Nonplanar Butterfly-Shaped π-Expanded Pyrrolopyrroles

Article abstract of DOI:10.1002/chem.201603282

Large aza-analogues of curved polycyclic aromatic hydrocarbons with a double-helicene structure present unique features for molecular photonics. We present the preparation and characterization of three such structures. The synthesis of these heterocyclic nanographenes involves only a few high-yield steps that use readily available starting materials. X-ray analysis revealed that each of these new dyes has three conformational isomers: one diastereoisomer in a meso form and two enantiomers in twisted forms [(P,P)] and [(M,M)]. The low energy barriers between the conformers, however, prevent their separation by using chiral HPLC, and the NMR spectra show only one set of signals for each of these curved compounds. Density functional theory (DFT) calculations quantify the small energy difference and the small energy barriers between the chiral and meso forms, which fully supports the experimental results. Their optical absorption lacks any sensitivity to the solvent environment, whereas their fluorescence features exhibit pronounced solvatochromism. This rarely observed solvatofluorochromism of centrosymmetric molecules without either electron-withdrawing groups or -donating substituents was probed by using time-resolved spectroscopy. These studies suggest that, similar to 9,9′-bianthryl, the nonpolar locally excited state shows negligible solvatochromism, whereas the charge-transfer state is sensitive to solvent polarity.

Full text of DOI:10.1002/chem.201603282

DOI: 10.1002/chem.201603282
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Fused-Ring Systems |Hot Paper|
Nonplanar Butterfly-Shaped p-Expanded Pyrrolopyrroles
Maciej Krzeszewski,[a, b] Takuya Kodama,[c] Eli M. Espinoza,[b] Valentine I. Vullev,*[b]
Takashi Kubo,*[c] and Daniel T. Gryko*[a]
Dedicated to Professor Yoshio Hisaeda on the occasion of his 60th birthday
Abstract: Large aza-analogues of curved polycyclic aromatic
hydrocarbons with double-helicene structure present
calculations quantify the small energy difference and the
a
small energy barriers between the chiral and meso forms,
which fully supports the experimental results. Their optical
absorption lacks any sensitivity to the solvent environment,
whereas their fluorescence features exhibit pronounced sol-
vatochromism. This rarely observed solvatofluorochromism
of centrosymmetric molecules without either electron-with-
drawing groups or -donating substituents was probed by
using time-resolved spectroscopy. These studies suggest
that, similar to 9,9’-bianthryl, the nonpolar locally excited
state shows negligible solvatochromism, whereas the
charge-transfer state is sensitive to solvent polarity.
unique features for molecular photonics. We present the
preparation and characterization of three such structures.
The synthesis of these heterocyclic nanographenes involves
only a few high-yield steps that use readily available starting
materials. X-ray analysis revealed that each of these new
dyes has three conformational isomers: one diastereoisomer
in a meso form and two enantiomers in twisted forms [(P,P)]
and [(M,M)]. The low energy barriers between the conform-
ers, however, prevent their separation by using chiral HPLC,
and the NMR spectra show only one set of signals for each
of these curved compounds. Density functional theory (DFT)
cells.[23] Their large p surfaces enable regular crystalline-state
packing and intermolecular coupling that has proven essential
for efficient electron and hole transduction. Control of the mo-
lecular packing in the solid state is a pivotal problem for the
effectiveness of functional materials.[24] The extension of p con-
jugation is responsible for molecular electronic and optoelec-
tronic properties, which are easily regulated by alteration of
the substituents on the periphery of the p-extended systems.
