Article pubs.acs.org/JPCC
Gigantic Electric Dipole Moment of Organic Microcrystals Evaluated in Dispersion Liquid with Polarized Electroabsorption Spectra Hung-Chu Chiang,† Toshifumi Iimori,†,‡ Tsunenobu Onodera,§ Hidetoshi Oikawa,*,§ and Nobuhiro Ohta*,†,‡ †
Graduate School of Environmental Science, Hokkaido University, Sapporo, 060-0810, Japan Research Institute for Electronic Science, Hokkaido University, Sapporo 001-0020, Japan § Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan ‡
ABSTRACT: Polarized electroabsorption spectra of the 4-N,Ndimethylamino-4′-N′-methyl-stilbazolium tosylate (DAST) microcrystal, which is well-known as an organic nonlinear optical material, were measured in decalin liquid using electric-field modulation spectroscopy. On the basis of the results, the magnitude of the electric dipole moment in the ground state of DAST microcrystals whose size was about 0.35 μm was determined to be as large as ∼4.5 × 104 D, and the magnitude of the difference in the dipole moment between the ground state and excited state was estimated to be ∼2.5 × 103 D, though the local field correction was not made. The transition moment of the DAST crystal was also found to be a little affected by application of electric fields.
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INTRODUCTION Organic crystal 4-N,N-dimethylamino-4′-N′-methyl-stilbazolium tosylate (DAST) has been widely studied as a promising candidate for organic nonlinear optical (NLO) materials since DAST takes a noncentrosymmetrically packed crystal structure, due to the Coulombic interactions, which is the basic requirement to provide the second-order NLO properties. The crystal structure of DAST is monoclinic with space group Cc. The crystallographic parameters are: a = 10.365 Å, b = 11.322 Å, c = 17.893 Å, β = 92.24°; the volume of a crystal cell with four molecules is about 2.1 nm3.1 The high efficiency of the second harmonic generation, which is ca. 1000 times higher than that of urea at 1907 nm, and wideband (0.1−1.5 THz) radiation have rendered the DAST crystal to be a promising material for technological applications such as high speed modulation and frequency mixing applications.2−5 The recent advances on organic microcrystals are very attractive for the photonic applications because of their sizedependent unique optical properties. Well-defined DAST microcrystals can be fabricated by a reprecipitation method, which provides the dispersion liquid of microcrystals, in which organic microcrystals can be easily oriented in a dispersion medium by applying an electric and/or magnetic field. In fact, Fujita et al.6,7 measured the field-induced absorbance changes of DAST microcrystals by the applied static electric field, and Kaneko et al.8 investigated the difference in absorbance with application of magnetic fields. The dispersion liquid of microcrystals can be said to have both crystal and liquid properties. The total dipole moment was estimated to be 2.8 × 107 D in one microcrystal whose size was 100 × 100 × 50 nm3, © 2012 American Chemical Society
supposing that the dipole moment of one DAST molecule is 30 D.7 However, it was remarkably difficult to experimentally evaluate the dipole moment of one DAST microcrystal. In the present study, we have clarified electro-optical properties of DAST at the micrometric scale and compared the results with the data calculated previously.7 Using electric-field modulation spectroscopy, we measured electroabsorption (E-A) spectra, i.e., electric-field-induced changes in the absorption spectrum. E-A spectroscopy is a powerful technique to probe the electronic structure straightforwardly because E-A spectra can be related both to the difference in dipole moment and to polarizability between the electronically excited state and the ground state and to the field-induced change in transition dipole moment. For an ensemble of mobile polar molecules, the parameters characterizing the field-induced orientation can also be obtained. The molecular parameters can be obtained by fitting the theoretical model to the E-A spectra. We have observed E-A spectra of DAST microcrystal suspension in the region of 350−700 nm, which is helpful in understanding not only molecular reorientation but also the physical properties such as the dipole moments in the ground state and in the excited state.
