Photophysics and redox properties of rylene imide and diimide dyes alkylated ortho to the imide groups

Article abstract of DOI:10.1021/jp908679c

Ruthenium-catalyzed C-H bond activation was used to directly attach phenethyl groups derived from styrene to positions ortho to the imide groups in a variety of rylene imides and diimides including naphthalene-1,8- dicarboximide (NMI), naphthalene-1,4:5,8-bis(dicarboximide) (NI), perylene-3,4-dicarboximide (PMI), perylene-3,4:9,10-bis(dicarboximide) (PDI), and terrylene-3,4:11,12- bis(dicarboximide) (TDI). The monoimides were dialkylated, while the diimides were tetraalkylated, with the exception of NI, which could only be dialkylated due to steric hindrance. The absorption, fluorescence, transient absorption spectra, and lowest excited singlet state lifetimes of these chromophores, with the exception of NI, are nearly identical to those of their unsubstituted parent chromophores. The reduction potentials of the dialkylated chromophores are a??100 mV more negative and oxidation potentials are a??40 mV less positive than those of the parent compounds, while the corresponding potentials of the tetraalkylated compounds are a??200 mV more negative and a??100 mV less positive than those of their parent compounds, respectively. Continuous wave electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) data on the radical anion of PDI reveals spin density on the perylene-core protons as well as on the ?2-protons of the phenethyl groups. The phenethyl groups enhance the otherwise poor solubility of the bis(dicarboximide) chromophores and only weakly perturb the photophysical and redox properties of the parent molecules, rendering these derivatives and related molecules of significant interest to solar energy conversion. ? 2010 American Chemical Society.

Full text of DOI:10.1021/jp908679c

1794
J. Phys. Chem. B 2010, 114, 1794–1802
Photophysics and Redox Properties of Rylene Imide and Diimide Dyes Alkylated Ortho to
the Imide Groups
Joseph E. Bullock, Michael T. Vagnini, Charusheela Ramanan, Dick T. Co, Thea M. Wilson,
Jay W. Dicke, Tobin J. Marks,* and Michael R. Wasielewski*
Department of Chemistry and Argonne-Northwestern Solar Energy Research (ANSER) Center,
Northwestern UniVersity, EVanston, Illinois 60208-3113
ReceiVed: September 8, 2009; ReVised Manuscript ReceiVed: December 21, 2009
Ruthenium-catalyzed C-H bond activation was used to directly attach phenethyl groups derived from styrene
to positions ortho to the imide groups in a variety of rylene imides and diimides including naphthalene-1,8-
dicarboximide (NMI), naphthalene-1,4:5,8-bis(dicarboximide) (NI), perylene-3,4-dicarboximide (PMI),
perylene-3,4:9,10-bis(dicarboximide) (PDI), and terrylene-3,4:11,12-bis(dicarboximide) (TDI). The monoimides
were dialkylated, while the diimides were tetraalkylated, with the exception of NI, which could only be
dialkylated due to steric hindrance. The absorption, fluorescence, transient absorption spectra, and lowest
excited singlet state lifetimes of these chromophores, with the exception of NI, are nearly identical to those
of their unsubstituted parent chromophores. The reduction potentials of the dialkylated chromophores are
∼100 mV more negative and oxidation potentials are ∼40 mV less positive than those of the parent compounds,
while the corresponding potentials of the tetraalkylated compounds are ∼200 mV more negative and ∼100
mV less positive than those of their parent compounds, respectively. Continuous wave electron paramagnetic
resonance (EPR) and electron nuclear double resonance (ENDOR) data on the radical anion of PDI reveals
spin density on the perylene-core protons as well as on the ꢀ-protons of the phenethyl groups. The phenethyl
groups enhance the otherwise poor solubility of the bis(dicarboximide) chromophores and only weakly perturb
the photophysical and redox properties of the parent molecules, rendering these derivatives and related molecules
of significant interest to solar energy conversion.
