Bis(n-octylamino)perylene-3,4:9,10-bis(dicarboximide)s and their radical cations: Synthesis, electrochemistry, and ENDOR spectroscopy

Article abstract of DOI:10.1021/jo052394o

1,6- and 1,7-bis(n-octylamino)perylene-3,4:9,10-bis(dicarboximide) were synthesized by reaction of n-octylamine with the corresponding dibromo compounds. These compounds display intense charge-transfer optical transitions in the visible spectrum (～550-750 nm) and fluoresce weakly (Φ_F < 0.06). Cyclic voltammetry reveals that each chromophore undergoes facile and reversible oxidation and reduction. Spectroelectrochemical studies show that the radical cations of these chromophores are stable and show no signs of deprotonation of the secondary amines. Electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) studies of the chemically generated radical cations of these chromophores corroborate the spectroelectrochemical data by showing that the radical cations persist for days at room temperature in methylene chloride solution. These experiments and complementary density functional theory (DFT) calculations provide a comprehensive picture of the molecular orbitals, spin density distributions, and geometries of the radical cations. The redox properties and stability of these alkylamino-functionalized perylene compounds make them a valuable addition to the family of robust perylene-based chromophores that can be used to develop new photoactive charge transport materials.

Full text of DOI:10.1021/jo052394o

Bis(n-octylamino)perylene-3,4:9,10-bis(dicarboximide)s and Their
Radical Cations: Synthesis, Electrochemistry, and ENDOR
Spectroscopy
Michael J. Ahrens, Michael J. Tauber, and Michael R. Wasielewski*
Department of Chemistry and Center for Nanofabrication and Molecular Self-Assembly,
Northwestern UniVersity, EVanston, Illinois 60208-3113
m-wasielewski@northwestern.edu
ReceiVed NoVember 20, 2005
1,6- and 1,7-bis(n-octylamino)perylene-3,4:9,10-bis(dicarboximide) were synthesized by reaction of
n-octylamine with the corresponding dibromo compounds. These compounds display intense charge-
transfer optical transitions in the visible spectrum (∼550-750 nm) and fluoresce weakly (ΦF < 0.06).
Cyclic voltammetry reveals that each chromophore undergoes facile and reversible oxidation and reduction.
Spectroelectrochemical studies show that the radical cations of these chromophores are stable and show
no signs of deprotonation of the secondary amines. Electron paramagnetic resonance (EPR) and electron-
nuclear double resonance (ENDOR) studies of the chemically generated radical cations of these
chromophores corroborate the spectroelectrochemical data by showing that the radical cations persist for
days at room temperature in methylene chloride solution. These experiments and complementary density
functional theory (DFT) calculations provide a comprehensive picture of the molecular orbitals, spin
density distributions, and geometries of the radical cations. The redox properties and stability of these
alkylamino-functionalized perylene compounds make them a valuable addition to the family of robust
perylene-based chromophores that can be used to develop new photoactive charge transport materials.
Introduction
solar cells,8,9 and light-emitting diodes.10,11 These chromophores
have generated great interest because of their high photochemi-
cal stability, ease of synthetic modification, and desirable optical/
redox characteristics. The special emphasis on stable, red to
Perylene-3,4:9,10-bis(dicarboximide) (PDI) and related arylene
dicarboximides and bis(dicarboximides) continue to receive
considerable attention for applications such as field effect
transistors,1,2 molecular electronics,3-5 light-harvesting arrays,6,7
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10.1021/jo052394o CCC: $33.50 © 2006 American Chemical Society
Published on Web 02/09/2006
J. Org. Chem. 2006, 71, 2107-2114
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Ahrens et al.
near-infrared (NIR) absorbing chromophores is driven by the
demands of optoelectronic and solar cell applications. For
example, chromophores with lengthened conjugated cores
(extended transition dipoles), such as derivatives of terrylene
and quaterrylene bis(dicarboximides), have excellent red and
NIR absorption characteristics.12-14
debromination. In an effort to expand the scope of chromophores
available for designing systems for self-assembly and charge
transport based on PDI, we have investigated substitution of
bay region bromines by flexible alkylamino substituents, since
previous studies from our laboratory and others17,18,20,28-38 have
used only sterically bulky cyclic amines as electron-donating
groups. We now report on the introduction of n-octylamino
groups at the 1,6- and 1,7-positions of PDI affording chro-
mophores that are either intense blue (1,6) or green (1,7) in
color and that readily undergo both one-electron oxidations and
reductions. In particular, the radical cations (1,6)•+ and (1,7)•+
are unusually stable.
