Facile bromine-termination of nonlinear optical chromophore: remarkable optimization in photophysical properties, surface morphology and electro-optic activity

Abstract

This paper describes how a small molecular structure modification can enhance the microscopic and macroscopic properties of chromophore. A new chromophore WJ10 is synthesized by applying bromine-termination to the remote donor of WJ6, an existing donor–π–acceptor chromophore. This small molecular change can significantly enhance the photophysical properties of chromophore and generate intriguing inverted solvatochromism in solutions. The absorption intensity of chromophore WJ10 in a guest-host electro-optic polymer film is 40–50% higher than that of the chromophore WJ6, which results in the increase of microscopic first-order hyperpolarizability of WJ10 in guest–host electro-optic (EO) polymer film. DFT calculations was carried out to explain this intriguing photophysical property in both solutions and in films. Bromine-termination also has the influence on macroscopic surface morphology of WJ10 in EO films, making WJ10 more homogeneously dispersed than WJ6. In EO activities, EO coefficient obtained with the WJ10 film is more than two times larger (211 pm V⁻¹) than benchmark value 104 pm V⁻¹ obtained from WJ6. The enhanced electro-optic activity with WJ10 is due to the enhancement of the microscopic hyperpolarizability and the better chromophore alignment in the poling process. This study demonstrates the structure–property relationship in bromine-termination of nonlinear optical chromophore, which can be further explored for the synthesis of new organic EO materials.

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Introduction

The study of organic and polymeric electro-optic (EO) materials has made tremendous progress for their potential applications in photonic devices.^1–3 For such applications, nonlinear optical (NLO) chromophores as active components in NLO polymers must possess large molecular nonlinearity and good thermal and chemical stability simultaneously. Various molecular engineering processes, including molecular structural optimization, self-assembly, binary chromophore doping, molecular glasses, and chromophore dendrimers, have been proposed to realize ultra large EO coefficients.^4–9 Alex. K-Y. Jen and Jingdong Luo have made great efforts on molecular optimization of polyene-based push–pull chromophores for the optimization of both the microscopic molecular hyperpolarizability (β) and the macroscopic EO coefficient.^10–14 Zhen Li et al. have proposed and demonstrated the use of suitable steric hindrance group to effectively attenuate the dipole–dipole interactions and enhance the EO coefficient.^15–22 Those endeavors show that molecular optimization is an effective means towards the development of new organic EO materials for device applications.

As an active component in EO material, a NLO chromophore should have a balance molecular structure shape for mobility and steric hindrance to prevent chromophore aggregation and improve the poling efficiency in thin films. It is well established that the hydroxyl group in a chromophore is an active site to conduct functionalization of the chromophore, such as post-functionalization of side-chain EO polymer. To prevent chromophore aggregation, a bulk steric hindrance group such as tert-butyldimethylsilyl moiety can be introduced into a chromophore through the reaction of the hydroxy group and tert-butyldimethylsilyl chloride.^12,23,24 On the other hand, the polar hydroxyl group might result in chromophore aggregation and affect the poling efficiency of EO films. In this case, the hydroxyl group in the chromophore is usually required to be substituted through etherification or esterification in the processing of EO polymer films.^19,25–28

In this paper, bromination reaction is implemented to produce the bromine-terminated chromophore WJ10 by substituting the hydroxyl group. The effects of bromine-termination on molecular microscopic photophysical properties, hyperpolarizability, macroscopic chromophore morphology, and EO activities are systematically investigated. This simple molecular modification makes an outstanding improvement in photophysical properties, including the highly enhanced absorption intensity and the intriguing inverted solvatochromism. A guest-host EO polymer film WJ10/APC is fabricated and its solid-state photophysical properties as well as the film surface morphology are investigated for the understanding of the effects of molecular bromination on molecular microscopic nonlinearity and macroscopic film surface morphology. These investigations confirm that molecular modification by bromination can enhance the molecular hyperpolarizability in the solid state and improve the macroscopic poling efficiency of chromophore alignment. As a result, an EO value as large as 211 pm V⁻¹ is achieved with WJ10, which is more than twofold of the benchmark value (106 pm V⁻¹) obtained from the hydroxy-terminated chromophore WJ6.