Introduction
The insolubility of these systems, especially the larger perylene
and terrylene derivatives, necessitates the use of bulky solubilizing
groups, which are commonly attached to the “bay regions” of the
aromatic core via heteroatoms. In addition to imparting solubility,
these substitutions affect the optical and electronic properties of
the dyes. Wu¨rthner and co-workers have reported di-, tri-, and tetra-
substituted NI-based chromophores with optical transitions that span
400-650 nm.7,11,13,14,42 They have also demonstrated numerous
substitutions in the bay region (1-, 6-, 7-, and 12-positions) of PDI
derivatives with various phenoxy groups.39 Rybtchinski and co-
workers have described the synthesis of water-soluble PDI deriva-
tives, which also possess alkyne connections.40,41,44,45 Separately,
they have reported PDI derivatives which possess transition metals
in the bay region. Furthermore, Huang et al. demonstrated several
examples of thiophenes attached to the bay region positions of
PDI,46 and Mu¨llen and co-workers have pioneered many of the
synthetic strategies for functionalizing the bay positions of TDI
with phenoxy, cyclic amine, and even water-soluble moieties.3,8,9
Some of our own studies include amino-substituted PDIs with
desirable electron-donating properties,47,48 and cyanated NIs and
PDIs,21,28 which have some of the highest electron mobilities
observed for organic semiconductors. Formation of various unsat-
urated C-C substituents on the aromatic core has been demon-
strated with NI, PMI, and PDI using palladium-catalyzed cross-
coupling reactions.12,45,49-51 The synthetic strategies for functionalizing
rylene imides are numerous and have enabled researchers to span
the optical absorption range from the blue to the near-infrared
region, as well as span redox potentials from -1.4 to 1.6 V vs
SCE. All the aforementioned substitutions require bromination or
chlorination of the aromatic core, which is typically performed
For decades, rylene imide dyes have been intensively studied
as useful light absorbers,1-15 fluorescent tags,6,16,17 and electron
donors/acceptors.2,4,18-30 The naphthalene derivatives, naphthalene-
1,8-dicarboximides (NMI) and naphthalene-1,4:5,8-bis(dicar-
boximides) (NI), have been used in artificial photosynthetic
systems,2,24,31,32 while the higher homologues, perylene-3,4-
dicarboximides (PMIs),3,6,8,17 perylene-3,4:9,10-bis(dicarbox-
imides) (PDIs),3,5,6 and terrylene-3,4:11,12-bis(dicarboximides)
(TDIs),3,8,9,33,34 have been extensively researched not only in
artificial photosynthetic systems24,35 but also in light-harvesting
systems36-38 and in solid-state devices such as organic field-
effect transistors21,28 and photovoltaics.18,20,22,23,25,29,30,39,40 In all
these systems, chemical functionalization through the imide
group has been shown to proceed efficiently starting from either
the dicarboxylic acid15 or the cyclic anhydride.11,17,18,34 Molecular
orbital calculations on these systems show that they all exhibit
a nodal plane in both their HOMO and LUMO that bisects the
molecules through the imide nitrogen(s), which decouples the
imide substituent from the π electronic structure of the rylene
imide or diimide chromophore.3,41-43 Thus, a diverse array of
alkyl and aryl imide derivatives of these dyes have been prepared
in which the N-alkyl or N-aryl groups have negligible impact
on the optical and electrochemical properties of the chro-
mophores. The covalent synthesis of multichromophoric arrays
demands exploitation of the imide functionality in such a way
that each chromophore retains its inherent properties.
* To whom correspondence should be addressed. E-mail:
m-wasielewski@northwestern.edu.
10.1021/jp908679c  2010 American Chemical Society
Published on Web 01/14/2010

Photophysics and Redox Properties of Rylene Imide
J. Phys. Chem. B, Vol. 114, No. 5, 2010 1795
under highly acidic conditions.14,15,21,31,44,47 Halogenation of PMI,
PDI, and TDI typically takes place in the bay region, and the steric
consequences of bay substitution disrupt the planarity of the
aromatic core, adversely affecting their electronic structures.27,52
To minimize this effect, synthetic strategies must avoid substitution
in the bay region and create saturated C-C bonds in lieu of
heteroatomic substitution. Until very recently,53 an efficient method
for functionalizing rylene imides with saturated C-C bonds, and
chemically accessing the position ortho to the imide, has not been
available due to the electron-withdrawing effect of the imides. Such
substitutions would be beneficial for solubility while maintaining
the optical properties of the parent chromophore.
