Organic Letters
Letter
show two major absorption bands, the high energy one below
differences and photophysical properties of the three
375 nm can be originated from the π−π* and n−π* transitions
of the conjugated skeleton, and the longer wavelength
absorption bands could be assignable to the HOMO−
LUMO transitions. The photoluminescence (PL) and the
absorption spectrum of the three QAO variants follow the
mirror image rule, and their Stokes shifts are small (ca. 1222−
−
1
2
334 cm ) as expected, embodying a singlet emission event
and the rigidity enforced by the fully structural constraint,
preventing nonradiative channeling of excitation energy. It is
noteworthy that three emitters exhibit narrow emissions at
4
65, 520, and 552 nm, with the fwhm as small as 0.17−0.22 eV
corresponding to 33−54 nm). From the maximum and the
onset of each low-temperature spectrum (Figure 3), the ΔE
(
ST
of three emitters were calculated to be 0.19, 0.16, and 0.16 eV,
respectively. To further explore the transition characteristics of
the excited state, we measured the UV−vis absorption and PL
polarity increased from nonpolar hexane to the highest polar
acetone, the absorption profiles of QAO, DQAO, and OQAO
change a little, while the PL spectra are more sensitive to
Figure 4. Density functional theory (DFT) simulations of DQAO,
OQAO, and SQAO.
2
7
different solvents. SQAO displays the most significant
solvatochromic effect of its fluorescent emission, and the
obvious bathochromic shifts was observed from green (509
nm) in n-hexane to orange (601 nm) in DMF, agreeing well
calculation (Table S7) and reflecting SQAO’s stronger ICT
HOMO electron clouds of DQAO are mainly concentrated
on the third bridging atoms, nitrogen atom, and its ortho- and
para-carbons relative to it, whereas the LUMOs are located on
the carbonyl groups and at the meta-carbons relative to the
nitrogen atom, which revealed the short-range intramolecular
charge transfer process between the nitrogen atom and
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films (Table S9). The maximum emission peaks of DQAO,
OQAO, and SQAO were observed at 482, 546, and 604 nm,
accompanying a significant bathochromic shift and fwhms
broadening (versus their emission bands in toluene solution).
Figure S32 shows how for the doped films of 8 wt % DQAO in
13,15
carbonyl group.
This multiple resonance effect reduces
the vibronic coupling and vibrational relaxation in the
14
molecules, thereby achieving efficient and narrow emission.
In addition, the situation is further complicated by enhancing
the electron-donating ability of the third bridging atoms, and
the HOMO electron clouds has the obvious tendency of
delocalization toward the third bridging position, reflecting the
reinforcement of ICT character, which weakens the MR effect
originated from the nitrogen atoms and carbonyl groups. For
three molecules, the distributions of LUMO are similar, which
agrees well with the result of electrochemical characterization.
To evaluate the function of DQAO, OQAO, and SQAO in
devices, we fabricated and optimized the TADF-based OLEDs
(Figure S36). The energy level alignment diagrams and current
with DQAO, OQAO, and SQAO, blue emission of 472 nm,
green emission of 532 nm, and yellow emission of 564 nm
were achieved, corresponding to Commission Internationale
de l’Eclairage (CIE) coordinates of (0.12, 0.18), (0.32, 0.65),
and (0.47, 0.52), respectively. Notably, the DQAO- and
OQAO-based OLED exhibited the extremely narrow fwhms of
34 and 45 nm, and OLED employing DQAO only showed
1
4
,3-bis(9H-carbazol-9-yl)benzene (mCP), 5 wt % OQAO in
,4′-bis(9H-carbazol-9-yl)biphenyl (CBP), and 1 wt % SQAO
in 9-(3-(9H-carbazol-9-yl)phenyl)-9H-carbazole-3-carbonitrile
(
mCPCN). This concentration was chosen to avoid
aggregation of three emitters, and the absolute PLQY of the
doped films was measured to be 59.3% for DQAO, 90.2% for
OQAO, and 65.4% for SQAO, respectively. As shown in
Figure S33, three emitters show microsecond-scale delayed
component with a lifetime of 110.58 μs for DQAO, 204.65 μs
for OQAO, and 78.38 μs for SQAO, demonstrating their
apparent TADF characteristics.
thermal stability with a high decomposition temperature (Td,
corresponding to 5% weight loss) of 316, 353, and 368 °C.
The electrochemical behaviors were investigated by cyclic
different electron-donating groups, the significantly distinct
oxidation potentials are observed (Eox, 1/2 = 1.76 for DQAO,
1
.50 for OQAO, and 1.31 V for SQAO), and the EHOMO values
1−
are calculated to be −5.96, −5.70, and −5.51 eV for DQAO,
OQAO, and SQAO, respectively. Similarly, originating from
the analogous half-wave reduction potentials (Ered, 1/2), the
ELUMO values are calculated to be −2.90/−2.93/−2.95 eV for
DQAO, OQAO, and SQAO, respectively.
We performed the DFT and TD-DFT calculations at the
B3LYP/6-31G(d) level to better understand the geometrical
EQEmax of 15.2% and power efficiency (PEmax) of 23.0 lm W .
In addition, SQAO achieved an EQEmax of 17.8% with yellow
emission, but the fwhm was much wider than DQAO and
OQAO, which may due to the large radius of the sulfur atom
making the molecular structure flabby. Ultimately, the device
employing OQAO exhibited maximum current efficiency
−
1
−1
(CEmax), PEmax, and EQEmax of 26.2 cd A , 31.6 lm W ,
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Org. Lett. 2021, 23, 958−962