Molecular Engineering of Structurally Diverse Dendrimers with Large

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Cite This: ACS Appl. Mater. Interfaces 2019, 11, 21058−21068

Molecular Engineering of Structurally Diverse Dendrimers with Large Electro-Optic Activities Huajun Xu, Delwin L. Elder,* Lewis E. Johnson, Bruce H. Robinson, and Larry R. Dalton* Department of Chemistry, University of Washington, Seattle, Washington 98195-1700, United States

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S Supporting Information *

ABSTRACT: To boost electro-optic (EO) performance, a series of multichromophore dendrimers have been developed based on higher hyperpolarizability (CLD-type) chromophore cores that have been used previously (FTC-type dendrimers). The multichromophore dendrimers were molecularly engineered to have either three arms, two arms, or one arm; long or short linkers; and a fluorinated dendron (FD) or tertbutyldiphenylsilyl (TBDPS) shell. The EO performance obtained by FDSD (poling efficiency = 1.60 nm2 V−2), based on succinic diester linkers, was higher than the analogue with longer adipic diester linkers and higher than the analogs with fewer chromophore moieties. Due to the shorter succinic diester linker and improved site isolation, the dendrimer chromophore with TBDPS groups exhibited enhanced glass-transition temperature (Tg = 108 °C) and comparable poling efficiency (1.62 nm2 V−2) to the FD-containing version. These neat EO dendrimers have a higher index of refraction (n = 1.75−1.84 at 1310 nm) than guest−host polymeric EO materials (n ≈ 1.6, 1310 nm) and FTC-type EO dendrimers (n = 1.73, 1310 nm), which is important, because a key metric for Mach−Zehnder modulators is proportional to n3. In addition, binary chromophore organic glasses (BCOGs) were prepared by doping a secondary EO chromophore at 25 wt % into neat dendrimers. Enhancements of EO performance were found in all BCOG materials compared with neat dendrimers due to the effect of blending. As a result of increased chromophore density, the n values of the BCOGs improved to 1.81−1.92. One BOCG, in particular, displayed the highest poling efficiency (2.35 nm2 V−2) and largest EO coefficient (r33) value of 275 pm V−1 at 1310 nm, which represents a high n3r33 figure-of-merit of 1946 pm V−1. The high poling efficiencies and n3r33 figure-of-merit combined with excellent film forming confirm these neat dendrimers and BCOGs based on them as promising candidates for incorporation into photonic devices. KEYWORDS: nonlinear optics, dendrimer, hyperpolarizability, binary chromophore organic glass, electro-optic

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INTRODUCTION

hybrid (SOH) devices and plasmonic−organic hybrid (POH) devices.7,13,19−26 OEO materials are traditionally composed of highly hyperpolarizable chromophores and nonlinear optically (NLO) inactive host matrix, such as a polymer.27,28 The chromophores possess a push−pull electronic structure in which π-conjugated bridge is end-capped by an electron donor and an electron acceptor. Obtaining high performance from OEO materials requires a combination of high chromophore hyperpolarizability (β), a high number of molecules per material unit volume (ρN), and high acentric ordering ⟨cos3 θ⟩ of chromophores throughout the active region of the device.29−31 Bulk acentric order is required for the Pockels effect, and in OEO materials, this is typically achieved by aligning the molecules with an electric field in a process called poling. Chromophores with large hyperpolarizabilities usually exhibit large dipole moments (∼30 Debye), which induce the molecules to aggregate in an antiparallel manner (centrosymmetric, therefore EO inactive) in the absence of a poling field. In a typical guest/host composite material, extensive

Over the last two decades, organic electro-optic (OEO) materials have drawn great attention because of their promising performance in applications related to telecommunications, computing, tetrahertz (THz) wave generation and detection, sensing, metrology, and other categories of devices for high-speed data/signal processing.1−7 The intrinsic response time of OEO materials (tens of femtoseconds), which result from π-electron oscillations, is much faster than those of inorganic EO materials.8 The Pockels EO coefficient, r33, of organic chromophores currently exceeds that of the industry standard lithium niobate by more than an order of magnitude.8−11 OEO materials can exhibit significantly higher EO activity that results in much improved Mach−Zehnder modulator performance including voltage-length product (VπL) as low as 40 V μm, low power consumption, and bandwidth from 100 to >500 GHz.12−17 Conventional synthetic methods can be used to satisfy specific requirements of the EO material such as the index of refraction, thermal stability, and processability.10,18 Solution processability is another essentially unique advantage of OEO materials, which facilitates the integration with silicon-based microelectronics and photonic circuit elements of silicon−organic © 2019 American Chemical Society

