Synthesis and properties of novel perylenetetracarboxylic diimide derivatives fused with BODIPY units

Article abstract of DOI:10.1016/j.dyepig.2010.08.014

Two novel fluorescent dyes based on perylenetetracarboxylic diimde (PDI) with methylpyridine or methylquinoline group at the carbonyl position were designed and synthesized. Their structures were confirmed by ¹H NMR, ¹³C NMR, MS, and elementary analysis. The resulted new compounds show longer wavelength absorption with relative high extinction coefficient and red-shifted emission with high fluorescence quantum yields. These compounds can further react with BF₃·Et₂O to form novel dyes containing BODIPY (difluoroboradiazaindacene) analogue unit, by which both the absorption and emission maximum have been red-shifted further. The minimized structures based on density function theory (DFT) calculation show planar configurations for the compounds. The calculated molecular orbital correlates well with their red-shifted absorption and emission spectra.

Full text of DOI:10.1016/j.dyepig.2010.08.014

Dyes and Pigments 89 (2011) 23e28
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Synthesis and properties of novel perylenetetracarboxylic
diimide derivatives fused with BODIPY units
,
,
*
Junqian Feng a b, Delou Wang a, Shuangqing Wang c, Liangliang Zhang a, Xiyou Li a
a Key Lab for Colloid and Interface Chemistry of Education Ministry, Department of Chemistry, Shandong University, Jinan 250100, China
b College of Shandong Policeman, Jinan 250014, China
c Key Laboratory of Photochemistry of CAS, Institute of Chemistry, Chinese Academy of Science, Beijng 100800, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Two novel fluorescent dyes based on perylenetetracarboxylic diimde (PDI) with methylpyridine or
methylquinoline group at the carbonyl position were designed and synthesized. Their structures were
confirmed by 1H NMR, 13C NMR, MS, and elementary analysis. The resulted new compounds show longer
wavelength absorption with relative high extinction coefficient and red-shifted emission with high
fluorescence quantum yields. These compounds can further react with BF3$Et2O to form novel dyes
containing BODIPY (difluoroboradiazaindacene) analogue unit, by which both the absorption and emis-
sion maximum have been red-shifted further. The minimized structures based on density function theory
(DFT) calculation show planar configurations for the compounds. The calculated molecular orbital
correlates well with their red-shifted absorption and emission spectra.
Received 30 June 2010
Received in revised form
20 August 2010
Accepted 24 August 2010
Available online 21 September 2010
Keywords:
Perylenetetracarboxylic diimide
Methylquinoline
Methylpyridine
BODIPY
Ó 2010 Elsevier Ltd. All rights reserved.
Synthesis
Photophysical properties
1. Introduction
show enhanced solubility and higher fluorescence quantum effi-
ciency in solid state [28].
Perylenetetracarboxylic diimides (PDIs) have found wide rang
applications in diverse fields of current research because of
their excellent thermal and photo-stability, high luminescence
efficiency, and novel optoelectronic properties [1e4]. Driven by the
demands of varies applications, the modification on molecular
structure of PDIs with intention to tune the photophysical proper-
ties has attracted a lot of research interest in the past decade
[5e12]. The modifications on molecular structure of PDI are usually
achieved by introducing side groups to the imide nitrogen atoms or
at the bay positions. Incorporation of substituents onto the imide
nitrogen atoms could improve the solubility of PDIs in solvents and
affect the packing behavior of PDIs in the solid state [13e22], but
did not change the photophysical properties significantly. However,
the introduction of substituents at the bay positions could change
the photophysical properties of PDIs as well as improve the solu-
bility of them in conventional organic solvents [7,23e27]. Recently,
direct alkylation of PDIs at 2,5,8,11-positions has been achieved by
the MuraieChatanieKakiuchi protocol. The resulted compounds
Fusing of aromatic rings with PDI conjugation system has turned
into another efficient method to modify the molecular structure of
PDI in the past several years, which could achieve significant prop-
erty variation [29e33]. The famous “amidine” derivatives of PDI,
developed by Langhals and Müllen groups [34e36], were prepared
by the condensation of perylenetetracarboxylic dianhydride with
diamino compounds. The absorption and fluorescence spectra of
these compounds are red-shifted even up to the near infrared region.
