Enhanced the electro-optic activity of optical chromophores via rigid isolation groups modification

Shuhui Bo; Fuyang Huo; Zefan Duan; Pengwei Li; Jiayi Lu; Zhuo Chen

doi:10.25259/AJC_168_2025

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Original Article

2025

:18;

1682025

doi:

10.25259/AJC_168_2025

Enhanced the electro-optic activity of optical chromophores via rigid isolation groups modification

Shuhui Bo^a,b, Fuyang Huo^c,, Zefan Duan^a, Pengwei Li^a, Jiayi Lu^a, Zhuo Chen^d,*

aOptoelectronics Research Centre, Engineering Research Centre of Photonic Design Software, Ministry of Education, School of Science, Minzu University of China, 27 Zhonguancun South Street, Beijing, 100081, China

bInstitute of National Security, Minzu University of China, Beijing 100081, China

cSchool of Chemistry and Chemical Engineering, Guangzhou University, No. 230, Outer Ring West Road, Guangzhou, 510006, China

dKey Laboratory of Bio-inspired Materials and Interfacial Science, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, 29 Zhongguancun East Road, Beijing, 100190, China

* Corresponding author: E-mail address: chenzhuo@mail.ipc.ac.cn (Z. Chen)

Received: 2025-02-19, Accepted: 2025-04-29, Epub ahead of print: 2025-07-29, Published: 2025-08-04

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Abstract

We engineered a conjugated organic chromophore, BHC-1, for second-order nonlinear optics, integrating a julolidine donor, a CF₃-Ph-TCF acceptor, and a diene bridge. To systematically study steric effects, the diene bridge was further modified with two types of distinct benzene derivative isolation groups, leading to the synthesis of chromophores BHC-2 and BHC-3. In this paper, the synthesized chromophores demonstrated robust thermal stability, as evidenced by decomposition temperatures above 230°C. Notably, UV-Vis analysis revealed that the chromophores BHC-2 and BHC-3 showed a stronger solvent effect, and they were more easily polarized than the chromophore BHC-1. By doping the three chromophores in amorphous polycarbonate (APC) at a significant weight fraction (35 wt%), the highest electro-optic (EO) coefficients (r₃₃) at 1310 nm can reach 70, 121, and 132 pm/V^-1. The results showed that polymers doped with chromophores bearing rigid sterically hindered groups exhibit higher EO activity. It suggests that the benzene derivative isolation groups are critical for mitigating dipole-dipole interactions through steric hindrance and enhancing the poling efficiency. The high EO coefficients and the good thermal stability validate its application in the field of nonlinear optics.

Keywords

Electro-optic coefficient

Nonlinear optics

Organic EO chromophores

Second-order nonlinear optical materials

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1. Introduction

Due to the accelerated advancement of materials and laser technology, some electro-optic (EO) materials are increasingly being used in photonics technology after they have been discovered by researchers [1-3]. Today, photonic technologies such as optical information, optical communication, and optical modulation have also made qualitative leaps in daily life and commercial practice [4-6]. It is imperative to search for a promising EO material. Organic second-order nonlinear optical (NLO) materials demonstrate superior performance relative to conventional alternatives, particularly in terms of their significantly higher EO coefficients (r₃₃), low dielectric constants, fast response times, and better processability and integration. In addition, organic NLO material is one of the most promising materials for large-scale integration in the future, because it is generally less expensive and easier to cut to shape [7,8]. The study of organic NLO materials is also becoming a hot topic of interest for contemporary scientists. One of the most widely researched materials is host-guest doped EO polymers, where the key to the performance of the material is the chromophore used for doping. Therefore, to meet the market demand of the material and to improve its EO properties, we urgently need to synthesize organic NLO chromophores with good microscopic molecular first-order hyperpolarizability (β), so that the microscopic β of the chromophore can be efficiently converted into a relatively high macroscopic r₃₃ value of the material.

Chromophores are central to the macroscopic EO properties of materials and are typically structured as a D-π-A system, comprising an electron donor, conjugated electron bridges, and acceptors [9]. In the mid-1970s, the relationship between the degree of charge transfer within a conjugated molecule and its nonlinear effects was first elucidated [10]. In the same period, [11] proposed the classical two-energy model theory, which more clearly illustrates that the dipole moment (μ) of molecular dipole moment and the β value both determine the non-linear properties of chromophores. Therefore, we can increase the value (μβ) of the molecule by increasing the electron-donating capacity of the donor, the electron-withdrawing capability of the acceptor, and increasing the length of the conjugated chain [12]. Furthermore, dipole-dipole interactions between chromophores can be attenuated through the introduction of bulky substituents as steric hindrance units. Such spatial isolation not only improves poling alignment but also boosts the r₃₃ value, a critical metric for NLO performance [13,14]. In summary, we have used the above methods to modify and design the chromophores as a means of obtaining organic NLO materials with large EO coefficients.

