Communication pubs.acs.org/cm
Matrix-Assisted Poling of Monolithic Bridge-Disubstituted Organic NLO Chromophores Delwin L. Elder, Stephanie J. Benight,† Jinsheng Song,‡ Bruce H. Robinson, and Larry R. Dalton* Department of Chemistry, University of Washington, Seattle, Washington 98195, United States S Supporting Information *
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econd-order nonlinear optical (NLO) materials have been utilized in commercial applications for computing, telecommunications, and sensing such as electronic/photonic integrated circuits, phased array radar, terahertz spectroscopy, etc. A common electro-optic (EO) device is the Mach−Zehnder modulator, which is very important commercially for translating ultrafast electrical signals into optical signals for fiber-optic telecommunications.1−3 The vast majority of commercial Mach− Zehnder modulators use inorganic lithium niobate as the EO material. However, organic EO materials have faster intrinsic response times and lower dielectric constants than lithium niobate leading to higher modulation bandwidths and efficiency.1 Second-order EO materials may also be compared by their Pockels4 EO coefficient (r33) which governs the size, voltage, and power requirements of a modulator (e.g., to the voltage-length product of a modulator). Developments over the past decade have identified organic materials that have an r33 that is more than an order of magnitude greater than that of lithium niobate when examined as thin film materials.5−7 The EO coefficient r33 is proportional to three critical parameters: EO chromophore number density, N, first molecular hyperpolarizability, βzzz, and the acentric order parameter, ⟨cos3 θ⟩. Many studies have focused on improving r33 by increasing βzzz.1,4,5 Recent proof of principle experiments have demonstrated a powerful EO chromophore molecular design methodology for increasing r33 by increasing N and the acentric order parameter.8 This molecular design entails attaching side chains to the chromophore that have liquid crystallike properties (e.g., alkoxybenzoylcoumarins).9 The side chains of C1 (Figure 1) have been shown to have some degree of selfassembly from coumarin−coumarin intermolecular interactions.8,10,11 Chromophores with these side chains can be used without a polymer host because the structure-directing side chains act as the matrix that (1) enhances the acentric order of the NLO portion of the chromophore upon poling and (2) increases the viscoelasticity of the material due to long-range intermolecular electrostatic interactions. This strategy has been named “matrix-assisted poling.”12 Our prior results show that attaching two of these side chains to a conventional “FTC” (thiophene/ polyene bridge)-based chromophore results in a poling efficiency (r33/poling field) increase from 0.45 nm2/V2 to 1.9 nm2/V2 and an estimated factor of 2 increase in ⟨cos3 θ⟩.8 In this paper, we improve upon the molecular design by switching to “CLD,”13 a polyene bridge chromophore that has a higher β,14 and changing the position of attachment of the alkoxybenzoylcoumarin side chains (molecule DLD164). We expected that the symmetrical placement of the side chains on the chromophore bridge might result in improved reduced dimensionality and therefore higher degrees of molecular © 2014 American Chemical Society
Figure 1. NLO chromophores. DLD164 and C1 have alkoxybenzoylcoumarin side chains (R1) shown in red. DLD136 has bulky TBDPS functional groups (R2) shown in red.
order. As a second discovery, we report a control molecule that replaces the alkoxybenzoylcoumarin side chains with bulky tertbutyl-diphenyl-silyl (TBDPS) functional groups (molecule DLD136). Although the TBDPS groups are expected to have no long-range cooperative interactions like the alkoxybenzoylcoumarins, their inclusion results in a chromophore that is filmforming and has a high poling efficiency as a single-component, monolithic chromophore despite its high number density. The two chromophores highlighted in this paper, DLD136 and DLD164 are based on a combination of structural elements found in YLD124 and C1. One key difference is that two substituents are attached to the same side of the central cyclohexenyl ring, which is a new synthetic achievement with respect to second order organic EO materials. The syntheses are described in detail in the Supporting Information. Their glass transition temperatures (Tg) were measured by differential scanning calorimetry (DSC) (see Table 1 and Supporting Information). Only Tg transitions and no crystallization peaks were observed for DLD136 and DLD164, suggesting that they are amorphous. They may be solvent cast into high-quality thin films that do not crystallize or crack when annealing or storing for months, as opposed to YLD124, which must be diluted in a host polymer to achieve optical quality films for poling. Furthermore, a neat material is simpler to handle and process consistently and has no Received: October 23, 2013 Revised: December 24, 2013 Published: January 3, 2014 872
