Synthesis and intramolecular FRET of perylenediimide-naphthalimide dendrons

Article abstract of DOI:10.1016/j.dyepig.2014.07.005

Two novel fluorescence resonance energy transfer-based perylenediimide- naphthalimide dendrons with perylenediimides as the energy acceptor, tertiary amines as the proton or metal receptor and naphthalimide as the energy donor were successfully synthesized. The dendrons exhibited high selectivity toward Fe(III) in the presence of various other metal cations and exhibited sensitivity to protons. A 1:2 stoichiometry was found for the complex formed by the probes and Fe(III) using a Job's plot and by non-linear least square fitting of the fluorescence titration curves. The probe molecules present abilities of fast fluorescence detection of Fe(III) and for the fluorescent detection of protons with high energy transfer efficiency of 96-98%.

Full text of DOI:10.1016/j.dyepig.2014.07.005

Dyes and Pigments 112 (2015) 317e326
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Synthesis and intramolecular FRET of perylenediimideenaphthalimide
dendrons
Huan-ren Cheng, Ying Qian*
School of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Two novel fluorescence resonance energy transfer-based perylenediimideenaphthalimide dendrons
with perylenediimides as the energy acceptor, tertiary amines as the proton or metal receptor and
naphthalimide as the energy donor were successfully synthesized. The dendrons exhibited high selec-
tivity toward Fe(III) in the presence of various other metal cations and exhibited sensitivity to protons. A
1:2 stoichiometry was found for the complex formed by the probes and Fe(III) using a Job's plot and by
non-linear least square fitting of the fluorescence titration curves. The probe molecules present abilities
of fast fluorescence detection of Fe(III) and for the fluorescent detection of protons with high energy
transfer efficiency of 96e98%.
Received 24 May 2014
Received in revised form
1 July 2014
Accepted 4 July 2014
Available online 4 August 2014
Keywords:
Fluorescence resonance energy transfer
Photoinduced electron transfer
Perylenediimide
© 2014 Elsevier Ltd. All rights reserved.
Naphthalimide
Fluorescent probes
Dendrons
1. Introduction
Most perylenediimide fluorometric sensors are designed to
sense photophysical changes produced upon complexation,
Perylene tetracarboxylic diimides (PDIs) have recently gener-
ated great interest in the field of photonic materials, because of
their excellent photo-stability, high luminescence efficiency, and
novel optoelectronic properties [1e3]. Perylenediimide dendrons
have attracted much attention because of their unique structures
which enable them to concentrate a high number of end groups in
one molecule. Perylenediimide dendrons have wide applications in
optical materials, photosynthetic models and chemosensors [4e7].
For example, Akkaya et al. reported a boron-dipyrrin functionalized
perylenediimide derivative with an energy transfer efficiency as
high as 99% [8]. PDI are also good electron acceptors with a low
reduction potential [9e12]. Several PDI probes should be promising
candidates for fluorophores in fluorescent probes based on
photoinduced electron transfer (PET). Recently several PDIs probes
based on PET mechanism have been developed [13e20]. Shen et al.
reported a multifunctional perylenediimide derivative, which can
be used as a recyclable specific Hg2þ ion sensor [21]. However,
fluorescence resonance energy transfer (FRET) and PET-based
including photoinduced electron transfer (PET), photoinduced
charge transfer (PCT) [7] and fluorescence resonance energy
transfer (FRET) [23]. Among numerous mechanisms, PET has been
widely used in the design of new sensors. A “turn-on” motif based
on PET, which changes from a non-fluorescent state to a fluorescent
state upon analyte binding, is widely employed in many per-
ylenediimide fluorometric sensors due to the inherently higher
sensitivity of PET as compared to the normal fluorescence
quenching motif [24e27]. Known to be sensitive, selective, and
adaptable to a wide variety of systems [28,29], FRET plays a more
significant role in natural photosynthetic process where solar en-
ergy is captured and delivered to the reaction center to initiate a
multistep electron-transfer cascade.
To make a useful probe, a compound must possess two func-
tional units: an ionophore responsible for selectively binding ions
and a fluorophore responsible for signal transduction [30e34].
Furthermore, a communication mechanism between the binding
and signaling sites must exist [35]. In this work, we exploited an
excellent ratiometric Fe(III) probe based on a Fe(III) complex with
nonconjugated “energy donorespacerereceptor” structure as
shown in Fig. 1, and further investigated the effect of NeFe(III)
binding on the energy transfer from donor to acceptor. Herein, we
synthesized two compounds with perylenediimide as energy
acceptor, 1, 8-naphthalimide derivative as peripheral fluorophore
dendritic perylenediimide fluorometric probes are still at
a
modest number [22].
* Corresponding author. Tel.: þ86 25 83793591; fax: þ86 25 83795857.
E-mail address: yingqian@seu.edu.cn (Y. Qian).
http://dx.doi.org/10.1016/j.dyepig.2014.07.005
0143-7208/© 2014 Elsevier Ltd. All rights reserved.

318
H.-r. Cheng, Y. Qian / Dyes and Pigments 112 (2015) 317e326
Fig. 1. Structures of the light-harvesting molecules PDI-PD and PDI-NB (Inset: Fluorescence spectra of PDI-NB (a) and PDI-PD (b) at various pH values).
and poly(amidoamine) (PAMAM) as linker. In addition, several
reference compounds (shown in Scheme 1), such as complex NB-1,
fluorescent donor, and fluorescent acceptor PDI-2, were also syn-
thesized in order to better understand the combined influence on
the excited-state properties of the complexes. Finally, the binding
of Fe(III) to PDI-NB or PDI-PD changes the excited-state properties
of the acceptor, inducing variation in FRET (fluorescence resonance
energy transfer) from donor to acceptor, and makes an excellent
ratiometric probe for Fe(III) and proton with switchable fluores-
cence signals.
