Achieving highly efficient aggregation-induced emission, reversible and irreversible photochromism by heavy halogen-regulated photophysics and D-A molecular pattern-controlled photochemistry of through-space conjugated luminogens

Article abstract of DOI:10.1039/d1sc02168k

It is extremely challenging but desirable to regulate the photophysical and photochemical processes of aggregation-induced emission luminogens (AIEgens) in distinct states in a controllable manner. Herein, we design two groups of AIEgens based on a triphenylacrylonitrile (TPAN) skeleton with through-space conjugation (TSC) property, demonstrate controlled regulation of photophysical emission efficiency/color and photochemical photochromic and photoactivatable fluorescence behaviours of these compounds, and further validate design principles to achieve highly efficient and emission-tuning AIEgens and to accomplish photo-dependent color switches and fluorescence changes. It is surprisingly found that the introduction of heavy halogens like bromine into a TPAN skeleton dramatically enhances the emission efficiency, and such an abnormal phenomenon against the heavy-atom effect is attributed to the specific through-space conjugation nature of the AIE-active skeleton, effective intermolecular halogen-bond-induced restriction of intramolecular motions, and heavy atom-induced vibration reduction. The incorporation of two electron-donating amino groups into the TPAN skeleton cause the luminogens to undergo a bathochromic shifted emission due to the formation of a D-A pattern. Apart from the regulation of photophysical processes in the solid state, the construction of the D-A pattern in luminogens also results in extremely different photochemical reactions accompanying reversible/irreversible photochromism and photoactivatable fluorescence phenomena in a dispersed state. It is revealed that photo-triggered cyclization and decyclization reactions dominantly contribute to reversible photochromism of the TPAN family, and the photo-induced cyclization-dehydrogenation reaction is responsible for the irreversible color changes and photoactivatable fluorescence behaviours of the NTPAN family. The demonstrations of multiple-mode signaling in photoswitchable patterning and information encryption highlight the importance of controlled regulation of photophysics and photochemistry of fused chromic and AIE-active luminogens in distinct states. This journal is

Full text of DOI:10.1039/d1sc02168k

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Achieving highly efficient aggregation-induced
emission, reversible and irreversible
Cite this: DOI: 10.1039/d1sc02168k
photochromism by heavy halogen-regulated
photophysics and D–A molecular pattern-
controlled photochemistry of through-space
conjugated luminogens†
All publication charges for this article
have been paid for by the Royal Society
of Chemistry
Zuping Xiong,
Hui Feng and Zhaosheng Qian
Xiaoxiao Zhang, Longxiang Liu, Qiaozhi Zhu, Zhenni Wang,
*
It is extremely challenging but desirable to regulate the photophysical and photochemical processes of
aggregation-induced emission luminogens (AIEgens) in distinct states in a controllable manner. Herein,
we design two groups of AIEgens based on a triphenylacrylonitrile (TPAN) skeleton with through-space
conjugation (TSC) property, demonstrate controlled regulation of photophysical emission efficiency/
color and photochemical photochromic and photoactivatable fluorescence behaviours of these
compounds, and further validate design principles to achieve highly efficient and emission-tuning
AIEgens and to accomplish photo-dependent color switches and fluorescence changes. It is surprisingly
found that the introduction of heavy halogens like bromine into a TPAN skeleton dramatically enhances
the emission efficiency, and such an abnormal phenomenon against the heavy-atom effect is attributed
to the specific through-space conjugation nature of the AIE-active skeleton, effective intermolecular
halogen-bond-induced restriction of intramolecular motions, and heavy atom-induced vibration
reduction. The incorporation of two electron-donating amino groups into the TPAN skeleton cause the
luminogens to undergo a bathochromic shifted emission due to the formation of a D–A pattern. Apart
from the regulation of photophysical processes in the solid state, the construction of the D–A pattern in
luminogens also results in extremely different photochemical reactions accompanying reversible/
irreversible photochromism and photoactivatable fluorescence phenomena in a dispersed state. It is
revealed that photo-triggered cyclization and decyclization reactions dominantly contribute to reversible
photochromism of the TPAN family, and the photo-induced cyclization–dehydrogenation reaction is
responsible for the irreversible color changes and photoactivatable fluorescence behaviours of the
NTPAN family. The demonstrations of multiple-mode signaling in photoswitchable patterning and
information encryption highlight the importance of controlled regulation of photophysics and
photochemistry of fused chromic and AIE-active luminogens in distinct states.
Received 19th April 2021
Accepted 3rd July 2021
DOI: 10.1039/d1sc02168k
rsc.li/chemical-science
challenging but desirable to greatly improve the emission effi-
ciency, alter the emission color and tune the emission lifetime
Introduction
making the luminogen more suited for distinct environments
and diverse applications. A great achievement has been made
on regulating the photophysics of traditional dye molecules in
a controlled way,1,2 and a diversity of dye molecules have been
broadly exploited in various applications such as in life science,
sensors and optoelectronics.3–6 These dye molecule-based uo-
rophores with a rigid structure and through-bond conjugation
(TBC) property are broadly employed in dilute solutions, but
they have to face a great challenge in use in their solid or
aggregation state due to their notorious photophysical process
of aggregation-caused quenching (ACQ). A new variety of
Rational regulation of photophysical processes of a luminogen
through structural design and functional modication is still
Key Laboratory of the Ministry of Education for Advanced Catalysis Materials,
Zhejiang Normal University, Yingbin Road 688, Jinhua 321004, People's Republic of
China. E-mail: qianzhaosheng@zjnu.cn
† Electronic supplementary information (ESI) available: Experimental and
computational details, UV-visible and PL spectra, NBOs, crystallographic and
photophysical data, calculated data, 1H NMR and 13C NMR spectra and mass
spectra. CCDC 2054237, 2054250, 2054251, 2054252, 2054253, 2054256,
2054258, 2054257, 2054254, 2054255 and 2054898. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/d1sc02168k
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luminogens with aggregation-induced emission (AIEgens) controlled dual absorption/emission changes.31,32 Fusion of
developed recently provide a great opportunity to overcome the a photo-active unit to a tetraarylethene skeleton provides
ACQ behaviours of traditional TBC dye molecules.7–9 In contrast a promising strategy to effectively integrate photochromism
to through-bond conjugated dyes, these AIEgens intensely and photo-triggered uorescence switching.33,34 Several recent
uoresce in the solid or aggregation state but exhibit almost no studies reported that tetraarylethenes aer proper modica-
emission in solutions, which can be attributed to the restriction tion demonstrate photoinduced absorption and uorescence
of intramolecular motions (RIM) in space-restricted environ- changes in the solid state; however, most of these molecules
ments.10–12 The reverse photophysical behavior of AIEgens to only undergo observable solid-state photochromism with low
TBC dyes originates from their distinct through-space conju- efficiency accompanying solid-state uorescence quenching,
gation (TSC) in the molecular structure and conformation in and almost no photochromic changes can be observed in
nature.13–15 Although diverse applications of AIEgen-based solution.35–39 Our recent studies reveal that photochromic and
materials have been accomplished,16–20 this growing research solid-state uorescent molecules can be rationally designed by
area is facing several important challenges, one of which is the introducing a traditional photoactive group such as a thienyl or
much lower emission efficiency of AIEgens (frequently <0.5) furyl group into a typical AIE molecular skeleton, and the shi
than the TBC dyes (mostly >0.8).21 Recent advances have of the dominant process between photophysics and photo-
revealed that the key factor for AIE is the control over the non- chemistry can be achieved by regulating the position of intro-
radiative decay (deactivation) pathway to improve the emission ducing units.40,41 However, it is still a formidable challenge to
efficiency,22 but operable and effective design strategies are still achieve simultaneous photophysical uorescence enhance-
extremely lacking now. Tailoring alkyl linkages has been proved ment in the solid state and photochemistry-controlled
to dramatically enhance the emission efficiency of chromism and uorescence switch based on a single AIEgen
tetraphenylethylene-based AIEgens without changing their with relatively bistable isomers in solution or dispersed state
emission color; however, this strategy is only limited to the AIE- upon photoirradiation.
