Ellipsometric determination of the electric-field-induced birefringence of photorefractive dyes in a liquid carbazole derivative

Chemical Physics 245 Ž1999. 407–415 www.elsevier.nlrlocaterchemphys

Ellipsometric determination of the electric-field-induced birefringence of photorefractive dyes in a liquid carbazole derivative E. Hendrickx

a,)

, B.D. Guenther a , Y. Zhang a , J.F. Wang a , K. Staub b, Q. Zhang b, S.R. Marder c , B. Kippelen a , N. Peyghambarian a a

b

Optical Sciences Center, The UniÕersity of Arizona, Tucson, AZ 85721, USA Molecular Resource Center, The Beckman Institute, California Institute of Technology, Pasadena, CA 91125, USA c Department of Chemistry, The UniÕersity of Arizona, Tucson, AZ 85712, USA Received 8 October 1998

Abstract We have used ellipsometric measurements on solutions of 2-dihexylamino-7-dicyanomethylidene-3,4,4a,5,6-pentahydronaphthalene and related polyenes in liquid 9-Ž2X-ethylhexyl.carbazole to measure the electric-field-induced birefringence at wavelengths of 690 nm and 830 nm. This electric-field-induced birefringence is related to the product of dipole moment squared and the polarizability anisotropy, m2D a , and to a large extent determines the dye’s photorefractive figure-of-merit ŽFOM.. We show that m2D a increases with the conjugation length and report one of the highest m2D a values observed so far. q 1999 Elsevier Science B.V. All rights reserved.

1. Introduction The photorefractive effect is caused by the absorption of light, followed by the diffusion and drift of photogenerated charges. If the spatial light intensity is nonuniform, the resulting nonuniform charge distribution leads to a space-charge field. This space-charge field subsequently modulates the refractive index w1x. The photorefractive effect can be used for a variety of applications, including optical data storage and optical correlation w2x. While first observed in inorganic crystals, it has been shown that photorefractivity also occurs in various types of other )

Corresponding author. Fax: q1-520-621-4442; E-mail: [email protected]

materials, such as organic polymers w3x, liquid crystals w4,5x, polymer dispersed liquid crystals w6x, and organic glasses w7x. Currently, due to their low cost, easy processability, high dynamic range, and subsecond response times, photorefractive polymers are actively being investigated. The highly efficient organic polymeric materials reported so far invariably contained up to 50 wt.% of a polar anisotropic dye. This dye usually is dissolved in a photoconductive matrix, such as polyvinylcarbazole ŽPVK.. If the materials are poled, have a high glass transition temperature ŽTg ., and if the dye has a nonzero hyperpolarizability, the refractive index is modulated by the Pockels effect only. In polymers that are close to Tg , the polar dyes can reorient in the electric field and the index modulation amplitude

0301-0104r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 1 - 0 1 0 4 Ž 9 9 . 0 0 0 4 9 - X

E. Hendrickx et al.r Chemical Physics 245 (1999) 407–415

408

will be strongly enhanced by birefringence w8x. For this class of low Tg polymers, a figure-of-merit ŽFOM. for the dye has been defined as w9,10x: FOM s

2 9kT

m2D a q mb

Ž 1.

where kT is the thermal energy, D a the polarizability anisotropy, m the dipole moment and b the first hyperpolarizability. The first term represents the contribution from birefringence to the index modulation, and the second term that of the Pockels effect. It has been shown that the FOM is optimized for dyes polarized beyond the cyanine limit w10x and that the contribution from birefringence is significantly larger than that of the Pockels effect. One of the highest FOM has been reported for the dipolar dye 2-dihexylamino- 7- dicyanomethylidene -3,4,4a,5,6-pentahydronaphthalene ŽDHADC–MPN. w11x. In a mix-

ture of DHADC–MPN : PVK : 9-ethylcarbazole ŽECZ. : 2,4,7-trinitro-9-fluorenone Ž25:49:25:1 wt.%. nearly complete internal diffraction could be observed at an applied field of 30 Vrmm and at a wavelength of 633 nm. By using the sensitizer Ž2,4,7-trinitro-9-fluorenylidene.malonitrile, the spectral response of the photosensitivity was extended to 830 nm and an external diffraction efficiency of 74% was obtained at an applied field of 59 Vrmm. An increase in the FOM would allow to further improve the dynamic range D n at lower applied field values. The first measurements of the FOM were done by means of frequency-dependent ellipsometry experiments on low-Tg polyvinylcarbazole polymer composites w12–14x. Here we report on ellipsometric measurements performed on solutions of DHADC– MPN and related chromophores in a liquid carbazole derivative, 9-Ž2X-ethylhexyl.carbazole ŽEHCZ.. The molecular structures are shown in Fig. 1. EHCZ was

