Complexation-induced circular dichroism and circularly polarised luminescence of an aggregation-induced emission luminogen

a Department of Chemistry, Institute for Advanced Study, Institute of Molecular Functional Materials, and Division of Biomedical Engineering, The Hong Kong University of Science & Technology (HKUST), Clear Water Bay, Kowloon, Hong Kong, China. Email: tangbenz@ust.hk b Department of Physics, HKUST, Clear Water Bay, Kowloon, Hong Kong, China. Email: phkswong@ust.hk c Guangdong Innovative Research Team, SCUT-HKUST Joint Research Laboratory, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology (SCUT), Guangzhou 510640, China d HKUST Shenzhen Research Institute, Nanshan, Shenzhen 518057, China


Introduction
Compared to standard photoluminescence spectroscopy, circularly polarised luminescence (CPL) spectroscopy, the emission analogue to circular dichroism (CD), is being less reported. 1From the spectroscopic CPL signals, we can obtain signicant information about the chirality of materials in the excited states, from which stereochemical, conformational and three-dimensional structure can then be identied. 2,3A typical example is using CPL as a tool to study the electronic structure of radioactive actinides. 1CPL active materials with efficient performances are especially suitable for optoelectronic applications, such as making devices for stereoscopic optical processing, display and storage, [4][5][6][7] chiral recognition in pharmaceutical industries and biological systems. 8In theory, luminophores arranged helically could produce CPL upon photoexcitation. 5With the selective emission of le-and righthanded circularly polarised (LCP and RCP) light from chiral molecular systems, CPL is characterized by differential spontaneous emission of LCP and RCP upon photo-or electro-excitation [DI(l) ¼ I L (l) À I R (l), where I L (l) and I R (l) denote the emission intensities of LCP and RCP components, respectively]. 2 The primary measurement of the degree of CPL, the emission dissymmetry factor g em , is dened as 2(I L À I R )/(I L + I R ) which indicates the degree of either one type of circularly polarised light (LCP or RCP) is preferentially emitted over another.The value of g em is between À2 to +2 and its absolute value equals 2 for a pure single-handed material. 5ntil now, there are two major chiral systems that have been reported with CPL properties: inorganic systems (lanthanide ion complexes, 9,10 transition metal complexes, [11][12][13] etc.) and organic systems (small organic molecules, 14-17 synthetic polymers [18][19][20][21][22] and biomacromolecules 4 ).To promote the real application of these materials, high emission efficiency and spectral stability in the condensed phase are necessary.However, most of the small organic candidates studied were measured in solution and found with only small g em values ($10 À5 to 10 À2 ).The performance of the organic candidates is even worse in the condensed phase because of the aggregation-caused quenching (ACQ) effect. 23Aggregation of chiral luminophores normally makes the excited species dissipate through nonradiative pathways with the formation of excimers or exciplexes induced by p-p stacking interaction. 23Consequently, the emission efficiency, spectral stability and processability are lowered.5][26] The upside of adopting AIE active materials in CPL are to overcome the serious limitations of ACQ in condensed phase: they are highly emissive in the solid state and thus more practical for real applications; they have better processability-no intricate process is required to interfere with the aggregation process during the fabrication of the solid thin lm.Decorating the AIE-active luminogens with chiral moieties may induce the ordered assembly and efficient CPL.Recently, we have proved this idea through attaching the AIE-active luminogen with chiral mannose-like moieties. 27The resultant molecules can self-assemble into right-handed helical nanostructures and give rise to simultaneous aggregation-induced CD (AICD) and a large dissymmetry factor with absolute values of 0.08-0.32 in the lm state.The value is about two orders of magnitude higher than those of commonly reported organic materials.The mechanism of that system is mainly based on self-assembly of molecules into a one-handed preferred architecture upon solvent evaporation.The CPL signal obtained from the screw-shaped structure, however, cannot be changed or modied.To expand the system, in this work, we employ a new strategy to produce CPL active material with controllable signals.We synthesize an AIE luminogen with thiourea linkers and chiral phenylethanamine groups (1, Scheme 1).This molecule has no CD or uorescence signals when molecularly dissolved.Upon aggregation in a poor solvent or fabricated as a thin lm in the solid state, 1 can give strong green uorescence upon photoexcitation, showing classical AIE characteristics but it is still CD-inactive.Due to the potential hydrogen-bonding interaction between the thiourea and carboxylic acid groups, complexation of 1 with specic chiral acids, herein mandelic acid, in the solid state can generate CD and CPL signals in the condensed phase, displaying a unique complexation-induced CD (CICD) characteristic.Interestingly, the predominant CPL can be RCP or LCP emission depending on the enantiomer of mandelic acid added.

