CPL emission of chiral BINOL-based polymers via chiral transfer of the conjugated chain backbone structure

Abstract

Two chiral BINOL-based polymer enantiomers R-P and S-P incorporating phenothiazine moieties in the main chain backbone can exhibit mirror image Cotton effects and emit green color circularly polarized luminescence (CPL), which can be attributed to chiral transfer of binaphthyl to chromophore phenothiazine moieties via the conjugated polymer chain backbone.

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Circularly polarized luminescence (CPL) as the emission analogue of circular dichroism (CD) is defined as the differential emission of left- versus right-circularly polarized light originating from a chiral molecular system and has been primarily applied to investigate structural information on the excited state of a chiral molecule. In the past few years, the chiral fluorescent organic molecules with CPL properties have been receiving considerable attention not only due to the valuable information on the chirality of the electronic excited states, but also its potential applications for photonic devices and biosensors.¹ Chiral conjugated polymers as promising CPL materials have attracted increasing interest due to their inherent flexible nature, tunable emission properties through molecular design at well-defined structure modification and easy fabrication.² Recently, Akagi group reported CPL emission properties of the conjugated disubstituted polyacetylene which could be dynamically switched by selective transmission originating from a thermotropic chiral nematic liquid crystal.³ Moreover, the luminescence intensity of most of the conjugated polymers usually becomes worse from solution to the condensed phase due to aggregation-caused quenching (ACQ).⁴ Therefore, it is of great significance on design of the chiral conjugated polymers with high CPL emission efficiency both in the solution and condensed phase.

Optically active binaphthalene derivatives as the most important C₂-symmetric compounds have been designed and used as chiral fluorescence chromophores for CPL emission materials.⁵ Recently, Kawai and co-workers have designed and synthesized a chiral binaphthalene compound, which CPL properties could be controlled by the length of the fibrous structures through the self-assembly.⁶ De la Moya's and Cheng's groups have also reported red color CPL emission of several novel chiral BINOL-based O-BODIPY organic small molecules, respectively.⁷ To the best of our knowledge, most investigations on CPL materials were mainly focused on chiral organic small molecules. So far, there have been few reports on chiral BINOL-based polymer CPL materials.^5a–d,8 Our group first reported aggregation-induced CPL of (R)-binaphthyl-based AIE-active conjugated polymer only at 3,3′-positions polymerization of 1,1′-binaphthene through the formation of self-assembly helical nanofibers from solution to aggregate state.⁹

Phenothiazines containing electron-rich sulfur and nitrogen heteroatoms can exhibit a high electron donating ability. Meanwhile, phenothiazines chromophore can also show relatively strong fluorescence emission. Most importantly, its unique non-planar butterfly conformation can effectively restrain molecular aggregation and the formation of intermolecular excimers.¹⁰ In this paper, two chiral BINOL-based conjugated polymers R-P and S-P incorporating phenothiazine moieties in the polymer main chain backbone can exhibit mirror image CPL signals in both common organic solvents and dichloromethane–hexane mixed solvents.

The detailed synthesis routes of the monomers and the chiral polymers are shown in Scheme 1. M-1 and R/S-M-2 could be prepared according to reported literatures.¹¹ R/S-P was synthesized by Pd-catalyzed Sonogashira coupling reaction of M-1 and R/S-M-2 in 76% yield. The detailed procedures and characterizations are described in ESI.† R/S-binaphthol moiety of R/S-P as a chiral group can orient a well-defined spatial arrangement in the regular polymer main chain backbone. Both polymers are air stable solids and have good solubility in common organic solvents, such as THF, DMF, CHCl₃ and CH₂Cl₂, which can be attributed to the nonplanarity of the twisted polymer backbone and the flexible n-octyl group substituents.

Scheme 1 Synthesis procedures of R/S-P.

We used dichloromethane–hexane mixed solvents for optical measurements of the enantiomers due to their good and bad solubility in the two solvents, respectively. We investigated their absorption and emission behaviors in the mixed solvents at a fixed concentration (1.0 × 10⁻⁵ mol L⁻¹). As shown in Fig. 1a, R/S-P exhibits a maximal absorption at 388 nm and two other absorption bands situated around 284 and 322 nm. The sharpest absorbance peak at 388 nm, which can be attributed to the effective π–π* conjugated segments of the polymer main chain backbone. With the addition of hexane, the absorption peaks at 388 nm have small red-shift.

