Comparison of nonlinear optical chromophores containing different conjugated electron-bridges: the relationship between molecular structure-properties and macroscopic electro-optic activities of materials

Abstract

In electro-optic (EO) materials, realization of large EO coefficients for organic EO materials requires the simultaneous optimization of chromophore first hyperpolarizability, acentric order, molecular shape etc. As these parameters are complicatedly inter-related, thorough analyses are required to understand the dependence of macroscopic EO activity upon chromophore structure and property. Herein, we presented the synthesis of three chromophores containing different conjugated electron-bridges by acidic and alkaline formylation. Electron-rich moieties thiophene and formyl-thiophene in the different positions of chromophores played the different roles of electron-bridge, site-isolator and electron-isolator, generating intriguing property variations of electron distribution of push–pull structure, intramolecular charge-transfer, solvatochromism, microscopic hyperpolarizability and related density functional theory calculation results. In addition these molecular structure–property relationships were rationally related to the EO activities to understand the impact of microscopic molecular property on macroscopic EO activities of materials.

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1. Introduction

The organic nonlinear optical (NLO) chromophores, which possessed the push–pull structure of donor-conjugated electron bridge-acceptor (D–π–A), have been widely reported as promising materials for photonic devices. Especially, the research of organic and polymeric electro-optic (EO) materials based on second order NLO chromophores has made tremendous progress.^1–4 Microscopically, the nonlinearity of chromophores was represented as the hyperpolarizability (β) and the dipole moment (μ). For the real application, it requires the macroscopic nonlinearity (EO coefficients: r₃₃) of materials and devices. Hence, NLO chromophores were incorporated into either the host polymer matrix, or covalently attach themselves into a suitable polymer substrate. Electrical field induced poling was applied to induce the acentric ordering of chromophores in polymers, which translated microscopic hyperpolarizability of chromophores into macroscopic EO coefficients of materials. Significant advances in the development of new-generation organic EO materials have been made through rational chromophore design, which has led to a successful demonstration of ultrahigh EO coefficients.^5–7

The design and synthesis of the NLO chromophores with large μ and β have been achieved. However, it was a problem to efficiently translate the molecular microscopic β value into macroscopic EO activities of the materials. In electrically induced poling, it was easy to induce dipole–dipole interactions of chromophores with large μβ values. These dipole–dipole interactions generated the antiparallel packing of chromophores to form the dimers, reducing the efficiency of translating the microscopic hyperpolarizability of the chromophores into macroscopic EO coefficients in materials. To attenuate the dipole–dipole interaction, Larry Dalton and Alex K-Y Jen proposed the strategy of site-isolation, which leads to distance the neighbour chromophores in EO polymers.^3,8–10 In this respect, the translation efficiency of molecular microscopic β value into macroscopic EO coefficients was significantly improved.^5,10,11 Based on the abovementioned work, Li Zhen proposed the concept of suitable isolation group and testified that introduction of suitable steric hindrance group could effectively attenuate the dipole–dipole interactions and improve the EO coefficients.^12–17 Series of chromophores containing suitable isolation such as pentafluorobenzene, dendritic group showed promising nonlinearity.^13,18–27

In our recent research, diene-bridge based chromophore WJ1 (Scheme 1) containing site-isolator thiophene perpendicular to the conjugated plane showed ultrahigh EO coefficients (337 pm V⁻¹) in guest–host EO materials.²⁸ Zhang ML et al. synthesized a diene-bridge based chromophore B (Scheme 1), but achieved rather different EO activities in guest–host EO materials.²⁹ Herein, as a follow-up research, this article presents the synthesis and property variation of three chromophores WJ1, WJ2 and WJ9 (Scheme 1) containing different conjugated electron-bridges. Comparing with conventional divinylthiophene-bridge based WJ9, chromophores WJ1 and WJ2 containing site-isolators thiophene and formyl-thiophene showed considerably different properties of intramolecular charge-transfer, solvatochromism, microscopic nonlinearity and related DFT calculation results. More intriguingly, thiophene and formyl-thiophene perpendicular to the conjugated plane of WJ1 and WJ2 were found to have different degree of electron-isolation function, which helped to attenuate the dipole–dipole interactions of chromophores and to achieve the high EO coefficients in guest–host EO polymeric materials even at high chromophore loading density.