The utility of these PAH p-extended systems, however, is limit-
ed by their planarity, which induces p-stacking and limits solu-
bility.[25] In recent years, nonplanar polyarenes have become
a focus for research and development.[26–31] Nonplanar PAHs
with curved (e.g., saddle-shaped or bowl-shaped) structures,
and PAHs with twisted p surfaces, such as [n]-circulenes,[32–35]
helicenes,[36–41] and sumanenes,[42] have recently attracted sig-
nificant attention from scientists. Curved polyarenes offer
a unique arena to study unprecedented structure–function re-
lationships, such as properties that emerge from the effects of
curvature on aromaticity.[43–44] Indeed, these PAHs exhibit
unique reactivity, such as supramolecular aggregation in the
anionic state and complexation with transition metals.[45] How-
ever, most of them are pure hydrocarbons and only a few con-
tain any aromatic heterocyclic moieties,[46–48] other than por-
phyrinoids.[49–51]
Introduction
At the beginning of the 20th century, when the mutagenic ef-
fects of pyrene were discovered, it seemed that the fate of
polycyclic aromatic hydrocarbons (PAHs) was sealed.[1] Howev-
er, the discovery of structures such as fullerene,[2] corranu-
lene,[3–4] and carbon nanotubes[5] and technologies such as or-
ganic field-effect transistors (OFETs)[6] and organic light-emit-
ting diodes (OLEDs)[7] have made PAHs favored molecules for
research.[8–11] Their heterocyclic analogues with p-expanded
structures have undergone a renaissance due to their high
charge-carrier mobility,[12–19] which renders these compounds
immensely useful for organic electronics, for example, in
OLEDs[20] and OFETs,[21] and energy applications, such as organ-
ic and hybrid photovoltaic devices[22] and dye-sensitized solar
[a] M. Krzeszewski, Prof. Dr. D. T. Gryko
Institute of Organic Chemistry, Polish Academy of Sciences
Kasprzaka 44–52, 01-224 Warsaw (Poland)
E-mail: dtgryko@icho.edu.pl
[b] M. Krzeszewski, E. M. Espinoza, Prof. Dr. V. I. Vullev
Department of Bioengineering, Department of Biochemistry
Department of Chemistry, and Materials Science and Engineering Program
University of California, Riverside, California 92521 (USA)
E-mail: vullev@ucr.edu
[c] T. Kodama, Prof. Dr. T. Kubo
Herein, we present the utility of one of the newest attractive
building blocks in heterocyclic chemistry, that is, pyrrole[3,2-
b]pyrroles,[52–55] in the construction of nonplanar butterfly-
shaped heterocycles with low energy barriers between confor-
mational transitions, which reflects the molecular mechanics
Graduate School of Science
Osaka University, Toyonaka, Osaka 560-0043 (Japan)
E-mail: kubo@chem.sci.osaka-u.ac.jp
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201603282.
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and engineering of new paradigms in organic electronics and
photonics.
tween the conformers (thus, the preferred structures) and of
the optical properties of this class of compounds.
We developed a general three-step protocol for the synthe-
sis of nonplanar highly conjugated pyrrolo[3,2-b]pyrroles 6a–
c by starting from 1-bromo-6-aryl(heteroaryl)benzaldehydes
3a–c, 4-tert-butylaniline (2), and diacetyl (1; see Scheme 1).
The initial step involves preparation of the respective 1-bromo-
6-aryl(heteroaryl)benzaldehydes 3a–c. A Suzuki–Miyaura cou-
pling by using phenylboronic acid and benzo[b]thien-3-ylbor-
onic acid gave aldehydes 3a and 3b. For the synthesis of alde-
hyde 3c, however, it was convenient to resort to a Stille cou-
pling by using 2-(tributylstannyl)thiophene. As a second sub-
strate, we used 2,6-dibromobenzaldehyde, obtained in one
step from 1,3-dibromobenzene.[56] With aldehydes 3a–c in
hand, we readily prepared a series of tetraarylpyrrolopyrroles
(TAPPs; 4a–c) in yields of 35 to 45% by applying a procedure
previously developed in our group.[52–53] The substrate designs
afford TAPPs 4a–c with bromine atoms in the ortho position
with respect to the pyrrolopyrrole core, which allowed
a smooth Pd-catalyzed intramolecular direct arylation[57] to give
Results and Discussion
Design and synthesis
Building upon our previous results on oxidative aromatic cou-
pling in the pyrrole[3,2-b]pyrrole series[54] and on geometrical
constraints, we designed a new class of p-expanded pyr-
role[3,2-b]pyrroles with large “wings” at the periphery
(Figure 1). An important feature of our wing design is the van
der Waals overlaps between spatially close hydrogen atoms
from different aromatic rings, which prevent structural planari-
ty and force the molecules to assume nonplanar conforma-
tions while maintaining their complete p conjugation. The key
design element that causes this steric constraint involves three
different heterocycles (benzene, thiophene, and benzothio-
phene), which allows for fine-tuning of the energy barriers be-
Figure 1. Structures of all synthesized heterocycles with the pyrrolopyrrole core.