2.1.2. Synthesis
2.1.2.1. Compound 2. Compound 2, 3 and AN-Boc were prepared in
the published method [36].
A
dichloromethane solution (500 mL) of di-tertbutyl
dicarbonate(8 g, 36.7 mmol) was added dropwise over 6 h to
vigorously stirred dichloromethane (50 mL) solution of ethyl-
enediamine (13.2 g, 220 mmol) at 0 ꢁC. The reaction mixture was
then left stirring overnight. The solvent was removed under a
reduced pressure to give an oily liquid which was then dissolved in
doubly distilled water (100 mL), filtered, and then extracted with
dichloromethane (2 ꢂ 300 mL). The final product obtained was a
viscous, colorless oil 2 (5.22 g). Yield: 88%. 1H NMR (CDCl3, ppm):
2. Experimental
d
5.24 (s, 1H, Boc-NHe), 3.10e3.07 (m, 2H, eNHeCH2e), 2.71e2.67
(m, 2H, eCH2eNH2), 1.35 (s, 9H, (CH3)3e). TOF-MS-ES: m/z. Calcu-
2.1. Experimental section
lated: ([MþH])þ ¼ 161.1, found: 161.2.
2.1.1. Materials and methods
1H NMR and 13C NMR spectra were recorded on a Bruker DMX
300 NMR spectrometer and a Bruker ADVANCE 500 NMR spec-
trometer in CDCl3 with tetramethylsilane (TMS) as internal stan-
dard. Chemical shifts are given in parts per million (ppm). Mass
spectra were recorded on Agilent Technologies 6530 Accurate-Mass
Q-TOF LC/MS. HRMS were recorded on an Ultraflex II MALDI-TOF
mass spectrometer. UVevisible absorption spectra were deter-
mined on a Shimadu UV-3600 spectrophotometer. Fluorescence
spectra were measured on a HORIBA FL-4 Max spectrometer.
Fluorescence life time was measured on an Edinburgh FLS920
spectrometer. FT-IR spectra were recorded on a Nicolet 750 series in
the region of 4000e400 cmꢀ1 using KBr pellets. All reagents used
were purchased from Aldrich, Fluka or Alfa Aesar and used without
further purification. All solvents used in spectroscopic measure-
ments were of analytical grade. Reactions were monitored by thin
layer chromatography using Merck TLC Silica gel 60 F254. Silica gel
column chromatography was performed over Merck Silica gel 60.
2.1.2.2. Compound 3. N-Boc-ethylenediamine 2 (5 g, 31 mmol) was
dissolved in methanol (150 mL); to this methyl acry-late (26.9 g,
310 mmol) dissolved in methanol (50 mL) was added slowly. The
reaction mixture was left stirring for 3 days at room temperature.
The solvent and excess methyl acrylate were removed under
reduced pressure at 60 ꢁC using a rotary evaporator. A more
rigorous purification was then carried out via column chromatog-
raphy (ethyl acetate) to give 3 (10 g), Yield: 97%. 1H NMR (CDCl3,
ppm): d 5.13 (s, 1H, Boc-NHe), 3.69 (s, 6H, OeCH3), 3.2e3.14 (m, 2H,
NHeCH2), 2.77e2.73 (t, 4H, eNe(CH2e)2, J ¼ 6.6 Hz), 2.54e2.52 (t,
2H, eCH2eNe(CH2e)2, J ¼ 6.0 Hz), 2.50e2.41 (t, 4H, eCH2eCO2Me,
J ¼ 6.6 Hz), 1.44 (s, 9H, (CH3)3e).
2.1.2.3. Compound AN-Boc. Compound 3 (5 g, 15 mmol) was dis-
solved in methanol (50 mL) then was added slowly to a cooled
(0 ꢁC) methanol solution of ethylenediamine (30 g, 500 mmol). The
reaction mixture was left stirring for 5 days at room temperature.
Scheme 1. Synthesis of the modules NB-1 and PD-1.

H.-r. Cheng, Y. Qian / Dyes and Pigments 112 (2015) 317e326
319
The solvent and the majority of the excess ethylenediamine were
removed under reduced pressure at 60 ꢁC using a rotary evaporator,
with a rigorous purification being carried out by using a heated
vacuum desiccator and a high vacuum mineral oil vacuum pump
(60 ꢁC) and a high vacuum mineral oil vacuum pump. The product
obtained was a thick, viscous honey-like oil (5.7 g). Yield: 99%. 1H
d
8.65e8.63 (d, 2H, H-1), 8.32e8.29 (d, 2H, J ¼ 9 Hz, H-5), 8.25e8.22
(d, 4H, J ¼ 9 Hz, H-3, H-9), 8.13e8.08 (d, 2H, J ¼ 15 Hz, H2C]C),
7.71e7.67 (m, 3H, H-2, 4), 7.65e7.60 (d, 2H, H-6), 7.51e7.46 (t, 2H,
COeNHeCH2e), 7.34e7.32(t, 2H, H-7), 7.22e7.20 (t, 2H, H-8),
7.13e7.08 (d, 2H, J ¼ 15 Hz, C]CH2), 5.67 (t, 1H, Boc-NHe), 4.19 (t,
4H, eCH2eNe(COe)2), 3.65e3.66 (q, 4H, eCOeNHeCH2e), 3.24 (q,
2H, Boc-NHeCH2e), 2.65 (s, 4H, eNe(CH2e)2), 2.51 (t, 2H,
eCH2eNe(CH2e)2), 2.33 (t, 4H, eCH2eCOeNHe), 1.41 (s, 9H,
NMR (CDCl3, ppm)
d 7.51 (s, 2H, COeNHeCH2e), 6.14 (t, 1H, Boc-
NHe), 3.11e3.06 (q, 4H, eCH2eCH2eNH2), 2.99e2.95 (q, 2H, Boc-
NHeCH2e), 2.65e2.61 (t, 4H, eCH2eCH2eNH2), 2.56e2.53 (t, 4H,
eNe(CH2e)2), 2.35e2.32 (t, 2H, eCH2eNe(CH2e)2), 2.17e2.14 (t,
4H, eCH2eCOeNHe), 1.24 (s, 9H, (CH3)3e). TOF-MS-ES: m/z.
