Morphology and emission behavior tuning of fluorescent composites using CO₂ expanded liquids

Abstract

CO₂-expanded liquids (CXLs) were used to develop a promising fluorescent probe, the self-assembled fluorescent composite (SAFC) formed between the dye molecule 4-(9-(2-(4-hydroxyphenyl)ethynyl)-7,10-diphenylfluoranthen-8-yl)-phenol (4) and the diblock copolymer P4VP-b-PS. The change in the solubility exhibited by P4VP-b-PS and 4 with increasing CXL pressure will affect the strength of the hydrogen bonds between P4VP and 4, together with the morphology and fluorescence (FL) performance of the SAFC. The structures and emission behaviors of the SAFC were strongly dependent on the CXL pressure and the molar ratio (R) of 4VP to 4. Meanwhile, the pressure dependence of the emission behavior of SAFC was attributed to hydrogen bonding and confinement acting as the major factors. The maximum FL intensity of SAFC at 5.41 MPa and R = 687 was enhanced by 14-fold in comparison with that of the pristine mixture. This work proves that the CXL method facilitates the modulation of the morphology and emission performance of the SAFC, and opens a new route for the development of efficient luminescent materials.

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Introduction

Block copolymer self-assembly has attracted the interest of numerous researchers due to their ability to self-assemble into various fascinating nanostructures such as spherical, cylindrical, and vesicular micelles, as well as other structures.¹ The morphologies of aggregates in solution are controlled by the free energy that is stabilized among three main parameters, which include the core stretching, inter-coronal interactions, and the interfacial tension between the solvent and the micellar core.² The factors influencing the above three terms have been investigated previously, including the concentration and composition of the copolymer, the selective solvent used, and the type and amount of the additives (e.g., acids, bases, or salts) to the copolymer solution.³ By tuning one of these three factors, the major forces stabilizing the micelles may be altered, leading to a morphological transition.³ Among these aggregates, vesicles have attracted considerable attention due to their unique morphological structure that provides them with a hollow cavity, making them attractive in various fields such as drug delivery,⁴ catalysis,⁵ and the controlled incorporation of hydrophobic species into vesicle walls⁶ or hydrophilic substances into its cavities.⁷

Many studies have focused on the self-assembly behavior of poly(styrene-block-4-vinylpyridine) (PS-b-P4VP) that takes place upon the addition of small molecules capable of forming hydrogen bonds with the P4VP block.^1,3d,8 Hydrogen bonds are thought to have an attractive role in the formation of supramolecular structures via self-assembly owing to their moderate binding energy.⁹ Additionally, the strength and extent of hydrogen bonding depends on the respective affinities between the hydrogen bond donors and acceptors.¹⁰ Chang et al. examined the complexation-induced phase behavior of the mixtures of PS-b-P4VP and octyl gallate (OG) in different solvents, and they found that the morphology of the complex could be mediated due to the different strengths of hydrogen bonds observed between PS-b-P4VP and OG in different common solvents.¹ Park et al.^8c studied the fluorescence (FL) emission behavior of Disperse Red 1 (DR1) in different domains of PS-b-P4VP micelles. They reported that when DR1 was confined in the core of the PS-b-P4VP micelles in toluene, this dye exhibited stronger FL than when it was localized in the soft corona due to the higher degree of confinement encountered within the core. The micellization behavior of a block copolymer/small molecule complex can be regulated by a variation in the amount of small molecules, as well as the environment affecting the interactions between copolymers and small molecules.¹¹ The micellization could even be induced by noncovalent interactions in the nonselective solvents for both blocks (e.g., THF) as the nonselective solvent becomes selective following a change in the P4VP block's solubility, which is induced by the interactions between the P4VP block and the small molecule.^8b

