Scientific paper
Synthesis, Crystal Structure, Photophysical Properties and Theoretical Study of a New Iridium(III) Complex Containing 2-phenylbenzothiazole Ligand
Yong-Pi Zeng,1 Cheng-Wei Gao,1 Liang-Jiang Hu,1 Hao-Hua Chen,1 Guang-Ying Chen,2 Gao-Nan Li1* and Zhi-Gang Niu12*
1 College of Chemistry and Chemical Engineering, Hainan Normal University, Haikou 571158, China
2 Key Laboratory of Tropical Medicinal Plant Chemistry of Ministry of Education, Hainan Normal University,
Haikou 571158, China
* Corresponding author: E-mail: ligaonan2008@163.com, niuzhigang1982@ 126.com
Received: 08-06-2015
Abstract
A new bis-cyclometalated iridium(III) complex [Ir(dmabt)2(bipy)][PF6] (3) (dmabt = 4-(benzo[d]thiazol-2-yl)-N,N-di-methylaniline, bipy = 2,2'-bipyridine) has been synthesized and fully characterized. The structure of complex 3 has been determined by X-ray analyses which shows that the central iridium(III) ion assumes distorted octahedral geometry. The photoluminescence spectrum exhibits orange emission maximum at 612 nm with quantum yield of 17% at 298 K. The frontier molecular orbital diagrams and the spin-allowed singlet-singlet electronic transitions of 3 have been calculated with density functional theory (DFT) and time-dependent DFT (TD-DFT), and the UV-Vis spectra are discussed based on the theoretical calculations.
Keywords: Iridium(III) complex; 2-phenylbenzothiazole; Syntheses; Crystal structure; Photoluminescence; DFT calculation
1. Introduction
In recent decades, organic light-emitting diodes (OLEDs) have received considerable attention as a promising technology for practical optoelectronic applica-tions.1-3 Particularly, luminescent iridium-based complexes play an important role in the fabrication of efficient OLEDs on account of their high stability, large quantum efficiency, short excited-stated lifetime and excellent color tunability.4,5 Moreover, substantial researches on iridium complexes have showed that the emission wavelength can be tuned by judicious selection of various li-gands, which can be further refined by altering the ligand substituents.6 In 2004, the research group lead by T. M. Chen et al. has synthesized and reported a series of iri-dium(III) complexes with 2-phenylbenzothiazole-based ligands. And they confirmed that varying the substituents (-CF3, -F, -Me, -OMe) on the 2-phenylbenzothiazole li-gands could fine-tune their solution-state photophysical properties.7 In this paper, we report on the introduction of
a stronger electron-donating group [-N(Me)2] as the sub-stituent. Based on the new 2-phenylbenzothiazole-based ligand (1), the corresponding Ir(III) complex 3 has been synthesized. The photophysical properties of 3 are investigated and the absorption spectra are rationalized on the basis of density functional theory (DFT) and time-dependent DFT (TDDFT).
2. Experimental
2. 1. Materials and Instrumentations
4-(Dimethylamino)benzaldehyde and 2-amino-benzenethiol were obtained from Sigma Adrich. IrCl3 ■ 3H2O was industrial product. All commercial chemicals were used without further purification unless otherwise stated. Solvents were dried and degassed following standard procedures. 1H NMR and 13C NMR spectra were recorded on a Bruker AM 400 MHz instrument. Chemical shifts were reported in ppm relative to Me4Si
as internal standard. ESI-MS spectra were recorded on an Esquire HCT-Agilent 1200 LC/MS spectrometer. FT-IR spectra were taken on a Nicolet 6700 FTIR spectrometer (400-4000 cm1) with KBr pellets. Elemental analyses for C, H, and N were performed on a Vario EL Elemental Analyser. UV-Vis spectra were recorded on a Hitachi U3900/3900H spectrophotometer. Fluorescence spectra were carried out on a Hitachi F-7000 spec-trophotometer.