The cooperation of aromatic rings to the bay positions of PDI ring by
DielseAlder reactions has generated a series novel PDI compounds
with longer wavelength absorption and emission [37]. A recent
report on this topic described the cooperation of imidazole with PDI
ring at the bay position by light driving cyclization [38]. The resulted
compounds show red-shifted absorption and emission spectra as
well as the structure tunable self-assembly. We have recently fused
a difluoroboradiazaindacene (BODIPY) type ring with the PDI ring
successfully by introducing 2-methylquinoline group at the imide
nitrogen position of PDI [39]. The resulted new molecules show
significantly red-shifted absorption and emission spectra with high
fluorescence quantum yield.
In this paper, we present another way for the molecular struc-
ture modification of the PDI by introducing methylquinoline or
* Corresponding author. Tel.: þ86 531 88564464; fax: þ86 531 88369877.
E-mail address: xiyouli@sdu.edu.cn (X. Li).
0143-7208/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2010.08.014

24
J. Feng et al. / Dyes and Pigments 89 (2011) 23e28
MSARn2S
MRASn2R
methylpyridine groups at the carbonyl position (1 and 2 in Fig. 1).
To the best of our knowledge, this is the first example that
substituent has been selectively introduced to the carbonyl group
of PDI. The resulted new compounds show significantly red-shifted
absorption and emission spectra with large extinction co-efficiency
and reasonable fluorescence quantum yield. More interestingly,
these novel compounds can further react with BF3$Et2O to intro-
duce BODIPY type structure characteristics into the PDI molecules
(3 and 4 in Fig. 1).
FS
FR
¼
(1)
where V represents Fluorescence quantum yield, and M, the
emission intensity, was calculated from the spectrum area,
A represents the optical density at the excitation wavelength and n
the refraction coefficient of each solvent. The superscripts “S” and
“R” refer to the sample and to the standard, respectively. Fluores-
cence quantum yield was determined on the basis of the absorption
and fluorescence spectra. N,N0-dicyclohexylperylene-3,4:9,10-tetra
carboxylic diimide was used as a reference (V ¼ 100% in CH2Cl2).
2. Experimental
2.1. General methods
2.2. Computation details
1H and 13C NMR spectra were recorded on a Bruker 300 MHz
NMR spectrometer with chemical shifts reported in ppm (in CDCl3,
TMS as internal standard). MALDI-TOF mass spectra were taken on
a Bruker/ultra flex instrument. Absorption spectra were measured
on HITACHI U-4100 spectrophotometer. Fluorescence spectra
and fluorescence lifetime were measured on an ISS K2 system.
The fluorescence lifetimes were measured with a phase modulation
model with a scattering sample as standard. The fluorescence
quantum yields were determined according to Equation (1).
The hybrid density function B3LYP (BeckeeLeeeYoungeParr
composite of exchange-correction functional) method [40,41] and
the standard 6e31G(d) basis set [42] were used for both structure
optimization and the property calculations. All the calculations were
performed using the Gaussian 03 program in the IBM P690 system at
the Shandong Province High Performance Computing Centre.
2.3. Materials
N,N0-dibutyl-1,6,7,12-tetra(4-tert-butylphenoxy)perylene-3,4:9,
10-tetra carboxylic diimide (6) [43] were prepared following the
literature method and fully characterized by 1H NMR and MALDI-
TOF mass spectra. All other chemicals are purchased from
commercial source. Solvents were of analytical grades and were
purified by the standard method before use.
2.3.1. N-n-Butyl-1,6,7,12-tetra(4-tert-butylphenoxy)-3,4:9,10-
perylenetetra-carboxdiimide (5)
A solution of N-n-Butyl-1,6,7,12-tetra(4-tert-butylphenoxy)per-
ylene-3,4-anhydride-9,10-imide [39] (1.0 g, 1.15 mmol) in meth-
anamide (100 ml) was purged with nitrogen for 15 min and then was
heated to reflux. The resulting mixture was kept at reflux continuously
for another 16 h, and then was poured into water. The solid was
collected by filtration and purified by column chromatography on
silica gel with chloroform as the eluent. Repeated chromatography
followed by recrystallization from a mixture of CHCl3 and MeOH gave
pure product 5 (0.62 g, yield 52%): mp. > 300 ꢀC; 1H NMR (300 MHz,
CDCl3) d 8.38 (s, 4H), 8.22 (s, 4H), 7.24 (m, 8H), 6.82 (m, 8H), 4.11 (t, 2H),
1.65 (m, 2H), 1.40(m, 2H), 1.29 (s, 36H), 0.93 (t, 3H); MALDI-TOF MS
(m/z) 1039.2, Calcd for C68H66N2O8 (m/z) 1039.3. Anal. Calcd. for
C68H66N2O8: C, 78.59; H, 6.40; N, 2.70. Found: C, 78.52; H, 6.38; N, 2.79.