Received: March 25, 2019 Accepted: May 22, 2019 Published: May 22, 2019 21058

DOI: 10.1021/acsami.9b05306 ACS Appl. Mater. Interfaces 2019, 11, 21058−21068

Research Article

ACS Applied Materials & Interfaces Chart 1. Structures of Dendrimers and Other Chromophores

coefficients.32,47 In the best examples, the dendrimers have been three-arm dendrimers with each chromophore based on an FTC-type chromophore that has a static hyperpolarizability βHRS(0) ≈ 1150 × 10−30 esu (extrapolated from Hyper− Rayleigh Scattering measurements at 1907 nm using a damped two-level model and a linewidth of 0.1 eV).37,48 One such dendrimer, PSLD41, when blended 3:1 with YLD124 is among the most successful chromophore systems with poling efficiency r33/Ep > 3 nm2 V−2 in bulk,49 index of refraction = 1.73 (1550 nm), and in-device r33 = 230 pm V−1.50 In this paper, our goal is to improve EO performance by evaluating dendrimers similar to PSLD41 but employing even higher hyperpolarizability EO-active units based on the CLD bridge chromophore YLD124 (βHRS(0) ≈ 2180 × 10−30 esu).37,48 Five dendrimers were designed and synthesized through the controlled synthesis of the shape, size, and the peripheral groups, as shown in Chart 1. We systematically analyzed the influences of linker length, dendritic configuration, and the alteration of peripheral functionality on Tg, index of refraction, linear and nonlinear optical properties. BCOGs were also prepared and studied by doping CLD-type chromophores YLD124 and JRD1, which have good EO performance in guest−host systems as the dopant chromophore.47,49,51,52

experimental and theoretical research has indicated that there is an optimum doping level for chromophore number density above which r33 decreases due to dipole−dipole interactions.32−35 Higher number density than optimum level can also result in increased optical loss caused by scattering due to guest/host phase separation and index of refraction heterogeneity.30,36 Therefore, OEO material systems need to be properly designed at the molecular level to achieve high number density while avoiding the potentially detrimental effects it can cause. In recent years, multichromophore dendrimers were introduced as an effective molecular architecture to combine the beneficial effects of polymer hosts (amorphous, film forming, site isolation to inhibit dipole−dipole interaction, and phase separation) and high chromophore density (combining high r33 and index of refraction).37−44 In these materials, chromophores were connected with an inert core to form a dendritic structure by a series of flexible sigma bonds, which play a critical role in restricting chromophore rotation and aggregation.45 Such attachments allow each individual chromophore to rotate more independently in response to the electric poling field. Multichromophore dendrimers can afford high chromophore loading and enhancement of n without crystallization and phase separation and the corresponding index of refraction inhomogeneity and optical loss from light scattering. Evaluation of multichromophore dendrimer families from one to five generations (and 2−62 chromophores per dendrimer) shows that the SHG coefficient and glass-transition temperature (Tg) increase with the number of chromophores up to d33 = 253 pm V−1 and Tg = 125 °C.42,43,46 However, the hyperpolarizability of each chromophore was low (∼200 × 10−30 esu), which may have limited the ultimate EO performance.42,43,46 An alternative strategy in the literature, in contrast, has been to use fewer chromophores per dendrimer, but use chromophores with much higher hyperpolarizabilities, and blend with a second small molecule chromophore in what is known as a binary chromophore organic glass (BCOG).47 BCOG have the same benefits as the multichromophore dendrimers and also have higher EO

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EXPERIMENTAL SECTION

Materials. Chemicals used were purchased from Sigma-Aldrich, Acros, Alfa Aesar, or Fisher Scientific and used without further purification unless otherwise noted. Chromatography on silica gel was carried out on Kieselgel (200−300 mesh). Benzocyclobutene (BCB) was purchased from Dow Corporation as a solution in mesitylene (Cyclotene 3022−46). 1,1,2-Trichloroethane (TCE) was dried over CaCl2 for several days then collected via vacuum distillation prior to use. ITO/glass slides were purchased from Thin Film Devices, Inc. Characterization. 1H NMR spectra were determined by a Bruker AVance 300M (300 MHz) NMR spectrometer (tetramethylsilane as the internal reference). All 13C NMR spectra were acquired on a Bruker AVance 500 MHz instrument. Optical profilometry measurements were carried out on a NT-2000 model profilometer, provided by WYKO Corporation. Electric field poling and Teng-Man 21059


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ACS Applied Materials & Interfaces Scheme 1. Synthesis Routes for Dendrimers

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ellipsometry were carried out on a custom apparatus.53,54 UV−visible absorption spectra were measured on a Varian Cary 5000 spectrometer. Gas chromatography with mass spectrometry detection (GC/MS) was carried out on an Agilent 7980A or a Hewlett-Packard 6890 gas chromatograph with a quadrupole mass detector. Electrospray ionization mass spectrometry (ESI−MS) was carried out on a Bruker Esquire ion trap mass spectrometer. Optical constants (n and k) were obtained by variable angle spectroscopic ellipsometry (VASE) analysis of unpoled EO material thin films on glass substrates using a J. A. Woollam M-2000 instrument. Data were acquired at 55, 65, and 75°, and fitting was done using the Woollam CompleteEASE software using an isotropic model. The decomposition temperatures (Td) were determined by TA5000-2950 TGA (TA Instruments) with a heating rate of 10 °C min−1 under the protection of nitrogen. Glass-transition temperature (Tg) was measured by differential scanning calorimetry (DSC) with a heating rate of 10 °C min−1 under the protection of nitrogen. Device Fabrication. Solutions of 9−12% w/w EO material in TCE were filtered through a 0.2 μm PTFE filter and spin-cast onto ITO/glass substrates, with or without BCB barrier layers. EO films were spin-cast in three stages, 500 rpm for 5 s, 850 rpm for 30 s, followed immediately by 2000 rpm for 30 s. The films were then dried on a hot plate at 65 °C for half an hour and then further cured in a vacuum oven at 65 °C overnight. The thickness of the EO film was then measured to be around 1−2 μm via optical profilometry. Finally, patterned gold electrodes were deposited on the top of the films by sputter coating through a shadow mask, thus completing a prototype device.

