Energy flow dynamics within cofacial and slip-stacked perylene-3,4-dicarboximide dimer models of π-aggregates

Article abstract of DOI:10.1021/ja507653p

Robust perylene-3,4-dicarboximide (PMI) π-aggregates provide important light-harvesting and electron-hole pair generation advantages in organic photovoltaics and related applications, but relatively few studies have focused on the electronic interactions between PMI chromophores. In contrast, structure-function relationships based on π-π stacking in the related perylene-3,4:9,10-bis(dicarboximides) (PDIs) have been widely investigated. The performance of both PMI and PDI derivatives in organic devices may be limited by the formation of low-energy excimer trap states in morphologies where interchromophore coupling is strong. Here, five covalently bound PMI dimers with varying degrees of electronic interaction were studied to probe the relative chromophore orientations that lead to excimer energy trap states. Femtosecond near-infrared transient absorption spectroscopy was used to observe the growth of a low-energy transition at ～1450-1520 nm characteristic of the excimer state in these covalent dimers. The excimer-state absorption appears in ～1 ps, followed by conformational relaxation over 8-17 ps. The excimer state then decays in 6.9-12.8 ns, as measured by time-resolved fluorescence spectroscopy. The excimer lifetimes reach a maximum for a slip-stacked geometry in which the two PMI molecules are displaced along their long axes by one phenyl group (～4.3 A). Additional displacement of the PMIs by a biphenyl spacer along the long axis prevents excimer formation. Symmetry-breaking charge transfer is not observed in any of the PMI dimers, and only a small triplet yield (<5%) is observed for the cofacial PMI dimers. These data provide structural insights for minimizing excimer trap states in organic devices based on PMI derivatives. (Figure Presented).

Full text of DOI:10.1021/ja507653p

Article
pubs.acs.org/JACS
Energy Flow Dynamics within Cofacial and Slip-Stacked Perylene-3,4-
dicarboximide Dimer Models of π‑Aggregates
Rebecca J. Lindquist,† Kelly M. Lefler,† Kristen E. Brown, Scott M. Dyar, Eric A. Margulies,
Ryan M. Young, and Michael R. Wasielewski*
Department of Chemistry and Argonne−Northwestern Solar Energy Research (ANSER) Center, Northwestern University, Evanston,
Illinois 60208-3113, United States
S
* Supporting Information
ABSTRACT: Robust perylene-3,4-dicarboximide (PMI) π-aggregates provide impor-
tant light-harvesting and electron−hole pair generation advantages in organic
photovoltaics and related applications, but relatively few studies have focused on the
electronic interactions between PMI chromophores. In contrast, structure−function
relationships based on π−π stacking in the related perylene-3,4:9,10-bis(dicarboximides)
(PDIs) have been widely investigated. The performance of both PMI and PDI
derivatives in organic devices may be limited by the formation of low-energy excimer trap
states in morphologies where interchromophore coupling is strong. Here, five covalently
bound PMI dimers with varying degrees of electronic interaction were studied to probe
the relative chromophore orientations that lead to excimer energy trap states.
Femtosecond near-infrared transient absorption spectroscopy was used to observe the
growth of a low-energy transition at ∼1450−1520 nm characteristic of the excimer state
in these covalent dimers. The excimer-state absorption appears in ∼1 ps, followed by
conformational relaxation over 8−17 ps. The excimer state then decays in 6.9−12.8 ns, as
measured by time-resolved fluorescence spectroscopy. The excimer lifetimes reach a maximum for a slip-stacked geometry in
which the two PMI molecules are displaced along their long axes by one phenyl group (∼4.3 Å). Additional displacement of the
PMIs by a biphenyl spacer along the long axis prevents excimer formation. Symmetry-breaking charge transfer is not observed in
any of the PMI dimers, and only a small triplet yield (<5%) is observed for the cofacial PMI dimers. These data provide structural
insights for minimizing excimer trap states in organic devices based on PMI derivatives.
relatively few studies have focused on the electronic
interactions between PMI chromophores in either covalent or
self-assembled systems.21,38−40 In contrast, the well-known
aggregation of the related family of perylene-3,4:9,10-bis-
(dicarboximides) (PDIs) based on π−π stacking of the
perylene core4,5,41−58 has inspired detailed investigations of
structure−function relationships in a variety of simple covalent
and intermolecular PDI aggregates.41,42,59−71 Among these
systems, dimer models provide particularly helpful fundamental
understanding of energy and electron transfer processes.
Importantly, excimer states have been observed for PDI
cyclophanes70,72 and H- and J-type dimers with xanthene
(Xan)59−61 and triptycene scaffolds.71
Here we report the synthesis and photophysics of a series of
five covalent cofacial and slip-stacked PMI dimers bridged by a
Xan scaffold (Figure 1). In molecules 1−3, the method of
attachment to the Xan bridge is varied to create different
degrees of displacement of the PMI π system along the short
molecular axis. In molecule 3, an additional small displacement
along the long axis is introduced by placing the two PMIs with
their dipole moments in opposite directions. In molecules 4
INTRODUCTION
■
The rational design of materials that incorporate self-assembly
of small-molecule building blocks for organic electronic and
photovoltaic applications necessitates an understanding of the
changes in optical and electronic behavior that result from the
relative orientations of chromophores.1−7 Strong electronic
coupling between chromophores can result in undesirable
formation of low-energy excimer trap states that inhibit more
desirable exciton diffusion,1,8,9 charge transfer and trans-
port,10,11 and singlet exciton fission.12−14
Perylene-3,4-dicarboximides (PMIs) are of particular interest
for self-assembled organic electronics and photovoltaics
because of their strong absorptivity in the midvisible spectral
region, high excited-state energies, and thermal and photo-
chemical stability.15−19 The relatively mild redox potentials of
PMI (EOX = 1.41 V vs SCE and ERED = −1.00 V vs SCE) make
it an attractive candidate as either an electron donor or electron
acceptor.20 PMI also offers interesting self-assembly oppor-
tunities because of the 6.4 D permanent ground-state dipole
moment directed along the long axis of the molecule,21 which
could enhance the bulk dipolar nature of an aligned
nanostructure.
