Novel fluorescence-enhancing effect based on light-induced E/Z isomerization of perylene diimides with Schiff-base groups on bay-positions

Article abstract of DOI:10.1016/j.dyepig.2017.01.048

Three novel perylene Schiff-base derivatives 5a-5c were synthesized and characterized in yields of 80–85%. They exhibited reversible trans/cis isomerization after UV-light irradiation and heating. The fluorescence-enhancing effects were firstly observed for perylene trans-to-cis isomerization. The increasing steric hindrance based on the restricted intramolecular rotation of trans-to-cis isomerization was responsible for the fluorescence-enhancing effect. The larger substituent on Schiff-based groups leaded to stronger fluorescence intensity and high quantum yield.

Full text of DOI:10.1016/j.dyepig.2017.01.048

Dyes and Pigments 140 (2017) 179e186
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Novel fluorescence-enhancing effect based on light-induced E/Z isomerization of perylene
diimides with Schiff-base groups on bay-positions
, b,
Mingguang Zhu a, Hongyu Guo a, Kaicong Cai a, Fafu Yang a *, Zusheng Wang a
a College of Chemistry and Chemical Engineering, Fujian Normal University, Fuzhou 350007, PR China
b Fujian Key Laboratory of Polymer Materials, Fuzhou 350007, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Three novel perylene Schiff-base derivatives 5a-5c were synthesized and characterized in yields of 80
e85%. They exhibited reversible trans/cis isomerization after UV-light irradiation and heating. The
fluorescence-enhancing effects were firstly observed for perylene trans-to-cis isomerization. The
increasing steric hindrance based on the restricted intramolecular rotation of trans-to-cis isomerization
was responsible for the fluorescence-enhancing effect. The larger substituent on Schiff-based groups
leaded to stronger fluorescence intensity and high quantum yield.
Received 16 August 2016
Received in revised form
26 November 2016
Accepted 19 January 2017
Available online 23 January 2017
© 2017 Elsevier Ltd. All rights reserved.
Keywords:
Perylene
Synthesis
Bay-substituent
Isomerization
Fluorescence
1. Introduction
chromophores into other structural units [22e24]. As to perylene
photochromic molecules, only several attempts by modifying per-
In recent years, the large-aromatic conjugated organic mole-
cules had been paid considerable attention due to their unique
optical and electrical properties [1e3]. Perylene diimide (PDI) and
its derivatives show excellent photophysical properties including
the large band-gap, high fluorescence quantum yields of photo-
luminescence and long singlet excited state lifetimes [4,5], which
makes them an ideal candidate for many optoelectronic devices
such as light-emitting diodes, electroluminescent devices, optical
switching and photovoltaics [6e9]. By covalent substitution on N-
imide position or bay-position, all kinds of perylene derivatives
were prepared and their interesting properties, such as optical and
electrical properties [10,11], self-assembly properties [12,13], liquid
crystalline properties [14e17], and ions-complexation properties
were investigated in details [18,19].
ylene with photoresponsive azobenzene moieties had been
accomplished to study the influences of trans-to-cis isomerization
on properties [25e28]. Würthner's group, Feng's group and Tao's
group reported perylene-azobenzene derivatives with light-
controlled self-assemble morphology changes, respectively
[25e27]. Gong and his co-workers investigated the photoinduced
electron-transfer in azobenzene-perylene diimide and found the
fast fluorescence quenching after being excited at characteristic
wavelengths [28]. Besides these perylene-azobenzene derivatives,
no other trans/cis photoisomeric perylene derivative was pre-
sented. Moreover, these studies suggested that, although the trans-
to-cis photoisomerization produced interesting changes for self-
assemble morphologies and absorption spectra [25e27], their
fluorescence emission of trans-to-cis photoisomerization only
showed the fluorescence quenching due to the stronger electron-
transfer properties of cis-structures [27,28]. Therefore, to better
understand the relationship of structures and properties of per-
ylene photoisomeric molecules, the theoretical and experimental
studies on more perylene photoisomeric molecules need to be
undertaken. For example, is it possible to attain fluorescence-
enhancing effect for trans-to-cis photoisomeric perylene deriva-
tive? Herein, we wish to report the first synthesis and fluorescence
properties of another kind of trans-to-cis photoisomeric perylene
Photochromic molecular switches [20,21], in which the molec-
ular structural changes can be addressed with light, possesses the
potential utilities in the development of reversible molecular-scale
information storage, optical switching strategies and nonlinear
optical waveguides by incorporating trans-to-cis photoisomeric
* Corresponding author. College of Chemistry and Chemical Engineering, Fujian
Normal University, Fuzhou 350007, PR China.
E-mail address: yangfafu@fjnu.edu.cn (F. Yang).
http://dx.doi.org/10.1016/j.dyepig.2017.01.048
0143-7208/© 2017 Elsevier Ltd. All rights reserved.

