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Switching on room-temperature phosphorescence of photochromic hybrid heterostructures by anion-π interactions
Journal Pre-proof Switching on room-temperature phosphorescence of photochromic hybrid heterostructures by anion-π interactions Ming-Hua You, Meng-Hua Li, Yi-Ming Di, Yi-Wen Wang, Mei-Jin Lin PII:
S0143-7208(19)32181-3
DOI:
https://doi.org/10.1016/j.dyepig.2019.107943
Reference:
DYPI 107943
To appear in:
Dyes and Pigments
Received Date: 15 September 2019 Revised Date:
30 September 2019
Accepted Date: 30 September 2019
Please cite this article as: You M-H, Li M-H, Di Y-M, Wang Y-W, Lin M-J, Switching on roomtemperature phosphorescence of photochromic hybrid heterostructures by anion-π interactions, Dyes and Pigments (2019), doi: https://doi.org/10.1016/j.dyepig.2019.107943. 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.
Graphical Abstract
Due to the compact contacts induced by the charge-assisted anion-π interactions, not only the photochromic speeds of three D-A hybrid heterostructures have been enhanced, but more interestingly their RTP emissions of have been switched on.
Switching on room-temperature phosphorescence of photochromic hybrid heterostructures by anion-π interactions Ming-Hua Youa, Meng-Hua Li a, Yi-Ming Di,a Yi-Wen Wang,a and Mei-Jin Lina* a
State Key Laboratory of Photocatalysis on Energy and Environment, and Key Laboratory of Molecule Synthesis and Function Discovery,
College of Chemistry, Fuzhou University, China, 350116. E-mail: [email protected].
Abstract: Room-temperature phosphorescence (RTP) has attracted much attention in the past decades due to its potential applications in optoelectronic devices and bioimaging. So far, most available RTP materials are pure inorganic or organic compounds, few are involved in the organic-inorganic hybrid complexes. The donor-acceptor (D-A) hybrid heterostructures are an emerging class of organic-inorganic hybrid complexes composed of semiconductive organic and inorganic tectons at the molecular level. For these unique hybrids, the primary properties are photochromism and photoinduced electron transfers. Herein, we have demonstrated that the combination of naphthalene diimide (NDI) tectons bearing two divergently oriented pyridyl protons with three polyoxometalate (POM) anions (SiW12O404-, PW12O403- or PMo12O403-) resulted in three isostructural D-A hybrid heterostructures with an alternate arrangement of segregated POM anions and 1-D H-bonded NDI networks as electron donors and acceptors, respectively. Due to the compact contacts between POM anions and NDI tectons induced by charge-assisted anion-π interactions, not only the photochromic speeds of three D-A hybrid heterostructures have been enhanced, but more interestingly their RTP emissions of have also been switched on, and their photoluminescence quantum yields are dependent on anion-π interaction strengths. Keywords: Room-temperature phosphorescence, Naphthalene diimides, Polyoxometalates, Photochromism, D-A heterostructures
1. Introduction Phosphorescent materials, particularly room-temperature phosphorescent (RTP) materials with long emission lifetimes arising from the long-lived intersystem crossing (ISC) have attracted considerable attention due to their
-1-
applications in optoelectronics,
[1]
chemical sensing,
rare-earth metal cation doped inorganic materials, aromatic organic molecules.
[6]
[4]
[2]
and bio-imaging.
[3]
So far, most available RTP materials are
noble metal (e.g. Ir, Ru and Os) organometallic materials
[5]
and
In these materials, RTP has mainly been achieved through the incorporation of heavy
atoms into the photosensitive systems or the establishment of intra- or intermolecular charge-transfer (CT) states to bridge the lowest local-excited states of photosensitive systems.
[7]
The incorporation of the heavy atoms into the
photosensitive systems affords the significant spin-orbital coupling (SOC), and thus enhances the efficiency of ISC, which is a simple but promising strategy to achieve long-lived RTP.
[8]
On the other hand, the installation of intra- or
inter-molecular CT states is more elaborate, which always combines both donor and acceptor components into photosensitive systems through the covalent bonds or non-covalent interactions, such as hydrogen bonds, π-π stacking and C-H⋅⋅⋅π interactions. [9] Currently it is of high interest to explore new CT RTP materials with high quantum yield, low cost and convenient operation through new non-covalent interactions. Anion-π interaction
[10]
is an emerging non-covalent force between the electron-deficient π-systems and anions,
which has potential applications in chemical sensing, [11] catalysis [12] and anion transport. [13] Recently, other groups [14] and us
[15]
have utilized such unique force for the construction of D-A hybrid heterostructures, whose semiconductive
organic and inorganic components are fabricated at the molecular level.
[16]
For such unique hybrid materials, the
primary properties of interest are photoinduced electron transfers (PIETs) and their resulting photochromism. Herein, we report our efforts towards the employ of anion-π interactions for novel highly emissive D-A hybrid heterostructures, which is anticipated to have the advantages of photochromism and RTP combined. Thus, three isostructural hybrid heterostructures 1-3 with an alternate arrangement of segregated POM anions and 1-D H-bonded NDI networks as electron donors and acceptors (Fig. 1), which have been obtained by the combination of N, N’-dipyridyl naphthalene
-2-
diimide (DPyNDI) and three POM anions, SiW12O404-, PW12O403- or PMo12O403-, in presence of a small amount of HI. We like to note that within the time frame of our project the similar hybrid heterostructure 4 with an unexpected RTP phenomenon has been reported by Lu and co-workers.
[17]
However, in Lu’s work, the quantum yield was rather low
(1.5 %) due to the weak anion-π interactions in the heterostructure. To enhance the anion-π interactions, the protons with simple H-bonds instead of metal ions with sterically hindered coordination groups were used. As expected, owing to the compact contacts between the POM and NDI components induced by charge-assisted anion-π interactions, besides photochromism, the RTP emissions of three isostructures have been switched on, and the photoluminescence quantum yields are dependent on their anion-π interaction strengths.
2. Experimental 2.1 Materials and measurements N-methylpyrrolidin-2-one
(NMP),
polyoxometalates (POMs,
including H4SiW12O40,
H3PW12O40,
and
H3PMo12O40), hydrogen iodide (HI, aqueous) and acetonitrile (MeCN) were obtained from commercial suppliers. All chemicals and reagents were used as received unless otherwise stated. N, N’-dipyridyl naphthalene diimide (DPyNDI) was obtained following the reported process.
