Dynamic dye emission ON/OFF systems by a furan moiety exchange protocol

Article abstract of DOI:10.1016/j.dyepig.2020.108652

Four triphenylamine-based dyes were synthesized by fluorescence turn on reactions. Optical behaviours, molecular arrangements, donor-to-acceptor charge transfer and dipole interactions of these functional dyes were investigated by UV–vis absorption and fluorescence spectroscopy, single-crystal X-ray diffraction and electrochemical cyclic voltammetry. The irreversible isomerization of itaconimide dye leads to an irreversible emission switch ON to OFF. Reversible Diels-Alder reaction of these dyes lead to a reversible emission switch OFF/ON. These luminescent dyes demonstrate dynamic dye molecular features by furan moiety exchanges to form energy-minimized and optimized dye molecular structures. In the dynamic molecular system, α-position furan-substituted dye was converted into more stable β-position furan-substituted dye according to ¹H NMR spectroscopic monitoring.

Full text of DOI:10.1016/j.dyepig.2020.108652

Dyes and Pigments 184 (2021) 108652
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: http://www.elsevier.com/locate/dyepig
Dynamic dye emission ON/OFF systems by a furan moiety
exchange protocol
Qi Zhang, Ying Wang**, Junbo Gong***, Xin Zhang*
School of Chemical Engineering and Technology, Collaborative Innovation Center of Chemistry Science and Engineering, Tianjin University, Tianjin, 300072, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Four triphenylamine-based dyes were synthesized by fluorescence turn on reactions. Optical behaviours, mo-
lecular arrangements, donor-to-acceptor charge transfer and dipole interactions of these functional dyes were
investigated by UV–vis absorption and fluorescence spectroscopy, single-crystal X-ray diffraction and electro-
chemical cyclic voltammetry. The irreversible isomerization of itaconimide dye leads to an irreversible emission
switch ON to OFF. Reversible Diels-Alder reaction of these dyes lead to a reversible emission switch OFF/ON.
These luminescent dyes demonstrate dynamic dye molecular features by furan moiety exchanges to form energy-
Dynamic covalent chemistry
Triphenylamines
Fluorescence
Supramolecular assembly
minimized and optimized dye molecular structures. In the dynamic molecular system,
α-position furan-
substituted dye was converted into more stable β-position furan-substituted dye according to 1H NMR spectro-
scopic monitoring.
1. Introduction
reprocessable liquid crystalline epoxy thermosets and elastomers with
dynamic covalent bonds [32]. Summerlin et al. utilized the reversible
Reversible cleavage and reformation of covalent bonds are the most
interesting features of dynamic covalent reaction, which allow the ex-
change of molecular moieties at equilibrium to achieve thermodynamic
minimum of system [1–5]. Compared with supramolecular weak
non-covalent bonds [6–9], dynamic covalent bonds are more robust
with response for the environments including temperature, light, me-
chanical stress, magnetic and electrical fields [10–12]. By dynamic co-
valent reactions, chemists have prepared a wide range of fascinating 1D,
2D and 3D molecular architectures such as polymers [13,14], molecular
knots [15], covalent organic frameworks [16–20], and macrocycles [21,
22]. Dynamic covalent reactions can be classified into two main re-
actions, i.e. exchange reactions including ester, disulfide, furan, amine
and acetal exchanges, and new dynamic covalent bond formation re-
actions including aldol reaction, imine condensation and cycloadditions.
On the basis of above outstanding dynamic properties, dynamic covalent
dye chemistry has shown various applications such as self-healing,
renewable and recyclable materials [23–25], chemical sensors [26,
27], biomedical [28], gas storage [29,30]. Recently, Zhang and Jin et al.
elegantly described recyclable plastics and thermosets based on dynamic
covalent bonds [31]. Ji et al. reported self-healable, recyclable,
properties of Diels-Alder reaction to prepare smart materials with
morphology transitions [33].