Results and discussion

Synthesis and structure analysis

Scheme 1 shows the synthesis of the chromophore WJ10, where remote hydroxyl group and bromine are linked with donor–acceptor conjugated backbone by hexyl chain. The ¹H NMR in Fig. 1 shows that the protons on the conjugated backbone of WJ10 have the same shift as those in WJ6. The only difference is the shift of the methylene protons on CH₂–OH and CH₂–Br, i.e., proton H_b of WJ10 shifts to a lower chemical shift (δ = 3.43 ppm in WJ10 vs. δ = 3.67 ppm in WJ6). The lower chemical shift is due to the stronger shielding effect generated from the less electron-negative of Br than OH.

Scheme 1 Synthesis of chromophore WJ10.

Fig. 1 ¹H NMR of chromophores in CDCl₃.

Photophysical properties in solutions

The maximum absorption wavelength λ_max should depend on the intramolecular charge-transfer of the conjugated system of the chromophore, unless that intermolecular interactions (such as chromophore–chromophore interactions or chromophore-solvent interactions) affected the intramolecular charge-transfer. WJ6 and WJ10 have the same conjugated backbone, so they are supposed to have the same band gap of intramolecular charge-transfer. However, bromine-termination changes the photophysical properties of WJ10, in particular, the absorption wavelength and intensity. The mechanisms that determine λ_max in a solvent are much more complicated. As shown in Fig. 2c, in a weak or medium polar solvent, such as dioxane, toluene, chloroform, or DCM, both chromophores show almost the same λ_max. The positive solvatochromisms of WJ6 and WJ10 from dioxane to chloroform are 79 nm and 77 nm, respectively. In a strong polar solvent, both chromophores show hypochromatic shifts. In acetone, WJ6 shows a λ_max of 655 nm, while WJ10 shows a λ_max of 665 nm. The solvatochromisms of WJ6 and WJ10 from DCM to acetone are −63 nm and −48 nm, respectively, which are inverted. The inverted solvatochromism indicates that WJ6 can be more easily polarized into the zwitterionic regime than WJ10 in the strong polar environment, which is chemically unstable for push–pull chromophores.¹⁰ The introduction of Br may create a stable dielectric environment to ensure that intramolecular charge-transfer is free from the interference of the strong polar molecules such as solvent and neighbor chromophores.

Fig. 2 Photophysical properties of chromophores in solution (0.02 mM).

In addition to the wavelength change, the introduction of Br significantly enhances the absorption intensity. As shown in Fig. 2d and Table 1, WJ10 shows an enhancement of the molar extinction coefficient (ε) in different solutions by about 40% to 50%, compared with WJ6. In the less polar solvent dioxane, WJ10 shows a 39% enhancement of the absorption intensity. As the polarity of the solvent increases, the absorption enhancement increases, up to about 50% in acetone. As a common feature of second-order NLO chromophores, the most-bathochromically shifted spectrum always exhibits the characteristic band shape with the strongest absorption and the narrowest bandwidth.¹⁰ Both absorption enhancement and inverted solvatochromism indicate that bromine-termination significantly optimizes the photophysical properties of chromophores in solution.

Table 1 Photophysical properties of chromophores in solution (0.02 mM)

Entry	WJ6	WJ10
a Unit of λmax: nm; ε: 10^4 L cm mol^−1.
λmax(ε) in dioxane^a	649 (3.2)	648 (4.4)
λmax(ε) in toluene^a	674 (3.0)	671 (4.5)
λmax(ε) in chloroform^a	728 (3.2)	725 (4.7)
λmax(ε) in DCM^a	718 (3.4)	713 (4.7)
λmax(ε) in acetone^a	655 (2.4)	665 (3.6)
λmax(ε) in acetonitrile^a	657 (2.8)	666 (4.0)

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Photophysical properties and hyperpolarizability in films