active tetraphenylethylene (TPE) skeleton without evidence of
Halogen bonds are broadly used to regulate molecular self-
expanding its effectiveness to other AIEgens.23 Our recent paper assembly processes in crystal engineering42 and thus are
on multi-halogenated TPE derivatives shows that even intro- frequently exploited to control the photophysical behaviors of
ducing multiple halogens like bromine into a TPE skeleton can organic optical materials involving halogen bonds.43–45 Inspired
greatly improve their emission efficiencies with the highest by such more ubiquitous halogen-induced interactions and the
value of 0.75, which is largely attributed to the effective RIM mechanism of AIEgens, we expect that it could be an
restriction of intramolecular motions due to strong intermo- effective and general way to tremendously enhance the emis-
lecular halogen–halogen interactions.24 However, general sion efficiency by introducing halogens to restrain intermolec-
design principles with broad applicability are still rare but ular motions of AIEgens due to the construction of halogen-
desperately needed to dramatically increase the emission effi- induced intermolecular interactions in the aggregation state.
ciency of AIEgens equivalent to those of TBC dyes.
The cyano group generally acts as an effective chromophore in
Photochemical reactions of a luminogen generally compete luminogens and an electron-acceptor in TICT molecules,46 thus
with its photophysical processes like uorescence and thus we aim to incorporate a cyano group into the classic AIEgen
only one aspect of them frequently dominates the photo- tetraphenylethene skeleton to activate photochemical activity
triggered behaviours of a luminogen. Photochromism origi- and reduce intermolecular motions and also to readily
nating from a specic photochemical reaction refers to construct donor–acceptor patterns by introducing electron-
a reversible transformation of a chemical compound between donating amino groups. To this end, we design two groups of
two forms with different absorption spectra and colors upon AIEgens based on a triphenylacrylonitrile skeleton, explore their
photoirradiation.25 Among various organic photochromic photophysical photochemical behaviours, and validate general
materials, the diarylethene family is the most famous and design principles to achieve high emission efficiency and
valuable because of their excellent photochromic performance regulate the photophysical process and photochemical reac-
such as the outstanding thermal stability of both isomers, good tions in different states. As shown in Scheme 1a, these designed
fatigue resistance and rapid response.26 They can also be luminogens consist of the 2,3,3-triphenylacrylonitrile (TPAN)
lighted up with bright uorescence in solutions with remote family
and
the
3,3-bis(4-(dimethylamino)phenyl)-2-
photoirradiation showing great potential in advanced anti- phenylacrylonitrile (NTPAN) family, which contain TPAN,
counterfeiting, super-resolution imaging and rewritable NTPAN and their respective halogenated compounds. As shown
photonic devices.27–29 In addition to classic photochromic in Scheme 1b, both TPAN and NTPAN families show a sharply
molecules, recent advances demonstrate that AIEgens are able increasing trend in the emission efficiency upon introducing
to undergo photochromic changes upon irradiation aer a heavier halogen from uorine to iodine, and the largest
proper modication, and the incorporation of photochromism emission efficiency is up to 0.73, validating that it is an effective
and aggregation-induced emission can realize photo- strategy to regulate the photophysics and enhance the emission
controlled dual-channel signaling.30 Direct linkage of tradi- efficiency in the solid state by the introduction of simple
tional photochromic units and classic AIEgens is simple, but halogen atoms into a through-space conjugated AIEgen. An
the interactions between the two units in such a molecule apparent red-shi in emission color for the NTPAN family is
frequently bring an apparently negative impact on photo- accomplished as compared to the TPAN family through
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Results and discussion
Synthesis and photophysical properties of
triphenylacrylonitrile (TPAN), bis(dimethylamino)
triphenylacrylonitrile (NTPAN) and their halogenated
derivatives
The classic Knoevenagel condensation reaction47,48 was
employed to synthesize TPAN and NTPAN derivatives using
a strong base such as sodium hydride, and all the ten
compounds were obtained in good yields. They consist of two
groups of compounds based on the molecular skeletons TPAN
and NTPAN. The TPAN group contains 2,3,3-triphenylacryloni-
trile
(TPAN),
2-(4-uorophenyl)-3,3-diphenylacrylonitrile
(TPAN-F), 2-(4-chlorophenyl)-3,3-diphenylacrylonitrile (TPAN-
Cl), 2-(4-bromophenyl)-3,3-diphenylacrylonitrile (TPAN-Br) and
2-(4-iodophenyl)-3,3-diphenyl-acrylonitrile (TPAN-I), while the
NTPAN group is composed of 3,3-bis(4-(dimethylamino)-
phenyl)-2-phenylacrylonitrile
(dimethylamino)phenyl)-2-(4-uorophenyl)-acrylonitrile
(NTPAN-F), 2-(4-chlorophenyl)-3,3-bis(4-(dimethylamino)
(NTPAN),
3,3-bis(4-
phenyl)acrylonitrile (NTPAN-Cl), 2-(4-bromophenyl)-3,3-bis(4-
(dimethylamino)phenyl)acrylonitrile (NTPAN-Br) and 3,3-bis(4-
(dimethylamino)phenyl)-2-(4-iodophenyl)acrylonitrile (NTPAN-
I). The detailed synthesis routes are depicted in Scheme S1.†
All the compounds were strictly puried using column chro-
matography and further characterized by 1H NMR and 13C NMR
spectroscopy and high-resolution mass spectrometry. To vali-
date their absolute structures and packing patterns in crystals,
their absolute structures were determined by the single-crystal
Scheme 1 Structures of halogenated TPAN/NTPAN derivatives (a), and
schematic illustration of controllable regulation of photophysics and
photochemistry of an AIE-active luminogen (TPAN) with through-
space conjugation by introducing halogens and constructing donor–
acceptor patterns (b). Introducing
a heavier halogen gradually X-ray diffraction technique using their respective single
increases the emission efficiency; constructing donor–acceptor
patterns tremendously alters the emission colors; TPAN without a D–A
pattern undergoes reversible photochromism upon irradiation; TPAN
with a D–A pattern (NTPAN) undergoes irreversible photochromism
and photoactivatable fluorescence upon UV irradiation.
crystal. Their determined absolute conformations and crystal-
lographic data are shown in Fig. S1 and Tables S1, S2,†
respectively, which absolutely conrm the designed structures
in Scheme 1. Through-space conjugation refers to the process of
the intramolecular adjacent conjugated subunits signicantly
interacting with each other through space instead of merely
forming covalent bonds in an organic molecule.13–15 From
Fig. S1,† it can be readily noted that these molecules show
similarly twisted conformations and three phenyl rings in
contact with each other or lying in a short distance (around 4.8
introducing electron-donating amino groups forming D–A
patterned molecules, but highly efficient solid-state uores-
cence dominates the photophysical processes of these
compounds in the aggregation state for the two groups. In
a photochemical aspect, the TPAN family can undergo revers-
ible photochromism upon UV irradiation through photo-
triggered cyclization and decyclization reactions in solution.