RESULTS Synthesis. In these multichromophore dendrimers (see Chart 1), the active chromophores were designed with an attachment to the core through the bridge (side-on) rather than the donor or acceptor (end-on), as this binding mode has been shown to produce better EO activity.55 For FDAD, a sixcarbon linker based on adipate diester was used to connect the chromophore and core. For other dendrimers, a shorter fourcarbon linker based on succinate diester was used. Two different units were explored at the periphery (donor end) of the chromophore: a tert-butyldiphenylsilyl (TBDPS) unit and bis(pentafluorobenzyloxy)benzoate (FD) unit.11 They are used, because chromophores with TBDPS groups have shown great success as monolithic EO materials, and dendrimer chromophores with FD units have shown great success as part of binary chromophore blends.47,56,57 1,1,1tris(4-hydroxyphenyl)ethane and bisphenol A (BPA) were adopted as the core of dendrimers to study the effect of different numbers of arms on EO properties. These dendrimers can be divided into three contrasting groups (FDAD/FDSD, FDSD/TBSD, FDSD/FDBD/FDMD) by the differences in linker length, peripheral groups, and the number of arms. Synthesis of the molecules (Scheme 1) began by a sodium ethoxide-promoted Knoevenagel condensation between diakylaminobenzaldehyde and 2-substituted isophorone 1 protected with tetrahydropyran (THP). Compound 1 was 21060


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ACS Applied Materials & Interfaces

Figure 1. UV−vis spectra of dendrimer chromophores and chromophore blends measured in chloroform (a) or measured in films (b, c, d).

Table 1. UV−Vis Absorption Data and Thermal Data for EO Dendrimers dendrimer FDAD FDSD FDBD FDMD TBSD YLD124 JRD1 PSLD41 PSLD33

λmax (nm) in 1,4-dioxane

λmax (nm) in acetone

λmax (nm) in chloroform

film λmax (nm)

solvent Δλmaxa (nm)

film-solvent Δλmaxb (nm)

Tg (°C)

Td (°C)

694 692 690 691 715 718 725

737 735 736 736 763 778 777

745 735 740 741 780 786c 788c 724d 759d

765 758 757 769 804 783c 800c

51 43 46 45 65 68 63

20 23 17 28 24 -3 12

71 81 77 75 108

210 219 229 202 211

Difference in λmax between 1,4-dioxane and chloroform. bDifference in λmax between chloroform and films. cReference 61. dReference 42.

a

tris(4-hydroxyphenyl)ethane core produced the three-arm dendrimer 10a,b and condensation with BPA produced the two-arm dendrimer 10c with high yields (>70%). Finally, Knoevenagel condensation with CF3-Ph-TCF acceptor was carried out to produce dendrimers. Acceptor attachment was executed last, because the highly polarized product is more susceptible to nucleophilic attack in any downstream reactions. For the synthesis of TBSD, the THP group of compound 5 was hydrolyzed by HCl/MeOH (pH = 1) at room temperature, leaving TBDPS intact, then followed the same procedures as the synthesis route of FDAD. For the synthesis of compound 16, esterification between 1,1,1-tris(4hydroxyphenyl)ethane and acetic anhydride could not be controlled by reactant equivalents and always resulted in triacetylation product 15. The deprotection with base, however, was easily controlled to generate one phenolic hydroxyl group (16). The same esterification technique (EDCI/DMAP) was used to attach 16 to 9b, then further

prepared by literature methods using commercially available isophorone.58 The hydroxyl group of 2 was protected as a TBDPS ether. The two hydroxyl groups in the donor and bridge were protected by different groups, THP and TBDPS, to selectively deprotect under different conditions later in the synthesis. THP groups could be cleaved by mild acidic solution (0.1 M HCl/MeOH, RT), but TBDPS survives these conditions. The Horner−Emmons reaction with n-butyllithium as deprotonation reagent was used to extend the conjugation of aminophenyldienone 3 to aminophenyltriene-nitrile 4. The nitrile was reduced by DIBAL-H to afford the aldehyde 5. The TBDPS group could be selectively cleaved by tetrabutylammonium fluoride (TBAF), leaving THP intact, followed by 1ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDCI/DMAP) esterification to introduce FD as 7. THP could be selectively removed using HCl/MeOH (pH = 1) in methanol followed by esterification to attach the 4-carbon or 6carbon diester linker producing 9a,b. Condensation with 1,1,121061


