Fluorescent macromolecular perylene diimides containing pyrene or indole units in bay positions

Article abstract of DOI:10.1016/j.dyepig.2009.11.005

Novel, symmetric and unsymmetric perylene diimide dyes with pyrene or indole units in the bay positions of the perylene ring were synthesized and characterized using FT-IR, ¹H and ¹³C NMR, MS, UV-Vis spectra and cyclic voltammetry. The λ_max in different solvents were in the range 526-585?nm and emission wavelengths of the dyes exhibited positive solvatochromism with increasing solvent polarity. Long wavelength emissions >750?nm of dyes with pyrene units displayed charge-separated state of perylene-pyrene system. Dyes with pyrene or indole units showed greater photostability in toluene than dyes which did not contain these bulky substituents. Incorporating electron-donating indole substituents lowered the band gap energies and, therefore, the HOMO energy levels were increased. The energy density and shape of the molecular orbitals were calculated theoretically.

Full text of DOI:10.1016/j.dyepig.2009.11.005

Dyes and Pigments 86 (2010) 32e41
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Fluorescent macromolecular perylene diimides containing pyrene
or indole units in bay positions
,
a
b
Haluk Dinçalp a , S¸ evki Kızılok , Sıddık Içli
_
*
a Department of Chemistry, Faculty of Art and Science, Celal Bayar University, Muradiye 45030, Manisa, Turkey
b Solar Energy Institute, Ege University, Bornova, 35100 Izmir, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
Novel, symmetric and unsymmetric perylene diimide dyes with pyrene or indole units in the bay
positions of the perylene ring were synthesized and characterized using FT-IR, 1H and 13C NMR, MS,
UV-Vis spectra and cyclic voltammetry. The lmax in different solvents were in the range 526e585 nm and
emission wavelengths of the dyes exhibited positive solvatochromism with increasing solvent polarity.
Long wavelength emissions >750 nm of dyes with pyrene units displayed charge-separated state of
peryleneepyrene system. Dyes with pyrene or indole units showed greater photostability in toluene than
dyes which did not contain these bulky substituents. Incorporating electron-donating indole substituents
lowered the band gap energies and, therefore, the HOMO energy levels were increased. The energy
density and shape of the molecular orbitals were calculated theoretically.
Received 13 September 2009
Received in revised form
26 November 2009
Accepted 28 November 2009
Available online 11 December 2009
Keywords:
Perylene diimide
Pyrene
Ó 2009 Elsevier Ltd. All rights reserved.
Solvatochromism
Charge transfer
HOMOeLUMO energy levels
DSSC
1. Introduction
accumulation at resembling electrodes and then to photocurrent
generation. In recent articles, to obtain new dyes having stronger
From a multifunctional perspective, perylene diimides (PDI)
have received much attention and are widely used in many appli-
cations owing to their high fluorescence quantum yield [1e3], large
molar absorption coefficient [3,4], high photo- and thermal
stabilities under visible light irradiation [5e7], high chemical
stability [8] and ease of their tunable absorption properties [9,10].
Both symmetric and unsymmetric derivatives of diimides have
been extensively used in dye sensitized solar cells (DSSC) [11e14],
organic light emitting diodes (OLED) [15], organic field effect
transistors (OFED) [16], liquid crystal displays (LCD) [17], dye lasers
[18], photosensitizers in chemical oxidations [7], photodynamic
therapy [19] and DNA G-quadruplex stabilization [20e22].
Many attempts have been made to enhance the light harvesting
properties and to improve the photophysical properties of PDIs.
Photoinduced energy and electron transfer processes in multi-
chromophoric PDI systems containing electron-donating and
electron-withdrawing groups are crucial point in the field of
material science, especially in the fabrication of DSSCs. In these
cells, an electron acceptoredonor system is required to enhance the
life time of charge-separated state which contributes to charge
intramolecular charge transfer and more promising absorptive
properties, PDIs have been functionalized at the 1- and 7-positions
using substituents which are suitable for solar light conversion
[23e26]. Absorption spectra of the synthesized dyes have to cover
the entire visible region of solar spectra to enhance the cell effi-
ciency. Also, the energy distribution of HOMO/LUMO levels of dyes
is significant subject for charge separation and charge injection
processes in DSSCs. If the HOMO/LUMO orbital separation is strong
on the same molecule, charge injection occurs immediately from
excited state of dye to the conduction band of metal oxide. Thus, the
photovoltaic performance of dye will be better.
This paper concerns the participation of electron donor, pyrene
or indole moieties, in the electron-acceptor perylene core for
improving solar cell efficiency. The synthetic routes are shown in
Schemes 1 and 2 for the preparation of the perylene series (P dyes).
2. Experimental
2.1. General procedures
1H NMR (400 MHz) and 13C spectra (100 MHz) were recorded
by a Bruker spectrometer. FT-IR spectra were recorded on a Perkin
Elmer-Spectrum BX spectrophotometer on KBr pellets. LC/MS
* Corresponding author. Tel.: þ90 236 2412151/2543; fax: þ90 236 2412158.
E-mail address: haluk.dincalp@bayar.edu.tr (H. Dinçalp).
0143-7208/$ e see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2009.11.005

H. Dinçalp et al. / Dyes and Pigments 86 (2010) 32e41
33
Scheme 1. Synthesis of the perylene diimide dyes P6 and P8.
analyses were performed with an Agilent 1100 MSD instrument.
The collision energies in positive and negative ion mode were
determined at 55 eV and 70 eV, respectively. UV-Vis spectra were
taken on an Analytic Jena Speedcord S-600 diode-array spectro-
photometer in the solutions. Fluorescence properties of the
synthesized compounds were obtained using a PTI QM1 fluores-
cence spectrophotometer. Their fluorescence quantum yields were
calculated with reference to fluorescence emission of perylene-
supporting electrolyte was 100 mM [TBA][PF6] in Me-CN. All dyes
were prepared in spectrophotometric grade Me-CN. All potentials
were internally referenced to the ferroceneeferrocenium couple
which was exhibited at about þ0.54 V. Redox potentials of the
dyes were calculated from reversible waves with the formula
[Ep(ox) þ Ep(red)]/2.
Molecular mechanics optimization of dye molecules was ach-
ieved by MMþ force field and the HOMOeLUMO orbital surfaces
were calculated by ZINDO/S (Single Point/CI) using Hyperchem
Package Version 8.0 computational software.