In this work, our chosen electron donor is julolidine, which has strong electron-donating ability and excellent thermal and photochemical stability performance in all aspects [15,16]. The electron acceptor CF₃-Ph-TCF was chosen because it has strong electron-accepting capability and good solubility. Moreover, the methoxyphenyl moiety and the N,N-dimethylaniline moiety are the substituted benzene derivative steric hindrance groups. To engineer enhanced NLO performance, bulky benzene derivative substituents were strategically incorporated into the polyene π-bridge. These groups serve dual roles: (1) as auxiliary electron donors to reinforce intramolecular charge transfer, and (2) as steric spacers to mitigate dipole-dipole interaction between chromophores. This synergistic design optimizes electric field-induced alignment (poling efficiency), ultimately amplifying the macroscopic EO response (r₃₃).

We developed two novel chromophores (BHC-2 and BHC-3) through molecular engineering of the π-conjugated bridge. For comparative analysis, chromophore BHC-1, a structural analogue lacking steric hindrance groups on the π-bridge, was synthesized as a performance reference. The thermal stability, UV-Vis, microscopic nonlinearity, and macroscopic EO activity of the chromophores were rigorously investigated and evaluated to elucidate the impact of molecular architecture in guiding the strategic engineering of high r₃₃ chromophores (Chart. 1). By doping the new BHC-1, BHC-2, and BHC-3 chromophores at a significant weight fraction of 35 wt% in APC, r₃₃ reach values of 70, 121, and 132 pm V^-1, demonstrating that the suitable steric hindrance groups can significantly enhance the EO performance. The simple and stable chromophore structure with a high r₃₃ exhibit the potential for high-speed EO device integration.

Molecular architecture of the D-π-A chromophores BHC-1, BHC-2c, and BHC-3.

2. Materials and Methods

2.1. Materials

All solvents, if not specified, were domestic analytical purity. Sodium borohydride, 2-bromomethyl-1,3-dioxopentane, triphenylphosphine, triphenylphosphine hydrogen bromide, p-methoxybenzyl alcohol, and 4-(N,N-dimethylamino)benzyl alcohol were bought from Sigma-Aldrich Company. The CF₃-Ph-TCF acceptor was synthesized following the procedures outlined in the literature [17,18]. ITO glasses were cleaned with several solvents, and then soaked in anhydrous isopropanol.

2.2. Measurements and instruments

Nuclear magnetic resonance (NMR) spectra (¹H, ¹³C) were acquired on a Bruker Advance NMR spectrometer. Absorption spectra in the UV-Vis region were obtained using a Cary 5000 spectrophotometer, while mass spectrometry (MS) was conducted on an AB SCIEX TripleTOF 4600 mass spectrometer. The differential scanning calorimetry (DSC) were executed on a DSC25 instrument. The thermogravimetric analysis (TGA) was conducted using a TA5000-2950 analyzer in a nitrogen environment, applying a heating rate of 10°C per minute. Additional equipment included a KW-4A spin coater for film deposition, a contact poling apparatus, an EO coefficient measurement system utilizing a simple reflection technique, and gold electrodes deposited via ion sputtering. The thickness of the film was measured utilizing a BRUKER DektakXT surface profiler.

2.3. Synthesis and characterization

Compounds 4a, 4b, and 4c were synthesized according to the published literatures [19,20].

2.3.1. The synthesis of chromophore BHC-1

A solution of compound 4a (0.21 g, 0.5 mmol) and the acceptor CF₃-Ph-TCF (0.15 g, 0.47 mmol) in 30 mL dry ethanol was heated to 70°C with vigorous stirring for 3hr. Following solvent evaporation under vacuum, the crude material was purified by flash chromatography over silica gel (gradient elution), affording the green solid chromophore BHC-1 in 42% isolated yield.

HRMS (ESI) (M+H)⁺ for (C₄₁H₄₂ClF₃N₄O₂+H)⁺: calcd: 715.3021; found: 715.2997.

¹H NMR (400 MHz, CDCl₃) δ 7.91 (t, J = 12.0 Hz, 1H), 7.51 (s, 5H), 7.36-7.31 (m, 2H), 6.82 (t, J = 16.0 Hz, 1H), 6.27 (d, J = 12.0 Hz, 1H), 3.79 (t, J = 8.0 Hz, 2H), 3.57 (t, J = 8.0 Hz, 2H), 3.44 (t, J = 4.0 Hz, 2H), 3.36 (s, 2H), 1.86 – 1.75 (m, 8H), 1.53 (s, 4H), 1.41 (s, 6H), 1.28 (s, 6H).

¹³C NMR (101 MHz, CDCl₃) δ 176.34, 160.81, 160.24, 152.99, 149.07, 148.48,130.95, 130.82, 129.41, 128.78, 126.78, 125.41, 123.77, 123.30, 122.99, 120.92, 118.08,113.09, 112.69, 112.43, 111.92, 95.33, 95.02, 90.20, 60.37, 54.05, 48.29, 47.62, 45.06, 38.59, 34.95, 32.51, 32.36, 32.08, 29.84, 29.69, 29.28, 26.74, 25.40.

2.3.2. The synthesis of chromophore BHC-2

The procedure for the reaction was analogous to the synthesis method used for the chromophore. BHC-1 to obtain the yellow-green crystalline solid BHC-2.

