Synthesis of BT-LC. The synthetic route of BT-LC is illustrated in Figure 1. The precursor 4,7-bis(2,4-dimethoxyphenyl)-2,1,3-benzothiadiazole (BT-M) was synthesized through Suzuki-Miyaura coupling reaction of 4,7-dibromo-2,1,3-benzothiadiazole and 2,4-dimethoxybenzeneboronic acid. 2,1,3-benzothiadiazole is a widely used building block in luminescent materials.41,42 BT-M is an ideal highly-emissive molecule, due to its D-A architecture with dimethoxyphenyl donor and 2,1,3-benzothiadiazole (BT) acceptor, where the push-pull system can improve the luminescence performance through the enhancement of intramolecular charge transfer (ICT).43-45 Subsequently, BT-LC was synthesized by a one-step condensation of BT-M with paraformaldehyde, catalyzed by Lewis acid of BF3•Et2O (52% yield). All chemical structures were confirmed by 1H and 13C NMR spectroscopy, high resolution mass spectra (HRMS) as well as single crystal X-ray diffraction.
Table 1. Photophysical properties of BT-LC
|
BT-LC
|
λabs(nm)
|
λem(nm)
|
ΦPL(%)
|
TOL
|
401
|
529
|
86
|
THF
|
407
|
537
|
86
|
CHCl3
|
406
|
545
|
83
|
DMF
|
410
|
561
|
89
|
Solid
|
-
|
562
|
99
|
λabs (nm): absorption maximum. λem (nm): fluorescence maximum. ΦPL: absolute PL quantum yield (λex = 418 nm).
|
Photophysical properties of BT-LC in solution. As depicted in Figure 2 and Table 1, its absorptions show negligible differences with λabs values of 401-410 nm. BT-LC exhibits high quantum yields values of 83-89%, and its emissions show remarkable bathochromic shift as the solvent polarity increases. Similar absorptions and emissions were observed for the monomer BT-M. This is due to intramolecular charge transfer (ICT) effect resulted by their distinguished D-A architectures.46-48 Particularly, both BT-LC and BT-M displayed large stokes shifts of 151 nm and 161 nm in DMF.
Macrocyclization-induced emission enhancement. From Figure 3a and Figure S10, the fluorescence intensity of BT-LC in THF/water mixtures becomes gradually reduced when the water fractions (fw) increase from 0 to 70%. Meanwhile, the emission peak shows a visible red-shift from 535 to 565 nm. This emissive phenomenon is attributed to the TICT effect.49-51 As the water fractions (fw) further increases from 70 to 95%, the fluorescence intensity sharply recovers, exhibiting a typical AIE property, and the emission peak blue-shifts to 547 nm. Such photo-physical behaviors demonstrate that BT-LC can be viewed as a dual-state emission (DSE) molecule.52-55 In sharp contrast, the emission intensity of the monomer BT-M swiftly decreases with increasing the fw values, indicating a typical ACQ phenomenon (Figure 3b and S9). The quantum yield of BT-LC in the solid state is as high as 99%, which is much higher than that for BT-M (65%). The above results suggest that the macrocyclization through methylene bridges is an effective way to enhance emission efficiency in aggregated or solid state. The MIEE effect should be attributed to the rigidity induced by macrocyclization and spatial separation from macrocyclic topologic structure.
