- Email: [email protected]
Synthesis and characterization of coumarin-biphenyl derivatives as organic luminescent materials
Journal of Photochemistry and Photobiology A: Chemistry 346 (2017) 10–16
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A: Chemistry journal homepage: www.elsevier.com/locate/jphotochem
Invited feature article
Synthesis and characterization of coumarin-biphenyl derivatives as organic luminescent materials Hui Zhanga , Qinglong Luoa , Yuzhong Maob , Yuling Zhaoc , Tianzhi Yua,* a
Key Laboratory of Opto-Electronic Technology and Intelligent Control (Ministry of Education), Lanzhou Jiaotong University, Lanzhou 730070, China School of Chemical and Materials Engineering, Yanching Institute of Technology, Langfang 065201, China c School of Chemical and Biological Engineering, Lanzhou Jiaotong University, Lanzhou 730070, China b
A R T I C L E I N F O
Article history: Received 19 March 2017 Received in revised form 22 May 2017 Accepted 23 May 2017 Available online 25 May 2017 Keywords: Coumarin Biphenyl derivative Photoluminescence Electroluminescence
A B S T R A C T
Three new coumarin derivatives containing biphenyl group have been successfully synthesized and characterized by EA, IR and NMR. The photophysical properties of all derivatives were investigated by UV–vis and photoluminescence spectroscopic analysis. Their thermal stabilities were demonstrated by TGA. The doped light-emitting devices using the coumarin derivatives as dopants were fabricated. The results show that one of coumarin derivatives with coumarin-biphenyl skeleton would serve as promising luminescent materials. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Coumarins are important class of organic heterocyclic molecules [1]. Many natural and synthetic coumarin derivatives have been extensively studied due to their several activities. In addition to a wide variety of biological activities [2–4], they exhibit favorable photophysical properties such as large Stokes shifts, high fluorescence quantum yields, excellentlight stability, visible excitation and emission wavelengths and nontoxicity [5,6]. Undeniably, the coumarin derivatives used extensively as potential fluorescence materials, and widely used as emissive dopants in organic light-emitting diodes (OLEDs) application [7,8]. The photophysical properties of the coumarin derivatives are strongly related to the electron-donating or electron-withdrawing capacity of the substituents attached to their core and the conjugation degree of molecules. The longer p-conjugation dye molecules generally achieve a longer absorption maximum and extend the absorption region [9]. The extension of the p-delocalized system will lead to molecules showing more promising fluorescent behavior [10]. As reported, biscoumarin analogues extend the absorption and emission wavelength ranges, by increasing the electron conjugated system [11,12]. Biphenyl exhibits planar configuration, which expects full delocalization of the p-system over the two rings [13]. Because of photophysical and opto-electronic properties, biphenyl derivatives
* Corresponding author. E-mail address: [email protected] (T. Yu). http://dx.doi.org/10.1016/j.jphotochem.2017.05.039 1010-6030/© 2017 Elsevier B.V. All rights reserved.
are widely used as hole-transport and host materials and sometimes as blue-emitting materials [14,15]. Combining biphenyl moiety with coumarin may afford new chemical entities, which would possess simultaneously the superior properties of biphenyl and coumarin. Furthermore, biphenyl with coumarin ring containing a diethylamino group as a donor moiety increases the conjugation of the chromophore. In view of the above mentioned findings, the present work describes the synthesis and characterization of three new coumarin-biphenyl derivatives (4,40 -di(coumarin-3-yl)-biphenyl, 4-(coumarin-3-yl)-40 -(7-diethylaminocoumarin-3-yl)-biphenyl and 4,40 -di(7-diethylaminocoumarin 3-yl)-biphenyl), aiming at obtaining new materials with higher fluorescent properties. The photophysical, electrochemical properties and thermal stabilities of the derivatives were investigated. Furthermore, the electroluminescence devices made using codeposition of the derivatives and TBADN films as emitters were fabricated to investigate the electroluminescence properties of the derivatives. The synthetic routes of the coumarin-biphenyl derivatives is outlined in Scheme 1. 2. Experimental 2.1. Materials and instruments All materials and reagents were of AR grade and directly used. All of the organic solvents used in this study were dried over appropriate drying agents and distilled prior to use.
H. Zhang et al. / Journal of Photochemistry and Photobiology A: Chemistry 346 (2017) 10–16
11
Scheme 1. Synthesis of coumarin-biphenyl derivatives.
IR spectra were taken with a Shimadzu IR Prestige-21 FT-IR spectrophotometer. 1H NMR spectra were obtained on Varian Mercury Plus 400 MHz and Agilent Technologies DDZ 600 MHz Spectrometer with tetramethylsilane as the internal standard. Element analyses were performed on an Vario-EL automatic elemental analysis instrument. Absorption spectra were recorded by a Shimadzu UV-2550 spectrometer. Photoluminescence spectra were obtained using a Perkin-Elmer LS-55 spectrometer. MS was obtained from a Thermo Scientific Orbitrap Elite mass spectrometer. Fluorescent lifetime and quantum yield were recorded on a FLSP920 type steady-state/transient fluorescence spectrometer (Edinburgh Instruments Ltd). Cyclic voltammetry (CV) was carried out on an CH Instruments 760B with a three electrode system (a Pt working electrode, a Pt-wire counter electrode, and a Ag/AgCl reference electrode) in the presence of n-Bu4NPF6 (0.1 mol L1) as a supporting electrolyte in CH2Cl2. 2.2. Synthesis and characterization 3-(4-Bromophenyl)coumarin and 3-(4-Bromophenyl)-7-(N,N0 diethylamino) coumarin were prepared according to the procedure as previously described [16]. 2.2.1. Preparation of 3-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2yl)phenyl)-coumarin (a1) The suspension of 3-(4-Bromophenyl)coumarin (1.5 g, 4.98 mmol), bis(pinacolato) diboron (2.0 g, 7.87 mmol), potassium acetate (0.8 g, 8.15 mmol) and Pd(dppf)Cl2 (0.1 g, 0.14 mmol) in absolute 1,4-dioxane (100 mL) was placed in a three-necked flask. The mixture was heated up to 95 C and stirred for 24 h under N2. After solvent was evaporated under vacuum, the mixture was dissolved in dichloromethane, washed with water (3 20 mL). The organic layer was dried over MgSO4 and concentrated in vacuum. The residue was chromatographed on silica, eluting with EtOAc/ ether(1:30-1:10 gradient, v/v) to form white solid (yield 84%). m. p.:178–179 C. Anal. Calcd. for C21H21BO4 (%): C 72.44, H 6.08. Found (%): C 72.31, H 6.10. 1H NMR (400 MHz, CDCl3) d: 7.89 (d, J = 8.0 Hz, 2H, Ar-H), 7.85 (s, 1H, coumarin H), 7.73 (d, J = 8.0 Hz, 2H, Ar-H), 7.54 (t, J = 8.7 Hz, 2H, coumarin H), 7.37 (d, J = 8.3 Hz, 1H, coumarin H), 7.30 (t, J = 7.4 Hz, 1H, coumarin H), 1.36 (s, 12H). 2.2.2. Preparation of 3-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2yl)phenyl)-7-(N,N0 - diethylamino)coumarin (a2) The preparation of a2 was similar to that described for a1, which was obtained through the reaction of 3-(4-Bromophenyl)-7-(N,N0 diethylamino)coumarin (1.0 g, 2.69 mmol) with bis(pinacolato)