The photophysical properties of guest–host EO polymer films that contain 20 wt% WJ6 and WJ10 chromophores in amorphous polycarbonate (APC), respectively, are also investigated for the understanding of the effects of bromine-termination on the solid-state photophysical properties. Because of different dielectric environments in solid and in solution, the hypochromatic shift of λ_max in respective films is about 20 nm larger than that obtained from solutions. Meanwhile, the absorption bands are also wider than those in solutions, as shown in Fig. 3. For the chromophores in EO films, ε = A/cL, where c is the molar concentration and L is the thickness of EO thin film. We can calculate the molar concentration according to the equation c = ρ × 25%/Mn, where ρ is the density of EO film, Mn is the molar weight of chromophore. Film WJ10/APC shows much stronger absorption intensity than WJ6/APC film (ε: 4.2 × 10⁴ vs. 2.5 × 10⁴).

Fig. 3 Absorption spectra of EO films before and after poling in 20 wt% chromophore loading density: solid curve: WJ10/APC (19.5%); dashed curve: WJ6/APC (11.0%); black: before poling; red: after poling.

On microscopic level, molecular second-order susceptibilities (hyperpolarizability, β) can be measured by Hyper-Rayleigh Scattering (HRS), Electrical-Field-Induced Second-Harmonic Generation (EFISHG), and solvatochromic method,^29–32 which are conducted in a dilute solution. However, the chromophores in a dilute solution and in an EO film have the different dielectric environments. As shown earlier, the chromophores have distinct absorption peaks, intensity and hence different β values in solid state and in solution. The existing HRS and EFISHG methods are both based on the solution environment. Here, the quantum-mechanical two-level model for the second-order polarizability β_CT in the solid state is used to qualitatively compare the microscopic β values of WJ6 and WJ10 in guest–host EO polymer. This approximate comparison based on the variations of chromophore's photophysical properties reflects the effects of the structural modification on the microscopic nonlinearity. In the two-level model,^33,34 β_CT can be expressed as

where Δμ = μ_e − μ_g is the difference between the dipole moments in the excited state and the ground state, and ω_eg is the transition frequency. To calculate β_CT, the microscopic quantities in eqn (1) have to be determined by spectroscopic measurements. The transition frequency ω_eg and the transition dipole moment μ_eg can be determined by recording the UV-Vis absorption spectrum of the chromophore in solution. ω_eg is given by the frequency of maximum absorption in the spectrum and μ_eg is related to the overall strength of the transition. According to general quantum mechanics, μ_eg can be determined from the area under the absorption band:^33,34where ω_eg is the transition frequency in the solvent, N_A is Avogadro's number, ε₀ is the vacuum permittivity, c is the speed of light, f is the local filed factor, and h is Planck's constant. The molar extinction coefficient ε is related to the measured absorption coefficient. Eqn (2) can be rearranged as

By substituting eqn (3) into eqn (1), the relation of β_CT and the molar extinction coefficient can be found:

which shows that β_CT is proportional to the integral area of the absorption band. By using the Lorentz approximation, the local-field factor f is given by^33,34where n is the refractive index of the EO film, which is equal to 1.6034 for the WJ10/APC film and 1.5914 for the WJ6/APC film. The introduction of Br increases the refractive index and the local field factor f, but the increase in f is too small (from 2.283 to 2.338) to be negligible.

Because the absorption wavelengths of the WJ10/APC film (702 nm) and the WJ6/APC film (704 nm) are close, as shown in Fig. 3, their difference can be ignored. The only components that affect the value of β_CT are Δμ and the area of the absorption band. The value of Δμ can be measured by solvatochromism in solution, which is related to the position, the shape, and the intensity of the absorption band and the emission band of the chromophore. Since there is no fluorescence of two chromophores in films and in solution, it is hard to compare the difference of Δμ. We assumed that the two chromophores in the EO films have the same Δμ value, although actual absorption data indicates that the Δμ value of WJ10 may be larger than that of WJ6. For this simplification, β_CT is proportional to the area of the absorption band of the chromophore in the EO film. To qualitatively compare the β_CT values for the two EO films, the area of the absorption band is calculated before poling. The absorption area of WJ10 in film is 1.47 times of WJ6, indicating that, under the approximations in the calculation, the β_CT value of WJ10 is 1.47 times larger than WJ6 in EO films. Although the comparison may be not so accurate, this discussion draws a clear relationship between the photophysical properties (especially the absorption intensity) and molecular microscopic hyperpolarizability. Bromine-termination increases the β_CT value of WJ10 in the EO polymer film, which may lead to an enhancement of the macroscopic EO activity.

