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This article can be cited before page numbers have been issued, to do this please use: M. Boominathan, S. Veerasamy, M.Nagaraj, N. Bhuvanesh, S. Muthusubramanian and R. Seenivasan, RSC Adv., 2013, DOI: 10.1039/C3RA42809E.

Aggregation Induced Emission characteristics of maleimide

derivative

Muthusamy Boominathan,a Veerasamy Sathish,

a Muthupandi Nagaraj,

a Nattamai Bhuvanesh

b

Shanmugam Muthusubramanian,a* and Seenivasan Rajagopal.

a*

One of the maleimide derivatives synthesised has been found to exhibit AIE characteristics by

the addition of water into THF solution.

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PAPER

This journal is © The Royal Society of Chemistry [year] [journal], [year], [vol], 00–00 | 1

Aggregation Induced Emission characteristics of maleimide derivative Muthusamy Boominathan,a Veerasamy Sathish,a Muthupandi Nagaraj,a Nattamai Bhuvaneshb Shanmugam Muthusubramanian,a* and Seenivasan Rajagopal.a*

Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x 5

A simple microwave assisted protocol for the construction of maleimides from various aniline and dialkyl acetylene dicarboxylate has been demonstrated. The AIE property of one of the maleimide derivatives has been studied in detail. The maleimide is AIE-active, whose color, fluorescence efficiency, quantum yield and fluorescence lifetime can be readily tuned by adding water to the THF solution. The compound 3a is found to be weakly luminescent when it is dissolved in THF, but its luminescence intensity increases 10

enormously by the gradual addition of water up to 90% with associated increase in quantum yield and fluorescence lifetime.

Introduction The maleimide motif occurs commonly in numerous natural products finding extensive applications as antibacterial, antiviral 15

and angiogenesis inhibition agents.1 Bisaryl-maleimides SB 216763 and SB 415286 are potent inhibitors of glycogen synthase kinase-3 (GSK-3) and the α-isoform of GSK-3.2 Besides, the above mentioned medicinal properties maleimide containing natural compounds also show promising biological activities.3 20

Maleimide based fluorophores have intense red color luminescent efficiency4 and are employed as organic light-emitting diodes (OLEDs).5 The optical property of maleimide is sensitive to receptor–anion interactions allowing that suitable for both colorimetric and fluorimetric detections. Hence maleimide 25

incorporated probes like DY-505-MAL, DY-555-MAL and DY-635-MAL are used as fluorescent-staining materials for protein labels.6a The absorption and emission properties exhibited by this class of compounds has been recently highlighted by Wang.6b

Because of their importance, great efforts have been focussed 30

towards developing simple synthetic approaches for the construction of these privileged structures. This article describes the synthesis of maleimide derivatives by a facile microwave assisted solvent free one pot method. Interestingly, one of the compounds is found to exhibit aggregation-induced emission 35

characteristics. There has been tremendous interest in organic materials that exhibit strong fluorescent emission in their aggregates or solid state and find applications in organic light emitting diodes7 and fluorescent chemo or biosensors.8 The light emission of organic 40

compounds is normally quenched due to Aggregation Caused Quenching (ACQ) effect9 and this is a spiny problem in the development of OLEDs10 and fluorescence sensing systems.11 So there is a huge demand for the development of fluorescent probes without ACQ. To overcome the ACQ effect in the condensed 45

phase, a number of chemical, physical and engineering methods

have been tried12 with only limited success. In this perspective, an unusual Aggregation-Induced Emission (AIE) process, in which nonluminescent molecules are induced to 50

emit by aggregate formation, has been noticed and this phenomenon has been extensively studied.13 Since then, numerous reports have appeared on the AIE active dyes, which include siloles,14 tetraphenylethane (TPE),15 9,10-bis(p-dimethylaminostyryl)anthracene (9,10-MADSA),16 1-cyano-55

trans-1,2-bis-(4′-methylbiphenyl)ethylene (CN-MBE),17 2,5-diphenyl-1,4-distyrylbenzene (DPDSB) derivatives,18

diphenyldibenzofulvene (DPDBF) derivatives,19 conjugate polymers,20 boron-dipyrromethene (BODIPY) derivatives,21 triazoles22 and others.23,24 Recently, AIE based probes are being 60

used in bioimaging,25 biosensors for proteins,26 sensing of CO2,27

detection of insulin fibrillation28 and recognition of fingerprints.29

Though the origin of AIE has been attributed to several factors including conformational planarization, J-aggregate formation, twisted intramolecular charge transfer, aggregation of long alkyl 65

chains and restriction of intramolecular rotation (RIR), RIR is the main cause for the AIE effect in luminophores.13,30