diboron (1.10 g, 4.33 mmol). The crude was purified by chromatography on silica gel using EtOAc/CH2Cl2/petroleum ether (1:8:30, v/v/v) to form yellow solid (yield 74%). m.p.: 212–214 C. Anal. Calcd. for C25H30BNO4 (%): C 71.61, H 7.21, N 3.34. Found (%): C 71.53, H 7.29, N 3.30. 1H NMR (400 MHz, CDCl3) d: 7.85 (d, J = 7.9 Hz, 2H, Ar-H), 7.73 (s, 2H, Ar-H), 7.71 (s, 1H, coumarin H), 7.32 (d, J = 8.8 Hz, 1H, coumarin H), 6.59 (dd, J = 8.8, 2.1 Hz, 1H, coumarin H), 6.53 (s, 1H, coumarin H), 3.43 (q, J = 7.0 Hz, 4H, CH2), 1.35 (s, 12H, CH3), 1.22 (t, J = 7.0 Hz, 6H, CH3). 2.2.3. Preparation of 4,40 -di(coumarin-3-yl)-biphenyl (b1) Absolute toluene (100 mL) was added under N2 to the mixture of a1 (1 g, 2.87 mmol), 3-(4-Bromophenyl)coumarin (1.4 g, 4.65 mmol), Na2CO3 (1 g, 9.43 mmol), TBAB (0.05 g, 0.16 mmol) and Pd(pph3)4 (0.2 g, 0.17 mmol). The suspension was heated up to 110 C and stirred vigorously. After 10 min, distilled water (2 mL) was added. The resulting mixture was stirred 110 C for 24 h. After solvent was evaporated under vacuum, the mixture was dissolved in dichloromethane, washed with water (3 20 mL). The organic layer was dried over MgSO4 and concentrated in vacuum. The residue was chromatographed on silica, eluting with EtOAc/ CH2Cl2/petroleum ether (1:100:300, v/v/v) to form white solid. Yield: 68%. m.p. > 280 C. Anal. Calcd. for C30H18O4 (%): C 81.44, H 4.10. Found (%): C 81.40, H 4.13. IR (KBr), cm1: 1722 (n C¼O), 1114 (n 1 C O), 1610 and 1455 (n C¼C), 809 (d Ar-H). H NMR (600 MHz, CDCl3) d: 7.82 (s, 2H, coumarin H), 7.62–7.57 (m, 8H, Ar-H), 7.57–7.54 (m, 4H, coumarin H), 7.38 (d, J = 8.7 Hz, 2H, coumarin H), 7.32 (d, J = 7.5 Hz, 2H, coumarin H). The preparation of 4-(coumarin-3-yl)-40 -(7-diethylaminocoumarin-3-yl)-biphenyl (b2) and 4,40 -di(7-diethylaminocoumarin-3yl)-biphenyl (b3) were similar to that described for b1. b2 was synthesized through the reaction of 3-(4-Bromophenyl) coumarin (0.5 g, 1.67 mmol) with a2 (0.46 g,1.09 mmol). The crude was purified by chromatography on silica gel using EtOAc/CH2Cl2 (1:100, v/v) to form yellow solid (yield 74%). m.p. > 280 C. Anal. Calcd. for C34H27NO4 (%): C 79.51, H 5.30, N 2.73. Found (%): C 79.53, H 5.31, N 2.71. IR (KBr), cm1: 2970 (n CH), 1715 (n C¼O), 1138 (n 1 C O), 815 (n Ar-H), 1275 (n CN). H NMR (400 MHz, CDCl3) d: 7.89 (s, 1H,coumarin H), 7.82 (dd, J = 8.4, 1.4 Hz, 4H, Ar-H), 7.78 (s, 1H, coumarin H), 7.71 (dd, J = 12.6, 8.5 Hz, 4H, Ar-H), 7.56 (dd, J = 15.6, 7.2 Hz, 2H, coumarin H), 7.40 (d, J = 8.7 Hz, 1H, coumarin H), 7.37– 7.28 (m, 2H, coumarin H), 6.64–6.49 (m, 2H, coumarin H), 3.45 (dd, J = 13.6, 6.5 Hz, 4H,CH2), 1.27-1.19 (m, 6H, CH3). MS (ESI) m/z 514.20 [M+H]+, calculated for C34H27NO4, 513.58. b3 was obtained from the reaction of 3-(4-Bromophenyl)-7-(N, N'-diethylamino) coumarin (0.46 g, 1.24 mmol) with a2 (0.4 g,0.95
12
H. Zhang et al. / Journal of Photochemistry and Photobiology A: Chemistry 346 (2017) 10–16
mmol). The crude was purified by chromatography on silica gel using EtOAc/CH2Cl2/petroleum ether (4:100:300, v/v/v) to form yellow solid (yield 78%). m.p. > 280 C. Anal. Calcd. for C38H36N2O4 (%): C 78.06, H 6.21, N 4.79. Found (%): C 78.07, H 6.24, N 4.77. IR (KBr), cm1: 2964(n CH),1715(n C¼O), 1262(n CN), 1095(n CO), 803(d Ar-H). 1H NMR (400 MHz, CDCl3) d: 7.64 (s, 2H, coumarin H), 7.60 (d, J = 8.9 Hz, 4H, Ar-H), 7.55-7.51 (m, 4H, Ar-H), 7.32-7.30 (m, 2H, coumarin H), 6.88 (d, J = 8.3 Hz, 4H, coumarin H), 3.43 (dd, J = 14.0, 6.9 Hz, 8H, CH2), 1.24–1.20 (m, 12H, CH3). 2.3. Crystallography A single crystal of b3 obtained from CH2Cl2 solution was selected for data collection that was performed on Bruker Smart Apex CCD diffractometer equipped with graphite-monochromatic Mo Ka radiation (l = 0.71073 Å) at 293 K. The structure was solved by direct methods and refined by full-matrix least-squares method on F2 using SHELXL. Single crystals of b1 and b2 have not obtained. 2.4. OLEDs fabrication and characterization To investigate the EL properties of the compounds b2 and b3, the multilayer OLEDs with a device architecture of ITO/TAPC (20 nm)/TBADN:compound b (x wt%, 30 nm)/TPBI (50 nm)/Liq (2 nm)/Al were fabricated by vacuum-deposition method. (ITO: Indium tin oxide; TAPC: 1,1-bis-(4-bis(4-methylphenyl)-aminophenyl)-cyclohexane; TBADN: 9,10-bis(2-naphthyl)-2-t- butylanthracene; TPBI: 2,20 ,200 - (1,3,5-Benzinetriyl)-tris(1-phenyl-1-Hbenzimidazole)). The emitting layers are consisted of host materials TBADN and dopants of b2 or b3 at different concentrations (x wt%). TAPC was used as the hole injection layer, TPBI was the electron transport layer, Liq and Al were used as the electron-injection layer and cathode, respectively. The background pressure of the chamber was under 2 105 Pa during the deposition process. The EL spectra and Commission Internationale de L'Eclairage (CIE) coordinates were measured on a Hitachi MPF-4 fluorescence spectrometer. 3. Results and discussion 3.1. X-ray crystallographic analyses The compound b3 crystallizes in the C2/c space group pertaining to monoclinic crystal system. The crystal structure of b3 is shown in Fig. 1. The crystal data and experimental details are given in Table 1. The selected bond lengths and bond angles are listed in Table 2. Biphenyl group have planar conformation. The intermolecular interactions may promote the coplanar arrangements of aromatic rings in the biphenyl compounds, which may be accountable for the noted conjugation[17]. Two coumarin rings are
Table 1 Crystallographic data for b3. Data
b3
Empirical formula Formula weight Temperature (K) Wavelength (Å) Crystal system, space group Unit cell dimensions a (Å) b (Å) c (Å) a ( ) b ( ) g ( ) Volume (Å3) Z, Calculated density (Mg/m3) Absorption coefficient (mm1) F(000) Crystal size (mm) Theta range for data collection ( ) Limiting indices
C38 H36 N2O4 584.69 298(2) 0.71073 Monoclinic, C2/c
Reflections collected/unique Completeness to theta = 25.02 Absorption correction Max. and min. transmission Refinement method Data/restraints/parameters Goodness-of-fit on F2 Final R indices [I > 2sigma(I)] R indices (all data) Extinction coefficient Largest diff. peak and hole (eÅ3)
20.9909(18) 6.4790(6) 23.584(2) 90 110.878(3) 90 2996.8(5) 4, 1.296 0.084 1240 0.40 0.35 0.13 2.23 to 25.02 24 h 18 7 k 7 27 l 28 7181/2636 [R(int) = 0.0466] 99.8% Semi-empirical from equivalents 0.9892 and 0.9672 Full-matrix least-squares on F2 2636/0/286 0.961 R1 = 0.0621, wR2 = 0.1785 R1 = 0.1426, wR2 = 0.2479 0.0005(5) 0.176 and 0.153
not in a coplane. The dihedral angles between coumarin ring and biphenyl group is 39.35 and 39.57, respectively. Fig. 2 shows the crystal packing of the compound b3. As depicted in Fig. 2, there are three intermolecular nonclassical C3 H3 O2, C15H15 O2, C18 H18A O2 hydrogen bonds. Also, in the lattice, two C H p stacking interactions are present. CH group and a coumarin ring with H p distance of 2.846 Å for C17 H17B Cg1, in which Cg1 is the centroid for the O1/C1/C2/ Table 2 Selected bond lengths and angles for b3. bond angles [ ]
bond lengths [Å] N(1)-C(7) N(1)-C(16) N(1)-C(18) O(1)-C(5) O(1)-C(1) O(2)-C(1) C(2)-C(10)
1.390(18) 1.435(13) 1.442(14) 1.372(4) 1.385(4) 1.206(4) 1.474(4)
C(7)-N(1)-C(16) C(7)-N(1)-C(18) C(16)-N(1)-C(18) C(5)-O(1)-C(1) O(2)-C(1)-O(1) O(2)-C(1)-C(2) O(1)-C(1)-C(2)
Fig. 1. ORTEP drawing of the molecular structure of b3 along with the atomic abeling scheme and H atoms are excluded.