[18]
The infrared spectra have been measured in the range of 400-4000
cm-1 by the use of a Perkin-Elmer FT-IR spectrophotometer. Powder X-ray diffraction (PXRD) patterns have been recorded on a Rigaku MiniFlex-II X-Ray diffractometer using Cu-Kα radiation. UV-Vis diffuse reflectance spectra were recorded at room temperature on a Varian Cary 500 UV-Vis spectrophotometer equipped with an integrating sphere. Photoluminescence spectra and lifetimes at room temperature were recorded on an Edinburgh FLS 980 luminescence spectrometer, while the photoluminescence quantum yield was measured on a Hamamatsu C11347−11 absolute luminescence quantum yield measurement system equipped with an integrating sphere apparatus and a 150 W
-3-
CW Xenon light source. Photochromic reaction was agitated using a continuous wavelength-irradiation by Xenon lamp (300W, 420-780 nm). Electron paramagnetic resonance (EPR) measurements were obtained using a Bruker A300 instrument operating in the X-band at the room temperature. 2.2 Synthesis Syntheses of hybrid complexes 1-3: A solution (0.5 mL) of NMP/MeCN (1:1, v/v) was carefully layered over a NMP (5 mL) mixture solution of DPyNDI (0.1mmol) and a small amount of HI (ca. 1 drop), then MeCN solution (5 mL) of H4SiW12O40 (H3PW12O40 or H3PMo12O40) (0.025mmol) was carefully added as a third layer. After standing in the dark for several days, suitable crystals of 1-3 for single-crystal X-ray diffraction analyses were collected, which were washed with mother liquid and then MeCN, and dried at ambient temperature. Hybrid complex 1: light yellow crystal, yield: 56% based on DPyNDI. IR data (KBr, cm-1): 1715(w), 1679(m), 1632(m), 1506(w), 1446(w), 1435(w), 1304(w), 1249(w), 1187(w), 1119(w), 1008(w), 965(w), 912(w), 878(w), 778(w), 748(w), 602(w), 514 (w), 477(w). Hybrid complex 2: light yellow crystal, yield: 63% based on DPyNDI. IR data (KBr, cm-1): 1720(w), 1633(m), 1582(m), 1505(w), 1445(w), 1403(w), 1337(w), 1248(w), 1191(w), 1118(w), 1076(w), 974(w), 891(w), 793(w), 756(w), 594(w), 506(w). Hybrid complex 3: deep blue crystal, yield: 38% based on DPyNDI. IR data (KBr, cm-1): 1717(w), 1631(m), 1581(m), 1501(w), 1445(w), 1402(w), 1329(w), 1247(w), 1188(w), 1146(w), 1056(w), 945(w), 783(w), 746(w), 577(w), 496(w). 2.3 X-ray crystallography study Crystal data for the hybrids 1-3 were collected on a Rigaku Saturn 724 CCD diffractometer with Mo-Kα radiation
-4-
(λ = 0.71073 Å) at 113 K under a cold nitrogen stream. The frame data were integrated and absorption correction using a Rigaku CrystalClear program package. All the structures were solved by the direct method and different Fourier syntheses, and the calculations were performed by full-matrix least-squares methods on F2 by using the SHELXTL program [19], all non-hydrogen atoms were refined with anisotropic thermal parameters and the hydrogen atoms were fixed at calculated positions and refined by a riding mode. For the complexes, some included solvents were disordered and thus their contributions were subtracted from the data using SQUEEZE software.
[20]
Crystallographic data have
been deposited with Cambridge Crystallographic Data Centre as supplementary publication number CCDC 1947100-1947102 for hybrids 1, 2 and 3, respectively, which can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
3. Results and discussions 3.1 Syntheses and structural description At room temperature, upon slow diffusion of a acetonitrile solution of H4SiW12O40 (H3PW12O40 or H3PMo12O40) into a NMP solution of DPyNDI and a small amount of HI with a buffer-layered solution of MeCN and NMP (1:1 in volume) in a long thin test tube, prism crystals of 1 (2 or 3, respectively) were obtained after several days in the dark. All three single crystals have been characterized by the single-crystal X-ray diffraction analyses, which revealed that they were their isostructures in spite of the slightly different cell parameters and volumes (Table 1, for details, see ESI). The purities of the obtained crystalline materials have been examined by the powder X-ray diffraction analysis (Fig. 2), which revealed only a single phase with a good fit between simulated and observed patterns for each complex, indicating their high crystalline purities. Moreover, almost same PXRD phases for crystalline materials 1-3 also corroborate their isostructural features.
-5-
Due to the isostructures, hybrid 1 is selected for structural descriptions. As shown in Fig. 3, each DPyNDI tecton is protonated at two pyridyl units in the longitudinal directions and then blocked by two NMP solvent molecules through H-bonding interactions (dO-H = 1.744 Å) between C=O units of NMP molecules and N-H units of the protonated pyridyl groups, but which is interconnected at the lateral positions through weak H-bonding interactions (dO-H = 2.873 Å) between C=O units of DPyNDI tectons and para-C-H units of pyridyl groups to form infinite 1-D H-bonded networks (Fig. 3a). However, in the perpendicular direction of the π-planes of DPyNDI tectons in the H-bonded networks, each DPyNDI is sandwiched by two silicotungstic anions through anion-π interactions (dO-π = 2.985-2.993 Å, Fig. 3b) to generate a 2-D hybrid network with an alternate arrangement of continuous 1-D H-bonded DPyNDI networks and segregated POM anions (Fig. 3c). In 3-D packing, such 2-D supromolecular networks are interconnected by the weak H-bonds between W=O in silicotungstic anions and C-H of NMP molecules in the range of 2.569-2.672 Å. Obviously, hybrid 1 possesses a typical structural feature of the D-A hybrid heterostructures with segregated POM anions and infinite 1-D H-bonded DPyNDI networks as electron donors and acceptors. Similar structural characteristics could also observed for hybrids 2 and 3, the major differences are the distances between the neighbouring POMs and DPyNDI units (dO-π = 2.991-3.268 Å for 2, 2.986-3.003 Å for 3, respectively). Compared to the reported hybrid 4 with segregated metal-coordinated DPyNDI complexes as electron acceptors,
[17]
the continuous H-bonded NDI networks
are obviously conducive to facilitate their electron transports. More importantly, due to the small steric hindrances, the distances between POM anions and the imide units of DPyNDI tectons in hybrids 1-3 are much shorter than those in metal-coordinated 4 (dO-π = 3.22-3.32 Å), [17] which will also help for the intermolecular charge transfers (ICTs), as well as photoinduced electron transfers (PIETs) between POM anions and DPyNDI tectons, which will be reflected and
-6-
magnified in the following photophysical and photochemical studies.