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EXPERIMENTAL SECTION DAST microcrystals were prepared by the reprecipitation method. An amount of 0.5 mL of DAST−ethanol solution (5 Received: January 16, 2012 Revised: March 9, 2012 Published: March 14, 2012 8230
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voltage was applied to the liquid cell. The strength of the applied electric field was evaluated from the applied voltage divided by the distance between two ITO layers. E-A and absorption spectra were measured using a spectrometer (Jasco, EMV-100). Linearly polarized light taken from a Xe lamp with a polarizer was irradiated to a sample film for excitation. The transmitted light intensity was detected by a photomultiplier. The dc component (T) was recorded by an analog-to-digital converter. The ac component (ΔT) was detected by a lock-in amplifier (SR830, SRS) at the second harmonic (2ω) of the frequency of the applied voltage. The detailed experimental procedures are described elsewhere.9
mM) containing dodecyltrimethylammonium chloride (5 mM) was injected into 50 mL of stirred Acrydic A-1381(surfactant)− decalin solution (0.1 wt %). The chemical structure of constitutes of a DAST crystal is shown in Figure 1, together
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Figure 1. Chemical structure of constituents of a DAST crystal (left) and a sample of DAST microcrystals dispersed in decalin liquid (right).
THEORETICAL BACKGROUND
The theoretical interpretations of E-A spectroscopy were already reported,10−13 and only the basic principles are briefly described here. Energy levels and transition dipole moments of a molecule are influenced by an electric field. In the presence of an applied electric field (F), the shift of the transition energy (ΔE) is related to the difference in dipole moment (Δμ) and molecular polarizability (Δα) between the excited state and the ground state.
with a DAST sample of decalin dispersion liquid. Measurements of scanning electron microscopy (SEM; JSM-6700F, JEOL) and dynamic light scattering (DLS; DLS-7000, Ootsuka Electronic Co.) were carried out to confirm the crystal shape and size. Before SEM observation, platinum was sputtered onto DAST microcrystals filtrated with a filter paper (JHWP, Millipore). A SEM image of DAST microcrystals filtrated on a filter paper is shown in Figure 2, which shows that DAST
ΔE = −Δμ·F − F ·Δα·F /2
(1)
The electric field dependence of the transition dipole moment can be written as d F = d + X ·F + F ·Y ·F
(2)
where d is the transition dipole moment vector in the absence of F, and X and Y are the transition dipole moment polarizability and hyperpolarizability tensors, respectively. As a result, optical spectra show a shift or/and a broadening upon the application of F. By assuming an isotropic distribution of the molecules in the absence of F, the intensity of the E-A spectrum at wavenumber ν̅, i.e., ΔA(ν̅), can be expressed as the sum of the zeroth, first, and second derivatives of the absorption spectrum as follows ⎧ ∂ ⎛ A( ν ̅ ) ⎞ ΔA( ν̅) = |fF |2 ⎨AχA( ν̅) + Bχ ν̅ ⎜ ⎟ ∂ν̅ ⎝ ν̅ ⎠ ⎩ ⎪
⎪
+ Cχ ν ̅
Figure 2. SEM image of DAST microcrystals.
microcrystals are rectangular. The coexistence of stabilizer Acrydic A-1381 caused the difficulty in clear observation of SEM images, and so the DLS measurements were used to determine the average size of the present DAST crystals. In fact, the crystal size of DAST microcrystals was estimated to be 348 ± 105 nm by the DLS method. Note that DLS is a technique that can be used to determine the size distribution of small particles in suspension or polymers in solution. The size calculated through the Stokes−Einstein equation is the size of a sphere that moves in the same manner as the scatterer. The sample solution of the DAST microcrystals was flowed into the sandwich-type liquid cell, which consisted of two ITOcoated quartz windows and a polymer spacer of 500 μm, with a rotatable holder. ITO layers were used as the semitransparent electrodes. A silicon−dioxide film was coated on the ITO layer as an insulator film with a thickness of 0.58 μm. The sinusoidal
∂ 2 ⎛ A ( ν ̅ ) ⎞⎫ ⎜ ⎟⎬ ∂ν̅ 2 ⎝ ν̅ ⎠⎭ ⎪
⎪
(3)