PMI has been incorporated into several donor−acceptor
Received: July 27, 2014
Published: September 23, 2014
systems for energy and electron transfer studies,22−37 but
© 2014 American Chemical Society
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Figure 1. Structures of 1−5. Both isomers of 2 are shown.
a
Scheme 1. Synthesis of Precursor Compounds and 1
a
Reaction conditions: (a) (1) KOH, t-BuOH, reflux, 2.5 h; (2) AcOH, RT, 30 min, 55%. (b) 2,7-Di-tert-butyl-9,9-dimethyl-4,5-diaminoxanthene,
pyridine, imidazole, reflux, 2 days, 52%. (c) Br2, CH2Cl2, reflux, 2 h, 93%. (d) Bis(pinacolato)diboron, KOAc, Pd(dppf)Cl2·CH2Cl2, dioxane, 85 °C,
16 h, 81%.
purification except where noted. Dichloromethane for all of the
and 5, phenyl and biphenyl spacers, respectively, are used to
further vary the π overlap along the long molecular axis. When
the PMIs are connected to the Xan bridge through the perylene
core, a 2-ethylhexyl group is attached to the imide nitrogen
atom to increase the solubility and minimize disruption of the
π−π stacking interactions. We investigate the nature of the
excimer states in the PMI dimers and make comparisons with
the energetics and dynamics of the analogous state in PDI
dimers using the red-shifted fluorescence emission and the
transient near-infrared (NIR) excimer absorption band similar
to those previously observed for PDI excimers70,71 and other
aromatic aggregates.73
spectroscopy experiments was dried using a Glass Contour solvent
1
system. Intermediates and final products were characterized with H
and 13C NMR spectroscopy and high-resolution mass spectrometry
(see the SI). 13C NMR spectra could not be obtained for molecules 4
and 5 because of their poor solubility. Compounds 1−5 and PMI were
further purified by HPLC for steady-state and time-resolved
fluorescence measurements.
X-ray Crystallography. Crystals of dimers 1 and 5 were grown by
slow diffusion of methanol vapor into a chloroform solution of the
compounds. The crystals were mounted on polymer loops with
Paratone oil, and the data were collected at 100 K on a Bruker Kappa
APEX II CCD diffractometer equipped with a Cu Kα IμS microfocus
source with MX optics and a Bruker Kappa APEX II CCD
diffractometer equipped with a Cu Kα IμS microfocus source with
Quazar optics, respectively. The data for dimer 1 were processed and
absorption-corrected using TWINABS, and the data for dimer 5 were
absorption-corrected with SADABS. The structures were solved with
SHELXT and refined with SHELXL using Olex2 software.
EXPERIMENTAL SECTION
■
Materials and Synthesis. The syntheses of 1−5 are depicted in
Schemes 1 and 2 and described explicitly in the Supporting
Information (SI). Reagents and spectrophotometric-grade solvents
were purchased from Sigma-Aldrich and used without further
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a
Scheme 2. Synthesis of 2−5
a
Reaction conditions: (a) (1) n-BuLi (1.6 M in hexanes), THF, −78 °C, 1 h; (2) CO2(g), −78 °C, 1 h, warmed to RT, 75%. (b) P3, Na2CO3,
Pd(PPh3)4, toluene, EtOH, H2O, 80 °C, 16 h, 33%. (c) (1) Diphenylphosphoryl azide, triethylamine, toluene, 15 min, 22 °C; (2) benzyl alcohol, 80
°C, 14 h, 97%. (d) P3, Na2CO3, Pd(PPh3)4, toluene, EtOH, H2O, 80 °C, 16 h, 48%. (e) Pd/C (10%), H2, EtOAc, RT, 3 h, 96%. (f) P1, imidazole,
140 °C, 1 day, 38%. (g) 4-Aminophenyl boronic acid pinacol ester (4) or 4-aminobiphenyl boronic acid pinacol ester (5), K2CO3, Pd(PPh3)4·
toluene, EtOH, H2O, 85 °C, 17 h, 67% (4), 78% (5). (h) Pd/C (10%), EtOH, EtOAc, H2 atmosphere, 22 °C, 3 h, quantitative. (i) P1, imidazole,
140 °C, 1 day, 31% (4), 33% (5).
Steady-State Optical Spectroscopy. Steady-state absorption
spectra were collected at room temperature in a 1 mm cuvette in
CH2Cl2 (optical density 0.3−0.7 at the maximum) using a Shimadzu
1800 spectrometer. Fluorescence measurements were made on a PTI
Quanta-Master 1 single-photon-counting fluorimeter in a right-angle
configuration with a 10 mm quartz cuvette. Quantum yield
measurements in CH2Cl2 (optical density 0.01−0.05 at the maximum)
used N-(2,6-diisopropylphenyl)perylene-3,4-dicarboximide in toluene
as a standard.74
Time-Resolved Optical Spectroscopy. Picosecond time-re-
solved fluorescence (TRF) measurements were made using a
laboratory-built cavity-dumped Ti:sapphire laser system with a streak
camera (Hamamatsu C4780 Streakscope) as previously described.14,75
Samples were prepared in 1 mm quartz cuvettes in dichloromethane
with optical densities of 0.1 at the excitation wavelength (416 nm). All
data were acquired in single-photon-counting mode using the
Hamamatsu HPD-TA software. The temporal resolution was
approximately 2% of the sweep window.