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ACS Applied Materials & Interfaces Table 2. Spectral Data, Optical Constants, and Poling Data chromophore

λmax (nm)a

ρNb

Tp (°C)c

n1310d

n1550e

FDAD 3:1 FDAD:JRD1 3:1 FDAD:YLD124 FDSD 3:1 FDSD:JRD1 3:1 FDSD:YLD124 FDBD 3:1 FDBD:JRD1 3:1 FDBD:YLD124 FDMD 3:1 FDMD:JRD1 3:1 FDMD:YLD124 TBSD 3:1 TBSD:JRD1 3:1 TBSD:YLD124

765 781 780 758 781 778 757 775 773 769 789 788 804 803 808

4.38 4.69 5.00 4.47 4.69 5.01 4.44 4.67 5.04 3.81 4.26 4.56 5.62 5.55 5.92

79 75 75 98 87 88 78 76 76 76 74 74 106 96 96

1.78 1.85 1.86 1.81 1.86 1.88 1.81 1.86 1.87 1.75 1.83 1.84 1.84 1.89 1.92

1.74 1.80 1.80 1.77 1.81 1.82 1.77 1.81 1.82 1.72 1.78 1.79 1.80 1.83 1.85

r33/Ep (nm2 V−2)

Δ((r33/Ep)/ρN)f

max. r33 (pm V−1)

max. n3r33 (pm V−1)

± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.82

71 132 145 147 186 221 86 118 141 89 105 127 176 213 275

400 836 933 872 1196 1468 510 759 922 477 643 791 1096 1438 1946

0.96 1.31 1.47 1.60 1.77 2.01 0.92 1.11 1.60 0.90 1.11 1.25 1.62 1.90 2.35

0.08 0.07 0.09 0.08 0.10 0.11 0.07 0.07 0.12 0.08 0.07 0.08 0.09 0.11 0.10

0.76

1.15

0.47

1.58

Maximum absorption peak measured in chloroform. bNumber density (assumes the mass density of 1 g cm−3, unit is 1020 molecules cm−3). Poling temperature. dIndex of refraction measurement at 1310 nm for the unpoled film. eIndex of refraction measurement at 1550 nm for the unpoled film. fSlope of r33/Ep vs ρN for each family of three samples, unit is 1010 pm V−1 molecule−1. a c

hyperpolarizability is expected to be similar to YLD124. In comparison with the absorption in solutions, the thin films of the dendrimers spin-coated on the glass substrates show a significant broadening of the half peak width and λmax redshifts of 17−28 nm for the FD-type chromophores and 24 nm for TBSD. In the UV−vis spectra of thin films, there were prominent long-wavelength shoulders in the range of 900− 1000 nm (Figure 1), which are typically observed in all thin film spectra of CLD-type chromophores with CF3-Ph-TCF acceptors (compare with spectra of nondendrimeric chromophores YLD124 and JRD1 in Figure 1b−d). It has been shown previously that the absorbance intensity of the long-wavelength shoulder has a positive correlation with the concentration when dispersed in polymer hosts such as PMMA or APC.60 Therefore, this kind of long-wavelength shoulder was generally attributed to electronic absorption characteristics associated with the J-aggregations between chromophores.52 The neat films of the FD-type dendrimers exhibit relatively weaker longwavelength shoulders compared to TBSD and presumably less aggregation. After doping in 25 wt % YLD124 or JRD1, the long-wavelength shoulders were of the same magnitude or slightly enhanced (Figure 1c,d), indicating the potential for slightly greater aggregation in the blends. However, the size of the shoulder is in line with other high-performance chromophores and not significant enough to expect suppression of poling-induced order. Glass-transition temperatures of multichromophore dendrimers were obtained by differential scanning calorimetry (DSC) and reported in Table 2 (and SI Figures). The Tg values are influenced by the combination of changes in linker length and flexibility, steric bulk, and mobility of the donor substituent. All dendrimers only show Tg, and no melting points were observed in DSC curves, indicating that the dendrimers are molecular glasses (amorphous), precluding light scattering caused by crystallization in an operational EO device. FDAD exhibited a relatively low Tg of 71 °C due to longer covalent linker between the core and chromophore fragments. When the 6-carbon adipate diester linkers are replaced by 4-carbon succinate diester, Tg increases 10 oC for FDSD (Tg = 81 °C). The increase of Tg for FDSD is attributed to the enhanced structural rigidity of shorter linkers, but is still

