Rigid Jeffamine-included polyrotaxane as hydrogen-bond template for salicylideneazine with aggregation-enhanced emission

Abstract

Because restricted molecular rotation is the main mechanism responsible for aggregation-induced emission (AIE), a Jeffamine-included polyrotaxane (JCD) was therefore used in this study as a rigid template to impose effective rotational restriction on the AIE-active luminogen of 1,2-bis(2,4-dihydroxybenzylidene)hydrazine (CN₄OH). Besides rigidifying the flexible Jeffamine chain, the β-cyclodextrin (β-CD) rings of JCD also provided hydroxyl (OH) groups to hydrogen bond to the OHs of CN₄OH, furnishing emissive CN₄OH/JCD(x/y) (x/y: molar ratio between CN₄OH and JCD) blends with emission efficiency higher than the pure CN₄OH itself. Dependent on the composition of the blends, CN₄OHs in the blends are arrayed differently to experience varied levels of rotational restriction and thus emit with an intensity correlated with the molecular arrangement of the CN₄OHs in the blends. The relationship between molecular arrangement, restricted molecular rotation and AIE-oriented emission behavior is the focus of this study.

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Introduction

Discovery of non-conventional luminogens with aggregation-induced emission (AIE)^1,2 properties has drawn lots of research interest^3–9 regarding their beneficial emissive behaviour in the aggregated state. Aggregation is considered to be detrimental to emission for the traditional organic luminognes with planar geometry but for AIE-active luminogens (AIEgens), aggregates formed in the solution and solid states nevertheless emit with higher intensity than the dilute solution state. With the beneficial strong emission of the aggregates, AIEgens constructed from different structural units, such as tetraphenylethylene,^10,11 phenothiazine,^12,13 anthracene,^14,15 triphenylethylene,^16,17 distyrylanthracene,^18,19 carbazolyl^20,21 groups, AIE-active metal complexes^22,23 and polymers,^24–26 had been prepared and characterized. Theoretical study concluded that the restriction of intramolecular rotations and vibrations (RIR and RIV)^9,27,28 are the main causes for the AIE phenomena observed in the propeller-shaped and shell-like AIEgens, respectively. Essentially, RIR is the main mechanism responsible for the AIE phenomenon since most of the AIEgens discovered nowadays are propeller-shaped molecules. By efficient RIR in the aggregated state, the non-radiative decay pathways of the exciton via vibrational/torsional energy relaxation can be largely reduced, resulting in the desired enhanced emission.

Inherent bulky substituent is no doubt effective in reinforcing RIR for the propeller-shaped AIEgens. However, external physical constraints also provide abundant resources for the deposition of rotational restriction on the AIEgens. Intuitively, rigid template is an ideal cornerstone for furnishing firm grips on AIEgens through favourable specific interactions, such as hydrogen bond (H bond)²⁹ between the AIEgen and the rigid template. Several H-bonded AIEgen/template blend systems^30–33 were therefore prepared in our laboratory. With effective H bond interactions to impose rotational restriction, AIEgens in the blends always emitted with higher emission intensity than pure AIEgens itself. The result clearly indicated that the key factor controlling the emission intensity is the effective rotational restriction rather than the amounts of the luminescent AIEgens in the blends.

Salicylideneazine^34–38 derivatives possess a six-membered, pseudo enolimine ring maintained by intramolecular H bond interaction between the hydroxyl (OH) group and the imine (C=N) nitrogen atom. For unbridged imine dyes, the C=N isomerization in the excited state is considered to be the predominant decay pathway but for enolimines with bridged C=N bonds (e.g. CN₄OH in Scheme 1) an excited-state intramolecular proton transfer (ESIPT)^39–46 process is responsible for its radiative emission behaviour. Upon photoexcitation, fast transfer of the OH protons to C=N nitrogen quickly converts the excited enol (E*) into the excited keto (K*), giving rise to emission band with a large Stoke shift. In solution, CN₄OH^36,47 emits weakly due to the twisted ketonic excited state whereas in the aggregated state, a planar dimer with the aromatic rings effectively locked by the intramolecular H bonds emits intensely. The OHs of CN₄OH can be conveniently H-bond to pyridine rings of poly(vinyl pyridine) (PVP).⁴⁸ Initially, preferable H-bonding between p-OHs of CN₄OH and pyridine rings of PVP occurs to result in strong emissive CN₄OH/PVP blends. However, when more PVPs are loaded in the blends, excess pyridine rings start to H-bond to the o-OHs of CN₄OH and rupture the enolimine rings to result in the reduced emission of the blends.