A very interesting class of dynamic covalent reactions is the Diels-
Alder [4 + 2] cycloaddition of electron-rich dienes and electron-poor
dienophile [34,35]. Diels-Alder furan-maleimide addition reaction can
occur at ambient temperature through relatively low activation barriers,
which leads to the only slightly exergonic adducts relative to starting
compounds, thus allowing the retro-Diels-Alder addition to occur under
appropriate conditions. The controllable characteristics for forward and
reverse reactivity of furan-maleimide addition enable chemists to create
dynamic molecular systems. In this work, we have designed and syn-
thesized four triphenylamine-based dyes. Triphenylamine chromo-
phores were chosen due to their propeller-like molecular structures and
functional electron-donating properties for organic photoelectric p-type
materials [36–38]. A high quality crystal was obtained, and its single
crystal X-ray diffraction reveals the supramolecular assembling struc-
tures. The optical properties of these dyes were investigated. In partic-
ular, we demonstrate dynamic dye systems with switchable ON/OFF
fluorescence behaviours.
* Corresponding author.
** Corresponding author.
*** Corresponding author.
E-mail addresses: yingwang@tju.edu.cn (Y. Wang), junbo_gong@tju.edu.cn (J. Gong), xzhangchem@tju.edu.cn (X. Zhang).
https://doi.org/10.1016/j.dyepig.2020.108652
Received 20 April 2020; Received in revised form 16 June 2020; Accepted 16 June 2020
Available online 14 August 2020
0143-7208/© 2020 Elsevier Ltd. All rights reserved.

Q. Zhang et al.
Dyes and Pigments 184 (2021) 108652
Fig. 1. Chemical structures of triphenylamine-based dyes 1 and 2 and Diels-Alder adducts 3 and 4. Dynamic Diels-Alder bonds are indicated in red, and triphe-
nylamine chromophores are indicated in blue. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of
this article.)
2. Experimental
δ = 3.51 (t, J = 2.1, 2.4Hz, 2H, -COCH2-), 5.74 (t, J = 1.5, 2.1Hz, 1H,
=CHH), 6.48 (t, J = 2.4, 2.1Hz, 1H, =CHH), 7.07 (t, J = 7.53Hz, 2H,
ArH), 7.11–7.31 (m, 12H, ArH). 13C NMR (DMSO‑d6, ppm): δ = 33.82,
119.8, 122.3, 123.6, 124.2, 124.5, 128.0, 129.7, 134.4, 146.9, 168.7,
147.1, 173.4. IR (KBr pellet, cmꢀ 1): 516, 615, 694, 756, 831, 888, 951,
1145, 1275, 1322, 1384, 1500, 1588, 1658, 1714, 2925. HRMS (APCI,
2.1. Synthesis
1-(4-(diphenylamino)phenyl)-maleimide (Dye 1) was synthesized
according to the previously reported procedure [39].
1-(4-(diphenylamino)phenyl)-itaconimide (Dye 2): Itaconic anhy-
dride (0.56 g, 5 mmol) and acetone (10 mL) were added to a 100 mL
flask, and the solution was heated to 50 ◦C. An acetone solution (8 mL)
of 4-aminotriphenylamine (0.98 g, 3.5 mmol) was added dropwise to the
reaction flask, and the mixture was reacted at 50 ◦C for 2 h. The reaction
mixture was filtered to give yellow precipitate. After drying under
vacuum, the yellow precipitate was added to a 100 mL flask. Sodium
acetate (0.42 g) and acetic anhydride (4.4 mL) were also added, the
reaction mixture was heated to 80 ◦C for 13 h. After cooling to room
temperature, the red solution was poured to deionized water (300 mL).
The product was extracted by dichloromethane for three times and dried
by anhydrous magnesium sulfate. The dichloromethane solution was
concentrated under reduced pressure. The crude product was purified by
silica gel column chromatography with dichloromethane/n-hexane (4/
3, v/v) as eluent to give yellow solid (0.47 g) with a yield of 38%. 2 was
recrystallized three times from hexane/dichloromethane (4/1, v/v) to
yield yellow crystals. Mp: 159–160 ◦C. 1H NMR (400 MHz, CDCl3, ppm):
Fig. S8): m/z calcd for
C
23H18N2O2[M+H]+: 355.1441; Found:
355.1444.