180
M. Zhu et al. / Dyes and Pigments 140 (2017) 179e186
derivative: perylene Schiff-base derivatives. The influences of
different Schiff-base substituents on fluorescence were investi-
gated. The fluorescence-enhancing effect was observed after tran-
to-cis isomerization by UV light irradiation for the first time. The
larger substituent on Schiff-base groups resulted in the stronger
fluorescence-enhancing effect. This fluorescence-enhancing phe-
nomenon based on tran-to-cis isomerization by UV light irradiation
open a new idea to design perylene photoisomeric molecules with
excellent optical properties.
crude product was further purified by column chromatography on
silica 100e200 mesh using CH2Cl2/petroleum ether (3:1, V/V) as
eluent to afford 4 as claret solid in yield of 74%. 1HNMR (400 MHz,
CDCl3)
d
: 9.99 (s, 2H, CHO), 9.42(d, J ¼ 8.0 Hz, 2H, Per-H), 8.62 (d,
J ¼ 8.0 Hz, 2H, Per-H), 8.35(s, 2H, Per-H),7.96 (d, J ¼ 8.0 Hz, 4H, ArH),
7.24 (d, J ¼ 8.0 Hz, 4H, ArH), 4.14 (t, J ¼ 6.4 Hz, 4H, NCH2), 1.71 (t,
J ¼ 8.0 Hz, 4H, CH2), 1.23e1.49 (m, 36H, CH), 0.86 (t, J ¼ 6.4 Hz, 6H,
CH3). MALDI-TOF-MS Calcd. for m/z
¼
966.4819, found: m/
z ¼ 1005.719 (MKþ). HR-MS(ESI) (C62H66N2O8) [MK]þ: Calcd.:
1005.4450. found: 1005.4402.
2. Experimental
2.2.3. Synthesis of target perylene diimides with Schiff-base groups
2.1. General
5a-5c
A
mixture of N,N0-didodecyl-1,7-di(p-formacylphenyl) per-
ylenediimide 4 (0.19 g, 0.2 mmol), corresponding anilines
All chemical reagents were obtained from commercial suppliers
and used without further purification. The other organic solvents
and inorganic reagents were purified according to standard anhy-
drous methods before use. TLC analysis was performed using pre-
coated glass plates. Column chromatography was performed us-
ing silica gel (200e300 mesh). NMR spectra were recorded in CDCl3
on a Bruker-ARX 400 instrument at 25 ꢀC. Chemical shifts are re-
ported in ppm, using tetramethylsilane (TMS) as internal standard.
MS spectra were obtained from Bruker mass spectrometer. Com-
pounds 2 were synthesized according to the published literature
[29].
UVeVis and fluorescence spectra were recorded on Varian
UVeVis spectrometer. Fluorescence spectra were measured in a
conventional quartz cell (10 ꢁ 10 ꢁ 45 nm) at 25 ꢀC on a Hitachi F-
4500 spectrometer equipped with a constant-temperature water
bath, with excitation and emission slits 10 nm wide. The excitation
wavelengths were 470 nm. The fluorescence absolute FF values
were measured using an Edinburgh Instruments FLS920 fluores-
cence spectrometer with a 6-inch integrating sphere. All calcula-
tions were carried out with the density functional theory (DFT)
method at the B3LYP/6-31G(d) level by Gaussian 09 program. XRD
(0.4 mmol) and three drops of glacial acetic acid were refluxed in
the solvent of CHCl3/MeOH (40 mL, 3:5(V/V)). The reaction was
monitored by TLC, indicating the disappearance of reactants in 4 h.
After cooling, the solution was concentrated by rotary evaporator.
Then, 30 mL of ethanol was added to afford precipitation. The crude
product was purified by recrystallization in CHCl3/MeOH to give
pure target compounds 5a-5c as carmine solid.
5a: Yield: 83%. 1H NMR (400 MHz, CDCl3)
d
: 9.52(d, J ¼ 8.0 Hz,
2H, Per-H), 8.65 (d, J ¼ 8.0 Hz, 2H, Per-H), 8.45 (s, 2H, CH¼), 8.39 (s,
2H, Per-H), 7.99 (d, J ¼ 8.0 Hz, 2H, ArH), 7.35 (t, J ¼ 8.0 Hz, 2H, ArH),
7.10e7.27 (m, 12H, ArH), 4.16(t, J ¼ 8.0 Hz, 4H, NCH2), 1.73 (bs, 4H,
CH2), 1.24e1.50 (m, 36H, CH2), 0.88 ((t, J ¼ 7.2 Hz, 6H, CH3). 13C NMR
(100 MHz, CDCl3)
dppm: 163.04, 162.59, 161.71, 159.23, 148.05,
145.37, 132.48, 131.38, 130.31, 124.58, 122.77, 122.63, 120.91, 119.40,
119.29, 119.17, 118.68, 118.49, 117.79, 117.31, 116.29, 114.91, 113.21,
40.78, 31.91, 29.62, 29.53, 29.34, 29.02, 28.92, 28.06, 26.94, 22.54,
14.20. MALDI-TOF-MS Calcd. for m/z ¼ 1184.5, found: m/z ¼ 1184.0
(Mþ). HR-MS (ESI) (C74H74Cl2N4O6) [MH]þ: Calcd.: 1185.5058.
found: 1185.5012.