where Aχ, Bχ, and Cχ are coefficients and f is the internal field factor. Note that χ represents the angle between the field direction and the polarization direction of the excitation light. If the angular motion of the sample is fast enough to follow the electric field variation and the Y term and the tensor term of XijXlk are negligible, each coefficient can be expressed as follows Aχ =
[
1
∑ (2diXijμj ) +
3kT |d|2 ij
(3 cos2 χ − 1) 30kT
(3 cos2 χ − 1) μ2 (3 cos2 ζ − 1) + 3(αm − α̅ )] + kT 15kT |d|2
∑ (3diXjiμj + 3diXjjμi − 2diXijμj ) ij 8231
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The Journal of Physical Chemistry C Bχ =
μΔμ cos γ (3 cos2 χ − 1) Δα̅ + + 3hckT 2hc 15hc ⎧ μΔμ(3 cos ζ cos η − cos γ) 3(Δαm − Δα̅ ) ⎫ ⎨ ⎬ + ⎩ ⎭ kT 2 +
(3 cos2 χ − 1) 15hc |d|2
− 2diXijΔμj ) +
Cχ =
Article
derived, as shown in Figure 3. The observed excitation spectrum, i.e., the b2 band, is very different in shape from the total absorption spectrum, and it is likely that the total absorption spectrum can be decomposed into different absorption bands. Then the b2 band was subtracted from the whole absorption spectrum. From the resulting spectrum, two broad bands with a peak at 21 150 and ∼17 000 cm−1, which are denoted by the b1 band and b3 band, respectively, were obtained. As a sum of the b1, b2, and b3 bands, the whole absorption spectrum of DAST microcrystals is reproduced, as shown in Figure 4. Thus, the observed absorption spectrum was
∑ (3diXjiΔμj + 3diXjjΔμi ij
2
∑ diXijΔμj
3hc |d|2 ij
(5)
Δμ2
Δμ2 + (3 cos2 χ − 1)(3 cos2 η − 1) 2 2 2 2 6h c 30h c (6)
where c is the velocity of light; k is Boltzmann’s constant; h is Planck’s constant; T is temperature; and μ and Δμ are the magnitudes of the ground state dipole moment and the change in the dipole moment following photoexcitation, respectively. The subscripts i and j refer to components of the vectors or tensors. αm is the polarizability in the ground state with respect to the direction of the transition dipole moment. α̅ represents the average of the trace of the polarizability tensor. ζ is the angle between μ and d. γ is the angle between μ and Δμ. η is the angle between Δμ and d.
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RESULTS AND DISCUSSION The absorption spectrum of DAST in decalin liquid is shown in Figure 3, where the DAST microcrystals show an absorption
Figure 4. Absorption spectrum of DAST microcrystals in decalin and the decomposition of the absorption spectrum into different components.
considered to be a superposition of different absorption bands of b1, b2, and b3, as shown in Figure 4. Then the E-A spectra were regarded as a sum of the contributions from these three components: ΔA = ΔAb1 + ΔAb2 + ΔAb3. Bhowmik et al14 reported the absorption spectrum of a single crystal thin film of DAST, which showed the absorption peak at 550 nm and fwhm ∼100 nm. Since both the position and shape of their spectrum are similar to those of the b2 band, the b2 band is ascribed to a single crystal like structure of the DAST microcrystal. Note that the location and shape of the b2 band result from the photoluminescence excitation spectrum, as mentioned above. The b1 and b3 bands may be ascribed to molecular arrangements in DAST microcrystals different from the single crystal like structure, but a detailed structure is unclear at the moment. However, it should be noted that the absorption spectrum of the b1 band with a peak at 473 nm is similar to the absorption spectrum of the DAST methanolic solution with a peak at 476 nm6 as well as the absorption spectrum of the DAST microcrystals with peaks at 425 and 475 nm.7 Each DAST microcrystal may have a polydomain structure, which is different from the bulk single crystal. The observed E-A spectra, which remarkably depend on χ, are shown in Figure 5. The E-A spectra could be analyzed using the decomposed three absorption bands and their derivatives. Actually, every E-A spectrum could be reproduced by a linear combination of the zeroth derivatives of b1, b2, and b3 and the first derivative of b2 (see Figure 6 for χ = 54.7° and for χ = 90°). The simulated E-A spectra at χ = 52, 55, 61, and 90° are shown in Figure 7, together with the observed spectra. The fact that the zeroth derivative component is not zero at the magic angle indicates that the transition moment is influenced by F.
