Efficient photoluminescence from the encumbered platinum(II) porphyrins

    Efficient Photoluminescence from the Encumbered Platinum(II) Porphyrins Wei Yuan, Yue Zhang, Hui Li, Qingjiang Ren, Yanxiang Cheng PII: DOI: Reference:

S1387-7003(14)00176-2 doi: 10.1016/j.inoche.2014.04.014 INOCHE 5553

To appear in:

Inorganic Chemistry Communications

Received date: Revised date: Accepted date:

3 March 2014 9 April 2014 11 April 2014

Please cite this article as: Wei Yuan, Yue Zhang, Hui Li, Qingjiang Ren, Yanxiang Cheng, Efficient Photoluminescence from the Encumbered Platinum(II) Porphyrins, Inorganic Chemistry Communications (2014), doi: 10.1016/j.inoche.2014.04.014

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ACCEPTED MANUSCRIPT Efficient Photoluminescence from the Encumbered Platinum(II) Porphyrins

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied

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a

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Wei Yuana,b, Yue Zhanga, Hui Li a,*, Qingjiang Rena, and Yanxiang Chenga,*

Chemistry, Chinese Academy of Sciences, Changchun 130022, P.R. China University of Chinese Academy of Sciences, Beijing 100039, P.R. China

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b

Completely enclosed

platinum(II)

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Abstract porphyrins

exhibit

the

dramatic

improvements

in

photoluminescence properties since the intermolecular π-π interactions and intramolecular vibrations are effectively suppressed by the encapsulating and steric effect of peripheral bulky and

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saturated substituents. The absolute photoluminescence quantum yields (PLQY) is up to 0.46-0.55 and significantly improved over their parent PtTPP.

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Keywords: photoluminescence; porphyrin; platinum; intermolecular interaction

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In the natural products and synthetic dendrimers, porphyrin units as bio-active sites are always embedded into the interior of molecules, in which the peripherally bulky substituents play a

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protection and assistance role for their biological activities [1]. Similarly, by mimicking these macromolecules, Pt(II) porphyrins as emitters should show better performance while inhibiting the energy transfer from the intermolecular interaction, especially the π-π interaction, although the molecular packing from the “naked” complexes is beneficial to charge mobility in field effect transistors [2]. In fact, a few researches have demonstrated the necessity of this method [3]. For example, Ikai et al reported the facially encumbered and sterically bulky meso-aryl Pt(II) porphyrin complexes that distinctly exhibit improved emissive performance [3a]. However, more remarkable development was expected to be comparable to the potential iridium complexes, if the self-quenching induced by the π-π stacking interaction could be avoided more effectively [4]. Herein, we report four Pt(II) porphyrin derivatives, in which the core was enclosed by the saturated and bulky substituents that are favour of removing the intermolecular π-π interaction 1

ACCEPTED MANUSCRIPT based on their non-aromatic character, and two corresponding precursors (Fig. 1). The preparation of complexes involved two steps [5]: the hydrolysis of methoxyl substituted Pt(II) tetraphenylporphyrin (PtTPP) using excess BBr3, then silylation in the presence of

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4-dimethylamiopyridine to give the partly and fully silylated mixtures, which could be isolated by careful column chromatography. These complexes show excellent solubility in organic solvents

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including petroleum ether. The structures and purities of all Pt(II) complexes were confirmed by 1

H-NMR and MALDI-TOF mass spectra. More fortunately, the crystals of complexes DB and TB

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were obtained and their structures were successfully determined by X-Ray diffraction. As shown in Fig. 1, the central Pt(II) porphrin unit is completely encapsulated by the peripheral groups,

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while it maintains the nearly ideal square planar coordination geometry. The complex TB exhibits an approximately layered-stacked molecular arrangement without the directly intermolecular Pt-Pt and π-π interactions between Pt(II) porphyrin rings. In contrast, a fairly weak face-to-edge π-π

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interaction is observed in DB with the distances of 3.936 and 3.992 Å between meso-phenyl rings (Fig. S6). Eight tert-butyldimethylsilyl groups are very tightly and nearly symmetrically located

