Unusual excitation wavelength tunable multiple fluorescence from organocyclo-phosphazene microspheres: Crosslinked structure-property relationship

Journal Pre-proof Unusual excitation wavelength tunable multiple fluorescence from organocyclophosphazene microspheres: Crosslinked structure-property relationship Majid Basharat, Yasir Abbas, Wei Liu, Zahid Ali, Shuangkun Zhang, Wenqi Zou, Zhanpeng Wu, Dezhen Wu PII:

S0032-3861(19)30948-6

DOI:

https://doi.org/10.1016/j.polymer.2019.121942

Reference:

JPOL 121942

To appear in:

Polymer

Received Date: 6 June 2019 Revised Date:

11 October 2019

Accepted Date: 23 October 2019

Please cite this article as: Basharat M, Abbas Y, Liu W, Ali Z, Zhang S, Zou W, Wu Z, Wu D, Unusual excitation wavelength tunable multiple fluorescence from organocyclo-phosphazene microspheres: Crosslinked structure-property relationship, Polymer (2019), doi: https://doi.org/10.1016/ j.polymer.2019.121942. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Unusual excitation wavelength tunable multiple fluorescence from organocyclophosphazene microspheres: Crosslinked structure-property relationship Majid Basharata, Yasir Abbasa, Wei Liua, Zahid Alia, Shuangkun Zhanga, Wenqi Zoua, Zhanpeng Wua* and Dezhen Wub* a

Key Laboratory of Carbon Fiber and Functional Polymers (Beijing University of Chemical

Technology) Ministry of Education, Beijing 100029, P.R. China. E-mail: [email protected] b

State Key Laboratory of Chemical Resource Engineering, Institute of Science, Beijing

University of Chemical Technology, Beijing 100029, P. R. China. E-mail: [email protected] Abstract Herein, we reported the organocyclophosphazenes (OCP) microspheres with excitation wavelength tunable fluorescence (EWTF) emission by reacting hexachlorocyclotriphosphazene and 4,4'-methylenedianiline via one-pot facile method. The as-prepared microspheres exhibit blue (λex 280 nm), green (λex 365/420 nm) and red (λex 546 nm) tunable fluorescence emissions in the ultraviolet (UV) and visible region. The microspheres have shown broadband semiconductors like absorptions in UV and visible regions indicate the multiple bandgaps in the microspheres. The role of the crosslinked structure and property demonstrated that stable interactions are conceivable in microspheres than branched polymer, which imparted EWTF property. This study explored the cyclotriphosphazene as a platform for the synthesis of EWTF materials, and their potential utilization in the full spectrum tunable fluorescence and other light related technologies. Keywords; Phosphazene; Luminescence; Microspheres.

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1. Introduction Fluorescent microspheres have been applying in the chemical diagnostics, as biomarkers, bioimaging and illumination techniques [1, 2]. Recently, a remarkable concern for the fluorescent materials based on inherently biocompatible and thermally stable cyclophosphazene (CP) has been noted [3-5]. For example, Xiaobin et al. and others have developed fluorescent microspheres with P-O-C or P-NH-C bonds [6] on various concepts for instance, intrinsic fluorescence, [7] aggregation induced emission [8] and by reducing the aggregation induced quenching [9] effect in small organic dyes [6]. However, rational synthesis of fluorescent materials with an excitation wavelength tunable fluorescence (EWTF) is highly challenging and demanding [10]. Basic understanding behind tunable fluorescence is still obscure, and its solution can open new directions for practicle applications [10]. CP is an inorganic heterocyclic ring (P=N) which readily undergoes nucleophilic substitution on P atoms by organic side groups, and produce inorganic-organic hybrids organocyclophosphazene, with an inherent structural stability,

biocompatibility,

and

high

thermal

stability

[4].

The

properties

of

organocyclophosphazene can be precisly tuned by organic sides groups, and allows the fabrication of a wide variety of materials with desired properties [4]. To meet the ever growing demands of polymeric materials with an intriguing intrinsic fluorescent properties, we foucesed on the developing and investigating visible light fluorescent polymeric materials with an intrinsic multiple fluorescence without changing their composition, by introducing small organic group CP. In this regard, we utilized of methylenedianiline (MDA) a simple non-conjugated bifunctional organic compound widely used as a monomer in polymer synthesis [11]. We