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ly gave butterfly-shaped compounds 6a–c in very good yields
of 85 to 90% (Scheme 1, Figure 1).
Although p stacking leads to pronouncedly poor solubility
of planar derivatives 5a–c in common organic solvents, addi-
tion of bulky tert-butyl groups partially addressed this issue
and allowed us to analyze all compounds. As predicted, com-
pounds 6a–c, with curved p systems, are considerably more
soluble than parent substrates 5a–c due to an induced dis-
placement of the structure by plane-weakening intermolecular
aggregation interactions.
Designed compounds 6a–c can be viewed as derivatives of
nitrogen-doped helical PAHs (Figure 1).[36–41] Because their
structures consist of double-helicene-like[36–41] units (Figure 2a),
compounds 6a–c should assume three different isomeric
forms: two twisted enantiomers (P,P)-6 and (M,M)-6 and
a folded meso form (P,M)-6 (Figure 3). The temperature-depen-
1
dent (TD) H NMR spectra recorded for 6a in [D8]toluene (see
the Supporting Information) indicate that in solution the trans-
formation between the conformational isomers is considerably
faster than the millisecond acquisition times, as observed from
the single average sets of signals for each compound, even at
temperatures as low as À408C. The transition-state energies
obtained from DFT calculations agree with the experimental
findings. The estimated energy barriers to interconversion be-
tween the conformers, which range from about 50 to
60 kJmolÀ1, are small enough to ensure rates of isomerization
that are faster than the NMR spectroscopy acquisition time-
1
scale. The TD H NMR spectra recorded for 6a in [D8]toluene
(see the Supporting Information), however, show only one set
of signals. At low temperatures the rate of the interconversion
between the isomers is slow enough to distinguish the
1H NMR signals of both isomers and the population of the en-
ergetically less stable isomer (folded form) becomes negligible
(<3%) and follows the Boltzmann distribution. Conversely, at
high temperatures the population of the folded isomer be-
comes appreciable (>5%), but rapid interconversion between
the isomers also takes place.[58] As a result, only one set of sig-
nals are apparent in the temperature range of À40 to +808C.
The relatively small energy barrier to interconversion
(56.0 kJmolÀ1) prevents separation of the isomers by using
chiral HPLC. Nevertheless, X-ray crystallography confirms the
existence of both isomers (folded and twisted) because both
forms co-crystallized (Figure 2).
X-ray crystallography
X-ray crystallography unequivocally confirmed the existence of
two diastereomeric forms of derivative 6a (Figure 2). Com-
pound 6a was crystallized from solution in dichloromethane
by slow addition of diethyl ether vapor at room temperature.
Both twisted and folded forms co-crystallized to form herring-
bone-like packing structures, with the monoclinic space group
P21/n in the unit cell (a=19.2789(9), b=(11.5527(4), c=
26.4311(11) ꢁ; a=90, b=105.645(3), g=908). The dihedral
angles between the wings reaches 41.3178 for the symmetric
folded form and 44.216 and 34.8888 for the highly distorted
asymmetric twisted form (Figure 2d). This structural difference
Scheme 1. General method for the synthesis of butterfly-shaped dyes 6a–c.
planar derivatives 5a–c in yields of 60 to 81% (Scheme 1,
Figure 1). Unfortunately, all these compounds exhibited poor
solubility in common organic solvents.
In the final step, compounds 5a–c underwent intramolecular
oxidative aromatic coupling. The application of a classical oxi-
dizing agent, that is, iron(III) chloride in nitromethane, smooth-
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Figure 3. Energy profile [kJmolÀ1] of the interconversion between the twist-
ed and folded forms of 6a’ calculated at the B3LYP/6-31G** level of theory
(zero-point energy corrected). tert-Butyl groups were omitted for reasons of
calculation time.
between the isomers has little effect on the electronic struc-
ture of 6a. The twisted and the folded isomers exhibit almost
the same distribution and energy of the molecular orbitals (see
the Supporting Information).
