Modulating optical and electrochemical properties of perylene dyes by twisting aromatic π-system structures

Article abstract of DOI:10.1016/j.dyepig.2021.109261

Three highly fluorescent perylene bisimide dyes were synthesized, where aromatic π-system structures are twisted to different degree by steric hindrance of two or four substitution groups at bay position. Light-emitting colours of these dye solutions can be modulated from green, yellow to red, and their fluorescence quantum yields increase up to approximate 100% with increasing the π-system twisting, which can be considered for new class of wavelength-tunable dye lasers. π-Twisted dyes are more sensitive to microenvironment changes. Thus, they are better fluorescence probe and sensory materials than planar dyes. Electrochemical cyclic voltammetry measurements revealed that these dyes are suitable for n-type optoelectrical devices and materials. These dye solids display near infra-red emission at 600–850 nm. Owing to strong π-π stacking interaction, planar dye solid loses its outstanding optical properties compared to its solution. In contrast, π-twisted dye solids retain their excellent optical properties including narrow emission bands and relatively high fluorescence quantum yields due to the suppression of π-π stacking interaction. Exceptional fluorescence polarization phenomena were observed for these π-twisted dye solids. These optical results revealed that π-twisted perylene bisimide dyes are more excellent optical materials than planar dyes.

Full text of DOI:10.1016/j.dyepig.2021.109261

Dyes and Pigments 189 (2021) 109261
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: http://www.elsevier.com/locate/dyepig
Modulating optical and electrochemical properties of perylene dyes by
twisting aromatic π-system structures
Ying Wang, Qi Zhang, 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:
Three highly fluorescent perylene bisimide dyes were synthesized, where aromatic
π-system structures are
Fluorescence
twisted to different degree by steric hindrance of two or four substitution groups at bay position. Light-emitting
colours of these dye solutions can be modulated from green, yellow to red, and their fluorescence quantum yields
Dyes
Perylene bisimides
Fluorescence quantum yields
Fluorescence probes
Polarization
increase up to approximate 100% with increasing the
wavelength-tunable dye lasers.
π-system twisting, which can be considered for new class of
π
-Twisted dyes are more sensitive to microenvironment changes. Thus, they are
better fluorescence probe and sensory materials than planar dyes. Electrochemical cyclic voltammetry mea-
surements revealed that these dyes are suitable for n-type optoelectrical devices and materials. These dye solids
display near infra-red emission at 600–850 nm. Owing to strong
π
-π
stacking interaction, planar dye solid loses its
-twisted dye solids retain their excellent
optical properties including narrow emission bands and relatively high fluorescence quantum yields due to the
suppression of stacking interaction. Exceptional fluorescence polarization phenomena were observed for
these -twisted dye solids. These optical results revealed that -twisted perylene bisimide dyes are more excellent
optical materials than planar dyes.
outstanding optical properties compared to its solution. In contrast,
π
π-π
π
π
1. Introduction
incorporation of large bulky substituent groups at imide or bay-position
of perylene dyes [32–34]. Although the syntheses of
π -twisted perylene
The information determining optical and optoelectronic properties
of functional dye materials is generally encoded in chemical structures,
molecular shapes and arrangements [1–6]. Therefore, a deep under-
standing of the structure-property relationship is significant to rational
dyes have been frequently reported from organic and supramolecular
chemists [35], the purpose of most of these syntheses is to improve the
solubility of perylene dyes. The systematic studies on the effect of aro-
matic
π-system twisting on optical behaviors are scarce.
design of
π
-system molecules to achieve high performance optical and
Generally, the twisting of aromatic
π
-systems is challenging, and a
optoelectronic devices and materials [7–10]. Optical materials based on
large degree of twisting may even destroy conjugated π-system structure
π
-system dyes recently have attracted much interest for their diverse
[36]. In our earlier work, we investigated the relationship between ag-
gregation morphologies and molecular structures, and reported the
precision morphology control of supramolecular polymers by twisting
applications such as bio-labeling [11,12], dye lasers [13], light-emitting
diodes [14], photovoltaic cells [15–21], organic field-effect transistors
[22,23], sensors [24–29]. Perylene bisimide dyes are polycyclic, aro-
conjugated π-system plane. Planar π-system dyes self-assembled into
matic, extended
rings, which contains the smallest
The strong tendency toward
π
-conjugated molecules consisting of five fused benzene
light-emitting rigid rod-like supramolecular polymers by strong
π-π
π
-system graphene fragment [30,31].
stacking [37], and twisted -system dyes self-assembled into flexible
π
π
-
π
stacking of aromatic -system dyes leads
π
worm-like supramolecular polymers [38]. We further developed their
functionalization and reported white-light emitting vesicles [25] and
superhelix [39]. These exceptional optical and optoelectronic properties
motivated us to explore the property-structure relationship. In this work,
we designed and synthesized three perylene dyes with different twist
to their low solubility and dramatically optical quenching in the solid
state, which severely limits their practical applications. Thus, a chemical
modification method was recently developed to obtain excellent optical
properties of solid states by preventing π-π stacking interaction, that is,
* Corresponding author.