Scheme 1 Chemical structure of chromophores. The synthesis and related data of chromophore B can be found in ref. 29.

2. Results and discussions

2.1 Synthesis and structure analysis

Scheme 2 shows the synthetic route of chromophores WJ1, WJ2 and WJ9. The formylation of donor-bridge compound 1 was preceded under acidic and alkaline conditions to prepare different donor-bridges 2, 3 and 4. In acidic Vilsmeier reaction, the intramolecular H-bond interaction and cis–trans isomerism changed the electron distribution of compound 1.²⁸ The formylation of compound 1 preferentially occurred at carbon–carbon double bond. Moreover, the deprotection of hydroxyl group and halogenation were carried out to form compound 3. In presence of excess phosphorus oxychloride (POCl₃), formylation of compound 3 at the ortho-position of thiophene facilitated compound 4. Table 1 demonstrates the feed ratio of POCl₃ and compound 1, as well as the yield of products 3 and 4. When less than 1 equivalent (eq.) POCl₃ reacted with compound 1 it only produced compound 3. As the molar ratio of POCl₃ and compound 1 increased to 2 : 1, the products were facilitated as the mixture of compounds 3 and 4. This result inferred that Vilsmeier formylation primarily reacted at the double bond and secondly reacted at the ortho-position of thiophene. The increased equivalent of POCl₃ (>2 eq.) resulted in the higher yield of compound 4 and lower yield of 3. However, the overall yield of compounds 3 and 4 was reduced, which indicates that excessive POCl₃ was not beneficial for the formylation. In alkaline formylation, compound 1 was formylated with n-butyl lithium and DMF to form compound 2. After the Knoevenagel condensation of compounds 3, 4 and tricyanofuran acceptor (TCF), chromophores WJ1 and WJ2 were obtained with the yield of 72% and 46%, respectively. The additional electron-withdrawing formyl group on thiophene made the electron density of compound 4 lower than 3, which might be the reason that the yield of WJ2 was lower than WJ1. In order to synthesize chromophore WJ9 containing the same modified donor moiety, chlorination using PPh₃ and CCl₄ was proceeded to facilitate the chloride-terminated chromophore WJ9. Hence, the three chromophores contained the same donor and acceptor, but different π-conjugated electron-bridges. This synthetic strategy contributed to compare their diverse properties caused by different π-conjugated electron-bridges. All the chromophores were confirmed by ¹H NMR, ¹³C NMR element analysis and mass spectrum.

Scheme 2 Synthesis of chromophores.

Table 1 Yield of products 3 and 4

POCl3: compound 1^a	Yield of compound 3	Yield of compound 4	Overall yield (3 and 4)
a Molar ratio of precursors.
0.5 : 1	41%	0	41%
1 : 1	89%	<1%	89%
2 : 1	71%	14%	85%
3 : 1	27%	44%	71%
4 : 1	16%	36%	52%
6 : 1	0	28%	28%

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DFT calculations using Gaussian 03 were carried out at the hybrid B3LYP level employing the split valence 6-31G* basis set.³⁰ The molecular configuration and geometries were optimized referred to the crystal conformation of WJ1 and analogy chromophore.^31,32 Fig. 1 showed that WJ1 and WJ2 contained a shorter divinyl bridge than divinyl-thiophene of WJ9. Chromophores with the longer conjugated plane were easier to generate dipole–dipole interactions or aggregation. Moreover, there was no steric hindrance on the conjugated bridge of WJ9, while WJ1 and WJ2 had the thiophene and formyl-thiophene, respectively, perpendicular to the electron-bridges playing as the steric hindrance to site-isolate chromophores. The crystal structure of WJ1 (Fig. 1d) confirmed that thiophene was perpendicular to the conjugated plane, playing as the site-isolator to attenuate the dipole–dipole interactions of chromophores.²⁸ Hence, in geometry analysis of conjugated electron bridges, WJ1 and WJ2 might have more effective steric hindrance than WJ9 to site-isolate chromophores and to attenuate the dipole–dipole interactions of chromophores. Formyl-thiophene is a larger site-isolator than thiophene, which was supposed that WJ2 should show better performance in site-isolation than WJ1.