calculated: [MþH]þ ¼ 389.2, Found: 389.3.
(CH3)3e). 13C NMR (CDCl3, ppm):
d 172.6, 164.3, 164.0, 156.0, 154.3,
149.9, 140.4, 133.7, 130.6, 129.9, 129.3, 128.1, 126.8, 126.5, 123.8,
123.3, 123.0, 122.2, 121.1, 52.6, 49.7, 39.9, 38.4, 33.2, 28.4. TOF-MS-
ES: m/z. Calculated: 954.4, Found: [MþH]þ
¼
955.4,
[MþNa]þ ¼ 977.3.
2.1.2.4. Compound PD-1. Compound v-1 was prepared as the
published method [37].
Compound PD-2 was prepared according the same method of
compound NB-2, a yellow solid was got. Yield: (80%) (m.p.
A
mixture of 4-Bromo-1,8-naphthalicanhydride (10 g,
132e134 ꢁC). FT-IR (KBr) cmꢀ1: 3367 (
n
NH); 2937e2842(
sNeC]O); 1571( NeC]O). 1H NMR
8.39e8.37 (q, 0.65H, H-6), 8.33e8.31(d, 1.4H, H-6),
nCH);
36.2 mmol) and piperidine (3.69 g, 43.5 mmol) in 100 mL 2-
methoxyethanol was heated under reflux for 12 h under N2. After
cooled to room temperature,100 mL water was joined, a majority of
yellow solid separate out, filtrated and then recrystallized by
ethanol. A bright yellow solid got. 9.2 g, Yield: (90%). 1H NMR
1698(
(CDCl3, ppm):
n
asNeC]O); 1657(
n
n
d
8.26e8.21 (q, 2H, H-10), 8.13e8.10 (q, 1.6H, H-8), 8.04e8.02 (d,
0.41H, H-8), 7.80e7.78 (q, 0.6H, H-9), 7.63e7.60 (q, 1H, H-9),
7.46e7.42 (q, 2.5H, NH), 7.31e7.26 (q, 0.41H, H-7), 6.97e6.92 (q,
1.4H, H-7), 5.59 (s, 1H, Boc-NHe), 4.22e4.18 (m, 4H,
COeNHeCH2CH2), 3.63e3.62 (m, 4H, eCH2eNe(CH2e)2), 3.20 (s,
2H,
CH2CH2eNeCH2CH2), 2.68e2.66 (m, 4H, COeNHeCH2$CH2), 2.48
(s, 2H, eCH2eNe(CH2CH2)2), 2.29e2.23(m, 4H,
(CDCl3, ppm):
d
8.53e8.51 (d, 1H, H-6, J ¼ 6.0 Hz), 8.45e8.41 (t, 2H,
J ¼ 6.0 Hz, H-10, 8), 7.7e7.65(t, J ¼ 9.0 Hz, 1H, H-9), 7.19e7.16 (t, 1H,
J ¼ 9.0 Hz, H-7), 3.31e3.28 (m, 4H, CH2eNeCH2, J ¼ 6.0 Hz),
1.90e1.89 (m, 4H, CH2CH2CH2), 1.77e1.75 (m, 2H, CH2CH2CH2).
MALDI-TOF-MS: m/z. Calculated: [MþH]þ ¼ 281.1, Found: 282.1,
([MþNa]þ) 314.1.
eCH2CH2eNe(CH2CH2)2),
3.10e3.06
(m,
5.7H,
eCH2eNe(CH2CH2)2). 1.83e1.82 (m, 6H, CH2CH2CH2), 1.70e1.69
Synthesis of 1,7-bis(4-tert-butylphenyloxy)perylene-3,4:9,10-
tetracarboxylic acid bisanhydride (PDI-2) [38].
(m, 3H, CH2CH2CH2), 1.39 (s, 9H, (CH3)3e). 13C NMR (CDCl3, ppm):
d
172.4, 164.6, 164.2, 157.1, 156.0, 132.8, 132.5, 131.6, 130.8, 130.7,
A mixture of 1,7-dibromoperylene-3,4:9,10-bis (dicarboxylic
anhydride) (2.0 g, 3.6 mmol), 4-tert-butylphenol (1.8 g, 12.0 mmol),
and cesium carbonate (2.38 g, 6.8 mmol) in dry DMF (120 mL) was
heated at the refluxing temperature for 4 h under an N2 atmo-
sphere. The reaction mixture was poured into water (100 mL) and
neutralized with aqueous 1.2 N HCl solutions. The formed precip-
itate was collected by filtration and washed with water and
methanol to give crude PDI-2 in 92% as this product showed poor
solubility in common organic solvents. Without further purification
it was used to the next reaction.