Figure 3. Absorption spectrum (black line), PL excitation spectrum (red line), b2 band deduced from the observed excitation spectrum (blue line), and the subtraction of the b2 band from the whole absorption spectrum (green line).
peak at 553 nm and a shoulder at ∼500 nm. Photoluminescence (PL) of DAST microcrystals dispersed in decalin liquid shows a broad band with a peak at 581 nm and with a bandwidth of ∼60 nm. Then, the emission excitation spectrum of the dispersed DAST microcrystal was observed by monitoring the emission at 750 nm. Note that the photoluminescence excitation spectrum is regarded as the same as the absorption spectrum, as far as the absorption intensity is weak. The obtained excitation spectrum shows a peak at 557 nm and a bandwidth much narrower than that of the observed absorption band (see Figure 3). From the excitation spectrum thus obtained, a smooth absorption spectrum, i.e., b2, was 8232
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Figure 7. Simulated E-A spectra (dotted line) and observed E-A spectra (solid line) at different angles of χ.
Figure 5. Polarized E-A spectra of DAST microcrystals dispersed in decalin observed with different angles of χ. Applied electric filed was 1 kV cm−1.
Table 1. Coefficients, Slopes, and Physical Parameters Obtained from the Simulation of the E-A Spectra of DAST Microcrystals in Decalina A54.7° (kV−2 cm2) slope (kV−2 cm2) B54.7° (kV−2 cm) ∑ij(2diXijμj)/|d|2 (CV−1 m2) μ (D) Δμ (D) a
b1
b2
b3
−0.023(±0.005) 0.133(±0.01) − −4.9(±1.0) × 10−32 4.5(±0.1) × 104 −
0.007(±0.002) 0.139(±0.006) 15(±1) 1.5(±0.4) × 10−32 4.6(±0.1) × 104 2.5(±0.1) × 103
0.020(±0.002) 0.127(±0.007) − 4.3(±0.4) × 10−32 4.4(±0.1) × 104 −
The experimental error is shown in parentheses.
Figure 6. (a) Absorption spectrum and the decomposed absorption bands, (b) the first derivative of each decomposed band, (c) E-A spectra of DAST microcrystals observed at χ = 90°, and (d) E-A spectra of DAST microcrystals observed at χ = 54.7°. The first derivative of the whole absorption spectrum (dotted line) is shown in (d) by a dotted line. Applied electric filed was 1 kV cm−1 in strength and 4 Hz in frequency.
Figure 8. Plots of Aχ as a function of (3 cos2 χ −1) for bands of b1, b2, and b3.
A54.7° corresponds to the first term in the right-hand side of eq 4. Thus, A54.7° gives ∑ij(2diXijμj)/|d|2, whose value at 298 K is shown in Table 1. It is noted that the observed E-A spectra could not be reproduced by a linear combination of the zeroth, first, and second derivatives of the whole absorption spectrum. As a reference, the first derivative of the whole absorption spectrum is shown in Figure 6d, to show how the E-A spectrum could not be reproduced with the derivatives of the whole absorption spectrum. As shown in Figure 8, plots of Aχ give a straight line as a function of (3 cos2 χ − 1) for each band of b1, b2, and b3, indicating that eq 4 is applicable for every band. The slope of
the straight line is shown in Table 1, together with the zeroth and first derivative components at the magic angle of χ, i.e., A54.7° and B54.7°. The coefficient A90° is about one order of magnitude larger than A54.7°, indicating that the zeroth derivative component resulting from the field-induced orientation is much larger than that from the field-induced change in transition moment. Then it was assumed that the third term in the right-hand side of eq 4 is much smaller than the second term in DAST microcrystals; Aχ − A54.7° was regarded as corresponding to the second term of eq 4. It was also assumed that the latter part of the second term in eq 4, which results from the anisotropy of polarizability, was negligible since the field-induced reorientation originating 8233
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frequency of applied ac voltage. The result is shown in Figure 9. The signal intensity rapidly decreased with an increase of the
from the large electric dipole moment in the ground state is considered to be much more efficient than the reorientation originating from the anisotropy of polarizability. It was confirmed that the E-A signal of the DAST microcrystals in a solid film, where the microcrystals were immobilized, was too weak to be detected, when field strengths used in the present study were applied. This fact shows the validity of the assumption that the E-A signal induced by a change in polarizability following absorption is negligible in the present EA spectra in dispersion liquid (see eq 5). Finally, Aχ − A54.7° obtained from the E-A spectra corresponds to the former part of the second term in eq 4 which includes μ2(3 cos2 ζ − 1). In space group Cc, the dipole moment of the DAST molecule is located in-of-a−c plane of the crystal. By assuming that the angle between μ and d is 35°,7 i.e., ζ = 35°, the ground state dipole moment, μ, was determined for each band of b1, b2, and b3, using the slope of the straight line in the plots of Aχ given in Table 