Fig. 1. Molecular structure of the molecules used: Ž1. 2-dihexylamino-7-dicyanomethylidene-3,4,4a,5,6-pentahydronaphthalene ŽDHADC– X X MPN.; Ž2. 2-Ž3 H .-dicyanomethylidene-7-dioctylamino-4,4a,5,6,10,10a-hexahydroanthracene; Ž3. 2-dihexylamino-7-Ž3 -phenylisoxazol-5 X X Y on-4 -ylidene.-3,4,4a,5,6-pentahydronaphthalene; Ž4. 2-N-methyl-N-w4 -N, N-diŽ4 -butylphenyl.-aminophenylx-7-dicyanomethylideneX X 3,4,4a,5,6-pentahydronaphthalene; Ž5. 2-N-methyl-N-Ž9 -hexylcarbazol-3 -yl.-7-dicyanomethylidene-3,4,4a,5,6-pentahydronaphthalene; Ž6. X 9-Ž2 -ethylhexyl.carbazole ŽEHCZ..

E. Hendrickx et al.r Chemical Physics 245 (1999) 407–415

used as solvent since it resembles the photoconducting PVK–ECZ polymer matrix.

2. Synthesis It is known that donor ŽD. and acceptor ŽA. substituted polyenes ŽFig. 2A. exhibit large dipole moments m and polarizability anisotropy D a w10,15,16x, which shows their good potential as dopants in photorefractive polymer composites. However, many of the most highly nonlinear chromophores exhibit only moderate thermal and photochemical stability, largely due to cis–trans isomerization of the double bonds. Several papers suggested that cis–trans isomerization causes enhanced reactivity towards electrophiles and oxidation processes w17,18x. We incorporated the polyene into a fused-ring bridge chain that will preclude cis–trans isomerization ŽFig. 2B.. Additional steric hindrance associated with the ring system is expected to reduce self-aggregation and thereby increase the compatibility of the chromophores with polymer hosts. These dyes exhibit a considerable charge transfer that is confined along the quasi-one-dimensional p-conjugated bridge. The ability to change alkyl groups allows us to control the melting temperature so as to reduce the speed of phase separation in the photorefractive polymer composites. The target compound 2 should exhibit interesting properties as a chromophore for photorefractive materials w11,19x. We synthesized this compound and compared it with previously studied compound 1 to understand the conjugation length effect on the photorefractive FOM. The synthesis of 2 has been achieved by combining two literature known procedures w16,19x for the

Fig. 2. Molecular structure of unbridged ŽA. and bridged ŽB. donor–acceptor substituted polyenes.

409

conformationally locked polyene, followed by two additional steps for the functionalization with the donor and the acceptor moiety ŽScheme 1.. Starting from commercially available 2,7-dihydroxyanthraquinone Ž7. we found that 2,7-dimethoxyanthracene Ž10. can be synthesized following a method of Hall et al. w17x. Reduction of 7 with Al P Hg gives 2,7-dihydroxyanthracene Ž8.. The alcohol is acylated with acetic anhydride. The resulting ester 9 can be converted to 10. The overall yield for these steps is 17%. The w16x Birch reduction following Heilig and Luttke ¨ proved to be successful and gave 1,4,5,8,9,10-hexahydro-2,7-dimethoxyanthracene Ž11. in 95% yield. The addition of the acceptor, malononitrile yielded 12, which is a crystalline compound. Exchange of the weak methoxy-donor functionality by a dioctylamine-group yielded the desired product 2. The synthesis of compounds 3–5 will be described elsewhere. 2.1. Experimental 1

