Constructing Diketopyrrolopyrrole-Based Fluorescent Porous Organic

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Constructing Diketopyrrolopyrrole-Based Fluorescent Porous Organic Polymer for Chromo Communication via Guest-to-Host Energy Transfer Shiming Bi, Yankai Li, Limin Wang, Jun Hu, and Honglai Liu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b12534 • Publication Date (Web): 09 Mar 2017 Downloaded from http://pubs.acs.org on March 11, 2017

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The Journal of Physical Chemistry

Constructing Diketopyrrolopyrrole-Based Fluorescent Porous Organic Polymer for Chromo Communication via Guest-to-Host Energy Transfer Shiming Bi,‡ † Yankai Li,‡ § Limin Wang,* † Jun Hu,* § and Honglai Liu§ †

Key Laboratory for Advanced Materials and Department of Chemistry, East China University

of Science and Technology, Shanghai, 200237, China. §

State Key Laboratory of Chemical Engineering and Department of Chemistry, East China

University of Science and Technology, Shanghai, 200237, China.

ABSTRACT: To obtain solid scaffolds exhibiting brilliant solid state fluorescence and serving as an arena for chromo communication via host-guest energy transfer, a series of diketopyrrolopyrrole moieties with different modified bulky groups were coupled into the porous polymeric networks via Sonogashira coupling reaction. Variation of the bulky substitution groups rendered the tunablity of porosity. With the improved porosity, it triggered the spatial isolation of the fluorophores and further enhanced the solid-state fluorescence of these polymers. More importantly, the brilliant nanopores can also serve to confine guest molecules to form a donor-acceptor system via energy transfer. After loading coumarin, outstanding energy transfer efficiency, was observed based on the calculation upon fluorescence decay measurements,

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spectral overlap function and the estimation of Föster radius. This study provided important insights of designing novel fluorescent POPs with efficient energy transfer flows.

Introduction The past few years have witnessed the drastically growing research focusing on porous organic polymers (POPs), since they lie at a crossroad of a wide range of applications, such as gas storage1, heterogeneous catalysis2 and hazards removal3. As an emerging organic-based material, POPs knit rigid monomers into three-dimensional crosslinking scaffolds, which give rise to impressive benefits of chemical and thermal stability, as well as intrinsic properties of abundant inner nanopores4-8. Moreover, versatility of available monomers renders feasible and easy functionalization upon POPs9-12. Specifically, a great number of fluorescent POPs were reported in recent years13-16, in which the photofunctionalization was mainly accomplished by the wide-extended conjugated structure (also known as CMPs)5,17-20 or the usage of peculiar aggregation-induced emission (AIE) moieties21-24. As the flexibility of polymerization facilitates fine manipulation of porosity as well as chemical nature of POPs25-31, there remains great potency to develop distinct strategies to exploit novel fluorescent POPs. In order to develop POPs with solid-state fluorescence, the intrinsic porosity of POPs could be designed to spatially isolate the fluorophores and avoid fluorophore stacking. It is an efficient way to circumvent the “aggregation caused quenching” (ACQ) effect32 and implement the strong fluorescence in solid state. The successful manipulation of the fluorescence for POPs in our previous work was accomplished by copolymerization of fluorescent dyes of 4diketopyrrolo[3,4-c]-pyrrole (DPP) into POPs33. Different from the pioneer work of DPP-based POPs by Skabara 34, in which the fluorescence intensity was arose from widely extended scaffold