Scheme 1 Synthesis of JCD and its complexation with CN₄OHs to prepare CN₄OH/JCD(x/y) blends and the possible H bonding fashions involved in the blends.

With the hollow truncated cone, cyclodextrin molecules (CDs) can be used as host materials for including small molecules^49–51 or linear polymers,^52–58 generating different polypseudorotaxane or polyrotaxane applied in controlled release of drugs in living organisms.^59–61 The hydrophobic cavity of CDs is also an ideal site for imposing efficient RIR required for AIE activity. A diboric acid tetraphenylethene (TPE) derivative⁶² can cooperatively bind to two pairs of alternative diols on a β-CD, resulting in highly-emissive solution due to the effective RIR of the CD-incorporated TPE derivative. Alternatively, TPE was linked to the α-, β- and γ-CDs⁶³ through ester bonds. Solution emission of the β-CD-linked complex is weaker than α-CD-incorporated complex but is stronger than the one linked by γ-CD. The highest emission efficiency of the α-CD-linked complex is owing to that the smaller cavity of α-CD restricts the motions of luminogen more efficiently than the larger-sized β- and γ-CD cavities. More recently, amphiphilic AIEgens containing TPE moiety⁶⁴ were prepared and reacted with γ-CD to show new insight for AIE behaviour. When included in the cavity of γ-CD, the amphiphilic AIEgens showed an enhanced monomer emission and a decreased aggregate emission. Recently, Jeffamine-included β-CD (JCD) with cationic ammonium terminals⁶⁵ was used as a rigid template to complex with the sulfonate anions of a tetraphenylthiophene derivative (TPS), resulting in an ionic complex, which emitted with superior emission intensity than complex from TPS and the flexible Jeffamine. The role of rigid template in imposing effective RIR on AIEgen was demonstrated in this study. All above examples illustrate the use of CDs in constructing new AIE-active systems.

In this study, a Jeffamine-included β-CD (JCD, Scheme 1) is used as rigid template to mix with different amounts of CN₄OHs, resulting in AIE-active CN₄OH/JCD(x/y) (x/y: molar ratio between CN₄OH and JCD) blends of varied emission efficiencies due to different levels of rotational restriction imposed on the CN₄OHs. The β-CD rings of JCD serve as rigid component to stiff the flexible Jeffamine (amino-terminated poly(propylene glycol)) chain, resulting in rigid template for imposing effective rotational restriction on the incorporated AIEgens. Besides as rigid component, the β-CD rings of JCD also provide OH groups for H bonding to the implanted AIEgens of CN₄OHs. Through favourable H-bond interactions between different amounts of JCD and CN₄OH, all solid blends of CN₄OH/JCD(x/y) are emitters with emission intensity higher than pure CN₄OH itself. The study also suggests that the emission efficiency of the blends does not follow a monotonous dependence on the content of AIEgen but rather it correlates with molecular arrangements of the CN₄OHs in the blends. As correlated to our previous study on the CN₄OH/PVP blends,⁴⁸ the molecular arrangements of CN₄OHs are related to their binding fashions to the β-CD rings of JCD (Scheme 1). When the H-bonding is between the p-OHs of CN₄OH and β-CD ring (route i), parallel arrays of CN₄OHs result in crystalline structure, within which AIEs are firmly fastened by the tightly-packed crystalline lattice and therefore emit intensely due to the effective rotational restriction. In contrast, if H-bonding takes place on the o-OHs of CN₄OHs and β-CD (route ii), the emission intensity of the blends will be largely reduced because the lateral binding fails in forming parallel, crystalline packing of CN₄OHs. When the o-OHs of CN₄OH was H bonded by JCD, the pseudo six-membered enolimine ring was also ruptured and the liberated C=N bond will be free to isomerize in the excited state, resulting in emission quenching. Factors of controlling molecular arrangement of CN₄OHs in the blends thus determine the rotational restriction and the emission efficiency of the AIE-active blends. The interrelationship between molecular arrangement, restricted molecular rotation and the AIE-related emission behaviour will be discussed following the manuscript.