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Communication
Table 1. Spectroscopic, Poling, and Physical Property Data chromophore densitye chromophore
solution λmax (nm)a
avg poling efficiency (nm2/V2)b
max poling field (V/μm)c
max r33 achieved (pm/V)b
Tg (°C)d
molecules × 1020/ cm3
wt %
DLD136 DLD164 25% YLD124 in PMMA
779 780 786g
1.7 ± 0.19 2.0 ± 0.13 1.27 ± 0.08h
87 75 80
170 137 118
72 66 105
4.95 3.95 1.71
58.0f 46.3f 25.0
In chloroform solution. bAverage r33/poling field measured using the ATR method at 1310 nm.17,18 cMaximum sustained poling field without electrical breakdown or shorting. dMeasured by differential scanning calorimetry on the second heating (heating/cooling rate of 10 °C/min). e Assumes a density of 1.0 g/mL based on measurement and calculations on similar molecules. fWeight percent of NLO active chromophore (in black in Figure 1) relative to total molecule with side chains. gSee ref 26. hSee ref 19. a
chromophores is due in part to their higher density (r33 is proportional to chromophore density). The density of YLD124 in PMMA is 1.71 × 1020 molecules/cm3. The density of the NLOactive portion of DLD136 is 4.95 × 1020 molecules/cm3. Higher densities of YLD124 in a host polymer are ineffective at boosting EO performance due to the well-known phenomenon of r33 saturation at high concentrations (due to increased centric ordering).1,2 Given the high density of DLD136, it is remarkable that it poles at all. The two bulky TBDPS bridge substituents may provide sufficient site isolation to inhibit dipole−dipole coupling common to high dipole moment molecules such as these.20 DLD136 is the first high β EO chromophore that we know of that poles efficiently21 as a single component system that is not a macromolecule or does not have structure-directing side chains (e.g., with aryl/perfluoroaryl interactions5 or other cooperative interactions such as in C18 or DLD164). DLD164 has a higher poling efficiency than DLD136 even though the density is lower (the density of the NLO-active portion of DLD164 is 3.95 × 1020 molecules/cm3, black in Figure 1). The improved performance of DLD164 is presumably due to the matrix assisted poling effect stimulated by the alkoxybenzoylcoumarin side chains.12 Though increased EO performance of a neat alkoxybenzoylcoumarin-containing chromophore has been observed recently relative to a control chromophore in a guest/host system,8 this is the first time the matrix-assisted poling concept has been compared with another relevant neat chromophore analog. We have integrated Teng-Man ellipsometry with our electric field poling apparatus to provide in situ (real-time) observation of the degree of poling.22 Teng-Man monitors Im, a signal intensity related to the change in index of refraction upon poling, and is proportional to r33 − r13. Figure 2a shows that as a sample of DLD164 is heated under a poling field of 63 V/μm (82 V), there is a sudden rise in Im upon reaching the chromophore Tg, which indicates that poling starts to occur. Figure 2b shows that at the same time that poling starts, there is a large current flow that reaches up to 175 μA (approximately 1.5 mA/cm2). Increased current flow or current spikes upon poling is common, but tends to be more severe with neat chromophore systems. For this reason, TiO2 was used as a barrier layer between the ITO electrode and the chromophore. Conductivity barrier layers, such as TiO2, have been shown to reduce conductivity in EO poling devices, and have become part of standard use.15,16,23 Even though TiO2 barrier was used for all of the poling results reported here, conductivity was still high and led to electrical breakdown and device failure which limited the highest poling field for DLD136 to 87 V/μm and to 75 V/μm for DLD164. When the poling induced current was high enough, there was a large corresponding drop in the effective voltage Veff even though the applied voltage (Vapp) remained the same (Figure 2c). For the
Figure 2. In situ poling data for DLD164 on ITO/TiO2. (a) Plot of Im (brown) and temperature (black). (b) Plot of current (green) and temperature. (c) Plot of effective voltage (blue) and temperature. The dashed red line indicates the start of the poling process.