** Corresponding author.
E-mail addresses: junbo_gong@tju.edu.cn (J. Gong), xzhangchem@tju.edu.cn (X. Zhang).
https://doi.org/10.1016/j.dyepig.2021.109261
Received 21 December 2020; Received in revised form 2 February 2021; Accepted 14 February 2021
Available online 23 February 2021
0143-7208/© 2021 Elsevier Ltd. All rights reserved.

Y. Wang et al.
Dyes and Pigments 189 (2021) 109261
angles between two naphthalene units, and investigated their optical
and electrochemical properties of solution and solid states. In particular,
we demonstrate the optical property modulation of functional dye ma-
purified by silica gel column chromatography with dichloromethane/n-
hexane (1/1) as eluent to give red solid (3.98 g). Yield: 72%. 1H NMR
(400 MHz, CDCl3, ppm, Fig. S10): δ = 8.15 (s, 4H, ArH), 7.26 (t, 8H,
ArH), 7.10 (t, 4H, ArH), 6.93 (d, 8H, ArH), 4.92 (t, 2H, 2-CH(CH2-)2),
2.44 (dd, 4H, 2-CH2-), 1.83 (d, 4H, 2-CH2-), 1.67 (d, 6H, 3-CH2-), 1.37
(dd, 6H, 3-CH2-). 13C NMR (CDCl3, ppm, Fig. S11): δ = 163.74, 155.90,
155.43, 132.82, 130.0, 127.08, 124.56, 123.21, 120.37, 120.03, 119.98,
54.05, 29.16, 26.55, 25.48. MS (MALDI-TOF, Fig. S12) calculated for
terials from π-twisting viewpoint.
2. Experimental section
2.1. Synthesis
C
60H46N2O8: 922.33; found: 922.11 [M+]. IR: ṽ (cmꢀ 1) = 3040.3,
N,N′-di(1,3-bis(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)prop-2-yl)-
perylene-3,4:9,10- tetra carboxylic bisimide (1): 2,5,8,11,15,18,21,24-
Octaoxa-13-pentacosane-amine (1.055 g, 2.75 mmol), perylene-
3,4:9,10-tetracarboxylic acid bisanhydride (400 mg, 1.02 mmol) and
zinc acetate (0.06 g) were mixed with imidazole (5 g). The reaction
mixture was stirred at 150 ◦C for 6 h under argon. After cooling to room
temperature, 2 N HCl (30 mL) was added to the above mixture. The
solution was extracted with 50 mL chloroform for three times. The
combined chloroform solution was dried with anhydrous MgSO4 and
then the solvent was evaporated. The crude product was further purified
by silica gel column chromatography with dichloromethane/methanol
((96/4) → (93/7)) as eluent to give final product (178 mg). Yield: 16%.
1H NMR (400 MHz, CDCl3, ppm, Fig. S2): δ = 8.48 (dd, 8H, ArH),
5.75–5.64 (m, 2H, 2-CH(CH2-)2), 4.20, 3.97 (dd, 8H, 2-CH(CH2-)2),
3.75–3.45 (m, 48H, 24-OCH2-), 3.31 (s, 12H, 4-OCH3). 13C NMR (CDCl3,
ppm, Fig. S3): δ = 163.81, 134.42, 131.40, 129.46, 126.25, 123.46,
123.02, 71.94, 70.51, 69.40, 58.98, 52.29. MS (MALDI-TOF, Fig. S4)
calculated for C58H78N2O20, 1122.51; found: 1145.63 [M + Na+]. IR: ṽ
(cmꢀ 1) = 2916.3, 2867.4, 1693.8, 1653.1, 1593.1, 1574.8, 1435.8,
1403.5, 1343.1, 1249.9, 1100.1, 857.7. UV–vis absorption (dichloro-
2929.1, 2852.5, 1691.2, 1649.9, 1585.9, 1486.4, 1412.3, 1341.1,
1285.1, 1199.7, 1166.8, 1075.9, 1024.9, 981.5, 891.1, 871.86. UV–vis
absorption (dichloromethane): λ
(
ε
, Mꢀ 1cmꢀ 1) = 569 nm (4.77×104),
ab
530 nm (3.04×104), 442 nm (1.65×104), [3] = 2×10ꢀ 6 M. Fluorescence
(dichloromethane, λex = 450 nm): λ = 603 nm, [3] = 2×10ꢀ 6 M.
em
3. Results and discussion
3.1. Design and synthesis of π-system perylene dyes
In this work, we synthesized planar perylene dye 1 and two π-twisted
dyes 2 and 3 (Fig. 1). Dye 1 is a rigid, planar -system structure. We
π
initially designed and synthesized a planar perylene bisimide dye with
the same cyclohexyl substituent group at imide position as dyes 2 and 3.
However, the solubility of planar perylene dye with the same cyclohexyl
group is too poor, which cannot be purified for further measurements.