5b: yield: 85%. 1HNMR (400 MHz, CDCl3)
d: 9.51(d, J ¼ 8.0 Hz,
experiments were performed on SEIFERT-FPM (XRD7), using Cu K
1.5406 Å as the radiation source with 40 kV, 30 mA power.
a
2H, Per-H), 8.63 (d, J ¼ 8.0 Hz, 2H, Per-H), 8.50 (s, 2H, CH¼), 8.36(s,
2H, Per-H),7.98 (d, J ¼ 8.0 Hz, 4H, ArH), 7.22e7.27(m, 8H, ArH), 7.12
(s, 2H, ArH), 6.96 (d, J ¼ 8.0 Hz, 2H, ArH), 4.15 (t, (d, J ¼ 7.2 Hz, 4H,
NCH2), 3.86 (s, 6H, OCH3), 1.72 (t, J ¼ 7.2 Hz, 4H, CH2), 1.22e1.50 (m,
36H, CH2), 0.88 (t, J ¼ 7.2 Hz, 6H, CH3). 13C NMR (100 MHz, CDCl3)
2.2. Synthesis
2.2.1. Synthesis of N,N0-didodecyl-1,7-dibromoperylene-diimide 3
dppm: 163.24, 158.37, 157.25, 156.80, 150.71, 144.75, 132.47, 130.91,
A mixture of 1,7-dibromoperylene-3,4,9,10-tertracarboxylic acid
diimide 2 (0.2 g, 0.36 mmol), dodecylamine (0.14 g, 0.76 mmol) and
a drop of glacial acetic acid in anhydrous dimethyl formamide
(DMF, 20 mL), was stirred for 5 h at 80 ꢀC under N2 atmosphere.
After being cooled to room temperature, 20 mL of distilled water
was added into the reaction system. The precipitate was formed
and filtered. The obtained solid was separated by column chro-
matography on silica 100e200 mesh using CH2Cl2/petroleum ether
(3:1, V/V) as eluent to afford crineous powder in yield of 84%.
130.45, 129.86, 129.00, 125.43, 124.95, 124.30, 124.05, 122.89,
122.54, 122.10, 119.16, 118.52, 116.39, 114.78, 114.44, 55.57, 40.74,
31.91, 29.62, 29.54, 29.34, 28.06, 27.13, 22.68, 14.12. MALDI-TOF-MS
Calcd. for m/z ¼ 1176.5, found: m/z ¼ 1176.585 (Mþ). HR-MS(ESI)
(C76H80N4O8) [MH]þ: Calcd.: 1177.6049. found: 1177.6034.
5c: Yield: 80%. 1HNMR (400 MHz, CDCl3)
d: 9.54(d, J ¼ 8.0 Hz, 2H,
Per-H), 8.65(d, J ¼ 8.0 Hz, 2H, Per-H), 8.57 (s, 2H, CH¼), 8.40(s, 2H,
Per-H), 8.35(d, J ¼ 8.0 Hz, 2H, ArH), 8.12 (d, J ¼ 8.0 Hz, 4H, ArH),
7.87(d, J ¼ 8.0 Hz, 2H, ArH), 7.75(d, J ¼ 8.0 Hz, 2H, ArH), 7.45e7.55 (m,
6H, ArH), 7.30(d, J ¼ 8.0 Hz, 4H, ArH), 7.08(d, J ¼ 8.0 Hz, 2H, ArH), 4.17
(t, J ¼ 7.2 Hz, 4H, NCH2), 1.74 (bs, 4H, CH2), 1.24e1.50 (m, 36H, CH2),
1HNMR (400 MHz, CDCl3)
d
: 9.46(d, J ¼ 8.0 Hz, 2H, Per-H), 8.90(s,
2H, Per-H), 8.68 (d, J ¼ 8.0 Hz, 2H, Per-H), 4.20 (t, J ¼ 8.0 Hz, 4H,
NCH2),1.75 (bs, 4H, CH2),1.24e1.49 (m, 36H, CH2), 0.88 (t, J ¼ 8.0 Hz,
6H, CH3).
0.88 (s, J ¼ 7.2 Hz, 6H, CH3). 13C NMR (100 MHz, CDCl3)
dppm: 163.17,
162.71, 158.72, 157.64, 154.21, 149.04, 133.99, 133.35, 133.11, 132.48,
131.36, 130.55, 129.23, 128.94, 128.75, 127.67, 126.46, 126.03, 125.81,
125.52, 124.90, 124.54, 124.17, 123.92, 122.57, 119.24, 112.70, 40.78,
31.92, 29.63, 29.56, 29.35, 28.07, 27.15, 22.69, 14.12. MALDI-TOF-MS
Calcd. for m/z ¼ 1216.6, found: m/z ¼ 1216.486 (Mþ). HR-MS(ESI)
(C82H80N4O6) [MH]þ: Calcd.: 1217.6150. found: 1217.6136.