2-(4-(diphenylamino)phenyl)-4-((2-(2-ethoxyethoxy)ethoxy)
methyl)-3a,4,7,7a-tetrahydro-1H-4,7-epoxyisoindole-1,3(2H)-dione
(Dye 3): Dye 1 (0.17 g, 0.5 mmol) and 2-{2-[2-(ethoxy)ethoxy]ethoxy}
methyl-furan (0.13 g, 0.6 mmol) were dissolved in chloroform (5 mL).
The reaction mixture was kept stirring for three days at room temper-
ature. The product was washed with deionized water and extracted by
dichloromethane for three times. After being concentrated, the crude
product was purified by silica gel column chromatography with ethyl
acetate/n-hexane (1/3, v/v) as eluent to give white solid (0.20 g) with a
yield of 47%. 1H NMR (400 MHz, CDCl3, ppm): δ = 7.26 (t, J = 7.4 Hz,
–
4H, ArH), 7.15–7.02 (m, 10H, ArH), 6.63 (d, J = 5.4 Hz, 1H, -CH CH-),
–
–
–
6.56 (d, J = 5.5 Hz, 1H, –CH CH-), 5.36 (s, 1H, -CH(OR)-), 4.30 (d,
J = 8.0 Hz, 1H, -CH(R)-), 3.97 (d, J = 8.0 Hz, 1H, -CH(R)-), 3.82–3.47
(m, 10H, 5-OCH2-), 3.10, 3.00 (dd, J = 6.4 Hz, 2H, -OCH2-), 1.20 (t,
J = 6.9 Hz, 3H,-CH3). 13C NMR (CDCl3, ppm): δ = 175.4, 174.0, 148.1,
Fig. 2. a) Synthetic route toward
α-position
substituted furan chain. Reagents and conditions: (i)
NaOH, THF/H2O, 0 ◦C, 8 h, 85%; (ii) NaH, THF,
70 ◦C, 2 days, 74%. b) Synthetic route toward β-po-
sition substituted furan chain. Reagents and condi-
tions:
(i)
Copper(I)
iodide,
tetrakis
(triphenylphosphine) palladium, anhydrous triethyl-
amine, 70 ◦C, 39%. (ii) CuI, Pd(PPh3)4, Et3N, Ar2,
140 ◦C, 6 h, 40%. (iii) Pd/C, CH3COOH, NaBH4,
Ethanol, r.t., 24 h, 75%. c) Synthetic route toward
dyes 3 and 4 from dye 1. Reagents and conditions: (i)
CHCl3, r.t., 3 days, 47%. (ii) CHCl3, r.t., 3 days, 60%.
2

Q. Zhang et al.
Dyes and Pigments 184 (2021) 108652
Table 1
UV–vis absorption maximum wavelength (λab, max), extinction coefficient (
ε
),
fluorescence emission maximum wavelength (λem, max), fluorescence quantum
yields (Φf) and lifetimes (
τ) of dyes 1–4 in 1, 2-dichloroethane.
Dyes
UV–vis absorption
Fluorescence
Stokes
Shifts/
nm
λab,
ε
/1 × 104 Mꢀ 1cmꢀ 1
λem,
Φf(%)
τ/ns
/
/
max
max
nm
nm
1
2
304
301,
429
307
307
2.56
a
a
a
a
2.33, 0.06
380,
537
374
374
1.81
0.81,
0.46
0.26
0.28
79, 108
3
4
2.35
2.46
18
24
67
67
[a] No fluorescence.
alcohol (Fig. 2a). β-position substituted oligo(diethylene glycol) furan
chain was synthesized by reduction reaction using palladium-carbon as
catalyst at room temperature from corresponding alkynyl furan deriv-
ative (Fig. 2b). The alkynyl furan derivative was synthesized by the
nucleophilic substitution reaction of 3-bromofuran with diethylene
glycol monopropynyl ether. Dyes 3 and 4 were finally obtained by
Diels-Alder [4 + 2] cycloaddition reactions of dye 1 and corresponding
α
- or β-position substituted oligo(diethylene glycol) furan chain without
catalyst under mild conditions (Fig. 2c).