To increase its solubility, we chemically attached long soft polyethylene
glycol (PEG) chains to the perylene core at imide position. For dyes 2
and 3, the perylene π-system planes are twisted by steric hindrance of
two and four substitution groups at bay (1,7 and 1,6,7,12) positions,
respectively. For dye 2, we initially designed and synthesized perylene
bisimide dye with the same phenoxy substituent group at bay position as
dye 3. However, the perylene dye with the same phenoxy group has a
methane): λab
(
ε
, Mꢀ 1cmꢀ 1) = 525 nm (8.43×104), 489 nm (5.13×104),
458 nm (1.92×104), [1] = 2×10ꢀ 6 M. Fluorescence (dichloromethane,
λex = 450 nm): λem = 534 nm, [1] = 2×10ꢀ 6 M.
N,N′-dicyclohexyl-1,7-di(3-methoxyphenoxy)perylene-3,4:9,10-tet-
racarboxylic acid bisimide (2): N,N′-dicyclohexyl-1,7-dibromoperylene-
3,4:9,10-tetracarboxylic dianhydride (1 g, 1.4 mmol), K2CO3 (606 mg,
4.39 mmol) and 3-methoxyphenol (1.51 g, 12.2 mmol) were added into
N-methyl pyrrolidone (NMP, 60 mL). The reaction mixture was stirred at
120 ◦C for 1 h, then was cooled to room temperature. HCl aqueous (100
mL, 8 vol-%) was slowly added dropwise to the reaction solution with
stirring, and the crude product precipitated out and was washed with
water then dried under vacuum. The crude product was purified by silica
gel column chromatography with dichloromethane/n-hexane (2/1) as
eluent to give red solid (0.73 g). Yield: 65%. 1H NMR (400 MHz, CDCl3,
ppm, Fig. S6): δ = 9.50 (d, 2H, ArH), 8.58 (d, 2H, ArH), 8.26 (d, 2H,
ArH), 7.37–7.28 (m, 2H, ArH), 6.84–6.75 (m, 2H, ArH), 6.71 (dd, 4H,
ArH), 5.08–4.88 (m, 2H, 2-CH(CH2-)2), 3.83 (s, 6H, 2-CH3), 2.61–2.39
(m, 4H, –CH(CH2-)2), 1.88 (d, 4H, –CH(CH2-)2), 1.78–1.21 (m, 12H, 6-
CH2-). 13C NMR (CDCl3, ppm, Fig. S7): δ = 163.80, 163.36, 161.61,
156.32, 155.02, 133.25, 131.02, 130.29, 129.22, 128.81, 125.35,
124.52, 124.40, 123.95, 122.88, 111.54, 110.52, 105.94, 55.60, 54.13,
29.18, 26.58, 25.50. HRMS (ESI, Fig. S8): calculated for C50H42N2O8:
798.2941; found: 799.3025 [M+H+]. IR: ṽ (cmꢀ 1) = 2917.0, 2851.9,
1697.9, 1651.7, 1597.9, 1489.0, 1448.8, 1407.3, 1326.5, 1257.1,
1197.6, 1171.5, 1133.1, 1035.9, 984.4, 944.5, 918.5, 893.1. UV–vis
π
-twisted angle of only 0–7.1◦. This twist angle is too small, which
cannot better distinguish with dye 1. Thus, 3-methoxyphenoxy was used
as the substituent group at bay position for dye 2, where the π-twisted
angle is 13.4◦. Finally, the average dihedral twist angles between two
naphthalene units are ~0◦, 13.4◦ to 27.0◦ for dyes 1, 2 and 3, respec-
tively, according to density functional theory calculations at B3LYP/6-
31G (d) level (DFT) (Fig. 1) [40,41].
Dye 1 was synthesized by a five-step reaction procedure (Fig. 2a).
The long soft chain PEG-amine was synthesized from the nucleophilic
substitution of triethylene glycol monomethylether with p-toluene-
sulfonyl chloride, and followed by Williamson ether synthesis of 2-(N,N-
di-benzylamino)-1,3-propanediol and 2-[2-(2-methoxyethoxy) ethoxy]
ethyl p-toluene sulfonate in the presence of sodium hydride and reduc-
tion reaction with the catalysis of Pd/C. Dye 1 was finally obtained
through the reaction of PEG-amine with perylene tetracarboxylic acid
dianhydride with the catalysis of zinc acetate [42]. The final step re-
action has a low synthetic yield of only 16%. The low solubility of
starting perylene dianhydrides, and the shielding effect of
amine-reactive site by long soft PEG chains may be responsible for low
synthetic yield.