3,4,9,10-tetracarboxylic-bis-N,N0-dodecyl diimide
(FF
¼
1.0 in
chloroform, lexc ¼ 485 nm) [27] and to fluorescence emission of
pyrene (FF ¼ 0.32 in cyclohexane, lexc ¼ 317 nm) [28]. The fluo-
rescence decay times were taken on a sub-nanosecond pulsed
LEDs. Details on the time-resolved measurements were reported
elsewhere [22]. Optical density of the solutions which were used
in the spectroscopic studies was below 0.1. Cyclic and differential
pulse voltammetry studies of the P dyes were performed with
a CH instruments (Electrochemical Workstation) and a potentio-
stat in a three electrode single-compartment cell which was
controlled by a computer. Surface of glassy carbon electrode which
was used as working electrode was polished prior to use in order
to obtain reproducible surfaces. Platinum and Ag/Agþ electrodes
were used as counter and reference electrodes, respectively. The
The following chemical reagents were purchased and used
without further purification: perylene-3,4,9,10-tetracarboxylic
dianhydride (Fluka), bromine (Merck), iodine (Merck), 2,6-dii-
sopropylaniline (Aldrich), propionic acid (Laboratory BDH
Reagent), hydroquinone (Merck), 2-aminonicotinic acid (Abcr
Gmbh), Zn(CH3COO)2 2H2O (Carlo Erba), 1-pyrene butyric acid
(Merck), N,N0-dicyclohexylcarbodiimide (Aldrich), 4-dimethyla-
minopyridine (Acros Organics), p-toluenesulfonic acid mono-
hydrate (Fluka), indole-3-butyric acid (Aldrich). Other chemical
reagents and organic solvents were analytical grade and used
without further purification. To investigate the solvent depen-
dent kinetics of the dye solutions, all dyes were studied in five

34
H. Dinçalp et al. / Dyes and Pigments 86 (2010) 32e41
Scheme 2. Synthesis of P7 and P9 dyes from P5.
different solvents ranging from low to high polarity. These
d
7.25 ppm]:
d
¼ 9.58 (2H, d, J ¼ 7.8 Hz, perylene H), 8.98 (2H, s,
solvents were toluene (
3
¼ 2.4), chloroform (
3
¼ 4.8), methanol
perylene H), 8.78 (2H, d, J ¼ 7.8 Hz, perylene H) ppm.
(3
¼ 32.7), acetonitrile (
3
¼ 35.9), and PBS (
3
¼ 80.2) [29] and
spectroscopic grade.
2.2.2. Synthesis of N,N0-bis(2,6-diisopropylphenyl)-1,7-
dibromoperylene-3,4,9,10-tetracarboxylic diimide (P2)
Following the published procedure [31], a solution of P1
(150 mg, 0.17 mmol) in 10 mL of propionic acid was treated with
2.2. Synthesis
2,6-diisopropylaniline (224 mL, 1.1 mmol). The reaction mixture was
2.2.1. Synthesis of 1,7-dibromo-perylene-3,4,9,10-tetracarboxylic
dianhydride (P1)
Following the published procedure [30], a 500 mg (0.91 mmol)
sample of perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA)
was dissolved in 4 mL of conc H2SO4. The solution was stirred at
room temperature under an air atmosphere for 16 h. Iodine
stirred at 130 ꢀC under a nitrogen atmosphere for 26 h. The hot
solution was filtered and the crude product was purified by column
chromatography on silica gel using n-hexane:ethyl acetate:acetic
acid (35:13:2) as an eluent to give the product as bordeaux solid.
C48H40Br2N2O4, Yield: 70%, FT-IR (KBr, cmꢁ1): 2962 (aliphatic nCeH),
2924 (aliphatic nCeH), 2868 (aliphatic nCeH), 1711 (nC]O), 1672 (nC]
O), 1590 (aromatic nC]C), 1465, 1386 (nCeN), 1335, 1240, 1198, 1144,
(11.8 mg, 46.5
mmol) and 144 mL (2.81 mmol) of bromine were
added to the solution. The solution was heated to 85 ꢀC for 10 h
with stirring. The mixture was cooled to room temperature, and the
product was isolated by filtration and washed with water until the
pH of the solution was reached to about 7. The crude product was
purified by column chromatography on silica gel using CHCl3:
CH3COOH (48:2) as an eluent to give the product as light red solid.
C24H6Br2O6, Yield: 84%, FT-IR (KBr, cmꢁ1): 3119 (aromatic nCeH),
1770 (nC]O), 1761 (nC]O), 1594 (aromatic nC]C), 1507, 1406, 1301
835 cmꢁ1. 1H NMR [400 MHz, CDCl3,
d
7.25 ppm]:
d
¼ 9.57 (2H, d,
J ¼ 7.8 Hz, perylene H), 9.02 (2H, s, perylene H), 8.81 (2H, d,
J ¼ 7.8 Hz, perylene H), 7.51 (2H, t, J ¼ 7.8 Hz, benzene H), 7.37 (4H,
d, J ¼ 7.8 Hz, benzene H), 2.74 (4H, septet, J ¼ 7.0 Hz), 1.20 (24H, d,
J ¼ 7.0 Hz) ppm. 13C NMR [100 MHz, CDCl3,
d 77.5 ppm (3 peaks)]:
162.6 (C]O), 145.8, 138.6, 133.4, 130.8, 130.3, 130.0, 129.8, 128.9,
124.3, 123.4, 121.4, 121.2, 29.5 (CH(CH3)2), 24.1 (CH(CH3)2) ppm.
LC/MS (ESI) m/z (%) ¼ 871 (4), 870 (3), MI peak 869 [Mþ] (5), 314
(35), 286 (43), 258 (24), 230 (15), 202 (100), 102 (10).
(
nCeO), 1235, 1122, 1024, 860, 733 cmꢁ1. 1H NMR [400 MHz, CDCl3,

H. Dinçalp et al. / Dyes and Pigments 86 (2010) 32e41
35
2.2.3. Synthesis of N,N0-bis(2,6-diisopropylphenyl)-1,7-bis
column chromatography on silica gel using chloroform:ethyl
acetate:methanol (40:8:2) as an eluent. C54H37N3O10, Yield: 60%,
FT-IR (KBr, cmꢁ1): 3523 (carboxylic acid OeH stretching), 3450,
3310 (phenol OeH stretching), 2957 (aliphatic nCeH), 2924
(aliphatic nCeH), 2868 (aliphatic nCeH), 1700 (imide nC]O), 1664
(carboxylic acid nC]O), 1653 (imide nC]O), 1627, 1588 (aromatic nC]
C), 1502, 1404, 1337 (nCeN), 1258 (phenol nCeO), 1188 (carboxylic acid
(4-hydroxyphenoxy)perylene-3,4,9,10-tetracarboxylic diimide (P3)
A mixture of 0.70 g (0.81 mmol) P2, 0.74 g (6.72 mmol) of
hydroquinone and 1.09 g (7.88 mmol) dry K2CO3 in 30 mL of dry
DMF was reacted at 150 ꢀC under an inert atmosphere for 1 h. After
the mixture was cooled to room temperature, 30 mL of water was
added to the solution. The precipitate was collected by suction
filtration, and then washed with cold methanolewater solution.
Chromatography on silica using chloroform:ethyl acetate (40:10)
system afforded a purple solid. C60H50N2O8, Yield: 60%, FT-IR
(KBr, cmꢁ1): 3329 (OeH stretching), 2963 (aliphatic nCeH), 2927
(aliphatic nCeH), 2869 (aliphatic nCeH), 1703 (nC]O), 1654 (nC]O),
1589 (aromatic nC]C), 1505, 1447, 1406, 1364, 1337 (nCeN), 1258
nCeO), 1068, 973, 808, 744, 553 cmꢁ1
.