Femtosecond vis−NIR transient absorption (fsTA) experiments
were conducted using a regeneratively amplified Ti:sapphire laser
system as previously described.76 Samples in dichloromethane were
irradiated at 490 nm in 2 mm glass cuvettes with 110 fs, 0.2−0.5 μJ
pulses focused to a 100 μm diameter spot. The optical density at the
excitation wavelength was maintained at 0.5−0.7. Transient spectra
were acquired for 5 s per pump−probe time delay point. The temporal
resolution was ∼120−280 fs.
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Nanosecond transient absorption (nsTA) experiments were
performed by exciting samples in dichloromethane with 7 ns, 1.5
mJ, 532 nm pulses from the output of an optical parametric oscillator
(Continuum Panther) pumped by the frequency-tripled output of a
Nd:YAG laser (Continuum Precision II 8000). The probe was
generated by a pulsed xenon flashlamp (EG&G Electro-Optics FX-
200). Kinetic traces were collected from 390 to 800 nm at 5 nm
intervals, omitting 520−540 nm. Spectra were constructed by merging
the kinetic traces (binned to 3 ns steps, 100 laser shots per kinetic
trace). Samples were prepared with optical densities of 0.5−0.7 and
were degassed with three freeze−pump−thaw cycles prior to
irradiation in 10 mm quartz cuvettes.
All of the fsTA data were corrected for group delay dispersion
(GDD, or “chirp”) and t0 prior to kinetic analysis. The three-
dimensional data sets from TRF (fluorescence counts vs time and
wavelength) were analyzed by singular value decomposition (SVD)
and global fitting to obtain the kinetic time constants and their decay-
associated spectra (DAS) using Surface Xplorer software, version 2.2.3
(Ultrafast Systems LLC, Sarasota, FL).77 These time constants, which
are too long to be accurately measured on the fsTA apparatus, were
input as the longest lifetimes and held constant when fitting the
combined visible and NIR fsTA data according to the same SVD and
global fitting procedure. The long-lived broad NIR features that
resulted from this treatment were fit to Gaussian functions for
comparison between dimers.
Computational Methods. The ground-state geometries of 1−5
were initially relaxed using molecular mechanics with the MMFF294
force field as implemented in the Avogadro 1.1.0 software.78 Starting
from this geometry, singlet ground-state structures were subsequently
relaxed using density functional theory (DFT) with Grimme’s D-3
dispersion correction79 as implemented in the QChem 4.0 software.80
All of the DFT calculations made use of the B3LYP exchange−
correlation functional with a split-valence double-ζ basis set with
added polarization functions (6-31G*). Implicit solvation was
implemented using a polarizable continuum model (CPCM/SWIG)
with 110 grid points on all atoms. A dielectric constant of 8.93
(dichloromethane) was used in all cases. Images of the optimized
structures were generated with PyMol 1.2r1.81 The 2-ethylhexyl
groups of molecules 2 and 3 were replaced with methyl groups to save
computation time.
condensation between P7 and P1 in molten imidazole afforded
3.15 Further, P5 was reacted with 4-aminophenylboronic acid
pinacol ester and 4-aminobiphenyl-4′-boronic acid pinacol ester
using Suzuki−Miyaura cross-coupling conditions to afford P8
and P9, respectively.69 Deprotection using the previously
described reductive conditions to give P10 and P1169 followed
by imide condensations with P1 yielded 4 and 5, respectively.15
Compound 2 was present as a mixture of isomers that could
be distinguished by 1H NMR spectroscopy using the
resonances of the 9-methyl and tert-butyl protons on the Xan
bridge. In 2A (Figure 1), the two methyl groups experience
different magnetic environments, while in 2B, the two methyl
1
groups experience the same magnetic environment. The H
NMR resonances in CDCl3 at 25 °C reflect both of these
structures with two singlets of equal intensity (3H) at 1.92 and
1.81 ppm (2A) and another more intense (6H) singlet at 1.90
ppm (2B) (Figure S1A in the SI). In CDCl3, two of these
singlets overlap with the multiplet from the two protons on the
tertiary carbons of the 2-ethylhexyl groups, but the ratio of
nonequivalent to equivalent methyl protons in toluene-d8
indicates a 2A:2B ratio of ∼9:10 (Figure S1B). Similarly, two
tert-butyl proton resonances from the two isomers are evident
1
at around 1.34 ppm. Variable-temperature H NMR measure-
ments up to 100 °C failed to interconvert the two isomers
(Figure S1C), and all attempts to isolate the two isomer bands
by HPLC yielded mixtures of isomers.
X-ray Crystallography. Dimer 1 crystallizes in the
monoclinic crystal system in the P21/c space group with one
molecule in the asymmetric unit and a total of four molecules in
the unit cell (Tables S3 and S4 in the SI). The unit cell
parameters are a = 23.3440 Å, b = 13.6417 Å, c = 15.9048 Å,
and β = 105.030°. The compound crystallizes with a total of
four molecules of chloroform and four molecules of methanol
in the unit cell. The two PMI subunits adopt a ∼70° angle
geometry relative to the Xan (φ in Figure 2) to adopt a shorter
π−π distance of 3.3 Å between PMIs. Dimer 5 crystallizes in
the triclinic crystal system in the P1 space group with two
̅
RESULTS
molecules in the asymmetric unit and a total of four molecules
in the unit cell (Tables S5 and S6). The unit cell parameters are
a = 17.3715 Å, b = 21.0110 Å, c = 21.0234 Å, α = 73.849°, β =
105.030°, and γ = 69.341°. The dimer structure has another
dimer intercalated, forming a strong association between the
PMIs of separate dimers. As a result, there is a distortion
between the PMIs within each dimer, resulting in a ψ angle of
26° and a larger π−π distance of 6.8 Å. The two crystal
structures are summarized in Figure 2, and pictures of the
structures are shown in Figure 3.