122.8(10) 124.3(11) 112.9(8) 122.7(3) 114.5(3) 127.8(3) 117.7(3)
H. Zhang et al. / Journal of Photochemistry and Photobiology A: Chemistry 346 (2017) 10–16
13
Fig. 2. Illustration of hydrogen bond interactions in molecular structure of b3.
C3/C4/C5 ring. CH group and a phenyl ring with H p distance 2.855 Å for C18 H18B Cg2, in which Cg2 is the centroid for the C10/C11/C12/C13/C14/C15 ring. The crystal structure has been deposited at the Cambridge Crystallographic Data Centre and allocated the deposition number of CCDC 1516115.
The UV–vis absorption and fluorescence spectra of b1, b2 and b3 in dilute CH2Cl2 solutions are presented in Figs. 3 and 4. Table 3 lists the UV–vis absorption maxima (la,max), emission maxima (le, max), Stokes shift (Dl), fluorescent quantum yield (F), fluorescent lifetime (t) and molar extinction coefficient (e) of the tested compounds. Fig. 3 shows the strong absorption bands of b1–b3 are at the 325–450 nm region, which correspond to the p-p* absorption induced by the larger conjugative biphenyl chain. It can be seen that the bathochromic shift observed is attributed to the increased number of diethylamino group, because diethylamino-
substitution with electron-donating ability at 7-position of coumarin may increase p-conjugation. Fig. 4 shows that the photoluminescence (PL) emission peaks in CH2Cl2 cover the 400–500 nm region. Compound b1 exhibits blue emission, while b2 and b3 exhibit blue-green emissions. The difference lies in the strong electron-donating diethylamino group in b2 and b3. This makes b2 and b3 have better conjugation than that of unsubstituted b1, and have red shifts of 48 and 54 nm compared with b1. It is also found from Table 3 that compound b3 has higher fluorescence quantum yield and longer fluorescence lifetime and red-shifted emission spectrum in contrast to b1 and b2. The higher quantum yield in fluorescence emission in b3 derives from the shorter Stokes Shift. The quantum yields determined were absolute values using an Integrating Sphere without reference standard [18]. As reported, there are some synthesized bis-coumarin analogues connecting two coumarin units using defferent bridge units, such as benzene [19], pyrazole [20], ketones [21], 1,3-diketo or 1,3diketofluoroborate [12]. The compound b3 utilazed biphenyl
Fig. 3. The UV–vis absorption spectra of b1, b2 and b3 in dilute (107 mol L1) CH2Cl2 solutions.
Fig. 4. The PL emission spectra of b1, b2 and b3 in dilute (107 mol L1) CH2Cl2 solutions.
3.2. Photophysical properties
14
H. Zhang et al. / Journal of Photochemistry and Photobiology A: Chemistry 346 (2017) 10–16
Table 3 Absorption, emission maxima, Stokes shift, fluorescence quantum yield, lifetime and molar extinction coefficient of b1, b2 and b3. Compound b1 b2 b3
la,max
le,max
nm
nm
Dl
cm1
341 409 417
430 478 484
6069 3529 3319
F 0.49 0.59 0.75
t
e
ns
L mol1 cm1 (105)
9.47 9.88 10.09
1.15 1.55 2.20
brigde connecting two coumarin units exhibits better properties of fluorescent quantum yield and fluorescent lifetime than other analogues reported earlier [19,21]. 3.3. Thermal properties Thermogravimetric analysis (TGA) measurement was performed in flowing dry nitrogen atmosphere at the heating rate of 10 C min1. The results of TGA measurement of tested compounds are shown in Fig. 5. Compound b3 displays good thermal stability up to 380 C (<2% weight loss). At 385 C, there is a sharp weight losses in its TGA curve, indicating that compound b3 begins to decompose at this temperature. The thermal stabilities of the b1 and b2 are lower than that of b3. The TGA curve of the b1 shows 2% weight loss at 204 C, and sharp weight loss at 250 C. b2 shows 2% weight loss at 267 C, and sharp weight loss at 276 C. The results indicate that compounds of b2 and b3 meet with the thermal stability requirement of fabrication of OLED luminescence.
Fig. 6. Cyclic voltammograms of ferrocene, b1, b2 and b3 (scan rate: 10 mV s1, solvent: dichloromethane).
absorption spectra, the optical band gaps of b1, b2 and b3 were estimated to be 412, 466, and 477 nm, which correspond to 3.01, 2.67, and 2.63 eV, respectively. The HOMO and LUMO energy levels can be calculated according to the relations reported [24]. The overall EHOMO, ELUMO, and Eg data of the compounds were listed in Table 4.
3.4. Electrochemical properties The electrochemical behaviors of b1, b2 and b3 were investigated by cyclic voltammetry (CV). The cyclic voltammograms of the compounds are shown in Fig. 6. The ionization potential (IP) may be regarded as HOMO energy level, and the electron affinity (EA) may be regarded as LUMO energy level [22,23]. To design optimized device structures, the information on HOMO and LUMO of the emitting layer are important. The Eg represent the band gap between HOMO and LUMO, and the value can calculated from optical absorption edge. From the UV–vis
Fig. 5. Thermogravimetric analyses (TGA) of b1, b2 and b3.
Table 4 Electrochemical data for the b1, b2 and b3. Compound
EHOMO/eV
Eg/eV
ELUMO/eV
b1 b2 b3
5.05 5.07 5.11
3.01 2.67 2.63
2.04 2.40 2.48
Fig. 7. EL spectra of devices ITO/TAPC (20 nm)/TBADN: b2 (x%, 30 nm)/TPBI (50 nm)/Liq (2 nm)/Al with different doping concentration (spectra were measured at the same current density in ca. 20 mA cm2).
H. Zhang et al. / Journal of Photochemistry and Photobiology A: Chemistry 346 (2017) 10–16
15
Fig. 9. EL spectra of devices ITO/TAPC (20 nm)/TBADN: b3 (x%, 30 nm)/TPBI (50 nm)/Liq (2 nm)/Al with different doping concentration (spectra were measured at the same current density in ca. 20 mA cm2).
Fig. 8. (a) L-I-V, (b) luminance-efficiency and (c) external quantum efficiency characteristics of devices ITO/TAPC (20 nm)/TBADN: b2 (x%, 30 nm)/TPBI (50 nm)/ Liq (2 nm)/Al with different doping concentration.
3.5. Electroluminescence properties Compound b1 has weak fluorescence, so the device based on b1 has not tested. Fig. 7 shows EL spectra of devices ITO/TAPC(20 nm)/TBADN: b2 (x%, 30 nm)/TPBI (50 nm)/Liq (2 nm)/Al. The doping concentration of b2 was 4, 7, 10 and 13%, respectively. All spectra were measured at the same current density in ca. 20 mA cm2. When the doping concentration was 4%, the device shows blue emission (464 nm) and the calculated CIE coordinate is (0.19, 0.22). When the doping concentration reached to 13%, the device shows green emission (512 nm) and the calculated CIE coordinate is (0.31, 0.50). It follows that with doping concentration increasing, the EL spectrum was red shifted, which might be caused by aggregation with increase in concentration.
Fig. 8 shows the L-I-V, luminance-efficiency, and EQE characteristics of those above devices. The device with 10% doped b2 has the highest current density of 230 mA/cm2 at 11.8 V (Fig. 8(a)), and external quantum efficiency (EQE) over 2% (Fig. 8(c)). The doped device at the concentration of 13 wt% b2 has the maximum brightness of 5135 cd/m2 at 12.8 V (Fig. 8(a)), the highest current efficiency of 4.83 cd/A at 219 cd/m2, and the highest power efficiency of 2.06 Lm/W at 2.65 cd/m2 (Fig. 8(b)). The EL spectra of the devices based on b3 with different dopant concentrations are shown in Fig. 9. The spectra were measured at the same current density in ca. 20 mA cm2. It can be seen that EL spectra differ from the PL spectra, which is attributed to excimer of b3 molecules at high concentration in the process of condensation from vapor phase in vacuum. Fig. 10 shows the L-I-V, luminance-efficiency, and EQE characteristics of devices based on b3. The device with 4% doped b3 shows current density of 152 mA/cm2 and the maximum brightness of 480 cd/m2 at 16 V (Fig. 10(a)), current efficiency of 1.60 cd/A and power efficiency of 0.71 Lm/W at 1.95 cd/m2 (Fig. 10(b)), and external quantum efficiency (EQE) over 1% (Fig. 10(c)). Obviously, electroluminescent performance of the devices based on b2 is substantially higher than that of the devices fabricated from b3. The aggregation of b3 due to strong intermolecular hydrogen bonds causes fluorescence quenching. From the above results, we can see that the compound b2 could be a good candidate for OLED devices. 4. Conclusion In summary, three coumarin-biphenyl derivatives have been developed. Compounds b2 and b3 have good photophysical properties and thermal stabilities. When b2 was fabricated as a dopant in OLED device with configuration of ITO/TAPC (20 nm)/ TBADN:b2 (x%, 30 nm)/TPBI (50 nm)/Liq (2 nm)/Al, the maximum emission band red shifted from 464 to 512 nm, accompanying with CIE coordinate changed from (0.19, 0.22) to (0.31, 0.50) as the doping concentration of b2 varied from 4 wt% to 13%. And the device with 13% doped concentration showed maximum luminance of 5135 cd/m2 at 12.8 V. Because of the fluorescence
16
H. Zhang et al. / Journal of Photochemistry and Photobiology A: Chemistry 346 (2017) 10–16
References
Fig. 10. (a) L-I-V, (b) luminance-efficiency and (c) external quantum efficiency characteristics of devices ITO/TAPC (20 nm)/TBADN: b3 (x%, 30 nm)/TPBI (50 nm)/ Liq (2 nm)/Al with different doping concentration.