3.2 Absorption and Photochromic properties In spite of the isostructures but due to the different POMs contained, the appearance colours for the as-synthesized hybrids 1-3 are remarkably different, specifically, dark golden coloured rods for 1, light-yellow for 2 (Fig. 4a), while black for 3. The colour of hybrid 3 with polyoxomolybdate anions is a bit darker than the first two with polyoxotungstate anions (hybrids 1 and 2), which is consistent with the darkness of their contained POMs (white for the polyoxotungstates, but yellow for the polyoxomolybdates). The different colours can be clearly reflected from their solid-state UV/Vis diffuse reflectance spectra (DRS). As shown in Fig. 4b-4d, in the range of 200-400 nm, almost same spectral shapes and intensities for hybrids 1 and 2, which can mainly be assigned to the π-π* transitions of H-bonding PDyNDI aggregates as well as those of a small amount of DPyNDI radical anions induced by natural light
[21]
(for
details, see the following). However, for hybrid 3, the enhanced absorption intensity from 300 to 400 nm is mainly attributed to the ligand-to- metal charge transfer (LMCT) transitions (Ob,c → Mo) within the PMo12O403- anions. [22] Besides these small variations, the remarkable difference for hybrids 1-3 is in the visible range from 400 to 800 nm. In detail, for hybrids 1 and 2, a weak shoulder band from 400-550 nm with an indistinct peak (a little clearer for hybrid 2) with the absorption maxima at 500-550 nm (Fig. 4b-4c) has been observed, which is mainly originated from three contributions: (1) an intermolecular charge transfer (ICT) transition (a characteristic band at ca. 505 nm [23]) from electron-rich POM anions to electron-poor DPyNDI tectons, which is strongly reflected in the following RTP studies; (2) the π-π* transitions of a small amount of partial reduced products of DPyNDI tectons (radical anions, besides a broad absorption band from 200-400 nm, also with a characteristic sharp band at ca. 600-800nm [21]); (3) an intervalene charge transfer (IVCT) transition of partial reduced products of POMs anions (heteropoly blue, two characteristic
-7-
absorption bands at 520 nm and 680-760 nm[24]). Indeed, the photoinduced formation of DPyNDI radical anions and reduced products of tungstic anions in hybrids 1-2 by the natural light (for the detailed generation mechanisms, see the PIET processes in the following) have been supported by their electron spin resonance (ESR) spectral studies (Fig. 5), which shows a weak typical characteristic signal of radial anions and WV species at the g value of ca. 2.0029 and 1.9296, respectively (Fig. 5a-5b). However, unlike the weak absorptions of hybrids 1-2 in the visible and NIR range, those of hybrid 3 exhibits an intense broad absorption band (Fig.4d), which seems to be governed by the IVCT transitions within the more reduced products of PMo12O403- anions. To our surprise, no clear signals for similar MoV species have been detected in its EPR spectrum (Fig. 5c), which is attributed to the formation of MoIV species without spin electrons. Indeed, the mother solution of 3 was turned into blue during the crystallization, which also confirms the formation of heteropoly blue. Due to the strong absorption of generated molybdenum blue, the absorption assigned to the ICT transitions in hybrid 3 is supposed to be overlapped, however which is reflected in the following RTP studies. Although the different initial appearance colours of hybrids 1-3, upon visible-light irradiation by Xenon lamp, their colours become darker over time (Fig.4a). The photoproducts are stable in air and can return to their original colours in a dark room for several hours at the room temperature. Such decolored samples can give colour changes upon another irradiation, which indicates their reversible photochromism. Indeed, such photochromic behavior can also be reflected in their solid-state DRS studies. As shown in Fig. 4b, the UV/Vis DRS reveal that the main influence on hybrid 1 by the visible-light irradiation is not the spectral changes in the ultraviolet range from 200 to 400 nm (almost no changes) but those in the visible region from 400 to 800 nm, including the absorption band aroused from the IVCT transition, which are increased gradually with the irradiation time. Similar spectral changes could also be observed for hybrids 2-3 (Fig. 4c-4d). Such remarkable changes might be attributed to the photoinduced formation of more
-8-
heteropoly blue species as well as DPyNDI radical anions,
[21]
which is confirmed by their ESR spectra showing the
enhancements of WV characteristic signals for hybrids 1-2 and NDI radical signals for all three complexes (Fig. 5). Taking all the above photoproducts into consideration, a plausible PIET process for hybrid 1-3 could be deduced. As shown in Fig. 6, upon irradiation (even by the natural light), a HOMO electron in the photoactive acceptor, H-bonded DPyNDI network, can be firstly excited to its LUMO orbit, and then its empty HOMO orbit will accept an electron from the HOMO orbit of electron-rich POM anions with higher energy levels. Due to the long-range order, the generated hole and electron will be transported freely along huge POMs and DPyNDI networks. During this process, the formed hole could again accept an electron from the sacrificial agent NMP solvent molecules. As a result, the intermolecular PIET process is from NMP via POMs to DPyNDI tectons to form the NDI radical anions. At the same time, some of the NDI radical species could also be further photoexcited to generate their excited states, which could photoreduce the POMs conversely to afford the reduced heteropoly blues.
[25]
Based on this PIET process, the PIET
speeds for hybrids 1-3 are mainly related to their initial absorptions, the distances between the donor and acceptor components, as well as the nature (HOMO and LUMO level energies, redox potentials) of the POM anions contained. However, from the UV/Vis DRS of hybrids 1-3 (Fig. 3b-3d), the increased absorption intensities at 800 nm in the first minute (photoresponse speeds) are in the order of 2 > 1 > 3, which is consistent with that of their initial absorptions. Therefore, PIET speeds for hybrids 1-3 are dominated by their initial absorptions. However, compared with the reported hybrid 4 bearing the same POM anions and similar NDI tectons, in spite of the close initial absorptions, the photochromic speed of hybrid 2 is relatively high, which is mainly resulted from its short anion-π distances (strong anion-π interaction strengths) (dO-π = 2.991-3.268 Å for 2, while 3.22-3.32 Å for 4).