The electronic characteristics of arylene imides and diimides
can be tuned to some degree using the imide functionality,15
but the nodes in both the HOMO and LUMO that bisect these
molecules through their long axes limit electronic coupling
through this linkage. The most effective method of changing
the electronic absorption characteristics of these molecules is
direct substitution of the conjugated aromatic core. Substitution
of the core with electron-donating groups has yielded alkyl-
amino-substituted naphthalene-1,8:4,5-bis(dicarboximide)s,16 pyr-
rolidinyl-substituted perylene-3,4:9,10-bis(dicarboximide)s,17,18
9-aminoperylene-3,4-dicarboximides,19 piperidinyl-substituted
perylene-3,4:9.10-bis(dicarboximide)s,20 and terrylene-3,4:11,12-
bis(dicarboximide)s.12 The amino-substituted arylene bis(dicar-
boximide)s display strongly red-shifted absorption and emission
bands and are oxidized and reduced with ease. By contrast, most
other arylene bis(dicarboximide)s are generally difficult to
oxidize due to stabilization of the HOMO by the electron-
withdrawing imide groups. Most examples in the literature report
enhancement of their good electron-accepting properties, the
most extreme example being the cyanated derivatives.21
Results and Discussion
Synthesis. The synthetic scheme for (1,6) and (1,7) is given
in Figure 1. Bromination of PDI leads to a mixture of isomers
in which the 1,7 isomer usually dominates.18,20 The isomeric
mixture N,N′-bis(n-octyl)(1,6- and 1,7-dibromoperylene-3,4:
9,10-bis(dicarboximide) was heated in neat n-octylamine to
obtain the corresponding secondary amines (1,6) and (1,7) as
well as the 1-(n-octylamino) derivative as a consequence of
partial debromination of the starting material. The 1-(n-
octylamino) derivative was separated from (1,6) and (1,7) by
column chromatography, while (1,6) and (1,7) were separated
from each other by preparative HPLC.
Photophysical Characterization. The ground-state absorp-
tion spectra of the green 1,7 isomer and the blue 1,6 isomer in
toluene are shown in Figure 2. The spectra are dominated by
very broad, and nearly structureless, absorption bands that span
a large part of the visible spectrum (475-750 nm for 1,6 and
550-750 nm for 1,7). This broad absorption is strongly red-
shifted relative to that of unsubstituted PDI, is very similar to
spectra of PDIs substituted at the 1,7 positions with secondary
cyclic amine substituents, and is characteristic of charge transfer
(CT) excited states.17 The main differences between the two
spectra are the absorption near 400 nm for the 1,7 isomer and
the peak at ∼545 nm in the spectrum of the 1,6 isomer. These
two features account for the large difference in color observed
by the naked eye.
This paper focuses on derivatives of perylene-3,4:9,10-bis-
(dicarboximide), PDI. To date, there are two principal methods
for introducing substituents onto the PDI core. The first method
is 4-fold symmetric chlorination of the 1, 6, 7, and 12
positions,22,23 while the second method is bromination of
perylene dianhydride,18,24 which yields the desired 1,7-dibro-
minated product but also produces a varying amount of the
difficult to separate 1,6-dibrominated side product. Replacement
of these halogens is readily achieved by traditional substitution
reactions or metal-catalyzed cross-coupling reactions,25-27
although the former are sometimes accompanied by extensive
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Both isomers are soluble in a wide range of solvents including
hexane, methylcyclohexane, toluene, chloroform, dichlo-
romethane, and butyronitrile. In previous work, preferential
solvation of long, aliphatic tails served to assemble various PDI
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2005, 17, 2208-2215.
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Mater. 2002, 14, 952-953.
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14, 2684-2686.
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Soc. 2004, 126, 12268-12269.
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Giaimo, J. M.; Gusev, A. V.; Goshe, A. J.; Tiede, D. M.; Wasielewski, M.
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2108 J. Org. Chem., Vol. 71, No. 5, 2006

Bis(n-octylamino)perylene-3,4:9,10-bis(dicarboximide)s
FIGURE 1. Synthetic scheme.
of 1,6 and 1,7 and compared these results to those for 1,7-bis-
(pyrrolidin-1-yl)perylene-3,4:9,10-bis(dicarboximide) (5PDI)30
and the monosubstituted N,N′-bis(n-octyl)-1-(n-octylamino)-
perylene-3,4:9,10-bis(dicarboximide). The emission λmax of each
compound was measured in eight solvents with dielectric
constants ranging from 2.02 to 36.7 and plotted against f(ꢀ,n)
) [(ꢀ - 1)/(2ꢀ + 1) - 0.5 (n2 - 1)/(2n2 + 1)], where ꢀ and n
are the static dielectric constant and the refractive index of the
solvent, respectively, to give the Lippert-Mataga plots shown
in Figure 3. The change in dipole moment from ground to
excited-state ∆µge determined from this plot is highly dependent
on the Onsager radius;41 therefore, we report a range of ∆µge
values that result from radii spanning 5.0-7.5 Å, with the upper
limit obtained from AM1 optimization using Gaussian 98. For
the 1,7 isomer, ∆µge ) 7.7-14.2 D, while for the 1,6 isomer,
∆µge ) 10.0-18.4 D. These values of ∆µge are very close to
those determined for 5PDI (7.4-13.6 D) and monosubstituted
N,N′-bis(n-octyl)-1-(n-octylamino)perylene-3,4:9,10-bis(dicar-
boximide) (7.7-14.2 D).