quenching due to aggregation of b3, electroluminescent performance of the devices based on b3 is lower than that of the devices fabricated from b2. We can see that the compound b2 could be a good candidate for OLED devices. In order to design a highperformance OLED, further modification and optimization of the EL devices having biscoumarin derivatives are in progress. Acknowledgements This work was financially supported by the Gansu Universities Research Fund (2015A-066) and Gansu Natural Science Foundation (1606RJZA018), and also supported by the National Natural Science Foundation of China (Grand 51563014).
[1] A. Dey, M.A. Ali, S. Jana, S. Samanta, A. Hajra, Palladium-catalyzed synthesis of indole fused coumarins via cross-dehydrogenative coupling, Tetrahedron Lett. 58 (2017) 313–316. [2] N.I. Aminudin, F. Ahmad, M. Taher, R.M. Zulkif, A.-D. Incrassamarin, Four new 4-substituted coumarins from Calophyllum incrassatum and their biological activities, Phytochem. Lett. 16 (2016) 287–293. [3] T. Abdizadeh, M.R. Kalani, K. Abnous, Z. Tayarani-Najaran, B.Z. Khashyarmanesh, R. Abdizadeh, R. Ghodsi, F. Hadizadeh, Design, synthesis and biological evaluation of novel coumarin-based benzamides as potent histone deacetylase inhibitors and anticancer agents, Eur. J. Med. Chem. 132 (2017) 42–62. ski, M. Misiak, A. [4] M. Dasko, M. Przybyłowska, J. Rachon, M. Masłyk, K. Kubin Składanowski, S. Demkowicz, Synthesis and biological evaluation of fluorinated N-benzoyl and Nphenylacetoyl derivatives of 3-(4-aminophenyl)coumarin-7-Osulfamate as steroid sulfatase inhibitors, Eur. J. Med. Chem. 128 (2017) 79–87. [5] M.R. Shreykar, N. Sekar, Stimuli-responsive luminescent coumarin thiazole hybrid dye: synthesis, aggregation induced emission, thermochromism and DFT study, Dyes Pigm. 142 (2017) 121–125. [6] A. Matta, V. Bahadur, T. Taniike, J.V.D. Eycken, B.K. Singh, Synthesis, characterisation and photophysical studies of oxadiazolyl coumarin: a new class of blue light emitting fluorescent dyes, Dyes Pigm. 140 (2017) 250–260. [7] T.Z. Yu, P. Zhang, Y.L. Zhao, H. Zhang, J. Meng, D.W. Fan, L.L. Chen, Y.Q. Qiu, Synthesis, crystal structure and photo- and electro-luminescence of the coumarin derivatives with benzotriazole moiety, Org. Electron. 11 (2010) 41– 49. [8] H. Zhang, H.F. Chai, T.Z. Yu, Y.L. Zhao, D.W. Fan, High-Efficiency blue electroluminescence based on coumarin derivative 3-(4-(anthracen-10-yl) phenyl)-benzo[5,6] coumarin, J. Fluoresc. 22 (2012) 1509–1512. [9] Y. Xie, L. Han, C.S. Ge, Y.H. Cui, J.R. Gao, Novel organic dye sensitizers containing fluorenyl and biphenyl moieties for solar cells, Chin. Chem. Lett. 28 (2017) 285–292. [10] J. Gordo, J. Avó, A.J. Parola, J.C. Lima, A. Pereira, P.S. Branco, Convenient synthesis of 3-vinyl and 3-styryl coumarins, Org. Lett. 13 (2011) 5112–5115. [11] H. Ammar, S. Fery-Forgues, R.E. Gharbi, UV/vis absorption and fluorescence spectroscopic study of novel symmetrical biscoumarin dyes, Dyes Pigm. 57 (2003) 259–265. [12] S.N. Margar, N. Sekar, Red and near-infrared emitting bis-coumarin analogues based on curcumin framework-synthesis and photophysical studies, J. Photochem. Photobiol. A: Chem. 327 (2016) 58–70. [13] J. Wang, G. Cooper, D. Tulumello, A.P. Hitchcock, Inner shell excitation spectroscopy of biphenyl and substituted biphenyls: probing ring-Ring delocalization, J. Phys. Chem. A 109 (2005) 10886–10896. [14] H.Z. He, C. Wang, T. Wang, N. Zhou, Z.Y. Wen, S. Wang, L.C. He, Synthesis, characterization and biological evaluation of fluorescent biphenylfurocoumarin derivatives, Dyes Pigm. 113 (2015) 174–180. [15] C.C. Zhu, K.P. Guo, W.B. Liu, Y.B. He, Z.M. Li, X.C. Gao, F.J. Deng, B. Wei, Synthesis of asymmetric biphenyl derivatives for optoelectronic applications, Opt. Mater. 35 (2013) 2095–2101. [16] H. Zhang, T.Z. Yu, Y.L. Zhao, D.W. Fan, Y.J. Xia, P. Zhang, Synthesis, crystal structure, photo- and electro-luminescence of 3-(4-(anthracen-10-yl)phenyl)7- (N,N0 -diethylamino) coumarin, Synth. Met. 160 (2010) 1642–1647. [17] T.V. Usha Gangan, S. Sreenadh, M.L.P. Reddy, Visible-light excitable highly luminescent molecular plastic materials derived from Eu3+-biphenyl based b-diketonate ternary complex and poly(methylmethacrylate), J. Photochem. Photobiol. A: Chem. 328 (2016) 171–181. [18] L. Porrès, A. Holland, L.O. Pålsson, A.P. Monkman, C. Kemp, A. Beeby, Absolute measurements of photoluminescence quantum yields of solutions using an integrating sphere, J. Fluoresc. 16 (2006) 267–272. [19] H. Ammar, S. Fery-Forgues, R.E. Gharbi, UV/vis absorption and fluorescence spectroscopic study of novel symmetrical biscoumarin dyes, Dyes Pigm. 57 (2003) 259–265. [20] K. Kandasamy, S. Ganesabaskaran, M.P. Pachamuthu, A. Ramanathan, A novel pyrazole biscoumarin based chemosensors for the selective detection of Cu2+ and Zn2+ ions, Spectrochim. Acta Part A: Mol. Biomol. Spectrosc. 148 (2015) 184–188. [21] R. Nazir, P. Danilevicius, A.I. Ciuciu, M. Chatzinikolaidou, D. Gray, L. Flamigni, M. Farsari, D.T. Gryko, p-Expanded ketocoumarins as efficient, biocompatible initiators for two-photon-induced polymerization, Chem. Mater. 26 (2014) 3175–3184. [22] H.Y. Fu, X.D. Gao, G.Y. Zhong, Z.Y. Zhong, F. Xiao, B.X. Shao, Synthesis and electroluminescence properties of benzothiazole derivatives, J. Lumin. 129 (2009) 1207–1214. [23] S.M. Kim, J.-S. Kim, D.-M. Shin, Y.K. Kim, Y. Ha, Synthesis and application of the novel azomethine metal complexes for the organic electroluminescent devices, Bull. Korean Chem. Soc. 22 (2001) 743–747. [24] K. Ezzayani, A.B. Khelifa, E. Saint-Aman, F. Loiseau, H. Nasri, Synthesis, spectroscopic characterizations, cyclic voltammetry investigation and molecular structure of the di-l-cyanato-N-bis (l-1,4,7,10,13,16hexaoxacyclooctadecane)bis (5,10,15,20-tetraphenyl porphyrinato) dimagnesiumdipotassium complex, Polyhedron 117 (2016) 817–825.
