1%, EK 25%, PTMS 24%, sample thickness 100 µm.

The refractive index grating formation was investigated in the PR composites using the four-wave mixing (FWM) technique (Fig. 2). The original Gaussian beam of a He–Ne laser (λ = 633 nm) was split into two beams of equal intensities (I_w1 = I_w2 = 0.5 W cm⁻²). These s-polarized beams wrote the holographic grating in the sample. The angle of incidence was 60° and the angle of intersection was 0.03 rad. The third wave with intensity I_r, obtained by reflecting the first writing wave backward from the mirror, read the grating. The wave diffracted on the grating I_d was recorded. The diffraction efficiency of the grating was determined as:

where I_r(z = 0) = I_w1 exp( − 2αl) and α is the absorption.

Fig. 2 The set-up for the FWM measurements.

Measurements were made both under zero-field conditions and with a high bias voltage applied to the cell. In the first case, the diffraction efficiency achieved was 0.12% (Fig. 3(a)). The lifetime of the grating depended on the illumination. When only one writing beam (I_w2) was blocked, the grating lifetime was found to be ca. 20 min (Fig. 3(a)), whereas when all the beams were blocked, the grating lifetime became greater than 1 h [Fig. 3(b)] (D was registered by use of short pulses of I_w1 and I_r with time intervals of 2 min). Such behaviour of the grating lifetime can be explained by a resonant or PR origin of the grating. It should be noted that any possible contribution of the small scale grating written by reading beam I_r and writing beams I_w1, 2 to the diffracted beam was negligible. This was verified by blocking the reading beam I_r and the writing beam I_w1. Under these conditions, no diffraction of the beam I_w2 to the beam I_d was observed. Thus, the measured diffraction efficiency is related to the large scale grating.

Fig. 3 Diffraction efficiency of the grating (D = I_d/I_r) as a function of time for two compositions consisting of PVK 45%, C₇₀ 1%, EK 25%, PTMS 24% and (A) PEAES(II) 5% or (B) PEAES(I) 5%. (a) t = t^* corresponds to switching off the writing beam I_w2, E₀ = 0, L = 60 µm; (b) at time t = t^*I_w1, I_w2 and I_r were switched off, D was registered by short pulses of I_w1 and I_r with time intervals of 2 min, E₀ = 0, L = 60 µm; (c) samples of composition (A) with higher conductivity 6 pS m⁻¹ (△) and lower conductivity 3.5 pS m⁻¹ (■); at time t = 0 the electric field was switched on, at time t = t^* the field was switched off, E₀ = 15 V µm⁻¹, L = 60 µm, α = 164 cm⁻¹.

The transmittance of the sample did not vary substantially during 2 h observation under illumination with beam intensity 1 W cm⁻², so that

where Δα is the decrease in absorption α due to saturation, L is the film thickness, and θ is the internal angle of incidence. Hence, the possible contribution of the absorption grating to the diffraction efficiency can be estimated from the following expression:²²where Δn and Δα are the magnitudes of the refractive index and absorption gratings, respectively. The second term is due to the absorption grating. Thus, the contribution of the absorption grating to the diffraction efficiency D is negligible being ca. 2.5 × 10⁻⁵, i.e. much less than the observed level (∼10⁻³).

Next, in order to investigate whether the observed diffraction was of PR origin or the result of resonant refractive index changes (or possibly of some photochemical reaction), further study of its origin was made by using the two-beam coupling (TBC) technique. On application of a high bias voltage, providing an external electric field E₀ = 16 V µm⁻¹, a fivefold increase in the diffraction efficiency was observed (Fig. 3(c)). When the applied voltage was switched off, the diffraction efficiency relaxed to the initial level thus demonstrating the PR response of the composites under an applied external field.