Dye 2 was synthesized in a three-step reaction procedure from 1,7-
dibromoperylene-3,4:9,10-tetracarboxylic dianhydride, which was ob-
tained through brominated reaction of commercially available 3,4:9,10-
perylenetetracarboxylic dianhydride. Imidization of 1,7-dibromopery-
lene-3,4:9,10-tetracarboxylic dianhydride with cyclohexylamine in the
presence of acetic acid in NMP gave corresponding dibromoperylene
bisimide. Dye 2 was finally obtained through the nucleophilic substi-
tution reaction of dibromoperylene bisimide with 3-methoxyphenol in
the presence of potassium carbonate with a yield of 65% (Fig. 2b). The
synthesis of dye 3 started from tetrachloroperylene, i.e. 1,6,7,12-tetra-
chloro-perylene-3,4:9,10-tetracarboxylic acid dianhydride (Fig. 2c)
[32]. By nucleophilic addition-elimination reaction of perylene tetra-
carboxylic acid dianhydride with cyclohexylamine, the perylene bisi-
mide precusor was obtained with a yield of 92%. The four phenoxyl
(dichloromethane): λab
(
ε
, Mꢀ 1cmꢀ 1) = 537 nm (5.40×104), 503 nm
(3.74×104), 398 nm (0.96×104), [2] = 2×10ꢀ 6 M. Fluorescence (λ
450 nm): λem = 567 nm (dichloromethane), [2] = 2×10ꢀ 6 M.
=
ex
N,N′-dicyclohexyl-1,6,7,12-tetraphenoxyperylene-3,4:9,10-tetra-
carboxylic acid bisimide (3): N,N′-dicyclohexyl-1,6,7,12-tetra-
chloroperylene-3,4:9,10-tetracarboxylic acid bisimide (4.14 g, 6 mmol),
K2CO3 (4.22 g) and phenol (2.82 g, 30 mmol) were added to N-methyl
pyrrolidone (NMP, 80 mL). The reaction mixture was stirred under
argon at 120 ◦C for 20 h. After cooling to room temperature, the
resulting reaction mixture was poured into HCl solution (400 mL, 8 vol
%) in water with stirring. The crude product precipitated out and was
washed with water then dried under vacuum. The crude product was
2

Y. Wang et al.
Dyes and Pigments 189 (2021) 109261
Fig. 1. Chemical structures of perylene bisimide dyes 1, 2, 3 with different π-twisted angles.
Fig. 2. a) Synthetic route of perylene bisimide dye 1. Reagents and conditions: (1) HSO3Cl, 0 ◦C, 4 h, 92%; (2) triethylene glycol monomethylether, Et3N, CH2Cl2, RT,
20 h, 60%; (3) 2-(N, N-di-benzylamino)-1,3-propanediol, NaH, THF, reflux, 16 h, 84%; (4) MeOH, Pd/C, H2, 16 h, RT, 72%. (5) Zn (OAc)2, imidazole, 150 ◦C, 6 h, Ar,
16%; b) Synthetic route of dye 2. Reagents and conditions: (1) H2SO4, I2, Br2, 85 ◦C, 24 h, 87%. (2) NMP, AcOH, cyclohexylamine, 85 ◦C, 8 h, Ar, 75%. (3) K2CO3, 3-
methoxyphenol, NMP, 120 ◦C, 1 h, Ar, 65%; c) Synthetic route of dye 3. Reagents and conditions: (1) AcOH, cyclohexylamine, NMP, 85 ◦C, 8 h, Ar, 92%. (2) K2CO3,
NMP, phenol, 120 ◦C, 20 h, Ar, 72%.
groups were then attached to bay position of peylene core through the
nucleophilic substitution reaction of the above tetrachloride perylene
bisimide precusor with phenol in the presence of potassium carbonate to
give the final dye 3. Chemical structures of dyes 1, 2 and 3 were fully
characterized by 1H and 13C NMR spectroscopy, high resolution or
MALDI-TOF mass spectroscopy, and IR spectroscopy.
3.2. Optical properties of perylene dye solutions
Highly fluorescent dye solutions have practical applications as op-
tical materials for dye lasers, fluorescence probes and sensors, biological
imaging. Thus, we first studied the optical properties of these dye so-
lutions. All these compounds 1, 2, 3 display excellent dye absorption
characteristics, i.e., high extinction coefficient (~105 Mꢀ 1cmꢀ 1) within
visible-light region. Planar
π-system dye 1 shows a well-defined S0–S1
electronic transition absorption band with a maximum wavelength of
3

Y. Wang et al.
Dyes and Pigments 189 (2021) 109261
525 nm (Fig. 3a). In contrast, the absorption spectra of π-twisted dyes 2
and 3 have no fine structures, and display spectroscopic broadening. An
absorption red-shift was observed from 525 nm (dye 1), 537 nm (dye 2)
to 569 nm (dye 3) with increasing the π-twisted angles. Thus, we can
observe different colours for these dyes under sunlight by naked eyes
(Fig. 3a inset). In addition, dye 3 exhibits S0–S2 transition absorption
band around 440 nm, which is symmetry-forbidden for planar perylene
dyes [43]. We attributed the gradual absorption red-shifts of these dyes
to the twisting of conjugated π-system structures, which increases the
highest occupied molecular orbital (HOMO) energy level and hence
decrease the energy gap between the HOMO and the lowest unoccupied
molecular orbital (LUMO) energy levels. The HOMO and LUMO energy
gaps (Eg) of these dyes can be obtained from their UV–vis absorption
wavelength according to the equation Eg(eV) = 1240/λ
[44].