Fig. 3. a) UV–vis absorption spectra of dyes 1–4 in 1, 2-dichloroethane,
c = 5 × 10ꢀ 5 M; b) Fluorescence spectra of dyes 1–4 in 1, 2-dichloroethane,
c = 5 × 10ꢀ 5 M, λex = 310 nm, Inset: CIE 1931 chromaticity diagram, three
points are emission colour coordinates of 2 (0.16, 0.34), 3 (0.16, 0.06), 4 (0.20,
0.10)). (For interpretation of the references to colour in this figure legend, the
reader is referred to the Web version of this article.)
Dye 1 shows UV–vis absorption maximum wavelength at 304 nm
with extinction coefficient (2.56 × 104 Mꢀ 1cmꢀ 1) (Fig. 3a), which is
assigned to
π-π* transition of triphenylamine chromophore [41,42]. Dye
2 shows the
π-π* transition absorption with a blue-shift of 3 nm and a
slightly lower extinction coefficient. A structureless broad lower-energy
absorption band at 380–520 nm was observed for dye 2. According to
Krijnen [43,44], this broad absorption band in the long-wavelength
147.3, 138.3, 136.7, 129.4, 127.2, 125.0, 123.6, 122.8, 91.0, 81.50,
71.2, 70.7, 70.4, 69.9, 68.5, 66.7, 49.9, 48.3, 15.2. IR (KBr pellet, cmꢀ 1):
3462, 2968, 2924, 2870, 1776, 1709, 1589, 1508, 1489, 1385, 1317,
1281, 1190, 1100, 806, 754, 698. HRMS (ESI, Fig. S9): m/z calcd for
region is attributed to the lowest π-π* transition of charge transfer be-
tween triphenylamine donor and itaconimide acceptor. Dyes 3 and 4
show nearly identical absorption spectra with the same maximum
wavelength at 307 nm.
C
33H34N2O6 [M+Na]+: 577.2309; Found: 577.2318.
2-(4-(diphenylamino)phenyl)-5-(3-(2-(2-ethoxyethoxy)ethoxy)pro-
These dyes show distinctly different fluorescence behaviours
(Fig. 3b). Dye 1 is non-fluorescent, and the fluorescence of the triphe-
nylamine donor is completely quenched by intramolecular charge
transfer interaction with strong electron acceptor. In contrast, dye 2 is
fluorescent with two emission bands (Fig. 3b). The first emission band at
380 nm is assigned to the emission of triphenylamine chromophore. For
the second broad emission band at 450–650 nm, we attributed it to
intramolecular charge transfer state between triphenylamine donor and
itaconimide acceptor according to its absorption and excitation spectra.
Dyes 3 and 4 both are fluorescent, and their fluorescence spectra are
similar with the emission maximum wavelength at 374 nm. The light-
emitting colour coordinates of dyes 2, 3 and 4 are (0.16, 0.34), (0.16,
0.06) and (0.20, 0.10) within blue light region of CIE 1931 chromaticity
diagram, respectively. The fluorescence quantum yields of dyes 2, 3 and
4 were measured to be 1.81%, 18% and 24%, respectively. The ab-
sorption and emission data of these dyes were listed in Table 1.