HRMS (ESI) (M+H)⁺ for (C₄₈H₄₈ClF₃N₄O₃+H)⁺: calcd: 821.3440; found: 821.3422.

¹H NMR (400 MHz, CDCl₃) δ 8.10 (s, 1H), 7.44 (ddd, J = 28.0, 16.0, 8.0 Hz, 6H), 7.03 (dd, J = 16.0, 8.0 Hz, 4H), 6.58 (s, 1H), 5.94 (d, J = 16.0 Hz, 1H), 3.87 (d, J = 8.0 Hz, 5H), 3.59 (t, J = 6.0 Hz, 2H), 3.29 (d, J = 4.0 Hz, 4H), 1.87 (s, 4H), 1.71 (s, 2H), 1.57 (s, 6H), 1.38 (s, 6H), 0.78 (s, 6H).

¹³C NMR (101 MHz, CDCl₃) δ 176.43, 161.40, 160.42, 159.65, 156.15, 148.21, 147.71, 136.16, 130.81, 130.79, 130.60, 129.32, 128.59, 128.07, 127.39, 126.40, 123.65, 122.64, 120.81, 117.54, 115.27, 112.38, 111.81, 95.21, 94.89, 90.37, 77.66, 77.37, 77.25, 77.05, 76.73, 55.48, 54.36, 48.06, 47.39, 45.16, 38.79, 34.98, 32.46, 32.40, 31.61, 29.90, 29.39, 29.36, 29.24, 29.20, 26.78, 25.46.

2.3.3. The synthesis of chromophore BHC-3

The acquisition of the green powdered solid BHC-3 entails a reaction process that resembles the synthesis method applied to the chromophore BHC-1.

HRMS (ESI) (M+H)⁺ for (C₄₉H₅₁ClF₃N₅O₂+H)⁺: calcd: 834.3756; found: 834.3745.

¹H NMR (400 MHz, CDCl₃) δ 8.02 (s, 1H), 7.35 (dt, J = 24.0, 8.0 Hz, 5H), 7.26 (s, 1H), 7.18 (s, 1H), 6.89 (d, J = 8.0 Hz, 2H), 6.74 (s, 2H), 6.57 (s, 1H), 5.99 (d, J = 12.0 Hz, 1H), 3.78 (t, J = 8.0 Hz, 2H), 3.50 (t, J = 8.0 Hz, 2H), 3.19 (s, 4H), 2.91 (s, 6H), 2.09 (s, 1H), 1.96 (s, 1H), 1.78 (d, J = 8.0 Hz, 4H), 1.62-1.48 (m, 2H), 1.30 (s, 6H), 1.17 (t, J = 8.0 Hz, 2H), 0.78 (d, J = 8.0 Hz, 1H), 0.70 (s, 6H).

¹³C NMR (101 MHz, CDCl₃) δ 207.01, 176.62, 161.53, 160.43, 160.28, 156.66, 150.62, 148.69, 148.54, 148.45, 148.31, 137.31, 131.09, 131.06, 130.84, 130.16, 129.38, 128.37, 127.23, 127.14, 126.60, 123.80, 122.35, 120.96, 117.72, 113.78, 112.74, 112.66, 112.15, 112.07, 111.95, 77.48, 77.16, 76.84, 48.06, 47.40, 45.27, 40.75, 39.02, 35.25, 35.23, 32.53, 31.71, 31.03, 30.03, 29.81, 29.50, 29.41, 26.90, 25.58.

3. Results and Discussion

3.1. Synthesis and characterization

Synthetic route of BHC-1, BHC-2, and BHC-3 has been shown in Scheme 1. Firstly, 1,6-dichlorohexane was substituted with julolidine with K₂CO₃ to give compound 2. 1-chloro-6-methoxyhexane flexible long-chain group can enhance the compatibility between the chromophore and the polymer. Then compound 2 underwent Wittig reaction with compound 3a, and compound 4a was produced by hydrolysis with dilute hydrochloric acid. The aldehydes 4b and 4c were produced when the compounds 3b and 3c underwent a reaction with POCl₃ and DMF. Finally, the Knoevenagel reactions of compounds 4a, 4b and 4c with the electron acceptors CF₃-Ph-TCF, led to the chromophores BHC-1, BHC-2 and BHC-3. All chromophores were thoroughly characterized via MS, ¹H NMR, ¹³C NMR, and UV-Vis spectroscopy. The analytical data exhibited excellent consistency with the proposed molecular structures. Detailed spectral assignments, including ¹H NMR, ¹³C NMR spectra for the three chromophores, have been provided in Supplementary Figures S1-S6 (Supporting Information).