1H NMR [400 MHz, CDCl3,
¼ 9.60 (2H, d, J ¼ 8.6 Hz, perylene H), 9.55 (1H, d,
d
7.19 ppm]:
d
J ¼ 7.8 Hz, pyridine H), 8.69 (1H, d, J ¼ 8.6 Hz, perylene H), 8.59 (1H,
d, J ¼ 8.6 Hz perylene H), 8.30 (1H, d, J ¼ 7.8 Hz, pyridine H), 8.27
(1H, s, perylene H), 8.17 (1H, s, perylene H), 7.38 (1H, t, J ¼ 7.8 Hz,
benzene H), 7. 27 (1H, s, pyridine H), 7.24 (2H, d, J ¼ 7.8 Hz, benzene
H), 6.98 (4H, d, J ¼ 8.6 Hz, phenol H), 6.83 (4H, d, J ¼ 8.6 Hz, phenol
H), 2.68 (2H, septet, J ¼ 7.0 Hz), 1.08 (6H, d, J ¼ 7.0 Hz), 1.06 (6H, d,
(
nCeO), 1190, 906, 838 cmꢁ1. 1H NMR [400 MHz, CDCl3,
d 7.25 ppm]:
d
¼ 9.70 (2H, d, J ¼ 8.6 Hz, perylene H), 8.72 (2H, d, J ¼ 8.6 Hz,
perylene H), 8.40 (2H, s, perylene H), 7.45 (2H, t, J ¼ 7.8 Hz, benzene
H), 7.31 (4H, d, J ¼ 7.8 Hz, benzene H), 7.07 (4H, d, J ¼ 8.6 Hz, phenol
H), 6.87 (4H, d, J ¼ 8.6 Hz, phenol H), 5.61 (2H, s, eOH), 2.73 (4H,
septet, J ¼ 7.0 Hz), 1.15 (24H, d, J ¼ 7.0 Hz) ppm. 13C NMR [100 MHz,
J ¼ 7.0 Hz) ppm. 13C NMR [100 MHz, methanol,
d 47.7 ppm
(7 peaks)]: 163.7 (C]O), 163.1 (C]O), 162.3 (C]O), 156.1, 155.5,
155.3, 148.7, 146.4, 146.2, 138.7, 136.8, 133.3, 132.0, 130.5, 129.9,
129.3, 128.8, 124.9, 123.8, 123.3, 123.2, 121.6, 121.3, 116.8, 29.0 (CH
(CH3)2), 23.1 (CH(CH3)2), 18.5 (CH(CH3)2) ppm. LC/MS (ESI) m/z
(%) ¼ 890 [Mþ þ 2] (1), 871 (16), 383 (30), 382 (100), 368 (47), 314
(24), 286 (37), 258 (15), 249 (22), 230 (29), 128 (29).
CDCl3, d 77.0 ppm (3 peaks)]: 163.9 (C]O), 148.9, 147.8, 145.7, 142.5,
131.2, 130.5, 129.5, 128.5, 126.4, 125.4, 124.8, 124.0, 121.1, 119.7, 117.0,
116.7, 28.9 (CH(CH3)2), 24.0 (CH(CH3)2) ppm. LC/MS (ESI) m/z
(%) ¼ 928 (1), MI peak 927 [Mþ] (2), 714 (20), 452 (63), 314 (18), 286
(18), 258 (11), 230 (16), 202 (100), 113 (26), 102 (11).
2.2.6. Synthesis of 4-(N,N-dimethylamino)pyridinium-4-
toluenesulfonate (DPTS)
2.2.4. Synthesis of N-(2,6-diisopropylphenyl)-1,7-bis(4-
hydroxyphenoxy)perylene-3,4,9,10-tetracarboxylic 3,4-anhydride
9,10-imide (P4)
Following the published procedure [33], a mixture of 0.1 g
(0.82 mmol) 4-dimethylaminopyridine and 0.156 g (0.82 mmol)
p-toluenesulfonic acid monohydrate was stirred in 10 mL of THF at
room temperature for 30 min. The resulting precipitate was
collected by suction filtration to give white solid. C14H18N2O3S,
Yield: 90%, FT-IR (KBr, cmꢁ1): 3064 (aromatic nCeH), 2918 (aliphatic
Following the published procedure [32], the mixture of 0.16 g
(0.17 mmol) P3 and 0.064 g (1.14 mmol) of KOH in 40 mL of t-BuOH
was reacted under reflux for 2 h. The reaction mixture was poured
into a solution of 30 mL of AcOH and 15 mL of 2 N HCl. The resulting
precipitate was collected by suction filtration, and then washed
with dilute NaHCO3 solution until the pH of the solution was
reached to about 7. The crude product was purified by silica column
chromatography eluted with CHCl3:CH3COOH (45:5). C48H33NO9,
Yield: 40%, FT-IR (KBr, cmꢁ1): 3310 (OeH stretching), 2963
(aliphatic nCeH), 2924 (aliphatic nCeH), 2851 (aliphatic nCeH), 1770
(anhydride nC]O), 1734 (anhydride nC]O), 1706 (imide nC]O), 1658
(imide nC]O), 1594 (aromatic nC]C), 1504, 1448, 1406, 1348 (nCeN),
þ
nCeH), 2789 (vAreN eH), 1647, 1600, 1563 (aromatic nC]C), 1451, 1401,
1216 (asymmetrical nS]O), 1158 (symmetrical nS]O), 1118, 1029 and
1006 (simetrik nSeO), 816, 682, 567 cmꢁ1 1H NMR [400 MHz,
.
methanol-d4,
d
4.82 (1) and 3.30 (5) ppm]: 8.07 (2H, d, J ¼ 6.2 Hz,
pyridine H), 7.70 (2H, d, J ¼ 7.8 Hz, benzene H), 7.22 (2H, d,
J ¼ 7.8 Hz, benzene H), 6.95 (2H, d, J ¼ 6.2 Hz, pyridine H), 3.21 (6H,
s, AreN(CH3)2), 2.34 (3H, s, AreCH3) ppm.
2.2.7. Synthesis of N,N0-bis(2,6-diisopropylphenyl)-1,7-bis
{4-[(4-pyrene-1-ylbutanoyl)oxy]phenoxy}perylene-3,4,9,10-
tetracarboxylic diimide (P6)
1258 (phenol nCeO), 1192 (anhydride nCeO), 1018 cmꢁ1
.