Experimentals

Materials

Jeffamine D-400 (diamino-terminated poly(propylene glycol), Alfa Assar) and β-cyclodextrin (TCI, 95%) were used directly. CN₄OH⁴⁸ and JCD⁶⁶ were synthesized according to the published procedures. Preparation of JCD was detailed below:

Preparation of Jeffamine-included-CD (JCD)

Solution of β-CD (0.38 g, 0.335 mmol) in deionized water (15 mL) was added into the stirred solution of Jeffamine D400 (2.92 g) in deionized water (4 mL). The whole solution was then under ultrasonic agitation for overnight. The white precipitant in the solution mixtures was then collected by filtration and was dried in the vacuum oven at 60 °C for 24 h. The resulting complex was then mixed with chlorotriphenylmethane (4.52 g) in DMF (30 mL) in prior to the addition of triethylamine (2 mL). The reaction mixtures were subjected to vigorous stirring at room temperature for 24 h. The reaction product was then precipitated from diethyl ether and dried in vacuum oven at 60 °C for 24 h. ¹H NMR (500 MHz, D₂O) (ppm): 7.32 (d, 12H, aromatic H), 7.34 (d, 12H, aromatic H), 7.19 (d, 6H, aromatic H), 5.75 (br, 14H overlapped), 4.8 (d, 7H), 4.45 (m, 6H), 3.64 (br, 28H overlapped), 3.08 (br, 14H overlapped), 1.16 (d, 3H, CH₃) (Fig. S1†).

Preparation of CN₄OH/JCD blends

The calculated amounts of CN₄OH and JCD were dissolved in THF and the whole mixtures were stirred vigorously at room temperature for 24 h. The resulting complex solution was then subjected to rotary evaporation to remove THF. The final product was obtained by drying in vacuum oven at 60 °C for 24 h.

Instrumentations

The ¹H NMR spectra were recorded by a Varian Unity Inova-500 MHz. UV-vis absorption spectra were obtained from an Ocean Optics DT 1000 CE 376 spectrophotometer. PL emission spectra were recorded by a LabGuide X350 fluorescence spectrophotometer using a 450 W Xe lamp as the continuous light source. Quantum yields (Φ_F) of the solutions were determined by using a quinine sulfate as a standard solution and integrating sphere was used for the solid sample. The wide-angle X-ray diffraction (WAXD) spectra were obtained from a Siemens D5000 diffractometer. Thermal transition of the blends was determined from a TA Q-20 differential scanning calorimeter (DSC) with a scan rate of 10 °C min⁻¹ under nitrogen. Particle sizes in selective solvent pairs were measured by dynamic light scattering (DLS) on a Brookhaven 90 plus spectrometer at room temperature. An argon ion laser operating at 658 nm was used as light source.

Results and discussion

According to Scheme 1, the rigid JCD template was primarily prepared from a two-step synthesis route. First, β-CD was ultrasonically shaken with Jeffamine to result in Jeffamine-included β-CD, which was then capped by chlorotriphenylmethane to obtain the desired JCD. The ¹H NMR spectrum (Fig. S1†) of JCD revealed an average of 3.1 β-CD rings per Jeffamine chain according to the integrated ratio between propylene methyl protons (proton a) of Jeffamine and acetal carbon proton (proton b) of β-CD. JCD was then mixed with different amounts of CN₄OHs, resulting in solid CN₄OH/JCD(x/y) (x/y: molar ratio between CN₄OH and JCD) blends with low (x/y < 1) and high (x/y > 1) contents of CN₄OHs for study.

Before discussion on the solid samples, we needed to identify the function of JCD as effective template for CN₄OH; for this, characterization on the solution mixtures of CN₄OH/JCD(x/y) was primarily conducted.

Solution mixtures of CN₄OH/JCD(x/y)

Solution mixtures of CN₄OH/JCD(x/y) containing excess JCD (x/y < 1) were used here to prove that JCD is an effective template in imposing rotational restriction on CN₄OH. With excess JCDs, CN₄OHs in the solutions are considered to be bound by JCDs and will emit with an intensity dependent on how well they are securely locked by the rigid JCDs. Preferable H bond interaction between CN₄OH and JCD should already take place in the solution state since solution mixtures of CN₄OH/JCD(x/y) exhibit a composition-dependent emission behaviour (Fig. 1). All the solution mixtures investigated in Fig. 1 contain constant amounts of CN₄OH (=10⁻³ M); therefore, the emission difference observed in Fig. 1 should be attributed to the amounts of JCDs added in the solutions. Solution of pure CN₄OH emitted dimly but the inclusion of JCD caused a large emission enhancement. Under the premise that all solutions contain the same amounts of CN₄OHs, the solution emission intensity nevertheless increases with increasing JCD content from an x/y = 1/3 to 1/12. The emission enhancement observed here is therefore related to the amounts of CN₄OHs anchored by JCDs in the solutions. By H bonding to the OH groups of rigid JCDs, the anchored CN₄OHs are clumsy in rotational motion, thus emitting efficiently with the operative AIE activity. More JCDs added in the solutions generate more of the anchored CN₄OHs, which contributes to the emission enhancement of the solution mixtures.