However, the existence of D–A pattern in the NTPAN family
greatly changes their photochemical behaviors; NTPAN
compounds have UV-dependent photochromic behaviours and
therefore undergo irreversible photochromism and exhibit an
interesting photoactivatable uorescence phenomenon upon
UV irradiation. The underlying photophysical processes and
photochemical reactions are systematically examined by
experimental and theoretical analyses to validate the regulation
of photophysics and photochemistry by molecular design.
These unique dual-mode, reversible and irreversible signaling
behaviors controlled by external irradiation make them suitable
for photoswitchable patterning of various surfaces and
advanced information encryption applications in a dual-
channel way.
˚
A), which indicates that through-space conjugation occurs for
these molecules. Taking TPAN and NTPAN as examples, the
independent gradient model (IGM) method is used on the basis
that a single crystal structure can be used to visualize such
through-space interactions. It is found that the apparent van
der Waals interactions shown in the green section occur among
three phenyl rings, further validating the occurrence of
through-space conjugation in these molecules. Their single-
crystal structures and packing patterns are valuable to analyze
the relationship between their solid-state uorescence proper-
ties and structural conformations/stacking patterns in addition
to verifying their absolute structures and through-space conju-
gation features.
Colorless solid forms and colorless solutions of TPAN groups
suggest that these ve compounds have selective absorption in
UV regions, which is veried by their UV-visible absorption
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spectra in DCM solvent (Fig. S2†). The major absorption peaks effect,49 and this observation also differs slightly from the anti-
are located around 240 nm and 312 nm for all of them, and heavy atom effect of halogenated tetraphenylethene proposed
almost no absorption above 400 nm can be observed. Their in our previous paper.24 Compared to the TPAN group, all
colorless solutions exhibit almost no emission upon UV exci- NTPAN derivatives exhibit yellow color in daylight, suggesting
tation, but their solids intensively emit blue uorescence, as that they have apparent absorption in the visible region in
shown in Fig. 1a. The major emission peak is located at 462 nm addition to UV absorption. Fig. S5† veries this implication
for TPAN, and its halogenated derivatives exhibit a similar blue from their major absorption peaks around 270 nm and 400 nm.
emission upon UV excitation without obvious change in emis- Resembling TPAN compounds, all NTPAN derivatives have
sion maxima. Their time-resolved decay curves in Fig. S3† almost negligible emission in solution upon UV excitation but
indicate that they have short-lived lifetimes in the range of 0.7– emit intense green/yellow uorescence in the solid state. Their
1.3 ns, suggesting that these emissions come from singlet states lifetimes fall in the range of 0.9–3.1 ns (Fig. S6†), and their
with uorescent nature. Aggregation-induced emission behav- aggregation experiments also conrmed similar aggregation-
iors of all the ve TPAN derivatives were veried by poor solvent- induced emission behaviors to TPAN compounds (Fig. S7†).
induced aggregation experiments (Fig. S4†), in which the blue However, the emission maxima of the NTPAN compounds have
emission was found to be progressively enhanced as the water a huge red-shi relative to the TPAN counterparts aer two
content was increased in the mixed solution for each of them. dimethylamino groups are introduced. All TPAN solids emit
However, it is surprisingly noted that their uorescence a blue emission upon excitation, whereas all NTPAN solids
quantum yields are gradually increased from 10.9% to 49.8% as exhibit green-to-yellow uorescence under UV light. As shown
the atomic number of halogen increases from TPAN-F to TPAN- in Fig. 1b, the emission maximum of NTPAN in the solid state is
I. This unusual heavy halogen-caused uorescence enhance- located at 580 nm, which is red-shied by nearly 100 nm relative
ment phenomenon disagrees with the traditional heavy atom to that of TPAN; the smallest difference in the emission peak
effect which describes that the introduction of a heavy atom like between NTPAN-I and TPAN-I is also up to 44 nm. These
bromine generally leads to weakening or quenching of the observations suggest that introducing dimethylamino groups
uorescence for conventional planar aromatic luminogens due into the TPAN skeleton is able to largely tune the emission to
to the heavy atom-induced notable spin–orbit coupling (SOC) long-wavelength regions, and this effect can be attributed to the
Fig. 1 (a) PL spectra and images of TPAN, TPAN-F, TPAN-Cl, TPAN-Br and TPAN-I in the solid state. (b) PL spectra and images of NTPAN,
NTPAN-F, NTPAN-Cl, NTPAN-Br and NTPAN-I in the solid state. (c) PL spectra of TPAN, TPAN-F, TPAN-Cl, TPAN-Br and TPAN-I homogeneously
dispersed in adamantane (AdH). (d) PL spectra and images of NTPAN, NTPAN-F, NTPAN-Cl, NTPAN-Br and NTPAN-I homogeneously dispersed
in adamantane (AdH). (e) Quantum efficiencies of TPAN, TPAN-F, TPAN-Cl, TPAN-Br and TPAN-I in the solid state and dispersed in adamantane.
(f) Quantum efficiencies of NTPAN, NTPAN-F, NTPAN-Cl, NTPAN-Br and NTPAN-I in the solid state and dispersed in adamantane.
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formation of donor–p–acceptor patterns in NTPAN compounds However, the contributions of halogens are still minor relative
where two dimethylamino groups act as the donor, while the to other moieties such as phenyl and acrylonitrile groups,
cyan group functions as the acceptor. Differing from TPAN whose contributions to HOMOs/LUMOs generally range
compounds, NTPAN-Cl has the largest uorescence quantum between 20% and 50%. Thus, a far-reaching enlightenment that
yield (73.1%) among the NTPAN derivatives, and the quantum can be drawn here is that the introduction of halogens in these
yield gradually decreased from NTPAN-Cl to NTPAN-I, indi- AIE molecules would not greatly enhance the spin–orbit
cating that the heavy atom effect plays an important role in coupling effect dominating the promotion of states with
NTPAN compounds. The main photophysical parameters of different multiplicities. This fact was further explained using
TPAN and NTPAN compounds in different states are summa- energy gaps between S1 and its nearest triplet state with TD-DFT
rized in Table S3,† which visually presents an abnormal pho- calculations. According to the perturbation theory,52 the inter-
tophysical phenomenon for TPAN and NTPAN compounds that system crossing rate (kisc) is proportional to the formula of the
the introduction of a heavy halogen greatly enhances the spin–orbit coupling constant (xST) and the energy gap (DEST):
emission efficiencies of these AIEgens in the solid or aggrega-
tion state.