130.4, 130.3, 129.6, 127.7, 125.8, 125.0, 122.5, 114.4, 54.3, 52.7, 49.7,
39.6, 38.5, 33.5, 28.4, 26.1, 24.2. TOF-MS-ES: m/z. Calculated: 914.4,
Found: [MþH]þ ¼ 915.4, [MþNa]þ ¼ 937.4.
2.1.2.7. Compound NB-3. Compound vi-1 (1.2 g, 1.3 mmol) was
placed into a round bottom flask to which dichloromethane (25 mL)
and trifluoroacetic acid (6 mL) was added. The mixture was stirred
vigorously and the reaction was followed by TLC (DCM:
Methanol ¼ 10:1). On completion of the reaction the excess trifl-
uoroacetic acid and dichloromethane were removed under reduced
pressure to give compound BN-3. Compound BN-3 was used for the
next reaction without further purification 98%. Compound PD-3
was prepared according the same method of compound PDI-NB.
Yield (99%).
2.1.2.5. Compound NB-1. A mixture of Pd(OAc)2 (22.5 mg,
0.10 mmol), tri(o-tolyl)Phosphine (70.5 mg, 0.20 mmol), K3PO4
(1.84 g, 5 mmol), 2-vinylpyridine (0.91 g, 8.69 mmol), and 4-Bromo-
1,8-naphthalicanhydride (2 g, 7.24 mmol) in DMF (100 mL) was
stirred for 24 h at 130 ꢁC, under N2. After cooled to room temper-
ature H2O (150 mL) were added to the reaction mixture. The pre-
cipitation was filtrated and washed with water and then recrystled
by methanol. A yellow solid got. 1.61 g, Yield: (74%). 1H NMR (CDCl3,
2.1.2.8. Compound PDI-NB. Compound NB-3 (1.1 g, 1.2 mmol),
compound PDI-2 (0.42 g, 0.6 mmol) and N(C3H7)3 was dissolved in
DMF (30 mL), the mixture was heated under reflux for 12 h under
N2. After cooled to room temperature the solvent was removed
under reduced pressure, a red-black solid got, washed with waters
(150 mL), dry in 60 ꢁC, A more rigorous purification was then car-
ried out via column chromatography (eluting with DCM/
Methanol ¼ 20:1) to give a purple solid 0.9 g, Yield: 75%. (m.p.
ppm):
d
8.85e8.82 (d, 1H, J ¼ 9.0 Hz, H-1), 8.77e8.75 (d, 1H,
J ¼ 6 Hz, H-5), 8.73 (s, 0.5H, H-3), 8.71e8.69 (d, 1H, J ¼ 6.0 Hz, H-9),
8.67 (s, 0.5H, H-3), 8.62 (s, 0.5H, H-4), 8.57 (s, 0.5H, H-4), 8.17e8.14
(d, 1H, J ¼ 9 Hz, H-2), 7.93e7.88 (m, 1H, H-6), 7.83e7.77 (m, 1H, H-
7), 7.52 (s, 0.5H, H-8), 7.49e7.48 (d, 1H, J ¼ 6 Hz, HC]C), 7.43 (s,
0.5H, H-8), 7.34e7.32(d, 1H, J ¼ 6 Hz, C]CH). TOF-MS-ES: m/z.
Calculated: 301.0, Found: [MþH]þ ¼ 302.1, [MþNa]þ ¼ 334.1.
208e210 ꢁC). FT-IR (KBr) cmꢀ1: 3422 (
1698(
asNeC]O); 1664(
(CDCl3, ppm):
9.25 (s, 2H, H-a, a0), 8.68 (s, 2H, H-1), 8.54e8.45(q,
n
NH); 2950e2863(
n
nCH);
n
n
sNeC]O) 1576( NeC]O). 1H NMR
d
4H, H-1, H-b, b0), 8.38e8.30(m, 6H, H-5, H-c, c0), 8.09e7.96(m, 9H,
H-3, H-9, H2C]C), 7.73e7.40(q, 20H, H2C]C, COeNHeCH2e, H-2,
4, 6, 7), 7.14e7.09 (m, 9H, H-7, 8, H-d, d0), 6.99e6.95 (s, 4H, H-d, d0),
4.22e4.08 (m, 12H, 6 ꢂ (CONCH2CH2)), 3.62 (s, 8H, 4 ꢂ (CON
CH2$CH2)), 2.86e2.72 (m, 12H, 2 ꢂ (CH2N(CH2CH2)2)), 2.40 (s, 8H,
2 ꢂ (CH2Ne(CH2CH2)2)), 1.37 (s, 18H, eC(CH3)3). 13C NMR (CDCl3,
2.1.2.6. Compound NB-2. A mixture of compound AN-Boc (0.8 g,
2.06 mmol), compound NB-1 (1.39 g, 4.94 mmol) was dissolved in
anhydrous ethanol (150 mL), refluxed for 48 h, after reaction was
finished (by TLC track), the ethanol was removed using a rotary
evaporator, The residue was subjected to silica gel column chro-
matography (eluting with DCM: methanol ¼ 10:1) to give a yellow
solid (m.p. 142e144 ꢁC). 1.54 g, Yield: (78%). FT-IR (KBr) cmꢀ1: 3382
ppm):
d 172.4, 172.3, 164.2, 154.0, 149.7, 139.9, 136.5, 133.3, 130.9,
130.4, 129.6, 127.5, 126.5, 123.5, 123.4, 123.2, 122.9, 119.1, 50.3, 50.2,
and
1657(
3294
(n
NH);
2978e2924(
n
CH);
1698(
n
asNeC]O);
39.9. MALDI-TOF-MS: m/z. Calculated: [MþH]þ
¼
2361.9039,
n
sNeC]O); 1576(
n
N-C]O). 1H NMR (CDCl3, ppm):
found: 2361.9045.