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(CMPs) and irrelevant to the porous nature, our strategy actually employed the porosity-based spatial effect to contribute the emission. However, the content of fluorephores should be carefully controlled in our previous work, because of the presence of a trade-off between the enhancement of the porosity (associated with the fluorescence) and the density of fluorophores in the network. As a result, developing a system to intensify fluorescence without the deterioration of fluorophores content remains a challenge. Apart from breeding the spatial isolation of the fluorophores in networks of POPs, the nanopores within the POPs can also serve to confine guest molecules22, 35-36. A donor-acceptor system for energy transfer can be formed when the light emission of guest molecules precisely match the absorbance of POPs network. Such combination of various light-absorbing moieties give rise to absorption union with extended coverage of light bands, which is in favor of effective energy utilization process37-38. Mimics of natural light harvesting system have attracted growing attention involving the funnelling of excitation energy through predesigned cooperative fluorophore communication39-41. Recently, a few POPs and their inorganic counterpart (MOFs) have been proved to be efficient mediators for light energy utilizations, by means of energy transfer triggered through delicate spatial arrangement of fluorophores22, 42-43. For example, πelectronic framework PP-CMP can efficiently transfer energy to guest molecule coumarin 6, accompanied with almost complete quenching of the fluorescence of PP-CMP network and strong green emission of coumarin 635. Typically, rational design of crystalline MOFs consisting ensemble of fluorophores with either ligand-ligand or host-guest energy transfer leads to outstanding energy transfer efficiency up to 65% and 72%, respectively44. However, the number of reported light-harvesting ensembles still remains limited, there is a wide scope in fabricating novel materials with excellent energy transfer efficiency.


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Herein, we designed POPs with strong fluorescence by coupling modified DPP moieties, and further exploited its potency for artificial light-harvesting arrays. As the side substituents on the building blocks have largely influenced the porosity, we anticipated that the rational design of the modified groups attached on the skeletons of POPs would result in specific functional performances. The porosity was either utilized to be the spatial void to avoid intermolecular aggregation of fluorophores in the skeleton or served to confine guest molecules to form donoracceptor systems. As a result, the nanopores within POPs network can perform as versatile arena, facilitating the design of novel materials. Experimental section Materials and methods. Unless otherwise stated, reagents were commercially obtained and used without further purification. All solvents were freshly distilled: THF from Na, toluene from CaH2, triethylamine from KOH. Anhydrous DMF were purchased. Reactions were monitored by TLC. Flash chromatography separations were carried out using silica gel (200-300 mesh). 1H NMR and

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C NMR spectra were collected on a Bruker Avance DPS-300 spectrometer using

CDCl3 as solvent and tetramethylsilane as internal reference, respectively. Mass Spectrometry was performed by Bruker Biflex III MALDI-TOF (both positiveand negative ion reflector mode). Solid state 13C NMR spectra were collected on Brukeravance III and tetramethylsilane as internal reference. Thermogravimetry analysis (TGA) were performed under N2 on a NETZSCH STA449F3, with a heating rate of 10 oC min-1. Nitrogen adsorption isotherms were measured at 77 K using Micromeritics ASAP 2020 static volumetric analyzer. Before adsorption measurements the polymer was degassed at 100 oC under vacuum. The Brunauer-Emmett-Teller (BET) surface area was calculated within the relative pressure range 0.05 to 0.30. Total volume was calculated at p/p0=0.98. Fluorescence spectrum was conducted on Hatachi F4500


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fluorescence spectrophotometer at room temperature. FTIR data were obtained using a Nicolet Magna-IR 550 spectrometer. Elemental analysis was determined using a Vario EL III Elemental Analyzer (Elementar, Germany). Field-emission scanning electron microscope (FESEM) images were taken by Nova Nanosem 450. Transmission electron microscopy (TEM) images were taken by JEOL JEM-2100 operating at 200 KV. Fluorescence microscope images are taken by inverted fluorescence microscope from carlzeiss (Axio Observer A1).Time-resolved fluorescence spectroscopy of solid samples are measured by Edinburgh Instrument (Model FLS 920) with a PMT detector (R928P Hamamatsu). Synthesis of DPP-PPN-m. The POPs (DPP-PPN-m) shown in scheme 1 were synthesized via Sonogashira coupling reaction using tetrakis(4-ethynylphenyl)methane (1 equiv) and the appropriate N,N’-disubstituted-3,6-Bis-(4-bromo-phenyl)-2,5-dihydro-pyrrolo[3,4-c]pyrrole-1,4dione

(Br-DPP-m)