3.3 Emission and photoinduced emission quenching properties
-9-
As we known, for the pure DPyNDIs and their aggregates, weak fluorescence but no RTP emissions can be observed due to the large singlet-triplet energy gap (△EST) and inefficient intersystem crossing (ISC) processes. However, the present three D-A hybrid heterostructures exhibit a strong emission with three emission bands from 600 to 800 nm under the excitation of UV light at room temperature in air (Fig. 7a and Fig.S2), which is also different from the traditional POMs-based hybrid materials usually with quenched emissions [26]. The emission lifetimes for three hybrids are approximately 11.246, 2.750 and 0.242 ms for hybrids 1-3 (Fig.7b and Fig.S3), respectively, indicating that these emissions are RTP. Similar as the reported hybrid 4, [17] the anion-π interaction induced ICT states between POM anions and DPyNDI tectons are decisive of the generations of RTP, which can bridge the large △EST and thus trigger a RTP under a suitable condition. As a result, the anion-π interactions play the most important role on the RTP emission lifetimes and quantum yields. Due to the strong anion-π interaction strengths, the emission lifetime for hybrid 2 (2.75 ms) is much shorter than that of the reported example 4 (13 ms). However, the shorter anion-π distances (dO-π = 2.985-2.993 Å) but longer lifetime (11.246 ms) for 1, as well as the slightly shorter anion-π distances (dO-π = 2.986-3.003 Å) but much shorter lifetime (0.242 ms) for 2, are also related to the heavy-atom effect. Similarly, due to the short anion-π distances, the photoluminescence quantum yields (PLQYs) for 1-3 are 7.2%, 2.4% and 1.6%, respectively, which is remarkably enhanced in comparison to that of the reported 4 (1.5%). In other words, the RTPs, particularly their PLQYs of the non-emissive NDIs could be switched on by anion-π interactions between POMs and DPyNDI tectons in photochromic POM-NDI hybrids, which can be further confirmed by their photoinduced emission quenching phenomena (see the following). The anion-π interaction induced ICT states answering for the RTP generations could also be reflected in the studies of photoinduced RTP spectral changes. As shown in Fig. 7a, the RTP intensities of hybrid 1 are decreasing
- 10 -
gradually with unchanged emission peak positions after the continuous irradiations by Xenon lamp. Similar spectral phenomena can be observed for hybrids 2 and 3 (Fig.S2). As stated above, during the photochromism, two components in the ICT states have been transformed into the DPyNDI radical anions and (or) reduced heteropoly blues, which will undoubtedly change the energy levels of ICT states and thus lead to the lower efficiencies of ISC processes and weak RTPs. Interestingly, the decreased speeds for the RTP intensities at the emission maxima for hybrids 1-3 are also in the order of 2 > 1 > 3, which is consistent with that of their photochromic speeds, further corroborates such an ICT-induced RTP generation mechanism. In addition, in comparison with the reported hybrid 4, the RTP decreasing speed of hybrid 2 is remarkably high, which is also attributed to its small anion-π distances. In a word, the compact contacts in the POM-NDI hybrid heterostructures also play a key role on their photochromic and photoinduced emission quenching properties.
4. Conclusions In summary, we have demonstrated three isostructural D-A hybrid heterostructural materials with the segregated POM anions and infinite 1-D H-bonded NDI networks as electron donors and acceptors, respectively. Due to the compact contacts between POM and NDI components induced by the charge-assisted anion-π interactions, not only the photochromic speeds of these unique isostructures have been enhanced, but more interestingly their RTP emissions of have been switched on, and their PLQYs are dependent on anion-π interaction strengths. This work provides a powerful strategy for the design of multifunctional hybrid materials, such as photochromic and RTP materials in the future.
5. Acknowledgements This work is supported by the National Natural Science Foundation of China (21572032, 21971041), Program for New Century Excellent Talents in Fujian Province University, Natural Science Foundation of Fujian Province
- 11 -
(2018J01431 and 2018J01690), Research Foundation of Education Bureau of
Fujian Province (JT180813) and
Foundation of Science and Technology on Sanming Institute of Fluorochemical Industry (FCIT201706GR).
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- 20 -
Table captions and Schemes, Figures: Fig. 1: Schematic representation of POM based continuous D-A hybrid heterostructures through charge-assisted anion-π interactions.
Table 1: Cell parameters (Å) and volume (Å3) for hybrid complexes 1-3.
Fig. 2: The observed PXRD patterns for hybrid complexes 1-3 and that of simulated 1.
Fig. 3: Portions of the X-ray structures of hybrid 1 showing the infinite 1-D H-bonded NDI networks and segregated POM anions.
Fig. 4: The sample colour charges of hybrid 2 (a, in each photo, the cloud was the sample, while the raindrops were drawn as black for comparison) and the solid-state diffuse reflectance spectra (b-d) for hybrids 1-3 depended on the irradiation time by xenon lamp (300W, 420-780 nm).
Fig. 5: Electron paramagnetic resonance (EPR) spectra of hybrids 1-3 (a-c) before and after the irradiation by xenon lamp for 15mins.
Fig. 6: Schematic representation of plausible PIET processes in hybrids 1-3.
Fig. 7: The emission spectral changes at the room temperature upon irradiation by Xenon lamp (a) (Ex=430nm), and the luminescence decay of hybrid 1 under ambient conditions (b).
- 21 -
Fig. 1:
Fig. 1: Schematic representation of POM based continuous D-A hybrid heterostructures through charge-assisted anion-π interactions.
- 22 -
Table 1: Complexes
1 (SiW12O404-)
2 (PW12O403-)
3 (PMo12O403-)
a (Å)
13.5043 (6)
12.9338 (7)
13.3063 (6)
b (Å)
13.5774 (6)
13.5150 (6)
13.5978 (7)
c (Å)
13.6210 (6)
13.9833 (6)
13.6457 (8)
α (deg)
75.091 (4)
97.664 (4)
76.185 (5)
β (deg)
85.497 (4)
100.906 (4)
88.859 (4)
γ (deg)
81.480 (4)
95.357 (4)
84.510 (4)
2384.61
2360.97
2386.57
3
V (Å )
Table 1: Cell parameters (Å) and volume (Å3) for hybrid complexes 1-3.
- 23 -
Fig. 2:
Fig. 2: The observed PXRD patterns for hybrid complexes 1-3 and that of simulated 1.
- 24 -
Fig. 3:
Fig. 3: Portions of the X-ray structures of hybrid 1 showing the infinite 1-D H-bonded NDI networks and segregated POM anions.
- 25 -
Fig. 4:
Fig. 4: The sample colour charges of hybrid 2 (a, in each photo, the cloud was the sample, while the raindrops were drawn as black for comparison) and the solid-state diffuse reflectance spectra (b-d) for hybrids 1-3 depended on the irradiation time by xenon lamp (300W, 420-780 nm).
- 26 -
Fig. 5:
Fig. 5: Electron paramagnetic resonance (EPR) spectra of hybrids 1-3 (a-c) before and after the irradiation by Xenon lamp for 15mins.
- 27 -
Fig. 6:
Fig. 6: Schematic representation of plausible PIET processes in hybrids 1-3.
- 28 -
Fig. 7:
Fig. 7: The emission spectral changes at the room temperature upon irradiation by Xenon lamp (a), and the luminescence decay of hybrid 1 under ambient conditions (b).
- 29 -
Highlights 1. Three isostructural D-A hybrid heterostructures with an alternate arrangement of segregated POM anions and 1-D H-bonded NDI networks as electron donors and acceptors have been synthesized.
2. Compact contacts between POM anions and NDI tectons induced by charge-assisted anion-π interactions results out the enhanced photochromic speeds.
3. RTP emissions of the three D-A hybrid heterostructures have been swiched on with a relatively high photoluminescence quantum yields.
4. An efficient way to tune RTP emission of hybrid and photochromic speed is described.
Conflict of Interest Statements Dear Editors Enclosed please find our manuscript “Switching on room-temperature phosphorescence of photochromic hybrid heterostructures by anion-π interactions”, which we wish to submit to Dyes and Pigments for consideration as Original Research Article. It is original and has not been published elsewhere. In addition, no conflict of interest exits in the submission of this manuscript, which is also approved by all authors. Thus, the authors declared that there are no conflicts of interest, including any commercial or associative interest, to this work.