320
H.-r. Cheng, Y. Qian / Dyes and Pigments 112 (2015) 317e326
Compound PDI-PD was prepared according the same method of
compound PDI-NB. A oxblood red solid got. 1.61 g, Yield: 76%. (m.p.
180e182 ꢁC). FT-IR (KBr) cmꢀ1: 3422 (
NH); 2971e2870( CH);
1696(
asNeC]O) 1657( sNeC]O) 1584( NeC]O). 1H NMR
(CDCl3, ppm):
9.42e9.39 (m, 1H, H-1), 9.36e9.33 (q, 1H, H-10),
vinylpyridine. Then, NB-2 was prepared via amidation reaction of
NB-1 with AN-Boc, which carries two terminal 8-vinylpyridine
naphthalimide units. After purification by column chromatog-
raphy, the amino protecting group Boc of NB-2 was removed in
trifluoroacetic acid, and used in the next reaction without purifi-
cation. Reaction of PDI-2 with a slight excess of NB-3 in dry DMF in
the presence of NEt3 yielded PDI-NB with four shorter-wavelength
8-vinylpyridine naphthalimide units NB-1. It was purified by col-
umn chromatography (DCM/methanol ¼ 9/1) to give a purple solid.
PDI-PD was prepared according to the same method.
n
n
n
n
n
d
8.41e8.33 (q, 4H, H-2, H-6), 8.27e8.25 (m, 2H, H-6), 8.20 (s, 1H, H-
10), 8.17e8.12 (m, 4H, H-10, 8), 8.09 (q, 1H, H-8), 8.06 (m, 1H, H-8),
8.04e8.03 (m, 1H, H-9), 8.03 (m, 1H, H-9), 8.0e7.96 (m, 2H, H-9),
7.67e7.64 (m, 3H, H-10), 7.50e7.48 (m, 7.4H, eCONHCH2e, H-10),
7.35e7.33 (m, 4H, H-5, 50), 7.12e7.10 (m, 4H, H-4, 40), 6.88e6.86 (m,
2.5H, H-3, 30), 4.17e4.12 (m, 12H, 6 ꢂ (CONCH2CH2)), 3.54e3.53 (m,
8H, 4 ꢂ (CON CH2CH2)), 3.01 (m, 10H, CH2CH2ArNCH2CH2),
All of the new compounds were fully characterized by FT-IR, 1H
NMR, 13C NMR and high-resolution mass spectrometry (HRMS-
MALDI-TOF).
2.79e2.77 (m, 12H,
2
ꢂ
(C H2N(CH2CH2)2)), 2.34 (m, 8H,
2 ꢂ (CH2N(CH2CH2)2)), 1.77e1.75 (m, 13H, CH2CH2ArNCH2CH2),
1.65e1.64 (m, 6H, CH2CH2ArNCH2CH2CH2), 1.37 (s, 18H, eC(CH3)3).
3.2. Absorption and fluorescence spectra of
perylenediimideenaphthalimide dendrons
13C NMR (CDCl3, ppm):
d 172.2, 164.1, 163.5, 163.2, 157.0, 156.5,
152.3, 148.5, 146.0, 133.4, 132.2, 131.6, 131.0, 130.8, 130.2, 128.5,
127.7, 127.6, 125.7, 125.0, 124.9, 123.3, 123.2, 122.5, 122.3, 119.2,
114.4, 54.3, 39.6, 37.8, 33.8, 31.4, 26.2, 24.2. MALDI-TOF-MS:
Calculated: m/z. [MþH]þ ¼ 2282.0291, Found: 2282.0297.
The dendritic naphthalimideeperylenediimide molecules PDI-
NB and PDI-PD were configured on the “donorespacerereceptor”
model, where the perylene-diimide core is the fluorophore func-
tioning as energy acceptor, the tertiary amines are the proton or
metal receptor [36] and the naphthalimide is energy donor. The
AN-Boc part between perylene-diimide core and naphthalimide
fluorophore serves as spacer that covalently separates the two
units. The maximum absorption spectra of PDI-NB were at 385 nm
and 556 nm (Fig. 2(a)), and the maximum molar extinction coef-
ficient (εmax) of NB units was nearly 2.5 times that of NB-2 units in
PDI-NB. The absorption spectra of PDI-NB indicates that the den-
drimer holds the donor (yellow-green emitting periphery 1, 8-
naphthalimide derivates) and the acceptor chromophores (red-
emitting core PDI-2) together.
3. Results and discussion
3.1. Synthesis of perylenediimideenaphthalimide dendrons
The novel perylenediimideenaphthalimide dendrons were
prepared in three parts (Schemes 1 and 2): synthesis of the func-
tional naphthalimide, PAMAM dendronization of the functional
naphthalimide to an amino-terminated naphthalimide PAMAM
dendron and peripheral decoration of the perylenediimide with
latter to the perylenediimideenaphthalimide dendrons.
In brief, NB-1 was synthesized using palladium-catalyzed Heck
In these probes, it was predicted that a PET process (an
electron transfers from the donor to the excited state of the
coupling
of
4-Bromo-1,8-naphthalicanhydride
with
2-
Scheme 2. Synthesis of perylenediimideenaphthalimide dendrons.