Fig. 1 Solution emission spectra of CN₄OH/JCD(x/y) mixtures with a constant concentration of CN₄OH (=10⁻³ M) in DMSO (λ_ex = 365 nm).

The ¹H NNR spectrometer is a powerful tool for evaluating the extent of rotational restriction in the solution state. It was suggested^27,67 that the fast conformational exchanges caused by the fast intramolecular rotation give sharp resonance peaks, whereas the slow exchanges due to the slow rotations broaden the resonance peals. The ¹H NMR analysis was then extended to polymer⁶⁸ and polymer blend³³ systems and the results suggested that the rotational restrictions in the blend systems are so effective that the corresponding resonance bands became reduced in intensity or disappeared due to the insufficient power of the magnetic field in igniting the molecular motion of the frozen bonds. We therefore applied ¹H NMR analysis on the present system but for the sake of eliminating the potential complication from concentration difference, the ¹H NMR analysis in Fig. 2 was performed using solutions with the same amounts of CN₄OHs (=10⁻³ M). Spectrum of pure CN₄OH contains three sets of large resonance peaks in correlation to the aromatic protons a, b and c, respectively. Addition of JCDs in the solutions resulted in the upfield shift and the large reduction of the resonance peaks in the spectra of CN₄OH/JCD(1/3), (1/9) and (1/12). Increasing amounts of JCDs in the solutions generated more anchored CN₄OHs, which are firmly held by the rigid JCD and are therefore clumsy in response to the ignition of the external magnetic field. The ¹H NMR analysis thus demonstrated that JCD is indeed an effective template in imposing the required rotational restriction on CN₄OH.

Fig. 2 ¹H NMR spectra of pure CN₄OH and the CN₄OH/JCD(x/y) mixtures with a constant amounts of CN₄OH (10⁻³ M) in d₆-DMSO.

Solid CN₄OH/JCD(x/y) blends

Solid blends containing low (x/y < 1) and high (x/y > 1) fractions of CN₄OH were prepared and their emission spectra were compared with pure CN₄OH in Fig. 3A and B, respectively. Consistent with our previous blend systems,^30–33 the CN₄OH/JCD (x/y) blends all emitted with higher intensity than pure CN₄OH. For the blends with fewer (x/y < 1) CN₄OHs, the resolved emission intensity (Fig. 3A) was increased upon increasing JCD content from the blends of CN₄OH/JCD(1/3) to (1/12). Quantum yield (Φ_F, Table 1) measured from integration sphere also follows the same result that increasing JCD content from CN₄OH/JCD(1/3) to (1/12) raised the Φ_F values from 17 to 28%. The blend containing less CN₄OHs (e.g. CN₄OH/JCD(1/12)) is superior in emission efficiency than the blend with more luminogens (e.g. CN₄OH/JCD(1/3)). Therefore, it is the rotational restriction, rather than the amounts of AIEgens, that controls the final emission efficiency of the AIE-active blends.

Fig. 3 Solid emission spectra of CN₄OH/JCD(x/y) with (A) x/y < 1 and (B) x/y > 1 (λ_ex = 365 nm).

Table 1 Quantum yield (Φ_F) and crystallinity of CN₄OH/JCD(x/y) blends

a Determined from integration sphere.b Calculated from the WAXD spectrum.
(A) CN4OH/JCD(x/y) with x/y < 1
x/y	0/1	1/3	1/6	1/9	1/12
wt% of CN4OH(%)	0	2.02	1.01	0.681	0.512
ΦF^a (%)	—	17	22	25	28
Crystallinity^b (%)	73	42	46	47	52
(B) CN4OH/JCD(x/y) with x/y > 1
x/y	1/0	3/1	6/1	9/1	12/1	15/1
wt% of CN4OH(%)	100	15.62	27.03	35.71	42.55	48.08
ΦF^a (%)	15	18	56	75	78	37
Crystallinity^b (%)	38	40	64	69	71	57

