1,7-And 1,6-regioisomers of diphenoxy and dipyrrolidinyl substituted perylene diimides: Synthesis, separation, characterization, and comparison of electrochemical and optical properties

Article abstract of DOI:10.1021/cm1018647

1,7-And 1,6-regioisomers of N,N′-dioctyl-di(2,4-di-tert-butylphenoxy) perylene diimide (1,7-5 and 1,6-5) and N,N′-dioctyl-dipyrrolidinylperylene diimide (1,7-8 and 1,6-8) dyesare prepared. These 1,7-and 1,6-regioisomers are successfully isolated from the regioisomeric mixture using conventional methods of separation and subsequently characterized, unambiguously, by 300 MHz ¹H NMR spectroscopy. This is the very first time when 1,6-regioisomers of diphenoxy and dipyrrolidinyl substituted perylene diimides are obtained in pure form. Optical and redox properties of these 1,6-regioisomers are examined extensively and compared with respective 1,7-regioiosmers. The optical and electrochemical characteristics of diphenoxy substituted isomers, 1,7-5 and 1,6-5, were found to be virtually the same. However, quite unexpectedly, crucial differences were observed in the properties of regioisomers of dipyrrolidinyl substituted PDIs, 1,7-8 and 1,6-8. Differential pulse voltammetry revealed that 1,7-8 has better electron-donating ability compare to that of 1,6-8. Pronounced differences were observed in the optical properties too. In addition to the lowest energy absorption band at around 700 nm, 1,6-8 exhibits another strong absorption band at ca. 560 nm, and consequently, covers larger part of the visible region relative to that of 1,7-8. Steady-state emission and fluorescence lifetime studies, carried out in solvents of different polarities, revealed that regioisomer 1,6-8 has inherently lower fluorescence quantum yields and lifetimes compared to that of 1,7-8. This fundamental information on the redox and optical properties of 1,6-and 1,7-regioisomers of diphenoxy and dipyrrolidinyl substituted PDIs will be of value especially for material chemists to develop more efficient systems and devices from bay-functionalized perylene diimides.

Full text of DOI:10.1021/cm1018647

778 Chem. Mater. 2011, 23, 778–788
DOI:10.1021/cm1018647
1,7- And 1,6-Regioisomers of Diphenoxy and Dipyrrolidinyl Substituted
Perylene Diimides: Synthesis, Separation, Characterization, and
Comparison of Electrochemical and Optical Properties†
Rajeev K. Dubey,* Alexander Efimov, and Helge Lemmetyinen
Department of Chemistry and Bioengineering, Tampere University of Technology,
P.O. Box 541, 33101 Tampere, Finland
Received July 7, 2010. Revised Manuscript Received August 9, 2010
1,7- And 1,6-regioisomers of N,N0-dioctyl-di(2,4-di-tert-butylphenoxy)perylene diimide (1,7-5 and
1,6-5) and N,N0-dioctyl-dipyrrolidinylperylene diimide (1,7-8 and 1,6-8) dyes are prepared. These 1,7-
and 1,6-regioisomers are successfully isolated from the regioisomeric mixture using conventional
1
methods of separation and subsequently characterized, unambiguously, by 300 MHz H NMR
spectroscopy. This is the very first time when 1,6-regioisomers of diphenoxy and dipyrrolidinyl
substituted perylene diimides are obtained in pure form. Optical and redox properties of these
1,6-regioisomers are examined extensively and compared with respective 1,7-regioiosmers. The optical
and electrochemical characteristics of diphenoxy substituted isomers, 1,7-5 and 1,6-5, were found to
be virtually the same. However, quite unexpectedly, crucial differences were observed in the
properties of regioisomers of dipyrrolidinyl substituted PDIs, 1,7-8 and 1,6-8. Differential pulse
voltammetry revealed that 1,7-8 has better electron-donating ability compare to that of 1,6-8.
Pronounced differences were observed in the optical properties too. In addition to the lowest energy
absorption band at around 700 nm, 1,6-8 exhibits another strong absorption band at ca. 560 nm, and
consequently, covers larger part of the visible region relative to that of 1,7-8. Steady-state emission
and fluorescence lifetime studies, carried out in solvents of different polarities, revealed that
regioisomer 1,6-8 has inherently lower fluorescence quantum yields and lifetimes compared to that
of 1,7-8. This fundamental information on the redox and optical properties of 1,6- and 1,7-
regioisomers of diphenoxy and dipyrrolidinyl substituted PDIs will be of value especially for material
chemists to develop more efficient systems and devices from bay-functionalized perylene diimides.
Introduction
arrays and artificial photosynthetic systems.8 In light of their
diverse and fascinating properties, such as easy fictionaliza-
tion, excellent electron acceptor ability, high molar extinc-
tion coefficient in the visible region, and high fluorescence
quantum yields, PDI dyes can also be considered as poten-
tial building blocks for many new applications.9
In the early nineties, there were two major challenges in
front of material chemists working with imide-substituted
PDIs (Figure 1). First, to increase the solubility of these
dyes, which was low because of the planar structure of the
perylenediimide core that facilitates the formation of
π-π aggregates. Second, to tune the photophysical and
redox properties, which are not affected by the chemical
modification at the imide position because the frontier
orbitals of these compounds have nodes at the imide
nitrogen atoms.9a
Perylene diimides (PDIs) represent a classical example
of an inherently robust and outstandingly versatile family
of organic compounds that have been extensively utilized
for a wide range of high technology applications including
field effect transistors,1 logic gates,2 organic light-emitting
diodes,3 organic solar cells,4 optical switches,5 photosen-
sitizers,6 sensors,7 and so forth. These compounds have also
been utilized as building blocks to construct light-harvesting
† Accepted as part of the “Special Issue on π-Functional Materials”.
*Corresponding author. Fax: þ358 3 31152108. E-mail: rajeev.dubey@
tut.fi.
(1) (a) Jones, B. A.; Ahrens, M. J.; Yoon, M.-H.; Facchetti, A.; Marks,
T. J.; Wasielewski, M. R. Angew. Chem., Int. Ed. 2004, 43, 6363. (b)
Chen, Z.; Debije, M. G.; Debaerdemaeker, T.; Osswald, P.;
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Wurthner, F. ChemPhysChem 2004, 5, 137.
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€
For these reasons, the discovery of Bohm et al. (1997) that
perylene-3,4,9,10-tetracarboxylic dianhyride (PTCDA) can
be selectively dibrominated to yield regioisomerically pure
€
(4) Schmidt-Mende, L.; Fechtenkotter, A.; Mullen, K.; Moons, E.;
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r
pubs.acs.org/cm
Published on Web 09/03/2010
2010 American Chemical Society

Article
Chem. Mater., Vol. 23, No. 3, 2011 779
Some of them took this issue into consideration,13 whereas
the majority of chemists still neglected the presence of
1,6-regioisomer.14 To our biggest surprise, the presence of
1,6-regioisomer is completely ignored even in very recent
reports.15
Among all substituents, linked to the PDI core until
date by substitution of bromine atoms, phenoxy and
pyrrolidinyl groups are of special interest. Basically, non-
bay-substituted PDIs are excellent n-semiconductors but
exhibit poor solubility because of π-π stacking when
employed in complex systems. The phenoxy groups were
attached at the bay region aiming to improve solubility of
the system in common organic solvents while retaining the
basic electron acceptor and optical characteristics.14c,16,13a On
the other hand, attachment of electron-donating pyrroli-
dinyl groups makes the dye a good electron donor.11a
Moreover, the lowest energy electronic transition moves
to substantially longer wavelengths and acquires signifi-
cant charge-transfer character.17 Later on, these 1,7-
dipyrrolidinyl-substituted PDIs (green PDIs) have been
used, extensively, as building blocks to prepare light
harvesting and photosynthetic systems.18 In addition,
the green PDI has also been applied in dye-sensitized
solar cells13b and photochromic systems.19 Recently,
prepared dyads of these chromophores with good elec-
tron acceptors, e.g., fullerene, which exhibited efficient
photoinduced electron transfer from PDI excited singlet
state to fullerene, are also a candidate for the photovoltaic
devices.14e,17 In view of their different absorption range in
the visible region, these 1,7-diphenoxy and 1,7-dipyrroli-
dinyl-functionalized PDIs were also linked together to
construct complex systems which exhibited better light-
harvesting and charge-separation properties.18b,17 In
most of the cases, both 1,7-diphenoxy and 1,7-dipyrroli-
dinyl PDI derivatives were prepared from a regioisomeric
mixture of 1,7- and 1,6-dibromoperylene diimide that was
synthesized according to the same, above-mentioned,
dibromination and imidization reactions described by
Figure 1. Chemical structure of imide-substituted perylene diimide
(PDI).