FIGURE 2. Ground-state absorption spectra for (1,7) (black) and (1,6)
(red) in toluene.
The ground-state dipole moments obtained by DFT calcula-
tions at the B3LYP/6-31G* level are 0.0 D for the centrosym-
metric 1,7 isomer, and 1.0 D for the asymmetric 1,6 isomer.
Therefore the ∆µge values obtained above reflect the formation
of a substantial dipole moment in the equilibrium excited state.
The most likely molecular origin of this dipole is charge transfer
from one amino substituent to the perylene core. This asym-
metric CT process readily explains why the centrosymmetric
1,7 isomer has an overall excited state dipole moment. The
hypothesis is also consistent with the nearly identical excited
state stabilization measured experimentally for the monosub-
stituted alkylamino-PDI and the 1,7 isomer. The formation of
substantial excited-state dipole moments in other symmetric
molecules in solution, due to asymmetric or local excitation,
chromophores into either H-32 or J-type39,40 aggregates. How-
ever, in this study, only the monomeric spectrum is observed
in all solvents investigated, indicating little or no aggregation
up to concentrations of ∼1 mM. The only observable difference
between solvents is a red shift in the absorbance spectrum upon
increasing solvent polarity: 1,7, λmax) 658 nm in hexane and
705 nm in DMF; 1,6, λmax) 624 nm in hexane and 675 nm in
DMF. We attribute the shift to increased stabilization of the
CT excited state.
Using the well-established fluorescence solvatochromic shift
method,41 we measured the stabilization of the excited-states
(39) van Herrikhuyzen, J.; Syamakumari, A.; Schenning, A. P. H. J.;
Meijer, E. W. J. Am. Chem. Soc. 2004, 126, 10021-10027.
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Kluwer Academic/Plenum Publishers: New York, 1999.
J. Org. Chem, Vol. 71, No. 5, 2006 2109

Ahrens et al.
isomer. Differential pulse voltammetry (DPV) was used to
determine the potential of the second irreversible oxidation
process for (1,6) at +1.06 V. The intensity of this peak was
lower than that of the first oxidation peak, most likely due to
irreversible decomposition of the dication. We note that the first
oxidation potential of (1,6) (+0.91 V) is considerably more
positive than that of (1,7) (+0.76 V), and that the reduction
potentials are identical within experimental error. The energies
of the LUMOs and HOMOs determined by the DFT calculations
(Table 2) closely reflect the experimental trends: the HOMO
of the 1,6 isomer is 0.11 eV lower in energy than that of the
1,7 isomer, and the LUMO energy of the 1,6 isomer is 0.01 eV
lower than that of 1,7; both of these calculated values correspond
well to the respective 0.15 and 0.01 V experimental differences.
Spectroelectrochemical oxidation of 1,7 shows the growth
of prominent new absorption features at 918 and 1068 nm,
which are attributed to the radical cation (1,7)•+, Figure 5A.
Full reversibility over as much as 40 min attests to the stability
of the radical cation. Nearly identical spectra of (1,7)•+ are
obtained upon chemical oxidation with NOSbF6 (Figure S1).
The stability of the chemically oxidized samples is remark-
able: the cation bands show only 50% diminishment after 6
days of storage at room temperature in a sealed tube. The NIR
bands resemble those previously observed for the cyclic amino-
substituted perylenes substituted at the 1,7 positions.30 The DFT
results help rationalize the appearance of the new features in
the NIR, since the calculated gap between (HOMO-1) and
HOMO of (1,7) is 8884 cm-1, close to the energy of the
experimentally observed transitions.
In the case of (1,6), spectroelectrochemical oxidation yields
a spectrum that differs only slightly from the neutral parent,
with the main change being the growth of a new band with a
maximum at 616 nm (Figure 5B). A chemically oxidized sample
shows similar spectral changes (Figure S1, Supporting Informa-
tion) and exhibits EPR and ENDOR signals (described below);
thus, we assign the new spectrum to the radical cation. The
prominent NIR absorption bands that are typical of the radical
cations of 1,7-substituted alkyl-amino PDIs are absent from all
spectra obtained after electrochemical or chemical oxidation of
the 1,6 isomer, including samples that are oxidized further than
those shown in Figure 5B (not shown). The DFT results support
the assignment of the new spectrum with a maximum at 616
nm to the radical cation. The calculated (HOMO-1)-HOMO
energy gap for (1,6) is only 5048 cm-1, which is consistent
with the lack of absorption features in the 800-1100 nm region
for (1,6)•+.
