Thermochromic Aggregation-Induced Dual Phosphorescence via

Temperature-Dependent sp3-Linked Donor-Acceptor Electronic

Coupling

Tao Wang,† Zhubin Hu,‡ Xiancheng Nie,† Linkun Huang,† Miao Hui,† Xiang Sun‡, and Guoqing

Zhang†

†Hefei National Laboratory for Physical Science at the Microscale, University of Science and Technology of China, Hefei,

China, 230026. ‡Division of Arts and Science, NYU-ECNU Center for Computational Chemistry, NYU Shanghai, Shanghai, China, 200122.

Department of Chemistry, New York University, New York, USA, 10003.

ABSTRACT: Aggregation-induced emission (AIE) has proven

to be a viable strategy to achieve highly efficient RTP in bulk by

restricting molecular motions. Here we show that by utilizing

triphenylamine (TPA) as an electronic donor which connects to

an acceptor via an sp3 linker, six TPA-based AIE-active RTP

luminophores were obtained. Both the TPA AIE-gen and the sp3-

linkage can suppress aggregation-caused quenching. Consequent-

ly, dual phosphorescence bands emitting from localized donor and

acceptor triplet states, respectively, could be recorded at lowered

temperatures; at room temperature, only a single RTP band corre-

sponding to the lowest triplet state is present, presumably due to

thermally assisted electronic coupling between the two states. The

reported molecular construct serves as an “intermediary case”

between a fully conjugated donor-acceptor system and a do-

nor/acceptor binary mix, which may provide important clues on

the design and control of molecular systems with complex excit-

ed-state dynamics.

Suppressing nonradiative decays (e.g., intramolecular motions)

and promoting the intersystem crossing rate (ISC) are crucial to

realize organic room-temperature phosphorescence (RTP) intense

enough for practical applications.1 Up till now, several RTP

design principles have been proposed, such as introducing bulky

hindrance groups2 and spiro linkers,3 constructing aggregates,4

polymers5 and self-assemblies,6 host-guest7 and bonding8

interactions, as well as tuning charge-transfer (CT) states.9

Recently, we put forward a new RTP design principle based on an

sp3-linker connected donor-acceptor dyad, where an angled

intramolecular CT state is believed to enhance spin-orbit coupling

due to electron circular motion during transition.10 For example, a

proton-activated “off-on” RTP molecular probe could be

developed based on this principle.10a However, most RTP emitters

still require stringent environmental factors such as low

temperature, solid matrix assistance, and/or oxygen exclusion,

which can be rather cumbersome for practical applications. Since

2001,11 aggregation-induced emission (AIE) luminophores have

been a research hotspot because of their enormous potentials in

solid-state display,12 bio-probes,13 circularly polarized

luminescence,14 and “on-off” sensors,15 with an advantage being

Figure 1. (a) Design concept of AIE-gen donor-sp3 linker-

acceptor thermochromic dual phosphorescent dyad via

temperature-dependent electronic coupling between two emissive

triplet excited states and (b) related chemical structures.

“emitting in their own microenvironments”. Therefore, a universal

design method for the development of RTP materials may be built

upon less environmental-sensitive AIE core structures with RTP’s

increasing use in chemical sensing,16 data storage,17 bioimaging18

and OLEDs.19 Herein, we present an RTP design strategy of mo-

lecular 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). We also show that the temperature-

dependent emission is largely dictated by how strongly the two

locally excited triplet states (i.e., donor triplet 3LED and acceptor

triplet 3LEA) associate (Figure 1a). The TPA moiety was selected

as an AIE activator as well as an electron donor with the acceptor

varying in chemical structures. Shown in Figure 1b, five TPA-

functionalized AIE-active RTP molecules were synthesized via

AIERoom-Temperature

Phosphorescence

TPA1, X= O

TPA2, X= CH2

TPA3, X= O

TPA4, X= CH2

TPA5

a

b

sp3 linker

Strong

Electronic

Coupling298 K

77 KDiminished

Electronic

Interaction

Thermochromic

Phosphorescence

ON OFF ONON

AIE activator AIE activator

Figure 2. Stick models (top) of TPA1 and TPA3-5 obtained from single-crystal XRD measurements and packing diagrams shown in

wireframe model (bottom) indicating no obvious strong π-π intermolecular interactions in the solid state.