In the early 1990s, Murai et al. pioneered the use of ruthenium
catalysts for highly efficient aromatic C-H bond activated
coupling to olefins.54-61 They discovered that Ru(H2)CO(PPh3)3
selectively coordinates with aromatic ketones to generate new
saturated C-C bonds R to the carbonyl and observed the greatest
reactivity with more electron-deficient substrates. The efficiency
and versatility of these catalytic reactions prompted their use
in polymer chemistry62-64 as well as in forming unsaturated
C-C bonds.65 Recently, Osuka et al. reported the first example
of using Ru(H2)CO(PPh3)3 to selectively alkylate the ortho
positions of PDI.53 We have now extended this work to include
a broad range of rylene imides and diimides of importance to
both organic electronics and solar energy conversion. Electro-
chemistry, steady-state, transient optical, electron paramagnetic
resonance (EPR), and electron nuclear double resonance (EN-
DOR)spectroscopy have been used to fully determine the effects
of this new substitution pattern on the electronic, photophysical,
and redox properties of these new chromophores.
Synthesis. Ruthenium-catalyzed C-H bond activation reac-
tions are generally performed in toluene at elevated tempera-
tures.66 However, since we were attempting 4-fold coupling
reactions, we used higher boiling mesitylene to decrease reaction
times.53 All parent dyes (1,7 3,7 5,17 7,19 and 99) were synthe-
sized according to published procedures. Styrene and
Ru(H2)CO(PPh3)3 were purchased from Aldrich and used
without further purification. All glassware was flame-dried and
kept stringently free from O2. All synthesized compounds were
analyzed at Northwestern University (IMSERC) using a Varian
500 MHz NMR, and high-resolution mass spectrometry was
performed on a Bruker MALDI-TOF instrument with polysty-
rene as an internal standard.
Electrochemistry. Electrochemical measurements were per-
formed using a CH Instruments Model 622 electrochemical
workstation. Measurements for 4, 6, 8, and 10 were performed
in dichloromethane containing 0.1 M tetrabutylammonium
hexafluorophosphate (TBAPF6) electrolyte. Measurements for
compound 2 were performed in benzonitrile with the same
electrolyte. A 1.0 mm diameter platinum disk electrode, platinum
wire electrode, and Ag/AgO+ reference electrode were em-
ployed. The ferrocene/ferrocenium redox couple (Fc/Fc+, 0.46
V vs SCE)67 was used as an internal standard. TBAPF6 was
recrystallized twice from ethyl acetate prior to use.
Optical Spectroscopy. Steady-state electronic absorption
spectra were recorded on a Shimadzu 1601 UV/vis spectrometer
with a 2 mm quartz cuvette. Fluorescence measurements were
performed using a PTI Quanta-Master 1 single-photon-counting
spectrofluorimeter in a right angle configuration with a 1 cm
quartz cuvette.
Experimental Section
Femtosecond transient absorption measurements were made
using a 2 kHz Ti:sapphire laser system as detailed previously,68
with the exception that the original oscillator has been replaced
with a Spectra-Physics Tsunami. The wavelengths used to excite
the samples were 390 nm (1-4), 416 nm (5-8), and 650 nm
(9-10). The instrument response function for the pump-probe
experiments is 150 fs. The transient spectra were obtained using
Solvents. Mesitylene was purchased from Aldrich, distilled
over CaH2, and stored over molecular sieves. Toluene
(Aldrich) and nonstabilized HPLC grade dichloromethane
(DCM) (Fisher) were dried using a Glass Contour solvent
system.

1796 J. Phys. Chem. B, Vol. 114, No. 5, 2010
Bullock et al.
SCHEME 1: NI Reaction Intermediate Illustrating the Steric Hindrance Impeding Incorporation of More than Two
Substituents
SCHEME 2: Alkylation Results for Compound 7
5 s of averaging at a given delay time. Glass cuvettes with a 2
mm path length were used and the samples were dissolved in
dry solvent and irradiated with 1.0 µJ pulses at the excitation
wavelength.