1,7-dibromoperylene-3,4,9,10-tetracarboxylic dianhyride
and subsequently, after imidization, bromine atoms can
also be exchanged by phenoxy and alkyne groups, estab-
lished a landmark in the developed field of PDI com-
pounds.10 This synthetic work not only helped to improve
the solubility of PDI-based systems but also opened an
efficient way to tune their electrochemical, optical, and
electronic properties through functionalization of the per-
ylene core with electron donor or acceptor groups. Later on,
these particular dibromination and imidization procedures
were repeated in several laboratories for many years, with-
out any skepticism, to prepare bay-functionalized PDIs
with interesting properties.2,11
€
A comprehensive study was carried out by Wurthner
et al., in the year 2004, on the procedure used by Bohm
€
et al. for the dibromination of PTCDA and subsequent
imidization.12 This study clearly revealed that dibromi-
nation reaction is not regioselective, rather, yields a
regioisomeric mixture of 1,7- and 1,6-dibromoperylene
dianhydride. Consequently, the subsequent imidization
with cyclohexyl amine also yields regioisomeric mixture
of 1,7- and 1,6-dibromoperylene diimides which can be
observed only by 600 MHz 1H NMR. They successfully
separated, and also unequivocally characterized, regioi-
somerically pure N,N0-dicyclohexyl-1,7-dibromoperylene
diimide from the regioisomeric mixture using repetitive
recrystallization. This repetitive recrystallization method
is the only existing method, until date, to obtain regioisome-
rically pure 1,7-dibromoperylene diimide. The method is
cumbersome and it is not possible to obtain 1,6-dibromo-
perylene diimide in pure form. As a consequence, regioi-
somerically pure 1,6-difunctionalized PDIs could not be
(13) (a) Baier, M. C.; Huber, J.; Mecking, S. J. Am. Chem. Soc. 2009,
131, 14267. (b) Shibano, Y.; Umeyama, T.; Matano, Y.; Imahori,
H. Org. Lett. 2007, 9, 1971. (c) Chao, C.-C.; Leung, M.-K. J. Org.
Chem. 2005, 70, 4323.
(14) (a) Chen, S.; Liu, Y.; Qiu, W.; Sun, X.; Ma, Y.; Zhu, D. Chem.
€
€
Mater. 2005, 17, 2208. (b) Mullen, K.; Kohl, C.; Muller, S.; Pisula,
W.; Nolde, F. Chem. Mater. 2006, 18, 3715. (c) Rodriguez-Mor-
gade, M. S.; Torres, T.; Atienza-Castellanos, C.; Guldi, D. M. J.
Am. Chem. Soc. 2006, 128, 15145. (d) Feng, J.; Zhang, Y.; Zhao, C.;
Li, R.; Xu, W.; Li, X.; Jiang, J. Chem.;Eur. J. 2008, 14, 7000. (e)
Shibano, Y.; Umeyama, T.; Matano, Y.; Tkachenko, N. V.; Lem-
metyinen, H.; Imahori, H. Org. Lett. 2006, 8, 4425.
€
prepared. On the basis of their study, Wurthner et al. also
predicted that the previously reported 1,7-difunctionalized
PDIs were contaminated with 1,6-regioisomer. However,
even after this clear demonstration, a mixed response has
been observed on this issue among the research groups.
€
(15) (a) Wonneberger, H.; Ma, C.-Q.; Gatys, M. A.; Li, C.; Bauerle, P.;
€
Mullen, K. J. Phys. Chem. B, DOI: 10.1021/jp911800q. (b) Shibano,
Y.; Imahori, H.; Adachi, C. J. Phys. Chem. C 2009, 113, 15454.
(16) (a) Rybtchinski, B.; Sinks, L. E.; Wasielewski, M. R. J. Phys. Chem.
A 2004, 108, 7497. (b) Wang, Y.; Chen, H.; Wu, H.; Li, X.; Weng,
Y. J. Am. Chem. Soc. 2009, 131, 30.
(17) Lukas, A. S.; Zhao, Y.; Miller, S. E.; Wasielewski, M. R. J. Phys.
Chem. B 2002, 106, 1299.
(18) (a) Perez-Velasco, A.; Gorteau, V.; Matile, S. Angew. Chem., Int.
Ed. 2008, 47, 921. (b) Rybtchinski, B.; Sinks, L. E.; Wasielewski,
M. R. J. Am. Chem. Soc. 2004, 126, 12269. (c) Giaimo, J. M.;
Gusev, A. V.; Wasielewski, M. R. J. Am. Chem. Soc. 2002, 124,
8530.
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(10) Bohm, A.; Arms, H.; Henning, G.; Blaschka, P. (BASF AG)
German Pat. DE 19547209 A1, 1997.
(11) (a) Zhao, Y.; Wasielewski, M. R. Tetrahedron Lett. 1999, 40, 7047.
(b) Ahrens, M. J.; Fuller, M. J.; Wasielewski, M. R. Chem. Mater.
2003, 15, 2684. (c) Liu, Y.; Li, Y.; Jiang, L.; Gan, H.; Liu, H.; Li, Y.;
Zhuang, J.; Lu, F.; Zhu, D. J. Org. Chem. 2004, 69, 9049. (d)
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Wurthner, F.; Thalacker, C.; Diele, S.; Tschierske, C. Chem.;Eur.
J. 2001, 7, 2247. (e) Serin, J. M.; Brousmiche, D. W.; Frechet,
J. M. J. J. Am. Chem. Soc. 2002, 124, 11848.
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€
(12) Wurthner, F.; Stepanenko, V.; Chen, Z.; Saha-Moller, C. R.;
€
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Kocher, N.; Stalke, D. J. Org. Chem. 2004, 69, 7933.
F. Angew. Chem., Int. Ed. 2008, 47, 6616.

780 Chem. Mater., Vol. 23, No. 3, 2011
Dubey et al.
€
Bohm et al., but dibromoperylene diimide and product
spectra were corrected using a correction function supplied by
the manufacturer. Fluorescence quantum yields were deter-
mined relative to fluorescein (Φf = 0.92 in 0.1 N NaOH aqueous
solution) and cresyl violet (Φf = 0.54 in methanol).20 The given
quantum yields are averaged from values measured at three
different excitation wavelengths. Optical densities at excitation
wavelengths were maintained at around 0.1 to avoid reabsorption.
Fluorescence decays of the samples in the nanosecond and
subnanosecond time scales were measured using a time-corre-
lated single photon counting (TCSPC) system (PicoQuant
GmbH) consisting of PicoHarp 300 controller and PDL 800-B
driver. The diphenoxy substituted PDIs (1,7-5 and 1,6-5) were
excited with the pulsed diode laser head LDH-P-C-485 at
483 nm, whereas, dipyrrolidinyl substituted PDIs (1,7-8 and
1,6-8) were excited with the pulsed diode laser head LDH-P-C-
650 at 648 nm. Fluorescence decays were measured at the wave-
length of emission maximum. The signals were detected with a
micro channel plate photomultiplier tube (Hamamatsu R2809U).