In addition, the nature of the solvent also plays an important role in regulating the type of complex that is formed. As described herein, a novel class of tunable solvents known as gas expanded liquids (GXLs) were used as selective solvents for the self-assembly process. These GXLs provide numerous advantages in materials processing in comparison with conventional solvents due to their tunable solvating power, fluidity, and various other properties that can be adjusted in response to changes to pressure.¹² CO₂-expanded liquids (CXLs) are the most commonly used GXLs, which combine the beneficial properties of compressed gases and conventional solvents, and are regarded as ideal solvents for virtually any given application while simultaneously reducing the environmental impact of a given process through the substantial replacement of organic solvents with eco-friendly CO₂.¹³ It should be stressed that CXLs processes can be utilized at milder processing pressures compared to their supercritical antisolvent (SAS) counterparts, and polymer processes involving CXLs have been used to regulate particle sizes and the morphologies of crystalline polymers.¹⁴ In addition, these systems have been employed to facilitate foaming and the impregnation of polymers by taking advantage of their melting-point lowering and viscosity-lowering effects.¹³

Herein, the molecule 4-(9-(2-(4-hydroxyphenyl)ethynyl)-7,10-diphenylfluoranthen-8-yl)-phenol (4)¹⁵ (Scheme 1) was used as the small fluorescent unit. 4 possesses both planar and non-planar moieties, which means that 4 may cause a system exhibiting aggregation-caused quenching (ACQ) to exhibit aggregation-induced emission (AIE) in certain situations.¹⁵ The intriguing luminescence-tuning behavior of 4 motivated us to further develop a new class of fluorescent materials via the complexation with P4VP-b-PS.

Scheme 1 Schematic representation of morphology and emission tuning of the fluorescent composites (P4VP-b-PS/compound 4) using CO₂ expanded liquids.

In this work, the self-assembled fluorescent composite (SAFC) was formed via complexation between 4 and P4VP-b-PS in CO₂-expanded toluene. The changes in the solubilities of P4VP-b-PS and 4 with increasing CXLs pressure will affect the strength of the hydrogen bonding that takes place between the P4VP block and 4, while also influencing the self-assembly behavior in CXLs, together with the FL performance of the resultant SAFC. As a result, the various micellization and emission behaviors of this SAFC can be realized in a facile manner by tuning the pressure of the CXLs. Meanwhile, hydrogen bonding and confinement were found to be major factors causing the differences in the pressure dependences of the emission behavior of the SAFC observed with different molar ratios of 4VP to 4.

Results and discussion

Firstly, we will provide proof for the formation of hydrogen bonding between P4VP blocks and 4 by FTIR data. In toluene or CO₂-expanded toluene where toluene was PS selective, the copolymer may self-assemble into nanostructures such as spherical, cylindrical, and vesicular micelles, etc., and P4VP should be located in the core portion of the spheres or the vesicle wall, and the molecule 4 could then be confined in the same location through hydrogen bonding with P4VP. Therefore, we speculate that both hydrogen bond and the confinement degree of 4 might have significant influence on the emission behavior of SAFC, and in this work a number of parameters including CXLs pressure, the concentrations of copolymer and 4, and the viscosity of solvent may have effect on the hydrogen bond and the confinement degree. To gain mechanistic insights, we further examined the effects of solution viscosity on the emission behaviors of 4. Then we investigated the effects of the molar ratio of 4-VP to 4, as well as the CXLs pressure on the morphology and FL property tuning of the new composite formed via self-assembly of P4VP-b-PS and 4.

FTIR analyses

Fig. 1 displays the stretching band (1570–1640 cm⁻¹, Fig. 1A) of the pyridine groups and the pyridine ring absorption region (980–1025 cm⁻¹, Fig. 1B) observed in infrared spectra of the pure P4VP-b-PS copolymer and the SAFC samples with R = 6.87 at different pressures in CO₂-expanded toluene. The pyridine band at 1597 cm⁻¹ shifted to higher wavenumbers upon the addition of 4, demonstrating that hydrogen bonds between the hydroxyl groups of 4 and the pyridine groups of the P4VP block were formed.^1,17 The characteristic band at 993 cm⁻¹ corresponded to the free pyridine ring absorption of pure P4VP-b-PS. Moreover, the new band observed near 1013 cm⁻¹ corresponded to hydrogen-bonded pyridine units,^1,17 and its intensity increased with decreasing CXLs pressure in comparison to the pure P4VP-b-PS. We could thus confirm that the self-assembled fluorescent composite (SAFC) of 4 and P4VP-b-PS did indeed form due to the hydrogen-bonding interactions between the hydroxyl groups of 4 and the pyridine groups of P4VP.

