Synthesis of asymmetric dendrimers with controllable chromophore concentration and improved electro-optical performance

Abstract

In this work, we firstly provided a simple work-up procedure to prepare a new kind of electro-optical dendrimer with an asymmetric configuration via a Cu-(I) catalyzed Huisgen-reaction. The resulting NLO dendrimers exhibited both good thermal stability and good film-forming ability. In order to investigate the influence of the asymmetric configuration on the electro-optic (EO) activities, UV-vis spectra, density functional theory (DFT) calculations and EO performance were studied. Due to their asymmetric spherical structure, two dendrimers showed weak dipole–dipole aggregation compared with traditional dendritic electro-optical materials (DESD), but they achieved better alignment after each poling process was carried out. As expected, HJ-2 exhibited 41 pm V⁻¹ in an EO coefficient test, which was higher than DESD and the guest–host polymeric system (C2 + PMMA) using the same chromophores.

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1. Introduction

Over the last two decades, polymeric and dendritic electro-optical (EO) materials have proven to be promising candidates for the development of photonic integrated devices.^1–10 In order to improve the EO activity, the doping level of chromophores must be optimized. Increasing the concentration of chromophores over the optimum level can lead to strong dipole–dipole interaction and optical loss due to a possible phase separation.^11,12 Thus, efficiently avoiding deleterious aggregation effects and translating the microscopic nonlinear optical activity (β) into macroscopic EO activity has already become the major focus in improving modern EO materials.^13–15

In recent years, multichromophore dendritic EO materials have been proven as an effective approach to inhibit interchromophore electrostatic interactions as well as to enhance the poling efficiency attributed to spherical configurations.^16–19 Based on the classic dendrimer’s structural motif (Scheme 1), the component chromophores are covalently attached to the dendrimer branches and can rotate freely if the linker is sufficiently long. However, if the chromophore density is too high this can lead to increased material conductivity, thus limiting the effectiveness of the poling field in reorienting the chromophores.² Symmetric multichromophore dendrimers (MCDs) may imply that the component chromophore dipole moments effectively cancel resulting in a low net dipole moment. Additionally, reorientation under a poling field is inhibited in symmetric MCDs (DESD; Chart 1) which have low net molecular dipole moments. In this article, we propose two kinds of novel EO multichromophore dendrimers with asymmetric configurations that were synthesized via the Huisgen reaction.^20–26 These materials were developed and tested in an effort to improve both the poling efficiency and the control of the material conductivity in organic EO materials.

Scheme 1 The synthesis of dendrimers HJ-1 and HJ-2.

Chart 1 The structure of the three dendrimers.

Chart 1 shows the asymmetric EO dendrimer structures, HJ-1 and HJ-2. Included in the dendrimer backbone are 3,5-dioxybenzyl ether units which possess enhanced rigidity and stability. In order to improve the molecular response to the applied poling field and to control the material conductivity, the chromophores were attached to a portion of the branches (1/3 or 2/3) using the Huisgen reaction, leaving the remaining branches functionalized with Fréchet dendrons as isolation groups.²⁷ During this process, the dendrimers maintained ellipsoidal configuration due to the control of the generation of different branches.

As shown in Scheme 1, the synthesis of HJ-1 and HJ-2 resulted in high yields (94.0% and 88.2%, respectively). The electron donor 2 was obtained by 4-((2-hydroxyethyl) (methyl)amino)-benzaldehyde (1) reacting with diphenylphosphoryl azide (DPPA), following a safe, high-efficiency route to turn the hydroxyl group of 1 into an azide group. By controlling the amount of 1,1,1-tris(4-hydroxy-phenyl)ethane, compounds 4 and 6 can be acquired via a different ratio (2 eq. and 0.5 eq., respectively). The combination of copper sulfate and sodium ascorbate was adopted as a catalyst, instead of copper (I) and amine which acts as a strong nucleophile causing acceptor ring opening, thereby causing chromophore decay.

2. Results and discussion

As shown in Scheme 1 and 2, the dendrimers were conveniently synthesized in high yields. Electron donor 2 was obtained by 4-((2-hydroxyethyl) (methyl)amino)benzaldehyde reacting with diphenylphosphoryl azide (DPPA), which was a safe, high-efficiency route to turn the hydroxyl group into an azide group. By controlling the number of equivalents of 1,1,1-tris(4-hydroxyphenyl)ethane added, compound 5 can be added to form different dendrons. As catalyst, we chose the combination of copper sulfate and sodium ascorbate instead of copper (I) and amine which will cause acceptor ring opening, further destroying the chromophore’s conjugated structure.

Scheme 2 The synthesis of the dendrimer DESD.