FIGURE 3. Lippert-Mataga plots for (1,7) (green dots, R2 ) 0.984),
(1,6) (black dots, R2 ) 0.991), 5PDI (red dots, R2 ) 0.947), N,N′-bis-
(n-octyl)-1-(n-octylamino)perylene-3,4:9,10-bis(dicarboximide) (blue
dots, R2 ) 0.975). The solvents are (1) methylcyclohexane, (2)
cyclohexane, (3) ethyl ether, (4) ethyl acetate, (5) tetrahydrofuran, (6)
butyronitrile, (7) acetone, and (8) DMF.
have been reported previously.42,43 An alternative explanation
for the excited-state stabilization, in which the symmetry of the
ground state is maintained, is local solvation differences between
the two equivalent aminoperylene dipoles oriented in opposition
along the short axis of perylene. The net stabilization in this
scenario is best described in terms of quadrupole-solvent dipole
interactions.44 Although we cannot rule out this explanation, it
is less likely since it requires a fortuitous coincidence in the
overall stabilization of the monosubstituted and 1,7 sym-
metrically disubstituted molecules.
The ground-state geometries determined by DFT(B3LYP/6-
31G*) calculations have a core twist angle of the perylene core,
i.e., the approximate dihedral angle between the two naphthalene
subunits attached to the central benzene ring, which is 14.9°
for the 1,7 isomer and 21.7° for the 1,6 isomer, Figure 4. For
comparison, the 1,7-dibrominated PDI derivative has a measured
core twist angle of 24°.2 There are very few literature examples
of 1,6-disubstituted PDIs and to our knowledge no measured
crystal structures. As a whole, the alkylamino PDI chromophores
have smaller calculated core twist angles than those obtained
from X-ray crystal structures of other PDI derivatives.2 The
experimental 620 cm-1 difference in energy at λmax between
the 1,6 and 1,7 isomers in hexane, Table 1, agrees reasonably
well with the calculated difference in HOMO-LUMO gaps
between the two isomers (985 cm-1, Table 2). As expected,
the calculation for the flatter 1,7 isomer yields a HOMO-
LUMO gap which is smaller than that of the more twisted 1,6
isomer.
Our observation of stable and reversible electrochemical/
chemical production of long-lived (1,6)+• and (1,7)+• species
is noteworthy. Generally, electrochemical oxidation of aliphatic
amines is irreversible because formation of the amine radical
cation leads to dealkylation, proton loss, or nucleophilic
attack.45-50 Substitution of the amine with aromatic groups
stabilizes the radical cation, as has been demonstrated in some
aniline derivatives.51 Maroz et al.52 have shown that geometry
Electrochemistry and Spectroelectrochemistry. The redox
potentials of the 1,6 and 1,7 isomers (Table 3) are obtained by
cyclic voltammetry. Both isomers show two oxidation and two
reduction waves. All are chemically reversible, with 60-90 mV
peak separations, except for the second oxidation of the 1,6-
(45) Portis, L. C.; Bhat, V. V.; Mann, C. K. J. Org. Chem. 1970, 35,
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1596.
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M. J. Phys. Chem. B 2004, 108, 16834-16840.
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Warman, J. M. J. Am. Chem. Soc. 2001, 123, 5337-5347.
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2110 J. Org. Chem., Vol. 71, No. 5, 2006

Bis(n-octylamino)perylene-3,4:9,10-bis(dicarboximide)s
FIGURE 4. DFT (B3LYP/6-31G*) geometry-optimized structures of (1,7) (left) and (1,6) (right) shown with view along the long axis. For the
purposes of the computation, ethyl groups replace the n-octyl amino groups and methyls replace the octyls at the imide positions. The “core-twist
angle” for (1,7) is 14.9° and for (1,6) is 21.7°.
TABLE 1. Absorption and Emission Maxima and Fluorescence
νENDOR ) |νn ( a/2| where νENDOR are the ENDOR transition
Quantum Yields (ΦF)
frequencies, νn is the proton Larmor frequency, and a is the
c
isotropic hyperfine coupling constant (hfcc).53 The ENDOR
spectrum of (1,7)+•, Figure 7, shows four distinct peak pairs,
of which the three largest are within 25% of hfcc’s determined
by DFT calculation, Table 4. The calculation leads to assignment
of the 3.51 G resonance to the two amine protons, and the 3.81
G resonance to the four methylene protons nearest the amine.
The experimental peak at 1.09 G is assigned to the bay region
proton. The smallest ENDOR peak at 0.14 G is likely due to
one of the two other perylene proton pairs, although definitive
assignment is not possible since the calculation suggests that
two resonances should be observed at 0.36 and 0.18 G. The
simulation of the CW-EPR spectrum of (1,7)•+ (Winsim)54 using
the ENDOR results to lock the proton assignments, allows
assignment of the nitrogen resonances. Only the hyperfine
coupling constant of the secondary amine nitrogens is large.