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on both faces of the porphyrin ring with the shortest H---H distance of 2.163 Å (in complex DB, the distance of the corresponding C---C linked with above H atoms is 3.606 Å), which is similar

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with the analogous Zn porphyrin complex [5c]. The encumbered structural characters definitely show that the bulky silyl groups can not only effectively protect the Pt(II) porphyrin emitting

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centre from the intermolecular interaction, but also restrict the meso-phenyl rings rotating freely to decrease the probability of non-radiative transition [3a, 5d, 6]. As depicted in Fig. 2, the UV-Vis absorption of the Pt(II) porphyrin complexes has the same characteristics as the other analogues [6, 7], showing an intense Soret band around 400 nm and two well-resolved Q bands in the range of 508-512 nm and 539-542 nm. Like their parent PtTPP, all complexes show a strong emission band around 655 nm and a weaker shoulder around 709-720 nm in the deep-red region. Furthermore, these emissions properties hardly change with either the solution concentration (1×10-6~2×10-5 M) or the solvent polarity (Fig. S9-S12). Hence, the saturated bulky groups have little influence on the absorption and emission. The lifetimes of Pt(II) porphyrin complexes ranging from 27 to 60 μs fall within the scope of the meso-aryl-substituted Pt(II) porphyrins reported in literatures (Table 1) [6, 8]. The instinct lifetimes of two emission bands are nearly identical, which are assigned to the same radiative 2

ACCEPTED MANUSCRIPT transition process from excited states (3LC) to ground state (0LC). Compared with other reported meso-aryl-substituted platinum(II) porphyrin complexes, four complexes DB, TB, DP and TP exhibit the superior PLQY (0.46–0.55 in degassed dilute solution of 2-MeTHF, Table 1). For

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example, the PLQY of octa-sites substituted complex DP is 0.55 and increases several times as high as the complexes DM (ΦPL = 0.08), TM (ΦPL = 0.10) and PtTPP (ΦPL = 0.045) [6, 8].

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Certainly, the enhancement of PLQY results from the encumbered molecular structure, in which the saturated and bulky groups not only effectively restrict the free rotation of the meso-phenyl

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rings due to the sterically hindrance effect, but also avoid from the intermolecular energy transfer due to their encapsulation. The two factors lead to the increasing of the rigidity of the Pt(II)

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porphyrin emissive core, reflecting in the rapidly increased radiative decay rate (Table 1). To further understand the improvement in photoluminescence due to introduction of the saturated and bulky substituents, the PL spectra and lifetimes at different doping concentration in

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PMMA and neat film were measured. Compared with the spectra in solution, four Pt(II) porphyrin complexes show a nearly identical profile with a very small red shift below 5 nm (Fig. 3 and Fig.

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S13–S18). The emission intensity gradually increases with the increasing of doping concentration and maximizes at neat film. In contrast, for complexes DM and TM, there is a sharp decrease in

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emission intensity, especially in higher doping concentration, while the emission is thoroughly quenched at neat film state.

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The lifetimes of the Pt(II) porphyrin complexes in the PMMA matrix and neat film showed a different downward trend with the increasing of the doping concentration. In Fig. 4, a sharp decrease of lifetime has been observed for complex DM due to aggregate and self-quenching [8a, 9], while the lifetime of complex DB decreased slowly under the same condition. The trend is similar to that observed for the PL intensity of DM, suggesting that the intermolecular interaction as a deactivation channel of the excite state for DM is easier to occur than DB, i.e. the increased knr lead to a rapid drop in the lifetime at high doping concentration and neat film for DM (Table 1). Other three Pt(II) porphyrin complexes (DP, TB and TP) also show slow lifetime roll-off at high doping concentration and neat film. Especially, the lifetimes of dode-sites substituted TB and TP display a smaller descent relative to the octa-sites substituted DB and DP, which indicate that more substituents are more effective to eliminate intermolecular interactions, as discussed in crystal structure analysis section. 3

ACCEPTED MANUSCRIPT In summary, four Pt(II) porphyrin complexes enclosed completely by the saturated and bulky silyl groups have been synthesized successfully and their photophysical properties were studied and compared to those of their parent complexes DM, TM and PtTPP. Dramatic improvement in

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luminescence including high PLQY, short lifetime and no self-quenching as well as good solubility from these complexes were observed. Non-aromatic character of the peripheral bulky

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substituens not only effectively eliminate the intermolecular π-π interactions but also restrict the free rotation of meso-phenyl, that is, increasing the rigidity of the central Pt(II) emitting core.