Fig. 1 FTIR spectra of the pyridine band of P4VP-b-PS and the SAFC in the range of 1550–1650 cm⁻¹ (A) and 980–1025 cm⁻¹ (B), respectively. (a) Pure P4VP-b-PS; SAFC (with R = 6.87) were obtained from the mixture of 4 (1 × 10⁻⁴ mol L⁻¹) and P4VP-b-PS (1 mg/2 mL) in CO₂-expanded toluene (313.2 K/varied pressure for 72 h) (b) SAFC of 5.41 MPa; (c) SAFC of 3.65 MPa; (d) SAFC of 0.1 MPa.

Effect of viscosity on the emission behavior of 4

As shown in Fig. 2a, the structure of compound 4 possesses both planar and non-planar moieties. Its planar moieties could aggregate due to π–π stacking interactions, which usually inhibit light emission through a process known as aggregation-caused quenching (ACQ). In contrast, the non-planar moieties exhibit the opposite behavior by promoting light emission due to the restricted intramolecular rotation (RIR) among the aggregates,^15,18 through a processes known as aggregation-induced emission (AIE). Therefore, molecule 4 can either exhibit an AIE or ACQ effect in different cases, or may cause a system to change from ACQ to AIE behavior in different situations. The fascinating luminescence-tuning behavior of 4 encouraged us to further explore its emission dependence on the solution viscosity. Glycerol is a very viscous liquid, with a viscosity of 934 cp at 25 °C, which is approximately 1720 times higher than that of methanol (0.544 cp).¹⁹ Fig. 2b shows the effect of the viscosity on the emission behavior of 4 in glycerol–methanol mixtures. It was found that the FL of 4 was dependent on the viscosity of the solution but its response to the viscosity was non-monotonic. The FL intensity of 4 swiftly increased with increasing viscosity when the glycerol volume fraction was increased from 0 to 40%, and reached a maximum at a glycerol volume fraction of 50%. The FL intensity exhibited by a solution of 4 in the 50% glycerol–methanol mixture is 1.21-fold higher than that observed in the methanol solution. However, further solution thickening (with increases in the glycerol volume fraction from 60% to 90%) led to a continuing decrease in the FL intensity. The addition of glycerol to a methanol solution of 4 generally exerts two contrary effects on the emission behavior of this fluorophore. Increasing the viscosity of the solution restricts the intra-molecular rotation of the fluorogen, whereas an increased viscosity promotes aggregation via π–π stacking interactions involving the planar moiety of 4. The former and latter effects enhance and weaken the emission of 4, respectively.¹⁵ When the glycerol volume fraction ranged between 0% and 50%, it is apparent that an increase in the solution viscosity predominantly exerted the former effect on 4, thus leading to a continuous increase in the emission intensity with increasing viscosity. In contrast, when the glycerol volume fraction exceeded 50%, the influence of the π–π stacking interactions of the planar moiety of 4 prevailed over that of the RIR process, leading to a continuous decrease in the emission intensity with increasing viscosity. This indicates that 4 may exhibit an ACQ effect when the glycerol volume fraction exceeds 50%, which suggests that the planar moiety of 4 exhibits the dominant effect when the solution of 4 becomes more viscous, although the detailed mechanism is unclear at the present stage.