2.2.2. Synthesis of N,N0-didodecyl-1,7-di(p-formacylphenyl)
perylenediimide 4
N,N′-didodecyl-1,7-dibromoperylenediimide3(0.4g, 0.45mmol),
p-hydroxybenzaldehyde (0.11 g, 0.9 mmol), and K2CO3 (0.18 g,
1.30 mmol) were added in 20 mL of DMF. The mixture was stirred for
12 h at 90 ꢀC under N2 atmosphere. The reaction was monitored by
TLC till the disappearance of reactants. Then the solution was cooled
toroomtemperature. The residuewastreatedwith30mLofHCl(10%)
and extracted with 30 mL of CHCl3. The organic layer was separated,
dried over anhydrous MgSO4, and then filtered, concentrated. The
3. Results and discussion
3.1. Synthesis and characterization
It was well-known that the steric hindrance of trans/cis-isomers

M. Zhu et al. / Dyes and Pigments 140 (2017) 179e186
181
is different and influences greatly on optical properties. The sub-
stituent on m-position of aromatic Schiff-base group would pro-
duce strong steric hindrance for intramolecular s-bonding rotation
of cis-structure. Thus, three aromatic molecules with gradually
increasing bulk of substituents on m-position (m-chloroaniline, m-
orange red under visible lamp. Moreover, under fluorescent lamp,
compound 5c exhibited far stronger fluorescence after UV-light
irradiation than the fluorescence before UV-light irradiation.
These results suggested that compound 5c with biggest substituent
on Schiff-base group possessed strongest fluorescence-enhancing
effect after UV-light irradiation. In addition, by heating at 80 ꢀC
for 12 h, the solution of 5c after UV-light irradiation could recover
to previous colour and absorption spectrum of trans-structure
(Fig. S28), indicating the reversible trans/cis isomerization by UV-
light irradiation and heating.
anisidine and
a-naphthylamine), were chosen as reactants for
Schiff-base condensation. The synthetic route was designed as
Scheme 1. According to the literature method [29], the 1,7-
dibrominated perylene bisanhydride 2 was obtained by treating
perylene tetracarboxylic anhydride with Br2, H2SO4 and I2. Then
compound 2 was used directly for next step without purification
due to the poor solubility. Perylene diimides 3 was synthesized by
the ammonolysis reaction of compound 2 with dodecylamine in
DMF at 80 ꢀC for 5 h. The yield for red solid 3 was as high as 84%
after column chromatography. Further, by treating compound 3
with 4-hydroxy benzaldehyde in K2CO3/DMF system, compound 4
was prepared in yield of 74% after column chromatography (CH2Cl2/
ethyl petroleum ether (3:1, V/V)). Finally, by reacting compound 4
with corresponding aromatic amine in CHCl3/EtOH (3:5, V/V), the
target perylene Schiff-base derivatives 5a-5c were synthesized in
yields of 80e85%. Due to the high efficiency of Schiff-base
condensation, the pure compounds 5a-5c was conveniently ob-
tained after recrystallization.
The structures of target compounds 5a-5c were confirmed by
NMR, and MS spectral analysis. In the mass spectrometry spectra,
the corresponding molecular ion peaks indicated the fulfillment of
Schiff-base condensation of two aldehyde groups. In their 1H NMR
spectra and 13C NMR spectra, only one kind of signal for proton of
HC]N suggested that all Schiff-base structures were in trans-iso-
mers, which were further supported by photoisomeric studies. The
other signals of protons in 1H NMR spectra and the signals of 13C
NMR spectra also agreed with the trans-isomers of compounds 5a-
5c.
3.3. The absorption spectral changes of E/Z isomerization
Furthermore, the changes of absorption spectra for compounds
5a-5c under UV-light irradiation were tested with interval times as
shown in Figs. 2e4. The obvious decrease of absorbency for trans-
Schiff-base groups and the increase of absorbency for cis-Schiff-
base groups were observed. The times attaining equilibrium of
trans/cis-isomerization were 40 min, 45 min and 75 min for com-
pounds 5a-5c, respectively. The absorbency for perylene core at
about 530 nm after UV-light irradiation increased a bit orderly.
Moreover, the maximal absorption wavelength of perylene core
showed blue shifts with 2 nm, 6 nm and 10 nm for compounds 5a-
5c, respectively. These results suggested that compounds 5a-5c
possessed the similar changes after UV-light irradiation. The larger
substituent showed stronger spectral change. These results might
be explained by that the larger substituent on Schiff-base group at
cis-isomerization exhibited stronger steric hindrance, which pro-
duced not only the longer time for isomeric equilibrium but also
more distortion of the aromatic conjugated systems, resulting in
more blue shifts for absorption spectra. The conversion efficiencies
of cis-isomers, based on the decrease of absorbency of trans-Schiff-
base groups at 316 nm, 343 nm and 360 nm, were calculated
approximately as 28%, 46% and 44% for compounds 5a, 5b and 5c,
respectively.
3.2. UV light-induced E/Z isomerization
The trans-to-cis isomerization of compounds 5a-5c were pre-
liminarily investigated by putting their CH2Cl2 solution under UV-
light irradiation for 2 h. The obvious chromatic changes were
observed as shown in Fig. 1. One can see that compounds 5a and 5b
showed slight chromatic changes after UV-light. However, com-
pound 5c presented strong chromatic changes from carmine to
3.4. The 1H NMR spectra change of E/Z isomerization
The 1H NMR spectra of compound 5c before and after UV-light
irradiation were studied as representative one to investigate the
trans/cis-isomerization. Fig. 5 illustrated the proton signals before
and after UV-light irradiation for compound 5c. Some new proton
Br
Br
O
N
O
O
O
O
O
O
O
O
O
O
O
O
O
O
dodecylamine
DMF
H2SO4
Br2, I2
C
12H25
N C12H25
O
Br
Br
1
2
3
R
H
OHC
N
O
O
O
N
O
O
O
N
O
O
HO
CHO
NH2-R
C12H25
N C12H25
O
C12H25
N C12H25
O
K2CO3, DMF
CHCl3, EtOH
O
O
5
4
CHO
Cl
N
R
H
OCH3
5b R =
5c R =
5a R =
;
;
Scheme 1. Synthesis of perylene diimides with Schiff-base groups on bay-positions 5a-5c.