1H NMR
¼ 9.43 (2H,
[400 MHz, methanol-d4, 4.80 (1) and 3.30 (5) ppm]:
d
d
d, J ¼ 8.6 Hz, perylene H), 8.54 (2H, d, J ¼ 8.6 Hz, perylene H), 8.24
(2H, s, perylene H), 7.43 (1H, t, J ¼ 7.8 Hz, benzene H), 7.32 (2H, d,
J ¼ 7.8 Hz, benzene H), 7.06 (4H, d, J ¼ 7.8 Hz, phenol H), 6.87 (4H, d,
J ¼ 7.8 Hz, phenol H), 2.69 (2H, septet, J ¼ 7.0 Hz), 1.13
(6H, d, J ¼ 7.0 Hz), 1.11 (6H, d, J ¼ 7.0 Hz) ppm. 13C NMR [100 MHz,
Following the published procedure [34], the mixture of 17 mg
(18.3 mmol) P3, 27 mg (93.6 mmol) 1-pyrene butyric acid, 33 mg
(0.11 mmol) DPTS and 67 mg (0.32 mmol) N,N0-dicyclohex-
ylcarbodiimide (DCC) in dry DMF/CH2Cl2 [7.5 mL (1:2)] was reacted
at room temperature under an inert atmosphere for 18 h. After the
mixture was cooled to room temperature, 15e20 mL of methanol
was added to the solution to precipitate the crude product. The
product was collected by filtration and then purified by silica
column chromatography eluted with chloroform:ethyl acetate
(40:10) to give P6. C100H78N2O10, Yield: 70%, FT-IR (KBr, cmꢁ1):
3445, 3014, 2924 (aliphatic nCeH), 2862 (aliphatic nCeH), 1734 (ester
nC]O), 1700 (imide nC]O), 1650 (imide nC]O), 1558 (aromatic nC]C),
1541, 1490, 1457, 1398, 1342 (nCeN), 1258 (phenol nCeO), 1177 (ester
acetone-d6,
d 206.3 (single peak) and 29.3 ppm (7 peaks)]: 163.3
(C]O), 162.8 (C]O), 159.6, 157.2, 156.3, 155.7, 147.1, 146.9, 146.2,
134.3, 132.9, 131.4, 130.0, 129.5, 129.1, 128.9, 126.6, 124.6, 123.9,
123.7, 123.3, 122.0, 121.6, 119.5, 117.3, 117.2, 28.7 (CH(CH3)2), 23.6
(CH(CH3)2), 13.7 (CH(CH3)2) ppm. LC/MS (ESI) m/z (%) ¼ 769 (4), MI
peak 768 [Mþ] (7), 543 (10), 452 (11), 314 (52), 286 (64), 258 (41),
230 (39), 202 (100), 113 (12), 102 (16).
2.2.5. Synthesis of N-(2,6-diisopropylphenyl)-N0-(3-carboxy-2-
pyridyl)-1,7-bis(4-hydroxyphenoxy)perylene-3,4,9,10-
tetracarboxylic diimide (P5)
nCeO), 1104 (ester nCeO), 1048, 869, 822, 668 cmꢁ1
.
1H NMR
[400 MHz, CDCl3, 7.25 ppm]:
d
d
¼ 9.57 (1H, d, J ¼ 7.8 Hz, perylene
H), 8.71 (1H, d, J ¼ 7.8 Hz, perylene H), 8.43 (1H, s, perylene H),
8.32 (1H, s, perylene H), 8.30 (2H, d, J ¼ 7.8 Hz, perylene H), 8.14
(10H, m, pyrene H), 8.02 (4H, s, pyrene H), 7.97 (2H, m, pyrene H),
7.87 (2H, m, pyrene H), 7.47 (2H, t, J ¼ 7.8 Hz, benzene H), 7.32 (4H,
d, J ¼ 7.8 Hz, benzene H), 7.16 (4H, s, phenol H) 7.14 (4H, s, phenol
H), 3.48 (2H, t, J ¼ 7.0 Hz), 3.39 (1H, t, J ¼ 7.8 Hz), 2.70 (7H, m),
2.46 (1H, t, J ¼ 7.0 Hz), 2.29 (5H, m), 1.16 (12H, d, J ¼ 6.8 Hz), 1.14
A mixture of 35 mg P4 (45.6
mmol), 20 mg (144.8
mmol)
2-aminonicotinic acid and 3 mg (13.7
mmol) Zn(CH3COO)2$2H2O
was stirred in 5 mL of quinoline at 180 ꢀC under a nitrogen atmo-
sphere for 18 h. The reaction mixture was poured into a solution of
10 mL of Me-OH and 10 mL of 2 N HCl. The resulting precipitate was
collected by suction filtration, and then washed with dilute Na2CO3
solution (10%) under vacuum. The crude product was purified by
(12H, d, J ¼ 6.8 Hz) ppm. 13C NMR [100 MHz, CDCl3,
d 77.3 ppm

36
H. Dinçalp et al. / Dyes and Pigments 86 (2010) 32e41
(3 peaks)]: 171.7 (C]O), 163.3 (C]O), 154.6, 153.3, 147.8, 147.3,
145.6, 135.2, 131.3, 130.8, 130.0, 128.7, 127.4, 127.3, 126.7, 125.8,
125.0, 124.9, 124.7, 124.0, 123.9, 123.4, 123.1, 119.8, 119.5, 118.4, 49.0,
33.9, 29.2 (CH(CH3)2), 25.6 (CH(CH3)2), 24.9 (CH(CH3)2), 24.0 ppm.
LC/MS (APCI, negative) m/z ¼ 1466 [M ꢁ H]ꢁ.
119.9, 119.5, 119.0, 116.1, 115.4, 111.3, 34.1, 29.9, 29.4 (CH(CH3)2),
25.4, 24.7 (CH(CH3)2), 24.2 (CH(CH3)2) ppm.
2.2.10. Synthesis of N-(2,6-diisopropylphenyl)-N0-(3-carboxy-
2-pyridyl)-1,7-bis{4-[(4-indole-3-ylbutanoyl)oxy]phenoxy}
perylene-3,4,9,10-tetracarboxylic diimide (P9)
2.2.8. Synthesis of N-(2,6-diisopropylphenyl)-N0-(3-carboxy-2-
pyridyl)-1,7-bis{4-[(4-pyrene-1-ylbutanoyl)oxy]phenoxy}perylene-
3,4,9,10-tetracarboxylic diimide (P7)
Following the published procedure [34], the mixture of 10 mg
(11.3 mmol) P5, 14 mg (48.6 mmol) 1-pyrene butyric acid, 17 mg
Following the published procedure [34], a solution of P5 (10 mg,
11.2
3-butyric acid (11 mg, 54.1
65.0 mol) and DCC (39 mg, 0.19 mmol) at room temperature under
an inert atmosphere overnight. Column chromatography using
mmol) in dry DMF/CH2Cl2 [6 mL (1:2)] was treated with indole-
mmol) in the presence of DPTS (19 mg,
m
(58.8
m
mol) DPTS and 34 mg (0.16 mmol) DCC in dry DMF/CH2Cl2
chloroform:ethyl acetate (45:5) on silica gave P9. C78H59N5O12,
[7.5 mL (1:2)] was reacted at room temperature under an inert
atmosphere for 20 h. After the mixture was cooled to room
temperature, 15e20 mL of methanol was added to the solution to
precipitate the crude product. The product was collected by filtra-
tion and then purified by silica column chromatography eluted
with chloroform:ethyl acetate (45:5) to give P7. C94H65N3O12, Yield:
70%, FT-IR (KBr, cmꢁ1): 3277 (carboxylic acid OeH stretching), 3210,
2963, 2924 (aliphatic nCeH), 2862 (aliphatic nCeH), 1759 (ester nC]O),
1731 (carboxylic acid nC]O), 1700 (imide nC]O), 1667 (imide nC]O),
1627, 1588 (aromatic nC]C), 1496, 1404, 1337 (nCeN), 1317, 1261, 1180
Yield: 75%, FT-IR (KBr, cmꢁ1): 3389 (carboxylic acid OeH stretch-
ing), 2963 (aliphatic nCeH), 2924 (aliphatic nCeH), 2868 (aliphatic
nCeH), 1756 (ester nC]O), 1728 (carboxylic acid nC]O), 1704 (imide
nC]O), 1665 (imide nC]O), 1591 (aromatic nC]C), 1496, 1406, 1343
(
nCeN), 1262, 1180 (ester nCeO), 1118 (ester nCeO), 906, 741 cmꢁ1. 1H
NMR [400 MHz, CDCl3,
d
7.25 ppm]:
d
¼ 9.63 (2H, d, J ¼ 7.8 Hz,
perylene), 9.58 (1H, s, pyridine), 8.70 (2H, d, J ¼ 7.8 Hz, perylene),
8.42 (1H, s, pyridine), 8.39 (1H, s, pyridine), 8.01 (2H, s, perylene),
7.64 (2H, s, indole NeH), 7.47 (1H, t, J ¼ 7.8 Hz, benzene), 7.33 (2H, d,
J ¼ 7.8 Hz, benzene), 7.19 (2H, d, J ¼ 7.6 Hz, indole phenyl), 7.16 (2H,
d, J ¼ 7.6 Hz, indole phenyl), 7.14 (4H, s, phenol), 7.12 (4H, s, phenol),
7.10 (2H, d, J ¼ 2.3 Hz, indole phenyl), 7.08 (2H, d, J ¼ 2.3 Hz, indole
phenyl), 7.01 (2H, s), 2.91 (4H, t, J ¼ 7.0 Hz), 2.72 (2H, septet,
J ¼ 6.8 Hz), 2.63 (4H, t, J ¼ 7.0 Hz), 2.18 (4H, pentet, J ¼ 7.0 Hz), 1.16
(6H, d, J ¼ 6.8 Hz), 1.15 (6H, d, J ¼ 6.8 Hz) ppm. 13C NMR [100 MHz,
(ester nCeO), 1124 (ester nCeO), 1060, 841, 805, 755, 668 cmꢁ1
NMR [400 MHz, CDCl3, 7.24 ppm]:
.