Energy gaps (Eg) decrease in the order of 1 (2.29 eV), 2ab(2, .o1n8seteV), 3
Fig. 4. Changes of optical properties (absorption maximum wavelength (λab),
emission maximum wavelength (λem), Stokes shifts and fluorescence quantum
(2.06 eV) with increasing π-twisted angles. These results were further
confirmed by following electrochemical measurements.
yields (Φ)of dyes 1, 2, 3 with increasing the π-twisted angles.
These perylene bisimide dyes are highly photoluminescent. Dye 1
has a planar, rigid chromophore structure with high molecular sym-
metry, which leads to well-resolved, finely structural, three narrow
emission bands at 534 nm, 575 nm and 623 nm (Figs. 3b and 4).
because that the more flexible conformations cannot be distinguished in
their excited states. The emission spectra of these dyes display approx-
imately mirror image relationship with their absorption spectra. The
emission bands of these dyes show gradual red-shifts from 534 nm to
603 nm, and Stokes shifts increase from 9 nm to 35 nm with increasing
π
-Twisted dyes 2 and 3 show structureless, broad emission bands. Dyes 2
and 3 have
π-twisted and flexible structures with lower molecular
symmetry, which makes them lose their fine structures of emission
the π-twisted angles. All these emission bands fall within the visible light
bands. Three emission bands cannot be well resolved in dyes 2 and 3
region. Thus, we can observe different light-emitting colours under UV
Fig. 3. a) UV–vis absorption spectra of dyes 1, 2, 3 in
dichloromethane, Conc: 2×10ꢀ 6 M. Inset: photo-
graphs of dichloromethane solutions of dyes 1, 2, 3
under sunlight, Conc: 4×10ꢀ 5 M; b) Fluorescence
spectra of dyes 1, 2, 3 in dichloromethane, Conc:
2×10ꢀ 6 M, λex
= 450 nm; c) Photographs of
dichloromethane solutions of dyes 1, 2, 3 under 365
nm UV lamp, Conc: 4×10ꢀ 5 M; d) CIE 1931 chro-
maticity diagram, three points are light-emitting
colour coordinates of 1 (0.38, 0.60), 2 (0.52, 0.48)
and 3 (0.63, 0.37) in dichloromethane, Conc: 2×10ꢀ 6
M.
4

Y. Wang et al.
Dyes and Pigments 189 (2021) 109261
lamp, specifically, green, yellow, red (Fig. 3c) for dyes 1, 2, and 3 with
light-emitting colour coordinates of (0.38, 0.60), (0.52, 0.48) and (0.63,
0.37), respectively (Fig. 3d). Changes of emission wavelength (colours)
with common fluorescence probes, the advantage of our dyes is high
sensitivity to environment changes, which is even directly observable by
naked eyes from luminescence colour changes within visible light
region.
of these dyes may be due to the increase in delocalized π-electron area
with increasing the
Interestingly, these dyes are becoming more and more bright with
increasing the -twisted angles, and their fluorescence quantum yields
π-twisted angles (Table 3).
3.4. Electrochemical properties of n-type perylene dyes
π
increase from 85.2% to close to 100% (Table 1, Fig. 4). Such highly light-
emitting efficiency within visible light region enable us to directly ‘see’
the emission colours of these dye solutions by naked eyes in the daytime
(Fig. 3a inset). Their fluorescence lifetimes slightly increase from 4.2 ns
to 6.2 ns from dyes 1 to 3. All optical data of these dye solutions are
listed in Table 1. From these optical data, we conclude that the twisting
We further explore electrochemical properties of our dyes as n-type
semiconductor or optoelectronic materials due to the electron-deficient
π
-system structures of perylene bisimide dyes. We performed cyclic
voltammetry measurements, these dyes exhibit two reversible reduction
waves at negative potentials, which are attributed to the formation of
radical anion and dianion of perylene bisimide rings (Fig. 6 and Table 2).
Reduction potentials of these dyes decrease in the order of 1 (ꢀ 0.67 eV)
> 2 (ꢀ 0.85 eV) > 3 (ꢀ 0.91 eV). These low reduction potentials show
that our perylene bisimide dyes are suitable candidate for n-type opto-
electronic materials.
of conjugated π-system structures has a significant influence on their
optical properties, especially for light-emitting colour changes. Our
light-emitting colour-tunable properties and high emission efficiencies
can be considered as a new class of wavelength-tunable dye lasers.
In addition, planar dye 1 do not show oxidation wave at positive
potentials, however, π-twisted dyes 2 and 3 show reversible oxidation
3.3. Environment-sensitive optical probes
waves. Dye 3 displays the lower oxidation potential (1.21 eV). Thus,
planar dye 1 is easiest to be reduced and hardest to be oxidized, in
We further investigated environment-sensitive ability of these per-
ylene dyes as new fluorescence probes. We measured UV–vis absorption
and emission spectra of these dyes in various polarity solvents at very
low concentration (2×10ꢀ 6 M) (Fig. 5 and Figs. S13–15). Environment-
sensitive ability of these dyes can be quantified by using the following
equation [47,48]:
contrast, π-twisted dye 3 is relatively hard to be reduced and relatively
easy to be oxidized. These conclusions are in good agreement with
molecular electrostatic potential calculations, in which the delocalized
electron density of these dyes increases in the order of 1 < 2 < 3.