182
M. Zhu et al. / Dyes and Pigments 140 (2017) 179e186
Fig. 1. The pictures of compounds 5a-5c before and after 365 nm UV-light irradiation (pictures were screened under visible lamp and fluorescent lamp, respectively).
5 min
3
2
1
0
5 min
10 min
15 min
20 min
25 min
30 min
35 min
40 min
45 min
50 min
55 min
60 min
65 min
70 min
75 min
2
1
0
10 min
15 min
20 min
25 min
30 min
35 min
40 min
528nm
for perylene core (blue shift)
530nm
for perylene core (blue shift)
523 nm
Cis- for C=N
for cis C=N
533 nm
Trans- for C=N
for trans C=N
300
400
500
600
700
Wavelength (nm)
300
400
500
600
700
Fig. 2. Time-dependent UV absorption spectra of compound 5a in CH2Cl2 (1 ꢁ 10ꢂ5 M)
Wavelength (nm)
under UV-light irradiation.
Fig. 4. Time-dependent UV absorption spectra of compound 5c in CH2Cl2 (1 ꢁ 10ꢂ5 M)
under UV-light irradiation.
5min
10min
15min
20min
25min
30min
35min
40min
45min
2
1
0
signals as labeled in Fig. 5 suggested the conversion of trans-to-cis
isomerization. Especially, the doublet for Ha on perylene core split
to four different groups of signals. This phenomenon could be
explained by that only part of the trans-structures turn over to the
cis-structures after UV-light irradiation, indicating the existence of
three kinds of isomers in solution including Schiff-base groups with
two trans-structures, two cis-structures, and one trans- and one cis-
structure, respectively. Furthermore, after calculation based on the
integration of four different signals of Ha, the conversion efficiency
of cis-structure was 46%, which was approximately in accordance
with the value in absorption spectrum.
525nm
for perylene core (blue shift)
531 nm
for cis C=N
for trans C=N
3.5. The fluorescence spectral changes of E/Z isomerization
300
400
500
600
700
Wavelength(nm)
As the tran-to-cis isomerization of compounds 5a-5c under UV
light irradiation was confirmed, the luminescent behaviors of
compounds 5a-5c were investigated. Fig. 6 illustrated the fluores-
cence spectra of compounds 3, 4 and 5a-5c. The emission wave-
lengths of compounds 4 and 5a-5c were red shift comparing with
Fig. 3. Time-dependent UV absorption spectra of compound 5b in CH2Cl2 (1 ꢁ 10ꢂ5 M)
under UV-light irradiation.

M. Zhu et al. / Dyes and Pigments 140 (2017) 179e186
183
Fig. 5. The 1H NMR spectra of compound 5c before and after UV-light irradiation for 2 h.
that of compound 3. These phenomena could be ascribed to the
large aromatic conjugated systems of compounds 4 and 5a-5c with
aromatic substituents on bay-positions. On the other hand, the
fluorescent enhancement for compound 5b was attributed to the
electronic effect of OCH3 groups. It was interesting that compound
5c showed far lower fluorescence intensity than other compounds.
This phenomenon could not be explained by the weak electronic
effect of naphthyl. But it could be elucidated by the restricted
intramolecular rotation mechanism which was popular in the field
of aggregation-induced luminescence [30e32]. The bay-
naphthyl consumed a large amount of energy, leading to much
reduction of fluorescence intensity.
Furthermore, the time-dependent fluorescence spectra of
compounds 5a-5c were studied under UV-light irradiation. The
results were shown in Figs. 7e9. After UV-light irradiation, the
fluorescence intensity increased obviously with the order of
5a < 5b < 5c. These phenomena were first observed by trans/cis-
isomerization of perylene derivatives. Especially, the fluorescence
intensity of compound 5c enhanced from 325 a.u. to 1850 a.u. In
addition, the emission wavelength of compound 5c shifted from
563 nm to 559 nm. These phenomena also could be explained by
the restricted intramolecular rotation mechanism [30e32]. After
substituent at the trans-structure could rotated around
s-bond
with little rotation limitation and consumed the excitation energy,
resulting in the vanishing of the fluorescence. The rotation of large
800
5min
10min
15min
1000
900
20min
25min
30min
35min
40min
45min
600
400
200
0
800
3 (550 nm)
4 (560 nm)
5a (560 nm)
5b (560 nm)
5c (563 nm)
700
600
500
400
300
200
100
0
450
500
550
600
650
700
-100
450
500
550
600
650
700
Wavelength(nm)
Wvelength(nm)
Fig. 7. Time-dependent fluorescence spectra of compound 5a in CH2Cl2 (1 ꢁ 10ꢂ5 M)
Fig. 6. Fluorescence spectra of compounds 3, 4 and 5a-5c in CH2Cl2 (1 ꢁ 10ꢂ5 M).
under UV-light irradiation.