Mechanism study. Single crystal analysis was examined to understand macrocyclic effects. From Figure 4a and S14, BT-LC has a rigid triangular geometry with vertex angles of 112°, 114° and 115°. The C(sp3) bridges spatially separated three luminophores in a single macrocyclic molecule, leading to the alleviation of the concentration quenching to a certain degree. Three series of torsion angles between the central benzothiadiazole planes and their adjacent phenyl rings are 55.29°, 77.35°; 53.02°, 60.92°; and 56.40°, 69.05, respectively. In comparison, both torsion angles for the monomer BT-M are 63.70° (Figure S11). Macrocyclization of luminophores would efficiently reduce the space of their rotation, therefore preventing their rotation. Furthermore, the intramolecular hydrogen bonds (C-H···N 2.70 Å, and C-H···S 2.75 Å, Figure 4b) between benzothiadiazoles could further limit the intramolecular motion. It should be noted that BT-LC are racemes possessing a pair of enantiomers with double-included dimer geometry, where dimethoxyphenyl group of one enantiomer was encapsulated in the cavity of the other one through multiple intermolecular interactions (C-H···O 2.74 Å, 2.67 Å, and C-H···N 2.95 Å, Figure 4c). For the monomer, adjacent benzothiadiazole units were packed in an antiparallel stacking with a separation of about 4.98 Å (Figure S11b); no C-H···N/S hydrogen bonds were found. Obviously, the tight packing arrangement of BT-LC is more beneficial to fluorescence than that of BT-M. Thus, the above results suggest that MIEE effect should be due to the alleviation of quenching caused by spatial separation and the restriction of intramolecular motion by macrocyclic topologic structure and intra/intermolecular interactions.
The electronic structures of BT-M and BT-LC were calculated by the B3LYP/6-31G (d) method. As depicted in Figure S17, the electron density of the HOMO is well distributed over BT-M, while the LUMO are predominantly dominated by the central BT acceptor. Since the macrocycle is formed by linking the monomers with non-conjugated methylene groups, BT-LC exhibits HOMO and LUMO orbitals similar to BT-M. Separation of space between the HOMO and LUMO indicates the existence of effective ICT, which is consistent with the bathochromic shift of emission peak of BT-LC in solutions.
Table 2. Summary of the EL data of devices A and B
Device
|
Emitting
layer
|
Dopant ratio (wt%)
|
Von
(V)a
|
Bmax
(cd/m2)b
|
CEmax
(cd/A)c
|
PEmax
(lm/w2)d
|
EQEmax
(%)
|
λem
(nm)f
|
CIE
(x, y)g
|
A
|
BT-M
|
5
|
3.98
|
2369
|
10.10
|
7.10
|
1.92
|
512
|
(0.29,0.53)
|
B
|
BT-LC
|
5
|
3.82
|
4355
|
9.93
|
8.25
|
2.82
|
534
|
(0.35.0.57)
|
aTurn-on voltage. bMaximum brightness (Bmax). cMaximum current efficiency (CEmax). dMaximum power efficiency (PEmax). eMaximum external quantum efficiency (EQEmax). fEL peak wavelength. gCommission International de I’Eclairage coordinates.
|
Electroluminescent (EL) properties. We then explore the possibility of applying BT-LC as OLED emitters. Thermogravimetric analysis (TGA) was first performed to assess its thermal stability. From Figure S24, the macrocycle has a higher decomposition temperature [Td (5 wt % loss) 423-426°C] than the monomer (286-291°C), demonstrating rigidity can increase the decomposition temperature. On the other hand, the spectralstabilities of BT-M and BT-LC were examined. The pristine BT-M displayed green fluorescence peaking at 491 nm. After grinding for 2 min, its maximum emission wavelength red-shifts to 505 nm and emits yellow-green fluorescence (Figure 3c). For BT-LC (Figure 3d), the emission wavelength and color of the pristine and ground sample were hardly affected by external stimuli, implying that macrocyclization can improve spectral stability.
High thermal decomposition temperatures suggested that the coevaporation technique is very suitable for purification of OLEDs. Subsequently, OLEDs were fabricated with a multilayer configuration: ITO (100nm)/HATCN (10 nm)/TAPC (40nm)/mCP:emitter (20nm)/TmPyPB (40nm)/Liq (2 nm)/Al (100nm). The device A fabricated with monomer and the device B fabricated with macrocycle at 5% doping concentration were compared to reflect the performance of OLEDs. As shown in Figure 5 and Table 2, device B exhibited higher Bmax and EQEmax than device A did. In other words, the macrocycle gives better device performance than the monomer which could be ascribed to improved quantum yield and rigidity caused by macrocyclization.