The time resolution of the TCSPC measurements was 110 ps for
483 nm excitation wavelength and 80 ps for 648 nm excitation
wavelength (fwhm of the instrument response function).
were both claimed to be pure 1,7-regioisomer.11a,17,18b,18c
We report herein the synthesis, separation, and unam-
biguous characterization of the novel 1,6- and 1,7-regioi-
somers of diphenoxy and dipyrrolidinyl substituted PDI
dyes. This article also includes a detailed comparative
study of optical and electrochemical properties of both
sets of regioisomers (1,7-5 & 1,6-5 and 1,7-8 & 1,6-8) in
different solvents. Electrochemical characteristics of
these isomers were investigated using differential pulse
voltammetry, whereas optical properties were examined
by steady-state UV-vis absorption and fluorescence
spectroscopy. In addition, fluorescence lifetime measure-
ments were also undertaken. This is the very first time
when 1,6-regioisomer of either diphenoxy or dipyrrolidi-
nyl-functionalized perylenediimide is obtained in pure
form and investigated in detail.
Experimental Section
Synthesis of N,N0-Dioctyl-dibromoperylene-3,4,9,10-tetracar-
boxy diimide (3). Crude dibromoperylene-3,4,9,10-tetracar-
boxylic dianhydride 2 (2.5 g, 4.54 mmol), obtained by the
dibromination of PTCDA 1, was suspended in propionic acid
(250 mL) and subsequently n-octylamine (11.7g, 90.76 mmol) was
added. The reaction mixture was refluxed under stirring for 48 h,
cooled to room temperature and poured into water (200 mL). The
precipitate was filtered off, thoroughly washed with several por-
tions of water, and dried to give the crude product. The crude
product was chromatographed on silica 60 with CH2Cl2 to yield
regioisomeric mixture of N,N0-dioctyl-1,7- and 1,6-dibromoper-
ylene diimides 3 (2.7 g, 77%) as major product and N,N0-dioctyl-
1,6,7-tribromoperylene diimide 4 (0.037 g, 1%) as a minor
product.
Materials. All the reagents utilized in the synthesis were
purchased from Sigma-Aldrich Co. and used as received.
Dibromoperylene-3,4,9,10-tetracarboxylic dianhydride 2 was
synthesized according to the literature procedure.12 The solvents
were of HPLC grade and purchased from VWR and used without
further purification. Thin-layer chromatography (TLC) was done
on aluminum sheets precoated with Silica 60 F254. The purification
and isolation of the products were performed by column chroma-
tography on silica gel 60, mesh size 40-63 μm or silica gel 100,
mesh size 63-200 μm. The sorbents for column chromatography
and TLC plates were purchased from Merck.
Instrumentation and Characterization. The proton and carbon
NMR spectra were recorded with Varian Mercury 300 MHz
spectrometer in CDCl3 at room temperature. All chemical shifts
are quoted relative to TMS (δ = 0.0 ppm); δ values are given in
ppm and J values in Hz. High-resolution mass spectra were
measured with Waters LCT Premier XE ESI-TOF benchtop
mass spectrometer. To obtain accurate mass value, we simulta-
neously infused the solution of the reference compound (leucine
enkephaline) with analyte, and processed the experimental
spectra according to the routine of accurate mass measurements
(peak centering and lock-mass TOF correction). Differential
pulse voltammetry was performed with Faraday MP, Obbligato
Objectives Inc., in three-electrode single-compartment cell con-
sisting of platinum in glass as working electrode, Ag/AgCl as
reference electrode and a graphite rod as counter electrode.
During the measurements, the values of pulse height, pulse
width, and step voltage were set to 20 mV, 20 ms, and 2.5 mV,
respectively. Benzonitrile containing 0.1 M tetrabutylammo-
nium tetrafluoroborate was used as solvent. The measurements
were done under continuous flow of nitrogen. A Fc/Fcþ couple
was used as an internal standard. The measurements were
carried out in both directions: toward the positive and negative
potential. The reduction and oxidation potentials were calcu-
lated as an average of the two scans.
1
N,N0-Dioctyl-1,7- and 1,6-Dibromoperylene Diimides (3). H
NMR (300 MHz, CDCl3, TMS): δ = 9.36 (d, J = 8.2 Hz, 2H),
8.80 (s, 2H), 8.59 (d, J = 8.2 Hz, 2H), 4.17 (t, J = 7.6 Hz, 4H),
1.72 (m, 4H), 1.49-1.18 (m, 20H), 0.88 (t, 6H). 13C NMR (75
MHz, CDCl3, TMS): δ = 163.0, 162.5, 138.2, 133.1, 132.9,
130.2, 129.3, 128.6, 127.1, 123.3, 122.9, 121.0, 41.1, 32.0, 29.6,
29.5, 28.3, 27.4, 22.9, 14.4. MS (ESI-TOF): [M]þ calcd for
C40H40Br2N2O4, 772.1339; found, 772.1321.
N,N0-Dioctyl-1,6,7-tribromoperylene Diimide (4). 1H NMR
(300 MHz, CDCl3, TMS): δ = 9.38 (d, J = 8.1 Hz, 1H), 8.87 (s,
1H), 8.78 (s, 2H), 8.65 (d, J = 8.1 Hz, 1H), 4.19 (m, 4H), 1.74 (m,
4H), 1.49-1.20 (m, 20H), 0.88 (t, 6H). 13C NMR (75 MHz,
CDCl3, TMS): δ = 162.8, 162.5, 162.1, 162.0, 137.4, 136.6,
136.5, 133.3, 133.2, 131.4, 131.2, 130.7, 129.9, 129.4, 127.9,
125.9, 125.1, 124.5, 124.0, 123.3, 122.9, 122.8, 122.5, 121.6,
41.2, 41.0, 29.6, 29.5, 28.3, 28.2, 27.3, 23.0, 14.3. MS (ESI-TOF):
[M]- calcd for C40H39Br3N2O4, 852.0428; found, 852.0475.
Synthesis of N,N0-Dioctyl-1,7- and 1,6-Di(2,4-di-tert-butylphe-
noxy)perylene-3,4,9,10-tetracarboxy Diimide (1,7-5 and 1,6-5).
A mixture of 2,4-di-tert-butylphenol (80.0 mg, 0.38 mmol),
K2CO3 (107 mg, 0.78 mmol) and 18-Crown-6 (401 mg, 1.55
mmol), in dry toluene (30 mL), was stirred for 20 min under argon
and subsequently N,N0-dioctyl-dibromoperylene-3,4,9,10-tetra-
carboxy diimide 3 (78 mg, 0.10 mmol) was added. The reaction
mixture was stirred for 4 h at 90 °C under argon atmosphere. After
being cooled to room temperature, the solvent was removed by
rotary evaporation. The solid residue was thoroughly washed with
several portions of water and dried. The crude product was dissolved
in chloroform and purified twice by column chromatography on
All the spectroscopic measurements were carried out at room
temperature. The absorption spectra were recorded with Shi-
madzu UV-2501PC spectrophotometer and the fluorescence
spectra using a Fluorolog-3 (SPEX Inc.) fluorimeter. The emission
(20) (a) Kreller, D. I.; Kamat, P. V. J. Phys. Chem. 1991, 95, 4406. (b)
€
Chen, Z.; Baumeister, U.; Tschierske, C.; Wurthner, F. Chem.;
Eur. J. 2007, 13, 450.