(2

equiv)

in

toluene/triethylamine(1:1,

v/v)

at

80

o

C

using

Palladium(0)tetrakis(triphenylphosphine) (0.14 mol%) and copper(I) iodide (0.26 mol%) as catalysis for 3d. After cooling to room temperature the precipitated polymers were isolated by filtration over a Büchner funnel and washed with 1 M hydrochloric acid (20 mL), followed with excess methanol, tetrahydrofuran and dichloromethane. Then the product was further extracted with dichloromethane by Soxhlet apparatus for 24 h. The solvent was removed under vacuum at room temperature to afford the DPP-PPN-m. The detailed synthesis and characterization of the DPP-PPN-m can be found in the ESI. Synthesis of courmarin@DPP-PPN-3. The 2-cyano-7- diethylamincoumarin molecules physically confined in the nanopores of DPP-PPN-3 to afford courmarin@DPP-PPN-3 via stirring the mixture of DPP-PPN-3 (10 mg) and 2-cyano-7- diethylamincoumarin (6.6 mg) in DMF at 80 oC for 24 hours. After cooling to room temperature the precipitated polymers were


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isolated by filtration over a Büchner funnel washed with excess DMF. The solvent was removed under vacuum to afford the material. Results and discussion As illustrated in Scheme 1, DPP monomers were pre-modified by grafting various substituents with different steric hindrances on the N sites of the pyrrole rings. The design of DPP monomers exhibited the competence to adjust the porosity of resulting POPs, so that the regulation of porosity can be realized with the enhanced density of fluorephores compared to our previous work. A series of DPP-PPN-m POPs were obtained via the Sonogashira coupling polymerization between Br-DPP-m (m=1~4) and tetrakis(4-ethynylphenyl)methane (TEPM), respectively, in which TEPM has been recognized as a preferable building block for constructing POPs with large BET surface area1. Scheme 1. Structural formulae of building blocks and preparation of DPP-PPN-m (m=1,2,3,4)


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The obtained DPP-PPN-m POPs were insoluble in common solvents. Scanning and transmission electron microscopy (SEM and TEM) images (Fig. 1a-h) revealed they were almost consisted of hollow tubes. The tubes of DPP-PPN-1 and DPP-PPN-2 were long and smooth, with the inner diameter of about 40 nm; whereas, for DPP-PPN-3 and DPP-PPN-4, the tubes were much shorter and aggregated together, so the internal structure was not clearly observed due to the difficulty for electron beams to transmit. Interestingly, the high magnification of TEM graphs revealed the amorphous porosity nature of DPP-PPNs (Figure 1e-h, inset). As illustrated in thermogravimetric analysis profiles (Fig. S1, ESI), DPP-PPN-1 began to degrade when the temperature rose to 275 oC, whereas DPP-PPN-2, DPP-PPN-3 and DPP-PPN-4 exhibited slightly lower thermo decomposition temperature (around 180 oC) due to the presence of ester groups. The chemical structures of DPP-PPN-m were analyzed by Fourier transform infrared (FTIR) spectroscopy (Fig. S2, ESI), in which the band around 2210 cm-1 and 1680 cm-1 were attributed to stretching vibration of alkynyl linkages and amide groups within DPP moieties, respectively. The bands around 1745 cm-1, which corresponded to stretching vibration of C=O bonds within ester groups, were observed for the spectra of DPP-PPN-2, DPP-PPN-3 and DPP-PPN-4. Furthermore, various ester groups could be differentiated by distinct C-O vibration at 1205 cm-1 for DPP-PPN-2, 1229 cm-1 for DPP-PPN-3 and 1200 cm-1 for DPP-PPN-4, respectively. The chemical structures of DPP-PPN-m were further confirmed by solid state 13C NMR spectra (Fig. 2). For all these polymers, the resonances at around 90 ppm corresponded to the triple bond linkages and the peaks around 160 ppm were ascribed to carbonyl carbon of DPP moieties. Besides, different substituents can be easily distinguished in

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C NMR spectra. The ethyl group

of DPP-PPN-1 was related to the resonances at 37 and 14 ppm. For the others, the emergence of peaks around 169 ppm indicated the existence of ester carbonyl group, whereas various ester


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groups could be differentiated (62, 14 ppm for ethyl ester in DPP-PPN-2; 82, 28 ppm for tertbutyl ester in DPP-PPN-3; 149, 129, 127, 33, 31 ppm for 4-tert-butylphenyl ester in DPP-PPN-4).