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These findings have been confirmed definitely by the variation tendency of PL intensity and lifetime at different doping concentration and neat film. Further application research including

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their electroluminescent performance is ongoing.

Acknowledgements

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The authors are grateful to the National Natural Science Foundation of China (No. 51073152 and

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21204083).

References

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[1] (a) W.-S. Li and T. Aida, Dendrimer Porphyrins and Phthalocyanines, Chem. Rev. 109(2009) 6047-6076; (b) S. Hecht and J. M. J. Frechet, Dendritic encapsulation of function: Applying nature's site isolation principle from biomimetics to materials science, Angew. Chem., Int. Ed. 40(2001) 74-91; (c) L.-L. Li and E. W.-G. Diau, Porphyrin-sensitized solar cells, Chem. Soc. Rev. 42(2013) 291-304. [2] C. M. Che, H. F. Xiang, S. S. Chui, Z. X. Xu, V. A. Roy, J. J. Yan, W. F. Fu, P. T. Lai and I. D. Williams, A high-performance organic field-effect transistor based on platinum(II) porphyrin: peripheral substituents on porphyrin ligand significantly affect film structure and charge mobility, Chem. Asian J. 3(2008) 1092-1103. [3] (a) M. Ikai, F. Ishikawa, N. Aratani, A. Osuka, S. Kawabata, T. Kajioka, H. Takeuchi, H. Fujikawa and Y. Taga, Enhancement of external quantum efficiency of red phosphorescent organic light-emitting devices ices with facially encumbered and bulky Pt-II porphyrin complexes, Adv. Fun. Mater. 16(2006) 515-519; (b) C. Huo, H. D. Zhang, H. Y. Zhang, H. Y. Zhang, B. Yang, P. Zhang and Y. Wang, Synthesis and assembly with mesoporous silica MCM-48 of platinum(II) porphyrin complexes bearing carbazyl groups: Spectroscopic and oxygen sensing properties, Inorg. Chem. 45(2006) 4735-4742; (c) J. M. Lupton, I. D. W. Samuel, M. J. Frampton, R. Beavington and P. L. Burn, Control of 4

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CE

[5]

PT

ED

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US

CR

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[4]