Fig. 1 Optimized structure of chromophores ((a): WJ1; (b): WJ2; (c): WJ9) and crystal conformation of WJ1 (d).

2.2 ¹H NMR analysis

Site-isolators of thiophene and formyl-thiophene also had influence on electron distribution and the chemical shift of protons. The two protons on double bond (near TCF) showed a difference in chemical shift Δδ (δ_downfield − δ_upfield), which was relative to the electron withdrawing ability of the acceptor, electron delocalization ability of conjugated electron-bridge and the electron donating ability of the donor. Fig. 2 shows the chemical shift of protons on the conjugated plane. Comparing with the divinyl-thiophene based WJ9, divinyl based chromophores WJ1 and WJ2 revealed totally different chemical shift of protons. The Δδ values of WJ1, WJ2 and WJ9 are 2.14 ppm, 1.31 ppm and 1.22 ppm, respectively. Large Δδ illustrated the extremely non-centrosymmetric structure of electron distribution, easiness of electron delocalization and polarizability of chromophores.^8,33 The electron cloud of electron-rich site-isolators thiophene and formyl-thiophene had a different deshielding effect, which was according to the electron density of site-isolators thiophene and formyl-thiophene. For WJ2, the introduction of electron-withdrawing formyl group on the thiophene decreased the electron density of thiophene and the conjugated system. The effects of deshielding and push–pull electron were weakening, showing less Δδ value than that of WJ1. Without deshielding effect of site-isolator, WJ9 showed less Δδ value of 1.22 ppm than WJ1 and WJ2. The electron clouds of site-isolators (thiophene and formyl-thiophene in WJ1 and WJ2) also had a strong deshielding effect to generate the distinct change of chemical shift of singlet proton in the donor for all the three chromophores. For D–π–A chromophore, if there were well-delocalized electron clouds out of (but quite close to) the conjugated plane to disturb the antiparallel packing of the chromophores, they might contribute to attenuate the formation of dimmer. Hence, this electron-rich site-isolator might be functionalized as the electron-isolator to prevent the antiparallel packing. Thiophene was more electron-delocalized and closer to conjugated plane than formyl-thiophene; thus, WJ1 was more effective than WJ2 in electron-isolation.

Fig. 2 ¹H NMR spectra of chromophores (solvent: CDCl₃).

2.3 Photophysical properties

In order to reveal the effect of different conjugated electron-bridges on the electronic structures of chromophores, UV-vis absorption spectra were measured to investigate the intramolecular charge-transfer. As shown in Fig. 3, all the chromophores showed the distinct intramolecular charge-transfer (ICT) absorption band shape and extinction coefficient (ε). From less polar solvents (dioxane and toluene) to more polar solvents (chloroform, dichloromethane, acetone and acetonitrile), it was observed that WJ1 had a noticeable spectral shape change showing a dominant low-energy absorption peak and a high-energy shoulder peak. Because of the introduction of electron-withdrawing formyl group, WJ2 showed two dominant absorption peaks and the high-energy peak had a stronger absorption. WJ9 had the broadest absorption band with a slight shoulder peak. In the respect of λ_max, WJ9 with longer conjugated bridge showed lower charge-transfer energy, which endowed WJ9 a larger λ_max. Chromophores B, WJ1 and WJ2 had the similar conjugated bridge, but they showed diverse change of λ_max in different solvents. These results could be attributed to the introduction of different electron-isolators. Moreover, it was accompanied by the change of absorption intensity (ε) for all chromophores. WJ1 showed the most intensified intramolecular charge-transfer for all the chromophores. This result was in agreement with the conclusion from the ¹H NMR analysis that the electron of WJ1 was more delocalized such that it was easier to have intramolecular charge-transfer, which reflected in the highest absorption intensity. Comparing WJ1 and B, it could be found that introduction of site-isolator effectively enhanced the intensity of intramolecular charge-transfer. While comparing WJ1 and WJ2, it could be found that introduction of electron-withdrawing formyl group had the opposite effect on the absorption intensity.

