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Nat. Commun., 2021, 12, 1364
Thermochromic aggregation-induced dual
phosphorescence via temperature-dependent
sp3-linked donor-acceptor electronic coupling
T. Wang, Z. Hu, X. Nie, L. Huang, M. Hui, X. Sun, and G. Zhang
October 19, 2021, Journal club
B4 K. Kawaguchi
Introduction
The authors present an RTP
design strategy of molecular
solids by combining the concept
of AIE and the donor–sp3 linker–
acceptor dyad molecular motif
and demonstrate that such a
design yields an unexpected
phenomenon of thermochromic
phosphorescence (TCP).
The authors also show that the
temperature-dependent emission
is largely dictated by how
strongly the two largely locally
excited triplet emitting states
(i.e., donor triplet 3LED and
acceptor triplet 3LEA) associate.
UV-vis absorption
Mostly LE state
Fig. 1a Normalized absorption spectra (left) of TPA1-5 in
optically dilute THF solutions and corresponding hole (blue)–
electron (green) distributions (right) calculated based on the
TD-DFT method.
Supplementary Table 1. Transition information of 4 lowest-lying
excited states (maximum peak) in THF
TD-DFT calculation
DFT functional corrected for the molecule with
long-distance charge transfer.
・ωB97X
・CAM-B3LYP
DFT basis function
Triple-zeta valence + polarization
Solvent effect
Supplementary Figure 3. UV-vis absorption spectra of (a) TPA1, (b) TPA2, (c) TPA3, (d) TPA4, (e)
TPA5, and (f) TPA6 in various optically dilute solvents.
Steady-state emission
Fig. 1b Steady-state emission spectra of TPA1-5 in a bicomponent
solution mixture showing the AIE process with different THF/water ratios
(0–95%, v/v) and related AIE photos under a hand-held UV lamp (water
fraction: 0 and 95%, TPA concentration: 2.0×10−3 mol/L).
AIE process
With increasing water fraction, a typical AIE process
can be observed due to the restriction of
intramolecular motions and possibly newly emerged,
lowest emissive states as aggregates.
Afterglow emission of TPA1
FESEM images
Always form
polycrystal
Nano crystal
Large particle size
Nano crystal
Large particle size
Nano crystal
Supplementary Figure 5. Scanning electron microscopy images of TPA1-6
Single-crystal XRD analysis
The donor and acceptor
moieties of TPAs adopt a
highly twisted conformation
due to the separation by a
sp3 linker (O or CH2),
resulting in a twist angle
within a range of ~110–
120°. As expected, the
combined effect of a
propeller-shaped TPA donor
and an additional twist
exerted by the sp3 linker is
apparent: it makes π–π
stacking essentially
nonexistent, while
suppressing various
molecular motions as
aggregates, which is a
typical AIE mechanism.
TPA1
TPA3
TPA4
TPA5
UV absorption and excitation spectra
Emission spectra and photos of the aggregates
Broad shoulder peak
Second emissive triplet excited state
(Not T2 state)
Phosphorescence
Fig. 4a Normalized emission spectra of TPA1-5 in air at
room temperature (excitation: 365 nm for TPA1, TPA2,
and TPA4; 430 nm for TPA3; and 450 nm for TPA5) and
corresponding photos showing TPA1-5 excited by 365nm UV light.
Decay profiles
Fluorescence
Phosphorescence
AIE-RTP
Fig. 4b Time-resolved decay profiles of TPA1-5 in air at
room temperature (right: fluorescence; left: RTP).
Time-resolved emission spectra ofTPA1
The shoulder peak intensity decays faster vs. that of the main RTP peak at room temperature
Fig. 4c Time-resolved emission spectra of TPA1. d Normalized
emission spectra of TPA1 at 77 K.
Supplementary
Figure
7.
Variation of the emission
intensity ratio of TPA1 between
485 and 525 nm at a different
delay time at room temperature.