Aggregation-induced emission (AIE)
The molecule 1 is soluble in polar organic solvents such as THF, chloroform, methanol and DMSO but insoluble in nonpolar solvents and water.The THF solution of 1, however, is faintly luminescent when excited at 370 nm (Fig. 1A).The uorescence remains very weak when up to $50 vol% poor solvent, herein water, is added to the solution (Fig. 1B).Aerwards, the uorescence intensity rises signicantly with the increase of water fraction in the mixture.The uorescence efficiency increases around 40-fold to its maximum intensity at 95 vol% water fraction.Since 1 is insoluble in water, aggregates must have been formed in the solvent mixture with high water fraction, which is further conrmed by particle size analysis (Fig. S4 †).This result demonstrates that 1 is AIE-active: it is practically non-emissive when molecularly dissolved in good solvents but emits intensely in the aggregate state.The AIE effect has  enabled 1 to emit efficiently in the solid state with a quantum efficiency of the thin lm as high as 95%.

Complexation-induced circular dichroism
CD spectroscopy measures the differential absorption of LCP and RCP light, [D3(l) ¼ 3 L (l) À 3 R (l), where 3 L (l) and 3 R (l) denote the molar extinction coefficients of LCP and RCP light, respectively] and reects the structural information of the ground electronic state of a system. 2,3Since the molecule 1 carries two chiral centres, we are curious about whether the chiral pendants can induce CD signals of the silole core.As shown in Fig. 2A, compound 1 itself, however, has no CD signal in solution at 370 nm, which is corresponding to the absorption band of the silole core (Fig. S5 †).The silent CD signals indicate the silole moiety of 1 cannot inherit the chirality from its chiral phenylethanamine pendants, most probably due to the weak intermolecular interaction under such conditions.To enhance intermolecular interaction, we fabricated a thin lm of 1 by casting and natural solvent evaporation.However, still no CD signals at the region of silole absorption band could be recorded, further conrming the weak self-assembly ability of 1 into chiral architecture.
The thiourea bridges of 1, though not favourable for selfassembly, may serve as proton donors in the hydrogen bonding interactions with hydroxy acids.Therefore, we examine whether a chiral hydroxy acid, e.g.mandelic acid, can trigger the chirality of the silole core.As shown in Fig. 2A, in the THF solution, no CD signal of 1 can be recorded at 370 nm even in the presence of enantiopure mandelic acid.Intriguingly, in the condensed phase (i.e.solid thin lm state), the addition of a large amount of mandelic acid to 1 (molar ratio 40 : 1) leads to the emergence of the Cotton effects at 370 nm (Fig. 2B).Note that a lower amount of mandelic acid can hardly induce the same effect (Fig. S6 †).This new peak, corresponding to the silole absorption band, is associated with chirality transfer from the chiral acids to 1 through the complexation probably between the thiourea groups with the chiral hydroxy acids.Such interactions can induce the silole cores to be helically arranged accordingly.We thus name this phenomenon as a complexation-induced CD (CICD) effect.The CD signals produced by the addition of mandelic acid enantiomers show opposite Cotton effects (Fig. 2B).
In order to understand the structural role of mandelic acids in the CICD effect of 1, we choose a number of chiral acids for comparison (Chart 1).In the solution state, none of these acids can induce the CD signals of 1 (Fig. 2A and S6A †).The peak at the short wavelength (<300 nm region) is referred to the chirality of the acid.For chiral aliphatic acids, such as malic and tartaric acids, there is no new peak found at 370 nm either in solution or solid states, indicating the importance of the phenyl group of mandelic acids in the CICD effect (Fig. 2 and S7 †).The silent CD spectrum of 1 with (R)-(À)-1-phenyl-1,2-ethanediol in both solution and solid states suggests the involvement of the carbonyl group of mandelic acids in the CICD effect.In addition to the phenyl and carbonyl groups, control experiments with phenylbutyric acid and dibenzoyl-tartaric acids reveal that both hydroxyl groups in mandelic acid are essential for generating the CICD effect.With a phenyl ring and the two hydroxyl groups, phenyllactic acid is able to induce the CD signals of 1 in the condensed phase (Fig. 2B).However, the presence of one more methylene spacer in its aliphatic chain compared to mandelic acid, greatly reduce the molar ellipticity of 1 at 370 nm.Thus, among the tested chiral acids, mandelic acids are the only acids that can generate the greatest CICD effect of 1 owing to their structural features.