2.4.1. Compound 1. A mixture of 5 (0.20 g, 0.19 mmol), 2-methyl-
quinoline (2 g,14 mmol) and fresh dried zinc chloride (2 g,15 mmol)
was heated to 200 ꢀC for 15 min. The reaction mixture was extracted
several times with dichloromethane. The combined extract was
dried over sodium sulfate for overnight and then was evaporated to
dry. The red solid collected was purified by column chromatography
on silica gel with dichloromethane as eluent. Compound 1 was
afforded as green powder (0.10 g, 46%): mp. > 300 ꢀC; UVevis
(CH2Cl2,
CDCl3)
3
) 626 nm (6.25 ꢁ 104 L molꢂ1 cmꢂ1); 1H NMR (300 MHz,
d
14.37 (s,1H), 8.20 (s,1H), 8.16 (s,1H), 8.15 (s,1H), 8.12 (d,1H),
8.00 (d, 1H), 7.84 (s, 1H), 7.69 (t, 2H), 7.44 (m, 1H), 7.41 (m,1H), 7.27
(m, 8H), 6.84 (m, 8H), 6.36 (s, 1H), 4.10 (t, 2H), 1.66 (m, 2H), 1.42 (m,
2H),1.31 (s, 36H), 0.94 (t, 3H); 13C NMR (100 Hz, CDCl3)
d 163.0,160.5,
155.7, 155.5, 154.8, 153.4, 152.6, 146.5, 146.3, 146.1, 139.1, 135.8, 133.3,
132.9, 129.6, 127.9, 126.8, 126.1, 125.7, 125.6, 124.6, 122.9, 121.1, 121.0,
120.8, 120.4, 119.1, 118.8, 118.7, 118.1, 117.5, 116.7, 112.1, 100.9, 39.8,
33.8, 31.0, 29.7, 29.2, 19.7, 13.3; MALDI-TOF MS (m/z) 1164.3, Calcd.
Fig. 1. Synthesis of compound 1e4.

J. Feng et al. / Dyes and Pigments 89 (2011) 23e28
25
For C78H73N3O7 (m/z) 1164.4. Anal. Calcd. For C78H73N3O7: C, 80.45%;
H, 6.32%; N, 3.61%. Found: C, 80.39%; H, 6.35%; N, 3.63%.
2.4.2. Compound 2. A mixture of 5 (0.20 g, 0.19 mmol), 2-methyl-
pyridine (2 g, 21 mmol) and fresh dried zinc chloride (2 g, 15 mmol)
was heated to 200 ꢀC for 30 min. The reaction cake was extracted
several times with dichloromethane. The combined extract was
dried over sodium sulfate for overnight and then was evaporated to
dry. The red solid collected was purified by column chromatog-
raphy on silica gel with dichloromethane as eluent. Compound 2
was afforded as green powder (0.09 g, 42%): mp. > 300 ꢀC; UVevis
(CH2Cl2,
CDCl3)
3
) 615 nm (5.35 ꢁ 104 L molꢂ1 cmꢂ1); 1H NMR (300 MHz,
d
13.67 (s, 1H), 8.63 (d, 1H), 8.21 (s, 1H), 8.17 (s, 1H), 8.13 (s,
1H), 7.80 (s,1H), 7.62 (m, 2H), 7.28 (s,1H), 7.25 (m, 8H), 6.83 (m, 8H),
6.26 (s, 1H), 4.11 (t, 2H), 1.64 (m, 2H), 1.43 (m, 2H), 1.38 (s, 36H), 0.93
(t, 3H); 13C NMR (100 Hz, CDCl3)
d 163.1, 160.4, 155.8, 155.6, 154.8,
154.7, 154.3, 153.4, 152.7, 147.7, 146.5, 146.4, 146.1, 137.4, 136.1, 133.3,
133.0, 126.2, 126.1, 126.0, 124.7, 124.2, 121.0, 120.9, 120.7, 120.4,
120.0, 119.5, 119.1, 118.9, 118.8, 118.7, 118.1, 117.1, 116.6, 111.9, 101.3,
39.8, 33.8, 31.0, 29.7, 29.2, 19.9, 13.3; MALDI-TOF MS (m/z) 1114.3,
Calcd. For C74H71N3O7 (m/z) 1114.4. Anal. Calcd. For C74H71N3O7: C,
79.76%; H, 6.42%; N, 3.77%. Found: C, 79.73%; H, 6.39%; N, 3.75%.