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envisioned that MDA could induce unique multiple fluorescence in the resulting crosslinked organocyclophosphazene due to a break in a conjugation and flexible structure [12]. 2. Experimental 2.1 Synthesis of Microspheres For a typical synthesis of the organocyclophosphazenes microspheres, we used molar ratio of hexacholorocyclophosphazene (HCCP): 4,4'-Methylenedianiline (MDA) 1:3; HCCP (0.174 g, 5 × 10-4 mol) and MDA (0.297g, 1.4 × 10-3 mol) dissolved in acetonitrile (50 mL) by ultrasonication (100 W) at room temperature. Afterward, TEA (2 mL) was added, and the reaction mixture was sonicated for 6 h. Yellow precipitates were collected by vacuum filtration, washed with deionized water and ethanol and vacuum dried at 60 °C for 6 h. The conditions optimization to control between the microspheres and the branched polymer are presented in (Table S1, Supporting information). 3. Results and discussion Excitation

wavelength

tunable

fluorescent

(EWTF)

organocyclophosphazene

(OCP)

microspheres were prepared by the reaction of HCCP and 4,4-methylenedianiline (MDA) via facile one-pot precipitation polymerization (Fig. 1). The control over the formation of microspheres and branched polymer (prepared for comparative purpose to understand unusual fluorescence in the crosslinked micropsheres) were achieved by tuning the monomers feed ratios (Table S1). The mechanism of control over branched polymer and microspheres is simple, at aceesive concentration of MDA to the HCCP branched polymer were formed, and were precipitated by the addition of ethanol in the reaction mixture. While at a lower or equivalent molar concetration to HCCP, the microspheres were obtained, followed a typical precipitation 3

polymerization [13, 14]. SEM image (Fig. 2a) revealed that the size of the microspheres ranges between 0.5-2 µm, while TEM image (Fig. 2b) showed that the microspheres are dense with the smooth surface. SEM-EDS mapping (Fig. 2c) of a single microsphere showed all constituent elements, with precise homogenious distribution confirmed the crosslinking reaction between HCCP and MDA. The formation and size tunability of the microspheres by variation in the feed ratios is presented in Fig. S1a-c, S2. The chemical structure and formation of bond characteristic P-NH was analyzed by FT-IR (Fig. 2d). The peak at 938 cm-1 attributes to the P-NH bond formation between HCCP and MDA for both OCP microspheres and branched polymer. The peaks around 3500 cm-1 are attributed to free primary or secondary amine groups. In the EDX survey (Fig. 2e) of the OCP microspheres, the appearance of all constituents elements further reinforcing the results of EDS mapping (Fig. 2c). There are still residual Cl atoms present in the microspheres might be due to the steric hindrance during the crosslinking reaction between cyclotriphosphazene and MDA. For OCP branched polymer, the appearance of single peak

31

P

NMR 2.51 ppm (Fig. S3) and H-NMR (Fig. S3) data confirmed the full substitution of Cl atoms on cyclotriphosphazene. Furthermore, GPC analysis of the typical branched polymer demonstrated that average highest molecular weight is 4366 with a PDI value about 1.5. Fig. 3a-d shows the fluorescence emission of OCP microspheres under different excitation wavelengths. When the OCP microspheres were excited at λex 280 nm, they emitted blue color, and produced dark green emission when excited at λex 365 nm. Furthermore, excitation at visible light of λex 420 nm, they emitted intense green color while their emission color changed to red at λex 546 nm. To testify the origin of emission, we have recorded solid state UV-Vis absorbance spectra (Fig. 3e) which showed semiconductors like broadband absorption (200-600 nm) with high-intensity absorption from 200-310 nm and a smooth decrease in

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intensity of absorption with uplifts around 430 and 550 nm. These absorptions depict the presence of multiple absorbing energetically different states [15]. Furthermore, the emission spectra were recorded (Fig. 3f) by tuning excitation wavelengths to comprehend this behavior. At λex 280, we observed four sharps peaks in emission spectra centered at 340, 380, 400, 430 nm, the multiple emission peaks originated from different emission bandgaps. At λex 365, a broad emission peak (420-700 nm) centered at 517 nm was observed. The tuning of excitation wavelength to the visible region λex 420 nm, a broad emission peak (460-700 nm) centered at 517 nm was observed, when the λex 546 nm, a broad emission peak (580-790 nm) centered at 625 nm was observed. These tunable emission spectral results further confirm the unusual emission colors shown in the Fig. 3a-d. The emission of OCP microspheres red-shifted with an increase of excitation wavelength, and broad emission peaks covered the entire visible region. This unusual fluorescence is were also observed in carbon dots, graphene dots and certain polymers have also been shown such an unsual behavior [12, 16, 17]. To investigate this unusual excitation wavelength tunable fluorescence (EWTF) in OCP microspheres, it is important to understand the fluorescence behavior of branched polymer. Comparative fluorescence data of MDA, the OCP microspheres and branched polymer may provide pathways to understand the fluorescence behavior in the microspheres. The absorption and fluorescence emission of MDA, OCP microspheres and branched polymer was recorded (in solvents) and depicted in Fig. S4ab and Fig. S5a-c. MDA and OCP branched polymer showed the absorption peak centered at 311 nm (Fig. S4a), and OCP microspheres showed absorption peaks at 250 and 288 nm and broad peak of lower intensity in the visible region (Fig. S4b). The absorbance of MDA and branched polymer attributed to n-л* and л- л* transitions, and showed no tunable emission when excited at the different wavelength with an emission maximum at 345 and 350 nm in the UV region (Fig.