Fig. 1 (a) UV-vis absorption spectra; (b) fluorescence spectra of R/S-P in dichloromethane–hexane mixtures (λ_ex = 377 nm), insert: plot of (I/I₀) values versus the compositions of the hexane fractions; (c) fluorescence spectra of R/S-P in common organic solvents. Solution concentration: 1.0 × 10⁻⁵ mol L⁻¹.

As is evident from Fig. 1b, the fluorescence spectra display emission band at 509 nm. In the dichloromethane, R/S-P can exhibited green light due to the extended π-electronic structure in the main chain backbone. With the hexane addition, the emissive intensity of polymer increases with the hexane fraction from 10% to 40%, while the fluorescence exhibits an obvious decrease with adding the hexane's ratio of the mixed solvent, which could be attributed to aggregation-caused quenching. Meanwhile, the maximum fluorescence emission of the polymer shows a clearly corresponding blue shift with the increase of hexane, and the blue shift can reach 22 nm with hexane fraction at 90% in the mixed solvents. Meanwhile, we have also investigated the polymer fluorescence emission change in the different common organic solvent. The fluorescence spectra of polymer in various solvents, such as CHCl₃, THF, DCM, DMF, and MeCN, are shown in Fig. 1c. The results indicate that polymer exhibits obvious emission changes of fluorescence spectra from 505 nm to 523 nm as changing solvent from THF to MeCN.

The CD spectra are shown in Fig. 2. R/S-P exhibited mirror-image CD bands in the dichloromethane. The conjugated chiral polymer R-P exhibits strong negative and positive Cotton effects at 264 nm and 296 nm arisen from the feature skeleton absorption of (R)-binaphthalene moieties in the polymer main chain, respectively. The negative Cotton effect at 403 nm can be regarded as the reflection of the extended conjugated structure in the repeating unit and the high rigidity of polymer backbone, indicating that the chirality of BINOL successfully transfers to the phenothiazine chromophore. We also investigated the CD spectra in the mixed solvents, we could find that the changes are little at 403 nm in the CD spectra with hexane fraction increased. Herein, we also measured the CD spectra in different common solvents, which also showed no apparent changes. The absorption dissymmetry factors (g_abs) of R/S-P obtained from the CD spectra were about 0.0014 at 403 nm.

Fig. 2 (a) CD spectra of R/S-P in dichloromethane–hexane mixtures; (b) CD spectra of R/S-P in common organic solvents. Solution concentration: 1.0 × 10⁻⁵ mol L⁻¹.

Herein, the resulting chiral polymer enantiomers R/S-P could exhibit strong fluorescence emission and CD absorption response behaviors, which inspired us to further investigate their CPL behaviors. As is evident from Fig. 3, it is clearly found that R-P and S-P showed mirror-image CPL signals which is due to effective chiral transfer from chiral binaphthalene moieties to chromophore phenothiazine moieties via the conjugated chain backbone structure. Interestingly, the CPL spectra did not show obvious changes upon changing the hexane ratios of the mixed solvents, which are coincident with those of CD. These results demonstrate that the chiral properties of the molecules could be retained. We also measured the CPL spectra in different common solvents as shown in Fig. 3b, which shows no marked changes.

Fig. 3 (a) CPL spectra of R/S-P in dichloromethane–hexane mixtures; (b) CPL spectra of R/S-P in common organic solvents. Solution concentration: 1.0 × 10⁻⁵ mol L⁻¹.

As the essential parameter of CPL, the optical anisotropy factor (g_lum) can be obtained from glum = 2(I_L − I_R)/(I_L + I_R), where I_L and I_R are the emission intensities of left and right circularly polarized luminescence, respectively.¹² The |g_lum| of R/S-P at 520 nm is about 0.001, which is in the range of most CPL materials from 10⁻⁵ to 10⁻².¹³

In summary, the resulting chiral conjugated polymers can exhibit strong Cotton effect and CPL emission signals due to the effective chiral transfer from chiral binaphthalene moieties to chromophore phenothiazine moieties via conjugated chain backbone structure. This work can provide critical information on a better understanding of chiral transfer mechanism for the design of novel CPL materials.

Acknowledgements

We would like to thank Prof. Zhiyong Tang and Dr Lin Shi at NCNST for their kind help on CPL measurement. This work was supported by the National Natural Science Foundation of China (21174061, 51173078, 21172106 and 21474048) and open project of Beijing National Laboratory for Molecular Sciences.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra23329a

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