Fig. 3 Solvatochromic behaviors of chromophores WJ1, WJ2 and WJ9 recorded in different solvents (2 × 10⁻⁵ M) at varying dielectric constants and their absorption spectrum of chromophores in guest–host EO polymer.

As shown in Table 2, bathochromic shift of λ_max from dioxane to chloroform showed that WJ1 and WJ9 both exhibited more bathochromic shift of +76 nm. WJ2 showed less bathochromic shift of +66 nm. To our surprise, chromophore B showed the most bathochromic shift from dioxane to chloroform. The most-bathochromically shifted spectra always exhibit the characteristic band shape of a cyanine dye with the most intensified absorbance (ε) and smallest FWHM (full width at half-maximum), suggesting that chromophores WJ1 and B, especially WJ1 with the most intensified absorption were easy to be polarized quite close to the cyanine limit in the polar solvents.

Table 2 Photophysical properties of chromophores

	WJ1	WJ2	WJ9	B^e
a The difference from dioxane to chloroform.b The difference from dichloromethane (DCM) to acetonitrile.c The full width at half maximum (FWHM) in chloroform.d The λ in guest–host EO polycarbonate films containing 25 wt% chromophores.e Data of chromophore B was selected from ref. 29 and N denotes not measured; The unit of the maximum absorption wavelength λ, Δλ and FWHM is nanometer (nm); the unit of ε is 10^4 L mol^−1 cm^−1; for the solvatochromism of Δλ, + represented the bathochromic shift and − represented the hypsochromic shift.
λ (ε)dioxane	626 (6.39)	587 (2.65)	648 (4.26)	624 (3.51)
λ (ε)toluene	667 (5.44)	606 (2.63)	671 (4.50)	636 (4.03)
λ (ε)chloroform	702 (9.96)	648 (2.86)	724 (5.56)	711 (7.09)
λ (ε)DCM	703 (9.48)	644 (2.71)	713 (4.86)	714 (6.92)
λ (ε)acetone	685 (7.49)	607 (2.39)	658 (4.40)	N
λ (ε)acetonitrile	692 (7.78)	610 (2.11)	659 (4.51)	699 (5.54)
Δλ^a	+76	+61	+76	+87
Δλ^b	−11	−37	−54	−15
FWHM^c	130	180	202	114
λ film^d	701	629	698	N

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From DCM to acetonitrile, the solvatochromism reversed to a hypsochromic shift, namely inverted solvatochromism.³⁴ The extent of inverted solvatochromism from DCM to acetonitrile (−11 nm, −30 nm, −54 nm and −15 nm for WJ1, WJ2, WJ9 and B, respectively) was quite disordered. For WJ1, electron-rich moieties thiophene, which was perpendicular to the conjugated planes, appeared inert to strong polar environment. In other words, the intramolecular electron distribution (electron-isolation) made the intramolecular charge-transfer inert to the neighbour strong polar solvents and chromophores, such that hypsochromic shift of λ_max was weaker than other chromophores. Hence, it might be predicted that chromophore with the strongest bathchromism from apolar solvent to polar solvent and the weak hypsochromism from polar solvent to strong polar solvent might be the best candidate to realize the attenuation of dipole–dipole interactions and large macroscopic EO coefficients of materials.

The absorption of chromophores in solid state was also measured. Table 2 shows the difference of λ_max in guest–host EO polymer films and in solution (chloroform) was in accordance with the variation trend of hypsochromic shift. WJ1 showed a small difference of λ_max (701 nm in film and 702 nm in chloroform). WJ2 and WJ9 had hypsochromic shifts of 19 nm and 26 nm. There might be intermolecular interactions or other factors to influence the intramolecular charge-transfer in solid films. However, it could be concluded that chromophore WJ1 in solid state was quite similar to that in solution.

