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Their emission spectra, however, are red-shifted with increasing solvent polarity, accompanying with decreased intensity (Fig. 2),
which is typically observed in ICT luminogens [25,28]. Taking DPA-DCDPP for an example, the emission peak shifts from 547 nm in
toluene to 608 nm in tetrahydrofuran (THF) and 623 nm in dichloromethane (DCM) [35]. Meanwhile, CZ-DCDPP, DPA-DCDPP and
DBPA-DCDPP emit bright green, greenish-yellow and yellowish-orange lights in low-polarity toluene, with emission peaks at 515,
5
47 and 565 nm (Fig. 2), respectively. Similar red-shifted emission is also observed for the luminogens in THF and DCM (Fig. 2,
Table S1 in Supporting information). Such trend in both low- and high-polarity solvents indicates that the effective conjugation length
and the ICT effect gradually increase in the order of CZ-DCDPP, DPA-DCDPP and DBPA-DCDPP. Owing to the highest ICT
strength, DBPA-DCDPP is most sensitive to the solvent polarity, and almost no emission is observed in THF (Fig. 2). These results
verify the effective modulation of the emission of the luminogens by tuning their effective conjugation length and ICT strength.
Their emission in THF and THF/water mixtures were further monitored to gain more insights into the ICT and aggregation effects.
As exampled in Fig. S5 in Supporting information, when dissolved in THF, DPA-DCDPP depicts red emission with moderate
intensity. Adding a small fraction of water (f
to the increased solvent polarity. Increasing of f
w
) of 10% into THF strikingly weakens its emission, making it almost nonemissive owing
does not change the intensity much until it reaches 70%, at which the intensity starts
w
to increase due to the predominant aggregation effect, which is helpful to the conformation rigidification and light emission. Further
addition of water affords more intensified emission at around 606 nm. DBPA-DCDPP exhibits similar emission behaviors to those of
DPA-DCDPP with more prominent AIE effect (Fig. S6 in Supporting information). However, CZ-DCDPP achieves its maximal
intensity at f = 60%, further addition of water reduces its emission (Fig. S7 in Supporting information). At f = 95%, the emission
w w
intensity is even lower than that in THF, which should be ascribed to the presence of detrimental exciton interactions in the
nanoaggregates.
Owing to the incorporation of AIE-active DCDPP moiety, the twisting structure and ICT nature, these luminogens are expected to
be highly emissive in the solid state [22–25,27]. Indeed, the recrystallized solid powders of CZ-DCDPP, DPA-DCDPP and DBPA-
DCDPP demonstrate intense green, yellow and red emission upon irradiation, with emission maxima/efficiencies of 527 nm/12.8%,
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77 nm/20.0% and 624 nm/43.0% (Fig. 3), respectively, indicative of their efficient and tunable emissions. Notably, among these
luminogens, the efficiency of CZ-DCDPP is the lowest, while that of DBPA-DCDPP is the highest, which appears to be
counterintuitive because luminogens normally tend to be less emissive with enhanced ICT effect. Despite the detailed reasons remains
unclear yet, it is speculated that various molecular structures might be accountable for it. While CZ is planar, DPA and DBPA units are
twisted, thus relatively less or blocked quenching by exciton interactions can be expected in the latter two luminogens because of steric
hindrance. Meanwhile, compared to DPA-DCDPP, conformation rigidity of DBPA-DCDPP in the neat solid might be enhanced due to
better conjugation and much greater steric hindrance, thus making it emits more efficiently even in the redder region.
The twisting structure and efficient solid emission of the luminogens make them highly promising as mechanochromic candidates
[
14,36–41], which can be used in security ink, data recording and anti-counterfeiting. Upon manual grinding, CZ-DCDPP and DPA-
DCDPP exhibit distinct emission color change from green/yellow to yellow/red (565/615 nm) with broadened full width at half
maxima (FWHM) (Fig. 3d), thus testifying their mechanochromism. DBPA-DCDPP, however, displays relatively slight variations in
color and emission peak. Such mechanochromism should be ascribed to the conformation planarization and potentially excimer
formation upon grinding, which is associated with the conversion from crystalline (twisted) to amorphous (planarized) states (Fig. 3e).
The small change of DBPA-DCDPP might be attributed to its larger steric hindrance and lower degree of conformation planarization,
as well as much lower crystallinity after recrystallization (Fig. 3e). Notably, the mechanochromism is also reversible. Upon fuming
with DCM vapor, the ground solids of DPA-DCDPP restore to its original green emission accompanying the recovery of emission
spectrum and crystalline XRD pattern (Fig. 3).
Thermal and morphological stabilities of the luminogens are crucial to the performance and lifetime in their optoelectronic
applications [42,43]. Thermal decomposition temperature (T
) of the luminogens were thus measured. As derived from the thermogravimetric analysis (TGA) and differential scanning
calorimeter (DSC) results, the T /T values forCZ-DCDPP, DPA-DCDPP and DBPA-DCDPP are 430/- [44], 372/104 and 427/151 °C
Figs. S8 and S9in Supporting information), respectively, suggestive of their good thermal and morphological stabilities.
d
, at which a sample loses its 5% weigh) and glass transition temperature
(T
g
d
g
(
Owing to the high solid efficiency, excellent thermal and morphological stabilities, as well as red light emission at amorphous state,
DPA-DCDPP and DBPA-DCDPP are promising for OLED applications. Their electroluminescence (EL) performances were thus
studied. We first tried the nondoped devices with the general configuration of ITO/HATCN/NPB/TCTA/X/TPBi/LiF/Al [45], where
ITO and LiF/Al are the anode and bilayer cathode, respectively, HATCN serves as an anode buffer layer to enhance the hole injection, NPB and
TPBi are the hole transport layer (HTL) and electron transport layer (ETL), respectively, TCTA acts as the electron-blocking layer (EBL),
and X represents the light emitting layer (LEL). The performances of devices I (X = DPA-DCDPP) and II(X = DBPA-DCDPP) are
shown in Fig. 4 and summarized in Table 1. Both devices turn on at a low voltage of 2.9 V, giving red EL at620 and 632 nm,
respectively, which are close to those of ground powders of DPA-DCDPP (615 nm) and DBPA-DCDPP(634 nm), suggestive of their
amorphous nature in the devices. The Commission Internationale de L’Eclairage (CIE) coordinates, maximalluminance(Lmax),current
efficiency (LEmax), power efficiency (PEmax) and external quantum efficiency (EQEmax) for devices I/II are (0.61, 0.38)/(0.64, 0.35),