Article
Chem. Mater., Vol. 23, No. 3, 2011 781
silica 60 using chloroform as eluent to afford the regioisomeric
mixture of product (72 mg, 72.4%).
cooled to room temperature, solvent was evaporated under reduced
pressure and solid residue was dissolved in chloroform. The crude
product was purified on silica 60 using chloroform as eluent. First,
the green band was collected and concentrated by rotary evapora-
tion to afford the product (1.56 g, 80%), which was found to be
regioisomeric mixture of 1,7- and 1,6-dipyrrolidinylperylene dii-
mide in a 74:26 ratio (according to 1H NMR analysis). The isomers
were separated by column chromatography on silica 100 using
DCM as eluent. First, a blue fraction was collected, and after that,
green. From the 1H NMR analysis, the blue fraction was identified
as 1,6-dipyrrolidinylperylene diimide 1,6-8, whereas the green frac-
tion was identified as 1,7-dipyrrolidinylperylene diimide 1,7-8.
N,N0-Dioctyl-1,7-dipyrrolidinylperylene-3,4,9,10-tetracarboxy
The regioisomers were isolated on the basis of their different
solubility in toluene. In a typical procedure, 400 mg of 70:30
mixture of N,N0-dioctyl-1,7- and 1,6-di(2,4-di-tert-butylphenoxy)-
perylene diimide was taken in 40 mL of toluene and sonicated for
10 min. The 1,6-isomer completely dissolved in toluene, whereas,
the 1,7-isomer remained as suspension. Within few hours, the
suspension of 1,7-isomer settled at the bottom of the vial. The
precipitate was filtered off and dried. 1H NMR spectrum of
precipitate showed the presence of only 13% of 1,6-isomer. The
procedure was repeated twice with 15 and 10 mL of toluene to
afford pure 1,7-isomer (160 mg, 40%). The mother liquor, which
contained 140 mg of 89:11mixture of 1,6- and 1,7-isomers (based
1
Diimide (1,7-8). Yield =1.15 g. H NMR (300 MHz, CDCl3,
TMS): δ = 8.36 (s, 2H), 8.31 (d, J = 8.1 Hz, 2H), 7.51 (d, J = 8.1
Hz, 2H), 4.21 (t, J = 7.5 Hz, 4H), 3.67 (br s, 4H), 2.74 (br s, 4H),
1
on H NMR analysis), was dried. The pure 1,6-isomer (85 mg,
21%) was obtained by repetition of above-mentioned separation
1.98 (br, 8H), 1.75 (m, 4H), 1.50-1.17 (m, 20H), 0.87 (t, 6H). 13
C
process two times with 10 mL of toluene.
N,N0-Dioctyl-1,7-di(2,4-di-tert-butylphenoxy)perylene-3,4,9,10-
tetracarboxy Diimide (1,7-5). 1H NMR (300 MHz, CDCl3, TMS):
δ = 9.65 (br d, J = 28.4 Hz, 2H), 8.56 (d, J = 8.3 Hz, 2H), 8.30 (s,
2H), 7.58 (d, J = 2.4 Hz, 2H), 7.24 (br, 2H), 6.86 (br d, J =
50.1 Hz, 2H), 4.13 (t, J = 7.6 Hz, 4H), 1.71 (m, 4H), 1.50 (d, J =
23.5 Hz, 18H), 1.39 (s, 18H), 1.37-1.18 (m, 20H), 0.86 (t, 6H). 13C
NMR (75 MHz, CDCl3, TMS): δ = 163.7, 163.4, 156.1, 151.7,
148.1, 140.8, 133.9, 130.3, 129.8, 128.3, 125.4, 125.2, 125.1, 124.4,
124.1, 122.2, 120.3, 40.9, 35.3, 35.0, 32.0, 31.9, 30.8, 29.9, 29.5,
29.4, 28.3, 27.4, 22.8, 14.4. MS (ESI-TOF): [M þ Na]þ calcd for
C68H82N2O6Na, 1045.6071; found, 1045.6035.
N,N0-Dioctyl-1,6-di(2,4-di-tert-butylphenoxy)perylene-3,4,9,10-
tetracarboxy diimide (1,6-5). 1H NMR (300 MHz, CDCl3, TMS):
δ = 9.60 (br d, J = 27.5 Hz, 2H), 8.63 (d, J = 8.6 Hz, 2H), 8.20 (s,
2H), 7.58 (d, J = 2.4 Hz, 2H), 7.25 (br, 2H), 6.85 (br d, J = 35.2
Hz, 2H), 4.19 (t, J= 7.6Hz, 2H), 4.07 (t, J= 7.5Hz, 2H), 1.70(m,
4H), 1.49 (d, J = 14.7 Hz, 18H), 1.39 (s, 18H), 1.37-1.18 (m,
20H), 0.86 (m, 6H). 13C NMR (75 MHz, CDCl3, TMS): δ =
163.9, 163.1, 157.2, 151.8, 148.2, 140.9, 134.1, 131.4, 128.9, 127.3,
127.1, 125.4, 125.2, 124.1, 123.2, 122.7, 122.4, 121.3, 120.3, 41.1,
40.8, 35.3, 35.0, 32.1, 32.0, 31.8, 30.9, 29.6, 29.5, 29.4, 28.4, 28.2,
27.4, 27.3, 22.9, 22.8, 14.4, 14.2. MS (ESI-TOF): [M þ Na]þ calcd
for C68H82N2O6Na, 1045.6071; found, 1045.6075.
NMR (75 MHz, CDCl3, TMS): δ = 164.3, 146.6, 134.3, 130.0,
126.8, 123.9, 122.2, 121.9, 120.9, 119.2, 118.2, 52.4, 40.8, 32.1,
29.7, 29.5, 28.5, 27.5, 26.1, 22.9, 14.4. MS (ESI-TOF): [M]þ calcd
for C48H56N4O4, 752.4302; found, 752.4284.
N,N0-Dioctyl-1,6-dipyrrolidinylperylene-3,4,9,10-tetracarboxy
1
diimide (1,6-8). Yield =0.41 g. H NMR (300 MHz, CDCl3,
TMS): δ = 8.68 (d, J = 8.1 Hz, 2H), 8.34 (s, 2H), 7.85 (d, J =
8.1 Hz, 2H), 4.24 (t, J = 7.5 Hz, 2H), 4.20 (t, J = 7.6 Hz, 2H),
3.71 (br s, 4H), 2.76 (br s, 4H), 2.00 (br, 8H), 1.84-1.67 (m, 4H),
1.53-1.16 (m, 20H), 0.87 (distorted triplet, 6H). 13C NMR (75
MHz, CDCl3, TMS): δ = 164.6, 164.3, 150.2, 135.9, 131.3,
130.4, 128.7, 128.5, 123.5, 123.1, 117.9, 117.8, 117.2, 117.1, 52.4,
40.9, 40.7, 32.0, 32.1, 29.8, 29.6, 29.4, 29.5, 28.5, 28.4, 27.5, 27.4,
25.9, 22.9, 22.89, 14.4. MS (ESI-TOF): [M]þ calcd for
C48H56N4O4, 752.4302; found, 752.4303.
Synthesis of 1,7-Dipyrrolidinylperylene-3,4,9,10-tetracarboxy
Dianhydride (1,7-9). N,N0-Dioctyl-1,7-dipyrrolidinylperylene-
3,4,9,10-tetracarboxy diimide 1,7-8 (200 mg, 0.27 mmol) was
taken in 2-propanol (20 mL) and subsequently KOH (1.9 g, 33.8
mmol) was added. The reaction mixture was stirred under an
argon atmosphere at reflux for 4 h. After being cooled to room
temperature, the reaction mixture was poured into acetic acid
(50 mL) and stirred overnight. The resulting green precipitate
was collected by filtration, washed with water and methanol,
and dried. The green solid was purified by column chromatog-
raphy (silica 100, CHCl3) to yield the product (61 mg, 43%). 1H
NMR (300 MHz, CDCl3, TMS): δ = 8.50 (s, 2H), 8.44 (d, J =
8.1 Hz, 2H), 7.60 (d, J = 8.1 Hz, 2H), 3.75 (br s, 4H), 2.85 (br s,
4H), 2.08 (br, 8H). MS (ESI-TOF): [M]þ calcd for C32H22N2O6,
530.1479; found, 530.1457.