danger of phase separation as does a guest/host system like YLD124/PMMA. Parallel-plate electric field poling devices were prepared according to standard methods (also see Supporting Information).8 The stacked device structure was as follows: glass/ITO/ TiO2/chromophore/gold. A thin (100 nm) sol−gel-derived TiO2 film was used as a barrier layer to reduce conductivity during electric field poling.15,16 The monolithic chromophores were spun cast onto TiO2-coated ITO from 1,1,2-trichloroethane and dried in a vacuum oven at 65 °C for at least 12 h yielding 1.0−1.6 μm thick films. A gold electrode (60 nm) was deposited on top by argon plasma sputter coating. Chromophore alignment was carried out by electric field poling: an electric field was applied across the electrodes while heating the samples from room temperature to the Tg at ∼10 °C per minute, holding at Tg for approximately 5 min, and then cooling slowly to well below the Tg before the electric field was turned off. The r33 was measured by the Attenuated total reflection (ATR) method at 1310 nm.17,18 The average results are shown in Table 1 (obtained from eleven devices of DLD136 and ten devices of DLD164). The poling efficiency (r33/poling field) is the best way to compare the EO performance of different chromophores, as it levels the effect of the poling field. DLD136 has a poling efficiency of 1.7 nm2/V2, and DLD164 has a poling efficiency of 2.0 nm2/V2, compared to 1.27 nm2/V2 for 25 wt % YLD124 in PMMA.19 Because the π systems of the donor-bridge-acceptors are nearly identical and the solution λmax values are very similar, it is expected that the β values are also very similar and not the cause for the differentiated EO performance. The higher performance of the neat EO 873
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sample shown in Figure 2, the Veff dropped very quickly from 82 to 54 V, and most of the poling occurred in this low voltage regime based on monitoring Im in Figure 2a. However, the poling voltage for determining poling field is Vapp for simplicity. The significance of this is that the poling efficiency of 1.7 nm2/V2 for DLD136 and 2.0 nm2/V2 for DLD164 are likely lower limits and an underestimation of the full potential of the chromophores (because r33/poling field would be higher if Veff was used in the calculation). Therefore, higher EO coefficients could potentially be achieved if better charge blocking layers were used. Recent work gives indication that nanoscale silicon slot waveguides and a coplanar poling electrode configuration can also attenuate conductivity effects sufficiently to allow the chromophore to withstand higher poling fields.24,25 In summary, two new monolithic organic EO chromophores are reported. DLD136, is novel in that it is the first high β chromophore that is reported to respond efficiently21 to electric field poling as a single component, neat molecule that is not a dendrimer or cross-linked polymer network and does not have functional side chains. It displays a maximum r33 of 170 pm/V and a poling efficiency of 1.7 nm2/V2. Its poling efficiency is 30% higher than an analogous EO molecule used in the standard guest/host configuration (YLD124/PMMA). DLD164 is molecularly engineered to have order-directing side chains. It is also film-forming, poles efficiently as a monolithic material, and has an even higher poling efficiency of 2.0 nm2/V2. This result is presumably due to the alkoxybenzoylcoumarin side chain induced matrix-assisted poling effect. Future studies will include estimation of both centric, , and acentric order, ⟨cos3 θ⟩, to quantify the effect of alkoxybenzoylcoumarin side chains relative to weakly interacting side groups like TBDPS.
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Washington Nanotechnology User Facility, a member of the NSF National Nanotechnology Infrastructure Network.