Figure 3. (a) Normalized emission spectra of TPA1-5 in air at room temperature (excitation: 365 nm for TPA1, TPA2 and TPA4; 430 nm

for TPA3; 450 nm for TPA5) and corresponding photos showing TPA1-5 excited by 365-nm UV light. (b) Time-resolved decay profiles

of TPA1-5 in air at room temperature (right: fluorescence; left: RTP). (c) Time-resolved emission spectra of TPA1. (d). Normalized emis-

sion spectra of TPA1 at 77 K. (e) 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; pro-

cess 2: intersystem crossing (ISC); processes 3&5: phosphorescence; process 4: internal conversion (IC).

simple Suzuki-Miyaura or Ullmann-type coupling reactions and

the AIE properties of TPA1-5 were investigated.

In solution, absorption spectra conducted in THF (Figure S1a)

are consistent with calculations by the time-dependent density

functional theory (TD-DFT) method with the optimally tuning

range-separated functional (LC-PBE*) and the TZVP basis set

(Figure S2, Table S1).20 According to the theoretical results, the

hole-electron distribution analysis performed by Multiwfn 3.7

(dev) program21 indicates that lower electronic transitions of all

TPAs are mainly contributed by local excitations, and the lowest

singlet excited state (S1) is the dark state with a CT character.

Therefore, it can be inferred that TPAs are non-emissive due to a

combination of excited-state energy dissipation via intramolecular

rotation (propeller-shaped TPA moiety) and a lowest forbidden

CT pathway. With increasing water fraction, a typical AIE

process (Figure S1b) can be observed due to restriction of

intramolecular motions and possibly newly emerged, lowest

emissive states other than CT as aggregates.22 Moreover, all TPAs

in THF/water (5/95, v/v) show long-lived emissions, and the

afterglow can even be observed for certain (e.g., TPA1)

aggregates (Figure S3). To assist further understanding of their

AIE behaviors, single-crystal measurements of TPA1 and TPA3-

5 were conducted to investigate the origin of the emission except

for TPA2, which always generates polycrystals. As shown in

TPA1 TPA3 TPA4 TPA5

117.07 119.01

111.17118.96

121.71

400 500 600 7000.0

0.2

0.4

0.6

0.8

1.0

Inte

nsity (

a.u

.)

Wavelength (nm)

77 K 190 K

110 K 210 K

130 K 230 K

150 K 250 K

170 K 270 K

180 K 298 K

f

b

0 1 2 3 4100

101

102

103

104

TPA1 306 ms@525 nm

TPA2 340 ms@568 nm

TPA3 7.6 ms@545 nm

TPA4 77.8 ms@525 nm

TPA5 31.1 ms@565 nm

Inte

nsity (

counts

)

Time (s)

400 500 600 7000.0

0.2

0.4

0.6

0.8

1.0

Inte

nsity (

a.u

.)

Wavelength (nm)

1ms

3ms

5ms

7ms

10ms

20ms

c d

400 450 500 550 600 650 700

Steady-state

Delayed

Em

issio

n Inte

nsity (

a.u

,)

Wavelength (nm)

UV on UV offa

FF+P= 31.2%

FP= 9.5%

FF+P= 49.8%

FP= 10.0%

FF+P= 16.8%

FP= 13.3%

FP= 13.6%

FF+P= 8.5%

FP = 8.2%

400 500 600 7000.0

0.2

0.4

0.6

0.8

1.0

Inte

nsity (

a.u

.)

Wavelength (nm)

Steady-state

Delayed

e

Flu

o.