condensation with CF3-Ph-TCF produced FDMD in 76% yield. Detailed synthesis procedures, spectrometric characterization, and official IUPAC names59 of compounds reported can be found in the accompanying Supporting Information. Characterization. UV−vis absorption spectroscopy was performed on the five dendrimers in three polar aprotic solvents and as thin films, and results are shown in Figure 1 and summarized in Table 1. UV−Vis spectroscopy is useful assessing how the chromophore attachments and dendrimer structure might affect the electronic structure and, therefore, hyperpolarizability. Four FD-type dendrimers in dilute solution exhibited similar absorbance spectra with slight differences in the wavelength of maximum absorbance λmax. In chloroform, the λmax values of FD-type dendrimers ranged from 735 to 745 nm, which were significant blueshifts relative to chromophore YLD124 (786 nm in chloroform) given their similar donor− bridge−acceptor structures. These blueshifts are partially ascribed to quadrupolar interactions between perfluorinated aromatic rings and electron-rich segments of the chromophore π-system and consistent with the blueshift of previously reported OEO materials (for example, comparing the FTC dendrimer analogues PSLD41 and PSLD33, the version with the perfluorinated periphery has its absorbance blueshifted by 35 nm in chloroform). 38 TBSD has the highest λ max absorbance (780 nm in chloroform). In general, the λmax of chromophores redshifts with the increase of the solvent dielectric constant due to the stronger electron polarization along the conjugated system in the neutral ground state. However, it has been observed that if the solvent dielectric increases above ∼15, the λmax may blueshift relative to the highest value in lower but moderate dielectric solvents.10,60,61 That is exactly what is observed in these dendrimers. λmax redshifts 43−65 nm from 1,4-dioxane (ε = 2.2) to chloroform (ε = 4.8). TBSD showed the largest λmax redshift of 65 nm. However, in the much higher polarity solvent acetone (ε = 21.0), λmax blueshifts relative to chloroform values by 4−10 nm for the FD-containing chromophores and by 17 nm for TBSD. The λmax values and solvatochromic shifts of TBSD are similar to those of YLD124 and JRD1, indicating that the attachment points and multichromophore nature of TBSD have little effect on individual chromophore electronic structure, and the 21062


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ACS Applied Materials & Interfaces lower than FTC-type PSLD33 (Tg = 85 °C) and PSLD41 (Tg = 103 °C). Slight Tg decreases were observed in the DSC curves of FDBD and FDMD compared with FDSD, presumably as a result of fewer arms. After replacing the peripheral FD group by TBDPS, TBSD displayed a Tg of 108 o C, which was higher than any of the other dendrimers including PSLD41.38 This may be attributed to the larger rigidity of TBDPS compared to the FD group. The decomposition temperatures (Td) of five dendrimers were measured by thermogravimetric analysis (TGA) while heating from room temperature to 420 °C at 10 °C per minute under nitrogen. The Td’s of the dendrimer ranged from 202 to 229 °C, which is typical of YLD124-type chromophores with CF3-Ph-TCF acceptors. The high Td’s mean that the molecules are stable enough to meet the requirements of device fabrication, poling, and long-term operation. Variable angle spectroscopic ellipsometry (VASE) measurements were performed on dendrimer thin films spin-coated on glass substrates to measure the optical constants (index of refraction, n, and absorption coefficient, k), as shown in Figure 2 and Table 2. A high index of refraction at two of the

polymer systems. For example, YLD124 blended in PMMA at 25 wt % has n ≈ 1.60 at the two telecom wavelengths.50,62 Upon doping JRD1 or YLD124 into the dendrimers, the binary materials displayed enhanced n values compared with neat dendrimers partly due to the increase in chromophore number density. The CLD-based dendrimer FDSD has a higher index than its FTC analogue PSLD41 (n = 1.73 at 1310 nm),38 and the FDSD/YLD124 BCOG has a higher index than the PSLD41/YLD124 counterpart (n = 1.77 at 1310 nm, n = 1.73 at 1550 nm).50 Poling Studies. To investigate the ability of translating microscopic hyperpolarizability into macroscopic EO response (r33), basic phase modulators were fabricated and poled. Dendrimers were dissolved in 1,1,2-trichloroethane (TCE) and spin-coated onto ITO electrodes with low reflectivity and good transparency to minimize the contribution from multiple reflections. It is worth noting that all of the dendrimers and binary materials form high-quality thin films either on bare ITO surface or cross-linked benzocyclobutene (BCB) polymer, which serves as a charge barrier layer to reduce leakage current during poling.63 The thicknesses of the EO thin films were in the range of 1.2−2.0 μm; film thickness was controlled by the solution concentration and spin-coating time and speed. The thin films were dried in a vacuum oven at 65 °C to remove the residual solvent, and then, a ∼60 nm gold layer was deposited by sputter coating through a shadow mask onto the films to form a top electrode. The films were contact poled by heating to their Tg for 5−10 min with an electric field applied. EO performance was evaluated using a modified single-beam reflection ellipsometry apparatus (Teng-Man simple reflection method), which can simultaneously monitor the EOmodulated signal intensity (Im) and unmodulated probe intensity (Ic) in real time during poling, further allowing in situ optimization of processing conditions.54,64 After cooling to room temperature, the absolute EO activities (r33 values) of poled films were measured by the Teng-Man technique at a wavelength of 1310 nm. The r33 measurement results and poling efficiencies (r33/Ep) for dendrimers and binary blends are summarized in Table 2 and Figure 3. Along with maximum r33 value achieved, poling efficiency is a good metric by which to compare EO materials, as it is an average of multiple poling experiments (9−13 samples) and independent of poling field. For the FD-type dendrimers, the neat FDAD, FDBD, and FDMD showed similar performances in poling efficiency (0.96, 0.92, and 0.90 nm 2 V −2 , respectively), which is close to the guest/host system 25 wt % YLD124 blended in PMMA (∼1.0 nm2 V−2). For FDSD, based on succinic diester linkers, the poling efficiency was 1.60 nm2 V−2, larger than the other FD-type dendrimers and FTCtype dendrimer PSLD41 (1.04 nm2 V−2), though they had similar chromophore loading densities (PSLD41: ρN = 4.6 × 1020 molecules cm−3). The difference in poling efficiencies of FDSD and PSLD41 can mainly be ascribed to the larger β of the CLD-type chromophore. Comparing FDSD, FDAD, and FDBD, FDSD has a significantly higher poling efficiency even though they all have the same chromophore backbone and nearly the same ρN (∼4.4 × 1020 molecules cm−3). With the shorter linker length of FDSD, the dendrimer core is able to provide better site isolation than for FDAD, which is more decoupled from the dendrimer core and more similar to a chromophore/polymer composite. Though the one-component chromophore FDMD and two-component chromophore FDAD have similar poling efficiencies, the performance FDSD