■
Synthesis and Structure. Syntheses of PMI,82 compounds
P4 and P5,83 and compounds P8−P1169 have been reported
previously. Precursors P1−P3 and molecule 1 were synthesized
according to literature procedures, as shown in Scheme 1. PMI
was hydrolyzed with KOH in tert-butyl alcohol to afford
compound P1.15 PMI was selectively brominated at the 9-
position with bromine in refluxing dichloromethane84 and then
converted to the boronic ester, compound P3, using bis-
(pinacolato)diboron with potassium acetate and [1,1′-bis-
(diphenylphosphino)ferrocene]dichloropalladium(II)·dichloro-
methane complex in dioxane.85 Molecule 1 was synthesized
using an imide condensation of P1 and 2,7-di-tert-butyl-9,9-
dimethyl-4,5-diaminoxanthene.15
Molecular Modeling. Structures computed using DFT
along with a polarizable continuum model for the solvent as
described in the Experimental Section were used to determine
the relative orientations of the PMIs with respect to each other
and the Xan bridge in CH2Cl2 (Tables S7−S12). Relevant
angles and distances are summarized in Figure 2, and pictures
of the structures are shown in Figures 3 and S19. The angle θ is
defined as the angle between the transition dipole moment for
the S1 ← S0 transition (in-plane with the long axis of PMI) and
a line connecting the geometric centers of the molecules, in
The syntheses of molecules 2−5 are shown in Scheme 2.
Molecule 2 was synthesized by a Suzuki−Miyaura cross-
coupling reaction between P3 and 2,7-di-tert-butyl-9,9-
dimethyl-4,5-dibromoxanthene using sodium carbonate and
tetrakis(triphenylphosphine)palladium(0) in a toluene/etha-
nol/water solution.86 The asymmetrical Xan spacers for 3−5
were synthesized according to conditions published by Malaise
and co-workers to give P4 and P5.83 A Suzuki−Miyaura cross-
coupling reaction of P5 with P3 using sodium carbonate and
tetrakis(triphenylphosphine)palladium(0) in a toluene/etha-
nol/water solvent mixture gave P6,86 which was deprotected
with 10% Pd/C under H2 in ethyl acetate to give P7.87 Imide
keeping with Kasha et al.88 The calculations showed that θavg
≥
́
66° for 1−3 and θavg = 54° and 34° for 4 and 5, respectively.
The large difference between θ1 and θ2 for 5 indicates that the
PMIs are tilted toward each other in that direction, while the
large ψ values for every molecule except 2B indicate a tilt
between the long axes of the PMIs in the perpendicular
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the PMIs are farthest from parallel in 5. The distance between
the 4- and 5-positions on the Xan bridge is 4.7 Å, but the PMIs
tilt toward each other in the θ and φ directions to achieve a
π−π stacking distance (d) of ∼3.6 Å at the closest points.
The X-ray structures and computed structures for 1 and 5 are
compared in Figures 2 and 3. Each of the crystal structures
yields nearly identical φ1 and φ2 compared with the much
larger difference in the computed geometries. The crystal
structure for 1 shows PMIs that are nearly parallel to each other
in both the θ and the ψ directions, which is again not true for
the computed geometries. Additionally, the intercalated crystal
structure of 5 has a unique bent Xan backbone. Because of
these significant differences, the crystal structures are assumed
to be poor approximations of the molecular geometries in
solution, and the computed solution structures are used in the
analysis.
Steady-State Optical Spectroscopy. The normalized
steady-state absorption and emission spectra of 1−5 and PMI
in CH2Cl2 are provided in Figures 4 and 5, and the spectral
properties are summarized in Table 1. PMI has an absorption
maximum at 481 nm (Figure 4A) that is assigned to the (0,1)
transition and a lower-energy vibronic band of nearly equal
intensity at 503 nm that corresponds to the (0,0) transition.
Cofacial π−π stacking of PMI in 1 results in a more intense
(0,1) absorption band at 476 nm relative to the redder band,
which is red-shifted to 518 nm. For the cofacially stacked PMIs
in 2 and 3, a similar though smaller intensity difference is also
observed at 485 and 478 nm compared with the red-shifted
bands at 531 and 522 nm, respectively. When a phenyl spacer is
inserted between the Xan bridge and one of the PMIs in 4, the
changes in the (0,1) band intensity at 483 nm relative to the
red-shifted band at 515 nm are small (Figure 5A). Dimer 5 with
a biphenyl spacer demonstrates a marked difference from any of
the other compounds, with a dominant (0,0) band at 511 nm
compared with the (0,1) band at 488 nm.
Figure 2. Schematic detailing the geometric parameters of covalent
PMI dimers. The structures of 1 and 4 are shown. PMI A is defined as
the PMI attached directly to the Xan backbone through the imide,
except for 2A and 2B, where the PMIs are identical and have no imide
attachment. δ is the distance between the PMI centers of mass (red
dots) along the long axis of PMI; θ is the angle between the transition
dipole moment (along the direction of the blue dotted line) and the
vector between the centers of mass; ψ is the rotational displacement
angle; φ is the dihedral angle between PMI and Xan; and d is the
smallest atom-to-atom distance between the PMIs. All angles are
reported in degrees. The reported values are for the computed
structures, except for the values in parentheses, which are from the
crystal structures.