The DFT result for the coupling constant is in rough agreement
with the value obtained from the simulation. The spin densities
at each atomic center show that the unpaired electron is most
strongly associated with the secondary amine nitrogens. The
greatest spin density on the perylene core of (1,7)•+ is distributed
centrosymmetrically on eight carbons, Figure 8. There are
distinct nodes in the spin density, which is fully consistent with
the small coupling constants for the imide nitrogens, as well as
outer protons 2, 5, 8, and 11 of the perylene core.
The EPR and ENDOR spectra of the (1,6)•+-isomer are shown
in Figures 6 and 7, respectively. Hfcc’s determined from these
spectra (Table 4) clearly show that the amine groups of (1,6)•+
have less unpaired spin density than (1,7)•+, as shown by the
smaller coupling constants of amino-alkyl protons and amine
nitrogens. On the other hand, there is considerable spin density
on the perylene core of (1,6)•+ which is concentrated on the
half of the aromatic system that is farthest from the amine
substituents. This results in a ∼4-fold increase in the hfcc’s of
the perylene bay protons of (1,6)•+, relative to (1,7)•+. The
asymmetric substitution also causes the imide nitrogen farthest
from the secondary amines to acquire additional spin density,
which is unusual since the imide nitrogens are a well-known
location for the position of a node in both the HOMO and
LUMO.55 It is clear from the experimental results, and the spin
density maps determined computationally (Figure 8) that the
substitution pattern strongly affects the electronic structure of
the radical cations of these amine-substituted PDIs.
λ
abs (nm)
λ
em (nm)
ΦF
toluene THF CH2Cl2 hexane toluene THF toluene THF
(1,6) 653.5
(1,7) 681
660
682
657
692
634
656.5
772a
743b
793a <0.01 <0.01
766b
0.06 0.02
a 580 nm excitation. b 550 nm excitation. c Oxazine 170 relative standard
(0.63 in methanol).
TABLE 2. Molecular Orbital Energies from DFT Calculations
(1,6) (eV)
(1,7) (eV)
LUMO (eV)
HOMO (eV)
-2.9973
-5.2556
-5.8815
-3.0069
-5.1430
-6.2445
HOMO-1 (eV)
energy gap (cm-1
)
18213a
5048b
17228a
8884b
a HOMO-LUMO. b HOMO-(HOMO-1).
TABLE 3. Oxidation and Reduction Potentials vs SCEa
compd
E-
E2-
E+
E2+
1/2
1/2
1/2
1/2
(1,6)
(1,7)
-0.84
-0.83
-1.03
-1.00
0.91
0.76
1.06b
0.95
a Values obtained by cyclic voltammetry (except as noted) in dry CH2Cl2
with 0.1 M Bu4NPF6 as the supporting electrolyte and Fc/Fc+ as internal
standard. b Value from minor peak of differential pulse voltammogram (not
shown).
plays an important role in stabilizing aromatic amines. In the
case of N-methylaniline, the planar geometry (proton in aromatic
plane) is not susceptible to deprotonation, whereas the perpen-
dicular geometry (proton out of aromatic plane) undergoes loss
of a proton at the diffusion-limited rate. In our case, DFT
calculations show that the amine protons of (1,6) and (1,7) are
nearly in the plane of the perylene core at their respective points
of attachment in both their neutral and oxidized states, which
is consistent with the stability to proton loss observed experi-
mentally.
Electron Paramagnetic Resonance. The EPR and ENDOR
spectra acquired upon reaction of (1,6) or (1,7) with NOSbF6
in CH2Cl2 at room temperature reinforce the conclusion that
these electron-rich PDIs form reasonably stable radical cations.
Broadened CW-EPR spectra are obtained at 270 K, with g
values of 2.0025 for (1,6)•+ and 2.0027 for (1,7)•+, Figure 6.
Remarkably, the same EPR spectra were acquired on these
samples after storage at room temperature for nearly one week,
albeit at reduced intensity. The signal of (1,6)•+ was diminished
by about 80%, while that for (1,7)•+ was reduced by only 50%.
Isotropic hfcc’s were obtained from ENDOR spectroscopy
in liquid CH2Cl2 solution using the ENDOR resonance condition
(53) Kurreck, H.; Kirste, B.; Lubitz, W. Electron Nuclear Double
Resonance Spectroscopy of Radicals in Solution; VCH: Weinheim, 1988.
(54) Duling, D. R. J. Magn. Reson. Ser. B 1994, 104, 105-110.
(55) Wu¨rthner, F. Chem. Commun. 2004, 14, 1564-1579.