With best regards, Mei-Jin Lin PhD, Professor College of Chemistry, Fuzhou University, Fuzhou 350108, P. R. China Email: [email protected]
S0143-7208(19)32181-3
DOI:
https://doi.org/10.1016/j.dyepig.2019.107943
Reference:
DYPI 107943
To appear in:
Dyes and Pigments
Received Date: 15 September 2019 Revised Date:
30 September 2019
Accepted Date: 30 September 2019
Please cite this article as: You M-H, Li M-H, Di Y-M, Wang Y-W, Lin M-J, Switching on roomtemperature phosphorescence of photochromic hybrid heterostructures by anion-π interactions, Dyes and Pigments (2019), doi: https://doi.org/10.1016/j.dyepig.2019.107943. 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.
Graphical Abstract
Due to the compact contacts induced by the charge-assisted anion-π interactions, not only the photochromic speeds of three D-A hybrid heterostructures have been enhanced, but more interestingly their RTP emissions of have been switched on.
Switching on room-temperature phosphorescence of photochromic hybrid heterostructures by anion-π interactions Ming-Hua Youa, Meng-Hua Li a, Yi-Ming Di,a Yi-Wen Wang,a and Mei-Jin Lina* a
State Key Laboratory of Photocatalysis on Energy and Environment, and Key Laboratory of Molecule Synthesis and Function Discovery,
College of Chemistry, Fuzhou University, China, 350116. E-mail: [email protected].
Abstract: Room-temperature phosphorescence (RTP) has attracted much attention in the past decades due to its potential applications in optoelectronic devices and bioimaging. So far, most available RTP materials are pure inorganic or organic compounds, few are involved in the organic-inorganic hybrid complexes. The donor-acceptor (D-A) hybrid heterostructures are an emerging class of organic-inorganic hybrid complexes composed of semiconductive organic and inorganic tectons at the molecular level. For these unique hybrids, the primary properties are photochromism and photoinduced electron transfers. Herein, we have demonstrated that the combination of naphthalene diimide (NDI) tectons bearing two divergently oriented pyridyl protons with three polyoxometalate (POM) anions (SiW12O404-, PW12O403- or PMo12O403-) resulted in three isostructural D-A hybrid heterostructures with an alternate arrangement of segregated POM anions and 1-D H-bonded NDI networks as electron donors and acceptors, respectively. Due to the compact contacts between POM anions and NDI tectons induced by charge-assisted anion-π interactions, not only the photochromic speeds of three D-A hybrid heterostructures have been enhanced, but more interestingly their RTP emissions of have also been switched on, and their photoluminescence quantum yields are dependent on anion-π interaction strengths. Keywords: Room-temperature phosphorescence, Naphthalene diimides, Polyoxometalates, Photochromism, D-A heterostructures
1. Introduction Phosphorescent materials, particularly room-temperature phosphorescent (RTP) materials with long emission lifetimes arising from the long-lived intersystem crossing (ISC) have attracted considerable attention due to their
-1-
applications in optoelectronics,
[1]
chemical sensing,
rare-earth metal cation doped inorganic materials, aromatic organic molecules.
[6]
[4]
[2]
and bio-imaging.
[3]
So far, most available RTP materials are
noble metal (e.g. Ir, Ru and Os) organometallic materials
[5]
and
In these materials, RTP has mainly been achieved through the incorporation of heavy
atoms into the photosensitive systems or the establishment of intra- or intermolecular charge-transfer (CT) states to bridge the lowest local-excited states of photosensitive systems.
[7]
The incorporation of the heavy atoms into the
photosensitive systems affords the significant spin-orbital coupling (SOC), and thus enhances the efficiency of ISC, which is a simple but promising strategy to achieve long-lived RTP.
[8]
On the other hand, the installation of intra- or
inter-molecular CT states is more elaborate, which always combines both donor and acceptor components into photosensitive systems through the covalent bonds or non-covalent interactions, such as hydrogen bonds, π-π stacking and C-H⋅⋅⋅π interactions. [9] Currently it is of high interest to explore new CT RTP materials with high quantum yield, low cost and convenient operation through new non-covalent interactions. Anion-π interaction
[10]
is an emerging non-covalent force between the electron-deficient π-systems and anions,
which has potential applications in chemical sensing, [11] catalysis [12] and anion transport. [13] Recently, other groups [14] and us
[15]
have utilized such unique force for the construction of D-A hybrid heterostructures, whose semiconductive
organic and inorganic components are fabricated at the molecular level.
[16]
For such unique hybrid materials, the
primary properties of interest are photoinduced electron transfers (PIETs) and their resulting photochromism. Herein, we report our efforts towards the employ of anion-π interactions for novel highly emissive D-A hybrid heterostructures, which is anticipated to have the advantages of photochromism and RTP combined. Thus, three isostructural hybrid heterostructures 1-3 with an alternate arrangement of segregated POM anions and 1-D H-bonded NDI networks as electron donors and acceptors (Fig. 1), which have been obtained by the combination of N, N’-dipyridyl naphthalene
-2-
diimide (DPyNDI) and three POM anions, SiW12O404-, PW12O403- or PMo12O403-, in presence of a small amount of HI. We like to note that within the time frame of our project the similar hybrid heterostructure 4 with an unexpected RTP phenomenon has been reported by Lu and co-workers.
[17]
However, in Lu’s work, the quantum yield was rather low
(1.5 %) due to the weak anion-π interactions in the heterostructure. To enhance the anion-π interactions, the protons with simple H-bonds instead of metal ions with sterically hindered coordination groups were used. As expected, owing to the compact contacts between the POM and NDI components induced by charge-assisted anion-π interactions, besides photochromism, the RTP emissions of three isostructures have been switched on, and the photoluminescence quantum yields are dependent on their anion-π interaction strengths.
2. Experimental 2.1 Materials and measurements N-methylpyrrolidin-2-one
(NMP),
polyoxometalates (POMs,
including H4SiW12O40,
H3PW12O40,
and
H3PMo12O40), hydrogen iodide (HI, aqueous) and acetonitrile (MeCN) were obtained from commercial suppliers. All chemicals and reagents were used as received unless otherwise stated. N, N’-dipyridyl naphthalene diimide (DPyNDI) was obtained following the reported process.