Synthesis of N,N0-Dioctyl-1,7- and 1,6-Di(4-tert-butylphenoxy)-
perylene-3,4,9,10-tetracarboxy Diimide (6). Prepared from 4-tert-
butylphenol (62.4 mg, 0.42 mmol), K2CO3 (111 mg, 0.83 mmol)
and 18-Crown-6 (422 mg, 1.66 mmol) and N,N0-dioctyl-dibromo-
perylene-3,4,9,10-tetracarboxy diimide 3 (80 mg, 0.104 mmol)
according to the procedure described above for compound 5. The
crude product was chromatographed on silica 60 using chloroform
as eluent to afford the regioisomeric mixture of product 6 (78.4 mg,
Results and Discussion
1
83%). H NMR (300 MHz, CDCl3, TMS): δ = 9.40 (d, J =
Synthesis and Characterization. The 1,7- and 1,6-regioi-
somers of diphenoxy and dipyrrolidinyl substituted PDIs
(1,7-5, 1,6-5, 1,7-8, and 1,6-8) were synthesized from per-
ylene-3,4,9,10-tetracarboxylic dianhydride 1 in three steps
according to the route depicted in schemes 1, 2, and 4. First,
dibromoperylene-3,4,9,10-tetracarboxylic dianhydride was
prepared by the I2-catalyzed bromination of perylene-
3,4,9,10-tetracarboxylic dianhydride1 in 96% sulfuric acid.
As mentioned in the literature, this reaction led to the crude
product 2, which was a mixture of 1,7- and 1,6-dibromo-
perylene dianhydride along with a very small amount of
1,6,7-tribromoperylene dianhydride.12 The presence of 1,7-
and 1,6-regioisomers can only be detected by high-field
(600 MHz) 1H NMR in concentrated D2SO4. Moreover, the
8.5 Hz, 1.4H), 9.33 (d, J = 8.5 Hz, 0.6H), 8.43 (d, J = 8.5 Hz,
0.7H), 8.40 (d, J = 8.5 Hz, 1.3H), 8.19 (s, 1.3H), 8.12 (s, 0.6H), 7.46
(d, J = 8.7 Hz, 4H), 7.08 (d, J = 8.7 Hz, 4H), 4.08 (t, J = 7.6 Hz,
4H), 1.69 (m, 4H), 1.38 (s, 18H), 1.32-1.21 (m, 20H), 0.86 (m, 6H).
13C NMR (75 MHz, CDCl3, TMS): δ = 163.4, 163.3, 162.9, 162.8,
156.6, 155.5, 152.7, 152.5, 148.6, 148.4, 133.4, 133.3, 131.1, 130.9,
130.1, 129.1, 128.8, 128.6, 127.6, 127.7, 127.4, 124.9, 123.8, 123.6,
123.5, 123.3, 122.4, 122.2, 122.1, 121.2, 119.6, 119.4, 40.9, 34.8, 32.1,
31.7, 29.6, 29.4, 28.3, 27.4, 22.9, 14.3. MS (ESI-TOF): [M þ Na]þ
calcd for C60H66N2O6Na, 933.4819; found, 933.4824.
Synthesis of N,N0-Dioctyl-1,7 and 1,6-Dipyrrolidinylperylene-
3,4,9,10-tetracarboxy Diimide (1,7-8 and 1,6-8). N,N0-Dioctyl-
dibromoperylene-3,4,9,10-tetracarboxy diimide 3 (2 g, 2.59 mmol)
was dissolved in 150 mL of pyrrolidine. The reaction mixture was
stirred at 55 °C under an argon atmosphere for 18 h. After being

782 Chem. Mater., Vol. 23, No. 3, 2011
Scheme 1. Bromination of Perylenetetracarboxy Dianhydride 1 and Subsequent Imidization
Dubey et al.
Scheme 2. Synthetic Route to Compound 1,7-5 and 1,6-5
Scheme 3. Structure of N,N0-Dioctyl-1,7- and 1,6-Di(4-tert-bu-
tylphenoxy)perylene Diimide 6
crude product 2 is insoluble in all organic solvents. There-
fore, we subjected the crude product to imidization without
characterization and further purification (Scheme 1).
Imidization of crude product 2 with n-octylamine in
refluxing propionic acid yielded perylene diimides 3 and
4. The product mixture was subjected to silica gel column
chromatography with CH2Cl2 as eluent. N,N0-dioctyl-1,6,7-
tribromoperylene diimide 4 was successfully separated from
the product mixture in ca. 1% yield and characterized by 1H
and 13C NMR and MS. However, 1,7- and 1,6-dibromoper-
ylene diimides could not be separated by column chroma-
1
tography. Additionally, 300 MHz H NMR spectrum of
mixture 3 could not differentiate the two isomers. Subse-
quently, mixture 3 was reacted with 2,4-di-tert-butylphenol
to afford regioisomeric mixture of N,N0-dioctyl-1,7- and 1,6-
di(2,4-di-tert-butylphenoxy)perylene diimide by the nucleo-
philic substitution of both bromine atoms (Scheme 2).
The product, after silica gel column chromatography
with CHCl3, was characterized by 1H NMR spectroscopy
(300 MHz, CDCl3) and mass spectrometry. The 1H NMR
spectrum exhibited two sets of signals with different intensities
in the aromatic region indicating the presence of 1,7- and
1,6-regioisomers in the product. Two well-resolved doub-
lets and singlets were observed in the region between 8.1
and 8.7 ppm (Figure 2a). In addition, two broad doublets
are overlapped at around 9.6 ppm. The integration areas
of the doublets at 8.56 and 8.63 ppm revealed that 1,7-5
and 1,6-5 are present in the product in a ratio 70:30. These

Article
Chem. Mater., Vol. 23, No. 3, 2011 783
Scheme 4. Synthetic Route to Compound 1,7-8 and 1,6-8
two regioisomers, 1,7-5 and 1,6-5, could not be separated
by conventional column chromatography. Fortunately,
these two regioisomers have shown very different solubi-
lity in toluene. The minor isomer 1,6-5 is well-soluble in
toluene and exhibit solubility as high as 15 mg/mL,
whereas the major isomer 1,7-5 has very poor solubility
in toluene (ca. 0.2 mg/mL). We successfully separated
them from each other on the basis of this pronounced
solubility difference. The process must be repeated at
least three times in order to achieve complete separation
of both regioisomers. The separation process was mon-
itored thoroughly by 300 MHz 1H NMR (see Figures S9
and S10 in the Supporting Information). An even more
pronounced solubility difference for the two isomers was
observed in benzonitrile, but we preferred toluene for the
separation process over benzonitrile because of its sim-
plicity of use. Such a large difference in the solubility of
these regioisomers may be attributed to the presence of
bulky 2,4-di-tert-butylphenoxy groups at bay region.
Because of the steric hindrance caused by bulky tert-butyl
groups located at ortho position, the orientation of
phenoxy groups is restricted relative to the perylene core.
This hypothesis of restricted orientation of phenoxy
groups is beefed up by two peculiar features observed in
1H NMR spectrum of both 1,7-5 and 1,6-5. First, the
signal of 6-H, 12-H protons (bay protons of the perylene
core) at 9.65 ppm is significantly broadened (Figure 2b,c).