Theoretical investigation of isomerization
DFT calculations at the B3LYP/6-31G** level of theory allow us
to estimate the energy profile for interconversion between the
twisted and the folded forms of the unsubstituted 6a (6a’; see
Figure 3). The transition-state (TS) energies provide an insight
into the kinetic trends of the transformations. Conversely,
energy differences between the conformers reveal the thermo-
dynamics of the interconversion and reflect their distribution
when equilibrium is attained. The calculations indicate that the
twisted form is more stable than the folded form by
8.92 kJmolÀ1. This small energy difference is consistent with
the coexistence of both forms in the crystalline state. The TS
structure between the most stable twisted form and the meta-
stable folded form was estimated by using the quadratic syn-
chronous transit (QST2) method at the PM3 level of calculation
and optimized at the B3LYP/6-31G** level. The calculated en-
thalpy of activation (DH°) for the twisted form to the TS is
56.0 kJmolÀ1. Structural changes upon isomerization take place
mostly on one of the helicene substructures, for which the
structure in the TS is nearly planar and slightly saddle-shaped
(Figure 3). The deformed helicene moiety adopts an intermedi-
ate structure between the TS structures of [4]helicene (planar)
and [5]helicene (saddle-shaped), and DH° is also intermediate
between them (32 kJmolÀ1 for [4]helicene and 100 kJmolÀ1
for [5]helicene).[59] For unsubstituted 6b and 6c, the twisted
forms are more stable than the folded form by 11.1 and
12.0 kJmolÀ1, respectively, and the energy differences between
Figure 2. Crystal structure of the folded and twisted forms of compound 6a
(hydrogen atoms and tert-butyl groups omitted for clarity): a) highlighted
helicene-like units; b) top view; c) side view along the x axis; d) side view
along the y axis; e) crystal packing, blue: folded form, red: twisted form.
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Table 1. Summary of optical and electrochemical properties of derivatives 5a–c and 6a–c.
opt
[c]
[d]
labs [nm][a] eꢂ10À3 [mÀ1 cmÀ1
]
lem [nm][a] lonset [nm] Eg [eV][b] SS [cmÀ1
]
Ffl
E1/2 [V][e] E1/2 [V][e] E1/2 [V][e] HOMO [eV][f] LUMO [eV][g]
ox1
ox2
ox3
5a 421; 446
5b 421; 445
5c 424; 444 21; 17
6a 405; 428 20; 25
6b 406; 426 26; 21
6c 406; 429 18; 24
20; 16
23; 18
511
492
524
498
461
486
472
468
467
451
447
448
2.63
2.65
2.66
2.75
2.78
2.77
2800
2100
3400
3300
1800
2700
0.32 À0.01
0.09 0.04
0.14 0.11
0.18 0.04
0.16 0.13
0.13 0.08
0.50
–
À4.79
À4.84
À4.91
À4.84
À4.93
À4.88
À2.16
À2.19
À2.25
À2.09
À2.15
À2.11
0.28
0.26
0.23
0.32
0.31
0.65
0.63
0.60
0.59
0.63
[a] Measured in dichloromethane. [b] Estimated from the onset of the UV/Vis spectrum by using Egopt =1242/lonset. [c] SS=Stokes shift. [d] Determined by
using quinine sulfate in H2SO4 (0.5m) as a standard. [e] Cyclic voltammograms were measured in a deoxygenated solution of 0.1m tetrabutylammonium
hexafluorophosphate in anhydrous dichloromethane, with a glassy-carbon working electrode, a Ag/AgCl reference electrode, and a platinum-foil auxiliary
electrode (scan rate v=250 mVsÀ1, Ar, 208C). Reduction potentials vs. Fc+/Fc. [f] EHOMO =À(EoxÀ(À0.48))À4.8 eV. [g] Determined from the optical band gap
and HOMO.
the TS and the twisted forms are 27.9 and 38.2 kJmolÀ1, re-
spectively (see the Supporting Information). The lower energy
barriers are a consequence of the fact that the six-membered
rings were replaced with five-membered rings, which decreas-
es steric hindrance.