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S5ab). While OCP microspheres exhibited excitation wavelength tunable emissions across UV to the visible region (Fig. S5c). The existence of excitation tunable emission attributed to the aggregation-induced emission (AIE), [18] and the presence of different energy levels; [12] as no such emission was observed in OCP branched polymer. The crosslinking in microspheres restrict the rotation of the units, which results in the excitation wavelength tunable emissions [19]. In Fig. 3g the relationship between MDA, OCP branched polymer and microspheres fluorescence emission behavior are summarized. As the excitation wavelength tuned the emission wavelength shifted to higher wavelength in OCP microspheres, while MDA and branched polymer emission observed at a single wavelength in the ultraviolet region. Fig. 3h shows the presence of different bandgaps in OCP microspheres and selective fluorescence emission from these bandgaps contribute to wavelength tunable emissions [15]. The performance under UV lamp λex 365 nm demonstrated green emission, (Fig. 3i) which is also corroborated to the emssion color in Fig. 3b. The multiple fluorescence emssion of the OCP microspheres are also compared with polyphosphazene based lumniscent materials (Table S2, Supporting information). The OCP microspheres have demonstrated unique fluorescence emssions. Hence, the aggregation of atoms, with non-bonding electrons and stacking of benzene rings produced the different bandgaps, which produce multiple fluorescence centers [12]. The selective excitation without transfer of energy, enable them to demonstrate EWTF property. 4. Conclusions Herein, first time we reported the organocyclophosphazenes (OCP) microspheres prepared by reacting HCCP and MDA. The as-prepared OCP microspheres showed the excitation wavelength tunable fluorescence. The emission color of OCP microspheres were tuned by changing the excitation wavelength to blue (λex 280 nm), green (λex 365/420nm) and red (λex 546 nm) with UV 6

to visible fluorescence without changing their composition. We proposed that the excellent tunable fluorescence of the as-prepared microspheres arise from the introduction of nonconjugated methylenedianiline, and the aggregation of both MDA and cyclotriphosphazene units. These microspheres will be potential candidates for applications in solar cells, LEDs and broad band fluorescence sensing. Conflicts of interest The authors declare no conflict of interests. Acknowledgments We acknowledge the National Natural Science Foundation of China for financial support for this research work under Project No. 51773010. References [1] K. Saralidze, L.H. Koole, M.L. Knetsch, Polymeric microspheres for medical applications, Materials, 3 (2010) 3537-3564. [2] K. Gardner, M. Aghajamali, S. Vagin, J. Pille, W. Morrish, J.G. Veinot, B. Rieger, A. Meldrum, Ultrabright Fluorescent and Lasing Microspheres from a Conjugated Polymer, Adv. Funct. Mater., (2018) 1802759. [3] M. Dewar, E. Lucken, M. Whitehead, 490. The structure of the phosphonitrilic halides, J. Chem. Soc., (1960) 2423-2429. [4] S. Rothemund, I. Teasdale, Preparation of polyphosphazenes: a tutorial review, Chem. Soc. Rev., 45 (2016) 5200-5215. [5] X. Li, B. Li, Z. Li, S. Zhang, Self-assembly of nanoparticles from cyclotriphosphazenes grafted with hexa-[p-(carbonyl tryptophan ethyl ester) phenoxy)] group, RSC Advances, 2 (2012) 5997-6004. [6] C. Wan, X. Huang, Cyclomatrix polyphosphazenes frameworks (Cyclo-POPs) and the related nanomaterials: Synthesis, assembly and functionalisation, Mater. Today Commun., 11 (2017) 38-60. [7] S.U. Dar, S. Ali, M.U. Hameed, Z. Zuhra, Z. Wu, A facile synthesis, structural morphology and fluorescent properties of cross-linked poly (cyclotriphosphazene-co-1, 3, 5-tri (4-hydroxyphenyl) benzene) hybrid copolymer microspheres, New J. Chem., 40 (2016) 8418-8423. [8] L. Meng, C. Xu, T. Liu, H. Li, Q. Lu, J. Long, One-pot synthesis of highly cross-linked fluorescent polyphosphazene nanoparticles for cell imaging, Polymer Chemistry, 6 (2015) 3155-3163. [9] X.-M. Hu, Q. Chen, D. Zhou, J. Cao, Y.-J. He, B.-H. Han, One-step preparation of fluorescent inorganic– organic hybrid material used for explosive sensing, Polymer Chemistry, 2 (2011) 1124-1128.