The steady-state emission spectrum of PMI displays a
vibronic progression with a maximum at 541 nm [the (0,0)
transition], a smaller peak at 578 nm, and a slight shoulder at
640 nm; the fluorescence quantum yield (ΦF) is 0.96 (Figure
4B).17 In contrast, the fluorescence from cofacial π-stacked
dimers 1−3 is highly quenched. The spectra are characterized
by the appearance of broad, featureless emission bands with
large Stokes shifts, and the mixture of 2A and 2B produces a
particularly broad emission spectrum. The vibronic structure on
the blue edge of the emission spectrum of 1 is attributed to a
minor monomeric impurity (∼0.1%). Compound 3 has a
somewhat higher fluorescence quantum yield (ΦF = 0.11) than
do 1 and 2 (ΦF = 0.02 and 0.03, respectively). As with the
direction. Values of φ, the dihedral angle between the plane of
the PMI π system and that of the Xan bridge, were obtained by
computing the angle using four atoms: the oxygen on the Xan
bridge, the Xan 4- or 5-carbon attached directly to the PMI or
the phenyl spacer, the PMI atom attached to the Xan bridge or
phenyl spacer (imide nitrogen or carbon at the 9-position), and
a nearby atom (the carbonyl oxygen or the carbon at the 10-
position). The difference between the two dihedral angles in 2A
indicates that the Xan bridge is bent, not that the PMIs are not
parallel. The calculated dihedral angles suggest that there is the
most π−π overlap with the PMIs closest to parallel in 2B, while
Figure 3. DFT-computed ground-state structures and crystal structures for 1 and 5.
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Figure 4. Normalized (A) absorption spectra and (B) emission spectra with excitation at 470 nm of 1 (red), 2 (green), and 3 (blue) compared with
PMI (black) in CH2Cl2.
Figure 5. Normalized (A) absorption spectra and (B) emission spectra with excitation at 470 nm of 4 (red) and 5 (green) compared with PMI
(black) in CH2Cl2.
Table 1. Steady-State Spectral Data
Table 2. Time-Resolved Kinetics
abs
max
em
max
em
molecule
λ
(nm)
λ
(nm)
λ (eV)
max
ΦF
fsTA
TRF
PMI
481
541
2.29
0.96
0.02
0.03
0.11
0.61
0.92
molecule
τexc (ps)
τrlx (ps)
τF (ns)
a
1
2
3
4
5
476
485
478
483
511
740
718
649
603
543
1.68
1.73
1.91
2.06
2.28
PMI
−
1.5 0.6
5.26 0.03
6.90 0.03
7.93 0.05
11.43 0.06
12.81 0.06
4.76 0.04
1
2
3
4
5
1.0 0.1
1.0 0.1
1.1 0.5
1.3 0.3
−
17
12
9
5
1
2
3
1
8
2
a
1
Vibrational cooling of the *PMI monomer.
absorption spectra, the emission spectra of molecules 4 and 5
are quite similar to that of monomeric PMI (Figure 5B),
though the fluorescence spectrum of 4 is slightly broadened and
red-shifted relative to that of PMI with a shoulder on the blue
edge. Dimers 4 and 5 show slightly quenched emission relative
to PMI with ΦF = 0.61 and 0.92, respectively.
Time-Resolved Optical Spectroscopy. Picosecond TRF
spectra and kinetic traces at the emission maxima in CH2Cl2 are
shown in Figures S2−S7 for PMI and 1−5. The fluorescence
lifetime of the PMI monomer is τF = 5.26 0.03 ns, while the
lifetimes of molecules 1 (τF = 6.90 0.03 ns) through 4 (τF =
12.81 0.06 ns) increase as the structures deviate from the
maximally overlapped cofacial structure. Conversely, the
fluorescence lifetime of 5 decreases slightly (τF = 4.76 0.04
ns) relative to that of PMI. The fluorescence lifetimes are
summarized in Table 2.
between the two experiments are not quantitatively scaled.
Photoexcitation of PMI results in a ground-state bleach at
430−560 nm, which is accompanied by a very small stimulated-
emission feature at 668−744 nm, a transient absorption feature
1
corresponding to *PMI at 611 nm with a much smaller band
at 850−1200 nm, and a small absorption tail extending to 1600
nm. SVD and global analysis of the combined visible and NIR
three-dimensional data set (ΔA vs time and wavelength)
yielded the decay-associated spectra presented in Figure S8 and
the lifetimes summarized in Table 2. The main 1*PMI
absorption exhibits a maximum at 620 nm immediately after
excitation, which blue-shifts to 611 nm in τrlx = 1.5 0.6 ps.
The maximum of the weaker absorption shifts from 1073 to
1056 nm on the same time scale.
The visible and NIR fsTA spectra of PMI in CH2Cl2
following 100 fs excitation at 490 nm are shown in Figure
6A. The data from the separate visible and NIR experiments are
corrected for GDD and t0, but the relative magnitudes of ΔA
Additional features and a lack of stimulated emission are
observed for the cofacial dimers 1−3 and the phenyl-spaced
dimer 4 (Figure 6). Biexponential evolution of the visible
spectra for 1 occurs with τexc = 1.0 0.1 ps and τrlx = 17 5 ps
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Figure 6. Femtosecond transient absorption spectra of PMI and 1−5 in CH2Cl2 following excitation with a 490 nm laser pulse.
as the main transient absorption feature sharpens and blue-
shifts (Figure S8). In the NIR, the feature at 1000 nm decays
and a new broad absorption appears with the same two time
constants. This feature can be fit to a Gaussian line shape
function with λmax = 1514 1 nm (0.82 eV) (Figure S10 and
Table 3). Time constants for the blue shift of the visible band
and the growth of the NIR transition for 1−4 are given in Table
2. The main long-lived transient absorption features in the
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shifted (0,0) transitions that overlap strongly with the (0,1)
transitions and increase the absorption at that wavelength,
consistent with H-aggregate behavior. Extensions to the exciton
model that incorporate vibronic coupling relax the selection
rules for transitions between the ground state and the exciton
states and allow the lowest-energy observed transition to be
assigned to the partially allowed (0,0) transition to the lower-
energy exciton state.89 In addition, transition density models
show that the “magic angle” criterion used to differentiate H-
from J-type aggregates is not always accurate.90−97 Molecule 1
has the largest intensity ratio of the high-energy allowed to low-
energy forbidden exciton transitions, which indicates stronger
electronic coupling than in either 2 or 3.