1H
d
d
¼ 9.55 (1H, d, J ¼ 8.6 Hz,
perylene H), 9.51 (1H, d, J ¼ 8.6 Hz, pyridine H), 8.70 (1H, d,
J ¼ 8.6 Hz, perylene H), 8.64 (1H, d, J ¼ 8.6 Hz, perylene H), 8.41 (1H,
s, perylene H), 8.36 (1H, s, pyridine H), 8.31 (1H, s, pyridine H), 8.28
(2H, s, perylene H), 8.11 (10H, m, pyrene H), 7.99 (4H, s, pyrene H),
7.95 (2H, t, J ¼ 7.8 Hz, pyrene H), 7.87 (2H, d, J ¼ 7.8 Hz, pyrene H),
7.46 (1H, t, J ¼ 7.8 Hz, benzene H), 7.30 (2H, d, J ¼ 7.8 Hz, benzene H),
7.14 (4H, s, phenol H), 7.12 (4H, s, phenol H), 3.46 (4H, t, J ¼ 7.8 Hz),
2.71 (4H, m), 2.30 (6H, m), 1.16 (6H, d, J ¼ 6.8 Hz), 1.15 (6H, d,
CDCl3, d 77.5 ppm (3 peaks)]: 172.2 (C]O), 163.5 (C]O), 161.9 (C]
O), 161.7 (C]O), 155.6, 145.8, 136.7, 136.6, 133.7, 131.0, 130.0, 129.8,
127.6, 127.3, 126.1, 125.7, 124.9, 124.4, 124.2, 122.9, 122.2, 121.8,
120.2, 120.0, 119.5, 119.0, 115.7, 115.4, 115.3, 112.9, 111.3, 34.1, 29.9,
29.4 (CH(CH3)2), 25.4, 24.6 (CH(CH3)2), 24.2 (CH(CH3)2) ppm.
J ¼ 6.8 Hz) ppm. 13C NMR [100 MHz, CDCl3,
d 77.5 ppm (3 peaks)]:
171.9 (C]O), 163.5 (C]O), 163.2 (C]O), 162.9 (C]O), 155.0, 152.6,
147.6, 145.8, 135.5, 133.7, 131.6, 131.0, 130.8, 130.6, 130.2, 129.8,
128.9, 127.7, 127.6, 127.5, 127.4, 126.9, 126.0, 125.3, 125.1, 125.0,
124.5, 124.2, 123.7, 123.4, 122.8, 120.0, 119.9, 34.0, 32.8, 29.9 (CH
(CH3)2), 26.8 (CH(CH3)2), 24.2 (CH(CH3)2), 14.3 ppm. LC/MS (APCI,
negative) m/z (%) ¼ 1429 [M ꢁ H]ꢁ.
3. Results and discussion
3.1. Stationary and time-resolved measurements
The UV-Vis spectra of P3, P6, P8 dyes in toluene are displayed in
Fig. 1. Also, long wavelength absorption data of all P dyes are given
in Table 1. All series of P dye show broad absorption band between
wavelength of 450 and 625 nm which belongs to 0e0 transitions. A
2.2.9. Synthesis of N,N0-bis(2,6-diisopropylphenyl)-1,7-bis
{4-[(4-indole-3-ylbutanoyl)oxy]phenoxy}perylene-3,4,9,10-
tetracarboxylic diimide (P8)
weak shoulder around 390 nm attributes to the pe
p* transition of
Following the published procedure [34], a solution of P3 (10 mg,
10.8
3-butyric acid (11 mg, 54.1
65.0 mol) and DCC (39 mg, 0.19 mmol) at room temperature under
an inert atmosphere overnight. Column chromatography using
mmol) in dry DMF/CH2Cl2 [6 mL (1:2)] was treated with indole-
mmol) in the presence of DPTS (19 mg,
m
chloroform:ethyl acetate (45:5) on silica gave P8. C84H72N4O10
,
Yield: 75%, FT-IR (KBr, cmꢁ1): 3327, 2957, 2924 (aliphatic nCeH),
2851 (aliphatic nCeH), 1765 (ester nC]O), 1703 (imide nC]O), 1661
(imide nC]O), 1627, 1591 (aromatic nC]C), 1496, 1406, 1339 (nCeN),
1261 (phenol nCeO), 1180 (ester nCeO), 1116 (ester nCeO), 911,
741 cmꢁ1. 1H NMR [400 MHz, CDCl3,
d
7.25 ppm]:
d
¼ 9.59 (2H, d,
J ¼ 8.6 Hz, perylene), 8.74 (2H, d, J ¼ 8.6 Hz, perylene), 8.42 (1H, s,
perylene), 8.35 (1H, s, perylene), 7.63 (2H, s, indole NeH), 7.46 (2H,
t, J ¼ 7.8 Hz, benzene), 7.32 (4H, d, J ¼ 7.8 Hz, benzene), 7.19 (2H, d,
J ¼ 7.8 Hz, indole phenyl), 7.15 (2H, d, J ¼ 3.1 Hz, indole phenyl), 7.13
(4H, s, phenol), 7.11 (4H, s, phenol), 7.08 (2H, d, J ¼ 3.1 Hz, indole
phenyl), 7.06 (2H, d, J ¼ 7.8 Hz, indole phenyl), 7.01 (2H, s), 2.90 (4H,
t, J ¼ 7.0 Hz), 2.73 (4H, septet, J ¼ 6.8 Hz), 2.62 (4H, t, J ¼ 7.0 Hz), 2.18
(4H, pentet, J ¼ 7.0 Hz), 1.16 (12H, d, J ¼ 6.8 Hz), 1.14 (12H, d,
J ¼ 6.8 Hz) ppm. 13C NMR [100 MHz, CDCl3,
d 77.2 ppm (3 peaks)]:
170.8 (C]O), 163.6 (C]O), 157.5, 141.6, 137.5, 135.7, 135.1, 133.7,
Fig. 1. UV-Vis absorption spectra of P3, P6 and P8 dyes in toluene at the concentra-
131.0, 129.9, 127.6, 125.4, 125.0, 124.2, 123.7, 122.5, 122.2, 121.9,
tions of 10ꢁ6 M.