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Increasing the content of AIEgen decreases the emission efficiency for the blends containing fewer (x/y < 1) CN₄OHs but such a trend is not followed in the blends containing large amounts (x/y > 1) of CN₄OHs. Initially, the resolved emission intensity (Fig. 3B) of the blends was increased upon with increasing CN₄OH content from the blends of CN₄OH/JCD(3/1) to (12/1) but further loading of CN₄OHs nevertheless resulted in the significant intensity reduction in the spectrum of CN₄OH/JCD(15/1). This result reflects subtle change of the molecular arrangement of the AIEgens in the blends and will be discussed next. The observed variations of the emission intensity are in line with the resolved Φ_Fs summarized in Table 1. CN₄OH/JCD(3/1) emits with a higher Φ_F than pure CN₄OH (18 vs. 15%) and increasing CN₄OH content eventually raises the Φ_F to the highest value of 78% for the blend of CN₄OH/JCD(12/1). However, raising content of CN₄OHs resulted in the lowering of Φ_F to a value of 37% for CN₄OH/JCD(15/1). From the blends of CN₄OH/JCD(12/1) to (15/1), molecular arrangement of the luminescent CN₄OHs must be under major transformation to result in such a large change on the emission efficiency.

WAXD spectra of the solid blend

The WAXD spectra of Fig. 4A and B provide information regarding the crystalline structures of the blends with low (x/y < 1) and high CN₄OH (x/y > 1) contents, respectively. Pure JCD and CN₄OH are semi-crystalline material with the clearly-visible crystalline diffraction peaks (Fig. 4A) over the broad amorphous background. For the blends containing fewer (x/y < 1) CN₄OHs (Fig. 4A), the incorporation of CN₄OHs tends to interrupt the crystalline structure of JCDs. Initially, certain fractions of crystalline JCDs are still present in the blends of CN₄OH/JCD(1/12); however, further incorporation of CN₄OHs in the blends of CN₄OH/JCD(1/9), (1/6) and (1/3) resulted in new structures with broad diffraction peaks in the range from 2θ = 12 to 18°. The sharp diffraction peaks of pure CN₄OH are all absent in the spectra of the corresponding blends; therefore, the minor CN₄OHs are considered to be completely dispersed by JCDs. Crystallinity calculated from the WAXD spectra was summarized in Table 1 and the results indicate that the blend's crystallinity slightly increases upon increasing JCD content in the blends. Favourable H bond interactions between JCD and CN₄OH must be prevail in the blend and act to rupture the crystalline arrays of CN₄OHs. Blends with higher crystallinity also emit with higher efficiency, which is reasonable considering crystals with less voids and free volumes are efficient in restricting the rotational motion of the AIEgens inside the crystalline lattices. This result is correlated with crystallization-enhanced emission (CEE) behaviour observed in several AIEgena.^69–74 For example, the AIE-active triphenylethylene derivative⁴⁸ emitted with lower intensity when it was transformed from crystalline to amorphous states by increasing the nonsolvent water fraction of the DMF/water mixtures.

Fig. 4 WAXD spectra of CN₄OH/JCD(x/y) blends with (A) x/y < 1 and (B) x/y > 1.

As illustrated in Fig. 4B, spectra of blends with excess (x/y > 1) CN₄OHs contain the characteristic diffraction peaks of crystalline CN₄OHs. We may assess the molecular packing of CN₄OHs by the main diffraction peak at 2θ = 27.2° (indicated by black arrow) that it gradually becomes larger in intensity (compared to the amorphous background) when more CN₄OHs are incorporated in the blends from CN₄OH/JCD(3/1) to (12/1). Crystalline structures beneficial for imposing effective rotational restriction are thus developed upon increasing the content of CN₄OH; nevertheless, the advantageous role of CN₄OHs is not always hold since a reduced intensity for the peak at 2θ = 27.2° was resolved in spectrum of CN₄OH/JCD(15/1). Careful inspection also revealed that the spectrum of CN₄OH/JCD(15/1) contains few new diffraction peaks at 2θ = 24.9, 26.2, 29.0° (indicated by red arrows), which indicates certain fractions of CN₄OHs in the blend of CN₄OH/JCD(15/1) were arrayed in a pattern different from the CN₄OHs in other blends. The calculated crystallinity (Table 1) is consistent with the WAXD spectra that crystallinity increased in blends from CN₄OH/JCD(3/1) to (12/1) but decreased in the blend of CN₄OH/JCD(15/1). Blends with higher crystallinity also emit with higher efficiency; therefore, the resolved Φ_Fs increases from 18 to 78% as the crystallinity increases from 38 to 71% from the blends of CN₄OH/JCD(3/1) to (12/1) but the reduced crystallinity of 57% results in the lower Φ_F of 37% for CN₄OH/JCD(15/1).