Electrophosphorescence in Conjugated Dendrimer Light-Emitting Diodes, Adv. Fun. Mater. 11(2001) 287-294. (a) J. Ding, B. Wang, Z. Yue, B. Yao, Z. Xie, Y. Cheng, L. Wang, X. Jing and F. Wang, Bifunctional Green Iridium Dendrimers with a "Self-Host" Feature for Highly Efficient Nondoped Electrophosphorescent Devices, Angew. Chem., Int. Ed. 48(2009) 6664-6666; (b) H. Li, J. Ding, Z. Xie, Y. Cheng and L. Wang, Synthesis, characterization and electrophosphorescent properties of mononuclear platinum(II) complexes based on 2-phenylbenzoimidazole derivatives, J. Organometallic Chem. 694(2009) 2777-2785; (c) J.-L. Chen, X.-X. Chen, X.-Z. Tan, J.-Y. Wang, X.-F. Fu, L.-H. He, Y. Li, G.-Q. Zhong and H.-R. Wen, A new luminescent platinum(II) chloride complex of 6-(5-trifluoromethyl1,2,4- triazol-3-yl)-4,4′-dimethyl-2,2′-bipyridine, Inorg. Chem. Commun. 35(2013) 96-99; (d) K.-J. Chen, H.-B. Xu, L.-Y. Zhang and Z.-N. Chen, Cyclometalated platinum(II) complex with C^N^N tridentate ligand as sensitizer for lanthanide luminescence, Inorg. Chem. Commun. 12(2009) 744-746; (e) X.-C. Hang, T. Fleetham, E. Turner, J. Brooks and J. Li, Highly Efficient Blue-Emitting Cyclometalated Platinum(II) Complexes by Judicious Molecular Design, Angew. Chem., Int. Ed. 52(2013) 6753-6756; (f) G. Cheng, P.-K. Chow, S. C. F. Kui, C.-C. Kwok and C.-M. Che, High-Efficiency Polymer Light-Emitting Devices with Robust Phosphorescent Platinum(II) Emitters Containing Tetradentate Dianionic O^N^C^N Ligands, Adv. Mater. 25(2013) 6765-6770; (g) N. Komiya, A. Yoshida and T. Naota, Synthesis and structure of vaulted trans-Bis[1(2-phenoxy)-imidazol-2-ylidene-C2,O]platinum(II) complex, Inorg. Chem. Commun. 27(2013), 122-126. (a) J. S. Lindsey and R. W. Wagner, Investigation of the synthesis of ortho-substituted tetraphenylporphyrins, J. Org. Chem. 54(1989) 828-836; (b) E. Tsuchida, T. Komatsu, E. Hasegawa and H. Nishide, Synthesis, characterization, and oxygenation of bis-fenced porphyrinato iron(II) and cobalt(II) complexes, J. Chem. Soc. Dalton Trans. 1990 2713-2718; (c) A. Sen and K. S. Suslick, Shape-Selective Discrimination of Small Organic, J. Am. Chem. Soc. 122(2000) 11565-11566; (d) R. C. Kwong, S. Sibley, T. Dubovoy, M. Baldo, S. R. Forrest and M. E. Thompson, Efficient, saturated red organic light emitting devices based on phosphorescent platinum(II) porphyrins, Chem. Mater. 11(1999) 3709-3713; (e) M. Fang, S. R. Wilson and K. S. Suslick, A four-coordinate Fe(III) porphyrin cation, J. Am. Chem. Soc. 130(2008) 1134-1135. W. Wu, W. Wu, S. Ji, H. Guo, X. Wang and J. Zhao, The synthesis of 5,10,15,20-tetraarylporphyrins and their platinum(II) complexes as luminescent oxygen sensing materials, Dyes and Pigments 89(2011) 199-211. P. Chen, O. S. Finikova, Z. Ou, S. A. Vinogradov and K. M. Kadish, Electrochemistry of Platinum(II) Porphyrins: Effect of Substituents and π-Extension on Redox Potentials and Site of Electron Transfer, Inorg. Chem. 2012, 51, 6200-6210; (a) F. Nifiatis, W. Su, J. E. Haley, J. E. Slagle and T. M. Cooper, Comparison of the Photophysical Properties of a Planar, PtOEP, and a Nonplanar, PtOETPP, Porphyrin in Solution and Doped Films, J Phys. Chem. A 115(2011) 13764-13772;

[6]

[7]

[8]

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(b) S. Drouet, C. O. Paul-Roth, V. Fattori, M. Cocchi and J. A. G. Williams, Platinum and palladium complexes of fluorenyl porphyrins as red phosphors for light-emitting devices, New J. Chem. 35(2011) 438-444. [9] A. K. Bansal, W. Holzer, A. Penzkofer and T. Tsuboi, Absorption and emission spectroscopic characterization of platinum-octaethyl-porphyrin (PtOEP), Chem. Phys. 330(2006) 118-129.

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0.8 0.6 0.4 0.2 0.0

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Absorbance (a.u.)

1.0

DP DB DM TP TB TM

1.0 0.8 0.6 0.4

PL Intensity (a.u.)

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Fig. 1 Molecular structures of Pt(II) porphyrins and crystal structure of TB (H atoms have been omitted for clarity).