Table 3. Gaussian Fits of the NIR Feature in the fsTA
Spectra
a
molecule
λmax (nm)
λmax (eV)
fwhm (eV)
1
2
3
4
1514
1457
1510
1485
1
2
2
2
0.82
0.85
0.82
0.83
0.19
0.23
0.19
0.11
a
Standard errors of the Gaussian fits are all <1 meV.
visible spectra are broader for 2−4 than for 1 or monomeric
PMI, and the blue shifts are more pronounced. The two
isomers of 2 show a broad NIR feature at 1457 2 nm, while 3
shows a maximum identical to that of 1 at 1510 2 nm. In
contrast to 1−3, the visible transient absorption feature of
compound 4 evolves into a very broad band that lacks a sharp
maximum with accompanying growth of only a weak NIR
feature at 1485 2 nm (Figures 6 and S9). Gaussian fits are
shown in Figures S11−S13 and summarized in Table 3. There
is a small residual transient absorption for 1−3 at 475−520 nm
The broad, featureless, red-shifted emission spectra observed
for 1−3 suggest that an excimer state in which the PMIs are
electronically coupled in the excited state is formed from the
initial excitonic state. The excimer emission energy reflects the
energy gap between the vibrationally relaxed excimer state and
the ground state of the molecule in the same conformation. It is
well-established that the energy of this transition depends on
both conformational relaxation and electronic coupling within
the dimer.70 Structural changes that stabilize the excimer
conversely serve to destabilize the ground state and thus
decrease the energy of the emission. The excimer is stabilized
by mixing of the two locally excited Frenkel (exciton) states,
derived from the configurations 1*PMIA−PMIB and
PMIA−1*PMIB, with the charge transfer (CT) configurations
3
resembling that of *PMI in the nsTA spectra (Figures S14−
S16) that is not present for 4 or 5 (Figures S17 and S18).27
The time constants for these decays are too close to the 7 ns
instrument response to be determined accurately. The
biphenyl-spaced PMIs of 5 behave almost identically to
monomeric PMI with a small stimulated emission feature at
688−745 nm, a blue-shift of the main transient absorption band
in τrlx = 2 1 ps, and a slightly shorter τF of 4.76 0.04 ns for
fluorescence decay (Figures 6 and S9).
−
+
+
−
PMIA −PMI2 and PMIA −PMIB to produce four states of
mixed Frenkel−CT character.98 Molecule 1 has the lowest
excimer emission energy of the series, which indicates that it
has the strongest coupling between the PMI molecules. This
correlates well with the steady-state absorption experiments,
which show that 1 has the largest exciton splitting as well.
Additionally, 1 has the largest angle ψ of all the dimers (Figure
2) and thus requires a greater structural rearrangement to reach
the relaxed excimer geometry, which is expected to have the
long axes of the PMI molecules parallel to one another.98
However, in the ground state, this relaxed geometry is
energetically higher, which decreases the emission energy
somewhat. The additional breadth of the emission spectrum for
2 is consistent with the presence of the two isomers 2A and 2B,
as they have differing degrees of coupling and stabilization of
their relaxed excimer states. On the basis of the energy-
minimized structures of 2A and 2B, we expect a lower-energy
emission band for 2B, which has a smaller π−π stacking
distance and more direct overlap between the π systems. The
average quantum yield for the isomer mixture, ΦF = 0.03, is
reported because the individual quantum yields for the two
isomers could not be obtained. However, the 1H NMR
spectrum of the mixture shows that 2A and 2B are present in
almost equal amounts, so both isomers must have very low
fluorescence quantum yields. Consistent with the steady-state
absorption spectra, the higher fluorescence quantum yield for
the excimer of 3 (ΦF = 0.11) compared with 1 (ΦF = 0.02) and
2 demonstrates that the PMI−PMI coupling is the weakest for
3 and that the nonradiative decay typical of strongly coupled H-
aggregates is less dominant.70
DISCUSSION
■
Steady-State Absorption and Emission Spectroscopy.
When chromophores with strongly coupled transition dipole
moments adopt a cofacial arrangement (H-aggregate regime,
54.7° < θ ≤ 90°; Figure 2), the excited state of the system splits
into two exciton states for which the electronic transition
between the ground and upper exciton state is symmetry-
allowed, resulting in an overall blue shift in the absorption
spectrum (Figure 7).88 When the transition dipoles of the
Figure 7. Schematic detailing the relative energy levels and
photophysical processes for molecules 1−5.
chromophores are offset along their long axes (J-aggregate
regime, 0° ≤ θ < 54.7°; Figure 2), the electronic transition
between the ground state and the lower-energy exciton state is
allowed, which results in an apparent red shift in the absorption
spectrum. On the basis of this zeroth-order molecular exciton
model, H-aggregate (1−3) and J-aggregate (4−5) behavior is
expected from the PMI dimer series studied here. Indeed, the
observed steady-state absorption spectra for 1−3 display blue-
The energy-minimized structures of 4 and 5 show that both
should behave as J-aggregates, with θavg = 54° and 34°,
respectively.88 Both exhibit a small red shift of the (0,0)
absorption band, and the band is slightly enhanced for 5, as
expected for a J-aggregate. However, molecule 4 shows a more
intense (0,1) band similar to those of H-aggregates 1−3, which
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indicates that the electronic coupling between the PMI
molecules in 4 is still significant.69,71 The emission spectrum
of 4 demonstrates that excimer formation occurs, but the
Frenkel−CT exciton mixing is weaker than that in 1−3 because
the excimer emission energy is higher, and there is a shoulder
on the blue edge of the spectrum resulting from emission from
the precursor lower exciton state, which is expected of J-
aggregates.99 Molecule 5 does not form an excimer and does
not have significantly quenched fluorescence, primarily because
of the long distance δ = 8.0 Å and the difference in φ between
the PMI molecules, which diminishes the electronic coupling
between them.