Thermal analysis of the solid blends

DSC traces of the CN₄OH/JCD(x/y) blends with x/y <1 and >1 are given in Fig. 5A and 6A, respectively, to provide information regarding possible molecular arrangements of the blends.

Fig. 5 (A) DSC thermogram of CN₄OH/JCD(x/y) with x/y < 1 (heating rate = 10 °C min⁻¹) and (B) the low-temperature melting enthalpy versus molar fraction of JCD in the blend.

Fig. 6 (A) DSC thermogram of CN₄OH/JCD(x/y) with x/y > 1 (heating rate = 10 °C min⁻¹) and (B) the low-temperature melting enthalpy versus molar fraction of CN₄OH in the blends.

As illustrated in Fig. 5A, the broad melting transition of pure JCD may be attributed to the large JCD aggregates of varied particle sizes. Through interchain H bond interactions between the OHs of the JCD chains, formation of large JCD aggregates of varied sizes is highly probable. For the blends containing more JCDs, the minor CN₄OHs implanted in the blends do not largely alter the JCD aggregates formed since DSC traces of the blends resulted in broad melting transition in the same temperature ranges. More conclusive support can be illustrated by the melting enthalpy (ΔH, Fig. 5B), involved in the melting transition, that it increased in linear proportional to the molar fraction of JCDs in the blends. Therefore, melting involved in the blends of low CN₄OH content mainly concerns the large JCD aggregates.

Molecules in the blends containing excess CN₄OHs (x/y > 1) are considered to have different packing manner from those in the blends of low CN₄OH content considering there are extra high-temperature thermal transitions in the DSC traces (Fig. 6A) of for the blends. The low- and high-temperature thermal transitions are attributed to the molecular motions involved in the JCD- and CN₄OH-rich phases, respectively. Compared to the broad melting transition observed in Fig. 5A, the low-temperature melting endotherms observed here are higher in temperatures and sharper in shape. It is then envisaged that the low-temperature melting in the blends of high CN₄OH content should involve the CN₄OH-anchored JCD particles. That the low-melting transition gradually shifted to lower temperatures with increasing CN₄OHs should correlate with the fact that the implanted CN₄OHs in the JCD-rich phase were already anchored by JCDs. The lowering of the melting transition from CN₄OH/JCD(3/1) to (15/1) is due to the size reduction of the JCD aggregates. By H bonding to JCDs, the small-mass CN₄OHs chopped the large JCD aggregates into smaller ones with lower melting temperatures.

The enthalpy (ΔH) adopted from the low-temperature melting transition are associated with the molar fraction of CN₄OH in the blends (Fig. 6B). Initially, ΔH is linearly proportional to the molar fraction of CN₄OHs in the blends from CN₄OH/JCD(3/1) to (12/1). However, further implantation of CN₄OHs in the blend of CN₄OH/JCD(15/1) lowered the ΔH significantly. The low ΔH indicates the presence of disordered, anchored CN₄OHs in the JCD-rich phase of CN₄OH/JCD(15/1). The low ΔH observed for CN₄OH/JCD(15/1) is in line with the low crystallinity evaluated from WAXD and is also correlated with the high-temperature thermal transition discussed next.

The high temperature thermal transition reflects the subtle phase behaviour of the CN₄OH-rich phase. For CN₄OH/JCD(3/1), the high-temperature melting endotherm occurred at temperatures higher than the pure CN₄OH. Extra CN₄OH molecules in the blend are supposed to follow the parallel packing of the pre-existing, anchored CN₄OHs (route i, Scheme 1), resulting in tightly-packed crystalline arrays with high melting temperature. Further loading of CN₄OHs resulted in complicated thermal behaviour regarding the multiple melting-recrystallization process shown in the DSC traces of CN₄OH/JCD(6/1), (9/1) and (12/1). The cause responsible for the multiple thermal transitions is hard to identify presently but fortunately, DSC trance of CN₄OH/JCD(15/1) is simple and contains only one single glass transition in the high-temperature region. Certain CN₄OHs in CN₄OH/JUCD(15/1) are therefore randomly packed to result in the glass transition. In addition, the high-temperature glass transition is considered to be associated with the low enthalpy involved in the low-temperature melting process. Through intermolecular H bond interactions, CN₄OHs in the CN₄OH-rich phase should be associated with packing manner of the anchored CN₄OHs in the JCD-rich phase. More crystalline JCD aggregates favor the formation of well-packed CN₄OHs in the CN₄OH-rich phase whereas misaligned, anchored CN₄OHs in the JCD-rich phase result in the amorphous CN₄OH-rich phase. Anchored CN₄OHs in the blends of CN₄OH/JCD(3/1), (6/1), (9/1) and (12/1) are packed regularly to result in crystalline CN₄OHs in the CN₄OH-rich phase but for CN₄OH/JCD(15/1) the disordered, anchored CN₄OHs generated certain amorphous CN₄OH-rich phase besides the regular crystalline lattices.