H NMR spectra were recorded at 300 MHz using a General Electric QE-300. Chemical shifts were referenced to the chemical shift of the residual protons of the solvent relative to tetramethylsilane. Elemental analyses were performed by Atlantic Microlabs. 2,7-Dihydroxyanthraquinone was purchased from Wilshire Chemical, Gardena, CA and used as received, even though it proofed not to be 90% pure. Ammoniumhydroxide solution, malononitrile and other chemicals were purchased from Aldrich and used as received. 2.1.1. 2,7-Diacetoxyanthracene (9) [17] 2 g Aluminum foil, chopped into small pieces is stirred in 100 ml water, containing 3 g HgCl 2 . After five minutes, the water is decanted and the aluminum–mercury couple is added to a refluxing suspension of 10 g Ž0.04 mol. 2,7-dihydroxyanthracene in 300 ml water–ethanol mixture Ž2:1.. 120 ml conc. NH 4 OH solution is added over a period of 10 min, followed by the addition of two portions of Al P Hg couple, made out of 1 g aluminum foil and 1.5 g HgCl 2 in 50 ml water each. The mixture is stirred at reflux for 30 min and then decanted hot into 10% aqueous HCl solution. The brown solid is collected and dried under vacuum in a desiccator to give 9.8 g

410

E. Hendrickx et al.r Chemical Physics 245 (1999) 407–415

Scheme 1.

crude 2,7-dihydroxyanthracene. The crude material is heated to reflux in 100 ml acetic anhydride. The solvent is removed at the rotary evaporator and the residue is dissolved in 400 ml dichloromethane and washed with 500 ml saturated NaCl solution. The aqueous layer is extracted twice with dichloromethane. The combined organic layers are dried over Na 2 SO4 and evaporated to dryness. The residue is separated on silica gel with dichloromethane as solvent to yield 2 g Ž6 mmol, 16%. orange solid which was used without further purification. 1 H NMR Ž300 MHz, CD 2 Cl 2 . d 8.48 Žs, 1H, CH.; 8.36 Žs, 1H, CH.; 8.04 Žd, J s 9.1 Hz, 2H, CH.; 7.69 Žd, J s 1.7 Hz, 2H, CH.; 7.26 Žd–d, J1 s 9.1 Hz, J2 s 2.2 Hz, 2H, CH.; 2.36 Žs, 6H, CH 3 .. 2.1.2. 2,7-Dimethoxyanthracene (10) [17] 2.0 g Ž6.8 mmol. 2,7-Diacetoxyanthracene are suspended in 250 ml MeOH. A catalytic amount of conc. H 2 SO4 is added and the mixture is heated to reflux for 4 h during which the initial suspension dissolved and later on, the product was precipitating. The mixture is cooled to ambient temperature and the solid is collected to yield 750 mg Ž3.1 mmol, 46%. of the desired product as slightly yellow solid. 1 H NMR see Ref. w16x. 2.1.3. 1,4,5,8,9,10-Hexahydro-2,7-dihydroxyanthracene (11) Has been successfully synthesized according to w16x. Heilig and Luttke ¨

2.1.4. 2 ( 3H ) -Dicyanomethylidene-7-methoxy4,4a,5,6,10,10a-hexahydroanthracene (12) 1.0 g Ž4.1 mmol. 1,4,5,8,9,10-Hexahydro-2,7-dihydroxyanthracene and 420 mg Ž6.3 mmol, 1.6 eq.. malononitrile are stirred together neat while heating the flask to 1508C oil bath temperature. After 20 min the flask is equipped with a distillation apparatus and the formed methanol is distilled under the aspiratorvacuum. The resulting brown solid is separated on silica gel with a gradient of cyclohexane–ethyl acetate Ž7:1–5:1–2:1.. The product is recrystallized from ethanol to yield 600 mg Ž2.2 mmol, 53%. purple crystals. 1 H NMR Ž300 MHz, d 6-benzene. d 6.36 Žs, 1H, CH.; 5.27 Žs, 1H, CH.; 5.04 Žs, 1H, CH.; 3.12 Žs, 3H, CH 3 .; 2.60–2.54 Žm, 1H.; 2.07– 2.00 Žm, 2H.; 1.82–1.57 Žm, 3H.; 1.27–1.21 Žm, 1H.; 1.12–0.92 Žm, 3H.; 0.71–0.51 Žm, 2H.. 13 C NMR Ž75 MHz d 6-benzene. d 168.5; 166.3; 158.8; 121.3; 118.1; 115.0; 113.8; 100.1; 79.9; 55.0; 36.4; 35.8; 35.8; 29.4; 29.2; 28.9 Ž2C.; 28.5. HRMS: calcd. m ŽH q .rz 279.1497; found m ŽH q .rz 279.1503. Elemental anal. calcd. C 77.67, H 6.52, N 10.06; found C 77.40, H 6.62, N 9.93. 2.1.5. 2(3H )-Dicyanomethylidene-7-dioctylamino4,4a,5,6,10,10a-hexahydroanthracene (2) 770 mg Ž2.8 mmol. 2Ž3 H .-Dicyanomethylidene7-methoxy-4,4a,5,6,10,10a-hexahydroanthracene is dissolved in 12 ml of pyridine and 8.4 ml Ž6.7 g, 28 mmol, 10 eq.. dioctylamine. Under nitrogen, the mixture is heated to 1058C for 3 h, after which, the