Second, at the same time, a clear broadening is observed
for the protons of o-tert-butyl groups in comparison to
that of p-tert-butyl groups (see Figures S7 and S8 in the
Supporting Information). This broadening of the signals
was presumably due to the spatial closeness of o-tert-
butyl group protons with 6-H and 12-H protons of the
perylene core, caused by rigidified conformation. In
addition, slowdown of the interconversion of two atropi-
somers caused by bulky 2,4-di-tert-butylphenoxy substi-
tuent may also be an additional contributing factor in the
above-mentioned broadening of NMR signals.21 To
further strengthen our hypothesis of restricted rotation
of the phenoxy group around the perylene core and to
correlate it with the solubility difference of two regioi-
somers, we synthesized a model compound 6 with 4-tert-
butylphenoxy groups at the bay region in place of
Figure 2. 1H NMR (300 MHz, CDCl3) spectra: (a) regioisomeric mixture
of 1,7-5 and 1,6-5, (b) pure 1,7-5 regioisomer, (c) pure 1,6-5 regioisomer,
(d) product 6 (1,7- and 1,6-regioisomers in a ratio 72:28).
2,4-di-tert-butylphenoxy groups (Scheme 3). In the case
of 6, the signal corresponds to the 6-H and 12-H protons
at 9.40 ppm that is observed to be sharp just like usual
disubstituted PDIs (Figure 2d). Moreover, the 1H NMR
spectrum (300 MHz, CDCl3) of6clearly revealed the presence
of 1,7- and 1,6-regioisomers in a 72:28 ratio (Calculated
from the integration areas of the doublets at 9.40 and 9.33
ppm). But, quite expectedly, these two regioisomers exhib-
ited the same solubility in toluene or benzonitrile.
To the best of our knowledge, this is the first time the
2,4-di-tert-butylphenoxy group has been linked to the
perylene core. Earlier, 4-tert-butylphenoxy, 3,5-di-tert-
butylphenoxy, and 4-1,1,3,3-tetramethylbutylphenoxy
groups were attached in which tert-butyl groups are
located away from the PDI core and consequently do
not cause restricted orientation of the phenoxy groups
around perylene core.13a,14c,16,19b
In view of the fact that PDI dyes readily form π-π aggre-
gates, which also causes broadening and shifting in the
1
NMR spectrum,16a variable-concentration H NMR mea-
surements of 1,7-5 were carried out in CDCl3 (20 mg/mL to
1 mg/mL), but no noticeable difference in the spectra was
observed. Additionally, no aggregation was observed for
both 1,7-5and 1,6-5in concentration-dependent steady-state
€
(21) Osswald, P.; Wurthner, F. J. Am. Chem. Soc. 2007, 129, 14319.

784 Chem. Mater., Vol. 23, No. 3, 2011
Dubey et al.
absorption and emission measurements in that concentra-
tion domain.
It is to be noted that the characteristic signals of the
1
regioisomers 1,7-5 and 1,6-5 in the H NMR spectra
(Figure 2), one singlet and two doublets of perylene core
protons, exhibit significant differences in the chemical
shift values (0.1 ppm for the singlets and 0.08 ppm for the
doublets at 8.53-8.66). Because of this difference, the two
regioisomers can be distinguished by 300 MHz 1H NMR.
However, a convenient unequivocal assignment of the
NMR spectrum to the individual regioisomers 1,7-5 and
1,6-5 was performed on the basis of the signal of methy-
lene protons next to the imide nitrogen at 4.13 ppm.
Because of the same chemical environment, all four
methylene protons of major regioisomer 1,7-5 appear as
one triplet at 4.13 ppm (Figure 2b). On the other hand, for
minor regioisomer 1,6-5 two separate triplets (at 4.19 and
4.07 ppm) were observed for the same four methylene
protons (Figure 2c). In this way, an unambiguous char-
acterization has been made successfully on the basis of
300 MHz 1H MNR.
Figure 3. 1H NMR (300 MHz, CDCl3) spectra of regioisomers 1,7-8, and
1,6-8.
Scheme 5. Conversion of 1,7-8 to Corresponding Dianhydride
Dipyrrolidinyl-substituted PDIs 1,7-8 and 1,6-8 were
synthesized from the regioisomeric mixture of N,N0-dioctyl-
1,7- and 1,6-dibromoperylene diimide 3 by substitution of
bromine atoms with pyrrolidine groups according to the
procedure described earlier in literature.12 As expected, the
substitution reaction yielded a mixture of 1,7- and 1,6-
dipyrrolidinyl perylene diimide derivatives. Separation of
1,7-8and 1,6-8regioisomers was performed by conventional
column chrmatogrphy on Silica 100 using DCM as mobile
phase. The minor regioisomer 1,6-8 came first and subse-
quently the major regioisomer 1,7-8 was collected.
Table 1. Redox Potentials (V vs Ag/AgCl) of 1,7-5, 1,6-5, 1,7-8, and 1,6-8
Obtained by DPVa
Both regioisomers were characterized on the basis of
1
300 MHz H NMR. Similar to previously mentioned
compd
E1ox
E2ox
E1red
E2red
1,7-5
1,6-5
1,7-8
1,6-8
þ1.47b
þ1.46
þ0.63
þ0.77
-0.61
-0.64
-0.83
-0.83
-0.85
-0.89
-0.99
-0.99
regioisomers 1,7-5 and 1,6-5, an unequivocal assignment
of the 1H NMR spectrum to the individual regioisomers
1,7-8 and 1,6-8 was based on the signal of methylene
protons next to the imide nitrogen at 4.21 ppm (Figure 3).
Additionally, in the aromatic region two doublets and
one singlet of the regioisomers 1,7-8 and 1,6-8 exhibit
large differences in chemical shift values (ca. 0.35 ppm).
Surprisingly, these aromatic signals for 1,6-8 appear in
different order (doublet, singlet, doublet) compared to
that of 1,7-8 (singlet, doublet, doublet). This different
pattern of appearance of singlet and doublets makes them
even more easily recognizable by 300 MHz 1H NMR.
The two regioisomers are very different in color too;
1,7-8 is deep green in color, whereas 1,6-8 is deep blue.
Both regioisomers are soluble in wide range of solvents
including hexane, toluene, chloroform, dichloromethane,
benzonitrile and ethanol. Keeping in mind that 1,7-di-
pyrrolidinylperylene dianhyride has been used as an
important building block for the synthesis of many useful
complex systems, we have also optimized the reaction
conditions to convert 1,7-8 to corresponding dianhyride
1,7-9 through saponification. We observed that complete
removal of n-octylamino groups of 1,7-8, in 2-propanol
with 100-fold excess of KOH, takes longer time compare
to cyclohexylamino groups of N,N0-dicyclohexyl-1,
þ0.77
þ1.28b
a Scan rate: 0.05 V/s. b Irreversible processes.
7-dipyrrolinylperylene diimide reported earlier.12 There-
fore, a 125-fold excess of KOH was used for the saponi-
fication reaction to afford 1,7-9 (Scheme 5).
Electrochemical Properties. To compare the relative
electron donor and acceptor capabilities of 1,6- and 1,7-
regioisomers, redox properties of both sets of isomers
(1,7-5 and 1,6-5; 1,7-8 and 1,6-8) were investigated by
differential pulse voltammetry in benzonitrile containing
0.1 M tetrabutylammonium tetrafluoroborate as sup-
porting electrolyte. Obtained redox potentials (V vs Ag/
AgCl) are summarized in Table 1 and differential pulse
voltammograms of 1,7-8 and 1,6-8 are shown in Figure 4.
The two regioisomers of diphenoxy substituted PDIs
(1,7-5 and 1,6-5), which are mainly known for their
electron acceptor capabilities, displayed very similar re-
dox characteristics. Both the isomers undergo two rever-
sible reductions, which reflect the first and the second
one-electron reductive process. The two reversible one-
electron reductions of 1,7-5 occur at -0.61 V and -0.85 V,
whereas for 1,6-5, both first and second reductions are

Article
Chem. Mater., Vol. 23, No. 3, 2011 785
Figure 4. Differential pulse voltammograms of 1,7-8 (green) and 1,6-8
(blue) in PhCN vs Ag/AgCl reference electrode.