The optical and optoelectronic properties of dyes are closely related
to their molecular topology 3D structures, which motivate us to explore
the crystal assembly structures of triphenylamine-based dyes. The single
crystal of dye 1 was prepared by slow evaporation of chloroform/
dichloromethane (1/1) solution within methanol atmosphere. The
crystal structure is orthorhombic system with a centrosymmetric space
group of P b c a [45]. One unit cell consists of 8 molecules. The crystal
density obtained from the X-ray diffraction is 1.275 g/cm3. The cell
length b of dye 1 is the nearly double the distance of lamellar stacking
pyl)-3a,4,7,7a-tetrahydro-1H-4,7-epoxyisoindole-1,3(2H)-dione (Dye 4)
was synthesized in the same manner as dye 3 except for starting mate-
rials (3-(3-{2-[2-(ethoxy)ethoxy]ethoxy}-propyl)-furan). The crude
product was purified by silica gel column chromatography with ethyl
acetate/n-hexane (3/2, v/v) as eluent to give white solid with a yield of
60%. 1H NMR (400 MHz, CDCl3, ppm): δ = 7.29–7.24 (m, 5H, ArH),
–
7.14–7.02 (m, 9H, ArH), 6.08 (q, J = 1.8 Hz, 1H, -CH C(R)-), 5.30 (s,
–
1H, -CH(OR)-), 5.13 (s, 1H, -CH(OR)-), 3.70–3.46 (m, 12H, -OCH2-),
3.04 (d, J = 8.0 Hz, 1H, -CH(R)-), 2.99 (d, J = 8.0 Hz, 1H, -CH(R)-), 2.34
–
(t, J = 7.6 Hz, 2H, –CH C(R)CH -), 1.88–1.73 (m, 2H, -OCH CH -),
–
2
1.21 (t, J = 7.0 Hz, 3H, -CH3). 13C 2NMR (CDCl3, ppm): δ = 175.9,2175.8,
151.9, 148.1, 147.3, 129.4, 128.7, 127.2, 125.1, 123.5, 122.8, 83.7,
82.3, 70.7, 70.6, 70.3, 70.3, 69.9, 66.7, 49.2, 47.2, 27.3, 24.1, 15.2. IR
(KBr pellet, cmꢀ 1): 3037, 2970, 2865, 1774, 1700, 1589, 1506, 1488,
1383, 1273, 1181, 1106, 933, 867, 750, 697. HRMS (ESI, Fig. S10): m/z
calcd for C35H38N2NaO+6 [M+Na]+: 605.2622; Found: 605.2628.
3. Results and discussion
3.1. Synthesis and optical behaviours
Four triphenylamine-based dyes 1–4 (Fig. 1) were synthesized and
fully characterized by NMR and IR spectroscopy and high resolution
mass spectrometry (HRMS). Dyes 1 and 2 were synthesized from either
maleic anhydride or itaconic anhydride, respectively via 5-member ring
opening and subsequently closing reactions [40]. The precursor hydro-
philic methylbenzenesulfonate chain was synthesized by the esterifica-
tion reaction of tosyl chloride with diethylene glycol monoethyl ether
interval (Fig. 4b). No hydrogen bonding or π-π stacking interaction was
observed in crystal packing structure. Triphenylamine chromophore
adopts a propeller–like conformation (Fig. 4a). In crystal structure, the
dye molecules are assembled in an alternating donor-acceptor head--
to-tail manner (Fig. 4b, c, d). Thus, the main driving force for the for-
mation of packing structure of dye 1 is the donor-accepter dipole-dipole
(Fig. 2a). The
α-position substituted oligo(diethylene glycol) furan chain
was synthesized by nucleophilic substitution reaction from furfuryl
3

Q. Zhang et al.
Dyes and Pigments 184 (2021) 108652
Fig. 4. a) Molecular structure (ORTEP) of dye 1 determined by single crystal X-ray diffraction. Thermal ellipsoids are drawn at 50% probability level. Crystal packing
diagrams of dye 1 along the a-axis (b), along the b-axis (c) and along the c-axis (d). Intermolecular donor-acceptor dipole interactions are indicated by red dotted lines
in (b) and (c). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 5. a) Cyclic voltammograms of dyes 1 and 2, c = 0.005 M; b) Cyclic voltammograms of dyes 3 and 4, c = 0.005 M. Solvent: dichloromethane (0.1 M Bu4NBF6),
Scanning rate: 100 mVsꢀ 1 (25 ◦C).
Table 2
Redox potentials of donors and acceptors (Eox(D), Ered(A)), highest occupied molecular orbital (EHOMO) and lowest unoccupied molecular orbital (ELUMO) energy levels,
energy gaps (ΔEg = ELUMO-EHOMO), excitation energy of donor (E00(D*)), photo-induced charge transfer free energy change (ΔG) of the dyes 1–4.