2
ðxST
Þ
kISC
f
2
DE
STÞ
eð
Mechanistic exploration of halogen-induced solid-state
uorescence enhancement and donor–p–acceptor-caused
red-shied emission
The excitation energies of the rst 10 excited states for all
TPAN and NTPAN derivatives were examined (Tables S6 and
S7†). Fig. 2c shows that the most efficient ISC of TPAN occurs
between S1 and T4, but the halogenated TPAN molecules
undergo the most efficient transitions from S1 to T3. The energy
gaps for halogenated TPAN molecules are slightly higher than
those of DEST for TPAN, whereas all the values for DEST of
halogenated TPAN are very close, ranging from 0.25 to 0.29 eV,
which clearly suggests that the introduction of heavy halogens
does not cause any apparent enhancement of ISC processes for
TPAN derivatives. Fig. 2d also shows a slightly declining
tendency in DEST following the order of NTPAN, NTPAN-F,
NTPAN-Cl, NTPAN-Br and NTPAN-I, and their differences in
DEST between the most efficient ISC process (S1 / T2) are really
small, which is similar to that of the TPAN group. More
specically, the largest difference in DEST is only 0.03 eV for
NTPAN-I as compared to NTPAN (Table S6, ESI†). As for these
AIE molecules, the presence of heavier halogens like bromine or
The heavy-atom effect predicts that the substitution of a heavy
atom with a large atomic number like bromine generally leads
to the uorescence quenching and simultaneous phosphores-
cence enhancement in solution for conventional dye molecules
because the heavy-atom effect greatly contributes to notable
spin–orbit coupling (SOC) and resultantly promotes intersystem
crossing processes.50,51 To test the impact of the classic heavy
atom effect on halogenated TPAN and NTPAN derivatives, low-
temperature experiments were rst performed as shown in
Fig. 2a and b. For the halogenated TPAN family, each of them in
THF at 77 K has only one emission peak at a location close to
their respective emission in the solid state without appearance
of new peaks; similarly, all halogenated NTPAN compounds in
THF at 77 K do not exhibit phosphorescence peaks. Major
emissions at 77 K for all halogenated derivatives have short-
lived lifetimes in the range of 0.9–6.0 ns (Fig. S8 and S9†).
These observations clearly suggest that these TPAN and NTPAN
compounds with heavy halogens do not follow the heavy atom
effect principle, and the introduction of heavy halogens plays
a negligible impact on promoting intersystem crossing and the
following phosphorescence, which is similar to the nding in
halogenated tetraphenylethene derivatives.24 To understand the
heavy halogen-induced solid-state uorescence enhancement of
these luminogens, some important visual information can be
obtained from NBO analysis rst. It is found that only a small
fraction of atomic components from halogens contributes to
HOMOs of TPAN and NTPAN derivatives, while almost no
contributions from halogens are observed in their LUMOs
(Fig. S10†), implying that halogens including heavy bromine
and iodine have a minor impact on transitions between HOMOs
and LUMOs. This deduction was further veried by quantitative
analysis of atomic contributions in Tables S4 and S5.† It is
noted that the atomic orbitals of all halogens contribute mainly
to the HOMOs of the halogenated TPAN, and their contribu-
tions progressively increased in the order of F (2.26%), Cl
(5.45%), Br (7.60%) and I (14.73%); with respect to the HOMOs
of the halogenated NTPAN, the contributions increased slightly
in the order of F (0.67%), Cl (1.24%), Br (1.38%) and I (1.95%).
iodine has little effect on the spin–orbit coupling constant (xST
)
and the involved energy gap (DEST), which leads to an insig-
nicant increment of the intersystem crossing rate (kISC). All the
theoretical data demonstrate that the introduction of heavy
halogens in TPAN and NTPAN molecules does not greatly
increase the intersystem crossing process, and no phospho-
rescence from the triplet state T1 can occur, which is consistent
with the experimental observation. Other parameters are also
critical to the uorescence quantum yield of luminogens, such
as the radiative constant (kr) and the nonradiative constant (knr),
as well as the intersystem crossing rate (kISC). Table S3 (ESI†)
reveals that the radiative constant (kr) gradually increased from
1.6 ꢀ 108 sꢁ1 for TPAN-F to 3.8 ꢀ108 sꢁ1 for TPAN-I and the
nonradiative constant (knr) experienced
a sharp decline,
whereas it decreased from 12.7 ꢀ 108 sꢁ1 for TPAN-F to 3.9 ꢀ
108 sꢁ1 for TPAN-I; the radiative constant (kr) gradually
increased from 1.1 ꢀ 108 sꢁ1 for NTPAN to 2.2 ꢀ 108 sꢁ1 for
NTPAN-Br and the nonradiative constant (knr) of NTPAN-F is
minimum (only 9 ꢀ 107 sꢁ1), but the kr decreased unceasingly to
3.9 ꢀ 108 sꢁ1 for NTPAN-I. All these parameters indicate that the
introduction of heavier halogens can effectively promote the
radiative process from S1 to S0 and can mostly prohibit the
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Fig. 2 (a) PL spectra of TPAN, TPAN-F, TPAN-Cl, TPAN-Br and TPAN-I in THF at 77 K. (b) PL spectra of NTPAN, NTPAN-F, NTPAN-Cl, NTPAN-Br
and NTPAN-I in THF at 77 K. (c) Energy profiles of excited states of TPAN, TPAN-F, TPAN-Cl, TPAN-Br and TPAN-I calculated with TDDFT. (d)
Energy profiles of excited states of NTPAN, NTPAN-F, NTPAN-Cl, NTPAN-Br and NTPAN-I calculated with TDDFT. (e) Electron density difference
maps of TPAN, TPAN-Cl, NTPAN and NTPAN-Cl at the S1 state.