H. Dinçalp et al. / Dyes and Pigments 86 (2010) 32e41
37
Table 1
Long wavelength absorption (
l/nm), emission maxima (lem/nm) and fluorescence quantum yields (FF) of P dyes in five solvents of increasing polarity.
P dyes
Solvents
Toluene
l
Chloroform
Methanol
Acetonitrile
PBSc, (pH 7.0)
lem
FF
l
lem
FF
l
lem
FF
l
lem
FF
l
lem
FF
P3
P5
P6
548
547
539
579
576
570
0.07a
0.17a
0.14a
0.14b
0.26a
0.34b
0.25a
0.27a
548
550
537
578
579
570
0.08a
0.12a
0.13a
0.09b
0.25a
0.19b
0.32a
0.24a
556
553
nd
635
565
nd
<0.01a
<0.01a
nd
550
548
540
628
566
573
<0.01a
<0.01a
0.04a
0.10b
0.11a
0.23b
0.08a
0.12a
574
585
567
630
615
640
<0.01a
<0.01a
<0.01a
0.06b
P7
539
566
540
571
nd
nd
nd
537
569
582
750
<0.01a
0.39b
P8
P9
541
539
570
562
540
526
571
565
540
nd
583
nd
0.02a
nd
537
534
575
564
561
566
600
605
<0.01a
<0.01a
nd: not determined.
a
Fluorescence quantum yields have been determined using perylene-3,4,9,10-tetracarboxylic-bis-N,N0-dodecyl diimide (lexc ¼ 485 nm).
Fluorescence quantum yields have been determined using pyrene (lexc ¼ 317 nm).
b
c
PBS: 70 mM potassium phosphate/100 mM KCl/1 mM EDTA.
the aromatic rings. Three vibronic maxima at 345, 330 and 315 nm
belong to the pyrene absorption for P6 and P7 dyes.
solution is highly quenched and structureless. Also, it gives
a marked red shift. This may be because the aggregation of the dye
in solution. Also, excited state relaxation of P9 dye in buffer solution
leads to an increase in dipole moment. Moreover, much larger
Stokes shift for P dyes is observed in higher polar solvents
compared to lower polar solvents.
We have studied the optical properties of P dyes in different
solvent of polarity. We choose low polar solvents such as toluene
and chloroform, and high polar solvents such as methanol and
acetonitrile. Also PBS, a more appropriate solution for biomedical
applications of perylene diimides, is prepared and used in optical
studies. In general, the absorption spectra of pyrene- or indole-
substituted P dyes give a small hypsochromic shift of 3e4 nm in
Me-CN compared to the corresponding spectra in toluene. This
observation is explained by the low polarizability of excited state of
the dyes in Me-CN. P6e9 dyes do not effectively solvated in Me-OH
because of their bulky aromatic groups which do not form
hydrogen bond with Me-OH molecules. However, the absorption
spectra of P3 and P5 dyes give a bathochromic shift of 3e4 nm in
Me-OH with respect to the same spectra in toluene. This may be
attributed to the formation of hydrogen bond between the solvent
molecule and the non-bonding electrons of hydroxyl group
attached to the benzene ring of the dyes [35]. The longest visible
wavelength shift for all the dyes was observed in phosphate buffer
solution among the studied solvents. Strong hydrogen bonding in
buffer solution stabilizes the charge-transfer excited state so that
absorption spectrum can shift to long-wavelength region [35].
Fig. 2 shows the solvent dependence of emission spectra of P9
dye at the excitation wavelength of 485 nm. Contrary to the organic
solvents, observed emission spectrum of P9 in phosphate buffer
When P7 dye in acetonitrile is excited at 317 nm, where the
pyrene unit absorbs strongly, high energy radiation of pyrene
monomer emission at 388 nm and low energy radiation of perylene
unit emission at 569 nm are observed. In addition, new emission
peaks around 758 and 788 nm appear (Fig. 3). Pyrenes are known to
give monomer emission peaks below 400 nm [36] and efficient two
excimer emission peaks between 440 and 480 nm [37]. No excimer
emission of pyrene unit of the molecule is observed in Fig. 3. The
red shifted emission peaks around 758 and 788 nm in the fluo-
rescent emission spectrum of P7 dye can be attributed to the
formation of charge-separated states in donoreacceptor system. In
comparison of the emission spectrum of the model compound P5
dye in acetonitrile at the excitation wavelength of 485 nm, the
compound gives the only emission from the perylene chromophore
at 566 nm. If the only process upon the excitation of pyrene unit
in P7 dye is the energy transfer process, maximum of emission
wavelength of the perylene unit should be the same as that of the
model compound P5 dye. Long wavelength emission peaks
displayed in Fig. 3 are the direct spectroscopic evidences of the
charge-separated state of P7 dye in acetonitrile. Also, fast decay
components obtained from the single photon counting experi-
ments may be attributed to the formation of a charge transfer
process in P7 dye (Fig. 4 and Table 2). The same results are obtained
Fig. 2. The normalized fluorescence spectra of P9 dye in different solvents of
increasing polarity (lexc ¼ 485 nm). aPBS: 70 mM potassium phosphate/100 mM KCl/
1 mM EDTA.
Fig. 3. The normalized UV-Vis absorption (with solid line) and fluorescence spectra
(with symbol) of P7 dye in Me-CN (lexc ¼ 317 nm).

38
H. Dinçalp et al. / Dyes and Pigments 86 (2010) 32e41
Fig. 4. Time-resolved fluorescence decay of P7 in toluene. The inset shows the
residuals. The time increment per channel amounts to be 42 ps.
Fig. 5. Linear fits of the values obtained from the photodegradation curves of P dyes in
toluene at the excitation wavelength of 254 nm for
1 h. Linear equations: P3:
(y ¼ ꢁ3.3 ꢂ 10ꢁ5$X ꢁ 1.9 ꢂ 10ꢁ4; R2: 0.99), P5: (y ¼ ꢁ2.1 ꢂ 10ꢁ5$X ꢁ 2.7 ꢂ 10ꢁ4; R2:
0.99), P6: (y
¼
ꢁ3.5
ꢂ
10ꢁ6$X
þ
3.1
ꢂ
10ꢁ4
;
R2: 0.96), P8:
(y ¼ ꢁ3.0 ꢂ 10ꢁ5$X ꢁ 0.00232; R2: 0.99), P9: (y ¼ ꢁ3.0 ꢂ 10ꢁ6$X ꢁ 1.1 ꢂ 10ꢁ4
R2: 0.96).
;
in other studied solvents. Also, excitation of P6 dye at 317 nm in the
studied solvents gives the long wavelength emissions above
750 nm (data not shown). Photophysical properties of four bay-
functionalized perylene diimides containing four attached pyrene
moiety have been studied using time-resolved emission and
femtosecond transient absorption in the literature [34,38]. Very fast
photoinduced energy transfer as well as efficient electron transfer
from the pyrene unit to the perylene diimide moiety has been
detected in high yield.