Dyes
Eox(D)/V
Ered(A)/V
EHOMOb/eV
ELUMOb/eV
ΔEg/eV
E00(D*)/eV
ΔG/eV
1
2
3
4
1.14
1.18
1.17
1.19
ꢀ 1.25
ꢀ 5.45
ꢀ 5.49
ꢀ 5.48
ꢀ 5.50
ꢀ 3.06
ꢀ 3.07
ꢀ 1.73
ꢀ 1.73
2.39
2.42
3.75
3.77
3.32
3.26
3.32
3.32
ꢀ 0.99
ꢀ 1.24
ꢀ 0.90
a
a
a
a
a No data was obtained due to reduction potentials beyond the electrochemical measurement windows.
b
Determined from cyclic voltammetry measurements. EHOMO = ꢀ [4.78 eV + Eox (D) - EFc/Fc+], ELOMO = ꢀ [4.78 eV + Ered (A) - EFc/Fc+], in which EFc/Fc+ is the
oxidation potential of ferrocene.
4

Q. Zhang et al.
Dyes and Pigments 184 (2021) 108652
Fig. 6. Energy levels of HOMO and LUMO, energy gaps (ΔEg), and electron cloud distribution of dyes 1–4 obtained by electrochemical CV measurements.
interaction [46]. The detailed crystal structure packing data of 1 were
provided in Supporting Information (Table S2).
3.2. Electrochemical properties
To investigate the donor-acceptor interaction of the driving force of
molecular assembly, we performed electrochemical cyclic voltammetry
(CV) measurements. All these dyes show reversible oxidation peaks of
triphenylamine donors at 1.14–1.19 V (Fig. 5). Dyes 1 and 2 display the
irreversible reduction peaks of acceptors at ꢀ 1.25 V (Fig. 5a). The
relative low reduction potentials of maleimide and itaconimide accep-
tors suggest that they both possess an electron-accepting ability. For
dyes 3 and 4, the reduction potentials of acceptors are calculated to be
ꢀ 2.55 V by molecular frontier orbital calculations, which is beyond the
electrochemical measurement windows (ꢀ 2 V–2 V) of solvent
dichloromethane. Thus, their reduction peaks are not shown in cyclic
voltammograms (Fig. 5b). The electrochemical data of these dyes 1–4
were listed in Table 2.
On the basis of the oxidation and reduction potentials of these dyes
obtained from electrochemical CV measurements, we thus obtained the
highest occupied molecular orbital (EHOMO) and the lowest unoccupied
molecular orbital (ELUMO) energy levels (Table 2 and Fig. 6). The HOMO
and LUMO energy levels are closely related to ionization potential and
electron affinity, which determine the chemical stability of these dyes.
The energy gap ΔEg between HOMO and LUMO energy levels is a sig-
nificant parameter in determining the molecular stability and the elec-
tron transport property. In general, a small energy gap ΔEg implies the
molecule possesses a strong polarity with a high chemical reactivity and
a low kinetic stability.
Fig. 7. Irreversible emission ON/OFF behaviours for isomerization of dye 2. a)
Isomerization of dye 2. Reagents and conditions: N,N-dimethyl acetamide
(DMAC), 120 ◦C, 3h. b) Fluorescence spectra in dichloromethane before and
after
isomerization
of
dye
2
in
1,
2-dichloroethane,
[2] = 5 × 10ꢀ 5 M, λex = 310 nm.
characteristic of dyes 1 and 2, and their strong acceptor suggest that
charge transfer interactions are relatively strong in 1 and 2 [47]. The
detailed measurement data were summarized in Table 2.
Rehm-Weller equation is usually used to estimate the charge transfer
ability of donor-acceptor molecules [48,49]. The free energy change of
charge transfer (ΔG) of these dyes were calculated by using
Rehm-Weller equation (1):
The HOMO energy levels of dyes 1–4 show approximate values due
to the same triphenylamine donors they possess, while, the LUMO en-
ergy levels are quite different. LUMO energy levels of dye 1 and dye 2
are relative low, thus, the HOMO and LUMO energy gap (ΔEg) of dye 1
and dye 2 are small enough for the formation of charge transfer state.