Molecular arrangement of the solid blends

As illustrated in Scheme 1, different H bond packing manners involved in CN₄OH/JCD(x/y) blends resulted in CN₄OHs of varied molecular arrangements, which in turn determine the emission efficiency of the blends. In blends containing excess JCDs (x/y < 1), the minor CN₄OHs are anchored and immobilized by the rigid JCDs but in blends containing less amounts of JCD (x/y > 1), majority of CN₄OHs form separate CN₄OH-rich phase with the molecular arrangement strongly associated with the packing manner of the anchored CN₄OH in the pre-existing JCD-rich phase. Particularly, molecular arrangement of the highly CN₄OH-loaded CN₄OH/JCD(15/1) is different from others and hence, needed to be considered separately. Accordingly, we classified possible molecular arrangements of the solid CN₄OH/JCD(x/y) blends into situations of x/y < 1, x/y > 1 and x/y = 15/1 (Scheme 2).

Scheme 2 Transformation of molecular arrangements upon increasing CN₄OHs in the blends.

Large JCD aggregates are the fundamental units existing in the blends containing fewer CN₄OHs (x/y < 1). For pure JCD, self-association of JCDs can be easily achieved through interchain H bond interactions between the inherent OH groups, resulting in large aggregated JCD particles with broad size distribution. For blends containing fewer CN₄OHs (x/y < 1), the minor CN₄OHs were considerably H bonded and dispersed by the majority of JCDs. The large, crystalline JCD aggregates are the fundamental structure units remained relatively intact in the presence of minor CN₄OHs. Because JCD aggregates are the main structures in blends containing fewer CN₄OHs, the resolved ΔHs are linearly proportional to the molar fraction of the loaded JCDs (Fig. 6B). Held by the rigid JCD templates, the anchored CN₄OHs are so immobile that they emit with intensity higher than pure CN₄OH itself.

For blends containing excess CN₄OHs (x/y > 1), the interrelating JCD- and CN₄OH-rich phases are responsible for the low- and high-temperature thermal transitions (Fig. 6A), respectively. The JCD-rich domains basically concern JCD aggregates and the anchored CN₄OHs in the surface areas of the JCD aggregates. Initially, the anchored CN₄OHs served as starting sites for further reaction with the implanted CN₄OHs in the CN₄OH-rich domains. Through favourable H bond interactions between the p-OHs of CN₄OHs (route i in Scheme 1), the anchored CN₄OHs directed the following CN₄OHs to pack in aligned and parallel fashion, thereby generating highly-emissive, crystalline CN₄OHs due to effective rotational restriction in the tightly-packed crystalline lattice. In contrast, H bond interactions to the lateral o-OHs of the anchored-CN₄OHs (route ii in Scheme 1) resulted in misaligned, amorphous CN₄OHs. Through intermolecular H bond interactions, molecular arrangement of the anchored CN₄OHs is considered to couple with the molecular packing of the excess CN₄OHs in the CN₄OH-rich phase. Anchored CN₄OHs in preferable crystalline packing lead to crystalline CN₄OHs in the CN₄OH-rich phase whereas misaligned, anchored CN₄OHs result in amorphous CN₄OH-rich phase.

