dihedral angles between phenyls and the silole core at the 1,3,4-positions are 67.99°, 82.55° and 80.01°, respectively. In addition, the
two substituents at 2,5-positions are unsymmetrical, where the dihedral angles between carbazoles and silole core at these two positions
are 47.83° and 9.39°, respectively. Further analysis for packing pattern of 3,3'-MTPS-CaP indicates that there are multiple
intramolecular and intermolecular C–H···π hydrogen bonds within the range of 2.648–3.039 Å (Fig. S2 in Supporting informaiton).
These weak interactions will rigidify molecular structure and restrict the intramolecular motions in the solid state. However, no π–π
stacking interactions in crystalline state of 3,3'-MTPS-CaP are found. A highly twisted conformation is helpful to prevent strong
intermolecular π‒π interaction, and suppress the emission quenching in the condensed phase.
The absorption maxima of 2,2'-MTPS-CaP, 3,3'-MTPS-CaP and 9,9'-MTPS-CaP in dilute THF solution (10–5 mol/L) are located at
398, 404 and 385 nm, owing to the π–π* electron transition (Fig. 3A). The optical band gaps of 2,2'-MTPS-CaP and 3,3'-MTPS-CaP are
relatively narrower than that of 9,9'-MTPS-CaP, estimated from the onset of absorption spectra, which indicate that 2,2'-MTPS-CaP and
3,3'-MTPS-CaP have better effective conjugation than 9,9'-MTPS-CaP. The photoluminescence (PL) emissions of 2,2'-MTPS-CaP,
3,3'-MTPS-CaP and 9,9'-MTPS-CaP in dilute THF solutions are very weak, with PL peaks at 525, 526 and 520 nm, and low ΦFs of
5.75%, 4.00% and 3.20%, respectively. After fabricated into solid films, they present slightly red-shifted and greatly enhanced PL peaks
within the range of 520–539 nm (Fig. 3B). Their ΦFs are significantly increased to 69.93%, 60.33% and 80.00% (Table 1), indicating
their AIE characteristics. To further assess the PL properties of 2,2'-MTPS-CaP, 3,3'-MTPS-CaP and 9,9'-MTPS-CaP, the fluorescence
lifetimes (τ), important parameters used to describe the excited-state decay processes, were measured and fitted. The intersystem
crossing (ISC) process had been ignored in the excited-state decay process of silole derivatives because of their fluorescent nature. The
ΦF and τ can be determined by radiative decay rate (kr) and nonradiative decay rate (knr) [12]. Thus, the decay rates of 2,2'-MTPS-CaP,
3,3'-MTPS-CaP and 9,9'-MTPS-CaP had been calculated, including in solution and solid states (Table S1 in Supporting information).
For these silole derivatives, once fabricated into solid films, there are great increases in their ΦF and τ, but small increases in kr.
However, the knr is decreased significantly. These results manifest that the RIM is triggered in the aggregated state, which blocks the
nonradiative decay channel, and thus boosts the PL emissions.
In order to gain an in-depth insight into their AIE property of these silole-based luminogens, their PL spectra in THF/water
mixtures were measured and the results are shown in Fig. 4. Since they are insoluble in water, their PL intensity was increased along
with the increase of water fraction (fw) in the mixture. Strong PL emissions were observed in the aggregated state with a high water
fraction (fw = 90%), further validating the AIE property of these silole derivatives.
To investigate the electronic structures of these new silole derivatives, their highest occupied molecular orbitals (HOMOs) and
lowest unoccupied molecular orbitals (LUMOs) were calculated by density function theory (DFT) calculation using a B3LYP/6-31G (d,
p) basis set on the Gaussian 09 program. The optimized structures and spatial distributions of HOMOs and LUMOs for 2,2'-MTPS-CaP,
3,3'-MTPS-CaP and 9,9'-MTPS-CaP are illustrated in Fig. 5. The HOMOs are located on the entire molecular backbones consisting of
central silole ring and carbazole substituents. Their LUMOs, however, are mainly concentrated on the silole ring. The exocyclic single
bonds at the 1,1-positions also contribute to the LUMOs, indicative of the unique σ*‒π* conjugation.
In order to get the experimental HOMO and LUMO energy levels of these novel silole-based luminogens, the electrochemical
properties were investigated by cyclic voltammetry (CV) in dichloromethane solution containing 0.1 mol/L tetra-n-butylammonium
hexafluorophosphate at a scan rate of 100 mV/s. The working electrode was platinum and the reference electrode was Ag/AgNO3
electrode. They showed a good electrochemical stability with reversible oxidation processes (Fig. 6). The values of Eox
and Ere
onset
onset
were represented by the onset oxidation and reduction potentials relative to Fc/Fc+. The onset potentials of oxidation (Eoxonset) of 2,2'-
MTPS-CaP, 3,3'-MTPS-CaP and 9,9'-MTPS-CaP occurred at 0.68, 0.51 and 0.82 V, respectively. Thus, the HOMO energy levels can
be determined as –5.20, –5.04 and –5.34 eV, respectively, according to the equation [HOMO = – (Eox
+ 4.8) eV]. While the onset
onset
potentials of reduction (Ereonset) of these silole derivatives occurred at –1.92, –2.21 and –2.09 V, respectively, and their LUMO energy
levels were calculated to be –2.70, –2.49 and –2.62 eV, respectively, from the equation [LUMO = – (Ere
+ 4.8) eV]. The variation
onset
tendency of energy band gaps between HOMOs and LUMOs are consistent with optical band gaps (Eg) (Table 1).
Given the high solid-state emission efficiency and favorable thermal stability of these new silole derivatives, their potentials as
light-emitting layers in non-doped OLEDs were evaluated. The non-doped OLEDs with a configuration of ITO/NPB (60 nm)/emitter
(20 nm)/TPBi (40 nm)/LiF (1 nm)/Al were fabricated, in which new silole derivatives worked as light-emitting layers, NPB (N,N'-di(1-
naphthyl)-N,N'-diphenyl-benzidine) functioned as the hole-transporting layer, and TPBi (1,3,5-tri(1-phenylbenzimidazol-2-yl)-benzene)
served as the electron-transporting layer. The EL performance data for the non-doped OLEDs based on these silole derivatives are
shown in Fig. 7 and Table 2. These non-doped OLEDs could be turned on at voltages of 2.9–5.3 V, and radiated green lights. The
maxima EL peaks of 2,2'-MTPS-CaP, 3,3'-MTPS-CaP and 9,9'-MTPS-CaP are located at 555 (CIEx,y = 0.244, 0.435), 552 (CIEx,y
=
0.399, 0.559) and 542 nm (CIEx,y = 0.376, 0.549), respectively, which are red-shifted by 16–22 nm in comparison with the PL emissions
in films. Actually, this is a common phenomenon for the luminescent materials because of the microcavity effect [13]. The devices of
2,2'-MTPS-CaP and 3,3'-MTPS-CaP showed comparable EL performances, with maxima luminance (ηL,max) of 83870 and 78780 cd/m2,
maxima current efficiencies (ηC,max) of are 12.72 and 12.44 cd/A, maxima power efficiencies (ηP,max) of 9.87 and 10.64 lm/W and
maxima external quantum efficiencies (ηext,max) of 4.01 and 3.57%, respectively. The device of 9,9’-MTPS-CaP exhibited the best EL
performance, affording high ηL,max, ηC,max, ηP,max and ηext,max of 91920 cd/m2, 17.59 cd/A, 12.55 lm/W, and 5.63%, respectively. The
excellent ηext,max is actually approaching the theoretical efficiency limit of fluorescent OLEDs.