E. Hendrickx et al.r Chemical Physics 245 (1999) 407–415

temperature is raised to 1258C. Stirring is continued at this temperature for 90 h. Then, the pyridine and dioctylamine are removed under vacuum. The dark purple residue is dissolved in dichloromethane and filtered over silica gel with dichloromethane as solvent. The filtered product is separated on silica gel with cyclohexane–ethanol acetate Ž8:2. as solvent to give the product as a dark purple solid. 1 H NMR Ž300 MHz, CDCl 3 . d 6.24 Žs, 1H, CH.; 5.79 Žs, 1H, CH.; 5.28 Žs, 1H, CH.; 3.27–3.15 Žm, 4H, NCH 2 .; 2.91–2.34 Žm, 1H.; 2.59–2.31 Žm, 4H.; 1.99–1.86 Žm, 3H.; 1.59–1.35 Žm, 8H.; 1.28 Žbr s, 20 H.; 0.87 Žt, J s 6.2 Hz, 6H, CH 3 .. 13 C NMR Ž75 MHz d 6-benzene. d 167.4; 161.5; 160.4; 157.3; 117.3; 116.4; 115.3; 113.3; 99.1; 63.3; 50.8; 36.7; 36.6; 35.8; 31.7; 29.6; 29.4; 29.2; 28.1; 27.3; 27.0; 22.6; 14.1; two aliphatic signals overlapping. HRMS: calcd. m ŽH q .rz 487.3926; found m ŽH q .rz 487.3933. 2.1.6. 9-(2X-Ethylhexyl)carbazole (6) To a solution of 5 g Ž29.9 mmol. carbazole and 6.76 g Ž35 mmol. 2-ethylhexylbromide in 20 ml dimethylformamide was added 2 g Ž69.9 mmol. 60% sodiumhydride in mineral oil in small portions. The reaction was carried out at room temperature for 4 h. 200 ml Water was added. The product was extracted with hexane. After removal of the solvent, the crude product was purified on a silica column using hexane as solvent to yield 7.4 g Ž88.7%. of pure 9-Ž2X-ethylhexyl.carbazole as colorless oil. Note that 6 has an asymmetric carbon atom and is a racemic mixture of the two optical enantiomers. 1 H NMR Ž250 MHz, CDCl 3 . d 8.09 Žd, 2H, CH.; 7.40 Žm, 4H, CH.; 7.18 Žm, 2H, CH.; 4.13 Žd, 2H, NCH 2 .; 2.08 Žm, 1H, CH.; 1.30 Žm, 8H, 4CH 2 .; 0.83 Žm, 6H, 2CH 3 .. Elemental Anal.: calcd. C 85.97, H 9.02, N 5.01; found C 86.20, H 9.13, N 5.01.

3. Measurements The measurements of the FOM were done by means of frequency-dependent ellipsometry experiments, as were previously described for low-Tg polyvinylcarbazole polymer composites w12–14x. Briefly, in this technique the sample is placed be-

411

tween crossed polarizers and the transmission is measured using linearly polarized light. If a periodic electric field superimposed to a DC field are applied to the sample, the modulated induced birefringence will lead to a modulated change in transmission that can be measured using a lock-in amplifier. The ellipsometric response function RŽ V . is calculated from the modulated intensity Im as w12,13x: RŽ V . s

3 Im Ž V . lGd

Ž 2.