2. 2. Synthesis of 4-(benzo[d]thiazol-2-yl)-N,N-dimethylaniline (dmabt, 1)
A mixture of 4-(dimethylamino)benzaldehyde (500 mg, 3.35 mmol) and 2-aminobenzenethiol (420 mg, 3.35 mmol) in DMSO (20 mL) was stirred at 100 °C for 10 h. After cooling, the solution was poured into ice water and extracted with DCM. The combined organic layer was washed with brine, dried over Na2SO4 and evaporated. The residue was purified by column chromatography (PE : EA = 20 : 1, Rj = 0.35) to afford pure product 1 (798 mg, yield: 93.6%) as a white solid. IR (KBr, cm-1): 2902 (w), 2809(w), 1610(vs), 1484(vs), 1430(s), 1186 (s), 1065 (m), 963(m), 817(s), 751(s); 1H NMR (400 MHz, CDCl3) 7.95Č7.99 (m, 3H), 7.84 (d, J = 7.6 Hz, 1H), 7.43 (t, J = 7.6 Hz, 1H), 7.30 (t, J = 7.6 Hz, 1H), 6.75 (d, J = 8.0 Hz, 2H), 3.06 (s, 6H); 13C NMR (100 MHz, CDCl3) 168.9, 154.6, 152.3, 134.7, 129.0, 126.1, 124.3, 122.4, 121.6, 121.5, 111.8, 40.3; MS-ESI: m/z 255.2 (M+1); Anal.
Calcd for C15H14N2S: C, 70.83; H, 5.55; N, 11.01; Found: C, 70.72; H, 5.50; N, 11.13.
2. 3. Synthesis of [Ir(dmabt)2(bipy)] [PFJ (3)
A mixture of IrCl3 ■ 3H2O (63 mg, 0.18 mmol) and the ligand 1 (100 mg, 0.39 mmol) in 9 mL of ethoxyetha-nol and H2O (v : v = 2 : 1) was refluxed for 12 h. Upon cooling to room temperature, the orange precipitate was collected by filtration and washed with cooled ether and MeOH. After drying, the crude product of chlorido-brid-ged dimer complex 2 was used directly in next step without further purification. A mixture of the above dimer complex 2 (80 mg) and 2,2'-bipyridine (21 mg, 2.5 equiv.) was dissolved in 6 mL of DCM and MeOH (v : v = 1 : 1) and was refluxed for 6 h under nitrogen. The orange-red solution was then cooled to room temperature, and NH4PF6 (5.0 equiv.) was added to the solution. The mixture was stirred at room temperature for 4 h, and then evaporated to dryness. The solid was purified by column chromatography with DCM / MeOH (100 : 1, Rj = 0.3) eluent to afford pure product 3 (54 mg, yield: 49.6%) as a yellow solid. IR (KBr, cm-1): 2963 (s), 2925(w), 2854(w), 1578(s), 1426(s), 1391(s), 1261 (vs), 1020 (vs), 799(vs), 755(s); UV-Vis (nm): 242, 268, 295, 335, 392, 418, 436; 1H NMR (400 MHz, CDCl3) 8.55 (d, J = 8.0 Hz, 2H), 8.13Č8.18 (m, 4H), 7.69 (d, J = 7.6 Hz, 2H), 7.59 (d, J = 8.8 Hz, 2H), 7.17 (t, J = 7.6 Hz, 2H), 6.98 (t,
J = 7.6 Hz, 2H), 6.38 (dd, J1 = 2.4 Hz, J2 = 8.8 Hz, 2H), 6.02 (d, J = 8.4 Hz, 2H), 5.61 (d, J = 2.0 Hz, 2H), 2.63 (s, 12H). 13C NMR (100 MHz, CDCl3) 179.0, 155.7, 151.9, 151.3, 150.0, 148.7, 138.8, 136.1, 129.4, 127.1, 126.9, 126.7, 123.7, 123.5, 122.0, 115.7, 113.6, 106.2, 38.6; MS-ESI: m/z 855.2 [MPF6]+. Anal. Calcd for C40H34F6IrN6PS2: C, 48.04; H, 3.43; N, 8.40; Found: C, 48.11; H, 3.269; IN, 8.53.
2. 4. Crystallographic Studies
X-ray diffraction data were collected with an Agilent Technologies Gemini A Ultra diffractometer equipped with graphite-monochromated Mo Ka radiation (X = 0.71073 À) at room temperature. Data collection and reduction were processed with CrysAlisPro software.8 The structure was solved and refined using full-matrix least-squares based on F2 with program SHELXS-97 and SHELXL-979 within Olex2.10 All non-hydrogen atoms were found in alternating difference Fourier syntheses and least-squares refinement cycles and, during the final cycles, refined anisotropically. Hydrogen atoms were placed in calculated positions and refined as riding atoms with a uniform value of Uiso.