2.4 DFT calculations

To obtain further understanding of the conjugated electron-bridge dependent microscopic properties of these chromophores, DFT calculations were carried out and chromophores were rotated into frame such that the x axis was aligned with the dipole axis. The relevant theoretical parameters, including HOMO and LUMO levels, dipole moment (μ), polarizability (α), zero-frequency molecular first hyperpolarizability (β) and bond-length alternation (BLA), are displayed in Table 3.

Table 3 Energy level (E), dipole moment (μ), polarizability (α), hyperpolarizability (β) and bond-length alternation (BLA) of chromophores^a

	WJ1	WJ2	WJ9
a More details about DFT calculations in ESI.
EHOMO (eV)	−1.967	−1.697	−0.402
ELUMO (eV)	0.0558	0.358	0.938
ΔE (eV)	2.0228	2.055	1.341
μx (Debye)	21.10	18.80	18.54
μtotal (Debye)	22.63	19.03	22.05
αtotal (esu)	192.77	196.45	250.35
βx (10^−30 esu)	252.98	241.81	825.64
βtotal (10^−30 esu)	254.66	246.87	831.20
μβ (10^−30 esu × D)	5763	4698	18 328
BLA (Å)	0.04812	0.04876	0.03761

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Because of different conjugated electron-bridge structure, all the chromophores showed the distinct HOMO and LUMO levels. Fig. 4 shows the electron distribution of conjugated plane. It was clear to see that, in HOMO level, electron distributed in the thiophene ring of WJ1 and lower density of electron in the formyl-thiophene of WJ2. Hence, thiophene and formyl-thiophene could be defined as the electron-isolator because of which chromophores were distracted to form antiparallel dimer. In addition, the energy gaps (ΔE) of HOMO and LUMO were estimated to be 2.0228, 2.055 and 1.341 eV, which was in accordance with the λ_max of intramolecular charge-transfer absorption.

Fig. 4 Electron distribution of HOMO and LUMO.

Dipole moment (μ) was also calculated for all three chromophores. The μ_total of all the three chromophores were estimated to be 22.63, 19.03 and 22.05 D, respectively, showing a noticeable change by replacing different conjugated electron-bridge. On the dipolar x axis, due to the high electron density on the conjugated plane, WJ1 showed the largest dipole moment (21.10 D). For WJ2, the electron-withdrawing of formyl group decreased the electron density on x axis and the μ_x decreased to 18.80 D, close to 18.54 D of WJ9.

In terms of the polarizability, chromophores with large α values were more sensitive to the high voltage electrical field. This property might contribute to the effectively acentric ordering of chromophores in the applied poling electrical field, but it was also likely to induce the dipole–dipole interactions to form the antiparallel dimers. It has been demonstrated that chromophore WJ9 with the strongest polarizability was more sensitive to the environment in strong polar solvent and in solid state, which caused the distinct change of λ_max in solution and in EO film.

All the three chromophores showed a decaying trend in molecular hyperpolarizability as the increasing trend of BLA values from WJ9 to WJ1 and WJ2. In this regard, we might conclude that divinylthiophene-conjugated chromophore WJ9 gas considerably better microscopic nonlinearity than WJ1 and WJ2. However, in the case of site-isolation and electron-isolation, WJ1 and WJ2 were assumed to have the better performance in the attenuation of dipole–dipole interactions than WJ9.