π
Compared with planar dye 1, the high
π electron density of dye 3 is
responsible for the large bathochromic shift in its absorption and
emission spectra.
2 + constant
(1)
(2)
2
(μE
ꢀ
μG)
The HOMO and LUMO energy levels of these dyes were further
calculated by using their redox potentials according to the equations
[49]: ELUMO = -[4.8 eV + E-1/2 - EFc/Fc+], EHOMO = ELUMO - ΔEg. On the
basis of cyclic voltammetry measurement data, HOMO energy levels for
dyes 1, 2, 3 were obtained to be ꢀ 5.95 eV, ꢀ 5.66 eV, and ꢀ 5.48 eV,
respectively. The LUMO energy levels of dyes 1, 2, 3 were obtained to be
ꢀ 3.66 eV, ꢀ 3.48 eV and ꢀ 3.42 eV, respectively, which well falls in the
range of organic n-type materials for efficient electron injection into the
semiconductor [50]. All these experimental data are close to the DFT
calculation results (Table 2). HOMO and LUMO energy gaps (ΔEg)
Δv = vA ꢀ vF
=
Δf
hc
a3
ε
ε
ꢀ 1 n2 ꢀ 1
ꢀ
Δf =
+ 2 n2 + 2
where, h is the Planck’s constant; c is the speed of light; νA and νF are the
wavenumbers (cmꢀ 1) of the absorption and emission, respectively; Δf is
the orientation polarizability of solvents; μE and μG are the dipole mo-
ments of dyes in the excited and ground states, respectively; a is the
radius of dye fluorophore resides, which is calculated to be 6.13, 6.13
decrease with increasing the π-twisted angles in the order of 1 (2.53 eV)
and 6.07 Å for dyes 1, 2, 3, respectively. Stokes shift Δ
all these dyes was plotted against polarity function Δf (Fig. 5). The plot
slopes increase from planar dye 1 to -twisted dyes 2 and 3 (Fig. 5b, d, f).
According to the plot slopes and equation (1), we obtained the dipole
ν (Δν = νA-νF) of
> 2 (2.46 eV) > 3 (2.34 eV) (Fig. 7). The energy gap (ΔEg) values are
fully consistent with those obtained from their UV–vis absorption
spectroscopy. The narrowest energy gap for dye 3 well agrees with the
energy of its largest absorption wavelength among these dyes.
π
moment changes (Δμ = |μE -μG|), specifically, 2.48 D, 3.89 D and 3.14 D
for dyes 1, 2 and 3, respectively. 2.48 D, 3.89 D, 3.14 D are corre-
3.5. Optical properties of dye solids
sponding to the dipole moments of a unit charge separation of 0.52 Å,
0.81 Å, 0.65 Å, respectively. The dipole moment changes (Δμ) of these
We further investigated the optical properties of these dye solids,
which is desirable for the preparation of optical and optoelectronic de-
vices and materials [51–53]. All these dye solids show emission at
600–850 nm (Fig. 8), which is beyond visible red-light region, and
extended to near infra-red (NIR) region. Emission of planar dye 1
extremely changes from its solution to solid state. The solid emission
spectrum of dye 1 loses its well-resolved fine structures (Fig. 8a). A new,
red-shifted, broad emission band appears at 600–850 nm. Full-width at
half maximum (FWHM) of emission spectrum dramatically increases
from 21 nm to 156 nm. Emission wavelength shows a large bath-
ochromic shift (131 nm) and fluorescence quantum yield significantly
decreases from 85.2% to 9.9% from its solution to solid state (Table 3).
All these dramatic optical changes and quenching are attributed to
dyes may be due to the decrease in symmetry of
the increase in -structural flexibility from planar to twisted dyes. A
larger dipole moment change (Δ ) means the higher environment
polarity-sensitive ability. Therefore, the -twisted perylene dyes are
π-system structures, and
π
μ
π
more sensitive to environment changes than planar dye and π-twisted
dyes are more excellent fluorescence probes than planar dyes. Compared
Table 1
Average
π-twisted angles, UV–vis absorption maximum wavelength (λab),
extinction coefficient (ε), fluorescence emission maximum wavelength (λem),
Stokes shifts, fluorescence quantum yields (Φ) and lifetimes (
τ) of dyes 1, 2 and 3
in dichloromethane, Conc: 2×10ꢀ 6 M.