Time-resolved emission spectra of TPA1
At 77 K, the steady-state and delayed emissions show a tremendous increase in the shoulder band,
which perhaps indicates a second emissive triplet state. This temperature-dependent
phosphorescence decay kinetics suggests that the second emissive triplet excited state is feeding
the lowest T1 at room temperature, whereas the communication is cut off at 77 K.
Emission in PMMA film
Visually, a color change from green to
sky blue in the afterglow could be noted.
T1L
T1 H
The temperature dependency
can therefore be interpreted
as: (1) a higher temperature
produces hotter excitons that
may prefer ISC favorable for
relaxation to the T1L site and
vice versa; (2)
communications among these
emitting states at local
minima by thermal motions
(e.g., vibrations) may be cut
off at frigid temperatures, so
that more distinct separation
in spectrum could be revealed.
Fig. 4e Normalized phosphorescence emission spectra of TPA1 dissolved in PMMA film (excitation: 375 nm;
concentration: 3%, w/w) at different temperatures. f Schematic illustration of the ternary emission process;
process 1: fluorescence; process 2: intersystem crossing at 77 and 298 K; process 3: favored intersystem
crossing at 298 K; process 4 and 6: phosphorescence; and process 5: thermally activated conformation
transformation.
Time-resolved emission spectra of TPA2
r.t.
77 K
Indicating a higher energy barrier between the two emitting states when the CH2
linker replaces O at room temperature.
Emission spectra in PMMA films
Stronger high-energy
shoulder peak
TPA4
TPA2
TPA3
TPA5
Excitation spectra in PMMA films
Emission spectra and photos of the aggregates
Due to the introduction of the carbonyl group in
TPA3–5, the fluorescence emission almost totally
turns into RTP because of a promoted ISC process,
and the fluorescence emission can only be measured
before 480 nm
Fig. 4a Normalized emission spectra of TPA1-5 in air at
room temperature (excitation: 365 nm for TPA1, TPA2,
and TPA4; 430 nm for TPA3; and 450 nm for TPA5) and
corresponding photos showing TPA1-5 excited by 365nm UV light.
Decay profiles
In the aggregated states, all three samples (TPA3–5) exhibit a broad RTP emission dominated by T1L
and much shorter RTP lifetimes due to strong electronic coupling between the TPA-localized 3π– π*
and the ketone-localized 3n–π* state.
Fluorescence
Phosphorescence
Fig. 4b Time-resolved decay profiles of TPA1-5 in air at
room temperature (right: fluorescence; left: RTP).
Time-resolved emission spectra of TPA3
Time-resolved emission spectra of TPA4
Time-resolved emission spectra of TPA5
Lifetime and quantum yield data
Properties of binary mixture
The results not only suggest that the two RTP bands do originate from the donor and acceptor,
respectively, but also point to the importance of the sp3 chemical linker: cutting off
communications at low temperature but not high temperature, something completely different
from blending.
Computational investigation
Geometry optimization & Frequency
calculation
UM06-2X-D3/6-31G(d)
Single-point calculation
PWPB95-D3(BJ)/def2-QZVPP
Vertical emission
TD-M06-2X/TZVP
Range-separation method
LC-ωPBE
Higher gap
Computational investigation
SOC: ωB97X-D3/TZVP
Geometry optimization
CAM-B3LYP/6-31G(d)
Geometry optimization
UCAMB3LYP/6-31G(d)
Emission energy
TD-LC-ωPBE*/TZVP
Emission energy
TD-LC-ωPBE*/TZVP
Application explorations
The rationale is that the electron-withdrawing pyridyl group is smaller than cyanobenzene, which should
yield an even more separated T1H and T1L energy gap to make the visual phosphorescence color change
more dramatic and spectroscopically more resolvable.
The lone pair electron in the pyridine moiety is likely to make molecular stacking even more difficult in
the solid state, so that no PMMA matrix is needed for the TPA6-based phosphorescence sensing module.
Fig. 7a Chemical structure of TPA6.