[18]
The infrared spectra have been measured in the range of 400-4000
cm-1 by the use of a Perkin-Elmer FT-IR spectrophotometer. Powder X-ray diffraction (PXRD) patterns have been recorded on a Rigaku MiniFlex-II X-Ray diffractometer using Cu-Kα radiation. UV-Vis diffuse reflectance spectra were recorded at room temperature on a Varian Cary 500 UV-Vis spectrophotometer equipped with an integrating sphere. Photoluminescence spectra and lifetimes at room temperature were recorded on an Edinburgh FLS 980 luminescence spectrometer, while the photoluminescence quantum yield was measured on a Hamamatsu C11347−11 absolute luminescence quantum yield measurement system equipped with an integrating sphere apparatus and a 150 W
-3-
CW Xenon light source. Photochromic reaction was agitated using a continuous wavelength-irradiation by Xenon lamp (300W, 420-780 nm). Electron paramagnetic resonance (EPR) measurements were obtained using a Bruker A300 instrument operating in the X-band at the room temperature. 2.2 Synthesis Syntheses of hybrid complexes 1-3: A solution (0.5 mL) of NMP/MeCN (1:1, v/v) was carefully layered over a NMP (5 mL) mixture solution of DPyNDI (0.1mmol) and a small amount of HI (ca. 1 drop), then MeCN solution (5 mL) of H4SiW12O40 (H3PW12O40 or H3PMo12O40) (0.025mmol) was carefully added as a third layer. After standing in the dark for several days, suitable crystals of 1-3 for single-crystal X-ray diffraction analyses were collected, which were washed with mother liquid and then MeCN, and dried at ambient temperature. Hybrid complex 1: light yellow crystal, yield: 56% based on DPyNDI. IR data (KBr, cm-1): 1715(w), 1679(m), 1632(m), 1506(w), 1446(w), 1435(w), 1304(w), 1249(w), 1187(w), 1119(w), 1008(w), 965(w), 912(w), 878(w), 778(w), 748(w), 602(w), 514 (w), 477(w). Hybrid complex 2: light yellow crystal, yield: 63% based on DPyNDI. IR data (KBr, cm-1): 1720(w), 1633(m), 1582(m), 1505(w), 1445(w), 1403(w), 1337(w), 1248(w), 1191(w), 1118(w), 1076(w), 974(w), 891(w), 793(w), 756(w), 594(w), 506(w). Hybrid complex 3: deep blue crystal, yield: 38% based on DPyNDI. IR data (KBr, cm-1): 1717(w), 1631(m), 1581(m), 1501(w), 1445(w), 1402(w), 1329(w), 1247(w), 1188(w), 1146(w), 1056(w), 945(w), 783(w), 746(w), 577(w), 496(w). 2.3 X-ray crystallography study Crystal data for the hybrids 1-3 were collected on a Rigaku Saturn 724 CCD diffractometer with Mo-Kα radiation
-4-
(λ = 0.71073 Å) at 113 K under a cold nitrogen stream. The frame data were integrated and absorption correction using a Rigaku CrystalClear program package. All the structures were solved by the direct method and different Fourier syntheses, and the calculations were performed by full-matrix least-squares methods on F2 by using the SHELXTL program [19], all non-hydrogen atoms were refined with anisotropic thermal parameters and the hydrogen atoms were fixed at calculated positions and refined by a riding mode. For the complexes, some included solvents were disordered and thus their contributions were subtracted from the data using SQUEEZE software.
[20]
Crystallographic data have
been deposited with Cambridge Crystallographic Data Centre as supplementary publication number CCDC 1947100-1947102 for hybrids 1, 2 and 3, respectively, which can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
3. Results and discussions 3.1 Syntheses and structural description At room temperature, upon slow diffusion of a acetonitrile solution of H4SiW12O40 (H3PW12O40 or H3PMo12O40) into a NMP solution of DPyNDI and a small amount of HI with a buffer-layered solution of MeCN and NMP (1:1 in volume) in a long thin test tube, prism crystals of 1 (2 or 3, respectively) were obtained after several days in the dark. All three single crystals have been characterized by the single-crystal X-ray diffraction analyses, which revealed that they were their isostructures in spite of the slightly different cell parameters and volumes (Table 1, for details, see ESI). The purities of the obtained crystalline materials have been examined by the powder X-ray diffraction analysis (Fig. 2), which revealed only a single phase with a good fit between simulated and observed patterns for each complex, indicating their high crystalline purities. Moreover, almost same PXRD phases for crystalline materials 1-3 also corroborate their isostructural features.
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Due to the isostructures, hybrid 1 is selected for structural descriptions. As shown in Fig. 3, each DPyNDI tecton is protonated at two pyridyl units in the longitudinal directions and then blocked by two NMP solvent molecules through H-bonding interactions (dO-H = 1.744 Å) between C=O units of NMP molecules and N-H units of the protonated pyridyl groups, but which is interconnected at the lateral positions through weak H-bonding interactions (dO-H = 2.873 Å) between C=O units of DPyNDI tectons and para-C-H units of pyridyl groups to form infinite 1-D H-bonded networks (Fig. 3a). However, in the perpendicular direction of the π-planes of DPyNDI tectons in the H-bonded networks, each DPyNDI is sandwiched by two silicotungstic anions through anion-π interactions (dO-π = 2.985-2.993 Å, Fig. 3b) to generate a 2-D hybrid network with an alternate arrangement of continuous 1-D H-bonded DPyNDI networks and segregated POM anions (Fig. 3c). In 3-D packing, such 2-D supromolecular networks are interconnected by the weak H-bonds between W=O in silicotungstic anions and C-H of NMP molecules in the range of 2.569-2.672 Å. Obviously, hybrid 1 possesses a typical structural feature of the D-A hybrid heterostructures with segregated POM anions and infinite 1-D H-bonded DPyNDI networks as electron donors and acceptors. Similar structural characteristics could also observed for hybrids 2 and 3, the major differences are the distances between the neighbouring POMs and DPyNDI units (dO-π = 2.991-3.268 Å for 2, 2.986-3.003 Å for 3, respectively). Compared to the reported hybrid 4 with segregated metal-coordinated DPyNDI complexes as electron acceptors,
[17]
the continuous H-bonded NDI networks
are obviously conducive to facilitate their electron transports. More importantly, due to the small steric hindrances, the distances between POM anions and the imide units of DPyNDI tectons in hybrids 1-3 are much shorter than those in metal-coordinated 4 (dO-π = 3.22-3.32 Å), [17] which will also help for the intermolecular charge transfers (ICTs), as well as photoinduced electron transfers (PIETs) between POM anions and DPyNDI tectons, which will be reflected and
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magnified in the following photophysical and photochemical studies.