Figure S1

Figure S2

Figure S3

Figure S4

Figure S5

Figure S6

3.2. Thermal stability

NLO chromophores require robust thermal stability (>200°C) to endure the harsh conditions of electric field poling and material processing. The thermogravimetric analysis (TGA) characterization results allow us to analyze the thermal stability of the three chromophores, whose thermal decomposition temperature (T_d) is the temperature corresponding to a 5 wt% thermal weight loss. In Figure 1(a) and Table 1, the T_d of the chromophores BHC-1, BHC-2 and BHC-3 were determined to be 238.23°C, 233.93°C and 230.53°C. The T_d of all three chromophores exceed 200°C, satisfying the thermal stability threshold required for device fabrication processes, and the similarity of the T_d of the three groups demonstrates that introduction of the rigid isolation groups does not destroy the thermal stability. Among them, the T_d of the BHC-2 and BHC-3 with the introduction of the rigid groups are similar to that of the original chromophore BHC-1, which makes it clear that the combination of strong donor structure and strong acceptor structure will affect the thermal stability of the material instead. Therefore, the selection of suitable electron donor and electron acceptor can ensure the good thermal stability.

Plots of (a) TGA and (b) DSC for BHC-1, BHC-2, and BHC-3.

Table 1. Thermodynamic and optical data of three chromophores.

Chromophore	T_d	T_g	λ_max^a	λ_max^b	λ_max^c	λ_max^d	λ_max^e	λ_max^f	Δλ^g
BHC-1	238	87.3	776	775	732	769	770	739	42
BHC-2	234	94.8	773	772	729	767	767	741	44
BHC-3	231	96.6	776	775	731	770	770	744	45

^a∼f (nm) were the maximum absorption wavelengths in acetone, acetonitrile, 1,4-dioxane, chloroform, THF, and toluene, respectively. ^g (nm) was the difference between λ_max^a or λ_max^c, and λ_max^d.

To investigate the glass transition temperature T_g, we specifically used DSC. In Figure 1(b) and Table 1, the T_gs of the three chromophores were 87.3°C, 94.8°C, and 96.6°C. The T_g of chromophore BHC-1 was lower than BHC-2 and BHC-3, indicating that the benzene derivative isolation groups lead to higher glass transition temperatures. The incorporation of rigid steric hindrance makes it difficult for molecular segments to move at lower temperatures. Furthermore, the T_g of chromophore BHC-3 was higher than BHC-2, which illustrated that the rigidity of the methoxyphenyl group is stronger than that of N,N-dimethylaniline group. The rigid steric hindrance was beneficial for improving the regularity of chromophores and increasing the thermal stability of molecular structure.

3.3. Optical properties

To investigate the impact of steric substituents on intramolecular charge transfer (ICT) in dipolar chromophores, the UV-Vis absorption profiles of the three synthesized chromophores were systematically studied in solvents spanning a range of dielectric constants, including 1,4-dioxane, toluene, chloroform, tetrahydrofuran, acetone, and acetonitrile (Figure 2). As summarized in Table 1, all chromophores displayed a characteristic π→π* transition, generating a strong absorption band in the visible region attributed to π-delocalization and ICT effects, with spectral positions exhibiting moderate solvent dependence, a hallmark of polar excited states. Notably, despite structural variations in steric hindrance groups, the absorption maxima of three chromophores remained a similar band. To systematically investigate the steric substituent effects on ICT dynamics in dipolar chromophores, solvatochromic analysis was conducted via UV/Vis-near infrared (NIR) spectroscopy across a dielectric constant continuum encompassing 1,4-dioxane, toluene, chloroform, THF, acetone, and acetonitrile. As systematically mapped in Figure 2 and quantified in Table 1, the three chromophores with different structures exhibited similar π - π * ICT absorption bands in the visible region. Observing the curves of different colors in the same spectrum, we found that the spectra showed first a red shift as the dielectric constant of the solvent gradually increases from 1,4-dioxane to acetone. The three chromophores reached the longest maximum absorption wavelengths (λ_max) in acetone, which were 776 nm, 771 nm, and 776 nm. When the chromophores are in polar solvents, the intramolecular charge separation (ICT) is further enhanced, meaning that electrons migrate from the donor to the acceptor, resulting in a larger polarity in the excited state compared to the ground state, which leads to a redshift in the absorption wavelength. However, it remains unchanged in acetonitrile, which has a higher dielectric constant (polarity). Due to the stronger donor-acceptor pairs in the three chromophores, they may both be polarized close to the cyanine limit, where the ground state is a resonance hybrid with equal contributions from the neutral and zwitterionic canonical structures. The reason why the waveforms and λ_max of the three chromophore spectra were close is that the introduced rigid groups did not change the conjugation system of the chromophores.

UV-Vis absorption spectra of (a) BHC-1, (b) BHC-2, and (c) BHC-3.

We know that julolidine possesses a strong ability to transfer charge, so the synthesized chromophores all have good nonlinear properties. In addition, we can show the solvation effect of the three chromophores by the λ_max differences (Δλ) of 42 nm, 44 nm, and 45 nm in different solvents in Table 1 [21,22]. The maximum absorption wavelength of the chromophores BHC-2 and BHC-3 in acetone solvent are redshifted by 44 nm and 45 nm compared to those in 1,4-dioxane, which are larger than those of the chromophore BHC-1. These indicate that the chromophores BHC-2 and BHC-3 with the rigid groups have stronger ICT capability and are more susceptible to polarization than the chromophore BHC-1 without the rigid groups.