Fluorescence quantum yields of P6 and P7 dye are higher than
that of the model compounds P3 and P5 in the studied solvents at
the excitation wavelength of 485 nm. This observation is explained
by the conformational relaxation of the perylene diimide chro-
mophore by introducing the pyrene or indole quenchers to the bay
position of the ring. However, a significant quenching of pyrene
emission in P6 and P7 dyes is observed as compared to the refer-
ence compound, pyrene itself (FF ¼ 0.32 in cyclohexane). This
behavior may be attributed to the formation of charge-separated
state.
In single photon timing experiments, analysis of the decays
revealed four-exponential decays at the emission wavelength of
570 nm for P6e9 dyes in toluene as given in Table 2. The highest
decay values for all the compounds may be attributed to the
stationary fluorescence of perylene diimides. Fast decay compo-
nents below 0.1 ns can be attributed to the photoinduced energy
transfer or electron transfer between the perylene diimide chro-
mophore and attached pyrene or indole moieties. As can be seen
from the data given in Table 2, fluorescence rate constants of pyrene
or indole-substituted perylene diimide dyes are higher than that of
model compounds P3 and P5 dyes. This means that the HOMO level
of the quencher lies below the HOMO level of perylene diimide.
Electron transfer occurs only from excited states of pyrene moiety
to the perylene diimide quencher.
Also, similar photophysical results have been observed for P8
and P9 dyes containing electron-withdrawing perylene diimide
group and electron-releasing indole moieties. The absorption and
emission spectra of both P8 and P9 dyes show similar blue shifts
with respect to the model compounds P3 and P5 in the studied
solvents. After introducing the indole quenchers into the phenol
hydroxyl group of the model compounds P3 and P5, nep* in the
phenol backbone will decrease. Therefore, remarkable blue shifts
were observed for P8 and P9 dye in the studied solvents. Fast decay
components obtained with single photon timing experiments, high
values of fluorescence rate constants and different emission
maxima as compared to the model compounds indicate that
photoinduced energy transfer as well as electron transfer occurs in
the studied solvents for P8 and P9 dyes. Familiar compounds,
peryleneebenzoindole dyes, were synthesized for solar photovol-
taic purposes in the literature [11]. Authors state that strong
Table 2
Fluorescence decay times (sf/ns), fluorescence lifetimes (sf/ns), radiative lifetimes
(
s
0/ns), fluorescence rate constants (krf ꢂ 108/sꢁ1), non-radiative rate constants
(knr
ꢂ
108/sꢁ1), and singlet energies (Es/kcal molꢁ1
) of P dyes in toluene
(
lexc ¼ 485 nm).a
P dyes
sf(1)
sf(2)
sf(3)
sf(4)
sf
s0
kfr
knr
Es
P3
P5
P6
P7
P8
P9
2.1
2.3
4.5
4.5
2.5
3.2
0.07
0.09
2.0
2.1
2.5
<0.01
<0.01
0.1
0.08
0.05
0.1
e
0.7
0.8
1.7
1.7
1.3
1.4
10.4
4.7
11.2
6.4
5.1
5.1
1.0
2.1
0.8
1.6
2.0
2.0
12.8
10.4
5.2
4.4
5.9
52.9
53.0
53.8
53.8
53.6
53.8
e
<0.01
<0.01
<0.01
<0.01
2.2
5.3
a
Photophysical parameters are calculated with the formulas: sf ¼ S
sf(n)/n, s0
¼
sf/
FF
,
Fig. 6. CVs of the synthesized P6 and P8 dyes on glassy carbon working electrode in
0.1 M [TBA][PF6]/Me-CN at 500 mV/s scan rate.
kf ¼ 1/sf ¼ krf þ knr, krf ¼ 1/
s
0, Es ¼ 0.0029/llong ꢂ 10ꢁ7 [39,40].

H. Dinçalp et al. / Dyes and Pigments 86 (2010) 32e41
39
Table 3
Cyclic voltammetry data for P dyes in 0.1 M [TBA][PF6]/Me-CN vs. Fc/Fcþ at 500 mV/s
scan rate.
P dyes Eꢀ
(V) Eꢀ
(V) Eꢀ
(V) Eꢀ
(V) Eꢀ
(V) Eꢀ
(V)
ox3
red3
red2
red1
ox1
ox2
P1
P2
P3
P4
P5
P6
P7
P8
P9
e
e
ꢁ0.45
ꢁ0.50
ꢁ0.71
ꢁ0.62
ꢁ0.69
ꢁ0.63
ꢁ0.64
ꢁ0.67
ꢁ0.65
ꢁ0.21
ꢁ0.29
ꢁ0.52
ꢁ0.44
ꢁ0.53
ꢁ0.43
ꢁ0.47
ꢁ0.48
ꢁ0.47
1.02
0.93
1.19
1.34
0.61
0.77
0.82
0.56
0.60
e
e
e
e
e
e
e
e
e
ꢁ1.00
e
e
1.34
1.22
1.30
1.13
1.15
ꢁ0.90
e
1.41
1.63
1.62
1.66
ꢁ0.94
e
electron-withdrawing nature of these dyes transfers electrons
under illumination towards the undesirable directions.
3.2. Photostability tests
Fig. 7. CVs of P7 dye on glassy carbon working electrode in 0.1 M [TBA][PF6]/Me-CN at
50 (1), 100 (2), 200 (3), 400 (4), 600 (5), and 800 (6) mV/s scan rates.
Photostability of toluene solutions of all series of P dye were
investigated. Specifically, toluene solutions were irradiated with Xe
lamp exposure in fluorescence spectrophotometer at the excitation
wavelength of 254 nm for 1 h. The emission data were collected at
the emission wavelength of 570 nm. Photostability tests were
performed with the detection of decrease in emission intensity at
570 nm. Photodegradation rate constants of the compounds were
obtained from the slopes of the linear region of the semilogarithmic
plot of for degradation decays of dyes vs. time [41], ln (I0/I) ¼ kp ꢂ t.
kp values of P3, P5, P6, P8, and P9 dyes were calculated to be
3.3 ꢂ 10ꢁ5, 2.1 ꢂ 10ꢁ5, 3.5 ꢂ 10ꢁ6, 3.0 ꢂ 10ꢁ5, 3.0 ꢂ 10ꢁ6, respec-
tively (Fig. 5). In general, it was understood from Fig. 5 that intro-
ducing the pyrene or indole units to the 1-, 7-bay positions of the
perylene ring might enhance dye photostability.
levels of P6 and P7 dyes containing pyrene moieties are calculated
to be about ꢁ5.03 and ꢁ5.08 eV (1 eV ¼ 1.602 ꢂ 10ꢁ19 J), respec-
tively. This means that electron-rich pyrene derivatives improve
the hole-injection capacity of the sensor dyes [42]. The corre-
sponding LUMO energy levels are estimated to be ꢁ3.83 and
ꢁ3.79 eV. The easy oxidation of P8 and P9 dyes at 0.56 and 0.60 eV
suggests that the least electron-withdrawing capacity among the
studied dyes is indole-substituted perylene diimides. The LUMO
energy levels of all the above synthesized dyes are higher than the
conduction band of TiO2. Therefore, these dyes with appropriate
molecular structures are suitable to n-type organic semiconductors
for DSSCs.