Fig. 2 (a) Chemical structure of 4, with a planar moiety and a propeller-like moiety. (b) Plot of fluorescence (FL) intensity of 4 at 490 ± 2 nm versus composition of glycerol–methanol mixture at 20 °C. Concentration of 4 in glycerol–methanol mixture at 1 × 10⁻⁵ mol L⁻¹, and λ_ex = 360 nm.

The morphology and FL properties of SAFC formed in toluene free of CO₂

The FL enhancement ratio of the SAFC at different reaction conditions are tabulated in Table 1. Firstly, it is found that after self assembly reaction in toluene free of CO₂, the emission intensity of the SAFC is increased (Table 1) compared to the corresponding pristine mixture, which proves that the self assembly of 4 and P4VP-b-PS is successful, and the SAFC are indeed more emissive than the pristine mixture. The reason could be, as 4 is confined in the inner portion of micells via hydrogen bonding with P4VP block, the increase in the confinement degree of 4 in the SAFC helps explain the FL enhancement after self-assembly in toluene.^8c It also indicates that the self assembly of 4 and P4VP-b-PS is an effective way of emission tuning of the promising sensor, the SAFC. Secondly, as shown in Table 1, the observed maximum FL intensity of SAFC at 0.1 MPa (free of CO₂) is about 5.2, 4.2 and 1.8-fold higher than that of the corresponding pristine mixture of P4VP-b-PS and 4 in toluene with R = 6.86, 68.7 and 687, respectively. That is, the FL intensity of SAFC is strongly R dependent and decreases with increasing R value. Thirdly, the morphology of SAFC transits from vesicles (R = 6.87, Fig. 3a) to vesicles plus less spheres (R = 68.7, Fig. 4a), then to less vesicles plus more spheres (R = 687, Fig. 5a), along with the FL enhancement of the resultant SAFC to be 5.2, 4.2 and 1.8-fold higher than that of the pristine mixture of P4VP-b-PS and 4 in toluene. It appears that the smaller the R value, the more amount of hydrogen bond between P4VP-b-PS and 4 due to more available molecules of 4, and the more vesicles among micells, and then the more emissive of the SAFC. As toluene was PS selective, 4 could be confined in the vesicle wall or the micellar core through coupling with P4VP. Although the confinement of 4 in the vesicle wall or the micellar core helps the emission enhancement due to the RIR effect of 4, a plausible conclusion could be that the confinement of 4 in the former case (vesicle wall) is more effective in the FL enhancement of SAFC than that in the micellar core based on the data of 0.1 MPa shown in Table 1.

Table 1 The FL enhancement of the SAFC at different reaction conditions

P (MPa)	FLSAFC/FLpristine mixture^a
	R = 6.87	R = 68.7	R = 687
a FLSAFC and FLpristine mixture represent the FL intensity of SAFC and that of the corresponding pristine mixture of P4VP-b-PS and 4 in toluene, respectively.
0.1 (free of CO2)	5.2	4.2	1.8
3.65	4.3	7.8	3.7
5.41	3.3	9.3	14.1

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Fig. 3 TEM images of the SAFC (with R = 6.87) obtained from the mixtures of 4 (1 × 10⁻⁴ mol L⁻¹) and P4VP-b-PS (1 mg/2 mL) in CO₂-expanded toluene (313.2 K/varied pressure for 72 h) (a) 0.1 MPa; (b) 3.65 MPa; (c) 5.41 MPa.

Fig. 4 TEM images of the SAFC (with R = 68.7) obtained from the mixture of 4 (1 × 10⁻⁵ mol L⁻¹) and P4VP-b-PS (1 mg/2 mL) in CO₂-expanded toluene (313.2 K/varied pressure for 72 h) (a) 0.1 MPa; (b) 3.65 MPa; (c) 5.41 MPa.

Fig. 5 TEM images of the SAFC (with R = 687) obtained from the mixture of 4 (1 × 10⁻⁵ mol L⁻¹) and P4VP-b-PS (5 mg mL⁻¹) in CO₂-expanded toluene (313.2 K/varied pressure for 72 h) (a) 0.1 MPa; (b) 3.65 MPa; (c) 5.41 MPa.

