Photoluminescent electrospun nanofibers were fabricated by incorporating a cyclopentadiene derivative AIE-active luminogen within the polymeric matrices of polyvinylpyrrolidone (PVP), poly(vinylidene fluoride)-co-hexafluoropropylene (PVDF-HFP), and poly(methylacrylate) (PMMA) under different solvents. Raman, infrared, and fluorescence spectroscopy showed the molecular integrity of the AIEgen is maintained during the electrospinning process while exhibiting a strong and uniformed phosphorescent radiative pathway of τaverage = 8.9 μs. This method allows for the rapid fabrication of photoluminescent nanofibers into a wide range of polymeric matrices without the need of chemical modifications. Taken together, the prepared electrospun phosphorescent nanofibers are a stepping stone for AIEgens to be applied in the digital manufacturing and design of smart textiles.

1. Introduction

Luminogens have gained tremendous interest because of their applicability in the fabrication of solid state emitters, such as organic light emitting diodes (OLED), required in display applications. However, traditional luminogens suffer from aggregation-induced quenching (AIQ) in the solid state form, mostly due to the formation of excimers and exciplexes species. Consequently, traditional luminogens have found limited applications in display devices because of its low dispersed concentration in films, providing inherently weak signals. In order to overcome this challenge, one strategy has been to chemically tailor luminogenic pendants to the backbone of polymers, refining polymeric architectures and granting optical capabilities, independent of conjugation as is the case in radical polymers [1]. Another strategy has been to synthetically modify polymeric backbones with pendants exhibiting aggregation-induced emission (AIE) properties [2, 3].

Recently, the fabrication of optical and electronic polymeric materials has been achieved through the use of the electrospinning technique, mainly due to its low cost and maintenance, flexible parametric tuning, green chemistry (use of small amounts of solvent), and high throughput [4]. One approach for the preparation of optical polymeric materials has been to electrospin polymer blends [5], such as polyfluorene derivatives/poly(methyl) methacrylate (PMMA) and phenylene vinylene derivatives/PMMA, using a single solution spinneret for the purpose of reducing AIQ to enhance luminescence efficiency. Results show an improved luminescence yield in comparison to spin-casted thin films, attributed to uniformed distribution due to geometrical constraints during the electrospinning process [6, 7]. In another approach, polymeric materials have been synthetically modified with AIE-active pendants and subsequently electrospun [8, 9] into flexible solid state emitters [10], bacterial sensor [11], and for oil adsorption [12].

In a different approach, inorganic germanium nanocrystals have been incorporated into electrospun polymeric fibers, resulting in fiber webs with unique optical properties rivalling solution photoluminescence [13, 14]. Similarly, CdSe, CdS, and ZnS quantum dots (QD) have been incorporated into electrospun poly(9-vinylcarbazole) matrices to produce uniformed orange and red color solid state mat emitters with superior luminescence than thin films, reducing QD aggregation and its quenching effects. These mats were subsequently used along luminogen C545T [15] to fabricate white light OLEDs [1517].

In this report, we undertake a different approach in the fabrication of photoluminescent electrospun polymeric nanofibers. An AIEgen [18] comprised of a cyclopentadiene head containing a pyridinium moiety as a tail, bridged by an 8-carbon alkane chain, is incorporated into polymeric matrices via electrospinning method. The AIEgen/polymer electrospun mat is subsequently characterized by various techniques. More specifically, we demonstrate that an AIEgen initially displaying low photoluminescence while dissolved in organic solvents can exhibit strong phosphorescence in an electrospun nanofiber when stimulated with UV light. This strategy eliminates the need to tether AIE-active pendants into polymeric backbones by chemical means, thus tremendously decreasing preparation time while minimizing potential solubility concerns.

1.1. Experimental

Figure 1(a) shows the molecular structure of the molecule displaying aggregation-induced emission (AIE) properties, 1-(1-(8-pyridiniumoctyloxy)-2,3,4,5-tetraphenylcyclopenta-2,4-dienyl) benzene chloride, hereafter referred to as C8, while Figure 1(b) exhibits the molecular structures of polyvinylpyrrolidone (PVP), poly(vinylidene fluoride)-co-hexafluoropropylene (PVDF-HFP), and poly(methylacrylate) (PMMA), the polymeric matrices used for electrospinning nanofibers.