Results and discussion The popular methods of generating maleimides have some serious limitation to the use of starting materials.3,31 Some 70

methods involve strong bases with volatile and toxic organic solvents or metal catalysts.3 Obviously any improvised atom economical green technology for the synthesis of maleimides would be greatly appreciated. The syntheses of heterocyclic compounds by multicomponent protocol and green chemical 75

routes have been targeted in our laboratory resulting in several successful results.32 In one such attempt, it is found that allowing excess aniline and diethyl acetylenedicarboxylate to react together under microwaves has led to one product predominantly over the others and the product obtained has been identified as a 80

maleimide derivative. When searched the literature for the

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formation of this compound by this route, it has been found that diethyl acetylenedicarboxylate, when allowed to react with excess aromatic amines at very high temperature (200 oC) yielded N-aryl-α-anilinomaleimides.33

However in the present investigation, following a modified 5

Heidel procedure,33 N-aryl-α-anilinomaleimides were obtained even when the aniline employed was just two fold. However, the reaction had to be conducted under microwaves irradiation with varying power (150-200W) keeping the temperature at 180oC (± 2°C). The reaction got completed in just 10 minutes. The 10

maleimide, 1-aryl-3-(arylamino)-1H-pyrrole-2,5-dione 3, was obtained as golden yellow solid in excellent yield ranging from 95-98%. All the synthesized maleimide derivatives (3a-e) (Scheme 1) have been completely characterized by IR, NMR and single crystal X-ray analysis (Figure 1, CCDC number 938824). 15

Whether the ester employed is dimethyl or diethyl acetylenedicarboxylate, the yield of the final substituted maleimide 3 is quantitatively the same. The maleimide derivative 3a shows excellent luminescent properties, which prompted us to study its photophysical properties in detail. 20

N

HN

O

O

O

OX

O

O

XNH2

R 180 ºC / 220W / 10 min

Microwave

RR

X = Me or Et95-97%a: R = H

b: R = OMec: R = Cld: R = Bre: R = F

1 2 3

Scheme 1 Microwave promoted green synthesis of maleimides (3a-e)

Fig. 1 Single Crystal X-ray structure of compound 3a

1-Phenyl-3-(arylamino)-1H-pyrrole-2,5-dione, 3a, is readily 25

soluble in organic solvents like acetone, THF, ethanol, methanol and DMSO, but it is insoluble in water. The UV-vis absorption spectra of 3a-e are shown in Figure 2. The compound 3a shows

an absorption maximum at 378 nm in THF and this band gets red shifted to 388 nm while adding water to the extent of 90% 30

(Figure 3). The levelling-off tail in the visible region of the absorption spectrum suggests the formation of nanoscopic aggregates of 3a, which is due to the Mie effect of the nanoparticles.34 The formation of nanoaggregates is confirmed by scanning electron microscopy (SEM) analysis indicating the 35

presence of nanoparticles with average size of ~100- 120 nm (Figure 4). When the water content was increased to 90%, the solution became little turbid and the SEM image clearly suggests that the formation of well defined spherical type aggregates. 40

Fig. 2 UV- Vis absorption spectra of 3a-e in THF (2×10-5M). Fig. 3 Shift in absorption spectrum of 3a (2 ×10-5M) in the THF/H2O.