Optical properties
Steady-state absorption and emission spectra revealed the
basic optical properties of parent planar derivatives 5a–c and
their p-extended nonplanar analogues 6a–c (Tables 1 and 2).
Table 2. Solvatofluorochromism of compounds 6a–c.
Figure 4. Absorption (solid) and emission (dotted) spectra for compounds
5a (blue line) and 6a (red line) measured in dichloromethane.
[b]
Solvent
labs
eꢂ10À3
[mÀ1 cmÀ1
lem
SS
[cmÀ1
Ffl
[a]
[nm]
]
[nm]
]
6a
6a
6a
6a
6a
6b
6b
6b
6b
6b
6c
6c
6c
6c
6c
cyclohexane
toluene
THF
DCM
DMSO
cyclohexane
toluene
THF
DCM
DMSO
cyclohexane
toluene
THF
DCM
DMSO
405; 428
407; 430
403; 426
405; 428
405; 427
407; 431
409; 432
405; 428
406; 426
406, 429
405; 428
408; 431
404; 427
406; 429
405; 428
19; 23
19; 23
21; 26
20; 25
16; 21
21; 17
25; 20
26; 21
26; 21
24, 20
13; 19
18; 24
18; 25
18; 24
18; 25
451
470
484
498
505
437
447
447
461
471
447
465
480
486
497
1200
2000
2800
3300
3600
300
0.25
0.17
0.19
0.18
0.32
0.18
0.15
0.14
0.16
0.16
0.18
0.12
0.13
0.13
0.16
ly, the longest wavelength absorption band of 6b originates
solely from a fully allowed HOMO–LUMO transition.
For compounds 5a–c, the molar extinction coefficient (e) is
about 30000 and 20000mÀ1 cmÀ1 for the first and second
major absorption bands, respectively.
800
1000
1800
2100
1000
1700
2600
2700
3200
These derivatives show emission maxima that ranged from
l=492 to 524 nm, which correspond to the colors cyan and
green. The fluorescence quantum yield (Ffl) of 5a is 0.32. The
presence of a thiophene, that is, an electron-donating moiety,
in 5b and 5c quenches their fluorescence and results in Ffl
values of 0.09 and 0.16, respectively. Surprisingly, extension of
the p conjugation, as achieved in 6a–c, gave rise to a slight
hypsochromic shift in the absorption (lꢀ15 nm). A plausible
factor that is attributed to this result is the breakage of the p
conjugation at the angular connectivity between the rings due
to the distortion from planarity.
[a] SS=Stokes shift. [b] Determined by using quinine sulfate in H2SO4
(0.5m) as a standard.
All examined compounds exhibited two major absorption
bands, one located at l=350 nm and the other between l=
400 and 450 nm (Figure 4 and the Supporting Information).
The absorption bands of dyes 6a and 6c that correspond to
the lower-energy transitions show two distinct peaks, ascribed
to a partially allowed HOMO–LUMO transition and a fully al-
lowed HOMO–LUMO+2 transition, as revealed by TDDFT cal-
culations (Figure 5 and the Supporting Information). Converse-
The values of e for the bands at l=350 nm increased signifi-
cantly, up to 80000mÀ1 cmÀ1, whereas for the bands between
l=400–450 nm, the values remained almost the same, that is,
e=20000mÀ1 cmÀ1
.
For these compounds, hypsochromic shifts in emission are
also apparent (lꢀ15–40 nm). Compounds 6a–c emit blue-to-
cyan light in dichloromethane (lmax =461–498 nm).The absorp-
tion properties of dyes 5a–c and 6a–c are comparable with
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Figure 6. Normalized emission spectra for compound 6a in different sol-
vents: cyclohexane (blue), toluene (green), THF (yellow), CH2Cl2 (orange),
DMSO (red).