2. 5. Computational Method
The geometry of complex 3 was optimized starting from the X-ray data by the DFT (density functional theory) method with B3LYP (Becke three-parameter Lee-Yang-Parr) hybrid density functional theory and the 6-31G* basis set. All calculations were carried out with Gaussian 09 software package.11
2. 6. Luminescence Quantum Efficiency
The luminescence quantum efficiencies were calculated by comparison of the fluorescence intensities (integrated areas) of a standard sample fac-Ir(ppy)3 and the unknown sample according to the equation.12-14
Where Фшк and Ф^ are the luminescence quantum yield values of the unknown sample andfac-Ir(ppy)3 solutions (Фй(1 = 0.4),14 respectively. Iunk and Istd are the integrated fluorescence intensities of the unknown sample and fac-Ir(ppy)3 solutions, respectively. Aunk and Astd are the absorbance values of the unknown sample and fac-Ir(ppy)3 solutions at their excitation wavelengths, respectively. The nunk and nstd terms represent the refractive indices of the corresponding solvents (pure solvents were assumed).
3. Results and Discussion 3. 1. Description of Crystal Structure
The crystal of 3 suitable for X-ray structural analysis was obtained by slow evaporation of CH2Cl2/MeOH solution. The crystallographic data and structure refinement details are given in Table S1; selected bond lengths and bond angles are collected in Table S2.
ORTEP view of complex 3 with the atomic numbering scheme is shown in Fig. 1. It crystallizes in triclinic space group P 1 and the asymmetric unit comprises two molecules. As seen, the Ir(III) ion resides in a distorted octahedral [(C'N)2Ir(N'N)]+ coordination geometry with the C and N atoms of two dmabt (CN) ligands and the two N atoms of bipy (N'N) ligand. Moreover, two cyclo-metalated dmabt ligands adopt cis-C,C and trans-N,N configuration.15 Among all the coordination bonds, the Ir-Nbipy bonds are the longest (2.114-2.133 À). It is not surprising to find that the Ir-C bonds (1.996-2.022 À) are shorter than Ir-Ndmabt bonds (2.044-2.056 À), suggesting the stronger coordination effect of Ir-C bonds than Ir-N bonds.16 After coordinated to Ir(III) ion, the phenyl ring (C18-C19-C20-C21-C22-C23) is almost fixed in the parallel position with the benzothiazole ring, and the dihedral angle between the two rings is 11.577°.
Fig. 1. ORTEP view of 3 with the thermal ellipsoids drawn at the 30% probability level. Hydrogen atoms, solvent molecules and PF6 anion are omitted for clarity. Selected bond length and angles: Ir(1)-N(1), 2.114(9); Ir(1)-N(2), 2.133(8); Ir(1)-N(3), 2.056(8); Ir(1)-N(5), 2.044(7); Ir(1)-C(19), 2.022(8); Ir(1)-C(38), 1.996(10); N(3)-Ir(1)-N(5), 170.2(3); C(19)-Ir(1)-N(2), 174.6(4); C(38)-Ir(1)-N(1), 173.7(3); N(5)-Ir(1)-N(2), 91.1(3); N(5)-Ir(1)-N(1), 101.4(3); N(5)-Ir(1)-C(19), 91.0(3); N(5)-Ir(1)-C(38), 80.1(3); C(19)-Ir(1)-C(38), 88.0(3); N(1)-Ir(1)-N(2), 76.5(3); N(3)-Ir(1)-N(2), 97.9(3).
3. 2. Electronic Absorption Spectra
The UV-Vis absorption spectra of 3 in CH2Cl2 solution are presented along with the calculated spectra (Fig. 2). The absorption band at 243 nm is assigned to the spin-
allowed ligand-centered 1n-n* transitions arising from N'N ligands. The next absorption bands up to ca. 320 nm are assigned to the spin-allowed 1MLCT transitions from the Ir(III) center to the C'N ligands. The strongest band at 436 nm is assigned to LLCT transitions from C'N ligands to N'N ligands.
The electronic absorption spectra of 3 were calculated using the time-dependent density functional theory (TD-DFT) method. The calculated results and spin-allowed electronic transitions are listed in Table 1, as well as compared with the experimental absorption spectra data. The molecular HOMO/LUMO orbital pictures are presented in Fig. 3. The electron density distributions are summarized in Table S3.