Figure 2. Optical constants k (a) and n (b) for chromophores used in this study (measured on unpoled thin films).

important telecom wavelengths (1310 and 1550 nm) is crucial for reducing the operating voltage (Vπ) of Mach−Zehnder modulators, in that the operating voltage is inversely proportional to the value of n3r33. The dendrimers and binary materials exhibited n ranging from 1.75 to 1.92 at 1310 nm and 1.72 to 1.83 at 1550 nm. These indices are significantly higher than those observed for typical guest−host, chromophore− 21063


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Figure 3. Poling curves (plots of r33 vs poling field, Ep); dashed lines are linear fits. The error in each r33 measurement is ∼5%. Average r33/Ep ± standard errors are shown.

with 25 wt % YLD124 or JRD1, which showed excellent film quality on ITO/glass as well as BCB barrier layer. The UV−vis to near IR absorption spectra were recorded for the BCOG samples (Figure 1). A long-wavelength shoulder was observed in the spectral region corresponding to the π−π* intramolecular charge transfer band similar to that of TBSD. It seems reasonable to assume that much of the observed broadening is due to a mostly linear combination of the spectral features associated with dendrimers and guest chromophores. The index of refraction also showed an increase (Table 2). The highest index of refraction was

is better indicating that a threshold has been passed for the three-component dendrimer. In addition to the lower poling performance, the longer linker of FDAD and lower number of arms of FDBD and FDMD result in lower Tg’s compared with FDSD. It has been observed that BCOGs can have higher EO performance than any of the individual constituents.38,49,52 Free chromophores YLD124 and JRD1 were chosen as the doping chromophores because of their high individual poling efficiencies, good compatibility, excellent film-forming properties, and high ρN. In total, 10 BCOG materials were prepared, 21064


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the effect of dendritic structure and binary blends on ρN and ⟨cos3 θ⟩, r33/Ep vs ρN is analyzed for the dendrimers and BCOGs in this study (Figure 4). All chromophore moieties are

observed for 3:1 TBSD/YLD124 (n = 1.92, 1310 nm) (plots of n and k are shown in the SI). As shown in Table 2, ρNs for the BCOGs were higher than the single-component dendrimers, except for TBSD:JRD1. Several samples of each BCOG composition were poled under increasing poling voltage, and the resulting r33 values plotted against Ep (see Table 2 and Figure 3). In all cases, the r33 values and poling efficiencies for the BCOGs were higher than the dendrimers. 3:1 blends, FDAD:YLD124 and FDBD:YLD124 showed great improvements in poling efficiencies (53 and 74% improvement, respectively) compared with FDMD:YLD124 (39% increase), but all three BCOGs still have lower poling efficiencies than those of neat FDSD and TBSD. FDSD/YLD124 and TBSD/YLD124 exhibited improved poling efficiencies to greater than 2 nm2 V−2. 3:1 blends FDSD/YLD124, TBSD/JRD1, and TBSD/YLD124 yielded higher than 200 pm V−1 maximum r33 values. The highest r33 value of 275 pm V−1 was achieved by 3:1 TBSD/YLD124, which is comparable with some of the best OEO materials reported in the literature, and higher than the maximum r33 achieved with a BCOG containing the FTC-based PSLD dendrimer and YLD124 (PSLD41/YLD124).52,56,57,63 n3r33 is an important metric for the operation of EO modulators and is inversely proportional to VπL. This metric heavily weights n, which is why high ρN and high n chromophores are favored in EO devices. It should be noted that the index measurements discussed herein are isotropic values, as opposed to the tensor component interacting with the optical field. The anisotropy of the index (birefringence) is heavily dependent on the amount of centrosymmetric order (⟨cos2 θ⟩) and has a significant influence on EO performance.62 However, VASE measurement of index anisotropy on poled OEO samples is challenging and highly model-dependent due to the influence of sputtered gold within the OEO film. For this reason and to compare with other values in the literature, the isotropic index is used for all samples. TBSD/YLD124 has the highest n3r33 figure-of-merit at 1946 pm V−1 (1310 nm) and is even higher than that of PSLD41:YLD124 (1580 pm V−1, 1310 nm). For this study, 3:1 blends were chosen since that ratio has been successful in previous dendrimer:chromophore studies and other BCOG evaluations. Further research will concentrate on fine tuning the ratio to improve the performance even more.