Time-Resolved Dynamics. The picosecond TRF measure-
ments yielded fluorescence lifetimes ranging from 4.76 to 12.81
ns for 1−5, compared with τF = 5.26 0.03 ns for 1*PMI. The
extended lifetimes and red-shifted emission for 1−4 suggest
that in each case emission occurs from an excimer state (Figure
7). The shortest fluorescence lifetime is observed for 1 and can
be explained by either increased vibrational interactions due to
the proximity and electronic coupling of the PMIs or by
enhanced internal conversion due to the lower energy gap
(energy gap law), as discussed above.100 Increasing the offset
between the PMIs along their short and long axes results in an
increase in the excimer-state lifetime from τF = 6.90 0.03 ns
for 1 to 12.81 0.06 ns for 4. As expected,101 these excimer
lifetimes are not significantly different from those observed for
extended PMI aggregates,40 so the conclusions drawn here
should apply to larger self-assembled aggregates and solid films
of similar morphologies. Molecule 5 shows no evidence of
excimer emission, so its fluorescence lifetime of 4.76 0.04 ns,
which is slightly shortened relative to that of PMI, may be
attributed to additional vibronic coupling to the Xan bridge,
which may increase the nonradiative decay rate.
the lower exciton state is independent of the conformation as
long as the coupling is sufficient for the process to occur, as the
time constants for all four molecules are τexc ≈ 1 ps. It is
important to note that this process is distinct from the observed
1
blue shift of the monomeric *PMI spectrum, which is most
likely due to vibrational relaxation. In support of this
conclusion, the NIR band near 1450−1500 nm begins to rise
with the same time constant and is similar to bands observed
previously for cofacial covalent PDI excimers.70,71,73 It is
assigned to the transition from the lowest PMI excimer state to
a PMI+−PMI− singlet charge transfer (1CT) state (Figure 7).
The second component in the biexponential kinetics of 1−4
is τrlx = 8−17 ps, which can be assigned to geometric
rearrangement from the initial unrelaxed excimer state to a
more relaxed excimer conformation (Figure 7).1,70,71 The blue
shifts in the NIR features accompanying these rises support this
assignment. Relaxation to a lower-energy excimer state would
1
necessarily raise the energy of the transition to the CT states.
The decreasing time constants for structural rearrangement in
going from 1 to 4 are consistent with the TRF data and support
the idea that steric hindrance to that motion is decreasing in the
series 1 to 4. The long decay time constant observed for 1 is
also consistent with the larger rotation in ψ required to reach
the relaxed excimer state, as discussed above. Thus, the
decreasing electronic coupling in the order 1 > 2 > 3 > 4
corresponds to an increasing ability of the PMIs to rearrange to
longer-lived excimer states.
Comparison to PDI Dimers. A comparison of the excimer
formation and decay lifetimes for 1−4 and the lack of excimer
formation in 5 reveal similar trends for PMI as for PDI. A
cofacial Xan-bridged PDI molecule that corresponds to 1 has
excimer formation time constants in toluene of τexc = 1.8 0.7
ps and τrlx = 14 4 ps, which are within the experimental error
Theoretical work on non-covalent PDI dimers by Schubert
and co-workers98,101 provides a framework with which the
excimer formation dynamics in 1−4 observed by fsTA can be
understood. Excitation to the upper Frenkel state is followed by
internal conversion to the lower exciton state via a transiently
populated CT state in ∼200 fs, from which excimer formation
occurs on a picosecond time scale. Such relaxation to the lower
exciton state prior to excimer formation has previously been
observed for PDI covalent dimers59 and α-perylene crys-
tals102,103 by temperature-dependent and time-resolved fluo-
rescence measurements.
of the excimer formation times of τexc = 1.0 0.1 ps and τrlx
17
=
5 ps observed for 1.70 The similarity in τexc is not
surprising given the comparable chromophore orientations.
The agreement in τrlx is consistent with its assignment to
structural rearrangement and the identical connectivity of the
Xan bridge to PMI in 1 and to PDI in the corresponding dimer.
We have also studied a pair of triptycene-bridged PDI dimers
analogous to 1 and 4, which eliminates the oxidation of Xan by
1*PDI that competes with excimer formation in the slip-stacked
Xan-bridged PDI dimer analogous to 4.71 Photoexcitation of
the triptycene-bridged cofacial PDI dimer forms the initial
Two features in the fsTA spectra allow us to follow the
formation of the excimer for 1−4. Stimulated emission of the
analogous PDI dimers lasts for only ∼200 fs before internal
conversion from the upper to the lower Frenkel state and
subsequent excimer formation in τexc occurs (Figure 7).98 If
PMI excimer formation follows a similar pathway, we would
not expect to observe any stimulated emission because of the
time resolution of our experiment. Indeed, dimers 1−4 show
no stimulated emission, while 5 displays a small amount at early
times. Thus, the early fsTA spectra for 1−4 are assigned to
formation of the excimer state from the lower Frenkel state. In
the series 1 to 4, the visible fsTA spectra become increasingly
broadened, which likely results from the increasing electronic
inequivalence of the two PMI chromophores due to their
different connectivity to the Xan bridge. Population of the
excimer state is observed as a blue shift and sharpening of the
S1 absorption spectrum, as has been observed previously by us
for self-assembled PMI excimers.40 Within the molecules
investigated here, the time scale of excimer formation from
excimer state in τexc = 2.2
subsequently relaxes in τrlx = 24 6 ps, while the triptycene-
bridged slip-stacked PDI dimer forms the excimer state in τexc
0.2 ps in CH2Cl2, which
=
12 3 ps with no readily observable distinction between the
initial excimer formation and its conformational relaxation.