Ph

os.2

Ph

os.1

T1

Tn

S1

TPA1

TPA2

TPA3

TPA4

TPA5

50 75 100 125 150100

101

102

103

104TPA1 1.0 ns@403 nm

TPA2 1.8 ns@437 nm

TPA3 5.0 ns@450 nm

TPA4 1.1 ns@420 nm

TPA5 0.6 ns@480 nm

Inte

nsity (

counts

)Time (ns)

T1Tn

Figure 2, the donor and acceptor moieties of TPAs adopt a highly

twisted conformation due to the separation by an 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 additonal twist exterted by the sp3 linker is apparent: it

makes - stacking essentially non-existent while suppressing

various molecular motions as aggregates.

These aggregates were then investigated by both steady-state

and delayed luminescence spectroscopy. For TPA1 and TPA2,

the steady-state emission exhibits a main AIE-fluorescence band

(em = 403 and 437 nm, respectively, Figure 3a) with nanosecond

lifetimes (Figure 3b, left). A low-energy band over 500 nm with a

lifetime of >300 ms (Figure 3b, right) is presumably due to the

AIE-RTP. When the delayed spectrum of TPA1 was examined,

we observed a high energy broad shoulder (~485 nm) that is

~1600 cm-1 above from lowest triplet excited state emission (T1).

To shed light on the origin of this shoulder peak, time-resolved

emission spectra were collected (Figure 3c), where the shoulder

peak intensity decays faster vs. that of the main RTP peak (Figure

S3 and S4). At 77 K, the steady-state and delayed emissions show

a tremendous increase in shoulder band (Figure 3d), which

perhaps indicate a new emissive triplet state.1b For delayed

emissions, the relative intensity ratio between the shoulder band

and the main peak maintains constant (Figure S6). This

temperature-dependent phosphoresence decay kinetics suggests

that an upper emissive triplet excited state (Tn, n>1) is feeding T1

at room temperature whereas the communication is cut off at 77 K.

To exclude the influence of ground-state aggregation and trace

impurity contamination (which can be signficant in molecular

solids), the phosphorescence spectra of single-crystal grade TPA1

dissolved in PMMA (t= 1 ms) were also recorded at various

temperatures, where a consistently blue-shifted trend could be

noted with decreasing temperature (Figure 3e), further indicative

of such a higher emissive Tn state being seperated from T1 without

going through internal conversion at low temperature. Visually, a

color change from green to sky blue in the afterglow could be

noted. A schematic illustration for this ternary emission

(fluorescence and dual phosphoresce) is presented in Figure 3f.

For TPA2 aggregates, although no Tn emission band is presented

at room temperature (Figure 3a and S6), a tiny new peak shows up

in the low-temperature phosphorescence spectrum (LTP, 77 K,

Figure S8) at 455 nm in addition to the main band (= 525 nm),

indicating even stronger coupling between the two states when the

CH2 linker replaces O at room temperature. Similarly, when

TPA2 was moleuclarly dissolved in the PMMA film, a much

stronger high energy shoulder peak (= 440 nm) gradually

emerged (Figure S9) compared to the aggregated state. The

experimental data above appear to indicate that Tn and T1

communicate at room temperature both intermolecularly and

intramolecularly, since the aggregates tend to give a much weaker

Tn phosphorescence band irrespectively of the temperature,

compared to monomers in PMMA. This is understandable since

both a higher degree of molecular aggregation and a higher

temperature offer more modes of molecular motions that couple

the two electronic states.

At this stage, we speculate that the T1 state is a TPA-localized

triplet state with Tn being an acceptor (cyanobenzene) centric

triplet one, given that the sp3 linker renders the two moieties more

spatially separated vs. an sp2 counterpart and thus less

electronically coupled at low temperature when vibrations are

inhibited. To verify the hypothesis, two important experiments

were conducted: 1) TPA3 to TPA5 with an aromatic ketone

acceptor were prepared. It is well-known that benzophenone

exhibits a prominent n-* triplet excited state (3n-*)23 with a

characteristic n-* vibrational progression in the phosphorescence

spectrum separated by ~1300-1600 cm-1. Indeed, such

spectroscopic patterns characteristic of benzophenone 3n-* were

unambiguously obtained (Figure S10) when the temperature is

reduced to or below 200 K for the two samples dissolved in

PMMA films (particularly for the CH2-linked TPA4 where no

oxygen conjugation interferes with the vibration), further

revealing that phosphorescence emissions consist of two emissive

triplet states from benzophenone and TPA subunits, respectively.