H.-r. Cheng, Y. Qian / Dyes and Pigments 112 (2015) 317e326
321
Fig. 2. Absorption spectra of PDI-NB (a) and PDI-PD (b) in H2O/DMSO, 1:1.
fluorophore acceptor) would quench fluorescence emission of
the perylenediimide unit [39e42]. The fluorescence spectra of
PDI-NB and PDI-PD in DMSO/H2O, obtained after excitation
within the spectral region of maximal absorption of the periph-
eral fluorophore (lex ¼ 385 nm or 415 nm), showed two emission
bands at 460/600 nm and 540/560 nm, corresponding to the
emission bands of the donor 1, 8-naphthalimide derivates' frag-
ments and the acceptor core in PDI-NB and PDI-PD as shown in
Fig. 3. The corresponding data was showed in Table 1. The fluo-
rescence quantum yield of NB-2 units in PDI-NB was lower than
that of compound NB-2. The weakened fluorescence emission
may be caused by the electron transfer of N atom from of 4-
replacing 1, 8-naphthalimide derivates [43e50]. On the other
hand the fluorescence quantum yield of PDI units in PDI-NB was
0.07 which is nearly negligible as compared with the fluores-
cence quantum yield of compound PDI-2. Nearly same result can
be obtained in probe PDI-PD. From the weakened fluorescence
emission of perylenediimide unit in probe, it was predicted that a
PET process (electron transfer from the nitrogen to the excited
state of the PDI core or 1, 8-naphthalimide derivates) quenched
fluorescence emission of the perylenediimide unit in PDI-NB.
In order to gain insight into the energy transfer process, the
fluorescence life time was evaluated by additional fluorescence
decay measurements. The fluorescence decay curves were showed
in Fig. 4, which were measured by single photon counting method
[51]. As showed in Table 1, the fluorescence life time of compound
PDI-NB was 2.33 ns (at 450 nm) and 3.23 ns (at 600 nm), respec-
tively. The fluorescence life time of compound NB-2 was 4.9 ns (at
450 nm), this value was nearly two times that of the life of NB units
respectively. The decreased value of fluorescence life of NB-2 shows
that the excited state energy transfer occurs to the PDI core in
compound PDI-NB.
3.3. FRET of perylenediimideenaphthalimide dendrons based on
Fe(III) or proton induced
Among biologically important metal ions, iron ion is an impor-
tant microelement in the human body. It is an oxygen carrier in
hemoglobin [53e55]. High doses of iron ions are, however,
dangerous and can be toxic because of their ability to promote
oxidation of lipids, proteins and other cellular components [56].
Due to the possible toxicity of iron ion, the design of fluorescent
iron ion probes is an area of active investigation, and a great deal of
effort has been devoted to the design of molecular receptors con-
taining binding sites to iron ion [57e65].
As discussed above, the tertiary amines in space quenched the
fluorescence emission of the perylenediimide unit in probes. The
protonation or the respective metal coordination of the tertiary
amines receptors would increase their oxidation potential, and as
such, thermodynamically disallow the electron transfer [21,22,42].
Consequently electron transfer from the donors to the excited state
of the fluorophore acceptor would be “switched off”. Thus we
expect the fluorescence signal of fluorophore serving as acceptor to
be amplified.
As showed in Fig. 5(a) and (b), under excitation at lex ¼ 385 nm
(within the maximal absorption of the peripheral fluorophore) the
probe PDI-NB showed a subtle emission in the red-green spectral
region, while fluorescence intensity of the donoreacceptor system
PDI-NB with Fe3þ or Hþ induced in the same region is more than 8
times higher due to the energy transfer from the periphery to the
focal chromophore. Fig. 5(a) and (b) represents the emission
spectra of light harvesting antenna PDI-NB under excitation in
periphery (lex ¼ 385 nm) and direct at the core (lex ¼ 556 nm). The
€
in PDI-NB. The energy transfer rate constant for the Forster
mechanism, КFen, can be estimated by using spectroscopic quantities
and according to the equation (1) showed in the Supporting
information [52]. The energy transfer rate constant was calcu-
lated to be 6.4 ꢂ 109 sꢀ1 for PDI-NB and 5.2 ꢂ 109
s
ꢀ1 for PDI-PD,
Fig. 3. Fluorescence spectra of PDI-NB (a) and PDI-PD (b) in H2O/DMSO, 1:1.

322
H.-r. Cheng, Y. Qian / Dyes and Pigments 112 (2015) 317e326
Table 1
Optical properties of compounds NB-2, PDI-NB and PDI-PD in CHCl3, 1/1 at 298 K.
b
d
ðε1Þ=l2maabxsðε2Þnma (105 Mꢀ1 cmꢀ1
)
l1flu
/
l2flu (nm)c
D
v1/D
v2 (cmꢀ1
)
Fe1x
/
Fe2xe
E1/E2 (%)f
t1F/
t2F (ns)g
max
labs
Compounds
l
PDI-NB-Fe(III)
PDI-NB
NB-2
385(0.65)/556 (0.25)
385(0.54)/556 (0.25)
385 (0.48)
460/600
460/600
460
4230/1313
4230/1313
4230
0.01/0.24
0.55/0.07
0.76
98
e
e
2.33/3.23
4.9
e
PDI-PD-Fe(III)
PDI-PD
PD-2
415(0.65)/529 (0.25)
415(0.54)/529 (0.25)
415 (0.48)
540/560
540/560
540
5571/6232
5571
5571
0.01/0.18
0.56/0.09
0.72
96
e
e
4.6/4.04
5.2
e
The fluorescence quantum yields were determined using Quinoline (
absorbance was used as the excited wavelength.
F
¼ 0.55 in sulfuric acid solution) and rhodamine 6G (
F
¼ 0.95 in ethanol) as a standard. The max
a
Absorption maximum.