Fig. 2. Normalized absorption spectra of compound 1e4 compared with that of 6 in
dichloromethane.
shorter reaction time compared with that of the literature method are
necessary. Product 1 and 2 were collected from the reaction mixture
with the yields of 46% and 42% respectively. Compound 1 or 2 reacts
further with BF3$OEt2 in toluene at 60 ꢀC in the presence of N-ethyl-N,
N-di-iso-propylamine for 2.5 h to give 3 and 4 in yields of 57%, 69%
respectively. All these compounds show good solubility in conven-
tional halogenated organic solvents, but small solubility in alcoholic
solvents. The structures of these compounds were fully characterized
by different spectroscopic methods, including 1H and 13C NMR,
MALDI-TOF mass spectrometry and elemental analysis. It is reason-
able to expect the formation of cis and trans isomers of compound 1
and 2 due to the newly formed carbonecarbon double bond. However,
the 1H NMR spectra did not show any evidence to support the pres-
ence of isomers of compound 1 and 2.
It is interesting that only the mono substituted compound 1 and
2 can be separated from the reaction mixture. Compounds with
more methylpyridine or methylquinoline groups attached were not
observed in the reaction mixture. This may be ascribed to the
steric hindrance introduced by the butyl group at imide nitrogen
and its electron donating nature, which reduce the reactivity of the
carbonyl groups at this end of the PDI significantly. Meanwhile, the
2.4.3. Compound 3. N-Ethyldiisopropylamine (0.5 ml) and BF3$Et2O
(1 ml) were added to the solution of compound 1 (0.1 g, 0.09 mmol)
in toluene (15 ml). The mixture was heated at 60 ꢀC and stirred under
nitrogen for 2.5 h. The residue was purified by silica gel column
chromatography with dichloromethane as eluent. Compound 3 was
collected as green powder (0.06 g, yield 57%): mp. > 300 ꢀC; UVevis
(CH2Cl2,
CDCl3)
3
) 648 nm (7.74 ꢁ 104 L molꢂ1 cmꢂ1); 1H NMR (300 MHz,
d
8.88 (d,1H), 8.25 (s,1H), 8.24 (s,1H), 8.17 (s,1H), 8.09 (d,1H),
7.97 (s, 1H), 7.80 (t, 1H), 7.70 (m, 1H), 7.50 (m, 1H), 7.30 (s, 1H), 7.22
(m, 8H), 6.90 (m, 4H), 6.78 (m, 4H), 6.35 (s, 1H), 4.13 (t, 2H), 1.66 (m,
2H), 1.40 (m, 2H), 1.30 (s, 36H), 0.94 (t, 3H); 13C NMR (100 Hz, CDCl3)
d
163.5,157.0,155.6,154.9,154.0,153.1,152.6,147.3,147.2,146.7,141.0,
132.7, 130.9, 128.9, 126.8, 125.0, 123.4, 122.1, 121.9, 120.2, 119.8, 119.5,
119.2, 118.6, 118.4, 40.4, 34.7, 32.7, 31.5, 30.2, 21.4, 20.4, 13.8; MALDI-
TOF MS (m/z) 1212.2, Calcd. For C78H72BF2N3O7 (m/z) 1212.2. Anal.
Calcd. For C78H72BF2N3O7: C, 77.28%; H, 5.99%; N, 3.47%. Found: C,
77.33%; H, 5.83%; N, 3.45%.
2.4.4. Compound 4. N-Ethyldiisopropylamine (0.5 ml) and BF3$Et2O
(1 ml) were added to the solution of compound 2 (0.1 g, 0.09 mmol)
in 15 ml toluene. The mixture was heated at 60 ꢀC and stirred under
nitrogen for 2.5 h. The residue was purified by silica gel column
chromatography using dichloromethane as eluent. Compound 4 was
collected as green powder (0.07 g, yield: 69%). mp. > 300 ꢀC; UVevis
(CH2Cl2,
CDCl3)
3
) 624 nm (4.87 ꢁ 104 L molꢂ1 cmꢂ1); 1H NMR (300 MHz,
d
8.61 (d,1H), 8.25 (s,1H), 8.21 (s,1H), 8.16 (s,1H), 7.91 (s,1H),
7.86 (m,1H), 7.27 (s,1H), 7.24 (m, 4H), 7.22 (m,1H), 7.21 (m, 4H), 6.87
(m, 4H), 6.77 (m, 4H), 6.34 (s, 1H), 4.11 (t, 2H), 1.63 (m, 2H), 1.40 (m,
2H),1.31 (s, 36H), 0.94 (t, 3H); 13C NMR (100 Hz, CDCl3)
d 163.6,156.9,
155.4, 154.9, 154.1, 153.1, 147.3, 147.1, 146.6, 141.6, 133.3, 131.2, 126.6,
123.5, 122.0, 121.3, 120.1, 119.7, 119.6, 119.4, 119.2, 115.0, 40.4, 34.3,
31.0, 30.2, 29.7, 20.4, 13.8; MALDI-TOF MS (m/z) 1162.2, Calcd.