Iip n 3 VB VAC

where Ii is the total intensity, l the experimental wavelength, d the sample thickness, n the refractive index, V B a constant bias voltage applied over the sample and VAC the amplitude of the modulated voltage Ž V Ž V . s VAC sinŽ V t .. superimposed on the bias voltage. For the experiments on the liquid samples, we used V B s 25 V and VAC s 25 V. G is a geometrical factor and is defined as: Gs

(

n n 2 y sin2 Ž u .

Ž 3.

sin2 Ž u .

u is the angle of incidence in air with respect to the sample normal Ž458.. At low frequencies of the modulating field, the response function is related to the molecular parameters by w12,13x: RŽ V . s

12p 3 n

4

ž

2

Bq

4 3

Cq2 D

/

Ž 4.

with Bs

2 45

m Nf` D a

C s Nf 0 f`2

kT

2

Ž 5.

mb

5kT g 2

D s Nf 02 f`

ž /

5

Ž 6. Ž 7.

N is the density of molecules, b the first hyperpolarizability, and g the second hyperpolarizability, f` s Ž n 2 q 2.r3 the Lorentz–Lorenz local-field factor, and f 0 s ´ Ž n 2 q 2.rŽ2 ´ q n 2 . the Onsager local field factor, with ´ the low-frequency dielectric constant. From the static hyperpolarizability of 1, b 0 s 63 = 10y3 0 esu w11x, a value of b s 220 = 10y3 0 esu at

E. Hendrickx et al.r Chemical Physics 245 (1999) 407–415

412

690 nm can be extrapolated using the two-level model for the Pockels effect w20x. With b s 220 = 10y3 0 esu and m s 12 = 10y1 8 esu, it can be verified that B s 50 C. Since B ) C ) D, Eq. Ž4. reduces to: RŽ V . s

4p 15

ž

n2 q 2 n4

m

2

/ ž / ND a

kT

Ž 8.

Note that this equation theoretically is valid only for pure solutions of chromophores or for guestrhost systems in which the host does not contribute to the measured response function. If several components contribute to the response function, Eq. Ž8. becomes: RŽ V . s

4p 15

ž

n2 q 2 n4

1

2

/ž / Ý kT

Ni m 2i D a

Ž 9.

i

where the summation runs over the components of the mixture. All the molecules were dissolved in 6. Sample cells consisted of ITO-coated glass slides. Glass spacer beads were used to ensure a uniform cell thickness of 111 mm. Since a difference of 0.2 in the refractive index of the liquid leads to a 20% change in the value of m2D a , the refractive index was measured for each solution separately. Because the distance between the glass slides of the cell was 111 mm over the entire surface and reflections occur at the glass–liquid interfaces, the cell has the transmission characteristics of a Fabry–Perot etalon. A series of interference fringes is observed in the transmission spectrum and, when the dispersion is small, the refractive index n can be calculated from:

l2 ns

2 LD l

Fig. 3. Interference fringes observed in the transmission spectrum of a 111 mm thick cell filled with 6. From the observed fringe spacing of 1.31 nm, a refractive index of 1.64 was calculated. Inset: Molar refraction of mixtures of 1 and 6 as a function of the mole fraction of 6. From the intercept and slope, and using Eq. Ž11., the trace of the polarizability tensor of 6 was calculated to be 3.0=10y2 2 esu.

pendence of the refractive index using the Lorentz– Lorenz equation w21x: n 2 y 1 M Ža. x Ža. q M Ž b. x Ž b. n2 q 2 s x Ža.

r

ž

Ža . Ža. Ža. a11 q a 22 q a 33

q x Ž b.

3

ž

/

Žb. Ž b. Ž b. a11 q a 22 q a 33

3

/

Ž 11 .

where x and M are the mole fractions and the molecular weights, respectively, and r is the density. The superscripts Ža. and Žb. refer to the guest and host molecule, respectively. The left-hand side term in Eq. Ž11. is also called the molar refraction.

Ž 10 .

where L is the cell thickness, l the wavelength and D l the spacing between the interference fringes. As shown in Fig. 3, typical fringe spacings are of the order of 1.3 nm. This method only requires small amounts of material, typically 30 mg, and can be used to determine the refractive index over a wide wavelength range. The trace of the polarizability tensor can be calculated from the concentration de-

4. Discussion The inset of Fig. 3 shows the value of the molar refraction as a function of the compound 1 mole fraction. For 1 at 690 nm, we found that a 11 q a 22 q a 33 s 3.0 = 10y2 2 esu. The evolution of the refractive index for a solution of 1 Ž N s 0.85 = 10 20 moleculesrcm3 in 6. with wavelength is shown in

E. Hendrickx et al.r Chemical Physics 245 (1999) 407–415

Fig. 4. Dispersion of the refractive index of a mixture of 1 and 6 as a function of wavelength. The solid line is a fit to the Sellmeier dispersion equation.