Fig. 7 shows the typical cyclic voltammograms of P7 dye at
various scan rates. The first and second reduction processes of P7
dye, recorded at ꢁ0.47 and ꢁ0.64 V, could be assigned to (0/ꢁ1)
3.3. CV measurements
It is important to get information about HOMO and LUMO
energy levels of the series of P dye for solar cell applications. The
electrochemical behavior of the dyes has been investigated by
cyclic voltammetry. Fig. 6 gives the typical voltammograms of P6
and P8 dyes. The reduction and oxidation potentials were given in
Table 3. Model compounds P3 exhibits three reversible reduction
and (ꢁ1/ꢁ2) species, respectively.
D
Ep values [Ep(anodic) ꢁ Ep(cathodic)
]
changed from 48 to 58 mV with the scan rates ranging from 50
to 800 mV/s for P7 dye. Also, P7 dye gives a significant change at
the peak current after 30 min electrolysis time as shown in Fig. 8.
The cathodic redox potentials of P7 dye show approximately
60 mV positive shift after 30 min electrolysis period with respect
waves (Eꢀ
¼ ꢁ0.52 V, Eꢀ
¼ ꢁ0.71 V, Eꢀ
¼ ꢁ1.00 V) and one
red1
red2
red3
reversible oxidation wave (Eꢀ
¼ 1.19 V), indicating the formation
ox1
of stable radical anions, dianions, trianions and radical cations of
PDI, respectively. Symmetric P6 and P8 dyes give extra two
oxidation peaks indicating the pyrene radical cations and indole
radical cations, respectively.
Taking the reduction onsets into account, the LUMO energy
levels have been calculated and listed in Table 4. The HOMO energy
Table 4
Molecular orbital energiesa (eV) of P dyes with respect to the vacuum level.
P
LUMO-2 LUMO-1 LUMO HOMO HOMO-1 HOMO-2 Egap2 Egap1 Egap
dyes
P1
P2
P3
P4
P5
P6
P7
P8
P9
e
e
ꢁ4.05 ꢁ5.28
ꢁ3.97 ꢁ5.19
ꢁ3.74 ꢁ5.45
ꢁ3.82 ꢁ5.60
e
e
e
e
e
e
e
e
e
e
e
e
e
e
1.23
1.22
1.71
1.78
e
e
e
e
e
e
e
e
e
e
ꢁ3.57
ꢁ3.63
ꢁ3.62
ꢁ3.59
ꢁ3.61
ꢁ3.73 ꢁ4.87 ꢁ5.60
ꢁ3.83 ꢁ5.03 ꢁ5.48
ꢁ3.79 ꢁ5.08 ꢁ5.56
ꢁ3.78 ꢁ4.82 ꢁ5.39
ꢁ3.79 ꢁ4.86 ꢁ5.41
e
2.03 1.14
ꢁ3.36
e
ꢁ5.67
e
2.31 1.85 1.20
1.94 1.29
2.56 1.80 1.04
1.80 1.07
e
ꢁ3.32
e
ꢁ5.88
e
e
Fig. 8. Anodic and cathodic (inset) current peaks which were observed after 30 min
electrolysis time for P7 on glassy carbon working electrode in 0.1 M [TBA][PF6]/Me-CN
at 500 mV/s scan rate.
a
HOMO and LUMO energy levels of the dyes were determined by the formulas:
ELUMO ¼ ꢁ(4.8 þ Eroendset), E
¼ Eꢀred ꢁ Eꢀox(ferrocene), EHOMO ¼ ELUMO ꢁ Egap [43].
onset
red

40
H. Dinçalp et al. / Dyes and Pigments 86 (2010) 32e41
Fig. 9. Optimized structures and molecular orbitals of energy densities of P4, P5, P7 and P9 dyes.
to the initial time. Also, a significant negative shift from 1.63 V to
1.50 V is observed at anodic potential of P7 dye at the end of the
electrolysis time. First anodic peak observed at 0.82 V for P7 dye
was disappeared after 30 min electrolysis period. These shifts may
be explained by the formation of charge transfer complex
between the electron-donor pyrene and electron-acceptor per-
ylene group of P7 dye. No obvious change at the peak currents has
been observed for indole-substituted perylene diimide dyes (P8
and P9) and other model compounds.
Electron density diagrams for HOMO and LUMO orbitals of P4,
P5, P7 and P9 dyes are depicted in Fig. 9. Both HOMO and LUMO
orbitals of P4, P5 and P7 dyes are delocalized over the whole per-
ylene ring. However, for P9 dye containing perylene-indole group,
HOMO level has the charge localized on the one of the indole rings
while LUMO level has distributed along the perylene ring. These
orbitals show a redistribution of the LUMO electron density
predominantly in the acceptor perylene core of the studied
compounds. HOMO orbitals of P9 dye are distributed over the one
of the indole groups which shows electron donating behavior.
HOMO orbitals are delocalized generally on the donor site of the
molecule, while LUMO orbitals are delocalized on the acceptor site
of the molecule. Similar studies with frontier orbitals on perylene
dyes have been studied in the literature and same explanations
about the HOMOeLUMO orbital partitioning have been cited by the
authors [10,44,45].
It is noted that bulky aromatic groups which show electron-
releasing activity in the 1-, 7-bay positions slightly narrow the band
gap energies with respect to the model compound P3. When the
bay substituents of the perylene diimides are changed, HOMO
energy levels increase while LUMO energy levels remain stable as
compared to the model compound P3. These results are in well
accordance with the literature data. Müllen and coworkers have
studied the HOMO/LUMO energy tuning of perylene dyes based on
N-(2,6-diisopropylphenyl)-perylene-3,4-dicarboximide and given
the similar results about the effect of bay substituent on the HOMO/
LUMO energy levels of the compounds [10]. An effective charge
injection from the LUMO level of the dye to the conduction band of
TiO2 realizes in high efficiency PV materials when strong orbital
partitioning occurs along the dye molecule [10,46].
4. Conclusions
We have synthesized and characterized new macromolecular
perylene diimides. In a detailed photophysical analysis of the
compounds, it is found that perylene diimide dyes containing
pyrene or indole moieties show fast photoinduced energy transfer
as well as efficient electron transfer from the quenchers to the
perylene diimide moiety. Fast decay components, low fluorescence
quantum yields at the excitation of pyrene quencher, long wave-
length emission peaks and high values of fluorescence rate
constants for P7 dye support the formation of charge-separated
states of P7 dye in solution.
Also, negative and positive shifts at redox potentials are the
evident for the formation of charge transfer complex between the
electron-donor pyrene and electron-acceptor perylene groups of
P7 dye. In a simplified view, pyrene or indole units in the 1-, 7-bay
positions slightly narrow the band gap energies and raise the
HOMO energy level of the perylene diimides as compared to the
model compound. For the indole-substituted perylene diimide,
the HOMO orbital is localized on indole group while LUMO is
extended on perylene core. Molecular structures of pyrene- or
indole-substituted perylene diimides with appropriate LUMO
energy values are suitable candidates for DSSCs.
Acknowledgements
This work was supported by the Research Council of Celal Bayar
University (BAP/2007-006) and the State Planning Organization of
Turkey (07 DPT 002).
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