SAFC morphology and FL properties tuning by CXLs pressure and molar ratio of 4VP to 4

The emission spectra of SAFC at different reaction conditions are given in Fig. 6–8. As shown in Fig. 6 (R = 687) and 7 (R = 68.7), we found that the emission of SAFC becomes enhanced with increasing CXLs pressure at molar ratio of 4VP to 4 (R) to be 687 and 68.7, respectively. At R = 687, the morphology of SAFC micells (Fig. 5) transits from less vesicles plus more spheres to more vesicles plus less spheres, then to thinner walled vesicles with increasing pressure at 0.1, 3.65 and 5.41 MPa, accompanied with the FL enhancement of SAFC being 1.8, 3.7 and 14.1-fold higher (Table 1) than the pristine mixture. And at R = 68.7, the morphology of SAFC micells (Fig. 4) switches from vesicles plus spheres to vesicles plus rods, then to smaller size dense vesicles plus rods with increasing pressure at 0.1, 3.65 and 5.41 MPa, together with the emission intensity of SAFC being 4.2, 7.8 and 9.3-fold enhanced (Table 1) than the pristine mixture. From above analysis, we could infer that the confinement of 4 in the vesicle wall is more efficient in the FL enhancement of SAFC than that in the micellar cores, as is shown in Fig. 5 and 6 (R = 687). Moreover, the confinement becomes more effective in the thinner vesicle wall and smaller sized vesicles, as is reflected in Fig. 4 and 7 (R = 68.7). Contrary to the pressure dependence of FL for the SAFC with R values of 687 and 68.7, the emission of SAFC is getting attenuated with increasing CXLs pressure when R is 6.87 (Fig. 8). At R = 6.87, the morphology of SAFC micells transforms from vesicles to vesicles plus rods, then to dense spheres (Fig. 3) with increasing pressure at 0.1, 3.65 and 5.41 MPa, along with the FL intensity of SAFC being 5.2, 4.3 and 3.3-fold enhanced (Table 1) than the pristine mixture.

Fig. 6 FL spectra of the SAFC (with R = 687) obtained from the mixtures of 4 (1 × 10⁻⁵ mol L⁻¹) and P4VP-b-PS (5 mg mL⁻¹) in CO₂-expanded toluene (313.2 K/varied pressure for 72 h). Physical mixture denotes the pristine mixture of 4 (1 × 10⁻⁵ mol L⁻¹) and P4VP-b-PS (5 mg mL⁻¹) in toluene without formation of the SAFC.

Fig. 7 FL spectra of the SAFC (with R = 68.7) obtained from the mixtures of 4 (1 × 10⁻⁵ mol L⁻¹) and P4VP-b-PS (1 mg/2 mL) in CO₂-expanded toluene (313.2 K/varied pressure for 72 h). Physical mixture denotes the pristine mixture of 4 (1 × 10⁻⁵ mol L⁻¹) and P4VP-b-PS (1 mg/2 mL) in toluene without formation of the SAFC.

Fig. 8 FL spectra of the SAFC (with R = 6.87) obtained from the mixtures of 4 (1 × 10⁻⁴ mol L⁻¹) and P4VP-b-PS (1 mg/2 mL) in CO₂-expanded toluene (313.2 K/varied pressure for 72 h). Physical mixture denotes the pristine mixture of 4 (1 × 10⁻⁴ mol L⁻¹) and P4VP-b-PS (1 mg/2 mL) in toluene without formation of the SAFC.

At constant pressure, the FL intensity of SAFC is found to be strongly R dependent, and the observed maximum FL intensity of SAFC at 5.4 MPa is about 3.3, 9.3 and 14.1-fold higher (Table 1) than that of the pristine mixture of P4VP-b-PS and 4 in toluene when R is 6.86, 68.7 and 687, respectively.