(52) Maroz, A.; Hermann, R.; Naumov, S.; Brede, O. J. Phys. Chem. A.
2005, 109, 4590-4596.
J. Org. Chem, Vol. 71, No. 5, 2006 2111

Ahrens et al.
FIGURE 5. Spectroelectrochemistry of (A) (1,7) and (B) (1,6) in CH2Cl2 with 0.1 M Bu4NPF6. The purple spectra are those of (1,7)+• and (1,6)+•
following complete one-electron oxidation of the samples.
FIGURE 7. ENDOR spectra of 0.23 mM (1,6)+• and (1,7)+• in CH2-
Cl2 at 270 K. An off-resonance background is subtracted, and spectra
are integrated. (1,6)+•: microwave power 39 mW, RF power 200-
600 W across scan range. (1,7)+•: microwave power 99 mW, RF power
FIGURE 6. CW-EPR spectra (black line) of (1,6)+• (top) and (1,7)+•
(bottom) in CH2Cl2 at 270 K and simulated fits (red line). Sample
concentration 0.23 mM of radical cation. Microwave power 6.2 mW,
modulation frequency 25 kHz, modulation amplitude 0.25 G. Simula-
tions of the CW spectra are constrained by ENDOR data for protons,
and the hfcc’s for nitrogen are optimized.
300-800 W across scan range. Frequency modulation νFM ) 25 kHz,
with modulation depth ∆νRF ) 100 kHz. 50 scans × 21 s/scan. Note:
the conversion factor 2.80 MHz/gauss for comparison with the data
presented in Table 4.
Experimental Section
Conclusions
Proton nuclear magnetic resonance spectra were obtained at 500
MHz using TMS as an internal standard. High-resolution mass
spectrometry was performed using a spectrometer equipped with a
FAB ion source. Laser desorption mass spectra were obtained with
a MALDI-TOF mass spectrometer using dithranol as a matrix.
Phase transitions were determined using a DSC system.
Synthesis/Purification. N,N′-Bis(n-octyl)(1,6- and 1,7-n-octyl-
amino)perylene-3,4:9,10-bis(dicarboximide). N,N′-Bis(n-octyl)-
(1,6- and 1,7-dibromoperylene-3,4:9,10-bis(dicarboximide)20 (0.080
g, 0.104 mmol) and n-octylamine (3.128 g, 24.2 mmol) were added
to a flame-dried 25 mL round-bottom flask. The contents were
heated at 150 °C for 2 h under a nitrogen atmosphere. The color of
the solution quickly turned from red to green. The hot solution
was poured into a solution of methanol/HCl (90/10 mL) and was
cooled at 0 °C overnight. The precipitate was collected by
centrifugation, washed several times with methanol, and dried with
nitrogen to give a green residue. The crude material was purified
via flash column chromatography on silica gel using methylene
chloride as the eluent to give a mixture of the 1,6 and 1,7 isomers
as a green solid, 0.054 g (0.062 mmol), 60%. Additionally, a
The novel chromophores (1,6) and (1,7) have photophysical
and redox properties that will make them useful in the design
of new photofunctional electron donor-acceptor systems. Their
oxidation and reduction at modest potentials make these
chromophores both good electron donors and acceptors. The
radical cations exhibit excellent stability at room temperature
as shown by cyclic voltammetry, spectroelectrochemistry, and
EPR/ENDOR spectroscopy. This stability is noteworthy in view
of numerous studies pointing toward oxidative degradation of
secondary N-alkyl-N-arylamines. We hypothesize that the
geometry of the secondary amine relative to the aromatic core
promotes the observed stability of the radical cations. These
compounds offer an additional avenue for expanding the already
rich chemistry of PDIs. Further work is underway to expand
the applicability of these chromophores to organic electronic
materials by functionalizing their imide positions.
2112 J. Org. Chem., Vol. 71, No. 5, 2006

Bis(n-octylamino)perylene-3,4:9,10-bis(dicarboximide)s
TABLE 4. Hyperfine Coupling Constants (Gauss) for (1,7)+• and (1,6)+• As Determined by Proton cw-ENDOR, Simulation of the cw-EPR
Spectrum, and DFT Electronic Structure Calculations of Model Compounds Described in the Text
H
H
H
H
N
N
N
ENDOR
(1,7)+
(1,6)+
(1,7)+
(1,6)+
(1,7)+
(1,6)+
(1,7)+
(1,6)+
3.81
2.43
3.5
2.14
1.09
4.34
0.14
1.53
CW-EPR simulation1
DFT Calculation
assignment2
3.81 × 4
2.43 × 4
+3.94
+2.56
CH2
3.5 × 2
2.14 × 2
-3.26
-2.1
N-H
N-H
1.09 × 2
4.34 × 2
-1.36
-4.72
6,12
0.14 × 2
3.46 × 2
1.94 × 2
+2.69
+1.76
N-H
0.12 x2
1.53 × 2
+0.18
+2.11
5,11
0.54 x1
+0.36
+0.15
2,8
-0.23
-0.10
3,4
-0.45
9,10
9,10
CH2
7,12
2,5
8,11
N-H
3,4
a Proton hfcc’s are from the ENDOR spectra; nitrogen hfcc’s are optimized in the simulation.