This result further give reasonable interpretation for completely fluo-
rescence quenching of dye 1 by intramolecular donor-to-acceptor charge
transfer. Adducts 3 and 4 both possess high LUMO values indicating that
electron affinity of acceptor is very weak. Thus, charge transfer inter-
action is weak for dyes 3 and 4. HOMO and LUMO distributions of all
dyes 1–4 are segregative (Fig. 6). The HOMO orbitals are mainly
distributed in electron-rich triphenylamine chromophores, while, the
LUMO orbitals are distributed in acceptors. The segregative
ΔG = Eox(D)-Ered(A)-E00(D*)-C
(1)
where, Eox(D) and Ered(A) are the redox potentials of the electron donor
(D) and acceptor (A), respectively; E00(D*) is the energy of the 0-
0 transition of the lowest excited state of the triphenylamine donor; C
is stabilization energy of solvents acting with ions (0.06 eV or 1.4 kcal/
mol). The negative or positive value of ΔG is regarded as a significant
parameter to judge if the photo-induced charge transfer can occur [50].
The ΔG data of dyes 1 and 2 were calculated and summarized in Table 2.
5

Q. Zhang et al.
Dyes and Pigments 184 (2021) 108652
Fig. 8. Reversible emission switch ON/OFF for
mutual conversion between dyes 1 and 3 by revers-
ible Diels-Alder reaction. a) Reversible Diels-Alder
addition from dye 1 to dye 3. b) Emission spectra
during Diels-Alder reaction from dye 1 to 3 in
toluene: 0, 5, 10, 20, 30, 40, 50 and 60 h from bottom
to top, respectively; c = 1.1 × 10ꢀ 5 M, λex = 310 nm.
c) Emission spectra during retro-Diels-Alder reaction
from dye 3 to 1 in toluene: 1, 2, 3, 4, 6, 8, 10, 12 and
72 h
from
top
to
bottom,
respectively;
c = 1.1 × 10ꢀ 5 M, λex = 310 nm. d) Photograph of the
dichloromethane solution of dyes 1 and 3 under ul-
traviolet lamp (365 nm); c = 0.1 M.
Dyes
1
and
2
both show negative ΔG value, revealing that
to OFF as the backward reaction proceeds (Fig. 8c). Therefore,
Diels-Alder addition is a fluorescence turn ON reaction, and retro--
Diels-Alder addition is a fluorescence turn OFF reaction. A reversible
emission switch ON/OFF was observed for the thermally reversible
Diels-Alder reaction. Recently, several classes of photo-induced charge
transfer dye molecules have been reported including crown ether,
anthracene, and pyrazoline derivatives for the detection of protons,
metal ions, and reactive oxygen et al. [52–54]. Most of the data storage
are inerasable because of irreversible emission properties. Our revers-
ible emission properties based on dynamic reaction provide a valuable
guideline for the rational design of a new class of fluorescence switches
and development of erasable data storage devices.
photo-induced charge transfer can occur in dyes 1 and 2.
3.3. Dynamic dye systems with switchable ON/OFF emission
In N,N-dimethyl acetamide (DMAC) at 120 ◦C for 3 h, itaconimide
acceptor of dye 2 is isomerized into more stable citraconimide (methyl
maleimide), which is a stronger acceptor (Fig. 7a). After the itaconimide
isomerization, the fluorescence was found to be completely quenched
(Fig. 7a and b). This can be reasonably explained from the charge
transfer quenching. The citraconimide acceptor has a stronger electron-
accepting ability than itaconimide acceptor. Therefore, the charge
transfer interaction becomes stronger after isomerization, which leads to
a significant fluorescence quenching. For the isomerization reaction of
dye 2, the fluorescence was switched from ON to OFF (Fig. 7b). How-
ever, the emission ON/OFF is irreversible because isomerization reac-
tion is irreversible. More stable citraconimide acceptor cannot be
converted back into itaconimide acceptor.
