Polymorph selectivity of an AIE luminogen under nano-confinement to visualize polymer microstructures

Despite the huge progress of luminescent molecular assemblies over the past decade, it is still challenging to understand their confined behavior in semi-crystalline polymers for constrained space recognition. Here, we report a polymorphic luminogen with aggregation-induced emission (AIE), capable of selective growth in polymer amorphous and crystalline phases with distinct color. The polymorphic behaviors of the AIE luminogen embedded within the polymer network are dependent on the size of nano-confinement: a thermodynamically stable polymorph of the AIE luminogen with green emission is stabilized in the amorphous phase, while a metastable polymorph with yellow emission is confined in the crystalline phase. The information on polymer crystalline and amorphous phases is transformed into distinct fluorescence colors, allowing a single AIE luminogen as a fluorescent marker for visualization of polymer microstructures in terms of amorphous and crystalline phase distribution, quantitative polymer crystallinity measurement, and spatial morphological arrangement. Our findings demonstrate that confinement of the AIE luminogen in the polymer network can achieve free space recognition and also provide a correlation between microscopic morphologies and macroscopic optical signals. We envision that our strategy will inspire the development of other materials with spatial confinement to incorporate AIE luminogens for various applications.

refinement package using Least Squares minimization. Non-hydrogen atoms were refined anisotropically while hydrogen atoms were refined using the riding model.
Preparation of TPE-EP embedded PLLA film. TPE-EP embedded PLLA film preparation procedures are as follows: TPE-EP was dissolved in THF to prepare a 1 mg mL −1 stock solution.
50 µL of the above TPE-EP solution was mixed with 50 mg of PLLA in chloroform (10 mg mL −1 ) under stirring. Unless specified otherwise, the content of TPE-EP in PLLA matrix was controlled to be 0.1 wt%. Afterward, the mixed solution was put in a covered weighing bottle with a radius and height of 2.5 cm and 3.0 cm, respectively. Crystallization of PLLA was occurred by controlling the solvent evaporation rate at ambient temperature. Then, the thin PLLA film was peeled off from the glass bottle for further study.
CFM imaging. CFM measurements were carried out on a Zeiss LSM7 DUO confocal microscope. For TPE-EP-embedded PLLA film with a mixture of amorphous and crystalline phases, CFM images were first obtained through two-channel scanning under laser excitation of 405 nm. Channel one was selected between 410-516 nm and channel two was selected between 518-688 nm. The obtained images were linearly unmixed [3] using ZEN software into two components, each of which was respectively assigned by green and yellow as shown in Figure S21 and S22. For TPE-EP-embedded crystalline PLLA film, CFM images were obtained under laser excitation of 405 nm, and the optical signal was selected between 400-700 nm ( Figure S26).
Measurements. Molecular weight and polydispersity index (PDI) of purified PLLA was determined by Waters Association Gel Permeation Chromatography (GPC) system equipped with UV detectors. Differential Scanning Calorimetry (DSC) measurements were conducted using a TA DSC Q1000 under nitrogen flow at a heating rate of 10 o C min −1 . Photoluminescence (PL) spectra were measured on a Horiba Fluorolog-3 spectrofluorometer. Quantum yields of the solid samples and polymer films were determined on a Horiba Quanta using a calibrated integrating sphere. Fluorescence lifetime measurements were conducted on a FLS 980 spectrometer. Fluorescent and optical microscopy images were captured on a Nikon Eclipse 80i microscope. WAXD analysis was conducted on a X'pert PRO, PANanalytical diffractometer using Cu-K radiation. Small-angle X-ray scattering (SAXS) experiments were performed on a SAXSess instrument (Anton Paar). Nanoindentation tests were conducted using a Hysitron TI950 TriboIndenter. SEM images were obtained on a JEOL-7100F SEM. Normal photographs were taken on Canon 7D camera under 365 nm UV illumination. CPL spectra were carried out on JASCO CPL-300, the measurement conditions of CPL are listed in Table S5.
Crystallinity measurements. Percentage crystallinities (χc) of the polymer films were estimated by DSC and XRD methods. For DSC measurements, crystallinities of the polymer films were calculated from DSC thermal profiles according to equation (1).
ΔHm is melting enthalpy and ΔHc is exothermal enthalpy arising from crystallization in the polymer film during heating process. ΔH 0 m is the melting enthalpy of PLLA crystal, 106 J g -1 . [4] DSC thermographs of tested films are shown in Figure S24b and thermal profiles indicating enthalpies and temperatures of melting and crystallization of the films are shown in Table S4.
For XRD measurements ( Figure S24a), crystallinities of the polymer films were calculated by dividing the area under the crystalline peaks to overall area of the diffractometer after subtraction of the background signal. Strong peak at 2θ of 16.7° from (200)/(110) planes and relatively smaller peak at 2θ of 19.0° from (203) planes were selected as crystalline peaks for fitting.
Lamellae thickness calculation. Lamellae thickness and long period were calculated from SAXS patterns using one-dimensional (1D) correlation function. [5] The normalized 1D correlation function was extracted from Lorentz-corrected experimental SAXS results by a Fourier transform by equation (2).
The long period L is the first maximum value of first oscillation, and the thickness of the thinner phase (Lt, determined as amorphous phase) is the intercept between level of the first minimum and the extrapolation of linear part of the correlation function as labelled in Figure S10.
Single crystal data. CCDC #1917740 for G-crystals and 1917741 for Y-crystals contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