The maximum molar extinction coefficients.
b
c
Emission maximum.
d
Stoke's shift ¼ (1/labs ꢀ 1/lem) ꢂ 107.
e
Fluorescence quantum yield.
f
Energy transfer efficiency. (
The efficiency of energy transfer.
l,
D
v,
F
, E,
t
)
1
¼ Naphthalimide unit in probe, (
l,
Dv,
F
, E,
t)
¼ Perylenediimide unit in probe.
2
g
Fig. 4. Fluorescence decay curves of PDI-NB (a, b) and NB-2 (c) in CHCl3 at (A) 450 nm and (B) 600 nm and (C) 450 nm.
emission intensity of the core, excited by energy transfer from
peripheral units (lex ¼ 385 nm), is 2 times higher than that of the
one excited within the maximal absorption of the focal fluorophore
The reason for fluorescence energy transfer of PDI-NB and PDI-
PD with Fe(III) or Hþ induced could be the possible photoinduced
electron transfer (PET) from the PAMAM bone to the core of probe
(PDI-NB and PDI-PD) as shown in Fig. 6. The poly(amidoamine)
architecture contains a tertiary amine proton receptor able to
involve the system in a PET process. In this particular probe, it was
predicted that the PET process (an electron transfers from the re-
ceptor to the excited state of the fluorophore) would quench fluo-
rescence emission of the perylenediimids units. The protonation of
the tertiary amine in the fluorophore-amine conjugates would in-
crease the oxidation potential of the receptor. Thus, we expect the
fluorescence signal of the light PDI-NB and PDI-PD to be a function
of Fe(III) or proton.
Fluorescence increase of the light harvesting dendrons PDI-NB
and PDI-PD as a function of Fe(III) and pH concentration were
observed only under excitation within the spectral region of
maximal absorption of the peripheral fluorophores. This is an
indication of the two amine receptors' involvement in PET
quenching of the perylenediimid core's excited state. Upon
(lex ¼ 556 nm). The difference between the harvested and direct
core emissions clearly demonstrates the great ability of the light
harvesting system PDI-NB with Fe3þ or Hþ induced to transfer
energy from its periphery to the core. This observation indicates
that the light harvesting antenna is more efficient than the core dye
at capturing photons from environment. At the same time the
emission intensity of the periphery in the light harvesting system
PDI-NB was decreased by 98% as compared with the emission of
the blue emitting reference 1, 8-naphthalimide at the same optical
density. The efficiency of energy transfer was evaluated according
to the equation: 100 ꢂ [1-(fluorescence intensity of the energy
donor in the cassette)/(fluorescence intensity of the free energy
donor)]% [66]. The same detection was conducted on PDI-PD
(detailed data was showed in Table
1 in the Supporting
information). Next, more detailed ratio detection was conducted
as showed below.
Fig. 5. FRET based fluorescence spectra of (a) PDI-NB-Fe(III), (b) PDI-NB-Hþ and (c) PDI-PD-Fe(III) in water (DMSO/H2O, 1:1, v/v) (c ¼ 10
mM).

H.-r. Cheng, Y. Qian / Dyes and Pigments 112 (2015) 317e326
323
Fig. 6. The possible binding mode of dyes with Fe(III) or Hþ
.
protonation or bonding with Fe(III) of this amine the quenching
process is substantially removed.
fold at 600 nm (Fig. 7(a)). The titration curve of fluorescent probe
PDI-NB to the concentration of Fe(III) ions shows that with the
concentration of Fe(III) ions enhanced from 2 mM to 20 mM there are
significant increases in fluorescence intensity (at 600 nm), followed
by a negligible increase in fluorescent intensity with the concen-
3.4. FRET efficiency of the probes as a function of Fe(III)
concentration
trations of Fe(III) ions at 24
mM and higher (Fig. 7(a)-inset). This
suggests that the binding of 20
m
M Fe(III) ions to PDI-NB (10 M) is
m
The Fe(III)-binding behaviors of fluorescent probes PDI-NB and
PDI-PD were investigated by absorption and fluorescence spec-
troscopy. Fluorescent probe PDI-NB displays weak fluorescence
with a fluorescence quantum yield of 8% in DMSO/H2O (1:1, v/v).
But this probe shows significant enhancement of fluorescence upon
the addition of Fe(III), which leads to a fluorescent increase by 20-
close to saturation, and that fluorescent probe PDI-NB interacts
with Fe(III) in 2 equivalent molar ratio. At the same time the probe
shows significant absorption increases at 385 nm upon the addition
of Fe(III) accompanied by a blue shift in the absorption spectrum
(Fig. 8(a)). The hypsochromic shift is due to the weakened electron-
Fig. 7. Fluorescence spectra of PDI-NB and PDI-PD in H2O/DMSO (c ¼ 10
m
M, lPDI-NB ¼ 385 nm, lPDI-PD ¼ 357 nm) in the presence of Fe(III) at concentration of 2e40
mM. Inset:
Relative fluorescent intensities of Iaccept/Idonor as a function of Fe(III) concentration).
Fig. 8. Absorption spectra of PDI-NB (a) and PDI-PD (b) as a function of Fe(III) concentration ([PDI-NB] ¼ 10
of 40 eq. of Fe(III) or proton under lamp 365 nm, A ¼ probe, B ¼ probe-Fe(III) or proton).
mM. Inset: Color change of PDI-NB and PDI-PD (10 mM) on the addition

324
H.-r. Cheng, Y. Qian / Dyes and Pigments 112 (2015) 317e326
Fig. 9. The linear relationship between the fluorescence intensity of probes and Fe(III) in concentration within the range 2e40 mM.
donor ability of the nitrogen atom at pyridine units whose electron
couple is engaged in the complex formation with Fe(III) ions.