FIGURE 8. Total spin density of model compounds for (1,6)+• (left) and (1,7)+• (right). The geometries of the radical cations are fully optimized
at the unrestricted B3LYP/6-31G* level. The value of S2 ) 0.75 for each calculated radical cation.
significant amount of monosubstituted product N,N′-bis(n-octyl)-
1-(n-octylamino)perylene-3,4:9,10-bis(dicarboximide) was isolated
and characterized (0.018 g, 0.024 mmol, 23.4%).
4H), 3.416 (m, 2H), 1.737, (m, 6H), 1.25-1.45 (m, 30H), 0.885
(m, 9H); mp 196-200 °C; C48H59N3O4 HRMS-FAB (m/z) 741.4506
(calcd 741.4506). Anal. Calcd for C48H59N3O4: C, 77.70; H, 8.01;
N, 5.66. Found: C, 77.46; H, 7.97; N, 5.52.
Separation of the 1,6 and 1,7 isomers was performed on a
preparative HPLC system equipped with a dual-wavelength absor-
bance detector and fitted with a macro-HPLC column (Si, 8 µm,
250 × 22 mm). The eluent was 3:1 hexanes/chloroform flowing at
12 mL/min. Typical injection volumes were 200 µL of a saturated
solution (hexanes) that had been passed through a 0.45 µm PTFE
filter prior to injection. Three fractions were collected from the
column; the first was pure 1,7 isomer, the second was a mixture of
1,7 and 1,6 isomers, and the third was pure 1,6 isomer. On the
basis of analytical HPLC analysis the ratio of 1,7:1,6 isomers can
vary from 90:10 to 80:20; see Figure S2, Supporting Information.
This reflects the isomeric ratio of the dibrominated starting material.
N,N′-Bis(n-octyl)-1,7-bis(n-octylamino)perylene-3,4:9,10-bis-
(dicarboximide): 1H NMR (CDCl3, 500 MHz, see Figure S3,
Supporting Information) δ 8.733 (d, J ) 8.1 Hz, 2H), 8.288 (d, J
) 8.1 Hz, 2H), 8.078 (s, 2H), 5.672 (s, 2H), 4.173 (t, J ) 7.6 Hz,
4H), 3.315 (m, 4H), 1.729 (m, 8H), 1.2-1.5 (m, 40H), 0.890 (m,
12H); mp 50-54 °C; C56H76N4O4 HRMS-ESI (m/z) [M + e]
868.5870 (calcd 868.5867). Anal. Calcd for C56H76N4O4: C, 77.38;
H, 8.81; N, 6.45. Found: C, 77.18; H, 8.81; N, 6.37.
N,N′-Bis(n-octyl)-1,6-bis(n-octylamino)perylene-3,4:9,10-bis-
(dicarboximide): 1H NMR (CDCl3, 500 MHz, see Figure S3,
Supporting Information) δ 8.668 (d, J ) 8.1 Hz, 2H), 8.594 (d, J
) 8.1 Hz, 2H), 7.833 (s, 2H), 6.009 (s, 2H), 4.225 (t, J ) 7.6 Hz,
2H), 4.178 (t, J ) 7.6 Hz, 2H), 3.364 (t, J ) 7.1 Hz, 4H), 1.78-
1.64 (m, 8H), 1.25-1.5 (m, 40H), 0.898 (m, 12H); mp 50-55 °C;
C56H76N4O4 HRMS-ESI (m/z) [M + e] 868.5870 (calcd 868.5867).
Anal. Calcd for C56H76N4O4: C, 77.38; H, 8.81; N, 6.45. Found:
C, 77.12; H, 8.66; N, 6.23.
Several arylene bis(dicarboximide)s having long alkyl chains are
liquid crystalline (LC).20,39,56,57 We explored the possibility of an
LC phase for both the 1,6 and 1,7 isomers using differential
scanning calorimetry. For both compounds there was no observable
liquid crystalline transition between -40 and +350 °C.
Optical Spectroscopy. Steady-state absorption and emission
spectra were acquired with a Shimadzu 1601 UV/vis spectropho-
tometer and PTI photon-counting spectrofluorimeter, respectively.
A 10 mm quartz cuvette was used for both the absorption and
fluorescence measurements. The fluorescence measurements were
performed in a right angle configuration, and the optical density at
λmax was maintained at 0.05-0.1 to avoid reabsorption artifacts.