184
M. Zhu et al. / Dyes and Pigments 140 (2017) 179e186
distortion of the plane aromatic conjugated systems, and then
produced obvious blue shift for fluorescence spectrum, which was
in agreement with the results of absorption spectrum.
5min
10min
15min
20min
25min
30min
35min
40min
45min
1200
1000
800
600
400
200
0
3.6. The energy difference for E/Z isomerization
In order to confirm the steric hindrance of trans-to-cis isomer-
ization, the energy of optimized noncoplanar and coplanar struc-
tures for trans- and cis-isomers were calculated by the density
functional theory (DFT) method at the B3LYP/6-31G(d) level with
Gaussian 09 program [36]. Fig. 10 illustrated the representative
structures and energy of trans- and cis-isomers of compound 5c.
Fig. 10 (a) and (c) were the optimized structures for trans- and cis-
isomers of compound 5c with lowest energy, respectively. Fig. 10
(b) and (d) were the coplanar structures with the highest energy
when Schiff-base groups rotated around s-bonding. After calcula-
tion, the energy difference between optimized noncoplanar struc-
ture and coplanar structure upon rotating were ꢂ0.0802, ꢂ0.1245
and ꢂ0.2963 Hartree for trans-structures of compounds 5a-5c,
and ꢂ0.1165, ꢂ0.1937 and ꢂ0.5734 Hartree for cis-structures of
compounds 5a-5c, respectively. One can see that the energy dif-
ferences for cis-isomers were bigger than that of trans-isomers,
which suggested the stronger rotation limitation for cis-isomers
than tran-isomers. Compound 5c showed the strongest energy
difference upon rotating at coplanar structure, which produced the
largest rotation limitation, resulting in the dramatic fluorescence-
enhancing effect.
450
500
550
600
650
700
Wavelength(nm)
Fig. 8. Time-dependent fluorescence spectra of compound 5b in CH2Cl2 (1 ꢁ 10ꢂ5 M)
under UV-light irradiation.
3.7. The IR spectra change of E/Z isomerization
The IR spectra were also employed to study the steric hindrance
effect of compound 5c before and after UV-light irradiation. Fig. 11
illustrated the corresponding IR spectra. One can see that, after UV-
light irradiation, although the most of peaks between 1000 and
3000 cmꢂ1 had no shift, the peaks from 400 to 1000 cmꢂ1 for CeH
bond of naphthyl and perylene core exhibited small change, indi-
cating the existence of new intramolecular electronic effect for CeH
bond of naphthyl and perylene core due to the influence of steric
hindrance. This IR result was also in accordance with the above
deduction of the restricted intramolecular rotation mechanism.
3.8. The changes of FF values and XRD curves for E/Z isomerization
Fig. 9. Time-dependent fluorescence spectra of compound 5c in CH2Cl2 (1 ꢁ 10ꢂ5 M)
under UV-light irradiation.
The fluorescence absolute FF values in CH2Cl2 solution were
measured by using an Edinburgh Instruments FLS920 Fluorescence
Spectrometer with a 6-inch integrating sphere. The FF values
before UV-light irradiation were 0.73, 0.80 and 0.26, and then
increased to 0.84, 0.90, 0.95 after UV-light irradiation for com-
pounds 5a, 5b and 5c, respectively. On the other hand, in thin solid
films, compounds 5a, 5b and 5c without UV-light irradiation
exhibited no fluorescence due to the ACQ effect. However, the thin
solid films, obtained by evaporating the solvents after UV-light
irradiation, showed fluorescence with the FF values of 0.05, 0.08
and 0.18 for compounds 5a, 5b and 5c, respectively. Normally, the
orderly aggregation in solid film caused strong fluorescence
quenching. Before UV-light irradiation, compounds 5a, 5b and 5c
possessed only trans-isomer, which could aggregated orderly and
resulted in ACQ effect. However, after UV-light irradiation, there
were three isomers as analyzed for 1H NMR spectral change of
Fig. 5. The mixture of three isomers was not favorable for the
orderly aggregation and then reduced the fluorescence quenching.
This speculation was also confirmed by the XRD data. Fig. 12
illustrated the XRD traces of compound 5c before and after UV-
light irradiation. It can be seen that, after UV-light irradiation, the
reflections at 2.8ꢀ, 4.8ꢀ 9.1ꢀ and 27.6ꢀ became weak obviously,
trans-to-cis isomerization under UV-light irradiation, the steric
hindrance of cis-structure was stronger and produced the larger
rotation limitation, leading to weaker rotation around s-bond and
less energy-consuming. As a result, the fluorescence intensity
increased obviously due to the less consumption of excitation en-
ergy by rotation. As the bulk of substituent of compounds 5a-5c
showed the order of Cl < OCH3 < naphthyl, their steric hindrance
also possessed same order and resulted in the same fluorescence-
enhancing order. Since the naphthyl is the large and rigid plane
structure with strong steric hindrance, the fluorescence intensity
increased dramatically for compound 5c. These similar
fluorescence-enhancing phenomena based on the restricted intra-
molecular rotation mechanism were also reported recently in the
research field of AIE [33e35].