For TPA5, the 4-oxygen substituted acetophenone acceptor is

expected to exhibit a mixed 3-* and 3n-* band for Tn in

PMMA at 77 K (em= 455 nm) (Figure S11). It has to be noted

that temperature-dependent excitation spectra of TPA1-5

dissolved in PMMA films also suggest the existence of two

different emissive triplet states (Figure S12). In the aggregated

states, all three samples (TPA3-5) exhibit a broad RTP emission

dominated by T1 (Figure 3a) and much shorter RTP lifetimes

(Figure 3b, Table S2) due to electronic coupling between the

TPA-localized 3-* and the ketone-localized 3n-* state; such

coupling is also present at 77 K based on the phosphorescence

spectra (Figure S13-15) since the Tn and T1 completely merged

into a consolidated phosphorescence state with both n-* and -

* properties possibly due to stronger intermolecular interactions

for these aggregates. 2) A binary molecular mixture between TPA

and the benzophenone precursor at 1:1 ratio was used to repeat the

same experiment (Figure S16), where it was found that the 1:1

physical mixture generates RTP bands belonging to the two

molecules irrespective of the temperature. 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.

Figure 4. Theoretical calculations of molecular orbitals of TPA1

(a) and TPA2 (b) and corresponding main transition

configurations (>5%) based on the optimized ground state (H=

HOMO, L= LUMO). Vertical emission energy levels of TPA1 (c)

and TPA2 based on optimized S1, T1 and T2, respectively.

To further back the proposed Tn-T1 dual phosphorescence

model, TPA1 and TPA2 were selected as examples to carry out a

series of theoretical calculations via TD-DFT at the LC-

PBE*/TZVP level in the gas state, which mimics the state of

monomers in PMMA. The calculated energy diagram of the verti-

cal excitation and spin-orbit coupling constants reveals multiple

ISC channels from S1 to a higher T state during RTP generation

(Figure S16). It can also be analyzed from the calculations that the

molecular orbital transitions of T1 does arise from a TPA-

2.0

2.5

3.0

3.5

En

erg

y level (e

V) S1 3.16eV

T1 2.43eV

T2 3.00eV

2.0

2.5

3.0

3.5

En

erg

y l

ev

el

(eV

) S1 3.24eV T2 3.20eV

T1 2.37eV

0.83eV 0.57eV

TPA1Transition

configuration Character

S1

H→L 13.7%

H→L+1 61.9%

H→L+4 14.8%

CT

LE+CT

LE+CT

T1 H→L+2 75.5% LE

T2

H→L+3 66.5%

H→L+4 5.2%

LE+CT

LE+CT

TPA2Transition

configuration Character

S1

H→L+1 36.2%

H→L+2 40.4%

H→L+4 14.5%

LE+CT

LE+CT

LE+CT

T1

H→L+2 44.8%

H→L+4 24.7%

LE+CT

LE+CT

T2 H→L+3 68.2% LE

H

L+2 L+3 L+4

L+1L H L+1 L+2

L+3 L+4

TPA1 TPA2a b

c d

localized state while T2 involves excitation on both TPA and

cyanobenzene, which is likely to relax to a cyanobenzene-

localized triplet state (Figure 4a). A similar scenario appears in

TPA2 with the composition of the two triplet states inversed

(Figure 4b). Moreover, the vertical emission calculations

demonstrate that 1) the S1 state is almost degenerate with the

acceptor T2 state, and 2) the energy gap between T1 and T2 states

is relatively large (Figure 4c and 4d). The two results correlate

well with experimental observations of 1) efficient RTP and 2)

inhibited T2→T1 internal conversion, respectively.