1. As shown in Table 1, the ground state dipole moment of the DAST microcrystal is confirmed experimentally to be as large as ∼4.5 × 104 D, irrespective of molecular arrangements in microcrystals. The first derivative component, i.e., Bχ, was confirmed for the b2 band, as already mentioned. This component is independent of χ, and its intensity decreased dramatically when ac frequency increased, as mentioned below, indicating that this component results from the field-induced reorientation. Then B54.7° is regarded as corresponding to the first term of the right-hand side in eq 5, and μΔμ cos γ can be obtained. Using the value of μ determined from Aχ, Δμ cos γ could be determined. The coefficients used to simulate the observed E-A spectra and the evaluated molecular parameters are shown in Table 1. By assuming that the direction of Δμ is the same as that of μ, i.e., γ = 0, Δμ was obtained for the b2 band. The result is shown in Table 1. The change in dipole moment following photoexcitation is about 10% of the original electric dipole moment. At the moment, it is not known why the first derivative component is not clear for the b1 and b3 bands. A brief description should be made for the second derivative term in the E-A spectra. This term is expected from the nonzero value of Δμ for the b2 band (see eq 6). The magnitude of ΔA induced by the above-mentioned value of Δμ is estimated to be about 1.6% of the observed value of ΔA, which is too small to be realized in the present experiments, and so the second derivative term was not included in the simulation. Negative point charge and positive point charge separated by 0.1 nm induce the dipole moment of 4.8 D. The size of DAST microcrystals used in the present study was 0.35 μm, as mentioned above. When the positive and negative charges are located with a separation of 0.35 μm, the resulting dipole moment is simply calculated to be ∼1.7 × 104 D, which is the same order of magnitude as the one determined in the present experiment. Then, a DAST microcrystal may be regarded as such a charge-separated material where the positive point charge is located near the one edge and the negative point charge is located near the opposite edge. It is noted that the internal field correction was not made in the present analysis; that is, f in eq 3 was assumed to be 1. The values of μ and Δμ given in Table 1 are multiplied by the correction factor f. For the quantitative comparison between the experimental value of μ and the calculated value, therefore, the internal field correction must be carefully considered. To examine the response rate of the sample reorientation to the frequency of F, we measured the dependence of ΔT/T on
Figure 9. Change in transmitted light intensity relative to the transmitted light intensity at zero field as a function of the frequency of the applied electric field. The strength of the applied electric field was 1 kV cm−1. The straight line was obtained by assuming a linear dependence in the frequency region below 1.5 Hz.
modulation frequency. The signal intensity at the intercept obtained by extrapolating the straight line onto the zero frequency was used to determine the physical parameters shown in Table 1, though the E-A spectra shown in Figure 5 were obtained with a modulation frequency of 4 Hz. Note that the change in frequency of the applied ac voltage does not induce a change in the E-A spectrum except for the signal intensity.
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CONCLUSIONS Electric field effects on absorption spectra of DAST microcrystals in dispersion liquid have been measured using E-A spectroscopy. The polarized E-A spectra of DAST microcrystals were obtained and simulated with the absorption spectrum deconvoluted with photoluminescence excitation spectrum. Then, the magnitude of the dipole moment in the ground state was evaluated to be as large as ∼4.5 × 104 D for DAST microcrystals having a diameter of 0.35 μm. Probably this is the first case that such a large dipole moment more than 10 000 Debye was experimentally confirmed in organic materials. The difference in the dipole moment between excited and ground states was evaluated to be ∼2.5 × 103 D for the absorption band corresponding to the single crystal of DAST. The change in transition dipole moment of DAST microcrystals was also confirmed with application of electric fields in the order of kV cm−1. It is also suggested that each DAST microcrystal has a polydomain structure, giving three different absorption bands.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected];
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was in part supported by a Grant-in-Aid for Scientific Research (No. 20245001) from the MEXT in Japan. This 8234
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research was also supported by the Nano-Macro Materials, Devices and System Research Alliance.
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