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A: Chemistry journal homepage: www.elsevier.com/locate/jphotochem
Invited feature article
Synthesis and characterization of coumarin-biphenyl derivatives as organic luminescent materials Hui Zhanga , Qinglong Luoa , Yuzhong Maob , Yuling Zhaoc , Tianzhi Yua,* a
Key Laboratory of Opto-Electronic Technology and Intelligent Control (Ministry of Education), Lanzhou Jiaotong University, Lanzhou 730070, China School of Chemical and Materials Engineering, Yanching Institute of Technology, Langfang 065201, China c School of Chemical and Biological Engineering, Lanzhou Jiaotong University, Lanzhou 730070, China b
A R T I C L E I N F O
Article history: Received 19 March 2017 Received in revised form 22 May 2017 Accepted 23 May 2017 Available online 25 May 2017 Keywords: Coumarin Biphenyl derivative Photoluminescence Electroluminescence
A B S T R A C T
Three new coumarin derivatives containing biphenyl group have been successfully synthesized and characterized by EA, IR and NMR. The photophysical properties of all derivatives were investigated by UV–vis and photoluminescence spectroscopic analysis. Their thermal stabilities were demonstrated by TGA. The doped light-emitting devices using the coumarin derivatives as dopants were fabricated. The results show that one of coumarin derivatives with coumarin-biphenyl skeleton would serve as promising luminescent materials. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Coumarins are important class of organic heterocyclic molecules [1]. Many natural and synthetic coumarin derivatives have been extensively studied due to their several activities. In addition to a wide variety of biological activities [2–4], they exhibit favorable photophysical properties such as large Stokes shifts, high fluorescence quantum yields, excellentlight stability, visible excitation and emission wavelengths and nontoxicity [5,6]. Undeniably, the coumarin derivatives used extensively as potential fluorescence materials, and widely used as emissive dopants in organic light-emitting diodes (OLEDs) application [7,8]. The photophysical properties of the coumarin derivatives are strongly related to the electron-donating or electron-withdrawing capacity of the substituents attached to their core and the conjugation degree of molecules. The longer p-conjugation dye molecules generally achieve a longer absorption maximum and extend the absorption region [9]. The extension of the p-delocalized system will lead to molecules showing more promising fluorescent behavior [10]. As reported, biscoumarin analogues extend the absorption and emission wavelength ranges, by increasing the electron conjugated system [11,12]. Biphenyl exhibits planar configuration, which expects full delocalization of the p-system over the two rings [13]. Because of photophysical and opto-electronic properties, biphenyl derivatives
* Corresponding author. E-mail address: [email protected] (T. Yu). http://dx.doi.org/10.1016/j.jphotochem.2017.05.039 1010-6030/© 2017 Elsevier B.V. All rights reserved.
are widely used as hole-transport and host materials and sometimes as blue-emitting materials [14,15]. Combining biphenyl moiety with coumarin may afford new chemical entities, which would possess simultaneously the superior properties of biphenyl and coumarin. Furthermore, biphenyl with coumarin ring containing a diethylamino group as a donor moiety increases the conjugation of the chromophore. In view of the above mentioned findings, the present work describes the synthesis and characterization of three new coumarin-biphenyl derivatives (4,40 -di(coumarin-3-yl)-biphenyl, 4-(coumarin-3-yl)-40 -(7-diethylaminocoumarin-3-yl)-biphenyl and 4,40 -di(7-diethylaminocoumarin 3-yl)-biphenyl), aiming at obtaining new materials with higher fluorescent properties. The photophysical, electrochemical properties and thermal stabilities of the derivatives were investigated. Furthermore, the electroluminescence devices made using codeposition of the derivatives and TBADN films as emitters were fabricated to investigate the electroluminescence properties of the derivatives. The synthetic routes of the coumarin-biphenyl derivatives is outlined in Scheme 1. 2. Experimental 2.1. Materials and instruments All materials and reagents were of AR grade and directly used. All of the organic solvents used in this study were dried over appropriate drying agents and distilled prior to use.
H. Zhang et al. / Journal of Photochemistry and Photobiology A: Chemistry 346 (2017) 10–16
11
Scheme 1. Synthesis of coumarin-biphenyl derivatives.
IR spectra were taken with a Shimadzu IR Prestige-21 FT-IR spectrophotometer. 1H NMR spectra were obtained on Varian Mercury Plus 400 MHz and Agilent Technologies DDZ 600 MHz Spectrometer with tetramethylsilane as the internal standard. Element analyses were performed on an Vario-EL automatic elemental analysis instrument. Absorption spectra were recorded by a Shimadzu UV-2550 spectrometer. Photoluminescence spectra were obtained using a Perkin-Elmer LS-55 spectrometer. MS was obtained from a Thermo Scientific Orbitrap Elite mass spectrometer. Fluorescent lifetime and quantum yield were recorded on a FLSP920 type steady-state/transient fluorescence spectrometer (Edinburgh Instruments Ltd). Cyclic voltammetry (CV) was carried out on an CH Instruments 760B with a three electrode system (a Pt working electrode, a Pt-wire counter electrode, and a Ag/AgCl reference electrode) in the presence of n-Bu4NPF6 (0.1 mol L1) as a supporting electrolyte in CH2Cl2. 2.2. Synthesis and characterization 3-(4-Bromophenyl)coumarin and 3-(4-Bromophenyl)-7-(N,N0 diethylamino) coumarin were prepared according to the procedure as previously described [16]. 2.2.1. Preparation of 3-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2yl)phenyl)-coumarin (a1) The suspension of 3-(4-Bromophenyl)coumarin (1.5 g, 4.98 mmol), bis(pinacolato) diboron (2.0 g, 7.87 mmol), potassium acetate (0.8 g, 8.15 mmol) and Pd(dppf)Cl2 (0.1 g, 0.14 mmol) in absolute 1,4-dioxane (100 mL) was placed in a three-necked flask. The mixture was heated up to 95 C and stirred for 24 h under N2. After solvent was evaporated under vacuum, the mixture was dissolved in dichloromethane, washed with water (3 20 mL). The organic layer was dried over MgSO4 and concentrated in vacuum. The residue was chromatographed on silica, eluting with EtOAc/ ether(1:30-1:10 gradient, v/v) to form white solid (yield 84%). m. p.:178–179 C. Anal. Calcd. for C21H21BO4 (%): C 72.44, H 6.08. Found (%): C 72.31, H 6.10. 1H NMR (400 MHz, CDCl3) d: 7.89 (d, J = 8.0 Hz, 2H, Ar-H), 7.85 (s, 1H, coumarin H), 7.73 (d, J = 8.0 Hz, 2H, Ar-H), 7.54 (t, J = 8.7 Hz, 2H, coumarin H), 7.37 (d, J = 8.3 Hz, 1H, coumarin H), 7.30 (t, J = 7.4 Hz, 1H, coumarin H), 1.36 (s, 12H). 2.2.2. Preparation of 3-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2yl)phenyl)-7-(N,N0 - diethylamino)coumarin (a2) The preparation of a2 was similar to that described for a1, which was obtained through the reaction of 3-(4-Bromophenyl)-7-(N,N0 diethylamino)coumarin (1.0 g, 2.69 mmol) with bis(pinacolato)