Figure 5. Steady-state absorption spectra of 1,6-5 (black) and 1,7-5 (red)
in chloroform.
slightly shifted to more negative values by 30 and 40 mV,
respectively. As far as oxidation potential is concerned,
only one irreversible oxidation peak was observed at
around þ1.47 V for both compounds. Thus, two regioi-
somers 1,7-5 and 1,6-5 have virtually the same oxidation
and reduction capabilities.
On the other hand, the two regioisomers of dipyrroli-
dinyl substituted PDIs (1,7-8 and 1,6-8), which are mainly
known for their electron donating capabilities, display
some interesting features and notable differences in their
oxidation characteristics (Figure 4). The regioisomer
1,7-8 exhibits two one-electron oxidation peaks at mod-
erate potentials (E1ox = þ0.63 V, E2ox = þ0.77 V). Both
the first and the second oxidations are reversible. In the
case of second regioisomer 1,6-8, the oxidation of both
pyrrolidinyl units takes place at the same potential,
i.e., þ0.77 V. This two-electron oxidation is reversible.
In addition, this blue isomer also exhibits an irreversible
one-electron oxidation peak at very high potential, i.e.,
þ1.28 V. Surprisingly, the first oxidation of 1,6-8 occurs
at exactly same potential at which second oxidation of
1,7-8 takes place, i.e., þ0.77 V. Significantly higher values
of oxidation potentials for 1,6-8 clearly indicating that the
removal of electrons from 1,6-8 is more difficult in
comparison to 1,7-8.
Both 1,7-8 and 1,6-8 exhibit two reversible one-electron
reduction peaks at exactly same potentials (E1red = -0.83 V,
E2red = -0.99 V) indicating that both regioisomers have
same electron acceptor abilities. The improved value for
first oxidation potential of 1,7-8(þ0.63V) obtainedbyusin
comparison to the literature value (þ0.72 V) clearly sug-
gests that the previously reported 1,7-dipyrrolidinyl sub-
stituted PDI was contaminated with 1,6-regioisomer.11a
Steady-State Absorption Studies. The ground-state ab-
sorption spectra of 1,7- and 1,6-diphenoxy substituted
PDIs (1,7-5 and 1,6-5) in chloroform are depicted in
Figure 5. The absorption spectra of both regioisomers
are dominated by characteristic π-π* transitions and
resemble with each other.
Figure 6. Steady-stateabsorptionspectraof1,7-8 (green) and1,6-8(blue)
in chloroform.
isomers is in the wavelength range 375-475 nm. For 1,7-5
the absorption band belongs to S0-S2 electronic transi-
tion is located at 408 nm, whereas in the case of 1,6-5, the
same band is centered at ca. 468 nm. The absorption
properties of these two regioisomers were also investi-
gated in nonpolar toluene and polar benzonitrile, but no
other noticeable differences were observed between the
two.
In contrast to 1,7-5 and 1,6-5, the dipyrrolidinyl-sub-
stituted regioisomers 1,7-8 and 1,6-8 display large differ-
ences in their absorptive features. The absorption spectra
in chloroform are shown in Figure 6.
For these two regioisomers the lowest energy bands,
which correspond to S0-S1 electronic transitions, are
highly red-shifted with respect to that of unsubstituted
PDIs (λmax = 527 nm),13c reflecting pronounced electronic
interaction between PDI core and pyrrolidinyl groups. In
the case of 1,7-8, the lowest energy band is centered at
702 nm and hasa small shoulder at ca. 650nm. Whereas for
regioisomer 1,6-8, this lowest energy band and the shoulder
have approximately same molar extinction coefficients. In
addition, this band is significantly weaker and blue-shifted
by ca. 15 nm with respect to that of 1,7-8. These lowest
energy bands of both the regioisomers are dependent on
polarity of the solvent and exhibit significant red shift upon
increase in the polarity of the surrounding medium (see
Figures S1 and S2 in the Supporting Information). In
addition to the lower energy absorption at ca. 700 nm,
isomer 1,7-8 exhibits a higher energy S0-S2 electronic
For both regioisomers, the absorption bands attributed
to S0-S1 electronic transition are at around 555 nm in
CHCl3. However, the molar extinction coefficient is sig-
nificantly lower for 1,6-5 in comparison to 1,7-5. The
main difference in the absorption characteristic of two

786 Chem. Mater., Vol. 23, No. 3, 2011
Dubey et al.
Figure 7. Normalized steady-state emission spectra of 1,7-5 (red) and
1,6-5 (black) in CHCl3 (λex = 474 nm).
Figure 8. Fluorescence decay time profiles of 1,7-5 (red, λmon = 587 nm)
and 1,6-5 (black, λmon = 593 nm) in CHCl3 (λex = 483 nm).
Table 2. Optical Properties of 1,7-5, 1,6-5, 1,7-8, and 1,6-8 in
Different Solvents
transition at 428 nm, but, corresponding band is almost
absent in the case of 1,6-8. We presume that the absence of
higher energy band is responsible for the deep blue color of
regioisomer 1,6-8. Quite surprisingly, 1,6-8 has one addi-
tional strong band centered at 550 nm, which looks very
similar to the lowest energy band of the diphenoxy sub-
stituted PDIs (1,7-5 and 1,6-5). Because of these peculiar
absorption characteristics, isomer 1,6-8 covers a larger part
of the visible region (450-750 nm) compared to that of
1,7-8. Therefore, 1,6-8 may be of particular interest for
some specific applications where larger absorption cross-
section in the visible region is required (e.g., photovoltaic
devices).
Steady-State and Time-Resolved Emission Studies.
Further insight into the optical properties of both set of
regioisomers (1,7-5 and 1,6-5; 1,7-8 and 1,6-8) was gained
by steady-state emission and time-correlated single
photon counting (TCSPC) methods. 1,7- and 1,6-diphe-
noxy substituted PDIs (1,7-5 and 1,6-5) were examined in
three different solvents, i.e., toluene, chloroform, and
benzonitrile. The emission spectra of these compounds
in chloroform are depicted in Figure 7 and fluorescence
decays are shown in Figure 8. The emission data are
summarized in Table 2.
As expected, on the basis of absorption studies, both
1,7-5 and 1,6-5 exhibit approximately similar emission
characteristics too. The emission bands of both regioi-
somers are approximately the mirror image of their S0-S1
absorption bands with Stokes shift of around 35 nm.
However, the emission band of 1,6-5 (λmax = 593 nm) is
slightly broader and red-shifted in comparison to that of
1,7-5 (λmax = 587 nm) in chloroform as shown in Figure 7.
The emission maxima of both isomers show small bath-
ochrmic shift (ca. 14-16 nm) upon moving from non-
polar toluene to moderately polar chloroform (Table 2).
Both regioisomers are highly emissive, in all the investi-
gated solvents, with fluorescence quantum yields greater
than 0.92. For fluorescence lifetime measurements, the
solutions of both regioisomers were excited at 483 nm and
emission time profiles were collected at corresponding
emission maximum. Similar to emission maxima and
quantum yields, both 1,7-5 and 1,6-5 also displayed
approximately same singlet-state lifetimes. However,
1,6-5 exhibited slightly higher fluorescence lifetimes in
compd
solvent
λabs
λem
Φf
τf (ns)
1,7-5
toluene
chloroform
benzonitrile
toluene
chloroform
benzonitrile
hexane
548
556
553
545
554
550
661
687
702
699
647
671
688
688
573
587
583
577
593
590
703
724
743
770
728
735
752
778
0.97
0.97
0.98
0.96
0.94
0.92
0.54
0.40
0.32
0.07
0.09
0.07
0.06
0.02
4.91
5.39
4.79
5.52
5.81
5.35
6.08
4.48
3.47
1.03
1.27
1.26
1.01
0.51
1,6-5
1,7-8
toluene
chloroform
ethanol
hexane
toluene
chloroform
ethanol
1,6-8
all the three solvents with respect to that of 1,7-5 (1,6-5,
τf [ns] = 5.52 in toluene, 5.81 in CHCl3, 5.35 in benzoni-
trile; 1,7-5, τf [ns] = 4.91 in toluene, 5.39 in CHCl3, 4.79 in
benzonitrile). It is noteworthy that for both of these
isomers fluorescence life times were found to be max-
imum in moderately polar chloroform in comparison to
nonpolar toluene and polar benzonitrile.