Figure 1. Scanning electron microscopy images of DPP-PPN-1 (a), DPP-PPN-2 (b), DPP-PPN-3 (c) and DPP-PPN-4 (d), scale bar: 5 µm and transmission electron microscope (TEM) images of DPP-PPN-1 (e), DPP-PPN-2(f), DPP-PPN-3 (g) and DPP-PPN-4(h), Inset: TEM images at a high magnification


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Figure 2. Solid state

13

C NMR spectra of DPP-PPN-1(a), DPP-PPN-2(b), DPP-PPN-3(c) and

DPP-PPN-4(d) DPP-PPN-m were red powders and exhibited quite similar light absorbance behaviors of considerately strong and broad bands from 200 nm to 600 nm, as suggested by UV-vis diffuse reflectance spectra (Fig. 3e). Interestingly, the solid state fluorescence properties of DPP-PPN-m were highly different. Upon excitation at 470 nm, DPP-PPN-3 exhibited the strongest emission centered at 635 nm; whereas the fluorescence intensity drastically decreased in sequence of DPPPPN-3, DPP-PPN-2 and DPP-PPN-1 (Fig. 3f). This phenomenon could be also intuitively seen from their fluorescence microscope images (Fig. 3a-c). The various fluorescence intensity of DPP-PPN-m (m=1,2,3,4) might not be ascribed to charge or electron transfer effect between distinct substitutions and DPP fluorophores, because the absence of conjugated linkages between them. To approve this speculation, fluorescence spectra of monomers Br-DPP-PPN-m were measured (Fig. S3. ESI), which suggested almost the same emission intensity for them. As a


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result, there remained other factors rather than interactions of these substitutions that influence the emission of DPP-PPN-m dramatically.

Figure 3. Fluorescence microscope images of (a) DPP-PPN-1, (b) DPP-PPN-2, (c) DPP-PPN-3, and (d) DPP-PPN-4, scale bar: 200 µm; (e) UV-vis diffuse reflectance spectra of DPP-PPN-m, (f) Solid state fluorescence spectra of DPP-PPN-m, λexitation= 470 nm. To take an insight into the mechanism of the fluorescence trend mentioned above, we made a comparative analysis between fluorescence quantum yield and BET surface area for DPP-PPN-m. N2 adsorption isotherms at 77 K indicated the quite different porosity of them (Fig. 4a and Table 1). There was a dramatic increasing trend of BET surface area in sequence of DPP-PPN-1, DPPPPN-2 and DPP-PPN-3, from 27.8 to 441.6 m2 g-1. For DPP-PPN-1, the flat DPP moieties had the tendency to pack tightly via π-π interactions and created interpenetrated networks, hence the poor porosity23. For DPP-PPN-2, the bulky substitution of ethyl ester in DPP moiety disrupted


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the intermolecular stack of fluorophores, and correspondingly, avoided the interpenetration of networks and enhanced the porosity. The grafted tert-butyl ester of DPP-PPN-3 was even more bulky stereo substitution, which enhanced the porosity more efficiently. Remarkably, the porosity acted on the fluorescence behaviour of DPP-PPN-m (m=1,2,3) significantly. For almost non-porous DPP-PPN-1, the intimate stack of DPP fluorophores led to ACQ effect and relatively weak emission. As the BET surface area grew, the DPP fluorophores began to be spatially isolated from each other and the fluorescence was gradually liberated from the ACQ effect. Time resolved decays (Fig. S6, ESI) for these polymers suggested a shorter lifetime of DPP-PPN-1 (0.541 ns) than those of DPP-PPN-2 and DPP-PPN-3 (0.923 and 1.085 ns), respectively. The reconvolution fit of the case of DPP-PPN-1 revealed a triexponential decay model whereas those of DPP-PPN-2 and DPP-PPN-3 indicated biexoponential decay models, which revealed the occurrence of ACQ effect for DPP-PPN-1 as well as the extrication of ACQ effect for others. The data plots of fluorescence quantum yield and BET surface area also exhibited correlated incremental trend (Fig. 4b), which certified the strategy of “porosity-induced emission” presented in our previous work. Notably, the strategy of tuning the porosity in this work, which is implemented by the substituents of DPP monomers was superior to the one implemented by copolymerization technique in our previous work33, for the former can accomplish strong solidstate fluorescence without compromising the density of DPP moieties.