diboron (1.10 g, 4.33 mmol). The crude was purified by chromatography on silica gel using EtOAc/CH2Cl2/petroleum ether (1:8:30, v/v/v) to form yellow solid (yield 74%). m.p.: 212–214 C. Anal. Calcd. for C25H30BNO4 (%): C 71.61, H 7.21, N 3.34. Found (%): C 71.53, H 7.29, N 3.30. 1H NMR (400 MHz, CDCl3) d: 7.85 (d, J = 7.9 Hz, 2H, Ar-H), 7.73 (s, 2H, Ar-H), 7.71 (s, 1H, coumarin H), 7.32 (d, J = 8.8 Hz, 1H, coumarin H), 6.59 (dd, J = 8.8, 2.1 Hz, 1H, coumarin H), 6.53 (s, 1H, coumarin H), 3.43 (q, J = 7.0 Hz, 4H, CH2), 1.35 (s, 12H, CH3), 1.22 (t, J = 7.0 Hz, 6H, CH3). 2.2.3. Preparation of 4,40 -di(coumarin-3-yl)-biphenyl (b1) Absolute toluene (100 mL) was added under N2 to the mixture of a1 (1 g, 2.87 mmol), 3-(4-Bromophenyl)coumarin (1.4 g, 4.65 mmol), Na2CO3 (1 g, 9.43 mmol), TBAB (0.05 g, 0.16 mmol) and Pd(pph3)4 (0.2 g, 0.17 mmol). The suspension was heated up to 110 C and stirred vigorously. After 10 min, distilled water (2 mL) was added. The resulting mixture was stirred 110 C for 24 h. After solvent was evaporated under vacuum, the mixture was dissolved in dichloromethane, washed with water (3 20 mL). The organic layer was dried over MgSO4 and concentrated in vacuum. The residue was chromatographed on silica, eluting with EtOAc/ CH2Cl2/petroleum ether (1:100:300, v/v/v) to form white solid. Yield: 68%. m.p. > 280 C. Anal. Calcd. for C30H18O4 (%): C 81.44, H 4.10. Found (%): C 81.40, H 4.13. IR (KBr), cm1: 1722 (n C¼O), 1114 (n 1 C O), 1610 and 1455 (n C¼C), 809 (d Ar-H). H NMR (600 MHz, CDCl3) d: 7.82 (s, 2H, coumarin H), 7.62–7.57 (m, 8H, Ar-H), 7.57–7.54 (m, 4H, coumarin H), 7.38 (d, J = 8.7 Hz, 2H, coumarin H), 7.32 (d, J = 7.5 Hz, 2H, coumarin H). The preparation of 4-(coumarin-3-yl)-40 -(7-diethylaminocoumarin-3-yl)-biphenyl (b2) and 4,40 -di(7-diethylaminocoumarin-3yl)-biphenyl (b3) were similar to that described for b1. b2 was synthesized through the reaction of 3-(4-Bromophenyl) coumarin (0.5 g, 1.67 mmol) with a2 (0.46 g,1.09 mmol). The crude was purified by chromatography on silica gel using EtOAc/CH2Cl2 (1:100, v/v) to form yellow solid (yield 74%). m.p. > 280 C. Anal. Calcd. for C34H27NO4 (%): C 79.51, H 5.30, N 2.73. Found (%): C 79.53, H 5.31, N 2.71. IR (KBr), cm1: 2970 (n CH), 1715 (n C¼O), 1138 (n 1 C O), 815 (n Ar-H), 1275 (n CN). H NMR (400 MHz, CDCl3) d: 7.89 (s, 1H,coumarin H), 7.82 (dd, J = 8.4, 1.4 Hz, 4H, Ar-H), 7.78 (s, 1H, coumarin H), 7.71 (dd, J = 12.6, 8.5 Hz, 4H, Ar-H), 7.56 (dd, J = 15.6, 7.2 Hz, 2H, coumarin H), 7.40 (d, J = 8.7 Hz, 1H, coumarin H), 7.37– 7.28 (m, 2H, coumarin H), 6.64–6.49 (m, 2H, coumarin H), 3.45 (dd, J = 13.6, 6.5 Hz, 4H,CH2), 1.27-1.19 (m, 6H, CH3). MS (ESI) m/z 514.20 [M+H]+, calculated for C34H27NO4, 513.58. b3 was obtained from the reaction of 3-(4-Bromophenyl)-7-(N, N'-diethylamino) coumarin (0.46 g, 1.24 mmol) with a2 (0.4 g,0.95
12
H. Zhang et al. / Journal of Photochemistry and Photobiology A: Chemistry 346 (2017) 10–16
mmol). The crude was purified by chromatography on silica gel using EtOAc/CH2Cl2/petroleum ether (4:100:300, v/v/v) to form yellow solid (yield 78%). m.p. > 280 C. Anal. Calcd. for C38H36N2O4 (%): C 78.06, H 6.21, N 4.79. Found (%): C 78.07, H 6.24, N 4.77. IR (KBr), cm1: 2964(n CH),1715(n C¼O), 1262(n CN), 1095(n CO), 803(d Ar-H). 1H NMR (400 MHz, CDCl3) d: 7.64 (s, 2H, coumarin H), 7.60 (d, J = 8.9 Hz, 4H, Ar-H), 7.55-7.51 (m, 4H, Ar-H), 7.32-7.30 (m, 2H, coumarin H), 6.88 (d, J = 8.3 Hz, 4H, coumarin H), 3.43 (dd, J = 14.0, 6.9 Hz, 8H, CH2), 1.24–1.20 (m, 12H, CH3). 2.3. Crystallography A single crystal of b3 obtained from CH2Cl2 solution was selected for data collection that was performed on Bruker Smart Apex CCD diffractometer equipped with graphite-monochromatic Mo Ka radiation (l = 0.71073 Å) at 293 K. The structure was solved by direct methods and refined by full-matrix least-squares method on F2 using SHELXL. Single crystals of b1 and b2 have not obtained. 2.4. OLEDs fabrication and characterization To investigate the EL properties of the compounds b2 and b3, the multilayer OLEDs with a device architecture of ITO/TAPC (20 nm)/TBADN:compound b (x wt%, 30 nm)/TPBI (50 nm)/Liq (2 nm)/Al were fabricated by vacuum-deposition method. (ITO: Indium tin oxide; TAPC: 1,1-bis-(4-bis(4-methylphenyl)-aminophenyl)-cyclohexane; TBADN: 9,10-bis(2-naphthyl)-2-t- butylanthracene; TPBI: 2,20 ,200 - (1,3,5-Benzinetriyl)-tris(1-phenyl-1-Hbenzimidazole)). The emitting layers are consisted of host materials TBADN and dopants of b2 or b3 at different concentrations (x wt%). TAPC was used as the hole injection layer, TPBI was the electron transport layer, Liq and Al were used as the electron-injection layer and cathode, respectively. The background pressure of the chamber was under 2 105 Pa during the deposition process. The EL spectra and Commission Internationale de L'Eclairage (CIE) coordinates were measured on a Hitachi MPF-4 fluorescence spectrometer. 3. Results and discussion 3.1. X-ray crystallographic analyses The compound b3 crystallizes in the C2/c space group pertaining to monoclinic crystal system. The crystal structure of b3 is shown in Fig. 1. The crystal data and experimental details are given in Table 1. The selected bond lengths and bond angles are listed in Table 2. Biphenyl group have planar conformation. The intermolecular interactions may promote the coplanar arrangements of aromatic rings in the biphenyl compounds, which may be accountable for the noted conjugation[17]. Two coumarin rings are
Table 1 Crystallographic data for b3. Data
b3
Empirical formula Formula weight Temperature (K) Wavelength (Å) Crystal system, space group Unit cell dimensions a (Å) b (Å) c (Å) a ( ) b ( ) g ( ) Volume (Å3) Z, Calculated density (Mg/m3) Absorption coefficient (mm1) F(000) Crystal size (mm) Theta range for data collection ( ) Limiting indices
C38 H36 N2O4 584.69 298(2) 0.71073 Monoclinic, C2/c
Reflections collected/unique Completeness to theta = 25.02 Absorption correction Max. and min. transmission Refinement method Data/restraints/parameters Goodness-of-fit on F2 Final R indices [I > 2sigma(I)] R indices (all data) Extinction coefficient Largest diff. peak and hole (eÅ3)
20.9909(18) 6.4790(6) 23.584(2) 90 110.878(3) 90 2996.8(5) 4, 1.296 0.084 1240 0.40 0.35 0.13 2.23 to 25.02 24 h 18 7 k 7 27 l 28 7181/2636 [R(int) = 0.0466] 99.8% Semi-empirical from equivalents 0.9892 and 0.9672 Full-matrix least-squares on F2 2636/0/286 0.961 R1 = 0.0621, wR2 = 0.1785 R1 = 0.1426, wR2 = 0.2479 0.0005(5) 0.176 and 0.153
not in a coplane. The dihedral angles between coumarin ring and biphenyl group is 39.35 and 39.57, respectively. Fig. 2 shows the crystal packing of the compound b3. As depicted in Fig. 2, there are three intermolecular nonclassical C3 H3 O2, C15H15 O2, C18 H18A O2 hydrogen bonds. Also, in the lattice, two C H p stacking interactions are present. CH group and a coumarin ring with H p distance of 2.846 Å for C17 H17B Cg1, in which Cg1 is the centroid for the O1/C1/C2/ Table 2 Selected bond lengths and angles for b3. bond angles [ ]
bond lengths [Å] N(1)-C(7) N(1)-C(16) N(1)-C(18) O(1)-C(5) O(1)-C(1) O(2)-C(1) C(2)-C(10)
1.390(18) 1.435(13) 1.442(14) 1.372(4) 1.385(4) 1.206(4) 1.474(4)
C(7)-N(1)-C(16) C(7)-N(1)-C(18) C(16)-N(1)-C(18) C(5)-O(1)-C(1) O(2)-C(1)-O(1) O(2)-C(1)-C(2) O(1)-C(1)-C(2)
Fig. 1. ORTEP drawing of the molecular structure of b3 along with the atomic abeling scheme and H atoms are excluded.