The pressure dependence of the emission behavior of SAFC was ascribed to hydrogen bonding and confinement acting as the major factor, respectively, subjected to different value of R. We think that both confinement and hydrogen bonding help the enhancement of the emission of SAFC, and the solubility (or solvated state) of P4VP chains has significant influence on the amount of hydrogen bonding between P4VP and 4. Apparently, the solubility of P4VP chains is getting worse with increasing CXLs pressure due to the anti-solvent effect of CO₂, along with the transition of P4VP chains from extended state to folded state, and a decline in the number of hydrogen bonding. For the self assembly reaction with R = 687 and 68.7, there are less hydrogen bonding formed due to less molecules of 4 available compared with R = 6.87, so it is reasonable that confinement is the major effect to influence the FL of the corresponding SAFC, whereas the SAFCs with smallest R are capable of forming more amount of hydrogen bonding due to more available molecules of 4, and thus hydrogen bonding plays major roles in the latter case. We believe that a reduction in hydrogen bonding amount with increasing CXLs pressure leads to less emissive of the SAFC at higher pressures when hydrogen bonding is predominating over confinement effect at R = 6.87. In contrast with SAFC at larger R value as 687 and 68.7, the quenching phenomena of SAFC with increasing CXLs pressure at R = 6.87 is owing to hydrogen bonding being the prevailing factor instead of confinement.

Experimental section

General information

The number-average molecular weight and polydispersity index of P4VP and P4VP-b-PS were determined by gel permeation chromatography (GPC, Waters 515 HPLC pump, Waters 2414 Refractive index detector; Waters Corporation, Milford, MA, USA). The morphology of the SAFC was characterized by transmission electron microscopy (TEM), using a JEOL JEM-2100 microscope with an accelerating voltage of 200 kV (JEOL Ltd, Tokyo, Japan). TEM samples were prepared by dropping solutions on carbon-coated copper grids, absorbing the solvent on filter paper, and evaporating the solvent at room temperature. The grid samples were then stained with iodine vapors for 6 h that selectively stained the P4VP block to enhance the contrast. Fourier transform infrared (FT-IR) spectra were recorded on a NEXUS-470 spectrometer. The fluorescence measurements were carried out on a Hitachi FL-4500 spectrofluorometer with a temperature controller. The emission spectra were recorded with an excitation wavelength at 390 nm, except the measurement in glycerol–methanol mixture excited at 360 nm.

Materials

Methanol, glycerol, and toluene for FL measurements were spectrophotometric or HPLC grade (≥99.9%) from Sigma-Aldrich and were used as received. 4-Vinyl pyridine (4VP, Alfa Aesar, Beijing, 96%) and styrene (Tianjin Chemical Reagent, 98%) were distilled under reduced pressure, respectively, and stored in a refrigerator. 2,2-Azobis (isobutyronitrile) (AIBN, Shanghai Chemical Reagents Co., purity 98%) was recrystallized from ethanol twice. Benzyl dithiobenzoate (BDTB) was prepared using the method described before.¹⁶ Compound 4 was synthesized in our lab according to a procedure reported previously.¹⁵ CO₂ (99.95%) was provided by Zhengzhou Shuangyang Gas Co. (Zhengzhou, China). Poly(4-vinylpyridine) (P4VP, M_n = 4300 g mol⁻¹) and P4VP-b-PS (M_n = 29 900 g mol⁻¹) used in this work were prepared in our lab using a method described previously,^14c and the detailed reaction conditions, the number-average molecular weight (M_n) and polydispersity index (M_w/M_n) of P4VP and P4VP-b-PS determined by GPC are tabulated in Table 2.