For C74H70BF2N3O7 (m/z) 1162.2. Anal. Calcd. For C74H70BF2N3O7: C,
76.48%; H, 6.07%; N, 3.62%. Found: C, 76.53%; H, 6.04%; N, 3.59%.
3. Results and discussion
The synthetic procedures of compound 1e4 are shown in Fig. 1.
Compound 1 and 2 were synthesized following a modified literature
method [39,44,45]. Condensation of 5 with 2-methylquinoline or 2-
methylpyridine was carried out without solvent using zinc chloride as
catalyst. To obtain compound 1 or 2, extra excess of zinc chloride and
Fig. 3. Normalized fluorescence spectra of 1e4 compared with that of 6 in dichloro-
methane (excited at 450 nm).

26
J. Feng et al. / Dyes and Pigments 89 (2011) 23e28
Table 1
and 2 are red-shifted for about 50 and 39 nm respectively compared
Photophysical properties of compound 1e4 and 6.
with that of 6 because the introduction of quinoline or pyridine
group at the carbonyl position of PDI has enlarged the conjugation
system of PDI ring. The red-shift of the maximal absorption band of
compound 1 is 11 nm larger than that of compound 2 because of the
larger conjugation system of quinoline compared with that of
pyridine. The maximal absorption bands of 3 and 4 were further
red-shifted to 648 and 624 nm respectively. These significantly
red-shifted maximal absorption bands relative to their precursors
may be attributed to the further extension of the conjugation system
because of the fixed coplanar conformation between quinoline or
pyridine and PDI ring.
The fluorescence spectra of this series compounds are recorded
in dichloromethane with excitation at 450 nm. Fluorescence
quantum yields (Ff) are calculated. The spectra and experimental
results are summarized in Fig. 3 and Table 1. Similar to that observed
in the absorption spectra, the maximal emission band of compound
1 and 2 red-shifted significantly relative to that of model compound
6. This can be ascribed to the extension of the conjugation system
Compounds labs
lem
Toluene Ff(%) CH2Cl2 Toluene
s(ns) CH2Cl2
THF
(nm) (nm)
THF
1
2
3
4
6
626
615
648
624
576
660
652
684
657
607
31.3
40.8
38.1
45.1
81.1
30.0 33.2
33.6 42.1
42.4 40.1
56.4 50.0
82.0 80.2
2.56
3.57
3.27
4.10
6.40
2.32 3.05
3.33 4.05
2.99 3.58
3.79 4.51
6.97 6.85
electron donating properties of methylpyridine or methylquinoline
reduced the reactivity of the carbonyl group left at the same end.
Therefore, only the mono substituted compounds can be isolated
from the reaction mixture.
Fig. 2 compares the electronic absorption spectra of compound
1e4 with model compound N,N0-dibutyl-1,6,7,12-tetra(4-tert-
butylphenoxy)perylene-3,4:9,10-tetra carboxylic diimide (6) [43] in
dichloromethane. All of these compounds show intense absorption
in the UVevis region. The maximal absorption band of compound 1
Fig. 4. The simplified molecular structure during the DFT calculation (A). The minimized structure of compound 1 (B) and 3 (C).

J. Feng et al. / Dyes and Pigments 89 (2011) 23e28
27
because of the incorporation of the quinoline or pyridine unit. The
maximal emission band of compound and further are
3
4
red-shifted to 684 and 657 nm respectively, which may be attrib-
uted to the further extension of the conjugation system because of
the bonding of boron fluorid. Quinoline group in compound 3 seems
more efficient on red-shifting the absorption and emission bands
than the pyridine group in compound 4 as revealed by the
comparison of the emission spectra of them, probably because of
the larger conjugation system of the former.