122.8(10) 124.3(11) 112.9(8) 122.7(3) 114.5(3) 127.8(3) 117.7(3)
H. Zhang et al. / Journal of Photochemistry and Photobiology A: Chemistry 346 (2017) 10–16
13
Fig. 2. Illustration of hydrogen bond interactions in molecular structure of b3.
C3/C4/C5 ring. CH group and a phenyl ring with H p distance 2.855 Å for C18 H18B Cg2, in which Cg2 is the centroid for the C10/C11/C12/C13/C14/C15 ring. The crystal structure has been deposited at the Cambridge Crystallographic Data Centre and allocated the deposition number of CCDC 1516115.
The UV–vis absorption and fluorescence spectra of b1, b2 and b3 in dilute CH2Cl2 solutions are presented in Figs. 3 and 4. Table 3 lists the UV–vis absorption maxima (la,max), emission maxima (le, max), Stokes shift (Dl), fluorescent quantum yield (F), fluorescent lifetime (t) and molar extinction coefficient (e) of the tested compounds. Fig. 3 shows the strong absorption bands of b1–b3 are at the 325–450 nm region, which correspond to the p-p* absorption induced by the larger conjugative biphenyl chain. It can be seen that the bathochromic shift observed is attributed to the increased number of diethylamino group, because diethylamino-
substitution with electron-donating ability at 7-position of coumarin may increase p-conjugation. Fig. 4 shows that the photoluminescence (PL) emission peaks in CH2Cl2 cover the 400–500 nm region. Compound b1 exhibits blue emission, while b2 and b3 exhibit blue-green emissions. The difference lies in the strong electron-donating diethylamino group in b2 and b3. This makes b2 and b3 have better conjugation than that of unsubstituted b1, and have red shifts of 48 and 54 nm compared with b1. It is also found from Table 3 that compound b3 has higher fluorescence quantum yield and longer fluorescence lifetime and red-shifted emission spectrum in contrast to b1 and b2. The higher quantum yield in fluorescence emission in b3 derives from the shorter Stokes Shift. The quantum yields determined were absolute values using an Integrating Sphere without reference standard [18]. As reported, there are some synthesized bis-coumarin analogues connecting two coumarin units using defferent bridge units, such as benzene [19], pyrazole [20], ketones [21], 1,3-diketo or 1,3diketofluoroborate [12]. The compound b3 utilazed biphenyl
Fig. 3. The UV–vis absorption spectra of b1, b2 and b3 in dilute (107 mol L1) CH2Cl2 solutions.
Fig. 4. The PL emission spectra of b1, b2 and b3 in dilute (107 mol L1) CH2Cl2 solutions.
3.2. Photophysical properties
14
H. Zhang et al. / Journal of Photochemistry and Photobiology A: Chemistry 346 (2017) 10–16
Table 3 Absorption, emission maxima, Stokes shift, fluorescence quantum yield, lifetime and molar extinction coefficient of b1, b2 and b3. Compound b1 b2 b3
la,max
le,max
nm
nm
Dl
cm1
341 409 417
430 478 484
6069 3529 3319
F 0.49 0.59 0.75
t
e
ns
L mol1 cm1 (105)
9.47 9.88 10.09
1.15 1.55 2.20
brigde connecting two coumarin units exhibits better properties of fluorescent quantum yield and fluorescent lifetime than other analogues reported earlier [19,21]. 3.3. Thermal properties Thermogravimetric analysis (TGA) measurement was performed in flowing dry nitrogen atmosphere at the heating rate of 10 C min1. The results of TGA measurement of tested compounds are shown in Fig. 5. Compound b3 displays good thermal stability up to 380 C (<2% weight loss). At 385 C, there is a sharp weight losses in its TGA curve, indicating that compound b3 begins to decompose at this temperature. The thermal stabilities of the b1 and b2 are lower than that of b3. The TGA curve of the b1 shows 2% weight loss at 204 C, and sharp weight loss at 250 C. b2 shows 2% weight loss at 267 C, and sharp weight loss at 276 C. The results indicate that compounds of b2 and b3 meet with the thermal stability requirement of fabrication of OLED luminescence.
Fig. 6. Cyclic voltammograms of ferrocene, b1, b2 and b3 (scan rate: 10 mV s1, solvent: dichloromethane).
absorption spectra, the optical band gaps of b1, b2 and b3 were estimated to be 412, 466, and 477 nm, which correspond to 3.01, 2.67, and 2.63 eV, respectively. The HOMO and LUMO energy levels can be calculated according to the relations reported [24]. The overall EHOMO, ELUMO, and Eg data of the compounds were listed in Table 4.
3.4. Electrochemical properties The electrochemical behaviors of b1, b2 and b3 were investigated by cyclic voltammetry (CV). The cyclic voltammograms of the compounds are shown in Fig. 6. The ionization potential (IP) may be regarded as HOMO energy level, and the electron affinity (EA) may be regarded as LUMO energy level [22,23]. To design optimized device structures, the information on HOMO and LUMO of the emitting layer are important. The Eg represent the band gap between HOMO and LUMO, and the value can calculated from optical absorption edge. From the UV–vis
Fig. 5. Thermogravimetric analyses (TGA) of b1, b2 and b3.
Table 4 Electrochemical data for the b1, b2 and b3. Compound
EHOMO/eV
Eg/eV
ELUMO/eV
b1 b2 b3
5.05 5.07 5.11
3.01 2.67 2.63
2.04 2.40 2.48
Fig. 7. EL spectra of devices ITO/TAPC (20 nm)/TBADN: b2 (x%, 30 nm)/TPBI (50 nm)/Liq (2 nm)/Al with different doping concentration (spectra were measured at the same current density in ca. 20 mA cm2).
H. Zhang et al. / Journal of Photochemistry and Photobiology A: Chemistry 346 (2017) 10–16
15
Fig. 9. EL spectra of devices ITO/TAPC (20 nm)/TBADN: b3 (x%, 30 nm)/TPBI (50 nm)/Liq (2 nm)/Al with different doping concentration (spectra were measured at the same current density in ca. 20 mA cm2).
Fig. 8. (a) L-I-V, (b) luminance-efficiency and (c) external quantum efficiency characteristics of devices ITO/TAPC (20 nm)/TBADN: b2 (x%, 30 nm)/TPBI (50 nm)/ Liq (2 nm)/Al with different doping concentration.
3.5. Electroluminescence properties Compound b1 has weak fluorescence, so the device based on b1 has not tested. Fig. 7 shows EL spectra of devices ITO/TAPC(20 nm)/TBADN: b2 (x%, 30 nm)/TPBI (50 nm)/Liq (2 nm)/Al. The doping concentration of b2 was 4, 7, 10 and 13%, respectively. All spectra were measured at the same current density in ca. 20 mA cm2. When the doping concentration was 4%, the device shows blue emission (464 nm) and the calculated CIE coordinate is (0.19, 0.22). When the doping concentration reached to 13%, the device shows green emission (512 nm) and the calculated CIE coordinate is (0.31, 0.50). It follows that with doping concentration increasing, the EL spectrum was red shifted, which might be caused by aggregation with increase in concentration.
Fig. 8 shows the L-I-V, luminance-efficiency, and EQE characteristics of those above devices. The device with 10% doped b2 has the highest current density of 230 mA/cm2 at 11.8 V (Fig. 8(a)), and external quantum efficiency (EQE) over 2% (Fig. 8(c)). The doped device at the concentration of 13 wt% b2 has the maximum brightness of 5135 cd/m2 at 12.8 V (Fig. 8(a)), the highest current efficiency of 4.83 cd/A at 219 cd/m2, and the highest power efficiency of 2.06 Lm/W at 2.65 cd/m2 (Fig. 8(b)). The EL spectra of the devices based on b3 with different dopant concentrations are shown in Fig. 9. The spectra were measured at the same current density in ca. 20 mA cm2. It can be seen that EL spectra differ from the PL spectra, which is attributed to excimer of b3 molecules at high concentration in the process of condensation from vapor phase in vacuum. Fig. 10 shows the L-I-V, luminance-efficiency, and EQE characteristics of devices based on b3. The device with 4% doped b3 shows current density of 152 mA/cm2 and the maximum brightness of 480 cd/m2 at 16 V (Fig. 10(a)), current efficiency of 1.60 cd/A and power efficiency of 0.71 Lm/W at 1.95 cd/m2 (Fig. 10(b)), and external quantum efficiency (EQE) over 1% (Fig. 10(c)). Obviously, electroluminescent performance of the devices based on b2 is substantially higher than that of the devices fabricated from b3. The aggregation of b3 due to strong intermolecular hydrogen bonds causes fluorescence quenching. From the above results, we can see that the compound b2 could be a good candidate for OLED devices. 4. Conclusion In summary, three coumarin-biphenyl derivatives have been developed. Compounds b2 and b3 have good photophysical properties and thermal stabilities. When b2 was fabricated as a dopant in OLED device with configuration of ITO/TAPC (20 nm)/ TBADN:b2 (x%, 30 nm)/TPBI (50 nm)/Liq (2 nm)/Al, the maximum emission band red shifted from 464 to 512 nm, accompanying with CIE coordinate changed from (0.19, 0.22) to (0.31, 0.50) as the doping concentration of b2 varied from 4 wt% to 13%. And the device with 13% doped concentration showed maximum luminance of 5135 cd/m2 at 12.8 V. Because of the fluorescence
16
H. Zhang et al. / Journal of Photochemistry and Photobiology A: Chemistry 346 (2017) 10–16
References
Fig. 10. (a) L-I-V, (b) luminance-efficiency and (c) external quantum efficiency characteristics of devices ITO/TAPC (20 nm)/TBADN: b3 (x%, 30 nm)/TPBI (50 nm)/ Liq (2 nm)/Al with different doping concentration.