2.5 NLO properties

EO coefficient, r₃₃, defining the efficiency of translating molecular microscopic hyperpolarizability into macroscopic EO activities, is described as follows:

r₃₃ = |2Nf(ω)β〈cos³ θ〉/n⁴|

where N represents the aligned chromophore number density and f(ω) denotes the Lorentz–Onsager local field factors. The term 〈cos³ θ〉 is the orientationally averaged acentric order parameter characterizing the degree of noncentrosymmetric alignment of the chromophores in materials and n represents the refractive index. The realization of large EO activity for dipolar organic chromophore-containing materials requires the simultaneous optimization of first hyperpolarizability (β), acentric order 〈cos³ θ〉, and number density (N).³⁴ Many sophisticated factors including intramolecular charge-transfer, intermolecular dipole–dipole interactions and molecular shape, as well as the polarizability and hyperpolarizability have influence on the efficiency of the translation of molecular microscopic nonlinearity into the macroscopic EO coefficients.

As DFT calculations results showed, WJ9 possessed better microscopic properties including BLA, μβ, and polarizability. In this respect, it was supposed that the EO polymer materials doped with WJ9 have the strongest EO activities. However, the EO coefficients of three guest–host doping materials did not fit the variation tendency of microscopic nonlinearity. EO materials of WJ1/APC, WJ2/APC and WJ9/APC containing 20 wt% respective chromophores showed r₃₃ values of 187 pm V⁻¹, 72 pm V⁻¹ and 37 pm V⁻¹, respectively. WJ9/APC, containing chromophores WJ9 with triple μβ values larger than WJ1 and WJ2, showed the lowest r₃₃ value. This contrast between microscopic nonlinearity and macroscopic EO coefficients implied that there might be more significant factors to influence the translation of microscopic nonlinearity into macroscopic EO activity.

Indeed, high β value of chromophores is important to determine the macroscopic EO activities. However, hyperpolarizability, no matter it was calculated by DFT or measured by hyper-Rayleigh scattering (HRS) in solution, could not accurately display the microscopic nonlinearity of chromophores in solid EO films. As UV-vis showed, chromophores showed different λ_maxs and absorption intensity in solution and in films, which was an important factor to determine the hyperpolarizability of chromophores. That meant the hyperpolarizability of chromophore in EO film was no longer the same as the measured in solution or the calculated β value. Hence, this result reminded us to focus more on other factors to improve the macroscopic EO activities.

For the EO materials, each chromophore with the confirmable push–pull structure has a certain intrinsic hyperpolarizability; thus, the uncertain factors to determine the EO coefficients were the acentric order 〈cos³ θ〉, and number of acentric ordering chromophores (N). The acentric order 〈cos³ θ〉 was related to the shape of chromophores, the degree of molecular mobility and steric hindrance. Until now, there is no direct and effective method to characterize the 〈cos³ θ〉 and N. However, undoubtedly, they significantly determine the r₃₃ value. Comparing with the β value, they played the key roles in determining r₃₃ values for WJ1/APC, WJ2/APC and WJ9/APC.

In structure analysis, Scheme 1 showed that there were no steric hindrance group as the site-isolator on the conjugated electron-bridge of chromophores WJ9 and B. Without site-isolator, they were easy to generate the dipole–dipole interaction and form the antiparallel dimmers, such that the true number of acentric ordering chromophores was low in electrical field induced poling. WJ9/APC and B/APC containing 20 wt% chromophores showed r₃₃ value of 37 pm V⁻¹ and 35 pm V⁻¹, respectively. For chromophores WJ1 and WJ2 containing thiophene and formyl thiophene perpendicular to the conjugated electron-bridge as the site-isolator groups, effective site-isolation distanced the neighbour chromophores. This isolation could attenuate the dipole–dipole interactions and increase the number of oriented chromophores in electrical field induced poling. Hence, WJ1/APC and WJ2/APC showed considerably higher r₃₃ value of 187 pm V⁻¹ and 72 pm V⁻¹, respectively in 20 wt% guest–host EO polymers.