Dyes
Twist
λab
nm
/
ε
/Mꢀ 1cmꢀ 1
λem
nm
/
Stokes
Φa
τ/ns
angles
shifts/nm
π
-stack formation for planar dye 1 from its solution to solid state
(Fig. 8b). Dye 1 has a planar -conjugated structure, which favors a large
degree of stacking between adjacent molecules in solid state as
observed from X-ray crystal structure of its model compound (Fig. 8b).
~0◦
13.4◦
27.0◦
525
537
569
8.4×104
5.4×104
4.8×104
534
567
603
9
0.852
0.90
4.2
4.7
6.2
π
1
2
3
30
35
π-π
0.978
a
In contrast, π-twisted dyes 2 and 3 retain their excellent optical
Fluorescein (Φ = 0.95 in 0.1 N NaOH aqueous solution [45]) and N, N’ -bis
(2,6-diisopropylphenyl)-1,6,7,12-tetraphenoxyperylene-3,4:9,10-tetracarbox-
ylic acid bisimide (Φ = 0.96 in chloroform [46]) were taken as references.
properties in the solid states. Emission spectroscopic shapes and FWHM
of dyes 2 and 3 do not obviously change from solution to solid states
5

Y. Wang et al.
Dyes and Pigments 189 (2021) 109261
Fig. 5. Environment-sensitive optical properties. Changes of emission spectra and Stokes shifts (Δ
ν
) of dyes 1 (a, b), 2 (c, d) and 3 (e, f) with increasing solvent
polarity (Δf). Solvents: cyclohexane (CYH), methylcyclohexane (MCH), diethyl ether (Et2O), methyl tert-butyl ether (MTBE), ethyl acetate (EtOAc), tetrahydrofuran
(THF), toluene (TOL), dichloromethane (CH2Cl2), chloroform (CHCl3) and n-propanol (n-BuOH), Conc: 2×10ꢀ 6 M, λex = 450 nm.
Fig. 6. Cyclic voltammograms of dyes 1, 2, 3, solvent: dichloromethane (0.1 M Bu4NBF6), scanning rate: 100 mVsꢀ 1 (25 ◦C), c = 0.005 M.
Table 2
Redox potentials (E+1/2, E-1/2, E2-1/2), HOMO and LUMO energy levels, and energy gap (ΔEg) of dyes 1, 2, 3.
E+
E-1/2
E2-
HOMOb
/eV
LUMOb
/eV
ΔEg
HOMOd
/eV
LUMOd
/eV
ΔEg
c
d
Dyes
/V1/2
/V
/V1/2
/eV
/eV
a
1
2
3
ꢀ 0.67
ꢀ 0.85
ꢀ 0.91
ꢀ 0.99
ꢀ 1.06
ꢀ 1.14
ꢀ 5.95
ꢀ 5.66
ꢀ 5.48
ꢀ 3.66
ꢀ 3.48
ꢀ 3.42
2.29
2.18
2.06
ꢀ 5.98
ꢀ 5.61
ꢀ 5.27
ꢀ 3.45
ꢀ 3.15
ꢀ 2.93
2.53
2.46
2.34
1.33
1.12
a
b
No data was obtained.
Determined from cyclic voltammetry measurements by using Fc/Fc+ as a reference. ELUMO = -[4.8 eV + E-1/2 - EFc/Fc+], EHOMO = ELUMO - ΔEg, in which EFc/Fc+ is the
oxidation potential of ferrocene.
c
ΔEg = 1240/λab, onset
.
d
Obtained by the Gaussian calculation based on 6-31G (d).
6

Y. Wang et al.
Dyes and Pigments 189 (2021) 109261
Fig. 7. Energy levels of HOMO and LUMO as well as energy gaps (ΔEg) of dyes 1, 2, 3 determined from experimental redox potentials of cyclic voltammetry
measurements, and electron cloud distribution of dyes 1, 2, 3 obtained by density functional theory calculations at B3LYP/6-31G (d) level.
Fig. 8. Emission spectra of dyes 1 (a), 2 (c) and 3 (e) in dichloromethane (Conc: 2×10ꢀ 6 M, λex = 450 nm) and solid states (λex = 466 nm). Inset in (a), (c), (e):
photographs of solids of dyes 1, 2, 3 under 365 nm UV lamp. Dye molecular packing structures in solid state determined from single crystal X-ray diffraction of the
model compounds [54–56] of dyes 1 (b), 2 (d) and 3 (f).
except for red-shifts in emission wavelength (Fig. 8c and e). The solid
2 and 3 remain still highly luminescent, and their fluorescence quantum
yields (Φ = 32.2% and 21.9%) do not significantly decrease in com-
emission of largest π-twisted dye 3 displays the smallest emission red-
shift (64 nm) from solution to solid states. Solid emission bands of
dyes 2 and 3 do not become broad, and remain relatively narrow as their
molecularly dissolved monomer state in solution. In the solid state, dyes
parison with their solutions (Table 3). For these π-twisted dyes from
their solutions to solid states, relatively, slightly optical changes reveal
no distinct structural changes between ground and excited states, and no
Table 3
Average twist angles between two naphthalene units, emission maximum wavelength (λem), Full-width at half maximum (FWHM) and fluorescence quantum yields (Φ)
and anisotropy (r) of dyes 1, 2, 3 in dichloromethane solution and solid states.