In the low-frequency limit, when VAC s VB , the ratio of RŽ V .rRŽ2 V . theoretically is equal to 4 w12x. As can be seen from Fig. 5, in the liquid samples the ratio of RŽ V .rRŽ2 V . is nearly constant from 10– 10,000 Hz. For the measurements on the liquid samples at a modulation frequency of 5 kHz, RŽ V .rRŽ2 V . were 3.8 " 0.2 and 4.3 " 0.1, at 690 nm and 830 nm, respectively. Note that our values of RŽ V . and RŽ2 V . have a factor of 62 to correct the rms values of Im taken from the lock-in amplifier. For 1, RŽ V . was measured as a function of the density of molecules at 690 nm and 830 nm. To analyze the data, the response function was divided by Ž n 2 q 2.rŽ n4 . and plotted as a function of molecular number density ŽFig. 6.: RŽ V .

Fig. 4. The data could be fitted to Sellmeier’s dispersion equation: 2

n y1sAq

B

l2 y l20

Ž 12 .

where l0 s 495 nm, the absorption maximum of 1, A s 1.45 " 0.03, B s 78,000 " 7000 nm2 . The typical evolution of RŽ V . when the frequency of the modulating field is scanned from 5 Hz to 100,000 Hz is shown in Fig. 5. In polymeric samples with a Tg close to room temperature, the response function decreases by a factor of 4 or more in the frequency range from 100–10,000 Hz, as the dye molecules can no longer reorient in the polymer matrix at high frequencies. For the liquid samples, RŽ V . is virtually independent of frequency from 100–10,000 Hz. In the region from 5–50 Hz a small decline in the response function can be seen, which we attribute to the migration of ionic species. Above 10,000 Hz, a sharp drop in the response function indicates that the dipoles can no longer reorient at these high frequencies. For all the measurements on the dilution series, the experimental frequency was set to 5 kHz. We have also measured the modulated intensity at twice the modulation frequency and calculated the response function as w12,13x: RŽ2 V . s

Im Ž 2 V . 3 lGd 2 Iip n 3 VAC

Ž 13 .

413

ž

n2 q 2 n4

4p s

/

15

1

ž / kT

2

Nm2D a q b

Ž 14 .

The units of RŽ V . and kT are m2rV 2 and J, respectively. m and D a can be converted from SI-units to the more frequently used esu-units with: D a ŽSI. Ž m3 . s 4p = 10y6D a Žesu . Ž cm3 . 1 mŽSI. s = 10y11mŽesu . 3

Ž 15 . Ž 16 .

Since the index corrected response function from pure 6 is small compared to the signal from the dye solutions, it was treated as a constant. The data were

Fig. 5. Response function of 4 in 6 Ž N s1.89=10 20 cmy3 ; ns1.86; l s690 nm. at the fundamental Žfull circle. and harmonic frequency Žopen circle..

414

E. Hendrickx et al.r Chemical Physics 245 (1999) 407–415

Fig. 6. Ž RŽ V ..rwŽ n 2 q2.rŽ n 4 .x at a modulation frequency of 5 kHz as a function of concentration for 1 in 6 at 690 nm Žfull circles. and 830 nm Žopen circles.. The slope is equal to 6.9=10 18 m2D a , where 6.9=10 18 is a conversion factor.