nonradiative process in TPAN derivatives, which is also appli- and this denotes that most intermolecular contacts are at
cable to most NTPAN compounds (from NTPAN to NTPAN-Br). a distance longer than the van der Waals distance. As a result,
One possible reason for such uorescence enhancement prob- weak intermolecular interactions among these mono-
ably comes from intermolecular interactions such as halogen- halogenated molecules may contribute to this anomalous heavy
induced weak interactions in crystals restricting the molecular atom-induced uorescence enhancement rather than the
motion, which can refer to the role of halogen–halogen inter- hardly observable strong interactions. Then, the important
action in multi-halogenated tetraphenylethene molecules.24 For quantitative information on the hydrogen–halogen weak inter-
these monohalogenated compounds, the halogen bonds are actions in these monohalogenated crystals was obtained by
relatively not obvious, so the contribution of those to the plotting two-dimensional (2D) ngerprint plots. It is found that
intermolecular interactions cannot be accurately given just a portion of the Hirshfeld surface area is covered by X (F, Cl, Br
according to the distance between atoms. Herein, the inter- and I)/H intermolecular contacts. These contacts comprise
molecular interactions in the crystal structures of mono- 10.5%, 12.1%, 12.5% and 13.1% to the total Hirshfeld surfaces
haologenated TPAN and NTPAN derivatives are investigated via for TPAN derivatives, respectively. In order to further investigate
Hirshfeld surface analysis. Fig. 3a–d rst represents the the differences in the weak intermolecular interactions caused
Hirshfeld surfaces of monohalogenated TPAN derivatives by the change of halogens, four pie charts are depicted in the
mapped over dnorm, where the larger red spots in map indicate bottom of Fig. 3a–d, which represent the percentage contribu-
the strong intermolecular interactions between the molecules, tions of various intermolecular interactions on the basis of
but the smaller spots are due to weak interactions. Clearly, the Hirshfeld surface analysis for the TPAN group. Noticeably, the
Hirshfeld surfaces of these TPAN derivatives are almost blue, contribution of halogen to all other atoms (including H, C and
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Fig. 3 Hirshfeld surfaces of TPAN-F (a), TPAN-Cl (b), TPAN-Br (c) and TPAN-I (d) mapped with the parameter dnorm along with 2-D fingerprint
plots with a specific interaction. Pie-charts: relative contributions of various intermolecular interactions to the Hirshfeld surface area. (e) A
comparison between TPAN and TPAN-Br at the same vibration mode: vibrational intensity differences of non-heavy-atom-substituted benzene
rings (left) and heavy-atom-substituted benzene rings (right).
N) increases signicantly from 10.5% for TPAN-F to 17.8% for crystals, as shown in Fig. 1. These compounds existing in a pure
TPAN-I, which coincides with the gradual increase of quantum solid form or homogeneously dispersed in adamantane have
efficiency from TPAN-F to TPAN-I. It implies that the halogen- similarly short-lived lifetimes in a ns scale (Fig. S12 and S13†);
induced weak intermolecular forces may promote the restric- however, comparing their small differences in emission spectra
tion of intramolecular motions (RIM), which lead to the solid- (Fig. 1c and d), the quantum yields of both TPAN and NTPAN
state uorescence enhancement.22 For NTPAN-Cl, the uores- compounds dispersed in adamantane become much lower than
cence enhancement may also be due to the enhancement of the those in crystalline form, which implies that various intermo-
contribution of halogen-induced intermolecular interactions lecular interactions are important to restrict the molecular
(Fig. S11†). However, the relatively signicant decrease in uo- motion and enhance the solid-state uorescence. This obser-
rescence for NTPAN-Br and NTPAN-I may be attributed to some vation cannot exclude the possibility of the change in the
degree of heavy-atom effect. On the other hand, the emission emission efficiency caused by the subtle variation in molecular
efficiencies are apparently decreased when these as-prepared conformation. In addition, this specic heavy halogen-induced
molecules are dispersed in adamantine compared to those in uorescence enhancement may be partly attributed to the
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effective restriction of intramolecular vibrations and rotations and is valuable to design mechanochromic materials with color
due to the large mass effect of heavy halogens.53,54 We compared regulation induced by the conformational change.55,56
the vibration intensities of TPAN and TPAN-Br using the same
vibration mode, as shown in Fig. 3e. Due to the introduction of
Reversible photochromism behaviours of
heavy atoms, the vibration frequency of TPAN (710.92 cmꢁ1
/
triphenylacrylonitrile (TPAN) derivatives and inreversible
photochromism behaviours of bis(dimethylamino)
triphenylacrylonitrile (NTPAN) compounds
709.79 cmꢁ1) becomes slightly smaller than that of TPAN-Br
(771.83 cmꢁ1/834.69 cmꢁ1) at the same vibration modes
involving a vibration mode with bromine and a vibration mode
without bromine. The vibration intensities of non-heavy-atom- TPAN and its halogenated derivatives exist as white solids,
substituted benzene rings for TPAN (43.19) and TPAN-Br indicating that no signicant absorption occurs in the visible
(36.42) are approximate, but there is a signicant decrease range, and this observation is consistent with their major
between the vibration intensity of heavy-atom-substituted absorption bands (around 240 nm and 310 nm) in DCM (Fig. S2
benzene rings for TPAN (36.62) and TPAN-Br (8.80). It and S5†). However, their solutions quickly changed from
provides solid evidence that the occurrence of heavy atoms like colorless to orange aer continuous irradiation with a hand-
bromine can effectively lower the molecular vibration strength held UV lamp emitting 365 nm UV light, and such color
and thus promote the emission of the luminogen. In brief, three changes can be easily observed by the naked eye (Fig. 4a–e).
important points including the negligible heavy-atom effect, Time-dependent UV-visible spectra show that each of them
halogen-induced weak interactions and heavy-atom-induced exhibits a new visible absorption peak around 490 nm aer
restriction of intramolecular vibration contribute to the continuously irradiating with 365 nm UV light for 60 s.
unusual heavy-halogen-induced solid-state uorescence Resembling those furan-/thienyl-containing tetraarylethene
enhancement and an enormous increase in emission compounds in our previous paper,40,41 these TPAN compounds
efficiencies.
also show a reversible photochromic behavior upon ceasing UV
It is obvious that the emission colors of NTPAN compounds irradiation. Aer various periods of standing, the orange colors
are largely red-shied compared to those of the TPAN group. of all solutions can fade out to their original colorless state.
This regulation of emission color can be attributed to the Among these compounds, the TPAN-F undergoes signicant
construction of
a donor–p–acceptor pattern in NTPAN coloration under the same UV irradiation conditions, and TPAN
compounds, where the cyano group acts as the donor and the shows the fast fading time of only 75 s, while the others require
dimethylamino groups function as the acceptor. Such a donor– 100 s to restore their original colorless state (Fig. 4f–j). The
p–acceptor pattern oen leads to a twisted charge-transfer fading of the colored solutions can also be achieved using
state, which has a longer emission maximum and is usually visible light treatment, and it needs a shorter time of only 5 s for
sensitive to the external environment.46 Fig. 2e displays electron total recovery. All these coloration and fading processes are
density difference maps of TPAN, NTPAN, TPAN-Cl and NTPAN- reversible, and no signicant fatigue can be observed aer 10
Cl at the S1 state. It is found that an apparent charge separation cycles in solution (Fig. 4k). Such a reversible photochromism of
between the cyano group and the dimethylamino group takes TPAN compounds demonstrates that these molecules undergo
place for NTPAN and NTPAN-Cl relative to those of TPAN and a UV-triggered cyclization reaction and the following ring–
TPAN-Cl. This difference in electron density at the S1 state opening reaction.