3.2 Absorption and Photochromic properties In spite of the isostructures but due to the different POMs contained, the appearance colours for the as-synthesized hybrids 1-3 are remarkably different, specifically, dark golden coloured rods for 1, light-yellow for 2 (Fig. 4a), while black for 3. The colour of hybrid 3 with polyoxomolybdate anions is a bit darker than the first two with polyoxotungstate anions (hybrids 1 and 2), which is consistent with the darkness of their contained POMs (white for the polyoxotungstates, but yellow for the polyoxomolybdates). The different colours can be clearly reflected from their solid-state UV/Vis diffuse reflectance spectra (DRS). As shown in Fig. 4b-4d, in the range of 200-400 nm, almost same spectral shapes and intensities for hybrids 1 and 2, which can mainly be assigned to the π-π* transitions of H-bonding PDyNDI aggregates as well as those of a small amount of DPyNDI radical anions induced by natural light
[21]
(for
details, see the following). However, for hybrid 3, the enhanced absorption intensity from 300 to 400 nm is mainly attributed to the ligand-to- metal charge transfer (LMCT) transitions (Ob,c → Mo) within the PMo12O403- anions. [22] Besides these small variations, the remarkable difference for hybrids 1-3 is in the visible range from 400 to 800 nm. In detail, for hybrids 1 and 2, a weak shoulder band from 400-550 nm with an indistinct peak (a little clearer for hybrid 2) with the absorption maxima at 500-550 nm (Fig. 4b-4c) has been observed, which is mainly originated from three contributions: (1) an intermolecular charge transfer (ICT) transition (a characteristic band at ca. 505 nm [23]) from electron-rich POM anions to electron-poor DPyNDI tectons, which is strongly reflected in the following RTP studies; (2) the π-π* transitions of a small amount of partial reduced products of DPyNDI tectons (radical anions, besides a broad absorption band from 200-400 nm, also with a characteristic sharp band at ca. 600-800nm [21]); (3) an intervalene charge transfer (IVCT) transition of partial reduced products of POMs anions (heteropoly blue, two characteristic
-7-
absorption bands at 520 nm and 680-760 nm[24]). Indeed, the photoinduced formation of DPyNDI radical anions and reduced products of tungstic anions in hybrids 1-2 by the natural light (for the detailed generation mechanisms, see the PIET processes in the following) have been supported by their electron spin resonance (ESR) spectral studies (Fig. 5), which shows a weak typical characteristic signal of radial anions and WV species at the g value of ca. 2.0029 and 1.9296, respectively (Fig. 5a-5b). However, unlike the weak absorptions of hybrids 1-2 in the visible and NIR range, those of hybrid 3 exhibits an intense broad absorption band (Fig.4d), which seems to be governed by the IVCT transitions within the more reduced products of PMo12O403- anions. To our surprise, no clear signals for similar MoV species have been detected in its EPR spectrum (Fig. 5c), which is attributed to the formation of MoIV species without spin electrons. Indeed, the mother solution of 3 was turned into blue during the crystallization, which also confirms the formation of heteropoly blue. Due to the strong absorption of generated molybdenum blue, the absorption assigned to the ICT transitions in hybrid 3 is supposed to be overlapped, however which is reflected in the following RTP studies. Although the different initial appearance colours of hybrids 1-3, upon visible-light irradiation by Xenon lamp, their colours become darker over time (Fig.4a). The photoproducts are stable in air and can return to their original colours in a dark room for several hours at the room temperature. Such decolored samples can give colour changes upon another irradiation, which indicates their reversible photochromism. Indeed, such photochromic behavior can also be reflected in their solid-state DRS studies. As shown in Fig. 4b, the UV/Vis DRS reveal that the main influence on hybrid 1 by the visible-light irradiation is not the spectral changes in the ultraviolet range from 200 to 400 nm (almost no changes) but those in the visible region from 400 to 800 nm, including the absorption band aroused from the IVCT transition, which are increased gradually with the irradiation time. Similar spectral changes could also be observed for hybrids 2-3 (Fig. 4c-4d). Such remarkable changes might be attributed to the photoinduced formation of more
-8-
heteropoly blue species as well as DPyNDI radical anions,
[21]
which is confirmed by their ESR spectra showing the
enhancements of WV characteristic signals for hybrids 1-2 and NDI radical signals for all three complexes (Fig. 5). Taking all the above photoproducts into consideration, a plausible PIET process for hybrid 1-3 could be deduced. As shown in Fig. 6, upon irradiation (even by the natural light), a HOMO electron in the photoactive acceptor, H-bonded DPyNDI network, can be firstly excited to its LUMO orbit, and then its empty HOMO orbit will accept an electron from the HOMO orbit of electron-rich POM anions with higher energy levels. Due to the long-range order, the generated hole and electron will be transported freely along huge POMs and DPyNDI networks. During this process, the formed hole could again accept an electron from the sacrificial agent NMP solvent molecules. As a result, the intermolecular PIET process is from NMP via POMs to DPyNDI tectons to form the NDI radical anions. At the same time, some of the NDI radical species could also be further photoexcited to generate their excited states, which could photoreduce the POMs conversely to afford the reduced heteropoly blues.
[25]
Based on this PIET process, the PIET
speeds for hybrids 1-3 are mainly related to their initial absorptions, the distances between the donor and acceptor components, as well as the nature (HOMO and LUMO level energies, redox potentials) of the POM anions contained. However, from the UV/Vis DRS of hybrids 1-3 (Fig. 3b-3d), the increased absorption intensities at 800 nm in the first minute (photoresponse speeds) are in the order of 2 > 1 > 3, which is consistent with that of their initial absorptions. Therefore, PIET speeds for hybrids 1-3 are dominated by their initial absorptions. However, compared with the reported hybrid 4 bearing the same POM anions and similar NDI tectons, in spite of the close initial absorptions, the photochromic speed of hybrid 2 is relatively high, which is mainly resulted from its short anion-π distances (strong anion-π interaction strengths) (dO-π = 2.991-3.268 Å for 2, while 3.22-3.32 Å for 4).
3.3 Emission and photoinduced emission quenching properties
-9-
As we known, for the pure DPyNDIs and their aggregates, weak fluorescence but no RTP emissions can be observed due to the large singlet-triplet energy gap (△EST) and inefficient intersystem crossing (ISC) processes. However, the present three D-A hybrid heterostructures exhibit a strong emission with three emission bands from 600 to 800 nm under the excitation of UV light at room temperature in air (Fig. 7a and Fig.S2), which is also different from the traditional POMs-based hybrid materials usually with quenched emissions [26]. The emission lifetimes for three hybrids are approximately 11.246, 2.750 and 0.242 ms for hybrids 1-3 (Fig.7b and Fig.S3), respectively, indicating that these emissions are RTP. Similar as the reported hybrid 4, [17] the anion-π interaction induced ICT states between POM anions and DPyNDI tectons are decisive of the generations of RTP, which can bridge the large △EST and thus trigger a RTP under a suitable condition. As a result, the anion-π interactions play the most important role on the RTP emission lifetimes and quantum yields. Due to the strong anion-π interaction strengths, the emission lifetime for hybrid 2 (2.75 ms) is much shorter than that of the reported example 4 (13 ms). However, the shorter anion-π distances (dO-π = 2.985-2.993 Å) but longer lifetime (11.246 ms) for 1, as well as the slightly shorter anion-π distances (dO-π = 2.986-3.003 Å) but much shorter lifetime (0.242 ms) for 2, are also related to the heavy-atom effect. Similarly, due to the short anion-π distances, the photoluminescence quantum yields (PLQYs) for 1-3 are 7.2%, 2.4% and 1.6%, respectively, which is remarkably enhanced in comparison to that of the reported 4 (1.5%). In other words, the RTPs, particularly their PLQYs of the non-emissive NDIs could be switched on by anion-π interactions between POMs and DPyNDI tectons in photochromic POM-NDI hybrids, which can be further confirmed by their photoinduced emission quenching phenomena (see the following). The anion-π interaction induced ICT states answering for the RTP generations could also be reflected in the studies of photoinduced RTP spectral changes. As shown in Fig. 7a, the RTP intensities of hybrid 1 are decreasing
- 10 -
gradually with unchanged emission peak positions after the continuous irradiations by Xenon lamp. Similar spectral phenomena can be observed for hybrids 2 and 3 (Fig.S2). As stated above, during the photochromism, two components in the ICT states have been transformed into the DPyNDI radical anions and (or) reduced heteropoly blues, which will undoubtedly change the energy levels of ICT states and thus lead to the lower efficiencies of ISC processes and weak RTPs. Interestingly, the decreased speeds for the RTP intensities at the emission maxima for hybrids 1-3 are also in the order of 2 > 1 > 3, which is consistent with that of their photochromic speeds, further corroborates such an ICT-induced RTP generation mechanism. In addition, in comparison with the reported hybrid 4, the RTP decreasing speed of hybrid 2 is remarkably high, which is also attributed to its small anion-π distances. In a word, the compact contacts in the POM-NDI hybrid heterostructures also play a key role on their photochromic and photoinduced emission quenching properties.