To reveal dynamic dye characteristics, we performed dynamic furan
exchange reaction in acetonitrile-d3 (CD3CN) at 75 ◦C. The dynamic
reaction process was monitored by 1H NMR spectroscopy (Fig. 9). As the
reaction proceeds, the 1H NMR peaks of 3 at δ 6.55–6.64 (He, f), δ 5.35
(Hg), δ 4.27–4.33 (Hh) and δ 3.93–4.00 ppm (Hi) and α-position furan
substitution of 3 at δ 6.28 ppm (Hb) gradually decrease. Meanwhile, the
1H NMR peaks of the protons of 4 at δ 5.12 (Ha’), δ 6.09 (Hb’), δ 5.30
(Hd’) and δ 4.50 ppm (Hj’) and β-position furan substitution of 4 at δ
6.30–6.36 ppm (He’, f’) gradually appear and increase as the dynamic
exchange reaction proceeds. These results reveal that β-position furan
The irreversible fluorescence isomerization is interesting, while, the
reversible fluorescence switch reaction is more attractive. The single
bond strength between diene and dienophile of the Diels-Alder adduct is
much lower than normal covalent single bonds [51], which readily leads
to a retro-Diels-Alder addition occurring at an elevated temperature.
Therefore, an attractive characteristic of this Diels-Alder reaction is its
thermal reversibility. Dye 1 was reacted with a furan derivative
(2-{2-[2-(ethoxy)ethoxy]ethoxy}methyl-furan) at ambient temperature
to give adduct 3 (Fig. 8a). Before the Diels-Alder addition, no fluores-
cence could be observable for this system, where the fluorescence of
triphenylamine chromophore was completely quenched. After the
Diels-Alder addition, the double bond acceptor that can quench fluo-
rescence was consumed (Fig. 8a). Therefore, as the forward reaction
proceeds, the fluorescence appears, and is turned ON (Fig. 8b). Inter-
estingly, the emission behaviour is reversible. When the Diels-Alder
adduct dye 3 was heated to 75 ◦C in toluene, a retro-Diels-Alder addi-
tion occurred according to 1H NMR spectroscopy (Supporting Informa-
tion Fig. S7). In this case, electron-accepting double bond was formed
again. The fluorescence was gradually quenched, and switched from ON
chain is more competitive than
change reaction, where β-position furan chain has reducing steric hin-
drance and optimizing spatial configuration. -position furan chain of 3
α-position furan chain in dynamic ex-
α
was finally replaced by β-position furan chain, thus converted into dye 4
by furan moiety exchanges (Fig. 9). The dynamic characteristic of the
dye reaction enables furan exchange continuously under thermody-
namic equilibrium condition to form energy-minimized and optimized
molecular structures.
4. Conclusion
In summary, four triphenylamine-based dyes were synthesized by
Diels-Alder cycloadditions. The studies on the optical and electro-
chemical properties of these dyes revealed the mechanisms of their
fluorescence switches. Isomerization reaction of itaconimide dye is
6

Q. Zhang et al.
Dyes and Pigments 184 (2021) 108652
Fig. 9. Dynamic furan moiety exchanges during the conversion of
α
-position furan-substituted dye 3 into β-position dye 4 monitored by 1H NMR spectroscopy .
[3] = 0.05 M. Reaction time are 0, 3, 6, 12, 24, 36, 48 and 72 h from top to bottom, respectively.
irreversible, which leads to an irreversible emission turn OFF behav-
iours. Diels-Alder reactions of these dyes are reversible, which leads to a
reversible emission turn OFF/ON behaviours. Furthermore, we
demonstrate the dynamic dye systems of furan moiety exchanges based
on dynamic emission turn ON reactions, where dye molecules were
mutually converted, and energy-minimized, more stable, optimized
molecular structure was finally formed. Our results presented here may
encourage the effort for development of dynamic light-emitting dye
systems and their potential applications for self-healing, recyclable,
renewable materials as well as erasable data storage devices.
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Acknowledgements
National Natural Science Foundation of China (grant number
21674079 and 21975177) is acknowledged for financial support.
Appendix A. Supplementary data
The supporting information is available free of charge on website.
Experimental procedures, synthesis, NMR spectra and HRMS of dyes 2,
3, 4 (PDF). X-ray crystallographic data of dye 1 (CIF).
CRediT authorship contribution statement
Qi Zhang: Data curation, Formal analysis, Investigation. Ying
Wang: Investigation, Methodology, Writing - original draft. Junbo
Gong: Conceptualization, Funding acquisition, Project administration,
Supervision. Xin Zhang: Conceptualization, Supervision, Writing - re-
view & editing.
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.dyepig.2020.108652.
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