Compound
Fluorescence , τi is an individual lifetime and ai is an amplitude of multi-exponential decay fit.
c) The radiative rate constant kr = ΦF/τ; the non-radiative rate constant knr = 1/τ -kr.  In Figure S1a, the first heating scan of G-crystals revealed a broad endothermic peak at ~134 o C, indicating the evaporation of included solvent in crystalline lattice. The disappearance of the broad peak at the second heat scan further demonstrates the exclusion of the solvent molecules. In Figure S1b, a thermal phase transformation from Y to G was initiated at 116 o C.
No reverse phase was observed when the sample was cooled. In Figure S1c, a similar thermal phase transformation from O to G was observed at 155 o C. The DSC analyses indicate that Ycrystals and O-crystals are metastable while G-crystals are thermodynamically stable. The luminescent color changes of TPE-EP polymorphs were shown in Figure S1d.               Bright rings bounding the spherulites comprise of both amorphous and crystalline arrangement based on the SEM data in Figure S23. The intermixed nature of the phase interface consequently causes TPE-EP to form mixture of green and yellow emissive state. These bright rings show emission intensity higher than amorphous and crystalline regions. The underlying reason for the observation is relatively higher amount of TPE-EP in this interface region due to accumulation of rejected TPE-EP molecules around edges of the spherulites. [7] Intensity profiles of red, blue, and green component of the fluorescent micrograph (along the red arrow) in Figure   S20 confirms that the edge area shows highest intensity in all three components. R, G, and B percentage of the intensity profile in the boundary ring is 29%, 53%, and 18%, respectively.
This set of value lies between the values of amorphous (25%, 59%, 16%) and crystalline (34%, 54%, 12%) region in terms of red and blue component, further validating that TPE-EP in the boundary rings exist as a mixture of TPE-EP with yellow and green emission.  After gold sputtering, the morphology of the film is presented as shown in Figure S23a.
In contrast, only ambiguous polymer crystalline shape can be observed in the SEM image. It has been reported that the density of amorphous PLLA (1.248 g ml -1 ) is less than that of crystalline states (1.290 g ml -1 ). [8] Acetone could dissolve the amorphous PLLA rather than the crystalline region. As a result, SEM images of 3D spherulites shown in Figure S23b, c and d were created.  Table S4.      The sensitivity of TPE-EP to distinguish polymer phases in other semi-crystalline polymers were studied. Melt crystallization method was used to obtain the polymer film with a mixture of crystalline and amorphous regions. The doping ratio of TPE-EP in PHBV and PEG was controlled to be 0.1 wt%. For PHBV (top), it is found the crystalline region shows dark emission, while amorphous region exhibits bright green emission. The distinct "On" and "Off" emission in respective amorphous and crystalline phase indicates the responsiveness of TPE-EP in polymer phases. In crystalline PHBV, TPE-EP molecules were excluded out from the spherulites due to the densely packed polymer lamellae, leading to significantly decreased intensity in the crystalline spherulites. We propose that TPE-EP molecules also form nano-aggregates with a similar structure of G-crystals in amorphous PHBV, which is attributed to the similar chemical structure of PHBV and PLLA.
For PEG (bottom), it found the crystalline region shows yellow emission, while amorphous exhibits orange emission. The result also shows potential of TPE-EP to distinguish the polymer phases of PEG. In contrast to the mechanism of TPE-EP in PLLA, the distinct emission color in amorphous and crystalline regions is originated from twisted intramolecular charge-transfer effect of the TPE-EP molecular rotors. In amorphous PEG, the hydrophilic network provides a polar microenvironment for TPE-EP to emit orange emission. We speculate the yellow emission of TPE-EP in crystalline PEG results from the confined space of crystalline lamellae and comparatively low polarity of crystalline PEG network.