1.1.1. Nanofiber Preparation

Polyvinylpyrrolidone (PVP), (MW: 130,000 Da), and ethanol (99.9% purity) were purchased from Sigma-Aldrich. The PVP solution was prepared by dissolving 0.3 g of PVP in 5 mL of ethanol and magnetic stirring for 1 h at room temperature. Subsequently, 4 × 10−3 g of C8 molecules were added to the above solution and stirred for 30 min to reach a concentration of 1.23 × 10−3 M. Synthesis and solution-based photoluminescence studies of C8 can be found in a previous work [18]. The resulting PVP/C8 solution was loaded into a plastic syringe equipped with a 21 G needle. A high voltage of 20 kV was applied between the needle tip and the collector placed at a distance of 13 cm from the needle tip. The feeding rate for the solution was set at 0.6 mL/h through a syringe pump. The electrospun nanofibers were electrospun for 30 minutes and collected on a glass substrate or an aluminium foil to be subsequently dried in an oven at 60°C for 2 h. Similarly, 0.3 g of poly(vinylidene fluoride)-co-hexafluoropropylene (PVDF-HFP), (MW: 110,000 Da), were dissolved in 5 mL dimethylformamide (DFM)/acetone (3.1 v/v) solution, followed by 30 min sonication and 30 min of magnetic stirring at room temperature. 0.3 g of poly(methylacrylate) (PMMA), (MW: 120,000 Da), were dissolved in 5 mL of dimethylformamide (DMF) solution, followed by magnetic stirring for 1 h. For both PVDF-HFP and PMMA solutions, 4 × 10−3 g of C8 molecules were added to reach a concentration of 1.23 × 10−3 M, and the overall solutions were magnetically stirred for 30 min. All chemicals were research grade, commercially available, and were used as purchased. For PMMA, a high voltage of 15 kV was used between a needle tip and the substrate at a distance of 15 cm, at a flow rate of 0.8 mL/h. For PVDF-HFP, the voltage applied was 15 kV, set at a distance of 15 cm between the needle tip and the collector, at a flow rate of 1.0 mL/h.

1.1.2. Nanofiber Mat Characterization

The electrospun nanofibers deposited on glass substrates were freshly characterized and subsequently stored in a low humidity environment (dry cabinet at ~20% humidity). The morphology of the electrospun nanofiber mats was observed using a scanning electron microscopy (SEM, JEOL JSM-7600 F). The particle size analysis of the C8 molecules in ethanol was obtained using a nanoparticle size analyser instrument (Nanosight NS300). Structural information of the electrospun nanofiber mats were gathered by Raman spectroscopy. Raman spectra were collected in a NT-MDT confocal Raman microscopic system with excitation laser wavelength of λex = 473 nm, whereby the Si peak at 520 cm−1 was used as a reference for wavenumber calibration. UV-Vis molecular electronic absorption of the solutions and the UV-Vis reflection studies of the nanofiber mats were measured using a UV-Vis spectrophotometer (PerkinElmer). A UV light source equipped with white light, λex = 254 nm, and λex = 365 nm excitation wavelength and a UV lamp with an excitation wavelength of λex = 254 nm and λex = 365 nm were used to irradiate the nanofiber mats to subsequently digitally photograph their luminescence. Photoluminescence and lifetime of the nanofiber mats were measured using a fluorescence spectrometer (FSP920, Edinburgh Instruments, Livingston, U.K.). The fluorescence images were produced using a confocal microscope equipped with an excitation filter λex = 330–380 nm and a barrier filter at λ = 420 nm (Nikon, UV-2A Filter). Digital photographs were taken with the camera of a Sony Experia Z3 handheld phone.

2. Results and Discussion

The morphology of the electrospun nanofibers are shown in Figure 2. The nanofibers displayed a diameter range of 300 to 500 nm for PVP/C8 mats, 80 to 150 nm for PVDF-HFP/C8 mats, and 120–250 nm for PMMA/C8 mats. At the 30,000 times magnification level (see Figure 2(a) inset in upper right corner), the nanofibers showed smooth surfaces without the presence of beads or any other structural features associated with the electrospinning method or as previously reported for a HPS/PMMA blend [19].