Ar+ laser (514.5 nm, 40 mW) beam elongated in one dimension
with a cylindrical lens. In all cases the photochemical reductions
using TEA formed solely monoanions of the PDI oligomers as
monitored by UV-vis. UV-vis spectra acquired through the
quartz tube match the spectra of PDI radical anions generated
electrochemically. A spline fit baseline correction was applied
to the ENDOR spectra following integration.
Density Functional Theory (DFT) Calculations. The struc-
ture of a model PDI radical anion with ethyl substituents at the
2, 5, 8, and 11 positions, as well as at the imide positions, was
first optimized using unrestricted DFT with Q-Chem 3.169 at
the B3LYP/6-31G* level. A single-point unrestricted DFT
calculation at this optimized geometry was performed to
calculate the isotropic hyperfine coupling constants (hfcc’s)
using a B3LYP functional with the expanded double-ꢁ EPR-II
basis set70 and Gaussian 98W.71
Time resolved fluorescence measurements were performed
at the Center for Nanoscale Materials at Argonne National
Laboratory using a frequency-doubled, mode-locked Ti:sapphire
laser system (Coherent Mira-900) with a 76.5 MHz repetition
rate to generate 416 nm pulses to excite 6 and 8. An optical
parametric oscillator (Coherent Mira-OPO) was used to generate
550 nm pulses to excite 9 and 10. Short- and long-pass filters
were used to remove residual pump radiation. Glass cuvettes
with a 1 cm path length were used, and the samples were
dissolved in dry solvent and irradiated with 100-300 µW of
pump light. The fluorescence emitted at right angles was
detected with a fiber-coupled avalanche photodiode (PDM from
Micro Photon Devices). The time-correlated single-photon-
counting module was a PicoHarp 300 from PicoQuant. The
instrument response function using 416 nm pulses was 270 ps,
while that using the 550 nm pulses was 35 ps. Analyses of the
kinetic data for both the transient absorption and fluorescence
lifetimeexperimentswereperformedbyusingaLevenberg-Marquardt
nonlinear least-squares fit to a general sum-of-exponentials
function convoluted with a Gaussian instrument response
function.
EPR and ENDOR Spectroscopy. Continuous wave EPR and
ENDOR spectra were acquired with a Bruker Elexsys E580
spectrometer, fitted with the DICE ENDOR accessory, EN801
resonator, and an ENI A-500 rf power amplifier. Rf powers
ranged from 240 to 400 W across the 7 MHz scanned range,
and microwave power was 2-6 mW. The sample temperature
of 290 K was controlled by a liquid nitrogen flow system. All
samples and solvents were handled in a nitrogen atmosphere
glovebox (MBraun Unilab). Samples were prepared in DCM
with 2% triethylamine (TEA) (w/w) and loaded into 1.4 mm
I.D. quartz tubes which were sealed with a 0.5-1.0 cm plug of
vacuum grease and wrapped tightly with Parafilm. TEA was
dried with CaH2 and filtered through dry silica gel prior to use
and storage in the glovebox. Photochemical reduction to form
monoanions was accomplished by exciting the sample with an
Results and Discussion
Synthesis. Reacting naphthalene monoimide 1 with styrene
in the presence of Ru(H2)CO(PPh3)3 produces the dialkylated
product 2 in 49% yield. Contrary to our expectations, however,
reacting the corresponding naphthalene diimide 3 under the same
conditions leads to a 1:1 isomeric mixture of 2,6- and 2,7-
disubstituted products in 48% yield. We surmise that steric
hindrance inhibits 4-fold substitution, as ruthenium-catalyzed
C-H bond activation reactions are significantly influenced by
steric factors.61,65 Even with extended reaction times,72 the Ru
catalyst cannot effectively bind the carbonyl group to activate
the proximal C-H bond, and hence realize styrene coupling in
the presence of an adjacent phenethyl group (Scheme 1). This
apparently prohibits styrene coordination to the ruthenium, a
prerequisite for C-C bond formation. The 2,7-disubstituted
isomer, 4, was isolated by HPLC to determine its photophysical
and redox properties. The dialkylated perylene monoimide 6
was readily prepared from 5 in 72% yield, while the corre-
sponding tetraalkylated PDI 8 was formed in 75% yield from 7
in agreement with the initial report of Osuka et al.53 In preparing
8, we also isolated a small percentage of an isomeric byproduct
containing a single 1-phenylethyl group (Scheme 2),65 which
could be separated from the target product using preparative

Photophysics and Redox Properties of Rylene Imide
J. Phys. Chem. B, Vol. 114, No. 5, 2010 1797
Figure 1. Photographs of saturated toluene solutions of (a) PMI, (b) PDI, and (c) TDI. In each photo, the left vial contains bare chromophore (5,
7, and 9) while the right vial contains alkylated chromophore (6, 8, and 10).