Electrochemistry and Spectroelectrochemistry. Electrochemi-
cal measurements were performed using an electrochemical work-
station. All measurements were performed on isomerically pure
compounds, confirmed by HPLC and NMR. The solvent used was
methylene chloride (unstabilized HPLC grade passed through two
columns of alumina) containing 0.1 M TBAPF6. The electrochemi-
cal cell was fitted with a 1.0 mm diameter platinum disk working
electrode, Ag/AgxO reference electrode, and platinum wire counter
electrode, and was continuously purged with nitrogen during the
measurements. The ferrocene/ferrocinium redox couple (Fc/Fc+,
0.475 vs SCE) is used as an internal reference. Spectroelectro-
chemical measurements were also performed in methylene chloride
with 0.1 M TBAPF6 as described earlier.21,58
(56) Debije, M. G.; Chen, Z.; Piris, J.; Neder, R. B.; Watson, M. W.;
Mu¨llen, K.; Wu¨rthner, F. J. Mater. Chem. 2005, 15, 1270-1276.
(57) Struijk, C. W.; Sieval, A. B.; Dakhorst, J. E. J.; van Dijk, M.;
Kimkes, P.; Koehorst, R. B. M.; Donker, H.; Schaafsma, T. J.; Picken, S.
J.; van de Craats, A.; Warman, J. M.; Zuilhohf, H.; Sudholter, E. J. R. J.
Am. Chem. Soc. 2000, 122, 11057-11066.
N,N′-Bis(n-octyl)-1-(n-octylamino)perylene-3,4:9,10-bis(dicar-
boximide): 1H NMR (CDCl3, 500 MHz) δ 8.667 (d, J ) 8.1 Hz,
1H), 8.448 (d, J ) 8.1 Hz, 1H), 8.350 (m, 1H), 8.216 (d, J ) 7.9
Hz, 1H), 8.153 (m, 1H), 8.053 (m, 2H), 5.899 (s, 1H), 4.139 (m,
J. Org. Chem, Vol. 71, No. 5, 2006 2113

Ahrens et al.
Electron Paramagnetic Resonance. EPR and ENDOR spectra
were acquired with an EPR spectrometer, fitted with an ENDOR
accessory, resonator, and RF power amplifier. Temperature was
controlled by a liquid nitrogen flow system. Experimental param-
eters are provided in the captions of Figures 6 and 7. All samples,
solvents, and oxidants were handled in a glovebox while purging
with dry nitrogen. Methylene chloride (unstabilized HPLC grade
passed through two columns of alumina) was stored over 3 Å
molecular sieves in the drybox. The oxidant NOSbF6 was used as
received. This insoluble oxidant was added to a 1 mM solution of
(1,6) or (1,7) in methylene chloride, and allowed to react for several
minutes while agitating. The oxidation was halted by withdrawing
the liquid, when ∼25% of (1,6) or (1,7) had reacted (see UV-vis
spectra, Figure S1, Supporting Information). A fraction of the
oxidized solution was loaded into a 2.4 mm i.d. quartz tube which
was then sealed in the glovebox with a 0.5-1.0 cm plug of vacuum
grease, and wrapped tightly with Parafilm. UV-vis spectra acquired
through the quartz tube match the spectra of the radical cations
generated electrochemically. After 4 h at room temperature there
was only a slight decrease in absorption of the radical cation as
seen by UV-vis spectroscopy.
tures.60 The molecular volumes (and Onsager radii) were estimated
by calculation at the AM1 level in Gaussian, employing the full
molecules with the n-octyl substituents.
Acknowledgment. This work was supported by the Office
of Naval Research under Grant No. N00014-05-1-0021. We
thank Brooks Jones for assisting with DSC measurements, Jovan
Giaimo for assistance with HPLC, Bryan Leavitt for assistance
with fluorescence measurements, and Josh Vura-Weis for advice
on the computations. M.J.T. acknowledges the donors of the
American Chemical Society Petroleum Research Fund for partial
support of this research. The Bruker E580 spectrometer was
purchased with partial support from NSF Grant No. CHE-
0131048.
Supporting Information Available: HPLC data, NMR, and
UV-vis spectra as well as details of the DFT calculations. This
material is available free of charge via the Internet at http://
pubs.acs.org.
JO052394O
Electronic structure calculations were performed with Gaussian
9859 installed on a Windows PC. Model compounds investigated
by DFT had N-ethylamine groups attached to the PDI core, and
methyl groups attached to the imide nitrogens. The structures of
the neutral/radical cation were optimized with restricted/unrestricted
DFT, incorporating the B3LYP functional and 6-31G* basis set.
The isotropic hyperfine coupling constants of the radical cations
were obtained after running a single point unrestricted DFT
calculations with the B3LYP functional using the expanded
double-ú EPR-II basis set on the optimized radical cation struc-
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2114 J. Org. Chem., Vol. 71, No. 5, 2006