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Figure 4. a) N2 adsorption isotherms of DPP-PPN-m under 77 K; b) Comparative analysis of data plots between quantum yield and BET surface area for DPP-PPN-m; c) Schematic representation of “porosity-induced emission” strategy

Table 1 Porous nature and fluorescence quantum yield of DPP-PPNs BET Total pore surface volume area (m2 g- (cm3 g-1) 1 )

Micropore volume (cm3 g-1)

Dominant pore size (nm)

Fluorescenc e quantum yield (%)

DPP-PPN-1

27.8

0.1267

0.0009

1.3, 1.7

3.59

DPP-PPN-2

169.5

0.1668

0.0583

0.7, 0.8, 1.1, 1.3

8.19

DPP-PPN-3

441.6

0.3263

0.1750

0.6, 0.8, 1.1, 1.5

18.54

DPP-PPN-4

180.8

0.1799

0.0645

0.8, 1.1, 1.3, 1.8

13.89


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Inspired by the tunable porosity and fluorescence intensity of DPP-PPN-m (m=1,2,3) with various substitutions on DPP moieties, we anticipated that further enhancement would be possible by the employment of even larger group of 4-tert-butylphenyl ester. To our surprise, the BET surface of DPP-PPN-4 did not increase, instead, the value was substantially lower (180.8 m2 g-1) than that of DPP-PPN-3 (441.6 m2 g-1). The observed poorer porosity might ascribe to two factors. Firstly, the larger substitutions would block the pore volume and restrict the accessibility within the pores45. Secondly, the fully polymerization of Br-DPP-4 and TEPM might be impaired by the excessively large substitution group, as well as the low solubility of BrDPP-4. These unfavourable nature of Br-DPP-4 impeded the further connection of additional TEPM monomers, consequently, DPP-PPN-4 was obtained as a less crosslinking and less polymeric network with the inclination of pore-collapse29. The decrease of BET surface or the possibility of pore-collapse for DPP-PPN-4 rose the liability of fluorophore aggregation, hence the decrease of fluorescence quantum yield, from 18.5 % for DPP-PPN-3 to 13.9 % for DPPDPP-4. The properties of DPP-PPN-4 manifested that the existence of upper limitation of size of substituents to achieve the optimized porosity of resulting POPs. More significantly, the trend of the fluorescence quantum yield was still correlated to the variation of BET surface area (Fig. 4b), further suggesting the relationship between porosity and fluorescence intensity mentioned above.


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Figure 5. a) Schematic representation of the donor-acceptor system; b) UV-vis diffuse reflectance spectrum of DPP-PPN-3 (purple dash line), emission spectrum of coumarin (solid cyan line) and emission spectrum of DPP-PPN-3 (solid orange line); c) emission spectrum of coumarin@DPP-PPN-3, inset: image of solid coumarin@DPP-PPN-3 under radiation of 365 nm by a portable UV lamb; d) Fluorescence decays of coumarin (donor lifetime) and coumarin@DPP-PPN-3 (donor lifetime in the presence of acceptor). In addition to breed the spatial isolation of fluorophores, the nanopores of these POPs also rendered the loading of dye molecules to deliver donor-acceptor system via guest-to-host energy transfer. Among numerous dyes, we chose 2-cyano-7-diethylamincoumarin as the guest molecule, because its emission precisely matched with the adsorption of DPP-PPNs. The coumarin dye molecules were physically confined in the nanopores of DPP-PPN-3 to afford courmarin@DPP-PPN-3 via stirring the mixture of DPP-PPN-3 and coumarin in DMF at 80 oC for 24 hours. We optimized the operation conditions to make coumarin saturated in DPP-PPN-3, and hence to obtain the highest energy transfer efficiency. The loading amount of coumarin was estimated as 23.4 mol% (coumarin/DPP moieties) through the standard absorption curve of