Fluorescent probe PDI-PD displays weaker fluorescence than
probe PDI-NB with a fluorescence quantum yield of 5% in DMSO/
H2O (1:1, v/v). This is due to more efficient fluorescence quenching
by photo-induced electron transfer from two tertiary amine groups
to the BODIPY core of the probe PDI-PD as compared to the tertiary
amine group of the probe PDI-NB. It displays similar responses to
Fe(III) ions in its absorption and fluorescence spectra as fluorescent
probe PDI-NB. Fluorescent probe PDI-PD shows increased blue
shifts in absorption spectra compared to fluorescent probe PDI-NB
The detection limit and quantification limit were measured to
be 1.2 ꢂ 10ꢀ7 M for PDI-NB (3
s/slope, s is the standard deviation of
the blank measurement) [68] and 1.8 ꢂ 10ꢀ7 M for PDI-PD,
respectively. In addition, the interaction of probes with Fe(III) was
completed in a few seconds, therefore 1 could be used for the real-
time and real-space analysis of Fe(III) ions, These results proved
that the probes possess a remarkably high binding affinity and
sensitivity to Fe(III).
3.6. FRET efficiency of the probes at various pH values
and the addition of 20 mM Fe(III) ions to fluorescent probe PDI-PD
led to 50 nm blue shift in the absorption spectrum at 415 nm
(Fig. 8(b)).
Protons play critical roles [69e71] in many cellular events such
as cell growth and apoptosis, calcium regulation, etc. So, protons
have become one of the most important sensing targets among the
interesting species [72]. According to published reports [39e42],
the fluorescence emission of PAMAM dendrons based on 1, 8-
naphthalimide is efficient “offeon” switcher for pH. The novel
light harvesting systems PDI-NB and PDI-PD are designed just
based on this theory. This was the reason to investigate the pho-
tophysical behavior of PDI-NB and PDI-PD at different pH values.
The pH dependence of the fluorescence properties of dendrimer
PDI-NB as a function of pH in water/DMSO (1:1, v/v) is illustrated in
Fig. 10(a). Weak fluorescence was observed at pH 8.00 with an
excitation wavelength of 385 nm. However, as the pH decreased
from 8.0 to 6.0, emission band centered at 600 nm increased
gradually and a distinct emission enhancement was observed with
the change of fluorescence intensity over 10-fold. The fluorescence
quantum yield of PDI-NB was also measured. In pH 10.0, the fluo-
3.5. Dissociation constant and the detection limit of probes
The apparent dissociation constant (Kd) was obtained by direct
fluorometric titration as a function of Fe(III) (in DMSO/H2O) using
the fluorescence emission spectra according to the reported
method [67]. The fluorescence intensity data (Fig. 9) was fitted to
eqn (1): Fmin and Fmax are the emission intensities with no addition
of Fe(III) and an addition of Fe(III) at 600 nm, respectively; n is the
binding stoichiometry between PDI-NB and Fe(III). A 2:1 stoichi-
ometry between probe and Fe(III) was assumed, and then the
binding stoichiometry kept fixed at 2 in the final curve fittings, The
value of Kd was found to be 5.3 ꢂ 10ꢀ11 M for PDI-NB and
2.5 ꢂ 10ꢀ10 M for PDI-PD.
n
rescence intensity was very low (
F
¼ 0.07) relative to rhodamine B
FminKd þ Fmax½Xꢃ
F ¼
(1)
n
(F
¼ 0.95 in MeOH) [73]. But when the pH was decreased to 6.00,
Kd þ ½Xꢃ
strong fluorescence was obtained immediately with a higher
Fig. 10. Fluorescence spectra of PDI-NB (a) and PDI-PD (b) at various pH values (c ¼ 10
in pH ¼ 1e13.
mM, lPDI-NB ¼ 385 nm, lPDI-PD ¼ 357 nm). Inset: Relative fluorescent intensities of Iaccept/Idonor

H.-r. Cheng, Y. Qian / Dyes and Pigments 112 (2015) 317e326
325
Fig. 11. Fluorescence responses of probe molecules to various metal ClOꢀ4 salts of metal cations such as Kþ, Fe3þ, Ca2þ, Mg2þ, Mn2þ, Cr3þ, Al3þ and Co2þ (3 eq.).
fluorescence quantum yield (
F
) of 0.24. Nearly same result can be
practical application in many sensing fields, such as environmental
obtained in probe PDI-PD (Fig. 10(b)). These changes are of such
magnitude that they can be considered as a pH “ONeOFF”, where
the fluorescence emission is “switched off” in alkaline solution and
“switched on” in acidic solution.
research and biology.
Acknowledgments
The light harvesting systems PDI-NB and PDI-PD are thus effi-
cient ‘‘offeon’’ switcher for pH. This switching process was also
found to be reversible. Taking the part of the graph in Fig. 5 located
between pH 3 and 9, the pKa values of the light harvesting dendron
PDI-NB and PDI-PD have been calculated by Eq. (2) [41]. The
calculated pKa value for PDI-NB was 6.02 and 6.25 for PDI-PD.
These results proved that the probes possess a high binding affinity
and sensitivity to proton.
This work was financially supported by the Fundamental
Research Funds for the National Natural Science Foundation of
China (No. 61178057) and for the Central Universities (No.
CXLX12_0085).
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.dyepig.2014.07.005.
Log½IFmax ꢀ IF=IF ꢀ IFminꢃ ¼ pH ꢀ pKa
(2)
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