Dyes
Twist angle
Delocalized
π
electron area/Å2
Solution
Solid
λem/nm
FWHM/nm
r
λem/nm
FWHM/nm
Φ
r
1
2
3
~0◦
36.1
36.5
36.8
534
567
603
21
71
65
0
0
0
665
644
667
156
104
95
0.099
0.322
0.219
~0
13.4◦
27.0◦
0.06
0.1
7

Y. Wang et al.
Dyes and Pigments 189 (2021) 109261
π
-stacking excimer emission species generate under excited state. Dyes 2
and 3 have twisted, -conjugated structures, which do not favor
π
π-π
stacking in solid state as observed from X-ray crystal structure of their
model compounds (Fig. 8d, f). Thus, the solids of dyes 2 and 3 retain
similar solution conformations. The emission specie of each molecule is
relatively isolated in solid state, and no excited state interaction occurs
between adjacent molecules. These prevent efficiently fluorescence
quenching. Thus, the solid emissions of
π-twisted dyes 2 and 3 still retain
outstanding optical properties as their molecularly dissolved monomer
states in solution. Our π-twisted dyes are excellent candidates as highly
luminescent optical materials. A recent study reported high fluorescence
quantum yield dye solids by attaching bulky substituent groups to per-
ylene core at one or two bay positions [34]. Our work is based on a
distinctly different viewpoint, that is, a large degree of π-twisting. In
particular, our work demonstrates tunable emission colours from green
to red, which are not achieved by the method of changing small sub-
stituent groups to larger ones at bay positions.
Fluorescence polarization is the phenomenon where light-emitting
intensity of a dye is different along different orientation of polariza-
tion, which is a very valuable property for optical materials such as
liquid crystal displays and optical storage devices [57]. Fluorescence
polarization phenomenon of optical dye materials can be quantified by
equations (3) and (4) [58]:
○
Fig. 9. Fluorescence anisotropy (r, ) spectra of dyes 1 (a), 2 (b) and 3 (c) in
solid states. IVV and IVH are the fluorescence intensities when the emission
polarizer is oriented parallel and perpendicular to the polarization direction of
the excitation light, respectively.
IVV ꢀ GIVH
r =
(3)
IVV + 2GIVH
IHV
IHH
G =
(4)
to the
optical properties with fluorescence polarization phenomenon in the
solid states due to the suppression of interactions. Our work provides
-twisting view-
π-π stacking. In contrast, π-twisted dyes retain their outstanding
where, IVV and IVH are the fluorescence intensities when the emission
polarizer is oriented parallel and perpendicular to the polarization di-
rection of the excitation light, respectively. G factor is the sensitivity
ratio of the detection system for vertically and horizontally polarized
light.
π-π
a rational molecular design guideline from aromatic
π
point to achieve high performance optical materials and optoelectronic
devices and materials.
In general, the anisotropy value of a fluorophore dye is within the
ranged between 0.4 (angle between absorption and emission dipole
moments is 0◦) and ꢀ 0.2 (angle between absorption and emission dipole
moments is 90◦) for single photon excitation. We measured fluorescence
polarization spectra of our dyes (Fig. 9). No polarization phenomenon
was observed for these dye solutions at room temperature. The solid
emission of planar dye 1 almost do not also display fluorescence po-
larization (Fig. 9a). This is because that solid emission of planar dye 1
Credit authorship statement
Ying Wang: Data curation, Formal analysis, Investigation, Writing -
original draft. Qi Zhang: Formal analysis, Investigation, Methodology.
Junbo Gong: Conceptualization, Funding acquisition, Project adminis-
tration, Supervision. Xin Zhang: Conceptualization, Supervision,
Writing - review & editing.
arose from closely π-π stacking, which leads to coherent energy migra-
Declaration of competing interest
tion/transfer and a depolarization effect. In contrast, high fluorescence
polarization was observed for dyes 2 and 3 (Fig. 9b and c). The anisot-
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
ropy values increase from 0.06 to 0.1 with increasing
π
-twisted angles.
-twisted
dyes can be reasonably interpreted as follows: In the solid states of dyes
2 and 3, their stacking degree is low, dye molecules are loosely
packed, and each one is relatively isolated and insulated, and no depo-
These interesting fluorescence polarization characteristics of
π
π-π
Acknowledgements
larization effect occurs. All these optical results revealed π-twisted per-
ylene bisimide dyes are more excellent optical materials than planar
dyes.
National Natural Science Foundation of China (grant number
21975177 and 21674079) is acknowledged for financial support.
4. Conclusion
Appendix A. Supplementary data
In summary, we synthesized three perylene bisimide dyes with
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.dyepig.2021.109261.
different
π-twisting degree between two naphthalene units. The twisting
of aromatic
π
-system structures has a remarkable influence on their
optical and electrochemical properties, especially for modulation of
light-emitting colours of these dyes from green, yellow to red. The op-
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9