Excimer formation in the triptycene-bridged slip-stacked PDI
dimer may be slowed by steric interference of the PDI alkyl tails
that preclude optimal π−π stacking, so the excimer state relaxes
at a rate comparable to that at which it initially forms. Dimers 1
and 4 do not have the alkyl tails that the analogous PDI dimers
have, so their PMI π systems can approach more closely. The
lifetimes of the relaxed excimer states of 1 and 4 are τF = 6.90
0.03 and 12.81
0.06 ns, while those of the corresponding
triptycene-bridged PDI dimers are τF = 12.0 0.3 and 24.1
0.4 ns, respectively.71 The shorter excimer-state lifetimes of 1
and 4 may result from stronger electronic coupling of the PMI
molecules relative to that of the PDI molecules, once again as a
result of better π overlap in the PMI dimers relative to the PDI
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Journal of the American Chemical Society
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dimers. Analogous to PDI,69 the biphenyl spacer between the
cofacial PMI molecules in 5 prevents excimer formation.
The NIR transition energies (ENIR) from the lowest
1*(PMI−PMI) states to the 1(PMI+−PMI−) 1CT states provide
an additional metric for comparing the excimer states of the
PMI dimers with those of the corresponding PDI dimers. For
example, comparison of the cofacial and slip-stacked PMI
dimers shows that the NIR transition energies for 1 (0.82 eV)
and 4 (0.83 eV) are nearly identical. Changes in ENIR can be
attributed to changes in the stability of the excimer state and/or
the 1CT state. Since the excimer-state energies (EXMR) of 1 and
4 are 1.68 and 2.06 eV, respectively, the fact that ENIR is similar
excimer state. Significantly increasing the offset by using a
biphenyl spacer to shift the PMI chromophores along their long
molecular axis in 5 diminishes the coupling between the PMI
chromophores enough to effectively prevent excimer formation.
The PMI excimer is formed in dimers having conformations
similar to those of the analogous PDI structures, which also
exhibit fast excimer formation. However, PMI has the benefit of
a much lower triplet yield due to competitive nonradiative
decay of the excimer states. These factors may provide
advantages for using PMI derivatives instead of PDI derivatives
in assemblies for some energy and charge transport
applications.
1
for these two molecules implies that the energies of the CT
ASSOCIATED CONTENT
states (ECT) differ by a comparable amount. ECT can be
■
estimated as ECT = EXMR + ENIR.
70 In this case, ECT = 2.50 and
S
* Supporting Information
2.89 eV for 1 and 4, respectively. In comparison, for the
corresponding cofacial and slip-stacked triptycene-bridged PDI
dimers, ENIR = 0.74 and 0.87 eV, respectively, and EXMR = 1.67
and 1.92 eV, respectively, giving ECT = 2.41 and 2.79 eV,
respectively.71 These data suggest that cofacial π−π stacking
Detailed synthesis and characterization of P1−P3, P6−P7, and
1−5; additional TRF and fsTA results; nsTA results; an
estimation of free energy changes for charge separation;
crystallographic data for 1 and 5 (CIF); and DFT-computed
ground state structures. This material is available free of charge
via the Internet at http://pubs.acs.org.
1
provides a similar significant degree of stabilization of the CT
state for both PMI and PDI dimers relative to slip-stacking.
Triplet State Formation. The excimer states of 1−3 decay
predominantly to the ground state with only low yields of
3*PMI, which most likely results from spin−orbit-induced
intersystem crossing (SO-ISC). Previous studies of cofacial
Xan-bridged PDI molecules found that SO-ISC for 1*(PDI)2 in
CH2Cl2 occurs in ∼23 ns.61 In that case, the triplet quantum
yields were estimated to be near 50% with competing excimer
decay to the ground state in 18 ns. The low triplet yield for the
PMI dimers can be explained by the relatively short excimer
lifetime, which kinetically outcompetes SO-ISC within the
excimer.
AUTHOR INFORMATION
■
Corresponding Author
m-wasielewski@northwestern.edu
Author Contributions
†R.J.L. and K.M.L. contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Chemical Sciences, Geo-
sciences, and Biosciences Division, Office of Basic Energy
Sciences, U.S. Department of Energy, under Grant DE-FG02-
99ER14999. The authors thank Drs. Charlotte L. Stern and
Amy A. Sarjeant for assistance with crystal structure refinement
and analysis as well as Dr. Raanan Carmieli for helpful
discussions.
Alternatives to SO-ISC can be ruled out on the basis of
energetic arguments. Charge separation resulting in either
radical pair intersystem crossing104−106 or spin−orbit charge-
transfer intersystem crossing107−110 are precluded as mecha-
nisms for 3*PMI formation because the free energies for charge
separation to produce Xan+•−PMI−• or PMI+•−PMI−• from
either 1*PMI or 1*(PMI)2 are very close to zero (Tables S1 and
S2).34,69 The data confirm this because charge separation and
recombination would be expected to produce a sharp, blue-
shifted absorption in a few picoseconds followed by
recombination in ∼1 ns.36,69 Instead, the observed excited-
state lifetimes are longer than that of the monomer, and the
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