3.4.Theoretical calculations

To examine the influence of chemical structure on the microscopic nonlinearity of chromophores, we conducted structural optimization through density functional theory (DFT) calculations. The geometries of chromophores were refined in a vacuum, utilizing the B3LYP functional paired with a 6-31G basis set. After optimization, vital microscopic nonlinearity such as the highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) energy gaps (ΔE), β, and μ were evaluated [23,24].

Frontline molecular orbitals are a typical method used to evaluate molecular reactivity and stability, as well as to provide qualitative understanding of their optical and electrical properties. Figure 3 shows the HOMO and LUMO energy levels of the chromophores in a vacuum. The electron density of the HOMO is mainly located on the donor, the conjugated bridge, and the substituted benzene derivative on the diene bridge. In contrast, the LUMO is primarily focused on the acceptor group and the conjugated bridge, particularly within the CF₃-Ph-TCF acceptor region. It indicates that the substituted benzene derivative steric hindrance groups play a role in electron donation, but 1-chloro-6-methoxyhexane groups on the donor have virtually no electron cloud distribution whether in the HOMO or LUMO levels.

The calculation data of HOMO and LUMO energies have been displayed in Table 2. The energy gaps of chromophores BHC-1, BHC-2, and BHC-3 is 3.8327 eV, 3.5848 eV, and 3.4311 eV, respectively. In comparison to BHC-1, chromophores BHC-2 and BHC-3 exhibit a smaller energy gap (ΔE), which can be explained by the electron-donating effects of the steric hindrance groups. Additionally, the reduced ΔE observed in chromophore BHC-3 versus BHC-2 directly correlates with the enhanced electron-donating strength of the N,N-dimethylaniline moiety relative to the methoxyphenyl substituent. DFT approach has proven to provide relatively reliable predictions of first-order hyperpolarizability for chromophores [25,26]. As noted in earlier studies, the β value is strongly influenced by factors such as substituents, steric hindrance, ICT, and the length of π-conjugation [27]. Relative to chromophore BHC-1, the β values of chromophores BHC-2 and BHC-3 showed increases of 5.4% and 14.9%, respectively. It is worth mentioning that chromophores BHC-3 showed significantly bigger values of 277×10⁻³⁰ esu than that of BHC-2 (254×10⁻³⁰ esu), alongside increased dipole moments of 25.55 D and 25.15 D. As a result, the chromophore BHC-3 exhibits the maximum μβ value. This can be explained by the incorporation of sterically bulky substituents, which enhance the β value of the chromophore through the strong electron-donating effect. The introduction of suitably rigid and flexible side chains, based on theoretical design, improves site isolation, enabling high hyperpolarizability, while keeping the dipole moment within an ideal range.

Table 2. DFT calculation data of chromophores BHC-1 to BHC-3.

Cmpd

E_HOMO (eV)

E_LUMO (eV)

ΔE (eV)

β_max

(10^-30 esu)

(D)

μβ

(10^-27esu D)

BHC-1

6.3620

2.5293

3.8327

241

24.14

5.82

BHC-2

6.0978

2.5130

3.5848

254

25.55

6.49

BHC-3

5.8026

2.3715

3.4311

277

25.15

6.97

3.5. Electro-optic (EO) performance

To study the correlation between microscopic hyperpolarizability and macroscopic EO effects, functional polymer films were fabricated by incorporating chromophore molecules at varying concentrations (15%, 25%, and 35% by weight) into an amorphous polycarbonate (APC) matrix, with dibromomethane serving as the solvent medium. The formulated mixtures were subsequently filtered via a 0.22 μm polytetrafluoroethylene (PTFE) membrane and deposited as uniform thin films onto indium tin oxide (ITO)-coated glass substrates using spin-coating techniques. Heat the doped polymer films overnight in a 60°C vacuum oven. To evaluate r₃₃, the polymer films underwent contact polarization within a ±10°C thermal window centered on the material’s glass transition temperature (T_g). Characterization was performed via the Teng-Man simple reflection method at a wavelength of 1310 nm using thin ITO glass as the bottom electrode, engineered with optimized optical transparency and reduced reflectivity for suppressing parasitic interference from multiple reflections [28,29]. Compared to silver or aluminum, gold foil, which is sputtered, has good conductivity and antioxidant and was selected as the upper electrode to achieve reflectivity. The r₃₃ and T_g values of films chromophore/APC with different concentration are in Table 3. The optimal electric poling conditions curves of chromophores BHC-2 and BHC-3 in APC (the thickness of films is about 2 μm), including poling temperature, time, voltage, and temperature were as shown in Figure 4 at the concentration of 35wt.%. Initially, when the temperature is relatively low, a small voltage is applied and the poling current is small. As the temperature rises to T_g, the system was subjected to poling electrical fields of 100∼130 V μm^-1 for 5∼10 min to allow the molecules to fully rotate. After polarization is completed, the temperature is lowered, while maintaining the voltage, so that the aligned molecules become fixed, thereby acquiring EO activity. This process is carried out in a nitrogen atmosphere.

Table 3. The r₃₃ and T_g values of films chromophore/APC with different concentration.