Circularly polarised luminescence
The AIE-active molecule 1 exhibits efficient uorescence and CICD effect in the condensed phase, which prompts us to investigate its CPL behaviour.The home-built epi-illumination (reection mode) optical system is applied to evaluate the CPL activity through recording the differential spontaneous emission, DI(l) ¼ I L (l) À I R (l).The details of the setup have been described in our previous publication. 27This setup is well suited for measuring the chirality of transparent solutions as well as opaque or nontransparent samples which have strong light scattering and low transmittance.Fig. 3 shows the dependence of DI and emission dissymmetry factor g em versus the wavelength.Since 1 is AIE-active, it emits weak uorescence in the solution state and thus we only measure the CPL in its condensed phase.As shown in Fig. 3A, the solid lm of 1 alone produces almost no CPL signals.The result, in line with the CD measurement, indicates the conformation of 1 is randomly arranged, together with weak or even no intermolecular interaction to introduce chiral architecture, therefore it shows no CD or CPL signals.In the presence of enantiopure mandelic acid, the DI signals become either negative or positive depending on the chirality of the guest acids (Fig. 3B and C).When complexed with R-(À)-mandelic acid, the RCP emission of 1 is dominant over the LCP emission in the whole monitored spectral region, which suggests that R-(À)-mandelic acid can induce the molecules of 1 to align as one-handed helical structures in the solid state.Interestingly, such an arrangement can be altered to the opposite orientation upon complexation with the enantiomeric S-(+)-mandelic acid.The g em values are clearly different, indicative of the distinct packing order in their respective helical assemblies.The g em values of R-(À)-mandelic acid and S-(+)-mandelic acid are about À0.01 and +0.01 on average, respectively, in the detected spectral window of 450-600 nm with little dependence on the emission wavelength.
To gain more information on the morphology of the complexes, we perform transmission electron microscopy (TEM) and powder X-ray diffraction (XRD) analysis (Fig. S9 and S10 †).The results show that upon complexation with mandelic acids, the morphology of the aggregates of 1 is changed with the emergence of new peaks in the XRD patterns.This implies a higher ordered packing mode has been adopted, mostly induced by the hydrogen-bonding interaction between the thiourea and carboxylic acid groups.The large amount of mandelic acid (40 times higher than 1 in molar ratio) in the solid thin lm may crystallize, which generates a chiral eld tted with 1 and induces the silole core to arrange in a helical manner.The presence of 1, however, may in turn cause the non-uniform lattice distortions of the mandelic acid crystals, which is reected in the broad patterns of XRD.

Conclusions
In this work, a small organic molecule 1 with complexationinduced chirality and CPL behavior has been reported.1 inherits the AIE characteristics of the silole core: it is nonemissive in the solution state and highly luminescent in the condensed phase.The uorescence quantum efficiency of 1 in solid state reaches as high as 95%.Although equipped with chiral phenylethanamine moieties, compound 1 can hardly selfassemble, resulting in negligible CD signals in either solution or solid states.Enantio-pure mandelic acid is able to complex with 1 and induces the CD signal of 1 at the silole absorption region, i.e. 370 nm, only in the condensed phase, showing a complexation-induced CD (CICD) effect.This complexation is structurally specic in that other hydroxy and carboxylic acids cannot induce the CD signals of 1 as efficiently as mandelic acids.Combining the AIE and CICD effects, we have then further investigated the CPL performance of the 1/mandelic acid complex in the condensed phase.Under these conditions, the complex generates a CPL dissymmetry factor at an appreciable value.The g em values of R-(À)-mandelic acid and S-(+)-mandelic acid are about À0.01 and +0.01 on average, respectively.These absolute values are much higher than most of the reported organic CPL emitters, which suffer from the aggregation-caused quenching effect.The AIE-active uorogens thus represent a class of desirable molecules for generating efficient CPL in the solid state.Moreover, whether the predominant CP emission is RCP or LCP emission can be designated upon complexation of 1 with either enantiomer of mandelic acid.Further study on the rational design of the AIE emitters to generate tunable CPL is ongoing in our laboratory.