TABLE 1: Solubilities of Alkylated vs Non-alkylated Rylene
Chromophores
TABLE 2: Redox Potentials (V vs SCE) of the Alkylated
and Unsubstituted Chromophores
chromophore
solubilitya (mg/mL)
chromophore
E+
E-
E2-
1/2
1/2
1/2
5
6
7
8
9
10
2
6
<1
2
<1
3
1
2
3
4
5
6
7
8
9
10
-
-1.40a
-1.53b
-0.48a
-0.61,
-1.00c
-1.13
-0.50c
-0.75
-0.63c
-0.83
-
-
-
-
-
-0.99a
-1.15
-1.49c
-1.43
-0.73c
-0.93
-
1.41
1.37
1.67
1.63
1.12
1.00
a Measured in toluene.
-
TLC. Reducing the amount of catalyst in the reaction did not
lower the yield of byproduct, which has been observed previ-
ously in C-H activation reactions of aryl ketones.65 The higher
homologue, terrylenediimide 9, was tetraalkylated to give 10
in 81% yield; however, no isomerization of the phenethyl groups
was observed in this reaction. The larger ring systems (5, 7,
and 9) all give much higher yields of substitution, most likely
a result of reduced steric hindrance.
The phenethyl groups have a significant effect on the
solubility of the larger ring systems (Figure 1). While compound
5 has significant solubility in toluene due to the branched alkyl
chain at the imide nitrogen atom, 6 has 3 times the solubility
with only two substituents (Table 1). A saturated solution of 7
in toluene is only weakly colored, yet exhibits a strong yellow
fluorescence under ambient light; in contrast, 8 dissolves to give
a significantly darker solution (Figure 1). Compound 9 possesses
two branched imide chains, but is similar to 7 in terms of
solubility. However, the addition of four phenethyl groups
significant increases its solubility. With its high extinction
coefficient (80 000 M-1 cm-1), saturated solutions of 10 in
toluene are virtually opaque.
Redox Potentials. Substitution of the phenethyl groups ortho
to the imide nitrogen atom renders these chromophores some-
what more difficult to reduce, but easier to oxidize (Table 2).
The reduction potentials of the disubstituted chromophores (2,
4, 6) are approximately 130 mV more negative than those of
their parent compounds. Correspondingly, the oxidation potential
of the disubstituted 6 is 40 mV less positive than that of 6.
Tetrasubstituted 8 and 10 have reduction potentials that are more
negative by ∼200 mV than their unsubstituted analogues. These
shifts in redox potentials reasonably arise from the electron-
donating nature of the phenethyl groups. The oxidation and
reduction potentials of all alkylated molecules shift in the same
direction by nearly the same amount, such that the corresponding
HOMO-LUMO energy gap remains essentially the same, as
is reflected in their electronic absorption and emission spectra
described below.
a Ref 5. b Obtained in benzonitrile with 0.1 M TBAPF6. c Ref 3.
shown in Figure 2a. The intensity of the S0 f S1 transition
increases with the size of the aromatic core, while the energy
decreases. The fluorescence spectrum of each chromophore is
shown in Figure 2b and summarized in Table 3 along with those
of compounds 1, 3, 5, 7, and 9. We see a similar trend in the
emission maxima, which move to longer wavelengths as the
size of the aromatic core increases. The electronic absorption
and fluorescence properties of all the alkylated chromophores
are very similar to those of the parent compounds. No observable
trend is apparent for the slight shifts in λmax; however, with the
exception of 2, all alkylated compounds have slightly lower
extinction coefficients than their parent compounds. The fluo-
rescence emission maxima of the alkylated compounds (Table
3) are similar to those of the parent compounds: 1,32 3,73 5,8 7,1
and 9.8 However, the phenethyl groups significantly reduce the
fluorescence quantum yields of only the naphthalene derivatives
2 and 4, while the yields for the higher rylenes are unaffected.