The distribution and size of the C8 aggregations within the PVP matrix could not be identified at this resolution, especially because C8 molecules dissolved in ethanol displayed a heterogeneous size aggregation distribution ranging from ~50 to 800 nm with two noticeable peak at 115 and 369 nm, as shown in Figure 3. Although further studies are warranted to discern the nanoaggregation size and distribution within the electrospun nanofibers, nonetheless, the photoluminescent observed suggests the presence of nanoaggregations within the PVP matrix.

Raman spectroscopy provided invaluable information on the molecular structure of the electrospun nanofibers and molecular integrity of C8 molecules within the matrix. Figure 4 shows Raman spectra in the range of 1400–1800 cm−1 for C8 powder (a), PVP nanofiber mat (b), and PVP/C8 nanofiber mat (c). C8 molecules displayed strong Raman-active bands at 1600 cm−1, 1580 cm−1, and 1564 cm−1 and weak bands at 1500 cm−1 and 1446 cm−1. The PVP nanofibers exhibited strong bands at 1664 cm−1, 1462 cm−1, 1446 cm−1, 1426 cm−1 and a weak band at 1500 cm−1. The PVP/C8 nanofiber mats showed peaks at 1664 cm−1, 1604 cm−1, 1580 cm−1, 1564 cm−1, 1495 cm−1, and 1450 cm−1, representing bands associated with C8 molecules and PVP. These results demonstrate that the molecular integrity of C8 molecules was not affected by the electrospinning process and were instead embedded in nanoaggregate form within the PVP polymeric matrix. This is illustrated by the band at 1600 cm−1 associated with ν(C=C) [20] in C8 molecules only, which is nonetheless observed in the PVP/C8 electrospun nanofibers. Similarly, the peak at 1664 cm−1 assigned to ν(C=O) [21] in the PVP structure is also present in the PVP/C8 nanofiber mats. In contrast, the band at 1446 cm−1, which corresponds to a symmetric ring deformation [22], is observed in all three spectra since both PVP and C8 molecules possess ring structures in their molecular structure.

The electronic absorption studies for solution and nanofiber mats are shown in Figure 5. Figures 5(a)–5(c) contrast UV-Vis absorbance spectra for C8, PVP, and PVP/C8 ethanol solutions. AIE-active C8 molecules with a concentration of 1.2 × 10−3 M showed a strong and broad absorption peak centered at 363 nm, which is an electronic π transition of the cyclopentadiene ring [23]. The strong absorption bands at λ = 254 nm and λ = 272 nm are associated with electronic π-π transitions of the pyridinium moiety [24]. Figure 5(b) corresponding to the PVP ethanol solution with a concentration of 4.6 × 10−4 M showed a featureless absorption throughout the spectral range, while the PVP/C8 mixture resembles the spectrum of C8 in ethanol solution in Figure 5(a). For the PVP/C8 ethanol solution, the broad band at λ = 363 nm has been blue shifted to λ = 358 nm presumably by the presence of the electron donating ability of the nitrogen atom in the PVP structure. The band at λ = 272 nm lost intensity and was not discernible, but the peak at λ = 254 nm associated with C8 molecules in the ethanol solution was observable.

Figures 5(d) and 5(e) depict the UV-Vis reflectance spectra of the PVP nanofiber and the PVP/C8 nanofiber mats, respectively. In this case, the broad band at λ = 365 nm and the peaks at λ = 254 nm and λ = 272 nm associated with the C8 molecules were clearly visible in the PVP/C8 nanofiber mat spectrum but absent in the PVP nanofiber mat spectrum. The UV-Vis absorbance studies lend further support to the idea that C8 molecules maintained molecular integrity and remained embedded within the PVP electrospun matrix.