On the other hand, due to the steric hindrance of cis-structure
was stronger than that of trans-structure, the plane aromatic con-
jugated systems was distorted more after trans-to-cis isomeriza-
tion, resulting the blue shift for emission wavelength. The largest
steric hindrance of cis-isomer of compound 5c resulted in strongest

M. Zhu et al. / Dyes and Pigments 140 (2017) 179e186
185
Fig. 10. The energy difference of optimized noncoplanar and coplanar structures for trans- and cis-isomers of compound 5c.
small change for C-H on naphthyl and perylene core
after UV light
before UV light
Fig. 12. XRD traces of compound 5c before and after UV-light irradiation.
3000
2500
2000
1500
1000
500
-1
wavenumber (cm )
heating effect. The conversion efficiencies of cis-structure were
28e46% by the absorption spectra and 1H NMR spectra. They
exhibited the fluorescence-enhancing effect after UV-light irradi-
ation, which was firstly observed for perylene isomeric derivatives.
The larger substituent leaded to stronger fluorescence-enhancing
effect. Compound 5c with naphthyl on Schiff-groups possessed
very high fluorescence intensity and good quantum yield after UV-
light irradiation. The phenomena were well explained by the
restricted intramolecular rotation mechanism, which was
confirmed by energy calculation, IR spectra and XRD analysis. This
paper supplied a new strategy to design perylene photoisomeric
Fig. 11. The IR spectrum of compound 5c before and after UV-light irradiation.
indicating the decrease of orderly aggregation for compound 5c.
4. Conclusion
In conclusion, three perylene Schiff-bases derivatives were
firstly synthesized in yields of 80e85%. These compounds showed
reversible trans/cis isomerization by UV-light irradiation and

186
M. Zhu et al. / Dyes and Pigments 140 (2017) 179e186
liquid crystals: synthesis and mesomorphism of galliceperyleneegallic tri-
molecules with excellent optical properties.
mers. New J Chem 2015;39:72e6.
[15] Kong XF, He ZQ, Zhang YN, Mu LP, Liang CJ, Chen B, et al. A mesogenic
triphenylene-perylene-triphenylene triad. Org Lett 2011;13:764e7.
[16] Gregg BA, Cormier RA. Doping molecular semiconductors: n-Type doping of
liquid crystal perylene diimide. J Am Chem Soc 2001;123:7959e60.
[17] Zhu MG, Guo HY, Yang FF, Wang ZS. Novel perylene liquid crystals: investi-
gation of the influence of bay- substituents on mesomorphism and photo-
physical property. Dye Pigments 2016;133:387e94.
[18] Funahashi M, Sonoda A. A liquid-crystalline perylene tetracarboxylic bisimide.
derivative bearing a triethylene oxide chain and complexation of the deriv-
ative with Li cations. Dalton Trans 2013;42:15987e92.
[19] Yagai S, Seki T, Karatsu T, Kitamura A, Würthner F. Transformation from H- to
J-aggregated perylene bisimide dyes by complexation with cyanurates.
Angew Chem Int Ed 2008;47:3367e71.
[20] Russew MM, Hecht S. Photoswitches: from molecules to materials. Adv Mater
2010;22:3348e60.
Acknowledgment
Financial support from the National Natural Science Foundation
of China (No: 21406036), Fujian Natural Science Foundation of
China (No. 2014J01034), the Program for Innovative Research Team
in Science and Technology in Fujian Province University and Pro-
gram of students innovative experimental projects of FJNU
(201310394016) were greatly acknowledged.
Appendix A. Supplementary data
[21] Yagai S, Karatsu T, Kitamura A. Photocontrollable self-assembly. Chem Eur J
2005;1:4054e63.
[22] Tang T, Zeng F, Wu SZ, Tong Z, Lu DB, She WL. Photo-induced birefringence
and all-optical switching effect in azobenzene-grafted polyurethanes. Opt
Mater 2004;27:585e90.
[23] Guang SY, Yin SC, Xu HY, Zhu WJ, Gao YC, Song YL. Synthesis and properties of
long conjugated organic optical limiting materials with different pi-electron
conjugation bridge structure. Dye Pigments 2007;73:285e91.
[24] Shi LL, Ran X, Li YJ, Li QY, Qiu WH, Guo LJ. Photoresponsive structure trans-
formation and emission enhancement based on a tapered azobenzene gelator.
RSC Adv 2015;5:38283e9.
[25] Yagai S, Hamamura S, Wang H, Stepanenko V, Seki T, Unoike K, et al. Un-
conventional hydrogen-bond-directed hierarchical co-assembly between
perylene bisimide and azobenzene-functionalized melamine. Org Biomol
Chem 2009;7:3926e9.
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.dyepig.2017.01.048.
References
[1] Newman CR, Frisbie CD, da Silva Filho DA, Bredas JL, Ewbank PC, Mann KR.
Introduction to organic thin film transistors and design of n-channel organic
semiconductors. Chem Mater 2004;16:4436e51.