fitted by linear regression and the product m 2D a was calculated from the slope. The results obtained for 1 and the other dyes in Fig. 1 are summarized in Table 1. The value of m 2D a found for 1 at 830 nm, m 2D a s 5.7 = 10y5 7 esu, is larger than the value deduced from four-wave mixing experiments Ž3.9 = 10y5 7 esu. and frequency-dependent ellipsometry measurements Ž3.3 = 10y5 7 esu. in a plasticized polyvinylcarbazole matrix at that wavelength, but in fairly good agreement w11x. The reduced efficiency in the polyvinylcarbazole matrix can possibly be attributed to an incomplete reorientation of the polar molecules or to dipolar aggregation effects in the polymer composite. Assuming a dipole moment of m s 12 = 10y3 0 esu for DHADC–MPN, we calculate D a s 6.8 = 10y2 3 esu and 3.9 = 10y2 3 esu at 690 nm and 830 nm, respectively. Note that the ratio of D a to a 11 q a 22 q a 33 is approximately 1r4. Similar ratios have been reported for other donor– acceptor substituted photorefractive dyes w22x. If the conjugated system of 1 is extended by one double bond as in 2, a red-shift in the absorption maximum is observed, together with a strong increase in m2D a . For 2, we found m 2D a s 14 = 10y5 7 esu at 690 nm, and m2D a s 8.1 = 10y5 7 at 830 nm. These values are among the highest that have been reported. Due to an increase in electron acceptor strength, the position of the absorption maximum of 3 is red-shifted compared to that of 1. Thus, for 3 the lmax is intermediate between that of 1 and

2, and m2D a follows the same tendency at both 690 nm and 830 nm. Previously we have shown that 1, when dissolved in an inert matrix, can simultaneously act as a charge-transport agent, charge generator and index modulating chromophore. The speed of these devices, however, was on the order of several tens of seconds w23x. Therefore we synthesized molecules 4 and 5, that retain the 1 unit, but are also functionalized with a hole-transporting unit, to improve the charge mobility. Analysis of the ellipsometric results shows that functionalization of the amino donor function of 1 with these bulky hole-transporting groups, reduces m 2D a . Given the more disc-like appearance of the molecule, this reduction can be partly induced by the decrease of the polarizability anistropy. Also note that the carbazole and triphenylamine groups have small dipole moments that do not coincide with the dipole moment of the 1 subunit. Finally, for all the molecules the m2D a values at 690 nm are larger than those at 830 nm due to the dispersion in the linear polarizability tensor. The reproducibility of the m2D a values listed in Table 1 is typically within 5%. However, considering the approximations involved in theory, such as the estimate of the local field factors, and the neglect of the mb contribution, an error of "15% is more realistic. Two other methods have been used for the characterization of chromophores for photorefractive applications. Ellipsometric measurements in common organic solvents using a pulsed electric field have been used to measure the polarizability anisotropy of 1Ž4-nitrophenyl.-2-pyrrolidinemethanol and 4-w N-Ž2-

Table 1 Absorption maxima Žnm. and m2D a values determined for molecules 1–5 in ethylhexylcarbazole Dye number

1 2 3 4 5

lma x in CHCl 3

m2D a Ž10y57 esu.

Žnm.

690 nm

830 nm

495 591 540 492 492

9.8 14 10 3.1 3.8

5.7 8.1 7.0 2.8 2.9

The values of m2D a are given in electrostatic units, i.e., m in Debye Ž10y1 8 esu or 10y18 statcoulomb cm. and D a in cm3 .

E. Hendrickx et al.r Chemical Physics 245 (1999) 407–415

hydroxyethyl.-N-ethylx-amino-4X-nitroazobenzene or disperse red 1 w24x. A combination of depolarized light scattering experiments, semi-empirical calculations, dipole moment measurements, and refractive index measurements gave D a values that were in good agreement with frequency dependent ellipsometry experiments w22x. The main advantages of the ellipsometric measurements in 6 are their speed, experimental simplicity and the straightforward calculation of the relevant parameter m 2D a . In summary, we have used ellipsometric measurements to determine the electric-field-induced birefringence of 1 and derivatives in 6. The refractive index of the solutions was measured independently. The m2D a value of 1 can still be improved by increasing the acceptor strength or by extending the conjugation length. Functionalization of the amine function with bulky hole-transporting groups such as carbazole and triphenylamine has a detrimental effect on m2D a value.

w3x w4x w5x w6x w7x

w8x w9x

w10x w11x

w12x w13x

Acknowledgements This research was supported by the U.S. Office of Naval Research Žthrough the MURI Center CAMP., NSF, an international CNRSrNSF travel grant, the U.S. Air Force Office of Scientific Research, the U.S. Ballistic Missile Defense Organization, and a NATO travel grant. E.H. is a postdoctoral research assistant of the Fund for Scientific Research, Flanders ŽBelgium.. K.S. thanks the Schweizerischer Nationalfonds for a stipend-fellowship.

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