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Fig. 4 SEM image of 3a (2×10-5M) in THF/H2O (1:9). In THF solution, the emission spectrum of 3a shows very weak-luminescence at 424 nm (Figure 5) with a quantum yield value of 5

0.024. Interestingly, the emission efficiency is improved

enormously when water is added to the THF solution. During the addition of water to the THF solution of maleimide 3a, the emission intensity remains unchanged up to 70% of water. But after 70%, further addition of water enhances the emission 10

intensity with a red shift up to 80 nm - from 424 nm to 500 nm, reaching maximum at 90% of water content (Figure 5). At this solvent composition of THF:H2O ratio at 1:9(v/v), the emission is enhanced by 240 fold compared to its value in 100% 15

THF due to the aggregation induced emission. Similarly, when the volume fraction of water in the THF-water mixture was increased to 90%, the quantum yield value is also increased to 45%, which was nearly 20 times higher than the emission quantum yield of the THF (2.4% in solution). This substantial red 20

shift in both UV-visible absorption and emission spectral data can be assigned to the J-aggregates of the maleimides in the solvent mixture. Similar J-aggregate formation has been noticed in several other cases.35

Fig. 5 Fluorescence spectra of 3a (2×10-5M) in different ratio of THF/H2O and plot of fluorescence intensity against the water fraction of compound 3a 25

The formation of J-aggregates in the THF/ H2O mixture with high volume fractions of water (i.e., 90%) can stiffen the molecular conformation and thus block the non-radiative relaxation channel, the excited state now decays radiatively. As a 30

result, 3a becomes highly emissive in the aggregation state. There is a visible change of color from colorless to greenish yellow under the UV light after the addition of 70% of water (Figure 6) and the plot of emission intensity against the solvent composition clearly distinguishes between the molecular solutions from the 35

aggregates (Figure 5). Reports are available where maleimide derivatives bearing other groups/chromophores exhibit AIE properties.36 Thus the Diels-Alder adduct of tris (4-maleimidophenyl) amine (TMPA) with furan, or Michael adduct of TMPA with piperidine shows 40

fluorescence, but TMPA itself is nonfluorescence. When the above Diels-Alder adduct/Michael adduct is heated, it undergoes

retro-Diels-Alder reaction and hence its AIE behaviour is lost. Also the maleimides have been utilised as Michael receptor in thiol related biosensor with AIE luminogenic probe like TPE and 45

silole derivatives. Functionalization of TPE by a maleimide group completely quenches the fluorescence process of TPE in both solution and in the solid state. The AIE activity, however, is recovered by the orthogonal hydrothiolation reaction of the maleimide unit with the free thiol group in the aggregate state 50

under ambient conditions.36c To the best of our knowledge; this is the first report where the maleimide itself shows AIE characteristics.

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Fig. 6 Compound 3a in THF–water mixtures containing different volume fractions of water (0, 10, 20, 30, 40, 50, 60, 70, 75, 80, 85, and 90). Photographs taken under illumination of a UV lamp.

The AIE behaviour of 3a has also been investigated by the 5

time-resolved fluorescence technique. In the absence of water, 3a in THF has two distinct lifetimes with biexponential decay of 1.8 ns (83.2%) and 0.7 ns (16.8%). After the addition of 90% H2O, increasing amplitude of single exponential decay with longer lifetime component (5.9 ns) is observed (Figure 7). The lifetime 10

of the compound is increased by three fold after increasing the water content in the solution. The RIR effect of phenyl ring attached to nitrogen atom in the 3a is responsible for this aggregation induced emission and it is confirmed by crystal data. But, it is unclear why the other maleimides (3b-e) have not 15

exhibited AIE property.

Fig. 7 Fluorescence decay of 3a in THF solution (2 ×10-5M) (red color) and in the nanoaggregates of 90% H2O (green color).

The solid state luminescence of 3a was studied using spin 20

coated plate. 3a exhibits a very strong greenish-blue luminescence at 500 nm (Figure 8A). When the solid state form of 3a is exposed to UV light, it illuminates with bright greenish yellow color (Figure 8B). Investigation of crystal structure of this compound gives further insight into the enhancement of emission 25

in solid state and RIR effect in solution state. The crystal packing diagram of 3a shows corrugates sheet with a tight intermolecular packing arrangements (Figure 9). Maleimide have non-planar configuration and twisted geometry in their crystals with CH····π interaction, the distance between C-H bond and π cloud being 30