DFT calculations

For electron-centrosymmetric chromophores with large dipole moment, intramolecular charge-transfer (ICT) is strongly affected by the chromophore-solvent interactions. Especially in strong polar solvent such as in acetone and acetonitrile, both chromophores shows hypochromatic absorption wavelength and weaken absorption intensity. As we reveal that bromine-terminated WJ10 showed much more intensified absorption and less inverted solvatochromism. To account for this intriguing photophysical properties, theoretical calculation of density functional theory (DFT) was carried out at the hybrid B3LYP level by employing the split valence 6-31g(d,p) basis set.³⁵ Dipole moment of chromophores was calculated after molecular configuration optimization (Table 2). In the direction from donor to acceptor (x axis), two chromophores show the similar dipole moment, owing to the same conjugated backbone of push–pull electron. In y axis, which is approximately along the direction of alkyl chain in donor, chromophore WJ10 shows smaller dipole moment of 4.51 D than 7.85 D of hydroxy-terminated WJ6. In this respect, introduction of electron-deficient bromine brings about the decrease of μ_y, leading to the less polarizable environment in y axis. It indicates that dipole–dipole interactions or chromophore–solvent interactions in the y axis are weaker than WJ6. Bromine has a larger van der Waals radius than hydroxy group (as shown in Fig. 4). So this effect of site-isolation may cover a large area around the conjugated backbone. Hence, chromophores WJ10 are more dispersed and weaker chromophore–solvent interaction in strong polar environment can be achieved, showing much higher absorption intensity and less inverted solvatochromism.

Table 2 Dipole moment of chromophores from DFT calculations

Entry	WJ6	WJ10
a μtotal = (μx^2 + μy^2 + μz^2)^1/2.
μx (D)^a	21.82	22.14
μy (D)	7.85	4.51
μz (D)	0.25	1.01
μtotal (D)^a	23.17	22.58

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Fig. 4 Optimized configuration of WJ6 (upper) and WJ10 (lower) from DFT calculations.

Surface morphology analysis

The surface morphologies of EO films before poling, measured by atomic force microscopy (AFM), are shown in Fig. 5. The guest–host EO films have the same chromophore loading density (20% in weight) and the host matrix is commercially available poly(bisphenol amorphous carbonate) (APC). WJ6/APC and WJ10/APC films have rather different surface morphologies. The surface of the WJ6/APC film is flat, but there are numerous compact arrays with a height of around 17 nm. The width of the compact arrays is around 2 nm. The surface of the WJ10/APC film is more homogeneous than that of the WJ6/APC film, and the arrays spread much more sparsely. It is difficult to identify the ingredients of the compact arrays, but it is apparent that the morphology variations are due to the different guest chromophore dopings. One possible reason is that different remote polar moiety OH and Br induce different degrees of intermolecular aggregation. As OH is a stronger polar group and can easily form hydrogen-bonding, stronger chromophore aggregation in the WJ6/APC film is produced. Consequently, film WJ6/APC shows a much weaker absorption intensity than the WJ10/APC film. The surface analysis by AFM may indicate the subdued aggregation of WJ10 in the WJ10/APC film. As DFT calculation reveals, bromination in remote donor played an important role in attenuating chromophore–chromophore interactions. Besides, as the site-isolation group, the radius of bromine was larger than hydroxy group, implying the better site-isolator of bromine to attenuate chromophore aggregation. This structural modification makes chromophore WJ10 more dispersed in the film and thus results in a higher absorption intensity, which may benefit the electrical-field induced poling for efficient chromophore alignment.

Fig. 5 AFM images of the EO films.