Table 2 Polymerization of P4VP and P4VP-b-PS at different reaction conditions

Sample	t (min)	Initiator	T (K)	BDTB/AIBN	Mn^a	Mw/Mn^a	Yield (%)
a The number-average molecular weight (Mn) and polydispersity index (Mw/Mn) of P4VP and P4VP-b-PS were determined by GPC in DMF at 313.2 K, flow rate 1 mL min^−1.
P4VP	55	AIBN	353.2	10 : 3	4300	1.356	72.8
P4VP-b-PS	300	AIBN	353.2		29 900	1.389	40.7

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Preparation of SAFC between P4VP-b-PS and 4 in CO₂-expanded toluene

The experimental apparatus was described in our previous work.^14a Typically, appropriate amount of 4 were dissolved in toluene. A certain amount of P4VP-b-PS were dispersed in toluene at a suitable temperature, the mixture was then ultrasonicated for 5 min in order to disperse the block copolymer homogeneously in the solution. The 4/toluene solution was then added into the dispersion in a tube. After equilibration, the tube containing the mixture was quickly transferred into a stainless-steel autoclave at the temperature of 313.15 K. CO₂ was then charged into the autoclave to achieve the desired pressure within a short time. After 72 h treatment in CO₂-expanded toluene, the system was slowly depressurized and the SAFC sample was collected and labeled.

To understand the molar ratio of 4VP/4 (the ratio of 4VP/4 is named R) dependence on morphology change and FL intensity of the SAFC, the reaction system were examined with R selected to be 6.87 (i.e., P4VP-b-PS and 4 in toluene at concentration of 1 mg/2 mL and 1 × 10⁻⁴ mol L⁻¹, respectively), 68.7 (P4VP-b-PS and 4 at concentration of 1 mg/2 mL and 1 × 10⁻⁵ mol L⁻¹) and 687 (P4VP-b-PS and 4 at concentration of 5 mg mL⁻¹ and 1 × 10⁻⁵ mol L⁻¹), respectively. The CXLs pressures were selected to be 5.41 and 3.65 MPa, and SAFC samples were prepared at 0.1 MPa (free of CO₂) for comparison.

Conclusions

CXLs-assisted hydrogen bonding between the P4VP block of P4VP-b-PS and 4 results in various anti-solvent- and complexation-induced micellization phenomena. FTIR spectra provided evidence of the formation of hydrogen bonds between the P4VP block and 4. TEM images and FL data show that the micellar structure and emission behavior of SAFC could be readily regulated by tuning the pressure of the CXLs and the molar ratio of 4VP to 4. P4VP blocks could be found either within the vesicle wall or the spherical cores depending on the micellar morphology of SAFC. In addition, the location of 4 could be controlled via coupling with P4VP. The molecules of 4 within the spherical micellar core showed relatively weak FL signals, whereas 4 in the vesicle wall exhibited FL enhancement due to the higher degree of confinement of 4 in the latter case. Meanwhile, the pressure dependence of the emission behavior of SAFC with different R values was attributed to hydrogen bonding or confinement acting as the major factor. The quenching phenomena of SAFC were observed with increasing CXLs pressure at R = 6.87 when hydrogen bonding was the dominant factor. In contrast, significant increases in the FL intensity of SAFC were found with increasing pressure at R = 68.7 and 687 when the confinement effect was predominant over hydrogen bonding interactions. The maximum FL intensity exhibited by the SAFC at 5.41 MPa and R = 687 is approximately 14-fold higher than that of the pristine mixture of P4VP-b-PS and 4 in toluene. In comparison with the emission of the pristine mixture of 4 and P4VP-b-PS, the as-prepared SAFC in toluene (free of CO₂) exhibited a maximum emission enhancement of 5.2-fold when hydrogen bonding acted as the major factor with R = 6.87, whereas in CXLs at 5.41 MPa a maximum FL enhancement of 14.1-fold was observed when the confinement effect was the dominant factor with R = 687. This work demonstrates that CXLs method facilitates the modulation of the morphology and the emission performance of the SAFC, and opens a new route for the development of efficient luminescent materials.

Acknowledgements

This work was supported by funds from the National Natural Science Foundation of China (No. 21543009, 21073167, J1210060), the China Scholarship Council [2011]3022, and the Innovative Research Grant for Undergraduate Students of Zhengzhou University (2012–2015).

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