Figure 4. Plot of r33/Ep vs ρN for neat dendrimers and 3:1 BCOGs. The dashed line is a guide to the eye.

based on the same CLD-like base structure and are assumed to have essentially the same β. YLD124 at 25 wt % in PMMA and neat YLD124 were used to represent the r33/Ep performance of the base chromophore, and the dashed line represents the expected performance based only on ρN. Data points that are above this line are assumed to have enhanced performance based on some structural factors or cooperativities of the blend resulting in higher ⟨cos3 θ⟩. There are several possible contributing factors to the increase or decrease of poling efficiency and the slope of r33/Ep vs ρN. Indeed, high chromophore concentrations tend to have higher dielectric permittivity, and β is positively correlated with dielectric permittivity, though we expect the contribution to be very small.32 The poling efficiencies of neat dendrimers FDAD, FDBD, and FDMD were close to 25 wt % YLD124/PMMA though the dendrimers had higher ρNs. The fact that they are below the line set by YLD124 implies a lower ⟨cos3 θ⟩ due to intermolecular interactions, which can be thought of as thermodynamic “traps.” TBSD shows slightly lower poling efficiency than the line set by YLD124. FDSD has a slightly higher poling efficiency, showing that of the new chromophores, only it has the right combination of linker length, number of dendrimer arms, and electrostatic interactions to yield a higher r33 and ⟨cos3 θ⟩ than consideration of ρN alone would predict. Upon doping in JRD1 or YLD124, the rate of increase in poling efficiency with increasing ρN was greater than the slope of the line set by YLD124 in all cases. This indicates that intermolecular electrostatic interactions between dendrimers and guest chromophores are acting to enhance ⟨cos3 θ⟩. In spite of the poling efficiency increases, the FDBD blends and FDAD blends stayed below the line set by YLD124, and FDBD:YLD124 only reached parity with the YLD124 line. Within the five families of blends in Figure 4, the nearly linear relationship between the three data points in each family tells us that the effect of YLD124 or JRD1 is nearly linear with ρN. One exception is with TBSD: Even though TBSD/JRD1 has lower ρN than neat TBSD, TBSD/JRD1 still exhibited enhanced poling efficiency and a higher maximum r33 value.

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DISCUSSION Electro-optic activity can be improved by increasing β, ρN, Ep, or ⟨cos3 θ⟩ or a combination of these factors.29,36,38 In the past, extensive efforts have been made to increase ρN of EO materials, design new chromophores with higher β, increase the ⟨cos3 θ⟩ of high ρN chromophores, or increase the Ep by decreasing the poling leakage current.10,56,63,65,66 For example, high ρN, neat multichromophore dendrimers, and chromophores were used directly to prepare EO devices instead of EO materials dispersed in EO-inactive polymer; chromophores with order-directing HD/FD or coumarin side chains were used to increase ⟨cos2 θ⟩, which was previously attributed to a corresponding increase in ⟨cos3 θ⟩ but may also be influenced by increased birefringence.62,67 Cross-linked BCB polymer or other materials were used as barrier layers to inhibit leakage current during poling. These factors, however, were not completely independent with each other; particularly, there is a trade-off for ρN and ⟨cos3 θ⟩ in that highest acentric order is usually not achieved at highest ρN.68,69 To better understand 21065


Research Article

ACS Applied Materials & Interfaces Author Contributions

The TBSD and FDSD BCOGs had poling performance well above the line set by YLD124. If we compare the slopes of r33/ Ep vs ρN for each of the five families of chromophores, the larger slope indicates a larger poling efficiency enhancement resulting from blending (Table 2). Of the FD-containing dendrimers, the two-arm FDBD family has the largest slope. However, this should not lead us to conclude that a two-arm dendrimer is a better design principle. The combination of neat dendrimer performance and enhancement from blending clearly concludes that the three-arm TBSD and FDSD are a superior overall design.

All authors have given approval to the final version of the manuscript. Funding

We gratefully acknowledge the financial support of the Air Force Office of Scientific Research (FA9550-15-1-0319 and FA9550-19-1-0069) and the National Science Foundation (DMR-1303080). 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. Dr. Huajun Xu thanks the StateSponsored Scholarship for Graduate Students from China Scholarship Council.