The behaviour of samples of the same composition (made from II) but with different conductivities owing to different drying times were compared (Fig. 3(c)). The more conductive sample with a dark conductivity at E₀ = 16 V µm⁻¹ of ca. 6 pS m⁻¹ showed a grating formation time of 20 min. The grating formation time increased to 100 min for the sample with lower conductivity (3.5 pS m⁻¹ at E₀ = 16 V µm⁻¹) resulting from increased drying time. The dependence of the lifetime on the conductivity can be explained by the PR origin of the grating.²³ The maximum value of D achieved was 5.5 × 10⁻³, which, from eqn. (3), corresponds to a magnitude for the PR refractive index grating of Δn = 1.5 × 10⁻⁴.

It should be noted that when long term exposure (grating writing) was applied, formation of an amplitude transmittance grating was observed. The character time for the written grating was ca. 7 h, a steady state grating magnitude being achieved after 14 h exposure. The maximum diffraction efficiency measured for the amplitude grating was 2 × 10⁻². There was no substantial decrease in the diffraction efficiency over 1 month under dark conditions. The formation of the amplitude grating may be the result of some intermolecular reaction such as the slow photoinduced radical crosslinking of the PEAES chromophore.

The PR nonlocal origin of the optical nonlinearities of the samples was verified by using a standard two-beam coupling (TBC) setup. The p-polarized pump and probe beams intersected in the sample at a variable angle in the range ca. 0.03–0.41 rad. The pump–to–probe ratio was β = I_pump/I_probe = 625. The two-beam coupling gain coefficient was determined from the expression:²

where γ = I_{probe, with pump}/I_{probe, without pump}, and l_eff = L cos⁻¹θ is the effective length.

A TBC gain was observed at zero bias field (Fig. 4). The magnitude of the TBC gain in the absence of an applied field depended on the grating spacing (Fig. 5). The TBC gain under zero-field conditions was found to be 90.5 cm⁻¹ at a large angle of intersection. It can be seen that the optimal grating period is 2.75 µm. Such dependence is evidence for the PR origin of the grating.²

Fig. 4 Oscillograms of the probe beam for compositions containing PEAES 5%, fullerene 1%, PVK 45%, EK 25%, PTMS 24%. At time t = 0 the pump beam (I_pump = 0.2 W cm⁻²) and bias field (if applied) were switched on, the angle of beam intersection for all plots being 0.03 rad. (×) PEAES(I)/C₆₀; Γ = 7 cm⁻¹, α = 87 cm⁻¹, E₀ = 0, l_eff = 70 µm. (△) PEAES(II)/C₇₀; Γ = 25 cm⁻¹, α = 194 cm⁻¹, E₀ = 0, l_eff = 100 µm. (▽) PEAES(IV)/C₇₀; Γ = 51 cm⁻¹, α = 190 cm⁻¹, E₀ = 0, l_eff = 90 µm. (⋄) PEAES(III)/C₇₀; Γ = 52 cm⁻¹, α = 194 cm⁻¹, E₀ = 16 V µm⁻¹, l_eff = 70 µm. (□) PEAES(I)/C₆₀; Γ = 67 cm⁻¹, α = 87 cm⁻¹, E₀ = 16 V µm⁻¹, l_eff = 70 µm. (○) PEAES(II)/C₇₀; Γ = 64 cm⁻¹, α = 194 cm⁻¹, E₀ = 16 V µm⁻¹, l_eff = 100 µm.

Fig. 5 The magnitude of the TBC gain as a function of the grating period Λ. PEAES(IV) 5%, PVK 45%, C₇₀ 1%, EK 25%, PTMS 24%, α = 190 cm⁻¹, E₀ = 0, L = 60 µm.

However, it is well known that according to PR theory an external electric field to remove inversion symmetry is necessary for PR effects in organic materials. TBC gain has been reported in some PR polymeric materials without an external field or mention of preliminary poling, but no explanation was given.²⁴ More recently, a high TBC gain was observed for a polymeric material in the absence of poling or an external field and it was suggested that the effect was due to some kind of coupling between the space charge field grating and the orientational grating caused by trans–cis isomerization processes.²⁵ However, the mechanism for such coupling between the two kinds of gratings is not yet clear and in our case no such trans–cis isomerization is possible.