As can be seen from Fig. 2, agreement between the calculated UV-Vis spectra and the experimentally determined spectra is quite satisfactory. According to the Fig. 2 and Table 1, electronic absorption spectra of 3 mainly exhibits four bands at 436, 295, 268 and 242 nm arising from corresponding electronic transition between HOMO/HOMO-1 and LUMO+1/LUMO+2/LUMO+3, HOMO-2 and LUMO+2, HOMO-5 and LUMO+1 orbitals, respectively. Unlike other iridium complexes,17,18 the highest molecular orbital (HOMO) is mainly localized on the dmabt ligand, whereas the lowest unoccupied molecu-

41 J
•J
о
»
Ш
HOMO (-5.351 eV)
LUMO (-2.703 eV)
Fig. 3. The frontier molecular orbital diagrams of complex 3 from DFT calculations.
lar orbital (LUMO) is mainly delocalized on the bipyridi-ne ligand. As a result, the lowest-energy electronic transition (436 nm) is not derived from HOMO ^ LUMO transition, which corresponds to ligand-ligand charge-transfer (LLCT, Lđmabt(n) ^ Lbipy(n*), Table 1).
3. 3. Emission Properties
Photoluminescence measurement was conducted in degassed CH2Cl2 solution at room temperature (Fig. 4). Upon photoexcitation at ca. 437 nm, complex 3 shows orange luminescence with the emission peak appeared at 612
Fig. 2. Comparison between the experimental and calculated UV-Vis absorption spectra of 3.
Fig. 4. The emission spectra of 3 in CH2Cl2 at room temperature.
Table 1 Main experimental and calculated optical transitions for 3.
Orbital Excitations	Character	Oscillation Strength	Calcd (nm)	Exptl (nm)
HOMO ^ LUMO+2 HOMO ^ LUMO+3	Ldmabt(n) ^ Ldmabt/Bipy(n )	0.2985	390	436
HOMO ^ LUMO+1 HOMO-1 ^ LUMO+3	Ldmabt(n) ^ Ldmabt/Bipy(n )	0.9249	389	
HOMO-2 ^ LUMO+2	Ir(dn)/Ldmabt(n) ^ Ldmabt(nt)	0.0912	335	295
HOMO-5 ^ LUMO+1	Ir(dn)/Ldmabt№ ^ Ldmabt(nt)	0.0738	304	268
HOMO-12 ^ LUMO	LBipy(n) ^ LBipy(n*)	0.3239	279	242
nm. From the results of UV-Vis spectra and DFT calculations, it can be seen that the emissive excited state arise from intraligand n-n* relaxation. In addition, phosphorescence relative quantum yield (Ф) in dichloromethane solution was measured to be 0.17 at room temperature by using typical phosphorescent /ac-Ir(ppy)3 as a standard (Ф = 0.40).14
4. Conclusions
In summary, a new bis-cyclometalated 2-phenylben-zothiazole-based iridium(III) complex 3 has been successfully prepared and characterized. Structural studies indicated that complex 3 adopts a distorted octahedral geometry around the iridium metal exhibiting cis-C,C' and trans-N,N' chelate disposition. The absorption and emission spectra have been investigated. The spin-allowed singlet-singlet electronic transitions are calculated with time-dependent DFT (TD-DFT), and a good agreement with the experimental data is observed.
5. Acknowledgements
This work was supported by the Natural Science Foundation of Hainan Province (No. 20152017), the Science and Research Project of Education Department of Hainan Province (Nos. Hjkj2013-25 and Hnky2015-27), Hainan Provincial Innovation Experiment Program for University Students (No. 20140053) and Hainan Normal University's Innovation Experiment Program for University Students (No. cxcyxj2015005).
6. Supplementary Material
Crystallographic data for the structural analyses have been deposited in the Cambridge Crystallographic Data Centre, CCDC reference number 1404920. Copies of this information may be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: +44 1223 336033; e-mail: deposit @ccdc.cam. ac.uk or ).
7. References
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Povzetek
Sintetiziran in okarakteriziran je nov bis-ciklometaliran iridijev(III) kompleks [Ir(dmabt)2(bipy)][PF6] (3) (dmabt = 4-(benzo[đ]tiazol-2-il)-N,N-dimetilanilin, bipy = 2,2'-bipiridin). Struktura kompleksa 3 je bila dolo~ena z rentgensko analizo, ki je razkrila, da ima centralni iridijev(III) ion popa~eno oktaedri~no geometrijo. Fotoluminiscen~ni spekter vsebuje oranžni emisijski maksimum pri 612 nm s kvantnim izkoristkom 17% pri 298 K. Izra~unani so diagrami mejnih molekulskih orbital ter spinsko dovoljeni singlet-singlet elektronski prehodi za spojino 3 po teoriji gostotnostnega funk-cionala (DFT) in ~asovno odvisni DFT (TD-DFT). Rezultati teoreti~nih izra~unov so bili uporabljeni pri interpretaciji UV-Vis spektrov.