2.1 ¹H-NMR and infrared spectra

¹H-NMR and FTIR spectra were used to identify the chemical structure of the obtained dendrimers. Fig. 1 and 2 show the ¹H-NMR spectra for HJ-1 and HJ-2 together with their precursors. The signals at 1.69 ppm for HJ-1 and HJ-2 are attributed to the protons in the methyl groups of the tricyanofuran (TCF) acceptor which undergo negligible displacement in comparison with those of the chromophore C1. In the spectra of G2-1 and G2-2, alkynyl protons showed triplet peaks at 2.49 ppm. After the Huisgen reaction, the peaks of alkynyl protons disappeared completely, which indicates that the starting materials are efficiently converted into the desired dendrimers HJ-1 and HJ-2. At the same time, a new broad single peak appeared at 5.09 ppm which belonged to 1,2,3-triazole.²⁷ Owing to the transformation of the azide groups in C1, the signals of two methylene protons in the donor moved from 3.57 and 3.69 triplet peaks to 3.94 and 4.57 singlet peaks in HJ-1 and HJ-2. Except for the signals of vinyl protons and aromatic protons which were overlapped by the proton signals of the dendrimer’s backbone, the other signals of C1 could be easily observed in the ¹H-NMR spectra. All of the chemical shifts were consistent with the proposed dendrimers’ structure. The measured ratios were in keeping with the ratios of the proposed structures, which proved that the content of chromophores in the dendrimers can be controlled accurately, and reproducibility of the dendrimers’ properties is guaranteed. FTIR spectra showed a strong absorption peak near 2225 cm⁻¹ indicating the presence of a nitrile group, and typical bands indicating aromatic rings at 1597 cm⁻¹ and 1523 cm⁻¹. The spectra also showed strong bands indicating a C–O–C group at 1284 and 1165 cm⁻¹, indicating the presence of benzyl ether.

Fig. 1 ¹H-NMR spectra comparison for C1, G2-1 and HJ-1.

Fig. 2 ¹H-NMR spectra comparison for C1, G2-2 and HJ-2.

2.2 Film-forming ability and thermal stability

HJ-1 and HJ-2 exhibited good solubility in 1,1,2-trichloroethylene and excellent performance of their film-forming properties. High-quality, smooth, homogeneous, and highly transparent films were obtained via spin-coating. To systematically examine the improvement of the materials’ properties brought about by the asymmetric dendritic structure, a traditional dendrimer DESD (Scheme 1) was synthesized and characterized in accordance with the methods used for HJ-1 and HJ-2. In addition, two guest–host polymer films, P-1 and P-2, were prepared using different concentrations of the host polymer PMMA blended with the same chromophore (C2) concentrations as HJ-1 and HJ-2. After one day’s stirring, nearly all of the chromophores had dissolved and a negligible amount of remaining solid was removed via filtration. Chromophore concentrations were estimated at 20% or below. Atomic force microscopy (AFM) is usually used to investigate the surface morphology of polymer films (Fig. 3). It could reveal the film-forming properties, and the differences in the film morphology before and after poling. Fig. 3 shows the AFM scans of the spin-coated films of two dendrimers before and after poling. After poling, the films were changed, and numerous hills and valleys appeared on the poled films, which could be seen as evidence of the chromophores’ alignment during the poling process. The AFM images further indicate that the synthesized EO dendrimers have excellent film-forming ability. They also demonstrate that the materials have excellent dielectric strength because their good film quality was retained after high electric field poling according to the AFM scans of the poled films. For thermoanalysis, both HJ-1 and HJ-2 began to decompose at 250 °C, the same as for chromophore C2, indicating that the decomposition of HJ-1 and HJ-2 at this temperature was induced by the decomposition of the chromophores.²⁸ HJ-2 exhibited the highest T_g value (116 °C) compared with HJ-1 and DESD which had T_g values of 106 °C and 104 °C, respectively.

Fig. 3 AFM images for spin-coated films.

2.3 UV-vis spectra of films and solutions

UV-vis spectra were measured to locate the charge transfer (CT) peak of the chromophore as well as to examine possible aggregation behaviour, typically indicated by additional low-energy (high-wavelength) absorption near the CT peak.^29–32 We measured the UV-vis spectra of five thin films (Fig. 4) and three polar aprotic solvents (detailed in the ESI†). Generally, the characteristic absorption band of the films will be broader than that of the solutions because of material inhomogeneity and chromophore interactions in the films.³² The nature of the chromophore aggregation that results in the noticeable bathochromic absorption shoulder appearing in the thin film spectra is not entirely certain, however, similar shifts have been previously attributed to the occurrence of J-aggregates and centrosymmetric aggregation.³³ To a certain extent, the absorption strength of the shoulder and the peak width of the CT-band reflect the degree of chromophore dipole–dipole interaction in materials containing the same type of chromophores. For example, DESD showed the widest CT-band and the most exaggerated shoulder. On the contrary, HJ-2 exhibited a narrow CT-band and a relatively weak shoulder. These results indicate that aggregation effects are more prominent in DESD than in HJ-2 or HJ-1. Furthermore, these results indicate that the flexible linkers performed poorly in preventing chromophore aggregations via site-isolation of each active unit within the internal free volume created by the dendrimer. Although P-1 and P-2 contained lower chromophore concentrations (≤20%), they also displayed more apparent shoulders in the right of the CT-band compared with HJ-1 and HJ-2. When doped into a PMMA matrix, chromophore C2 possessed a similar rotating degree of freedom to DESD, thereby improving the chromophores’ effective response to an applied field. Chromophores exhibit a greater tendency to form anti-parallel (i.e., centrosymmetric) aggregations which can defeat the strength of the poling field to re-align (Table 1).