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(1) Sun, S.-S.; Dalton, L. R. Introduction to Organic Electronic and Optoelectronic Materials and Devices; Taylor & Francis: New York, 2008. (2) Shi, Y.; Zhang, C.; Zhang, H.; Bechtel, J. H.; Dalton, L. R.; Robinson, B. H.; Steier, W. H. Science 2000, 288, 119−122. (3) Dalton, L. R.; Sullivan, P. A.; Bale, D. H. Chem. Rev. 2010, 110, 25− 55. (4) Nalwa, H. S.; Miyata, S. Nonlinear Optics of Organic Molecules and Polymers; CRC Press: Boca Raton, FL, 1997. (5) Luo, J.; Zhou, X.-H.; Jen, A. K.-Y. J. Mater. Chem. 2009, 19, 7410− 7424. (6) Kim, T.-D.; Luo, J.; Cheng, Y.-J.; Shi, Z.; Hau, S.; Jang, S.-H.; Zhou, X.-H.; Tian, Y.; Polishak, B.; Huang, S.; Ma, H.; Dalton, L. R.; Jen, A. K.Y. J. Phys. Chem. C 2008, 112, 8091−8098. (7) Pereverzev, Y. V.; Gunnerson, K. N.; Prezhdo, O. V.; Sullivan, P. A.; Liao, Y.; Olbricht, B. C.; Akelaitis, A. J. P.; Jen, A. K.-Y.; Dalton, L. R. J. Phys. Chem. C 2008, 112, 4355−4363. (8) Benight, S. J.; Johnson, L. E.; Barnes, R.; Olbricht, B. C.; Bale, D. H.; Reid, P. J.; Eichinger, B. E.; Dalton, L. R.; Sullivan, P. A.; Robinson, B. H. J. Phys. Chem. B 2010, 114, 11949−11956. (9) Tian, Y.; Kong, X.; Nagase, Y.; Iyoda, T. J. Polym. Sci., Part A: Polym. Chem. 2003, 41, 2197−2206. (10) Knorr, D. B.; Benight, S. J.; Krajina, B.; Zhang, C.; Dalton, L. R.; Overney, R. M. J. Phys. Chem. B 2012, 116, 13793−13805. (11) Benight, S. J.; Knorr, D. B.; Johnson, L. E.; Sullivan, P. A.; Lao, D.; Sun, J.; Kocherlakota, L. S.; Elangovan, A.; Robinson, B. H.; Overney, R. M.; Dalton, L. R. Adv. Mater. 2012, 24, 3263−3268. (12) Dalton, L. R.; Benight, S. J.; Johnson, L. E.; Knorr, D. B.; Kosilkin, I.; Eichinger, B. E.; Robinson, B. H.; Jen, A. K.-Y.; Overney, R. M. Chem. Mater. 2011, 23, 430−445. (13) Zhang, C.; Dalton, L. R.; Oh, M.-C.; Zhang, H.; Steier, W. H. Chem. Mater. 2001, 13, 3043−3050. (14) As an example, the βzzz(−ω;0,ω) of YLD124 is 7600 × 10−30 esu in CHCl3, and the “FTC” bridged analog is 3500 × 10−30 esu (measured at 1310 nm). The enhanced beta is due to more favorable bond length alternation and reduced aromaticity of the π bridge. Marder, S. R.; Cheng, L.-T.; Tiemann, B. G.; Friedli, A. C.; Blanchard-Desce, M.; Perry, J. W.; Skindhøj, J. Science 1994, 263, 511. (15) Huang, S.; Luo, J.; Jin, Z.; Zhou, X.-H.; Shi, Z.; Jen, A. K.-Y. J. Mater. Chem. 2012, 22, 20353−20357. (16) Huang, S.; Kim, T.-D.; Luo, J.; Hau, S. K.; Shi, Z.; Zhou, X.-H.; Yip, H.-L.; Jen, A. K.-Y. Appl. Phys. Lett. 2010, 96, 243311−243311−3. (17) Herminghaus, S.; Smith, B. A.; Swalen, J. D. J. Opt. Soc. Am. B 1991, 8, 2311. (18) Davies, J. A.; Elangovan, A.; Sullivan, P. A.; Olbricht, B. C.; Bale, D. H.; Ewy, T. R.; Isborn, C. M.; Eichinger, B. E.; Robinson, B. H.; Reid, P. J.; Li, X.; Dalton, L. R. J. Am. Chem. Soc. 2008, 130, 10565−10575. (19) Sullivan, P. A.; Dalton, L. R. Acc. Chem. Res. 2010, 43, 10−18. (20) Hammond, S. R.; Clot, O.; Firestone, K. A.; Bale, D. H.; Lao, D.; Haller, M.; Phelan, G. D.; Carlson, B.; Jen, A. K.-Y.; Reid, P. J.; Dalton, L. R. Chem. Mater. 2008, 20, 3425−3434. (21) In this case, poles efficiently means that r33/poling field is on par with that of an analogous chromophore in a guest−host blend at a common concentration used in organic EO devices (∼20−30 wt %). (22) Teng, C. C.; Man, H. T. Appl. Phys. Lett. 1990, 56, 1734. (23) Sprave, M.; Blum, R.; Eich, M. Appl. Phys. Lett. 1996, 69, 2962− 2964. (24) Song, R.; Yick, A.; Steier, W. H. Appl. Phys. Lett. 2007, 90, 191103. (25) Koeber, S.; Palmer, R.; Heni, W.; Elder, D. L.; Korn, D.; Woessner, M.; Alloatti, L.; Koenig, S.; Schindler, P. C.; Yu, H.; Bogaerts, W.; Dalton, L. R.; Freude, W.; Leuthold, J.; Koos, C. Nat. Photonics 2013, submitted. (26) Kosilkin, I. V. Organic Materials for Electro-Optic and Optoelectronic Applications: Understanding Structure−Property Relationships. Ph.D. Thesis, University of Washington, Seattle, WA, 2012.
ASSOCIATED CONTENT
* Supporting Information S
Detailed synthesis, characterization, and electric field poling details are reported. This material is available free of charge via the Internet at http://pubs.acs.org/.
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REFERENCES
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Present Addresses †
S.J.B. is currently at Department of Chemical Engineering, Stanford University, Stanford, CA, U.S.A. ‡ J.S. is currently at Henan University, Kaifeng, China Author Contributions
All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This material is based upon work supported by the National Science Foundation under Grants DMR-1303080 and DMR0120967). Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation. The authors acknowledge partial financial support from the Air Force Office of Scientific Research (FA9550-09-10682). The authors also thank Denise Bale for many helpful discussions and Antao Chen for use of equipment in his laboratory. Part of this work was conducted at the University of 874
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