A possible mechanism for the reduced fluorescence quantum
yields of 2 and 4 relative to those of 1 and 3, respectively, will
be discussed below.
The absorption spectrum of the radical anion of 8 was
obtained by photoreduction in the presence of triethylamine in
CH2Cl2. As 8 is reduced, absorption bands at 691 (shoulder 645
nm), 774 (shoulder 796 nm), 824, and 910 nm appear and the
vibronic progression of the neutral species in the visible region
decreases in intensity (Figure S1, Supporting Information). The
absorption spectrum of 8•- is similar to that of other PDI radical
anion derivatives.5
Transient Absorption and Emission Spectroscopy. The
transient absorption spectra of naphthalene monoimide 2 are
shown in Figure 3. The S1 f Sn absorption is characterized by
a single broad band near 575 nm, which decays monoexponen-
tially with a time constant τD ) 34 ps. This is similar to the
time constant obtained for other NMI derivatives4 and is
attributed to rapid intersystem crossing leading to the triplet
state of 2. The transient absorption spectra and monoexponential
Electronic Absorption and Emission Spectra. The ground-
state electronic absorption spectra for 2, 4, 6, 8, and 10 are

1798 J. Phys. Chem. B, Vol. 114, No. 5, 2010
Bullock et al.
Figure 2. (a) UV-vis absorption and (b) fluorescence spectra of rylene chromophores in toluene.
TABLE 3: Electronic Absorption (λmax) and Emission Maxima (λem), S1 Energies, Fluorescence Quantum Yields (OF) and
Fluorescence Lifetimes (τS1) in Toluene
chromophore
λmax (nm) (ε (M-1cm-1))
λem (nm)
E(S1)c (eV)
φF
τS1 (ns)
1
2
3
4
5
6
7
8
9
350 (10647)a
353 (15000)
382 (14500)a
390 (14000)
505 (29970)a
501 (27000)
526 (80000)a
525 (58000)
650 (93000)b
649 (80000)
386a
394
3.38
3.34
3.15
3.09
2.38
2.43
2.34
2.35
1.88
1.90
0.25d
0.08
<0.005i
0.034 ( 0.001
407a
415
0.0018e
0.0005
0.98f
0.98
<0.002i
0.0016 ( 0.0009
3.5 ( 0.1j
6.8 ( 0.1
539a
521
535a
533
0.99g
0.99
4.5 ( 0.1
5.1 ( 0.1
673b
661
0.95h
0.95
3.5 ( 0.1
10
3.4 ( 0.1
a Reference 5. b Reference 34. c Determined from the average energy of the absorption and emission maxima. d Reference 32. e Reference 73.
f Reference 8. g Reference 1. h Reference 8. i Reference 4. j Reference 33.
Figure 3. Transient absorption spectra of compounds 2, 6, 8, and 10 in toluene. Insets: transient absorption kinetics at the indicated wavelengths.
The sharp negative features observed in the spectra of 6, 8, and 10 at 416, 416, and 650 nm, respectively, are due to scatter from the excitation laser
pulse.
S1 lifetimes of the perylene derivatives 6 and 8 are similar to
those reported by us earlier for other PMI and PDI derivatives,4,74
and are illustrative of the minimal perturbation that the (two)
four alkyl substituents have on the electronic structures of these
molecules (Table 3). The transient absorption spectrum of a
terrylenediimide has not been reported previously and is shown

Photophysics and Redox Properties of Rylene Imide
J. Phys. Chem. B, Vol. 114, No. 5, 2010 1799
Figure 4. Transient absorption spectra of 4 in (a) toluene, (b) MTHF, and (c) PrCN.