By favourable intermolecular H bond interactions, large JCD aggregates were dissociated and chopped by CN₄OHs into smaller aggregates with lower melting transitions. Smaller JCD aggregates possess larger surface areas for accepting more CN₄OHs but the small particles also limited inter-aggregates spaces leaving for the accommodation of implanted CN₄OHs. For the blends from CN₄OH/JCD(3/1) to (12/1), loading CN₄OHs in the blends resulted in the proportional increase of the low-temperature ΔH, which refers to the continuous growth of the tightly-packed, anchored CN₄OHs. The crystalline anchored CN₄OHs also led to crystalline CN₄OHs in the CN₄OH-rich phase. Because the molecular motions are securely locked in the crystals, corresponding blends therefore emitted intensely. For CN₄OH/JCD(15/1) with the highest content of CN₄OH, there are limited inter-aggregates space available for the large amounts of CN₄OHs. Therefore, excess CN₄OHs were forced to penetrate into the free spaces between β-CDs of JCD and started to bond to the lateral o-OHs of the anchored CN₄OHs, resulting in misaligned CN₄OHs responsible for the new diffraction peaks observed in WAXD spectrum (Fig. 3B) and the lowering of the low-temperature ΔH. The misaligned CN₄OHs in the JCD-rich phase are also responsible for the disordered CN₄OHs in the CN₄OH-rich phase. Besides the crystalline, parallel CN₄OHs with the H-bonded p-OHs, the CN₄OH-rich phase of CN₄OH/JCD(15/1) also contains the misaligned, amorphous CN₄OHs formed through lateral H-bond interactions. All the disordered CN₄OHs in the blend of CN₄OH/JCD(15/1) are responsible for its lower Φ_F compared to other blends with higher crystallinity (i.e. CN₄OH/JCD(6/1), (9/1) and (12/1)), considering the inefficient rotational restriction in the loosely-packed amorphous regions.

The results suggested that JCD served as efficient rigid template in reinforcing rotational restriction of CN₄OH if appropriate amounts of CN₄OHs were applied in the blends. With fewer CN₄OHs (x/y < 1), most of the CN₄OHs were securely held by JCDs and emitted efficiently with the operative rotational restriction. With more CN₄OHs (x/y > 1), well-packed CN₄OHs in the JCD- and CN₄OH-rich phases led to crystalline blends with the inherent CN₄OHs firmly held in the crystalline lattices. The corresponding blends are therefore strong emitters with the operative rotational restriction in the crystalline blends. With the limited inter-aggregates spaces, the highly CN₄OH-loaded CN₄OH/JCD(15/1) contains certain misaligned, amorphous CN₄OHs and its emission efficiency is comparably lower than the CN₄OH/JCD(6/1), (9/1) and (12/1). Molecular arrangement of CN₄OHs are therefore the decisive factor controlling the emission efficiency of the CN₄OH/JCD(x/y) blends.

Conclusion

A Jeffamine-included polyrotaxane (JCD) was prepared and served as rigid template to H bond to different amounts of CN₄OH, resulting in CN₄OH/JCD(x/y) blends with solid emission intensity dependent on the amounts of CN₄OH loaded in the blends.

Solution emission behaviour clearly demonstrates the role of JCD in imposing effective rotational restriction on CN₄OHs. Solution emission of the CN₄OH/JCD mixtures was increased with increasing JCDs in the solutions. Increasing JCD content of the CN₄OH/JCD mixtures also reduced the aromatic resonance peaks of the ¹H NMR spectra due to the increasing rotational restriction on CN₄OHs. Therefore, JCDs are rigid templates effective in imposing rotational restriction on CN₄OHs.

Emission efficiencies of the solid CN₄OH/JCD(x/y) blends are all higher than pure CN₄OH itself. It is then the rotational restriction, rather than the amounts of the AIEgens, controlling the final emission behaviour of the AIE-active blends. With fewer (x/y < 1) CH₄OHs, the emission efficiency of the blends was increased with increasing JCDs loaded in the blends. Though H bond interactions, the CN₄OHs are firmly-held by the JCD aggregates and emit intensely due to the operative rotational restriction. For the blends containing excess (x/y > 1) CN₄OHs, the emission efficiency was increased with increasing CN₄OH content. The tightly-packed crystalline CN₄OHs in both the JCD- and CN₄OH-rich phases are responsible for the strong emission of the blends. Crystallinity of the blends is raised by the CN₄OHs loaded in the blends and so is the emission efficiency of the blends. Nevertheless, further loading of CN₄OHs result in the adverse emission reduction of the CN₄OH/JCD(15/1) blend. Inside this highly-loaded blend, lateral binding to the o-OHs of CN₄OH occurred to result in misaligned CN₄OHs, which emit weakly due to the loose packing of the disordered CN₄OHs. The correlation between molecular arrangement, rotational restriction and AIE-related emission behaviour are therefore illustrated in this study.

Acknowledgements

We appreciate the financial support from the Ministry of Science and Technology, Taiwan, under the contract no. NSC 102-2221-E-110-084-MY3.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Fig. S1–S2. See DOI: 10.1039/c5ra05215g

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