Chromophore

r₃₃/T_g

15wt%

r₃₃/T_g

25wt%

r₃₃/T_g

35wt%

BHC-1

45 pm V^-1/151°C

62 pm V^-1/138°C

70 pm V^-1/126°C

BHC-2

47 pm V^-1/163°C

98 pm V^-1/152°C

121 pm V^-1/143°C

BHC-3

49 pm V^-1/165°C

105 pm V^-1/155°C

132 pm V^-1/147°C

The poling curves for (a) BHC-2/APC and (b) BHC-3/APC with the concentration of 35wt%.

The experimental r₃₃ are governed by three interdependent factors: hyperpolarizability (β), chromophore number density (N), and polarization efﬁciency, which is defined by the order parameter <cos³ θ> (Eq. 1) [30].

(1)

r_{32} = | 2 N f (ω) β 〈 \cos^{3} θ 〉 / n^{4} |

The optical property parameters n (film refractive index) and f(ω) (field-enhancement factor) reflect electric-ﬁeld factors. These two parameters are relatively constant for chromophores with similar loading concentration. The acentric ordering, quantified by the cos³(θ) parameter, reflects polar orientation anisotropy within the Landau-de Gennes formalism, where θ denotes the angular deviation between chromophores’ intrinsic dipole moments and the poling field vector. The EO coefficient exhibits a linear scaling relationship with four key parameters: chromophore number density (N), molecular dipole moment (μ), hyperpolarizability (β), and poling field strength (E), when dipole-dipole interaction is ignored. The r₃₃ values of all chromophores increase as the doping concentration rises in Table 3. However, when the concentration increases from 25 wt% to 35wt%, the magnitude of the increase in the r₃₃ value of the chromophores is smaller than that observed when the concentration rises from 15% to 25%. Elevated molecular dipole moments in such systems promote strong dipole-dipole correlations that favor antiparallel supramolecular organization. Consequently, the effective concentration of chromophores attaining the desired noncentrosymmetric alignment becomes severely diminished.

Additionally, the increase in magnitude for chromophores BHC-2 and BHC-3 is larger than that for chromophore BHC-1, indicating that steric hindrance groups can enhance the distance between molecules, thereby weakening dipole-dipole interactions. Thus, in molecular optimization, the introduction of stronger electron acceptors to increase the β and the introduction of a steerable blocking group to isolate a neighboring molecule are the preferred ways to maximize the EO coefficient of NLO materials [31,32,33].

The chromophores within the EO material initially exhibited random orientation due to thermal motion, resulting in no observable EO activity prior to poling treatment. This disordered state was subsequently modified through an electrical poling process, where an applied external electric field induces non-centrosymmetric alignment of the chromophore molecules, thereby creating the necessary asymmetric organization for EO functionality. We generally used contact polarization. The polymer film was placed in the contact polarization device, and a voltage was applied to create an electric field between the Au electrode and the ITO electrode, allowing the chromophore molecules to orient. Following systematic optimization of key poling parameters, specifically time, temperature, and electric field intensity, the highest EO coefficients of three chromophore-doped APC films were obtained as 70, 121, and 132 pm/V at the concentration of 35 wt%. The Chromophore BHC-3 possesses the highest r₃₃, which is attributed to the combined effects of its large microscopic first-order hyperpolarizability, large steric hindrance, and electron-donating substituted benzene derivatives groups, and long-chain 1-chloro-6-methoxyhexane groups that can improve compatibility.

Obviously, the introduction of two rigid groups, namely the methoxyphenyl and the N,N-dimethylaniline units, on the electronic bridge in the chromophores BHC-2 and BHC-3 introduce are extremely valuable. As the benzene ring has a rigid structure, the atoms on the benzene ring belong to the same plane, so its molecular chain is difficult to rotate, and the introduced rigid groups are perpendicular to the conjugation plane of the chromophores, thus playing a very good isolation role. In addition, the introduction of a rigid group with a certain electron-giving capacity can increase the β of the chromophore, thus increasing the r₃₃ of the material. As a result, the polymer films doped with the chromophores BHC-2 and BHC-3, with the rigid isolation groups, have better EO properties and larger EO coefficients than the chromophores BHC-1 without the rigid isolation groups.

4. Conclusions

This study successfully synthesized three novel organic chromophores (BHC-1, BHC-2, and BHC-3) with enhanced NLO properties, focusing on the impact of rigid benzene-derived isolation groups integrated into π-conjugated diene bridges. Systematic characterization revealed exceptional thermal stability (T_d >230°C) across all chromophores. UV-Vis analysis demonstrated that BHC-2 and BHC-3, featuring rigid groups, exhibited stronger solvation effects, superior ICT efficiency, and enhanced polarizability compared to BHC-1. When incorporated into host-guest EO polymers, BHC-2 and BHC-3 achieved record EO coefficients (r₃₃) of 121 pm V^-1 and 132 pm V^-1, respectively, 1.73 times and 1.88 times that of BHC-1 (70 pm V^-1). Crucially, molecular engineering of sterically hindered substituents on the π-bridges suppressed dipole-dipole interactions, enabling unprecedented hyperpolarizability translation efficiency. This work establishes a structure-function paradigm where spatial modulation via rigid isolation groups optimizes both thermal robustness and EO performance, advancing the design of high-efficiency nonlinear optical materials.