2.791, 2.713Å and the other side 2.752 and 3.003Å respectively. This clearly shows the propeller shaped arrangement (not disc

pile up arrangement) of molecules, which is one of the essential conditions for AIE activity in solid state (Figure 10). The enhanced emission with red shift of the aggregates is attributed to 35

the restriction of intramolecular rotation of the phenyl ring attached to nitrogen atom in the compound. On the other hand, this compound in the monomer form in THF allows active intramolecular rotations, which effectively deactivate its excitons via non-radiative pathway, thereby leaving the molecules non-40

emissive in the solution state.13

A

B 45

Fig. 8 (A) Solid-state emission spectrum of compound 3a. (B) Photograph of compound 3a taken under absence and presence of UV light. The H-bond formation between N−H···O is anti parallel to 50

each other and the bond distance of each molecule is 2.131 and 2.121Å (Figure 11). The twisted structure of this compound rules out the face-to-face π − π interactions, which generally quenches fluorescence. The compound 3a has no parallel molecular stacking, allowing the crystal to exhibit strong fluorescence. This 55

confirms the formation of J-aggregates, where the molecules are arranged in head-to-tail direction (Figures 10 and 11). The transition from the lowest excited state of molecule to ground state is hence allowed with relatively high fluorescent efficiency along with the bathochromic shift of absorption and emission 60

bands.16,17a,37 This formation of the ordered J-aggregates stimulates the efficient yellowish green AIE of the aggregates in the solvent mixtures and in the solid state. Thus the solid state X-ray crystal data are helpful in describing the AIE of 3a which is corroborative to other reports.18,38

65

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Fig. 9 Crystal packing diagram of compound 3a projected along b-axis showing corrugates sheets.

Fig. 10 Compound 3a showing C−H···π distances of neighbouring molecules and propeller shed arrangement of molecules.

Fig. 11 Ball and stick representation of chains of compound 3a showing N-H···O hydrogen bonding distances. Chains formed by different molecules of the asymmetric unit are represented with different bond colors.

Conclusion 5

In conclusion, the AIE properties of a maleimide derivative, synthesized by a facile one pot synthesis with microwave technique, have been studied. The maleimide 3a is AIE-active, whose color, fluorescence efficiency, quantum yield and fluorescence lifetime can be readily tuned by adding water. To 10

the best of our knowledge, this is the first report on the unique Aggregation Induced Emission characteristics of maleimide and it opens up new vistas for the applications in the field of optical devices, biosensor and design of new molecular materials.

15

Experimental section General procedure for the synthesis of maleimide derivatives

(3a-e):

The mixture of substituted aniline (2 mmol), and diethyl/dimethyl 20

acetylenedicarboxylate (1 mmol) was thoroughly mixed at room temperature and then the reaction mixture was poured into microwave vial, which was tightly locked with Teflon cap and subjected to irradiation at 180oC, 220 Watt for 10 minutes in a microwave chamber. After the reaction, maleimide was obtained 25

as golden yellow solid. The product was washed with dichloromethane and the pure maleimide was filtered. The yield of the product was excellent ranging from 95-98 %. All the reactions were carried out in a CEM Discover Microwave synthesizer. 30

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Photophysical measurements Electronic absorption spectra are recorded on Analytik Jena Specord S100 spectrophotometer using 1 cm path length cuvette. Emission spectra are measured using JASCO FP6300 spectrofluorimeter. HPLC grade THF and double distilled water 5

were employed in all photophysical and photochemical measurements. Emission bandpass used in these measurements was 5 nm. Emission quantum yields are measured at quinine sulphate in 0.1 N H2SO4 solution as reference.39 The morphology of the nanoaggregates were studied using field emission scanning 10

electron microscopy (FESEM) (Carl Zeiss) equipped with energy dispersive X-ray analysis (EDX).

Excited state lifetime measurement Fluorescence decays were recorded using time correlated single photon counting (TCSPC) method. A diode pumped millena CW 15

laser (Spectra Physics) 532 nm was used to pump Ti:Sapphire rod in Tsunami picosecond mode locked laser system (Spectra Physics). The 750 nm (8 MHz) line was taken from the Ti:Sapphire laser and passed through a pulse picker (Spectra Physics, 3980 2s) to generate 80 kHz pulses. The second 20

harmonic output (375 nm) was generated by a flexible harmonic generator (Spectra Physics, GWU 23 ps). The vertically polarised 375 nm laser was used to excite the sample. The fluorescence emission at the magic angle (54.7º) was dispersed in a monochromator (f/3 aperture), counted by a MCP PMT 25