A possible explanation follows from the fact that the presence of the internal polarization field due to a longitudinal intensity gradient can produce local poling. The longitudinal intensity gradient produces a charge displacement and hence a space-charge field in the same direction. This internal field could give rise to local poling of the oligomer, thus removing the inversion symmetry. Such an effect of breaking the symmetry by longitudinal field formation has been observed experimentally²⁶ and explained theoretically²⁷ in photorefractive liquid crystal composites. Indeed, the possibility of observing such an effect in PR polymeric materials was suggested.²⁷

When an external voltage was applied, the probe beam was observed to increase in the presence of a pump beam and a bias field with definite direction (application of the opposite external voltage resulted in the absence of the weak increase in the probe beam) (Fig. 4).

The greatest TBC gain achieved at E₀ = 16 V µm⁻¹ was Γ = 67 cm⁻¹ for I, without any net gain due to the relatively high absorption coefficient, α = 87 cm⁻¹. Thus the PR nonlinearity is demonstrated by the presence of TBC with an external field. The achievement of a high net gain may be possible by applying a higher bias field, up to 100 V µm⁻¹. Indeed, high values of net gain observed in PR organic materials normally require electric fields greater than 60 V µm⁻¹.²

Direct measurements of the phase shift of the refractive index grating Φ from the light intensity pattern were carried out. As the grating was written, the sample was moved along the grating wave-vector (x-axis). The dependence of the probe beam power on the x-shift was studied (Fig. 6). A maximum in the TBC gain was observed at x = 0 thus showing there to be a π/2 phase shift between the grating and the interference field. The relation between the probe beam gain Γ and the phase shift of the refractive index grating Φ is well known (Γ ∼ sin Φ). Thus the theoretical prediction of measured dependence γ(x) can be written as:

where the grating period Λ = 22 µm.

Fig. 6 The dependence of the probe beam power on the x-shift of the sample PEAES(III) 5%, PVK 45%, C₇₀ 1%, EK 25%, PTMS 24%. (○) Experimental data and theoretical curve.

The theoretical curve γ(x) eqn. (6), assuming Φ = π/2 is shown in Fig. 6. It can be seen that there is good agreement between theory and experiment thus proving the PR origin of the TBC gain.²

In the presence of an applied electric field, the TBC gain observed for II was greater than that for III (Fig. 4) suggesting that silicon hexacoordination rather than pentacoordination is more favourable to photorefraction. Finally, it should be noted that improved properties for compositions containing longer chain homologues of the oligomeric chromophores can be expected.²⁸

In conclusion, the photorefractive properties of composites based on oligomeric PEAES as optical chromophores, poly(9-vinylcarbazole) as photoconductor, N-ethylcarbazole and phenyltrimethoxysilane as plasticizer, and fullerenes as charge generators have been investigated. The nonlinear optical effects, including four-wave mixing and two-beam coupling were studied at 633 nm. The photorefractive origin of the refractive index changes was confirmed by the achievement of a high two-beam gain under an externally applied electric field. Refractive index gratings affording two-beam gain can also be written under zero-field conditions. We suggest as a possible explanation for this unique property of the materials that chromophore orientation asymmetry can be induced by the longitudinal intensity gradient.

Acknowledgements

Financial support from INTAS (Brussels) [Open Call 97-1785], the National Russian Foundation (grants 99-03-32911, 00-15-97439 and 00-02-81206) and EOARD (London) is gratefully acknowledged. We thank Prof. P. M. Johansen and Mr. K. G. Jespersen for their investigation of the thermal transition temperatures of our samples.

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Footnote

† Present address: Canadian Space Agency, Space Technologies, 6767 route de l'Aéroport, St-Hubert, Québec, Canada J3Y 8Y9.

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