between TPAN and NTPAN series indicates that the emission of
Among TPAN compounds, TPAN-Cl was used as an example
TPAN stemmed from the local excited state, whereas the emis- to probe their photochromic mechanisms experimentally and
sion of NTPAN comes from the charge transfer state, thus the theoretically. To explore the underlying reaction nature, the 1H
emission of NTPAN compounds should be sensitive to their NMR spectra of TPAN-Cl in CDCl3 before and aer continuous
structural conformations in different packing crystals. Fortu- irradiation of 365 nm UV light for 10 s and 20 s were compared
nately, we obtained two kinds of crystals for NTPAN-I from two rst, as shown in Fig. 5a. Before UV irradiation, all TPAN-Cl
different solvents (Fig. S14†). The NTPAN crystal cultured in molecules exist in their original structures, and thus all their
DCM was denoted G-type crystal due to its bright green uo- 1H MNR signals fall in the scope of 6.8–7.8 ppm originating
rescence, while the crystal grown in acetone was denoted Y-type from aromatic hydrogen atoms. However, four additional 1H
due to its yellow colored emission. Aer analyzing their struc- NMR peaks appear aer 20 s of UV irradiation, and they are
tures, we found that their absolute structures are the same, but located at 3.8, 6.1, 6.2 and 6.3 ppm. It is evident that the two
their structural conformations are different. In G-type confor- peaks at 3.8 ppm do not belong to the aromatic hydrogens but
mation, the angles of three phenyl groups relative to the central to the alkyl counterparts, suggesting the generation of alkyl
ethane plane are 62.7ꢂ, 30.2ꢂ and 36.8ꢂ, whereas those angles carbons and the formation of the cyclized ring under UV irra-
changed to 54.0ꢂ, 41.8ꢂ and 30.5ꢂ in Y-type conformation. Such diation. Due to the cyclization of the central ethane and two
a subtle change in structural conformation probably leads to neighboring benzene units, three new signals of aromatic
their distinct emission colors with a large difference of 43 nm in hydrogens of the product also emerge around 6.0 ppm. These
the emission maximum. As a result, the introduction of results provide solid evidence that a portion of molecules of
electron-donating dimethylamino groups into the triphenyla- TPAN-Cl are cyclized aer UV irradiation. To further demon-
crylonitrile skeleton is capable of tuning the emission colors strate the underlying nature of cyclization-depending photo-
chromism of these TPAN compounds, the calculated absorption
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Fig. 4 (a–e) Time-dependent UV-vis absorption spectra of TPAN (a), TPAN-F (b), TPAN-Cl (c), TPAN-Br (d), and TPAN-I (e) in DCM (2.0 mM)
under continuous UV irradiation at 365 nm. (f–j) Time-dependent UV-vis absorption spectra of colored solutions of TPAN (f), TPAN-F (g), TPAN-
Cl (h), TPAN-Br (i), and TPAN-I (j) in DCM (2.0 mM) under natural standing conditions. (k) Recycling times of UV-triggered coloration and white-
light-induced decoloration of TPAN-F in DCM (2.0 mM). (l) PL spectra of TPAN-F in DCM (2.0 mM): the original solution (before 365 nm UV), the
photochromic solution (after 365 nm UV irradiation) and the complete decoloration solution (placed naturally for 100 s) respectively. Fluo-
rescence photographs of these solutions were taken upon UV excitation.
wavelengths for S0 / S1 before and aer UV irradiation were only a slight change in UV-vis absorption can be detected, as
compared to those values calculated with DFT calculations. shown in Fig. 5b. Such a slight red-shi in absorption peaks
Table S8† depicts the experimental and calculated S0 / S1 implies that the molecular change might occur under 365 nm
absorption wavelengths of TPAN compounds and their cyclized UV irradiation. However, when the UV light changed to 254 nm
products in DCM. The calculated values of S0 / S1 absorption from 365 nm, an apparent color change from light yellow to
wavelengths for TPAN compounds in DCM are very close to green can be readily observed. Their UV-vis spectra show that
those experimental data observed in DCM, and the differences a new absorption peak at 625 nm appears aer 254 nm UV
are only about 40 nm, indicating that these calculations are irradiation, and the large absorbance at the new absorption
accurate to estimate the absorption wavelengths. By compar- band is in accord with the deep green color of the irradiated
ison, the calculated S0 / S1 absorption wavelengths for cyclized solution. The green colored solution can also fade apparently
products of TPAN compounds are much longer than those of out when irradiated by white light, which suggests that UV
the respective TPAN compounds and are consistent with those irradiation with 254 nm makes possible the NTPAN cyclization
data observed aer photochromism. These observations further accompanying the color change. Aer irradiated with 254 nm
verify that TPAN compounds can cyclize to their cyclized prod- and 365 nm light, it can be observed that the yellow color of the
ucts under UV irradiation, and these cyclized products can also solution rst turns to deep green and then to a green-yellow
turn back to their original structures.
color. It is predicted that a different photo-induced reaction
For NTPAN group, their photochromic behaviours are from the cyclization reaction occurs aer 365 nm UV irradiation
apparently different from those of TPAN compounds due to the for NTPAN compounds, and a dehydrogenation reaction fol-
introduction of the D–A pattern in the NTPAN structure. Under lowed the cyclization might take place with 365 nm UV irradi-
365 nm UV irradiation, NTPAN shows a slight color change from ation, as depicted in Fig. 5c. Using NTPAN as an example, most
yellow to orange, which is consistent with the observation that proton signals of the original molecule fall in the range of 6–8,
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are much shorter than those aer 254 nm UV irradiation (at
around 625 nm) from the experimental data. Table S9† displays
the comparison between experimental and calculated data for
S0 / S1 absorption wavelengths of NTPAN compounds, their
cyclized products and their dehydrogenated products. The
small differences in the rst absorption peaks between the
observed and calculated data for NTPAN compounds in DCM
solution validate the TDDFT calculations. Although a direct
comparison between experimental and calculated data for the
cyclized products and dehydrogenated products shows
apparent variations varying in tens of nm in wavelength, it is
absolutely evident that the calculated S0 / S1 absorption
wavelengths of the cyclized products of NTPAN are much longer
than those of their respective dehydrogenated products, which
is consistent with the experimental observations that the
absorption of NTPAN compounds irradiated with 254 nm UV
light is remarkably red-shied relative to those irradiated with
365 nm UV light. This consistence in absorption peaks observed
in the experiment and predicted by theory further validates our
prediction that UV-dependent photochromic behaviours of
NTPAN compounds originate from two distinct reactions, i.e.
the cyclization reaction and the cyclization–dehydrogenation
reaction.
The 254 nm UV light was rst selected to perform photo-
chromic experiments for all NTPAN compounds. As shown in
Fig. 6a–e, a deep green color can be observed for all NTPAN
compounds aer 110 s of UV irradiation, and new absorption
peaks at around 625 nm can be detected in their UV-vis spectra.
Interestingly, the colored solutions of NTPAN compounds aer
UV irradiation with 254 nm hardly fade out, keeping the green
color even aer a long time of standing. Only a very slight
decrease at 625 nm can be detected in the UV-vis spectra during
the process, providing an implication that the products during
such a cyclization reaction are quite stable. It is interesting to
note that these green solutions exhibit bright blue uorescence
under UV excitation, as shown in Fig. 6h–l. The generation of
bright blue uorescence of irradiated NTPAN solution can be
attributed to the cyclization-induced rigidication of large
conjugates containing two phenyl rings and a central ethane.