(Hamamatsu R 3809) and processed through CFD, time-to-amplitude converter (TAC) and multi channel analyzer (MCA). The instrument response function for this system is ≈ 52 ps and the fluorescence decay was analyzed by using the software provided by IBH (DAS-6) and PTI global analysis software. 30

Acknowledgment

We thank DST-IRHPA for NMR measurements. M. Boominathan thanks to UGC-SRF for generous funding. The authors thank Prof. P. Ramamurthy, Director, NCUFP, University of Madras, Chennai for his help in the TCSPC. For SEM analysis, 35

Centre for Nanotechnology, Bharathidasan University, Thiruchirapalli is gratefully acknowledged.

Notes and references aSchool of Chemistry, Madurai Kamaraj University, Madurai – 625 021, India. Fax: 91-452-2459139; Tel: 91-452-2458246; E-mail: 40

muthumanian2001@yahoo.com (S. Muthusubramanian) rajagopalseenivasan@yahoo.com (S. Rajagopal) bX-ray Diffraction Laboratory, Department of chemistry, Texas A&M University,College station, Texas 77842, United States 45

† Electronic Supplementary Information (ESI) available: [Crystallographic data and refinement for compound 3a, data and IR and NMR spectra for all maleimide derivatives]. See DOI: 10.1039/b000000x/ 50

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DOI: 10.1039/C3RA42809E

This journal is © The Royal Society of Chemistry [year] Journal Name, [year], [vol], 00–00 | 7

21 (a) R. Hu, C. F. A. G. Duran, J. W. Y. Lam, J. L. B. Vazquez, C. Deng, S. Chen, R. Ye, E. P. Cabrera, Y. Zhong, K. S. Wong and B. Z. Tang, Chem. Commun., 2012, 48, 10099; (b) K. Perumal, J. A. Garg, O. Blacque, R. Saiganesh, S. Kabilan, K. K. Balasubramanian and K. Venkatesan, Chem. Asian J., 2012, 7, 2670. 5

22 (a) J. Wang, J. Mei, R. Hu, J. Z. Sun, A. Qin and B. Z. Tang, J. Am. Chem. Soc., 2012, 134, 9956; (b) J. Wang, J. Mei, R. Hu, J. Z. Sun, A. Qin and B. Z. Tang, J. Am. Chem. Soc., 2012, 134, 9956.

23 (a) N. B. Shustova, B. D. McCarthy and M. Dinca, J. Am. Chem. Soc., 2011, 133, 20126; (b) N. B. Shustova, T.-C. Ong, A. F. 10

Cozzolino, V. K. Michaelis, R. G. Griffin and M. Dinca, J. Am. Chem. Soc., 2012, 134, 15061.

24 B. Manimaran, P. Thanasekaran, T. Rajendran, R. J. Lin, I. J. Chang, G. H. Lee, S. M. Peng, S. Rajagopal and K. L. Lu, Inorg. Chem., 2002, 41, 5323. 15

25 (a) S. Chen, Y. Hong, Y. Liu, J. Liu, C. W. T. Leung, M. Li, R. T. K. Kwok, E. Zhao, J. W. Y. Lam, Y. Yu and B. Z. Tang, J. Am. Chem. Soc., 2013, 135, 4926. (b) M. Faisal, Y. Hong, J. Liu, Y. Yu, J. W. Y. Lam, A. Qin, P. Lu and B. Z. Tang, Chem.-Eur. J., 2010, 16, 4266; (c) W. C. Wu, C. Y. Chen, Y. Tian, S. H. Jang, Y. Hong, Y. 20

Liu, R. Hu and B. Z. Tang, Y. T. Lee, C. T. Chen, W. C. Chen and A. K. Y. Jen, Adv. Funct. Mater., 2010, 20, 1413.