TABLE 4: Kinetics of Exciplex Formation (τER) and Decay
(τED) and the Decay of the Ion Pair (τCR) in Compound 4
for 10 in Figure 3. The bleaching of the ground-state absorption
bands is accompanied by a strong stimulated emission feature
at 725 nm, as well as weak positive absorption changes at
400-540 nm and 765-800 nm. All of these transient spectral
solvent
τER (ps)
τED (ps)
τCR (ps)
PrCN
MTHF
toluene
<1
<1
<1
1.2 ( 0.5
1.4 ( 0.6
1.6 ( 0.9
2.5 ( 0.5
1.5 ( 0.7
11.5 (1.0
changes decrease with a monoexponential decay time of τS1
)
3.4 ns, which is very similar to that observed previously for 9
using time-resolved fluorescence measurements.33
through the imide nitrogen atoms, so that it is likely that
photoinduced electron transfer occurs very rapidly from one of
the appended phenyl groups to NI in 4. Following photoexci-
tation of 4 in all three solvents, a broad feature at 450-500 nm
forms within the 150 fs instrument response function, which is
assigned to 1*NI. This is followed by rapid, solvent-independent
formation (<1 ps) and decay (<2 ps) of a distinct absorption
band at 580 nm leading to a longer-lived feature with an
absorption maximum near 480 nm, which is characteristic of
NI•- formation.5 The absorption band of NI•- decays much more
slowly in low polarity toluene relative to higher polarity MTHF
and PrCN (Table 4).
We hypothesize that following photoexcitation of NI, rapid
electron transfer from one phenyl group to 1*NI occurs resulting
in a sandwich-like conformation characteristic of exciplex or
tight ion pair formation, which absorbs at 580 nm. Rapid
relaxation of this exciplex results in full charge separation to
form Ph•+-NI•-, which produces the transient absorption
characteristic of NI•- near 480 nm. The bandwidth of the NI•-
absorption narrows in the more polar solvents that better solvate
the Ph•+-NI•- ion pair. Charge recombination of Ph•+-NI•-
occurs more rapidly in more polar MTHF and PrCN than in
toluene as predicted by energy gap law considerations based
on Marcus electron transfer theory.76 To further test this
hypothesis, we performed transient absorption experiments on
4 in MTHF at low temperatures to limit the motion of the phenyl
groups relative to NI. The transient absorption spectral features
at 193 K (Figure 5a) and 143 K (Figure 5b) are similar to those
Time-resolved fluorescence experiments were also conducted
in order to more accurately determine the singlet excited-state
lifetimes for 6, 8, 9, and 10 (Table 3), given that the maximum
pump-probe delay time available using our femtosecond
transient absorption apparatus is about 6 ns. As expected, there
is no significant solvent dependence (Table S1, Supporting
Information). The fluorescence lifetimes of 8 and 10 are very
similar to those of their respective unsubstituted analogues 7
and 9, while that of 6 is only about twice as long as that of 5.
Comparing the fluorescence lifetimes of the perylenediimides
7 and 8 with those of the terrylenediimides 9 and 10, since the
fluorescence quantum yields of the terrylenediimides are slightly
smaller than those of the perylenediimides, the somewhat shorter
S1 lifetimes of the terrylenediimides result largely from a small
increase in their nonradiative decay rates due to their smaller
S0-S1 energy gap.
In contrast to the other phenethyl-substituted rylenes, pho-
toexcitation of the NI group within 4 results in a complex series
of solvent-dependent transient absorption spectra and kinetics.
Experiments were performed in a series of solvents ranging from
low to high polarity, toluene, 2-methyltetrahydrofuran (MTHF),
and butyronitrile (PrCN) (Figure 4). Photoexcited naphthalene
diimide derivatives have been shown to oxidize phenyl rings
covalently bonded to them through their imide nitrogen atoms
due to their high excited state energies (ES1 ) 3.2 eV).75
Chaignon et al.31 have demonstrated that electron transfer
reaction rates from donors attached to the 2-position of NI are
about 1000-fold faster than the rates from donors attached

1800 J. Phys. Chem. B, Vol. 114, No. 5, 2010
Bullock et al.
Figure 5. Transient absorption spectra of 4 in MTHF at (a) 193 K and (b) 143 K. (c) Transient kinetics of 4 at 580 nm in MTHF at the indicated
temperatures.
Figure 7. 1H-ENDOR spectra of 8•- in CH2Cl2 with ∼2% TEA (w/
w) at 290 K. Microwave power was 6 mW, and rf power was 240-400
W with a frequency modulation depth of 50 kHz.