Figure 5. (a) Chemical structure of TPA6. (b) Normalized steady-

state and RTP emission (t = 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) Normalized temperature-

dependent phosphorescence emission spectra of TPA6. (e)

Relationship between the emission intensity ratio of 455 and 425

nm (I455/425) and temperature. (f) Image showing delayed

emission (> 50 ms) color change at different temperatures.

Finally, to showcase the application value of the AIE

thermochromic dual phosphorescence molecules, we synthesized

TPA6 with a TPA donor and a pyridine acceptor (Figure 5a). The

rationale is that the electron-withdrawing pyridyl group is smaller

than cyanobenzene which should yield an even more separated T2

and T1 energy gap to make the visual phosphorescence color

change more dramatic and spectroscopically more resolvable.

Meanwhile, 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. Figure 5b shows that the TPA6

solid exhibits a fluorescence emission at 425 nm (= 0.7 ns).

Meanwhile, an obvious afterglow can be observed by the naked

eye and an RTP emission band can be collected at 550 nm with a

lifetime of 83 ms in air at room temperature. As the temperature

decreases, a new phosphorescence band (T2) appears with an

emission maximum at 455 nm in the steady-state emission

spectrum (Figure 5c and 5d), which can be used as a ratiometric

phosphorescence/fluorescence (P/F) sensing scheme (Figure 5e)

demonstrated in many previous reports.24 However, here we show

for the first time that a delayed spectrum can further become

sensitive to an external stimulus and responds to temperature

change dramatically (Figure 5f).

In summary, by introducing a TPA-based AIE-gen into the

donor-sp3 linker-acceptor structure, several ternary emissive AIE-

active RTP molecules with prompt fluorescence and dual

phosphorescence were obtained. The phenomenon is explained by

the presence of a TPA-localized T1 state, which couples to a

higher acceptor T2 state at room temperature but cuts the

electronic coupling off at lowered temperatures. Therefore, a

universal principle for designing a dual-phosphorescence

thermochromic material can be deduced and demonstrated. We

expect many more structurally different molecular can be used in

this strategy with a possibility of generating more than two

phosphorescence bands to obtain kinetically more complex

molecular systems.

ASSOCIATED CONTENT

AUTHOR INFORMATION

Corresponding Author

T. Wang: chemwang@ustc.edu.cn

G. Zhang: gzhang@ustc.edu.cn

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT

We would like to thank the National Key R&D program of China

(2017YFA0303500 to G. Z.) and the National Natural Science

Foundation (GG2340000364 to G. Z.), and the National Natural

Science Foundation of China (21903054 to X.S.).

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50 100 150 200 250 3000.8

1.0

1.2

1.4

1.6

I 455

/42

5

Temperature (K)

400 450 500 550 600 6500.0

0.2

0.4

0.6

0.8

1.0In

tensity (

a.u

.)

Wavelength (nm)

Steady-state

Delayed

400 450 500 550 600 650 7000.0

0.2

0.4

0.6

0.8

1.0

Inte

nsity (

a.u

.)

Wavelength (nm)

77 K 190 K

100 K 200 K

130 K 230 K

150 K 260 K

170 K 280 K

180 K 298 K

400 450 500 550 600 6500.2

0.6

1.0

1.4

1.8

Inte

nsity (

a.u

.)

Wavelength (nm)

77 K

100 K

130 K

150 K

170 K

180 K

190 K

200 K

230 K

260 K

280 K

298 K

UV on

UV off

77 K 150 K 180 K

298 K260 K200 K

a b

c

e f

d

TPA6FF+P=14.2%

FP =7.9%

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6

TOC

Strong

Electronic

Coupling298 K

77 KDiminished

Electronic

Interaction

Thermochromic

Phosphorescence

ON OFF ONON

AIE activator AIE activator