Macroscopic electro-optic activities

The effects of bromine-termination have been displayed in the enhancement of the photophysical properties. In the EO measurement, the poled WJ10/APC film shows an EO coefficient of 211 pm V⁻¹ at 1310 nm with the chromophore loading density 20 wt%, which is much higher than value (104 pm V⁻¹) of the WJ6/APC film. This significant enhancement of the EO activity is achieved by simple bromination of the chromophore WJ6. The EO coefficient, which defines the efficiency of translating the molecular microscopic hyperpolarizability into the macroscopic EO activity, is described aswhere N represents the aligned chromophore number density and f(ω) denotes the Lorentz–Onsager local field factor. The term 〈cos³ θ〉 is the orientationally averaged acentric order parameter that characterizes the degree of centrosymmetric alignment of the chromophores, and n is the refractive index. Realization of large EO activity for dipolar organic chromophore-containing materials requires simultaneous optimization of the chromophore first hyperpolarizability β, the acentric order 〈cos³ θ〉, and the number density N.³⁶

Table 3 listed the data of EO films WJ6/APC and WJ10/APC. More than twofold enhancement of EO activity was achieved in poled film WJ10/APC. This enhancement can be attributed to two aspects, which influenced the β value and aligned chromophore number N in eqn (6). According to the quantum-mechanical two-level model in eqn (4) the β value is proportional to the integration area of ε.³³ With other factors ignored, the strong absorption intensity of film WJ10/APC may result in a β value about 50% larger than that of WJ6/APC. The qualitatively approximate comparison in the solid state may be not so accurate, but it explicitly shows that the hyperpolarizability of WJ10 in the solid state is significantly enhanced. In this respect, the enhanced hyperpolarizability can be regarded as the first factor that contributes to the enhancement of the macroscopic EO activity in WJ10/APC.

Table 3 Data of EO films

Entry	WJ6/APC	WJ10/APC
a Measured in 1310 nm.
λmax (nm)	704	702
ε (10^4 L cm mol^−1)	2.6	4.2
Poling efficiency	11.0%	19.5%
Refractive index^a	1.5914	1.6034
r33 (pm V^−1)^a	104	211

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The other factor that contributes to the enhancement of the EO activity is the poling efficiency, which affects the parameter N in eqn (6). The absorption spectra of the films were measured after poling for the calculation of the poling efficiency of chromophore alignment. Poling efficiency is calculated by the absorption intensity change ratio before and after chromophore alignment.^10,37 The WJ10/APC film shows a much higher poling efficiency of 19.5%, which indicates that the number of aligned chromophores (WJ10) is much larger than that of WJ6. This improvement of poling efficiency directly increases the chromophore density N. As the AFM analysis shows, the change in the film morphology enhances the solid-state photophysical properties and leads to less chromophore aggregation. It ensures more emancipation of the chromophore WJ10 for alignment, which contributes to the high poling efficiency in electrical-field induced poling. Owing to the higher poling efficiency and the larger β value, the WJ10/APC film achieves a much higher macroscopic EO coefficient.

Conclusions

A simple chromophore structure modification by bromine-termination can significantly enhance the photophysical properties, in particular, the absorption intensity of the chromophore in solution. The intriguing inverted solvatochromism of the chromophore in strong polar solvents indicates that, with regard to intramolecular charge-transfer in a strong polar environment, the bromine-terminated chromophore WJ10 is much more stable than the hydroxyl-terminated chromophore WJ6. DFT calculations reveals that μ_y of WJ10 was decreased from 7.85 D to 4.51 D, indicating the weaker intermolecular interactions and more dispersed for more intensified ICT. In guest–host EO films, the enhancement of photophysical properties contributes to the enhancement of the molecular microscopic hyperpolarizability of the chromophore WJ10. Bromine-termination also produces a better surface morphology for the WJ10/APC film, which reveals that more homogeneous chromophore disperse for efficient poling. Thanks to the enhancement of the molecular microscopic hyperpolarizability and the much higher poling efficiency (19.5% for WJ10/APC and 11.0% for WJ6/APC), the WJ10/APC film achieves a much higher EO coefficient of 211 pm V⁻¹, compared with WJ6/APC (104 pm V⁻¹). These results confirm that the structure modification by bromine-termination significantly improves the microscopic molecular nonlinearity and the macroscopic film morphology and weakens the intermolecular interaction. This structure–properties relationship represents a new strategy of molecular optimization for the synthesis of organic EO materials.