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CONCLUSIONS In summary, a series of novel EO dendrimers with high-β, CLD-type EO moieties are synthesized and systematically characterized. Through careful molecular engineering of linker length, the number of fragments, and peripheral groups, the dipole−dipole interactions between chromophore fragments are effectively attenuated. These factors show significant effects on material Tg’s and EO activities. Relative to the base chromophore in a polymer host, the dendrimers (FDSD and TBSD) have larger poling efficiencies because of the isolating ability of the dendritic structure. The neat films of FDSD and TBSD exhibited higher poling efficiencies in comparison with those of PSLD41 and PSLD33, which was mainly due to the higher β value of the CLD-type chromophores relative to the FTC-type. Upon doping guest chromophores into dendrimers, the rate of increase of poling efficiency with increasing number density was greater than the rate set by YLD124. This enhancing effect is ascribed to favorable intermolecular electrostatic interactions between free chromophore and dendrimer host that allow a higher ⟨cos3 θ⟩ to be attained. The poled EO film of TBSD/YLD124 with BCB barrier layer exhibits a maximum r33 of 275 pm V−1 at 93 V μm−1 poling field, n3r33 of 1946 pm V−1, and an average r33/Ep of 2.35 ± 0.10 nm2 V−2. The r33 and n3r33 values were higher than those for the BCOG based on the FTC-type dendrimer, PSLD41. The combined excellent film-forming abilities, high poling efficiencies, and large EO activities indicate that these new EO dendrimers and blends can be promising candidates for highspeed, energy efficient EO devices.

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Part of this work was conducted at the Molecular Analysis Facility, a National Nanotechnology Coordinated Infrastructure site at the University of Washington which is supported in part by the National Science Foundation (awards NNCI-1542101, 1337840 and 0335765), the National Institutes of Health, the Molecular Engineering & Sciences Institute, the Clean Energy Institute, the Washington Research Foundation, the M. J. Murdock Charitable Trust, Altatech, ClassOne Technology, GCE Market, Google and SPTS.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b05306.

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REFERENCES

Experimental details; synthesis and characterization of EO chromophores by NMR and mass spectrometry; UV/vis spectra, optical constants; and DSC data.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (D.L.E.). *E-mail: [email protected] (L.R.D.). ORCID

Huajun Xu: 0000-0002-5267-5910 Delwin L. Elder: 0000-0001-9302-3858 Lewis E. Johnson: 0000-0002-7412-073X Bruce H. Robinson: 0000-0002-5579-953X Larry R. Dalton: 0000-0002-6461-0145 21066


Research Article

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(62) Johnson, L. E.; Elder, D. L.; Kocherzhenko, A. A.; Tillack, A. F.; Isborn, C. M.; Dalton, L. R.; Robinson, B. H. Poling-Induced Birefringence in Oeo Materials under Nanoscale Confinement. Organic and Hybrid Sensors and Bioelectronics; International Society for Optics and Photonics, 2018; Vol. XI, pp 107381A. (63) Jin, W.; Johnston, P. V.; Elder, D. L.; Tillack, A. F.; Olbricht, B. C.; Song, J.; Reid, P. J.; Xu, R.; Robinson, B. H.; Dalton, L. R. Benzocyclobutene Barrier Layer for Suppressing Conductance in Nonlinear Optical Devices During Electric Field Poling. Appl. Phys. Lett. 2014, 104, 243304−243304-5. (64) Olbricht, B. C.; Sullivan, P. A.; Wen, G.-A.; Mistry, A. A.; Davies, J. A.; Ewy, T. R.; Eichinger, B. E.; Robinson, B. H.; Reid, P. J.; Dalton, L. R. Laser-Assisted Poling of Binary Chromophore Materials. J. Phys. Chem. C 2008, 112, 7983−7988. (65) Kang, H.; Facchetti, A.; Zhu, P.; Jiang, H.; Yang, Y.; Cariati, E.; Righetto, S.; Ugo, R.; Zuccaccia, C.; Macchioni, A.; Stern, C. L.; Liu, Z.; Ho, S. T.; Marks, T. J. Exceptional Molecular Hyperpolarizabilities in Twisted Pi-Electron System Chromophores. Angew. Chem., Int. Ed. 2005, 44, 7922−7925. (66) Liu, S.; Haller, M. A.; Ma, H.; Dalton, L. R.; Jang, S. H.; Jen, A. K.-Y. Focused Microwave-Assisted Synthesis of 2,5-Dihydrofuran Derivatives as Electron Acceptors for Highly Efficient Nonlinear Optical Chromophores. Adv. Mater. 2003, 15, 603−607. (67) Robinson, B.; Johnson, L.; Elder, D.; Kocherzhenko, A.; Isborn, C.; Haffner, C.; Heni, W.; Hoessbacher, C.; Fedoryshyn, Y.; Salamin, Y.; et al. Optimization of Plasmonic-Organic Hybrid Electro-Optics. J. Lightwave Technol. 2018, 36, 5036−5047. (68) Tillack, A. F.; Johnson, L. E.; Rawal, M.; Dalton, L. R.; Robinson, B. H. Modeling Chromophore Order: A Guide for Improving Eo Performance. MRS Proc. 2014, 1698. (69) Tillack, A. F.; Robinson, B. H. Toward Optimal Eo Response from Onlo Chromophores: A Statistical Mechanics Study of Optimizing Shape. J. Opt. Soc. Am. B 2016, 33, E121−E129.

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