The conjugation of most parts of these molecules enables the
transformation of their photophysical behaviours from AIE to
ACQ, which indirectly proves that the regulation of intra-
molecular motions determines the photophysical properties of
the luminogens and the RIM mechanism of AIEgens.10–12 The
green color of NTPAN and NTPAN-I is much deeper than the
others, indicating that the transformation rates for NTPAN and
NTPAN-I are greater; however, the uorescence of irradiated
NTPAN solution is much brighter than that of the irradiated
NTPAN-I solution. This is due to the uorescence quenching
induced by the heavy atom effect of iodine in the cyclized
product of NTPAN-I compound. The photochromic behaviours
of NTPAN compounds irradiated with 365 nm light were also
examined to demonstrate UV-triggered irreversible coloration
and the accompanying uorescence turn-on phenomenon.
Fig. S15† shows photochromic changes of NTPAN compounds
under the continuous irradiation of 365 nm light. It is observed
that new absorption bands emerge at around 520 nm aer 120 s
Fig. 5 (a) Comparison of 1H NMR spectra of TPAN-Cl in CDCl3 before
(0 s) and after UV irradiation (10 s and 20 s). Inset: demonstration of
photo-controlled cyclization and ring-opening reactions using TPAN-
Cl as an example. (b) Time-dependent UV-vis absorption spectra of
NTPAN in dichloromethane with a concentration of 2 mM; (1) before
UV irradiation; (2) after irradiation with 254 nm UV light for 30 s; (3)
after irradiation with 365 nm UV light for 80 s; and (4) after irradiation
with 254 nm and 365 nm UV light successively. Inset: the corre-
sponding pictures of photochromic solutions in daylight. (c) Demon-
stration of the UV-dependent cyclization reaction and the cyclization–
dehydrogenation reaction using NTPAN as an example and their
changes in 1H NMR spectra.
and no other peaks can be detected except for the signal from
methyl groups. Aer irradiation with 254 nm UV light, at least
four new small signals can be identied, especially the two
signals at 4.0 and 4.1 ppm belong to alkyl hydrogens, which
provide solid evidence for the occurrence of alkyl hydrogens of
the resulting product aer UV irradiation. The appearance of
two alkyl hydrogen atoms in different chemical environments
clearly proves that the cyclization reaction takes place to
transform the aromatic hydrogens to alkyl hydrogens under
such UV excitation. Compared to the 1H NMR spectrum of
NTPAN irradiated with 254 nm UV light, no observable signals
at around 4.0 ppm appear in the spectrum of NTPAN solution
aer 365 nm irradiation, indicating that no alkyl hydrogens are
generated during such a process. Accordingly, the new signals
at 8.6, 8.3, 7.8 and 7.7 ppm are in accord with the structure of
the dehydrogenated product of NTPAN, suggesting that the
original NTPAN molecules undergo consecutive cyclization and
dehydrogenation reactions to produce a small portion of the
dehydrogenated product. To further verify this hypothesis, the
absorption maxima for S0 / S1 of NTPAN compounds, their
cyclized products and their dehydrogenated products were
calculated and compared with their respective experimental
data. It is obvious that the new absorption maxima of NTPAN
compounds irradiated with 365 nm UV light (at around 520 nm)
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Fig. 6 (a-e) Time-dependent UV-vis absorption spectra of NTPAN (a), NTPAN-F (b), NTPAN-Cl (c), NTPAN-Br (d), NTPAN-I (e) in DCM (2.0 mM)
under continuous UV irradiation at 254 nm. (f) Change in the UV-vis absorption spectra of NTPAN-F in DCM (2.0 mM) under various conditions:
the original solution (before 254 nm UV irradiation), the photochromic solution (after 254 nm UV irradiation) and the photochromic solution (left
standing under natural light for 10 min), respectively. (g) Change in the PL spectra and images of NTPAN-F in DCM (2.0 mM) under various
conditions. (h–l) Time-dependent PL spectra of NTPAN (h), NTPAN-F (i), NTPAN-Cl (j), NTPAN-Br (k), and NTPAN-I (l) in DCM (2.0 mM) under
continuous UV irradiation at 254 nm.
of UV irradiation on the solutions of NTPAN compounds. detectable uorescence decrease can be recorded, indicating
Although the color of the solution only slightly changed from that these dehydrogenated products are extremely stable. In
yellow to orange, such a change can also be identied by the comparison with the fast recovery of colored solutions of TPAN
naked eye for most solutions. Among these compounds, NTPAN compounds aer irradiation with UV light, the colored solu-
has the sharpest color change from its large absorption tions of NTPAN compounds almost barely fade out under
enhancement at the new absorption band, suggesting that various conditions, suggesting that their photochromic behav-
NTPAN is favorable to undergo the successive cyclization– iours are irreversible relative to those of TPAN compounds.
dehydrogenation reaction. It is also easily concluded that the These observations demonstrate that the introduction of
introduction of heavy halogens counteracts such chemical dimethylamino groups into the TPAN skeleton not only
changes and progressively decreasing absorbance at the new constructs a unique D–p–A pattern and red-shied emissions
absorption bands from NTPAN-F to NTPAN-I. Similar to the but also greatly turns their photochemical behaviours from
cyclized products, these dehydrogenated products also exhibit unstable and reversible cyclization to UV-dependent stable and
bright blue uorescence, and as the irradiation increases the irreversible cyclization and dehydrogenation.57,58
uorescence intensity is gradually enhanced. By comparison,
the enhanced uorescence of the dehydrogenated product
progressively declined from NTPAN to NTPAN-I, and this fact
resembles the observation of the cyclized products, both of
Photoswitchable reversible and irreversible patterning on
various surfaces and multiple information encryption
them highlighting the uorescence quenching effect on planar
conjugated luminogens due to the heavy atom effect. Aer
a long time of monitoring of uorescence brightness, it is found
that the uorescence can always be kept, and no obviously
Most diarylethene-based photochromic molecules with uores-
cence turn-on behaviors were examined in their solution states,
and a few of them exhibited photoswitchable patterning on the
solid surface upon irradiation.26–29 Herein, we demonstrate that
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TPAN compounds with reversible photochromism possess the
ability to undergo photo-triggered reactions and the resulting
sharp color changes in the diverse matrix and on various
surfaces. TPAN was used as an example to show this unique
reversible photochromism in various solid environments
including in porous cotton, in a cellulose blend and on a polymer
surface (Fig. 7a). In such distinct solid environments, the TPAN
molecule can also respond to two different irradiation conditions
showing two sharply different colors, indicating that irradiation-
triggered cyclization and decyclization reactions of these TPAN
molecules can efficiently and easily take place in different solid
matrixes and surfaces, which will greatly expand their practical
applications in various photo-controlled processes. The fast
response to external stimuli is also illustrated in Fig. 7b, where
TPAN molecules were homogeneously distributed on a paper-
based bird model. Upon only 5 s of UV irradiation, the white
bird quickly changes to an orange bird; and aer 6 min of
standing under an ambient environment, the orange bird grad-
ually turns back to its original white. This color shiing can be
run many times in a reversible way, which grants these TPAN
molecules great potential in photoswitchable patterning and
information storage.59
The experimental and theoretical results indicate that the
TPAN groups are able to toggle conditionally between two
discrete open- and closed-ring-form states upon alternate irra-
diation with UV and visible light; due to stable cyclized products,
the NTPAN compounds can undergo irreversible photochromic Fig. 8 (a) A demonstration of photo-controlled patterning application
on a filter paper by combining reversible and irreversible photochro-
mism using the blend of TPAN and NTPAN as an example. The right
half was treated with TPAN, while the left half was distributed with
NTPAN; a phoenix pattern was used to generate controlled patterning,
transformation under UV light. Thus, these two classes of similar
compounds can be used to combine reversible and irreversible
photochromic behaviours as key elements of various light-driven
information encryption and molecular switches. A combination
and 365 nm UV light and white light were employed to trigger the
of TPAN and NTPAN was rst used as an example to demonstrate color change or erase the generated color. (b) Illustration of the
geographic coordinate encryption as a prototype combining revers-
ible (TPAN)/irreversible (NTPAN) photochromism based on a code
encryption design. (c) Demonstration of photoactivatable fluorescent
patterning using NTPAN as an example. A viscous solution containing
the photo-controlled patterning on a solid matrix. Fig. 8a shows
a successive change in a phoenix pattern upon irradiation of
different lights from an unobservable state to full emergence and
nally to a half pattern. Half of the paper was treated with TPAN
NTPAN and sucrose octaacetate (1 : 100) in DCM was used as the
and the other was distributed with NTPAN; when a phoenix mask background and then the background was irradiated with a beam of
UV light at 365 nm through different letter masks (ZJNU).