26 (a) X. Xu, J. Li, Q. Li, J. Huang, Y. Dong, Y. Hong, J. Yan, J. Qin, Z. Li and B. Z. Tang, Chem. Eur. J., 2012, 18, 7278; (b) H. Lu, B. Xu, Y. Dong, F. Chen, Y. Li, Z. Li, J. He, H. Li and W. Tian, 25

Langmuir, 2010, 26, 6838. (c) Y. Hong, C. Feng, Y. Yu, J. Liu, J. W. Y. Lam, K. Q. Luo and B. Z. Tang, Anal. Chem., 2010, 82, 7035; (d) M. Wang, G. Zhang, D. Zhang, D. Zhu and B. Z. Tang, J. Mater. Chem., 2010, 20, 1858 and references there in.

27 Y. Liu, Y. Tang, N. N. Barashkov, I. S. Irgibaeva, J. W. Y. Lam, R. 30

Hu, D. Birimzhanova, Y. Yu and B. Z. Tang, J. Am. Chem. Soc., 2010, 132, 13951.

28 Y. Hong, L. Meng, S. Chen, C. W. T. Leung, L. T. Da, M. Faisal, D. A. Silva, J. Liu, J. W. Y. Lam, X. Huang and B. Z. Tang. J. Am. Chem. Soc., 2012, 134, 1680. 35

29 Y. Li, L. Xu and B. Su, Chem. Commun., 2012, 4109. 30 (a) H. Shi, J. Liu, J. Geng, B. Z. Tang and B. J. Liu, Am. Chem. Soc.,

2012, 134, 9569; (b) A. Qin, J. W. Y. Lam and B. Z. Tang, Prog. Polym. Sci., 2012, 37,182; (c) Z. Zhao, J. W. Y. Lam and B. Z. Tang, Curr. Org. Chem., 2010, 14, 2109. 40

31 (a) M. A. Walker J. Org. Chem., 1995, 60, 5352; (b) G. B. Gill, G. D. James, K. V. Oates and G. Pattenden, J. Chem. Soc. Perkin Trans.1. 1993, 2567; (c) A. Alizadeh, F. Movahedi and A. A. Esmaili, Tetrahedron Lett., 2006, 4, 4469.

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Krishnakumar, Tetrahedron, 2011, 67, 6057; (b) S. Chitra, N. Paul, S. Muthusubramanian and P. Manisankar, Green Chem., 2011, 13, 2777; (c) M. Nagaraj, M. Boominathan, S. Muthusubramanian and N. Bhuvanesh, Synlett, 2012, 23, 1353.

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34 B. Z. Tang, Y. Geng, J. W. Y. Lam, B. Li, X. Jing, X. Wang, F. Wang, A. Pakhomov and X. X. Zhang, Chem. Mater., 1999, 11, 1581.

35 (a) B.-K. An, J. Gierschner and S. Y. Park, Acc. Chem. Res., 2012, 45, 544; (b) J. Yi, Z. Chen, J. Xiang and F. Zhang. Langmuir, 2011, 55

27, 8061; (c) H. Tong, Y. Hong, Y. Dong, Y. Ren, M. Haussler, J. W. Y. Lam, K. S. Wong and B. Z. Tang, J. Phys. Chem. B., 2007, 111, 2000.

36 (a) X. Wang, J. Hu, T. Liu, G. Zhang and S. Liu, J. Mater. Chem., 2012, 22, 8622; (b) X. Zhang, Z. C. Li, K. B. Li, F. S. Du and F. M. 60

Li, J. Am. Chem. Soc., 2004, 126, 12200; (c) W.Y. Lam, Y. Liu, B. Z. Tang and Y. Yu, US patent, US20120237964 A1, 2012.

37 C.-T. Lai and J. -L. Hong, J. Phys. Chem. B., 2010, 114, 10302. 38 (a) Z. Zhao, S. Chen, J. W. Y. Lam, Z. Wang, P. Lu, F. Mahtab, H.

H. Y. Sung, I. D. Williams, Y. Ma, H. S. Kwok and B. Z. Tang, J. 65

Mater. Chem., 2011, 21, 7210; (b) H.-H. Fang, Q.-D. Chen, J. Yang, H. Xia, B.-R. Gao, J. Feng, Y.-G. Ma and H.-B. Sun, J. Phys. Chem. C., 2010, 114, 11958.

39 J. N. Demas and G. A. Crosby, J. Phys. Chem., 1971, 75, 991. 70

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DOI: 10.1039/C3RA42809E