Synthesis, crystal structure, photophysical properties and theoretical study of a new iridium(III) complex containing
2-phenylbenzothiazole ligand
Yong-Pi Zeng,1 Cheng-Wei Gao,1 Liang-Jiang Hu,1 Hao-Hua Chen,1 Guang-Ying Chen,2 Gao-Nan Li1^ and Zhi-Gang Niu1,2^
1 College of Chemistry and Chemical Engineering, Hainan Normal University, Haikou 571158,
China
2 Key Laboratory of Tropical Medicinal Plant Chemistry of Ministry of Education, Hainan Normal
University, Haikou 571158, China
* Corresponding author: E-mail: ligaonan2008@163.com, niuzhigang1982@126.com
Supporting materials
Table S1 Crystallographic data and structure refinement details for 3.
	Empirical formula	C40H34F6IrN6PS2	
	Mr	1000.02	
	Crystal system	Triclinic	
	Space group	P1	
	Wavelength / Ä	0.71073	
	X-radiation (graphite monochromator)	Mo-Ka	
	T / K	293(2)	
	a (Ä)	10.5274(3)	
	b (Ä)	15.7581(6)	
	c (Ä)	23.8577(7)	
	a(°)	88.861(3)	
	ß(°)	85.958(3)	
	Ю	82.435(3)	
	V (Ä3)	3913.4(2)	
	Z	4	
	Dcalcd (Mg/m3)	1.697	
	F(000)	1976	
	^(Mo-Ka)/ mm-1	3.627	
	index ranges	-13 < h < 13	
		-19 < k < 19	
		-27 < l < 29	
	Rint	0.0881	
	GOF (F2)	0.890	
	Ra wR2b (I >2a(I))	0.0616, 0.0781	
	Ri", wR2 (all data)	0.1507, 0.1041	
	a R1 = ZIFol - |Fc||/2|Fo|. b wR2 = [Zw(Fo2 - Fc2)2/Zw(Fo2)]1/2		
Table S2 Selected bond distances (Ä) and angles (◦) for complex 3.			
Ir(1)-N(1)	2.114(9)	C(5)-C(6)	1.477(14)
Ir(1)-N(2)	2.133(8)	C(17)-C(18)	1.413(11)
Ir(1)-N(3)	2.056(8)	N(4)-C(21)	1.341(11)
Ir(1)-N(5)	2.044(7)	N(4)-C(24)	1.451(12)
Ir(1)-C(19)	2.022(8)	S(1)-C(16)	1.720(11)
Ir(1)-C(38)	1.996(10)	S(1)-C(17)	1.715(8)
N(3)-Ir(1)-N(5)	170.2(3)	N(5)-Ir(1)-C(19)	91.0(3)
C(19)-Ir(1)-N(2)	174.6(4)	N(5)-Ir(1)-C(38)	80.1(3)
C(38)-Ir(1)-N(1)	173.7(3)	C(19)-Ir(1)-C(38)	88.0(3)
N(5)-Ir(1)-N(2)	91.1(3)	N(1)-Ir(1)-N(2)	76.5(3)
N(5)-Ir(1)-N(1)	101.4(3)	N(3)-Ir(1)-N(2)	97.9(3)
Table S3 Frontier orbital energy and electron density distribution for 3.
Orbital	Energy (eV) -
Ir
LUMO+3	-1.724	5.11
LUMO+2	-1.835	3.12
LUMO+1	-1.842	2.77
LUMO	-2.703	4.70
HOMO	-5.351	0.21
HOMO-1	-5.371	4.25
HOMO-2	-6.247	45.69
HOMO-5	-6.523	31.09
HOMO-12	-7.630	17.85
Composition (%)
Ph-R	benzothiazole	2,2'-bipyridine
6.43	13.04	75.46
28.04	50.40	19.23
34.59	59.50	4.24
4.31	0.27	90.78
91.62	21.72	1.06
87.40	22.17	1.06
51.27	5.12	4.87
6.12	56.55	8.12
11.50	3.39	69.78
Fig. S1. :H NMR of dmabt (1) in CDC^.
Fig. S2. 13C NMR of dmabt (1) in CDCI3.
Fig. S3. 1H NMR of [Ir(dmabt)2(bipy)][PF6] (3) in CDCI3.
Fig. S4. 13C NMR of [Ir(dmabt)2(bipy)][PF6] (3) in CDCI3.