Fig. 4 UV-vis absorptions for five films.

Table 1 The data of maximum absorption wavelength for three dendrimers in solutions and the film

	λmax^a/nm	λmax^b/nm	λmax^c/nm	λmax^d/nm
a Measured in acetone.b Measured in chloroform.c Measured in DMF.d Measured in the film.
HJ-1	552	554	568	549
HJ-2	551	556	569	544
DESD	556	563	573	545

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2.4 DFT calculations and EO performance

To obtain a further understanding of the effect of the asymmetric structure on molecular movement and rotation, density functional theory (DFT) calculations using Gaussian 09 were carried out on three dendrimers at the PM3 level, employing the split valence 3-21 basis set.³⁴ All calculations converged to a RMS error in the density matrix of <10⁻¹¹ au. Geometry optimized structures at this level of theory for HJ-1 and HJ-2 possess well dendronized configurations which are close to ellipsoidal and are proposed to be effective in inhibiting anti-parallel dipole–dipole aggregation (Fig. 5). On the contrary, DESD showed a rod-like optimized structure with two chromophores aggregated together. Rod-like structures have a poor performance in isolating chromophores and, thus, are undesirable for this study.

Fig. 5 Geometry optimized structures of three dendrimers.

The EO coefficients of HJ-1 and HJ-2 were 28 pm V⁻¹ and 41 pm V⁻¹ (1310 nm), respectively. Following the same process of preparation, the maximum r₃₃ exhibited by the poled DESD-films was 19 pm V⁻¹ for a doping concentration of nearly 63%. Despite this being the maximum r₃₃ achieved for this material, the high chromophore doping concentration likely induced severe dipole–dipole interactions, thus partly accounting for the reduction in the EO performance of DESD-films. The EO activity obtained by the poled HJ-2-film was almost four times larger than the chromophore-doped films P-1 and P-2, for which the maximum r₃₃ value was approximately 7 pm V⁻¹. The great improvement of the EO activity of HJ-2 indicates that the intermolecular electrostatic interactions were effectively suppressed in this polymer system, and these NLO chromophores could be well isolated on the polymer chain. After 300 h at 75 °C, the EO activities can be retained at above 78% of the initial data for HJ-1 and HJ-2, which were higher than for DESD (75%) and P-1 (72%) (detailed in the ESI†). The improvement of the thermal stability for EO activity should be attributed to the rigid structure which helped to restrain chromophore depolarization (Table 2).

Table 2 Glass transition temperatures and EO performances

	Tg/°C	r33^a/(pm V^−1)	r33^b/r33^a
a The maximum EO coefficient.b The EO coefficient measured after poling for 300 h at 75 °C.
HJ-1	106	28	78%
HJ-2	116	41	79%
DESD	104	19	75%
P-1	114	7	73%

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3. Conclusions

In conclusion, we have developed a facile route to synthesize a new kind of EO dendrimer with unique asymmetric structures. Via Cu-(I) catalyzed Huisgen reactions, the chromophores (C1) were attached to the dendrimer backbone with high efficiency (final step yield ≥ 90%). The surface topography and photophysical experiments showed the excellent film-forming abilities of the asymmetric dendrimers HJ-1 and HJ-2 along with weak chromophore dipole–dipole aggregation. After corona poling, a large EO coefficient of 41 pm V⁻¹ was achieved by HJ-2, which was more than twice as large as that of DESD containing the same chromophores. The asymmetric dendrimers also exhibited excellent thermal stability such that these materials retained above 78% activity after 300 h at 75 °C. The asymmetric EO dendrimers could effectively improve the response of chromophores to an applied poling field and could be used in EO device fabrication.

Acknowledgements

We are grateful to the National Natural Science Foundation of China (No. 11104284, No. 61101054 and No. U1533121) for the financial support. We also thank Dr Kerry Garrett for many useful comments and manuscript revision.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Details of synthetic procedures, UV-vis spectra, thermoanalysis data and EO activity stability are available. See DOI: 10.1039/c6ra03288e

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