was subject to 30 s of irradiation of 365 nm UV light, a full orange
phoenix pattern emerged on the paper. Aer only 5 s of irradia-
tion of white light, one half of the pattern disappeared and the
other half remained orange. Such different responses to distinct
irradiation lights using the combined reversible and irreversible
photochromism are valuable for multiple information encryp-
tion.
A demonstration of information encryption on the
geographic coordinates of our university is depicted in Fig. 8b,
where two papers treated with reversibly photochromic TPAN
and irreversibly photochromic NTPAN were used to decode the
true geographic coordinates. The misleading geographic coordi-
nates (23.1ꢂ N, 13.5ꢂ E) on a yellow paper emerges rst aer
365 nm UV irradiation, the false coordinates directing to some
place in Africa. The key information to decode the true coordi-
nates (29.1ꢂ N, 119.6ꢂ E) can be exhibited upon 365 nm UV
irradiation on a white paper. When the two papers are combined,
the true geographic coordinates (29.1ꢂ N, 119.6ꢂ E) for ZJNU can
Fig.
7 (a) Demonstration of reversible photochromism of TPAN
compounds in various environments including in porous cotton, in
a solid blend and on a polymer surface using TPAN as an example. (b)
Demonstration of the fast response of reversible photochromism of
TPAN compounds on a paper-based bird model.
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be read. Such a true coordinate pattern can be easily encrypted photochromism and photoactivatable uorescence of the NTPAN
using white light irradiation and also decoded by UV light, family with a D–A pattern stemmed from the photo-induced
illustrating the multiple information encryption ability by the cyclization–dehydrogenation reactions upon UV-dependent irra-
combination of reversible and irreversible photochromism diation in solution. Compared to the previous fused molecules of
behaviours of the two groups of molecules. In comparison with AIE and photochromism, these molecules show distinct domi-
the photo-controlled patterning in color, NTPAN compounds nant photophysical emission enhancement in the solid state and
have special photo-triggered uorescence turn-on behaviours photochemistry-dominating color and emission changes in the
and valuable use in photoactivatable uorescent patterning dispersed state to accomplish controllable regulation of photo-
applications. Fig. 8c shows a demonstration of photoactivatable physics and photochemistry of luminogens. The demonstration
uorescent patterning using NTPAN as an example, where of controlled regulation of photophysics and photochemistry of
a viscous solution of NTPAN containing a great amount of fused chromic and AIE-active luminogens in distinct states
sucrose octaacetate was used to retain the designed patterns. provide strong evidence that they have great potential for various
When the masks of designed letters such as ZJNU were used to applications involved in multiple-mode signaling and diverse
block part of a beam of UV light, the other parts of the solutions surfaces such as photoswitchable patterning and information
uoresce in green with gradually increasing intensity. Then, the encryption.
designed letters ZJNU clearly appear on the bright green uo-
rescent background under the UV light and becomes clearer as
the irradiation time increases. Such a dual-mode signaling of
Data availability
these compounds gives them great advantage to be used in The datasets supporting this article have been uploaded as part
applications of advanced encryption and storage of information of the ESI.
due to their specic reversible and irreversible photochromism
and photoactivation of uorescence. These demonstrations
illustrate the remarkable potential of these TPAN-based/NTPAN-
Author contributions
based AIEgens with photocontrolled photochromism and pho- Z. Xiong conceived of the study, collected the experimental data
toactivatable uorescence in optical memory media and infor- and wrote the initial dra of this article; Z. Qian contributed to
mation encryption.60
the design of central ideas, carried out additional analyses and
nalized this article; and H. Feng contributed to the revisions.
The remaining authors contributed to the discussion of the
results and rening of the ideas.
Conclusions
In summary, two groups of triphenylacrylonitrile derivatives with
different functionalities were rationally designed based on an
AIE-active skeleton with through-space conjugation property, and
Conflicts of interest
controllable regulation of photophysical and photochemical There are no conicts to declare.
processes in distinct states were achieved by altering the func-
tionals and molecular polarities of the skeleton molecule. Since
the discovery of the rst AIEgen, AIEgens have been facing a great
Acknowledgements
challenge of much lower emission efficiency than their counter- We gratefully acknowledge nancial support from the National
parts through-bond conjugated dyes, but few strategies were Natural Science Foundation of China (Grant No. 21775139 and
proposed to resolve this problem in a general and effective way. 21675143) and Natural Science Foundation of Zhejiang Prov-
In this work, we propose a new strategy of introducing heavy ince (Grant No. LR18B050001).
halogens into suitable positions to dramatically improve the
emission efficiency in a widely applicable manner, which is ex-
pected to be expanded to other AIE-active systems. This anoma-
Notes and references
lous emission enhancement induced by heavy halogens against
the traditional heavy-atom effect on through-bond conjugated
dyes is attributed to the specic through-space conjugation
nature of the AIE-active skeleton, effective intermolecular
halogen-bond-induced restriction of intramolecular motions,
and heavy atom-induced vibration reduction by evident data and
rational analyses. The construction of a donor–acceptor pattern
in the AIE skeleton not only leads to photophysical emission
alteration to a red-shied direction but also tremendously
changes the dominant photochemical reactions and the accom-
panying photochromism and photoactivatable uorescence
behaviours. It has been proved that the reversible photochro-
mism of the TPAN family originated from photo-triggered cycli-
zation and decyclization reactions, but the irreversible
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