Experiments

The NMR spectra were determined by a Varian Gemini 300(400 MHz) NMR spectrometer. The MS spectra were obtained on a MALDITOF (Matrix Assisted Laser Desorption/Ionization of Flight) on BIFLEXIII (Broker Inc.) spectrometer. The UV-Vis spectra were performed on cary 5000 photo spectrometer. Atomic Force Microscope (AFM) was characterized by multimode Bruker. The thicknesses of the films were measured with an Ambios Technology XP-1 profilometer. The refractive indices of the thin films were measured with a Metricon Model 2010 prism coupler.

All commercially available chemicals were used without further purification unless stated. The DMF was freshly distilled prior to its use. The chromophore WJ6 and the 2-dicyanomethylene-3-cyano-4-methyl-2,5-dihydrofuran acceptor were prepared by the processes reported in the literature.^8,28

Synthesis of the chromophore WJ10

Excessive tetrabromomethane was dropwise added to a mixed tetrahydrofuran (30 mL) solution of 0.33 g (0.5 mmol) of the chromophore WJ6 and 0.25 g (0.6 mmol) triphenylphosphine. The solution turned purple and was stirred for 2 hours at room temperature. The solution was then poured into saturated brine and extracted by dichloromethane, dried overnight by MgSO₄. After removal of the solvent under reduced pressure, the crude product was purified by silica chromatography, eluting with (EA : hexane = 1 : 4) to give the chromophore WJ10 as dark-green powder (yield: 94%, 0.34 g). MS (MALDI-TOF), m/z: 725.247 1H NMR (400 MHz, CDCl₃) δ 7.69 (d, J = 15.6 Hz, 1H), 7.32 (d, J = 4.0 Hz, 1H), 7.23 (d, J = 15.4 Hz, 2H), 6.95 (dd, J = 16.2, 9.9 Hz, 2H), 6.49 (d, J = 15.6 Hz, 1H), 3.77 (t, J = 6.6 Hz, 2H), 3.36 (t, J = 6.7 Hz, 2H), 3.15 (d, J = 27.9 Hz, 4H), 1.91–1.79 (m, 4H), 1.69 (d, J = 8.1 Hz, 10H), 1.51 (d, J = 6.6 Hz, 4H), 1.36 (s, 6H), 1.24 (s, 6H). ¹³C NMR (101 MHz, CDCl₃) δ 174.81, 171.80, 156.22, 154.43, 138.18, 136.71, 136.13, 130.58, 127.82, 125.76, 122.20, 114.81, 111.35, 110.56, 110.47, 110.09, 95.84, 94.76, 74.63, 64.54, 54.71, 46.65, 46.11, 38.66, 35.11, 32.94, 31.71, 31.28, 29.91, 29.01, 28.96, 27.15, 25.62, 24.54. Anal. calcd (%): WJ10, C₄₀H₄₅BrN₄O₂S: C 66.19; H 6.25; N 7.72; found: C 66.52; H 6.49; N 7.86.

Thin-film fabrication and EO coefficient measurements

Guest–host polymers were prepared by doping respective chromophores (20% wt) into amorphous polycarbonate (APC) using dibromomethane (CH₂Br₂) as the solvent. The resulting solutions were filtered through a 0.22 μm Teflon membrane filter and spin-coated onto indium tin oxide (ITO) glass substrates. The films of doped polymers were baked in a vacuum oven at 40 °C to remove the residual solvent and dried in vacuum at 40 °C overnight to thoroughly remove the residual solvent. Both films were prepared by the same procedure to the same thickness around 1.6 μm. The poling process was carried out at a temperature of about 10 °C above the T_g of the polymer. The EO values were measured by the Teng-Man reflection technique at the wavelength 1310 nm.³⁸

Acknowledgements

The research is supported by the National Natural Science Foundation of China under Projects 61505020, 11104284 and 61101054.

Notes and references

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