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coumarin dyes. Photophysical characteristics were conducted to certify the occurrence of energy transfer process (Fig. 5b-c). The emission of coumarin molecules (donor) was in the range of 440-550 nm with λmax=464 nm. Remarkably, the absorption band of DPP-PPN-3 exhibited a wide coverage up to 600 nm, providing the necessity of spectral overlap of acceptor’s absorbance and donor’s emission. Coumarin@DPP-PPN-3 emitted strong red emission with its spectral profile centered at approximate 620 nm. Remarkably, in the fluorescence spectrum of coumarin@DPP-PPN-3, the almost complete disappearance of donor emission (470 nm) suggested the efficient energy transfer process that funnelling photons from wide wavelength coverage to the emission band of DPP-PPN-3 skeleton. To quantitative analyze the energy transfer efficiency, time-resolved fluorescence decay measurements were conducted (Fig. 5d). The donor lifetime in the presence and absence of acceptors, which were examined within the donor emission range, were used to calculate the energy transfer efficiency (ΦET). The fluorescence decay of confined coumarin in DPP-PPN-3 (with acceptors) was characterized to be a more rapid decrease profile than that of pristine coumarin (without acceptors), with corresponding average lifetime drastically dropped from 2.38 ns to 0.82 ns. Accordingly, the ΦET value was calculated to be 67 %. Furthermore, we also calculated the spectral overlap function (J) and Förster critical radius (R0) to address the possibility of energy transfer. The J value was obtained from the experimental donor emission and acceptor absorption to be 6.25×1014 M-1cm1

nm4 (Fig. S8. ESI). Förster critical radius (R0) value was further calculated to be 31 Å from the J

value, which is far beyond the host-guest distance considering the dominant pore diameters (6, 8 and 11 Å). Therefore, taking the fluorescence data and calculated J and R0 values as account, we can certify the presence of efficient energy transfer process between DPP-PPN-3 and confined coumarin.


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Conclusion A series of fluorescent POPs were fabricated by coupling DPP derivatives and TEPM via Sonogashira reaction. The BET surface area of POPs can be tuned by varying the size of substitutions on DPP moieties. Remarkably, we discovered that the growing BET surface area gave rise to the enhancement of solid-state fluorescence, since the presence of nanopores can breed the spatial isolation of DPP moieties to circumvent the ACQ effect. Therefore, delicate design of the bulky substitution group of DPP monomers can afford POPs with optimized porosity and fluorescence behaviour. In addition to avoiding the intermolecular aggregation of DPP moieties, the nanopores can also provide the confinement for guest coumarin dyes to form a donor-acceptor system. The emission of confined coumarin matched well with the absorbance of DPP-based POPs, hence the efficient energy transfer process. ΦET value was calculated to be as high as 67 %, based on the fluorescence decay data, the spectral overlap function and Förster critical radius. Thus, the size of nanopores in POPs was larger enough to prevent the fluorophores from the ACQ effect, while still within the threshold that allowing for efficient fluorophore communications. Our work demonstrated the usage of POPs to design novel materials with excellent solid-state fluorescence and their capacity to confine guest dyes to form light-harvesting arrays with outstanding energy transfer efficiency.

ASSOCIATED CONTENT Supporting Information. Details of synthesis of monomers, fluorescence spectra of monomers; TGA, FTIR spectra, fluorescence decay profiles of DPP-PPNs; Förster analysis of


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coumarin@DPP-PPN-3 are available in supporting information. This material is available free of charge via Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * Liming Wang, E-mail: [email protected] * Jun Hu, E-mail: [email protected]

Tel: +86 21 64252630

Author Contributions ‡ These authors contributed equally to this work and should be considered co-first authors. ACKNOWLEDGMENT The work was supported by the National Nature Science Foundation of China (No. 21272069, 20672035, 91334203, 21376074), the National Basic Research Program of China (2013CB733501), the Key Laboratory of Organofluorine Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, the 111 Project of China (No. B08021) the Fundamental Research Funds for the Central Universities and the project of FP7-PEOPLE-2013IRSES (PIRSES-GA-2013-612230). Notes The authors declare no competing financial interest. REFERENCES (1) Lu, W.; Yuan, D.; Zhao, D.; Schilling, C. I.; Plietzsch, O.; Muller, T.; Bräse, S.; Guenther, J.; Blümel, J.; Krishna, R., et al. Porous Polymer Networks: Synthesis, Porosity, and Applications in Gas Storage/Separation. Chem. Mater. 2010, 22, 5964-5972.


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Constructing Diketopyrrolopyrrole-Based Fluorescent Porous Organic

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