quenching due to aggregation of b3, electroluminescent performance of the devices based on b3 is lower than that of the devices fabricated from b2. We can see that the compound b2 could be a good candidate for OLED devices. In order to design a highperformance OLED, further modification and optimization of the EL devices having biscoumarin derivatives are in progress. Acknowledgements This work was financially supported by the Gansu Universities Research Fund (2015A-066) and Gansu Natural Science Foundation (1606RJZA018), and also supported by the National Natural Science Foundation of China (Grand 51563014).
[1] A. Dey, M.A. Ali, S. Jana, S. Samanta, A. Hajra, Palladium-catalyzed synthesis of indole fused coumarins via cross-dehydrogenative coupling, Tetrahedron Lett. 58 (2017) 313–316. [2] N.I. Aminudin, F. Ahmad, M. Taher, R.M. Zulkif, A.-D. Incrassamarin, Four new 4-substituted coumarins from Calophyllum incrassatum and their biological activities, Phytochem. Lett. 16 (2016) 287–293. [3] T. Abdizadeh, M.R. Kalani, K. Abnous, Z. Tayarani-Najaran, B.Z. Khashyarmanesh, R. Abdizadeh, R. Ghodsi, F. Hadizadeh, Design, synthesis and biological evaluation of novel coumarin-based benzamides as potent histone deacetylase inhibitors and anticancer agents, Eur. J. Med. Chem. 132 (2017) 42–62. ski, M. Misiak, A. [4] M. Dasko, M. Przybyłowska, J. Rachon, M. Masłyk, K. Kubin Składanowski, S. Demkowicz, Synthesis and biological evaluation of fluorinated N-benzoyl and Nphenylacetoyl derivatives of 3-(4-aminophenyl)coumarin-7-Osulfamate as steroid sulfatase inhibitors, Eur. J. Med. Chem. 128 (2017) 79–87. [5] M.R. Shreykar, N. Sekar, Stimuli-responsive luminescent coumarin thiazole hybrid dye: synthesis, aggregation induced emission, thermochromism and DFT study, Dyes Pigm. 142 (2017) 121–125. [6] A. Matta, V. Bahadur, T. Taniike, J.V.D. Eycken, B.K. Singh, Synthesis, characterisation and photophysical studies of oxadiazolyl coumarin: a new class of blue light emitting fluorescent dyes, Dyes Pigm. 140 (2017) 250–260. [7] T.Z. Yu, P. Zhang, Y.L. Zhao, H. Zhang, J. Meng, D.W. Fan, L.L. Chen, Y.Q. Qiu, Synthesis, crystal structure and photo- and electro-luminescence of the coumarin derivatives with benzotriazole moiety, Org. Electron. 11 (2010) 41– 49. [8] H. Zhang, H.F. Chai, T.Z. Yu, Y.L. Zhao, D.W. Fan, High-Efficiency blue electroluminescence based on coumarin derivative 3-(4-(anthracen-10-yl) phenyl)-benzo[5,6] coumarin, J. Fluoresc. 22 (2012) 1509–1512. [9] Y. Xie, L. Han, C.S. Ge, Y.H. Cui, J.R. Gao, Novel organic dye sensitizers containing fluorenyl and biphenyl moieties for solar cells, Chin. Chem. Lett. 28 (2017) 285–292. [10] J. Gordo, J. Avó, A.J. Parola, J.C. Lima, A. Pereira, P.S. Branco, Convenient synthesis of 3-vinyl and 3-styryl coumarins, Org. Lett. 13 (2011) 5112–5115. [11] H. Ammar, S. Fery-Forgues, R.E. Gharbi, UV/vis absorption and fluorescence spectroscopic study of novel symmetrical biscoumarin dyes, Dyes Pigm. 57 (2003) 259–265. [12] S.N. Margar, N. Sekar, Red and near-infrared emitting bis-coumarin analogues based on curcumin framework-synthesis and photophysical studies, J. Photochem. Photobiol. A: Chem. 327 (2016) 58–70. [13] J. Wang, G. Cooper, D. Tulumello, A.P. Hitchcock, Inner shell excitation spectroscopy of biphenyl and substituted biphenyls: probing ring-Ring delocalization, J. Phys. Chem. A 109 (2005) 10886–10896. [14] H.Z. He, C. Wang, T. Wang, N. Zhou, Z.Y. Wen, S. Wang, L.C. He, Synthesis, characterization and biological evaluation of fluorescent biphenylfurocoumarin derivatives, Dyes Pigm. 113 (2015) 174–180. [15] C.C. Zhu, K.P. Guo, W.B. Liu, Y.B. He, Z.M. Li, X.C. Gao, F.J. Deng, B. Wei, Synthesis of asymmetric biphenyl derivatives for optoelectronic applications, Opt. Mater. 35 (2013) 2095–2101. [16] H. Zhang, T.Z. Yu, Y.L. Zhao, D.W. Fan, Y.J. Xia, P. Zhang, Synthesis, crystal structure, photo- and electro-luminescence of 3-(4-(anthracen-10-yl)phenyl)7- (N,N0 -diethylamino) coumarin, Synth. Met. 160 (2010) 1642–1647. [17] T.V. Usha Gangan, S. Sreenadh, M.L.P. Reddy, Visible-light excitable highly luminescent molecular plastic materials derived from Eu3+-biphenyl based b-diketonate ternary complex and poly(methylmethacrylate), J. Photochem. Photobiol. A: Chem. 328 (2016) 171–181. [18] L. Porrès, A. Holland, L.O. Pålsson, A.P. Monkman, C. Kemp, A. Beeby, Absolute measurements of photoluminescence quantum yields of solutions using an integrating sphere, J. Fluoresc. 16 (2006) 267–272. [19] H. Ammar, S. Fery-Forgues, R.E. Gharbi, UV/vis absorption and fluorescence spectroscopic study of novel symmetrical biscoumarin dyes, Dyes Pigm. 57 (2003) 259–265. [20] K. Kandasamy, S. Ganesabaskaran, M.P. Pachamuthu, A. Ramanathan, A novel pyrazole biscoumarin based chemosensors for the selective detection of Cu2+ and Zn2+ ions, Spectrochim. Acta Part A: Mol. Biomol. Spectrosc. 148 (2015) 184–188. [21] R. Nazir, P. Danilevicius, A.I. Ciuciu, M. Chatzinikolaidou, D. Gray, L. Flamigni, M. Farsari, D.T. Gryko, p-Expanded ketocoumarins as efficient, biocompatible initiators for two-photon-induced polymerization, Chem. Mater. 26 (2014) 3175–3184. [22] H.Y. Fu, X.D. Gao, G.Y. Zhong, Z.Y. Zhong, F. Xiao, B.X. Shao, Synthesis and electroluminescence properties of benzothiazole derivatives, J. Lumin. 129 (2009) 1207–1214. [23] S.M. Kim, J.-S. Kim, D.-M. Shin, Y.K. Kim, Y. Ha, Synthesis and application of the novel azomethine metal complexes for the organic electroluminescent devices, Bull. Korean Chem. Soc. 22 (2001) 743–747. [24] K. Ezzayani, A.B. Khelifa, E. Saint-Aman, F. Loiseau, H. Nasri, Synthesis, spectroscopic characterizations, cyclic voltammetry investigation and molecular structure of the di-l-cyanato-N-bis (l-1,4,7,10,13,16hexaoxacyclooctadecane)bis (5,10,15,20-tetraphenyl porphyrinato) dimagnesiumdipotassium complex, Polyhedron 117 (2016) 817–825.