General information
Tetrahydrofuran (THF) was distilled from sodium benzophenone ketyl immediately prior to use.Dichloromethane (DCM) was distilled over calcium hydride.Lithium wire, naphthalene, dichlorobis(triphenylphosphine)palladium(II), ZnCl 2 $TMEDA, copper(I) iodide, triphenylphosphine, and other chemicals and solvents were all purchased from Aldrich and used as received without further purication.
1 H and 13 C NMR spectra were measured on a Bruker ARX 400 NMR spectrometer using methanol-d 4 or chloroform-d as solvent with tetramethylsilane (TMS) as internal reference.UV absorption spectra were taken on a Milton Ray Spectronic 3000 array spectrophotometer.CD spectra were recorded on a JASCO J-810 spectropolarimeter in a 1 mm quartz cuvette using a step resolution of 0.1 nm, a scan speed of 200 nm min À1 , a sensitivity of 0.5 nm, and a response time of 1 s.Photoluminescence (PL) spectra were recorded on a Perkin-Elmer LS 55 spectrouorometer.Emission efficiencies of cast thin lms of the molecule 1 were measured by a calibrated integrating sphere.High-resolution mass spectra (HRMS) were recorded on a Finnigan MAT TSQ 7000 Mass Spectrometer System operating in a MALDI-TOF mode.Morphologies and structures of the aggregates were investigated by high-resolution JEOL 2010F transmission electron microscopy (TEM).Circular photoluminescence spectra were measured with a home-made CPL spectroscopy system which has been published in a previous paper by our group. 26
Preparation of 1. Into a Schlenk tube equipped with a magnetic stirrer were added 0.14 g (0.25 mmol) of 7 and 0.73 mL (0.553 mmol) of 8 in 6 mL of distilled THF protected under nitrogen, the setup was stirred at room temperature for 30 hours.Aer solvent evaporation under reduced pressure, the residue was puried by a silica-gel column using gradient DCM/ hexane (80/20 to 100/0, v/v) mixture as eluent.A yellow solid was obtained in 97.6% yield. 1  Thin lm preparation.The thin lms for CD and CPL measurement were prepared by dissolving 1 and enantiopure chiral acid (1 : 40 molar ratio) in THF.The solution was cast on a quartz plate and solvent was then naturally evaporated.

Fig. 1 (
Fig. 1 (A) PL spectra of compound 1 in THF-water mixtures with different water fractions.Excitation wavelength: 370 nm.(B) Plot of I/I 0 versus water content of a THF-H 2 O mixture of 1, where I 0 denotes the emission intensity in pure THF solution.Concentration of 1: 10 À5 M. Excitation wavelength: 370 nm.

Fig. 2
Fig. 2 CD spectra of 1 in the absence and presence of chiral hydroxyl acids in (A) THF solution and (B) solid thin film states.[1] ¼ 1 mM; [acid] ¼ 40 mM.Chart 1 Structures of compound 1 and enantiomeric acids used in this study.

Fig. 3
Fig. 3 Plots of PL intensity and CPL dissymmetry factor (g em ) versus wavelength for 1 in the (A) absence and (B and C) presence of R-(À) or S-(+)-mandelic acid ([1]/[acid] ¼ 1 : 40 by mole) in the solid film state, respectively.g em ¼ 2(I L À I R )/(I L + I R ) and PL intensity ¼ I L + I R , where I L and I R denote the left-and right-handed emission intensity, respectively.Excitation wavelength: 325 nm (0.5 mW).