[2] Chen L, Li C, Müllen K. Beyond perylene diimides: synthesis, assembly and
function of higher rylene chromophores. J Mater Chem C 2014;2:1938e56.
€
[3] Zagranyarski Y, Chen L, Jansch D, Gessner T, Li C, Müllen K. Toward perylene
dyes by the hundsdiecker reaction. Org Lett 2014;16:2814e7.
[4] Peumans P, Uchida S, Forrest SR. Efficient bulk heterojunction photovoltaic
cells using small-molecular-weight organic thin films. Nature 2003;425:
158e62.
[26] Feng W, Feng YY. Photo-responsive perylene diimid-azobenzene dyad:
photochemistry and its morphology control by self-assembly. Opt Mater
2008;30:876e80.
[27] Ma LL, Jia J, Yang TY, Yin GZ, Liu Y, Sun X, et al. Light-controlled self-assembly
and conductance: from nanoribbons to nanospheres. RSC Adv 2012;2:
2902e8.
€
[5] Schmidt-Mende L, Fechtenkotter A, Müllen K, Moons E, Friend RH,
MacKenzie JD. Self-organized discotic liquid crystals for high-efficiency
organic photovoltaics. Science 2001;293:1119e22.
[6] Kelley RF, Taube MJ, Wasielewski MR. Intramolecular electron transfer
through the 20-position of a chlorophyll a derivative: an unexpectedly effi-
cient conduit for charge transport. J Am Chem Soc 2006;128:4779e91.
[7] Beckers EHA, Meskers SCJ, Schenning APHJ, Chen Z, Würthner F, Janssen RAJ.
Charge separation and recombination in photoexcited oligo(p-phenylene
vinylene): perylene bisimide arrays close to the marcus inverted region.
J Phys Chem A 2004;108:6933e7.
[8] Zagranyarski Y, Chen L, Zhao Y, Wonneberger H, Li C, Müllen K. Facile trans-
formation of perylene tetracarboxylic acid dianhydride into strong donor-
acceptor chromophores. Org Lett 2012;14:5444e7.
[28] Feng WK, Feng YY, Wang SF, Feng W, Yi WH, Gong QH. Intramolecular
photoinduced electron-transfer in azobenzene-perylene diimide. Chin Phys B
2010;19:113401e5.
€
[29] Würthner F, Stepanenko V, Chen Z, Saha-Moller CR, Kocher N, Stalke D.
Preparation and characterization of regioisomerically pure 1,7-disubstituted
perylene bisimide dyes. J Org Chem 2004;69:7933e9.
[30] Hong Y, Lam JWY, Tang BZ. Aggregation-induced emission. Chem Soc Rev
2011;40:5361e88.
[31] Wang M, Zhang G, Zhang D, Zhu D, Tang BZ. Fluorescent bio/chemosensors
based on silole and tetraphenylethene luminogens with aggregation-induced
emission feature. J Mater Chem 2010;20:1858e67.
[32] Hong Y, Lam JWY, Tang BZ. Aggregation-induced emission: phenomenon,
mechanism and applications. Chem Commun 2009:4332e5.
[33] Dong J, Solntsev KM, Tolbert LM. Activation and tuning of green fluorescent
protein chromophore emission by alkyl substituent-mediated crystal packing.
J Am Chem Soc 2009;131:662e70.
[34] Wu L, Burgess K. Syntheses of highly fluorescent GFP-Chromophore ana-
logues. J Am Chem Soc 2008;130:4089e93.
[35] Naumov P, Kowalik J, Solntsev KM, Baldridge A, Moon JS, Kranz C, et al.
Topochemistry and photomechanical effects in crystals of green fluorescent
protein-like chromophores: effects of hydrogen bonding and crystal packing.
J Am Chem Soc 2010;132:5845e57.
[9] Thorley KJ, Würthner F. Synthesis and properties of
angular perylene imide dimer. Org Lett 2012;14:6190e3.
[10] Neuteboom EE, Meskers SCJ, Beckers EHA, Chopin S, Janssen RAJ. Solvent
mediated intramolecular photoinduced electron transfer in fluorene-
a covalently linked
a
perylene bisimide derivative. J Phys Chem A 2006;110:12363e71.
€
[11] Hippius C, Schlosser F, Vysotsky MO, Bohmer V, Würthner F. Energy transfer
in calixarene-based cofacial-positioned perylene bisimide arrays. J Am Chem
Soc 2006;128:3870e1.
[12] Ogi S, Stepanenko V, Sugiyasu K, Takeuchi M, Würthner F. Mechanism of self-
assembly process and seeded supramolecular polymerization of perylene
bisimide organogelator. J Am Chem Soc 2015;137:3300e7.
[13] Bhavsar GA, Asha SK. Pentadecyl phenol- and cardanol-functionalized fluo-
rescent, room-temperature liquid-crystalline perylene bisimides: effect of
pendant chain unsaturation on self-assembly. Chem Eur J 2011;17:12646e58.
[14] Meng L, Wu QM, Yang FF, Guo HY. Novel room-temperature thermotropic
[36] Cox RA. Gaussian 09, gaussian, inc. 2009. Wallingford CT.