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[10] Z. Gan, H. Xu, Y. Hao, Mechanism for excitation-dependent photoluminescence from graphene quantum dots and other graphene oxide derivates: consensus, debates and challenges, Nanoscale, 8 (2016) 7794-7807. [11] E.K. Gibson, J.M. Winfield, K.W. Muir, R.H. Carr, A. Eaglesham, A. Gavezzotti, D. Lennon, A structural and spectroscopic investigation of the hydrochlorination of 4,4′-methylenedianiline, Phys Chem Chem Phys, 12 (2010) 3824-3833. [12] K.M. Lee, W.Y. Cheng, C.Y. Chen, J.J. Shyue, C.C. Nieh, C.F. Chou, J.R. Lee, Y.Y. Lee, C.Y. Cheng, S.Y. Chang, Excitation-dependent visible fluorescence in decameric nanoparticles with monoacylglycerol cluster chromophores, Nature Communications, 4 (2013) 1544. [13] M. Basharat, W. Liu, S. Zhang, Y. Abbas, Z. Wu, D. Wu, Poly(cyclotriphosphazene-co-tris(4hydroxyphenyl)ethane) Microspheres with Intrinsic Excitation Wavelength Tunable Multicolor Photoluminescence, Macromolecular Chemistry and Physics, 0 1900256. [14] Y. Abbas, Z. Zuhra, M. Basharat, M. Qiu, Z. Wu, D. Wu, S. Ali, Morphology Control of Novel CrossLinked Ferrocenedimethanol Derivative Cyclophosphazenes: From Microspheres to Nanotubes and Their Enhanced Physicochemical Performances, The Journal of Physical Chemistry B, 123 (2019) 4148-4156. [15] S. Kim, K. Lee, S. Kim, O.-P. Kwon, J.H. Heo, S.H. Im, S. Jeong, D.C. Lee, S.-W. Kim, Origin of photoluminescence from colloidal gallium phosphide nanocrystals synthesized via a hot-injection method, RSC Advances, 5 (2015) 2466-2469. [16] Q. Xu, Q. Zhou, Z. Hua, Q. Xue, C. Zhang, X. Wang, D. Pan, M. Xiao, Single-Particle Spectroscopic Measurements of Fluorescent Graphene Quantum Dots, Acs Nano, 7 (2013) 10654-10661. [17] Y. Dong, J. Shao, C. Chen, L.I. Hao, R. Wang, Y. Chi, X. Lin, G. Chen, Blue luminescent graphene quantum dots and graphene oxide prepared by tuning the carbonization degree of citric acid, Carbon, 50 (2012) 4738-4743. [18] C. Shang, N. Wei, H. Zhuo, Y. Shao, Q. Zhang, Z. Zhang, H. Wang, Highly emissive poly(maleic anhydride-alt-vinyl pyrrolidone) with molecular weight-dependent and excitation-dependent fluorescence, Journal of Materials Chemistry C, 5 (2017) 8082-8090. [19] H. Nie, K. Hu, Y. Cai, Q. Peng, Z. Zhao, R. Hu, J. Chen, S.-J. Su, A. Qin, B.Z. Tang, Tetraphenylfuran: aggregation-induced emission or aggregation-caused quenching?, Mater. Chem. Front., 1 (2017) 11251129.

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Fig. 1. Schematic illustration of organocyclophosphazene (OCP) microspheres synthesis and their performances under different lights.

Fig. 2. (a) SEM image (b), TEM image (c), SEM-EDS mapping (d), comparative FTIR spectra and (e) EDX survey and elemental composition of the OCP microspheres.

Fig. 3. Fluorescence colors at (a) UV 280 nm, (b) UV 365 nm, (c) blue light, (d) greenlight, (e) UV-Vis absorbance spectra, (f) emission spectra at different excitations wavelengths, (g) 9

relationship among MDA, OCP microspheres and branched polymer excitation and emission, (h) bandgap calculation through extra-polation (i) OCP microspheres under UV Lamp 365 nm.

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Highlights: •

A novel morphology controllable fluorescent microsphere based on CPs were synthesized.

•

EWTF properties of the microspheres were achieved and the influence of crosslinkedstructures was investigated.

•

The mechanism of introducing non-conjugated structures attribute to multiple fluorescence emission due crosslinking was proposed.

Declaration of conflict of interest The authors declare no conflict of interest.