Acknowledgement

This work was supported by the National Natural Science Foundation of China (62175267), Beijing Natural Science Foundation (International Scientists Project, grant no. IS24038), National Foreign Expert Individual Human Project (Type Y, Y20240234, Y20240225), and the National Youth Talent Support Program (grant no. SQ2022QB03114) for the financial support.

CRediT authorship contribution statement

Fuyang Huo: Writing–original draft, Validation, Formal analysis, Data curation. Shuhui Bo: Writing–review & editing, Funding acquisition, Project administration, Supervision. Zhuo Chen: Writing–review & editing, Methodology, Investigation, Formal analysis. Zefan Duan: Software. Pengwei Li, Jiayi Lu: Performed some experiments, Data analysis. Kamran Hasrat and Fahim Ullah: Writing–review & editing.

Declaration of competing interest

The author declare that there are no competing interests associated with this work. There is no financial, professional, or personal conflict of interest that could influence the research outcomes or interpretation of the findings.

Declaration of Generative AI and AI-assisted technologies in the writing process

The authors confirm that there was no use of artificial intelligence (AI)-assisted technology for assisting in the writing or editing of the manuscript and no images were manipulated using AI.

Supplementary data

Supplementary material to this article can be found online at https://dx.doi.org/10.25259/AJC_168_2025.

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Show Sections

[1] Dalton L.R., Leuthold J., Robinson B.H., Haffner C., Elder D.L., Johnson L.E., Hammond S.R., Heni W., Hosessbacher C., Baeuerle B., De Leo E., Koch U., Habegger P., Fedoryshyn Y., Moor D., Ma P.. Perspective: Nanophotonic electro-optics enabling THz bandwidths, exceptional modulation and energy efficiencies, and compact device footprints. APL Materials. 2023;11:050901. https://doi.org/10.1063/5.0145212
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[2] Xu H.J., Delwin L.E., Johnson L.E., Coene Y., Hammond S.R., Ghinst W.V., Clays K., Dalton L.R., Robinson B.H.. Electro-optic activity in excess of 1000 pm/V⁻¹ achieved via theory-guided organic chromophore design. Advanced Materials. 2021;33:2104174. https://doi.org/10.1002/adma.202104174
[Google Scholar]

[3] Wang S., Zhu T., Sabatini R., Najarian A.M., Imran M., Zhao R., Xia P., Zeng L., Hoogland S., Seferos D.S., Sargent E.H.. Engineering electro-optic BaTiO₃ nanocrystals via efficient doping. Advanced materials (Deerfield Beach, Fla.). 2022;34:e2207261. https://doi.org/10.1002/adma.202207261
[Google Scholar]

[4] Messner A., Moor D., Chelladurai D., Svoboda R., Smajic J., Leuthold J.. Plasmonic, photonic, or hybrid? Reviewing waveguide geometries for electro-optic modulators. APL Photonics. 2023;8:100901. https://doi.org/10.1063/5.0159166
[Google Scholar]

[5] Yuan S., Hu C., Pan A., Ding Y., Wang X., Qu Z., Wei J., Liu Y., Zeng C., Xia J.. Photonic devices based on thin-film lithium niobate on insulator. Journal of Semiconductors. 2021;42:041304. https://doi.org/10.1088/1674-4926/42/4/041304
[Google Scholar]

[6] Lu G.W., Hong J.X., Qiu F., Spring A.M., Kashino T., Oshima J., Nawata H., Yokoyama S.. High-temperature-resistant silicon-polymer hybrid modulator operating at up to 200 Gbit s⁻¹ for energy-efﬁcient datacentres and harsh-environment applications. Nature Communications. 2020;11:4224. https://doi.org/10.1038/s41467-020-18005-7
[Google Scholar]

[7] Eppenberger M., Messner A., Bitachon B.I., Heni W., Blatter T., Habegger P., Destraz M., De Leo E., Meier N., Del Medico N., Hoessbacher C., Baeuerle B., Leuthold J.. Resonant plasmonic micro-racetrack modulators with high bandwidth and high temperature tolerance. Nature Photonics. 2023;17:360-367. https://doi.org/10.1038/s41566-023-01161-9
[Google Scholar]

[8] Ayata M., Fedoryshyn Y., Heni W., Baeuerle B., Josten A., Zahner M., Koch U., Salamin Y., Hoessbacher C., Haffner C., Elder D.L., Dalton L.R., Leuthold J.. High-speed plasmonic modulator in a single metal layer. Science (New York, N.Y.). 2017;358:630-632. https://doi.org/10.1126/science.aan5953
[Google Scholar]

[9] Zhang D., Chen W., Zou J., Luo J.. Critical role of non-classical intermolecular hydrogen bonding in affecting the π–π stacking and nonlinear optical properties of tricyanofuran-based push–pull heptamethines. Chemistry of Materials. 2021;33:3702-3711. https://doi.org/10.1021/acs.chemmater.1c00704
[Google Scholar]