For EO materials WJ1/APC and WJ2/APC, both chromophores WJ1 and WJ2 had a site-isolator to attenuate the dipole–dipole interaction to effectively transfer molecular microscopic nonlinearity to macroscopic EO coefficients of materials. However, different site-isolator group of thiophene and formyl-thiophene indeed generated different molecular properties in ¹H NMR analysis and photophysical property analysis. In addition the site-isolation, electron of thiophene and formyl-thiophene was found to have the function of electron-isolation. Thiophene was more electron-delocalized and more electron-rich than formyl-thiophene, such that the electron-isolation of thiophene in chromophore WJ1 was more effective than formyl-thiophene in WJ2. This variation of electron-isolation generated more intensified absorption, weaker inverted solvatochromism and more non-centrosymmetric electron distribution of conjugated plane of WJ1 than WJ2. Moreover, in EO film, the property of chromophore WJ1 in solid state was similar to that in solution, indicating that function of electron-isolation could be sufficiently displayed in EO film to distract the antiparallel packing of chromophores. Thus, WJ1/APC showed much higher r₃₃ value of 187 pm V⁻¹ than WJ2/APC of 72 pm V⁻¹ in 20 wt% guest–host EO polymers.

In terms of the relationship of EO coefficients and chromophore loading density in Fig. 5, those guest–host EO materials contained the higher chromophore density (>25 wt%), WJ2/APC and WJ9/APC showed a decreased r₃₃ values, but WJ1/APC showed ultrahigh r₃₃ value of 337 pm V⁻¹ as the chromophore density raised to 40 wt%. The ultrahigh EO coefficient and rarely high chromophore loading density in guest–host EO materials were the evidences to testify the hypothesis that electron-isolation and site-isolation, especially electron-isolation, might effectively attenuate the intermolecular dipole–dipole interactions of chromophores to achieve the ultrahigh EO coefficients in high chromophore loading density in EO materials.

Fig. 5 The relationship of EO coefficient and chromophore concentration.

In EO activities, WJ1/APC and WJ2/APC containing chromophores with site-isolator and electron-isolator showed higher r₃₃ values than WJ9/APC containing WJ9 without isolator in conjugated electron-bridge. Moreover, it was indicated that chromophore WJ1 had more excellent effect on the electron-isolation, so that WJ1/APC showed a much higher r₃₃ value and it allowed the higher chromophore loading density of 40 wt% to achieve ultrahigh r₃₃ value of 337 pm V⁻¹.

3. Conclusion

Three chromophores with different conjugated bridges were synthesized through the formylation under different conditions. In acidic formylation, we synthesized the diene-conjugated chromophores WJ1 and WJ2 containing thiophene and formyl-thiophene, respectively, perpendicular to the conjugated planes. As the site-isolators, steric hindrances of thiophene and formyl-thiophene were more effective to attenuate the dipole–dipole interactions for chromophores WJ1 and WJ2 than divinyl-thiophene conjugated chromophore WJ9. More intriguingly, the site-isolators had significant influence on the photophysical properties, intramolecular charge-transfer, and electron distribution and theoretically calculated molecular microscopic properties. It was confirmed that site-isolators also played as the electron-isolators, which greatly contributed to generate the weak inverted solvatochromism in strong polar solvents and made the photophysical properties of chromophores in EO films as excellent as in solution. In EO activities, it showed that site-isolation and electron-isolation were the more important factors than molecular microscopic hyperpolarizability to influence the EO coefficients of materials. Moreover, comparing the EO materials WJ1/APC and WJ2/APC, it was confirmed that WJ1 containing thiophene as a more effective electron-isolator had the most powerful effect to attenuate the dipole–dipole interactions of chromophores in EO films. WJ1/APC showed the ultrahigh r₃₃ value at high chromophore density. These comparisons inspired us that introduction of electron-delocalized moiety into the chromophore, functionalized as the electron-isolation and site-isolation, might be an effective path to attenuate the dipole–dipole interactions of chromophores and further to efficiently translate the microscopic nonlinearity into macroscopic EO coefficients.

Acknowledgements

We are grateful to the Directional Program of the Chinese Academy of Sciences (KJCX2.YW.H02), Innovation Fund of the Chinese Academy of Sciences (CXJJ-11-M035) and the National Natural Science Foundation of China (no. 61101054) for financial support.

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Footnote

† Electronic supplementary information (ESI) available: Details of experiment and DFT calculations. See DOI: 10.1039/c4ra09368b

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