4. Conclusions In summary, we have demonstrated three isostructural D-A hybrid heterostructural materials with the segregated POM anions and infinite 1-D H-bonded NDI networks as electron donors and acceptors, respectively. Due to the compact contacts between POM and NDI components induced by the charge-assisted anion-π interactions, not only the photochromic speeds of these unique isostructures have been enhanced, but more interestingly their RTP emissions of have been switched on, and their PLQYs are dependent on anion-π interaction strengths. This work provides a powerful strategy for the design of multifunctional hybrid materials, such as photochromic and RTP materials in the future.
5. Acknowledgements This work is supported by the National Natural Science Foundation of China (21572032, 21971041), Program for New Century Excellent Talents in Fujian Province University, Natural Science Foundation of Fujian Province
- 11 -
(2018J01431 and 2018J01690), Research Foundation of Education Bureau of
Fujian Province (JT180813) and
Foundation of Science and Technology on Sanming Institute of Fluorochemical Industry (FCIT201706GR).
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Table captions and Schemes, Figures: Fig. 1: Schematic representation of POM based continuous D-A hybrid heterostructures through charge-assisted anion-π interactions.
Table 1: Cell parameters (Å) and volume (Å3) for hybrid complexes 1-3.
Fig. 2: The observed PXRD patterns for hybrid complexes 1-3 and that of simulated 1.
Fig. 3: Portions of the X-ray structures of hybrid 1 showing the infinite 1-D H-bonded NDI networks and segregated POM anions.
Fig. 4: The sample colour charges of hybrid 2 (a, in each photo, the cloud was the sample, while the raindrops were drawn as black for comparison) and the solid-state diffuse reflectance spectra (b-d) for hybrids 1-3 depended on the irradiation time by xenon lamp (300W, 420-780 nm).
Fig. 5: Electron paramagnetic resonance (EPR) spectra of hybrids 1-3 (a-c) before and after the irradiation by xenon lamp for 15mins.
Fig. 6: Schematic representation of plausible PIET processes in hybrids 1-3.
Fig. 7: The emission spectral changes at the room temperature upon irradiation by Xenon lamp (a) (Ex=430nm), and the luminescence decay of hybrid 1 under ambient conditions (b).
- 21 -
Fig. 1:
Fig. 1: Schematic representation of POM based continuous D-A hybrid heterostructures through charge-assisted anion-π interactions.
- 22 -
Table 1: Complexes
1 (SiW12O404-)
2 (PW12O403-)
3 (PMo12O403-)
a (Å)
13.5043 (6)
12.9338 (7)
13.3063 (6)
b (Å)
13.5774 (6)
13.5150 (6)
13.5978 (7)
c (Å)
13.6210 (6)
13.9833 (6)
13.6457 (8)
α (deg)
75.091 (4)
97.664 (4)
76.185 (5)
β (deg)
85.497 (4)
100.906 (4)
88.859 (4)
γ (deg)
81.480 (4)
95.357 (4)
84.510 (4)
2384.61
2360.97
2386.57
3
V (Å )
Table 1: Cell parameters (Å) and volume (Å3) for hybrid complexes 1-3.
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Fig. 2:
Fig. 2: The observed PXRD patterns for hybrid complexes 1-3 and that of simulated 1.
- 24 -
Fig. 3:
Fig. 3: Portions of the X-ray structures of hybrid 1 showing the infinite 1-D H-bonded NDI networks and segregated POM anions.
- 25 -
Fig. 4:
Fig. 4: The sample colour charges of hybrid 2 (a, in each photo, the cloud was the sample, while the raindrops were drawn as black for comparison) and the solid-state diffuse reflectance spectra (b-d) for hybrids 1-3 depended on the irradiation time by xenon lamp (300W, 420-780 nm).
- 26 -
Fig. 5:
Fig. 5: Electron paramagnetic resonance (EPR) spectra of hybrids 1-3 (a-c) before and after the irradiation by Xenon lamp for 15mins.
- 27 -
Fig. 6:
Fig. 6: Schematic representation of plausible PIET processes in hybrids 1-3.
- 28 -
Fig. 7:
Fig. 7: The emission spectral changes at the room temperature upon irradiation by Xenon lamp (a), and the luminescence decay of hybrid 1 under ambient conditions (b).
- 29 -
Highlights 1. Three isostructural D-A hybrid heterostructures with an alternate arrangement of segregated POM anions and 1-D H-bonded NDI networks as electron donors and acceptors have been synthesized.
2. Compact contacts between POM anions and NDI tectons induced by charge-assisted anion-π interactions results out the enhanced photochromic speeds.
3. RTP emissions of the three D-A hybrid heterostructures have been swiched on with a relatively high photoluminescence quantum yields.
4. An efficient way to tune RTP emission of hybrid and photochromic speed is described.
Conflict of Interest Statements Dear Editors Enclosed please find our manuscript “Switching on room-temperature phosphorescence of photochromic hybrid heterostructures by anion-π interactions”, which we wish to submit to Dyes and Pigments for consideration as Original Research Article. It is original and has not been published elsewhere. In addition, no conflict of interest exits in the submission of this manuscript, which is also approved by all authors. Thus, the authors declared that there are no conflicts of interest, including any commercial or associative interest, to this work.
With best regards, Mei-Jin Lin PhD, Professor College of Chemistry, Fuzhou University, Fuzhou 350108, P. R. China Email: [email protected]