0.2 0.0

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300 350 400 450 500 550 600 650 700 750 800

Wavelength(nm)

600 5% 8% 10% 25% 50% 75% neat film

500

PL Intensity

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Fig. 2 The normalized UV-vis and emission spectra of the Pt(II) porphyrins (in 2-MeTHF).

400 300 200 100 0 600

650

700

Wavelength(nm)

750

800

Fig. 3 The emission spectra of DB at different doping concentration in PMMA and neat film excited at 400 nm.

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PL Intensity(a.u.)

100

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DB DM 0

0.1

25

50

75

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10

Lifetime(us)

10

100

100

Doping concentration (wt%)

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Fig. 4 The PL intensity (solid) and lifetime (open) curve of DB (solid line) and DM (dash line) at different doping

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concentration in PMMA and neat film.

Table 1: Spectroscopic and photophysical data of the platinum(II) porphyrins in 2-MeTHF.

DM

TP

TM

652/36.43,

542/3.71

400/5.12, 508/4.10,

655/40.32,

539/3.71

709/42.82

404/5.18, 510/4.19,

654/31.30,

540/3.79

719/33.82

405/5.35, 512/4.35,

656/53.13,

542/3.90

720/51.07

404/5.25, 509/4.29,

652/57.25,

540/3.77

712/60.22

540/3.91 403/5.06, 511/4.03,

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TB

τ (μs)

[b]

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DB

401/5.27, 510/4.26,

λem (nm)[a]/

718/38.69

654/27.53, 718/27.39

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DP

λabs (nm)/log(ε)[a]

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Complex

ΦPL[b]

kr 4 -1 [c]

(×10 s )

knr 4 -1 [c]

(×10 s )

emission lifetime τ (μs) τ1 (8%)[d]

τ2 (neat film)[d]

0.55

1.53

1.25

33.03

20.68

0.46

1.64

1.93

20.41

7.95

0.08

0.20

2.30

25.89

0.06

0.50

1.61

1.65

11.35

7.67

0.48

0.91

0.98

11.44

11.06

0.10

0.18

1.57

38.08

0.09

[a] Measured at room temperature (10-5 M in 2-MeTHF); [b] Determined in degassed 2-MeTHF using the optically dilute solution at room temperature; [c] The kr and knr values are calculated by followed equations on the assumption that ΦISC is 1.0. (1) ФPL = ΦISC[kr/(kr+knr)]; (2) τ = 1/(kr+knr); [d] Performed in 8% doped PMMA and neat film at room temperature excited at 410 nm (detected at 655 nm).

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ACCEPTED MANUSCRIPT Efficient Photoluminescence from the Encumbered Platinum(II) Porphyrins

250 200

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PL intensity (a.u.)

300

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Wei Yuan, Yue Zhang, Hui Li, Qingjiang Ren, and Yanxiang Cheng

150 100

2 x10-5 1 x10-5 8 x10-6 6 x10-6 5 x10-6 4 x10-6 3 x10-6 2 x10-6 1 x10-6 TP in Tol, 400nm ex

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0

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600

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Graphical abstract

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650

700

Wavelength (nm)

750

ACCEPTED MANUSCRIPT Efficient Photoluminescence from the Encumbered Platinum(II) Porphyrins

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Wei Yuan, Yue Zhang, Hui Li, Qingjiang Ren, and Yanxiang Cheng

The encumbered Pt(II) porphyrins display the dramatic improvements in

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photoluminescence compared to their parents, and their emission gradually increases

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Graphical Abstract - Synopsis

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with the increasing of concentration.

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ACCEPTED MANUSCRIPT Highlights

(1)

We have successfully synthesized the octa-sites and dode-sites substituted Pt(II)

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tetraphenylporphyrin complexes, in which the Pt(II) porphyrin emitting centre can completely encapsulated by the peripheral saturated and bulky silyl groups. The introducing of these peripheral substituents can more effectively suppress

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(2)

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the intermolecular π-π interactions and intramolecular vibrations of meso-phenyl, and decrease the probability of non-radiative transition.

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The novel complexes exhibit the dramatic improvements in photoluminescence

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relative to their parents based on the increase of the molecular rigidity.

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(3)

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