The emission properties of 1,7- and 1,6-dipyrrolidinyl-
substituted PDIs (1,7-8 and 1,6-8) were investigated in
four different solvents, i.e., hexane, toluene, chloroform,
and ethanol. The emission spectra of these compounds in
chloroform are depicted in Figure 9 and fluorescence
decays are shown in Figure 10. The emission data are
summarized in Table 2.
In contrast to 1,7-5 and 1,6-5, the 1,7- and 1,6-regioi-
somers of dipyrrolidinyl PDIs (1,7-8 and 1,6-8) display
large differences in emission properties. Additionally, for
these compounds, emission properties namely emission
maximum, fluorescence quantum yield and fluorescence
lifetime were found to be dependent on polarity of
surrounding medium due to the presence of electron
donating pyrrolidine groups at the perylene core
(Figure S3, S4, S5 and S6, Supporting Information).
Emission maxima of both isomers exhibit significant
bathochromic shift upon increasing the solvent polarity:
1,7-8, λmax = 703 nm in hexane and 770 in ethanol; 1,6-8,
λmax = 728 nm in hexane and 778 in ethanol (Table 2). In
both polar and nonpolar solvents, the emission band of

Article
Chem. Mater., Vol. 23, No. 3, 2011 787
Conclusions
To conclude, we have synthesized, separated, and
characterized 1,6- and 1,7-regioisomers of diphenoxy
and dipyrrolidinyl substituted PDI dyes. The regioi-
somers of diphenoxy substituted PDIs (1,7-5 and 1,6-5)
were separated easily utilizing their very different solubi-
lity in toluene. On the other hand, regioisomers of dipyr-
rolidinyl substituted PDIs (1,7-8 and 1,6-8) were
separated by conventional silica gel column chromatog-
raphy. Our studies have also shown that these 1,7- and
1,6-isomers can readily be characterized even by 300 MHz
1H NMR. Subsequently, a comparative study of their
electrochemical and optical properties was also carried
out. Eventually, 1,7-dipyrrolidinylperylene diimide 1,7-8
was successfully converted to corresponding dianhydride
1,7-9, which can be used as building block for the synth-
esis of complex systems.
Figure 9. Normalized steady-state emission spectra of 1,7-8 (green) and
1,6-8 (blue) in CHCl3.
One of the rationale of this work was to clear the chaos
present in literature over the presence of 1,6-regioisomeric
impurity in 1,7-difunctionalized perylene diimide dyes.
€
Our results further confirm the observation of Wurthner
et al. that the bromination and subsequent imidization of
perylene dianhydride 1 leads to a mixture of regioisomeric
1,7- and 1,6-dibromoperylene diimides rather than only
1,7-regioisomer. Additionally, comparison of redox po-
tentials of regioisomerically pure 1,7-dipyrrolinylpery-
lene diimide 1,7-8 with published values confirm their
prediction that the previously reported difunctionalized
PDIs were contaminated with 1,6-regioisomers.
Figure 10. Fluorescencedecaysof1,7-8(green, λmon =724nm) and1,6-8
(blue, λmon = 735 nm) in tolune (λex = 483 nm).
Our studies have also shown that introduction of
phenoxy groups at the 1,7- and 1,6-positions of PDI core
leads to the regioisomers that are very similar in redox
and optical properties. Therefore, a mixture of regioiso-
meric 1,7- and 1,6-diphenoxy PDIs may be used for
various applications, i.e., construction of light harvesting
systems, photovoltaic devices, etc. On the contrary, in the
case of dipyrrolidinyl-substituted PDIs, redox and optical
properties depend significantly on the positions of two
pyrrolidine groups relative to each other. Consequently,
the two regioisomers obtained by the substitution of
pyrrolidine groups at the 1,7- and 1,6-positions exhibit
profound differences in their electrochemical and optical
characteristics. We found that 1,6-dipyrrolidinylperylene
diimide has significantly diminished electron donor abil-
ity and inherently low emission characteristics compared
to respective 1,7-regioisomer. In view of our results, we
therefore strongly emphasize that impurity of 1,6-regioi-
somer must be removed from 1,7-dipyrrolidinylperylene
diimide prior to any use, otherwise it will significantly
reduce the efficiency of the whole system. The final
conclusion that can be drawn from this work is that 1,6-
difunctionalized PDIs, depending on the substituents,
may or may not have characteristics similar to respective
1,7-derivative. We expect these results and findings will
further motivate material chemists to take especially
cautious approach to this regioisomeric impurity pro-
blem particularly when a strong electron-donating
groups, e.g., pyrrolidine, are attached to the PDI core.
1,6-8 is found to be broader compare to that of 1,7-8. In
addition, the emission maxima of regioisomer 1,6-8 is
bathochromically shifted by ca. 10 nm relative to that of
1,7-8 in polar and weakly polar solvents (i.e., ethanol,
chloroform, and toluene). On the other hand, in non-
polar hexane, the emission maxima of 1,6-8 is bath-
ochromically shifted by 25 nm compare to that of 1,7-8.
The fluorescence quantum yields of both isomers were also
determined in solvents ranging from nonpolar to polar.
Quite unexpectedly, a large difference is observed in the
quantum yields of these two isomers. In nonpolar and
weakly polar solvents, 1,7-8 has high to moderate quantum
yields (Φf = 0.54 in hexane, 0.40 in toluene, and 0.32 in
chloroform), whereas emission quantum yields of the
regioisomer 1,6-8 are found to be significantly lower than
that of 1,7-8 (Φf = 0.09 in hexane, 0.07 in toluen, and 0.06
in chloroform). However, both regioisomers have weak
emission in polar ethanol (1,6-8, Φf = 0.02; 1,7-8, Φf =
0.07). For fluorescence lifetime measurements, solutions of
both regioisomers were excited at 648 nm and emission
time profiles were collected at corresponding emission
maximum. Similar to fluorescence quantum yields, fluor-
escence life times of isomer 1,6-8 were also found signi-
ficantly lower with respect to that of 1,7-8 in all the
investigated solvents (1,6-8, τf [ns] = 1.27 in hexane, 1.26
in toluene, 1.01 in CHCl3, and 0.51 in ethanol; 1,7-8,
τf [ns] = 6.08 in hexane, 4.48 in toluene, 3.47 in CHCl3,
and 1.03 in ethanol).

788 Chem. Mater., Vol. 23, No. 3, 2011
Dubey et al.
1
Acknowledgment. The authors are grateful to Academy of
Finland for the financial support.
solvents (S4); H NMR spectra of aliphatic region of 1,7-5 and
1,6-5 in CDCl3 (S5); 1H NMR spectroscopic monitoring of separa-
tion of regioisomers 1,7-5 and 1,6-5 (S6); 1H and 13C NMR
spectra of all synthesized compounds (S7-S13) (PDF). This
material is available free of charge via the Internet at http://
pubs.acs.org.
Supporting Information Available: Steady-state absorption
and emission spectra of 1,7-8 and 1,6-8 in different solvents (S2 &
S3); fluorescence decay time profiles of 1,7-8 and 1,6-8 in different