The photoluminescent characteristics of the electrospun PVP/C8 nanofiber mats are shown in Figure 6. Figure 6(a) reveals that the observed photoluminescent in the form of a broad band centered at λmax = 460 nm is due to the embedded C8 AIEgen within the polymeric matrix, presumably through nanoaggregations since PVP is nonphotoluminescent. Furthermore, the photoluminescent of PVP/C8 nanofiber mats is highly uniformed, as shown in Figure 6(b), whereby fluorescence microscope images of the PVP and PVP/C8 nanofiber mats are presented. The image mean intensity for the photoluminescent nanofiber mat was calculated to be 171.6 ± 4.4, while the PVP nanofiber mat image mean intensity was 0.01 ± 0.004, amounting to a coefficient of variance of 2.6% and 40% for the PVP/C8 and PVP nanofiber mats, respectively, suggesting the surface of the PVP/C8 electrospun nanofiber mats displays remarkably uniformed photoluminescent. These findings strongly indicate that the C8 AIEgen is uniformly distributed in the PVP polymer matrix.

Figure 7 displays a lifetime photoluminescent curve for the embedded C8 molecules. The time-resolved curve is best fitted with a double exponential decay, indicating the presence of two energy relaxation pathways for the excited molecules. A relatively fast relaxation time is shown in the upper inset with τ1 = 5 μs (fraction of molecules f1 ≅ 0.85), while the slow relaxation time can be found in the lower inset with τ2 = 31 μs (f2 ≅ 0.15). The weighted photoluminescent average was calculated to be τaverage = f1τ1 + f2τ2 = 8.9 μs, which falls in the range of a phosphorescent phenomenon associated with AIE-active molecules, indicating the fabrication process did not affect the emission lifetime. Photoluminescent lifetimes are related to radiative and nonradiative processes. It is not clear at this early stage the mechanism behind each of the two relaxation pathways; however, since the radiative lifetime is an intrinsic property of the luminogen, the nonradiative decay mechanism can be modified to become radiative by tailoring the polymer matrix/luminogen interaction, which within the AIE model means inhibiting the phenyl ring vibrational modes. We leave this analysis for a later manuscript.

The advantage of this fabrication method is the ability to incorporate AIEgens into a variety of solvents and polymeric matrices rather readily, obviating the need to tether AIE-active pendants into polymeric backbones by chemical means while minimizing solubility concerns. This improvement is demonstrated in Figure 8(a) where digital photographs of PVP and PVP/C8 nanofiber mats under white light and UV irradiation are shown. The PVP nanofiber mats show no photoluminescent while the PVP/C8 mats do exhibit a strong photoluminescence under both λex = 365 nm and λex = 254 nm excitation. Figure 8(b) shows digital photographs of electrospun nanofiber mats of poly(vinylidene fluoride)-co-hexafluoropropylene (PVDF-HFP)/C8 (top panel) and poly(methylacrylate) (PMMA)/C8 (bottom panel) displaying strong photoluminescence under λex = 365 nm and λex = 254 nm excitation. It is indeed quite a rapid and flexible photoluminescent nanofiber synthesis methodology.

3. Summary

We have demonstrated a successful and readily incorporation of an AIEgen within various polymeric matrices, such as PVP, PVDF-HFP, and PMMA, under various solvents to fabricate electrospun nanofiber mats (textiles) with smooth morphology without the need of chemical modification of the polymers and with minimal concerns in solubility. Both, the Raman and UV-Vis spectra, revealed that the AIEgen’s molecular integrity remains intact within the polymeric electrospun nanofibers after electrospinning. The fluorescence emission spectrum indicated a strong and uniformed photoluminescence when excited with λex = 254 nm and λex = 365 nm, while the lifetime photoluminescence studies suggest the nanofibers follow a phosphorescent energy emission pathway. Taken together, the prepared electrospun phosphorescent nanofibers are a stepping stone for AIEgens to be applied in the digital manufacturing and design of smart textiles.

Conflicts of Interest

The authors declare no competing interests.


Associate Professor Wu Ping at SUTD and the MIT-SUTD International Design Centre (IDC) is acknowledged for instrument support. Leng L. Chng is supported by the Institute of Bioengineering and Nanotechnology (Biomedical Research Council, Agency for Science, Technology and Research, Singapore). PhD candidate Jeck Chuang Tan at EPD, SUTD, is gratefully acknowledged for helpful discussions on the subject.