[10] Bo S.H., Li Y., Liu T.T., Huo F.Y., Xiao H.X., Zhang H., Chen Z.. Systematic study on the optimization of a bis(N,N-diethyl)aniline based NLO chromophore via a stronger electron acceptor, extended p-conjugation and isolation groups. Journal of Materials Chemistry C. 2022;10:3343-3352. https://doi.org/10.1039/D1TC05684K
[Google Scholar]

[11] Xu H.J., Liu F.G., Elder D.L., Johnson L.E., Coene Y., Clays K., Robinson B.H., Dalton L.R.. Ultrahigh electro-optic coeﬃcients, high index of refraction, and long-term stability from Diels−Alder cross-linkable binary molecular glasses. Chemistry of Materials. 2020;32:1408-1421. http://dx.doi.org/10.1021/acs.chemmater.9b03725
[Google Scholar]

[12] Rahaman A., Zhang W.J., Bo S.H., Ali A., Wahab R., Shehzad K., Liu. F.. Boosted electro-optic performance in second-order nonlinear optical chromophores featuring thiophen-2-amine-derived donor groups. Dyes and Pigments. 2024;231:112379. 112379.https://doi.org/10.1016/j.dyepig.2024.112379.
[Google Scholar]

[13] Khalid M., Yasmeen S., Ahmed S., Asghar M.A., Imran M., Braga A.A.C., Ojha S.C.. Improved the optical nonlinearity of carbazole based chromophores via molecular engineering: A DFT approach. Arabian Journal of Chemistry. 2024;17:105753. https://doi.org/10.1016/j.arabjc.2024.105753
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[Google Scholar]

[16] Liu G.C., Liao Q.Y., Deng H.Y., Zhao W.J., Chen P.Y., Tang R.L., Li Q., Li Z.. Janus NLO dendrimers with different peripheral functional groups: convenient synthesis and enhanced NLO performance with the aid of the Ar–Ar^F self-assembly. Journal of Materials Chemistry C. 2019;7:7344-7351. https://doi.org/10.1039/C9TC01724K
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[18] Jin W., Johnston P.V., Elder D.L., Manner K.T., Garrett K.E., Kaminsky W., Xu R., Robinson B.H., Dalton L.R.. Structure–function relationship exploration for enhanced thermal stability and electro-optic activity in monolithic organic NLO chromophores. Journal of Materials Chemistry C. 2016;4:3119-3124. https://doi.org/10.1039/c6tc00358c
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[20] Wu J., Wu B., Wang W., Chiang K.S., Jen A.K.Y., Luo J.. Ultra-efficient and stable electro-optic dendrimers containing supramolecular homodimers of semifluorinated dipolar aromatics. Materials Chemistry Frontiers. 2018;2:901-909. https://doi.org/10.1039/C8QM00006A
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[22] Liu F., Wang H., Yang Y., Xu H., Zhang M., Zhang A., Bo S., Zhen Z., Liu X., Qiu L.. Nonlinear optical chromophores containing a novel pyrrole-based bridge: Optimization of electro-optic activity and thermal stability by modifying the bridge. Journal of Materials Chemistry C. 2014;2:7785-7795. https://doi.org/10.1039/c4tc00900b
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[24] He Y.L., Chen L., Zhang H., Chen Z., Huo F.Y., Li B., Zhen Z., Liu X., Bo S.. A novel bichromophore based on julolidine chromophores with enhanced transferring efficiency from hyperpolarizability β to electro-optic activity. Journal of Materials Chemistry C. 2018;6:1031-1037. https://doi.org/10.1039/c7tc04928e
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[29] Teng C.C., Man H.T.. Simple reflection technique for measuring the electro‐optic coefficient of poled polymers. Applied Physics Letters. 1990;56:1734-1736. https://doi.org/10.1063/1.103107
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[32] Isborn C.M., Leclercq A., Vila F.D., Dalton L.R., Brédas J.L., Eichinger B.E., Robinson B.H.. Comparison of static first hyperpolarizabilities calculated with various quantum mechanical methods. The Journal of Physical Chemistry. A. 2007;111:1319-1327. https://doi.org/10.1021/jp064096g
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Enhanced the electro-optic activity of optical chromophores via rigid isolation groups modification

Abstract

Keywords

Electro-optic coefficient

Nonlinear optics

Organic EO chromophores

Second-order nonlinear optical materials

1. Introduction

2. Materials and Methods

2.1. Materials

2.2. Measurements and instruments

2.3. Synthesis and characterization

2.3.1. The synthesis of chromophore BHC-1

2.3.2. The synthesis of chromophore BHC-2

2.3.3. The synthesis of chromophore BHC-3

3. Results and Discussion

3.1. Synthesis and characterization

3.2. Thermal stability

3.3. Optical properties

3.4.Theoretical calculations

3.5. Electro-optic (EO) performance

4. Conclusions

Acknowledgement

CRediT authorship contribution statement

Declaration of competing interest

Declaration of Generative AI and AI-assisted technologies in the writing process

Supplementary data

References

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