Both the fluorescence quantum yields and fluorescence lifetimes
of compound 1 and 2 are distinctively reduced relative to that of
standard compound 6 because of the introduction of quinoline or
pyridine. This might be attributed to the increased flexibility of the
molecule after the introduction of side groups, which induced
massive non-radiative decay for their excited states. The fluores-
cence quantum yields of compound 3 and 4 increased significantly
relative to their precursor 1 and 2. This can be attributed to the more
rigid molecular structure of compound 3 and 4 after the bonding of
boron, which reduces the non-radiative decay of the excited states.
To enhance our understanding of the relationship between
molecular structures and the electronic spectra of these new PDI
derivatives, we carried out structure optimization and molecular
orbital (MO) calculations on compound 1e4 based on a simplified
model with density functional theory (DFT) on the level of B3LYP.
The minimized structure is shown in Fig. 4 and the calculated
molecular orbital (HOMO and LUMO) energies are shown in Fig. 5.
For comparison purpose, model compound 6 has also been
calculated.
The minimized structure of compound 1 revealed that the
quinoline group at carbonyl position presents a coplanar confor-
mation with the PDI ring. This indicates that the quinoline group
conjugated with the PDI ring efficiently through the double bond.
This result is in accordance with the remarkable red-shifted
absorption spectra of compound 1 relative to model 6. Similar
coplanar conformation was also observed in the minimized struc-
ture of compound 2. The bonding of boron in compound 3 and 4 did
not vary the coplanar conformation of them.
Fig. 5. Molecular orbital energy levels of compound 1e4 and 6.
model 6 is dominantly caused by the double bond between the
quinoline and PDI ring. In another word, the conjugation system of
PDI ring does not extend to the whole quinoline ring, but is limited to
the double bond between quinoline and PDI ring. Similar results can
be deduced from the molecular orbital maps of compound 2
(Supporting information Fig. S1). This calculated result explains well
the larger red-shift on the absorption maximum of compound 1 and
2 relative to that of model compound 6, while the difference of the
maximum absorption wavelength between compound 1 and 2 is
small. The introduction of boron in compound 3 and 4 does not vary
the distribution of HOMO and LUMO significantly relative to those of
compound 1 and 2. This is why the energy levels of HOMO and LUMO
in compound 3 and 4 are close to that in compound 1 and 2.
The molecular energy levels of the frontier molecular orbital
shown in Fig. 5 revealed that the introduction of quinoline group at
the carbonyl position has raised the energy levels of HOMO and
LUMO simultaneously for compound 1 relative to that of the model
compound 6. But the increase of the HOMO is larger than that of
LUMO, therefore, the energy gap between HOMO and LUMO
decrease significantly. This corresponds well to the experimentally
recorded red-shifted absorption spectra of compound 1. Introduc-
tion of pyridine group has also increase the energy levels for both
LUMO and HOMO, but the energy gap between HOMO and LUMO is
relative larger than that in compound 1, therefore, the absorption
maximum in the absorption spectra of compound 2 is in the blue
side relative to that of compound 1.
Further bonding of boron in compound 3 and 4 do not change
the planar conformation of the molecules a lot, but reduce the
energy levels of HOMO and LUMO significantly. For instance, the
energy level of the HOMO of compound 1 is at ꢂ4.89 eV, and it
decreases to ꢂ5.12 eV in compound 3. Similar changes are found for
the energy levels of LUMOs. The energy gaps between the HOMO
and LUMO of compound 3 and 4 are little bit smaller than that of
compound 1 and 2 respectively, which is in accordance with the
red-shifted absorption spectra of compound 3 and 4 relative to that
of compound 1 and 2.
The frontier molecular orbital maps of compound 1 as shown in
Fig. 6 revealed that the contribution of the quinoline group to the
HOMO and LUMO is small, but the linkage between quinoline and
PDI ring contributes to the HOMO and LUMO significantly. Therefore,
the red-shift on the absorption maximum for compound 1 relative to
Fig. 6. Molecular orbital maps of compound 1 (A) and 3 (B).

28
J. Feng et al. / Dyes and Pigments 89 (2011) 23e28
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In summary, we have successfully synthesized two novel fluores-
cent dyes with longer wavelength absorption and emission by intro-
ducing methylpyridine or methylquinoline groups at the carbonyl
position. The new compounds can further react with BF3$Et2O to form
novel dyes with further red-shifted absorption and emission spectra.
Quantum calculation suggested that the extension of the conjugation
system of PDI ring is caused predominantly by the double bond
between side group and the PDI ring. The nature of the side group can
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Acknowledgements
Financial support from the Natural Science Foundation of China
(Grant No. 20771066 and 20640420467), Ministry of Education,
Shandong University are acknowledged.
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