Figure 6. EPR spectra of 8•- in CH2Cl2 with ∼2% TEA at 290 K.
The microwave power was 2 mW with a modulation amplitude of 0.1
G at 25 kHz. The simulation is constrained by ENDOR data for protons,
and the nitrogen hfcc is optimized.
obtained at 290 K is presented in Figure 7 and exhibits three
line pairs with hfcc’s of 4.9, 1.4, and 0.2 MHz. The largest
hfcc has been assigned to that of the bay-region protons
(positions 1, 6, 7, and 12) on the perylene core and is similar
in magnitude to previously reported PDI derivatives.26,78 The
two smaller proton hfcc’s are attributed to the ꢀ-protons on the
phenethyl groups. The 0.2 MHz splitting could also be due to
the octyl group attached to the imide nitrogen atom.
These assignments were confirmed using unrestricted DFT
to calculate the hfcc’s using B3LYP functionals with the
expanded double-ꢁ EPR-II basis set. The computed proton hfcc’s
were 5.4 (perylene protons 1, 6, 7, 12), 1.6 (ꢀ-phenethyl
protons), and 0.3 (ꢀ-phenethyl and ꢀ-octyl protons) MHz and
1.7 MHz for the nitrogens. A simulation of the EPR spectrum
of 8•- (Winsim)79 using the ENDOR results to lock the hfcc
values allows determination of the nitrogen hfcc to be 1.4 MHz,
which is also in reasonable agreement with the DFT calculation.
The EPR and ENDOR spectra reveal that the spin (and charge)
observed at 295 K; however, the 580 nm band becomes more
pronounced as the temperature decreased. This result suggests
that a ground-state charge transfer interaction may fold the
phenyl into an exciplex-like conformation with NI prior to
excitation as the temperature is lowered. Following photoex-
citation of NI at low temperatures, the conformation of the
exciplex (tight ion pair) relaxes to the fully solvated ion pair
state much more slowly than it does at room temperature, which
results in the much longer lifetimes for the 580 nm exciplex
absorption band observed at low temperatures (Figure 5c).
EPR and ENDOR Spectroscopy. The EPR spectrum of 8•-
(Figure 6) is inhomogeneously broadened due to the large
number of hyperfine coupling constants (hfcc’s). Isotropic hfcc’s
were obtained from ENDOR spectroscopy in liquid CH2Cl2 at
the ENDOR resonance condition ν(ENDOR ) |νn ( aH/2|, where
ν(ENDOR are the ENDOR transition frequencies and νn is the
proton Larmor frequency.77 The ENDOR spectrum of 8•-

Photophysics and Redox Properties of Rylene Imide
J. Phys. Chem. B, Vol. 114, No. 5, 2010 1801
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The catalytic addition of phenethyl groups to rylene imides
and diimides yields solublized imide dyes without the need for
strongly electron donating heteroatoms or bay-region substitu-
ents that may twist the aromatic core and alter the photophysics
and redox properties. The versatility of the present alkylation
process lends itself to addition of a wide variety of terminal
alkenes,72 providing a broad range of options for tuning rylene
solubility and solid-state properties. These new rylene imides
and diimides significantly broaden the scope of redox-active
chromophores of interest in molecular systems for solar energy
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Acknowledgment. This work was supported by the Chemical
Sciences, Geosciences, and Biosciences Division, Office of
Basic Energy Sciences, DOE under grants DE-FG02-99ER14999
(M.R.W.) and DE-FG02-08ER46536/A000 (T.J.M.). Use of the
Center for Nanoscale Materials was supported by the U.S.
Department of Energy, Office of Science, Office of Basic Energy
Sciences, under Contract No. DE-AC02-06CH11357. We thank
Dr. J. Vura-Weis for assistance with the DFT calculations and
Dr. D. Gosztola for assistance with the time-resolved fluores-
cence measurements.
Supporting Information Available: Synthesis and charac-
terization of 2, 4, 6, 8, and 10, time-resolved fluorescence spectra
for 6, 8, 9, and 10, and low-temperature transient absorption
studies of 4. This material is available free of charge via the
Internet at http://pubs.acs.org.
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