Application explorations
Fig. 7b Normalized steady-state and RTP emission (Δt = 3 ms) spectra of TPA6 in air at room temperature; inset:
photo showing TPA6 solid excited by 365-nm UV light. c Relative steady-state emission spectra of TPA6 at
different temperatures. d Relationship between the emission intensity ratio of 455 and 425 nm (I455/I425) and
temperature.
Application explorations
Fig. 7e Normalized temperature-dependent phosphorescence emission spectra of TPA6. f Image showing delayed
emission (>50 ms) color change at different temperatures.
Synthesis
Synthesis
Synthesis (C-N coupling)
Synthesis of 4-methoxy-N, Ndiphenylaniline (TPA-OMe).
Diphenylamine (1 equiv., 0.95 g), 4iodoanisole (1.2 equiv., 1.34 g), 2,2'dipyridyl (0.02 equiv., 0.018 g), CuI
(0.02 equiv., 0.021 g) and potassium
tert-butylate (t-BuOK, 1.5 equiv., 0.94
g) were added into a round-bottom flask
containing 20 mL of toluene. The
reaction was heated to reflux under N2
for 4 h. After the reaction finished and
cooled down to room temperature, the
reaction solvent was wash by deionized
water twice (30 mL×2). Then the
organic layer was collected, and the
solvent was removed by the rotary
evaporator in vacuum. The obtained
crude product was further purified by
column chromatography, giving the
white solid (95%, 1.42 g).
Synthesis (Demethylation)
Synthesis of 4-(diphenylamino)phenol (TPA-OH). To a solution of TPA-OMe (1 equiv., 1.4 g) in dichloromethane (45 mL)
was slowly added BBr3 (1.5 equiv., 1.91 g) at 0C. The mixture was allowed to be stirred at 0C for 0.5 h under the
protection of N2, and then the system temperature was heated to 25 C. After being stirred at 25C for 10 h, a saturated
NH4Cl aqueous solution (60 mL) was added into the reaction system to quench the reaction. Then the resulting mixture was
extracted with dichloromethane three times (60 mL×3), and the organic layer was collected. The dichloromethane was
removed by the rotary evaporator under reduced pressure, and the obtained crude product was purified by column
chromatography, affording TPA-OH as the off-white solid (85%, 1.13 g).
Synthesis (Ullmann ether synthesis)
Synthesis of 4-(4-(diphenylamino)phenoxy)benzonitrile (TPA1). TPA-OH (1 equiv., 0.32 g), 4- bromobenzonitrile (1.5
equiv., 0.33 g), caesium carbonate (Cs2CO3, 2 equiv., 0.80 g) were added into the round-bottom flask containing 4 mL of DMF.
The reaction mixture was heated to reflux and allowed to react overnight. After the reaction finished, 20 mL of deionized
water was added to quench the reaction. And then mixture was extracted with dichloromethane three times (20 mL×3). The
organic layer was collected and removed under reduced pressure. The obtained crude product was purified by column
chromatography, giving the white crystal (92%, 0.39 g).
Synthesis (Suzuki-Miyaura coupling)
Synthesis of 4-(4(diphenylamino)benzyl)benzonitrile
(TPA2). 150 mL of degassed THF and 15
mL of Na2CO3 aqueous solution (2 mol/L)
were added into a round-bottom flask and
protected by N2. Then 4-cyanobenzyl
bromide (1 equiv., 0.8 g), 4(diphenylamino)phenylboronic acid (1.2
equiv., 1.54 g), Pd(PPh3)4 (0.03 equiv., 0.15
g) were added to the flask. Then the reaction
system was heated to reflux for 24 h. After
the reaction system cooled down to room
temperature, the solvent was removed under
reduced pressure. Then the collected solid
was re-dissolved in dichloromethane (100
mL) and washed with deionized water (100
mL) twice. The organic layer was collected
and removed by the rotary evaporator in
vacuum. The crude product was further
purified by column chromatography to afford
TPA2 as the white solid (65%, 0.95 g).