pounds, such as these butterfly-shaped polycyclic heterocy-
cles.[61–64] In the latter case, distinct electron-donating and -ac-
cepting moieties usually need to be present in the molecular
structure; a notable exception is 9,9’-bianthryl. The solvato-
fluorochromism of this nonpolar and highly symmetric mole-
cule has been extensively studied[65–69] and shows that the ob-
served dual fluorescence and the large redshift in the long-
wavelength band in polar media is associated with symmetry
breakage in the excited state after intramolecular charge trans-
fer (ICT). Similarly, fluorescence solvatochromism of centrosym-
metric compounds 6a–c can also be ascribed to the different
excited-state geometries assumed in various solvents. In non-
polar solvents, the first excited state (S1) is predominantly a lo-
cally excited (LE) state with very little or no charge-transfer
(CT) character. The relatively small Stokes’ shifts and the vi-
bronic structure of the emission bands for 6a–c in low-polarity
media are consistent with radiative deactivation from the LE
states of aromatic conjugates. In polar solvents, conversely, ef-
ficient intramolecular photoinduced electron transfer leads to
low-lying CT states that are inaccessible through direct excita-
tion from the ground states due to symmetry restrictions. In
polar solvents, therefore, dyes 6a–c exhibit redshifted, struc-
tureless fluorescence bands.
Figure 5. Energy diagrams for twisted 6a’ and meso-6a’ calculated at the
B3LYP/6-31G** level of theory. Excitation energies were computed by using
TD-DFT at the same level.
those of p-extended indolo[3,2-b]indoles[54] and 7,14-dihydro-
benzo[g]benzo[6,7]indolo[3,2-b]indoles.[60] The angular fusion
between the rings in all these classes of heterocycles results in
absorption that spans over the UV spectral region and extends
into the visible region to approximately l=450 to 470 nm.
Unless strong electron-withdrawing substituents are present,
all of these compounds[54,60] show emission maxima located at
around l=500 nm in CH2Cl2 (DCM). The current study also
shows that the introduction of a double-helical motif com-
bined with further p extension of compounds with a 1,4-dihy-
dropyrrolo[3,2-b]pyrrole core induces dramatic decreases in the
Ffl value, as compared with previously obtained TAPPs[52–53]
and their p-extended analogues.[54] Interestingly, all the butter-
fly-shaped dyes exhibited solvatofluorochromism (Figure 6 and
the Supporting Information). Compound 6a exhibits the most
distinctive shift in emission maximum (l=54 nm). It emits blue
light when dissolved in cyclohexane and green light in DMSO
(lC H =451, lTol =470, lTHF =484, lDCM =498, lDMSO =505 nm).
Time-resolved spectroscopy offers a key means for probing
such excited-state dynamics. Despite the pronounced changes
in the shapes of the fluorescence spectra and the significant
wavelength shifts, time-correlated single-photon counting
(TCSPC) shows that the excited-state lifetimes (t) of these
double-helical conjugates manifest little solvent dependence,
that is, tTol =16.8 and tDMSO =18.9 ns for 6a; tTol =2.48 and
tDMSO =2.84 ns for 6b; tTol =5.12 and tDMSO =7.17 ns for 6c.
These trends for the lifetimes of the radiative excited states are
consistent with the observed lack of solvent effect on the fluo-
rescence quantum yields.
6
12
Unlike in the majority of other known cases, a decrease in the
fluorescence quantum yield does not accompany the solvato-
fluorochromism of compounds 6a–c, that is, redshifts in emis-
sion induced by the increase in media polarity. In contrast, Ffl
remains more or less the same for the different solvents we
used. For compound 6a, Ffl varies over a narrow range of 0.17
to 0.32. The fluorescence quantum yields of 6b and 6c show
even smaller variations, with values that ranged from 0.12 to
0.18 (Table 2).
Transient absorption (TA) spectroscopy reveals the trends in
the picosecond dynamics prior to the nanosecond radiative
deactivation of the excited states of the butterfly-shaped dyes.
In nonpolar media, such as toluene, photoexcitation of com-
Although fluorescence solvatochromism is quite well report-
ed in dipolar molecules, it occurs rarely in quadrupolar com-
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pounds 6a–c generates transients, ascribed to the LE state,
with a broad absorption centered around l=550 nm and with
nanosecond lifetimes (Figure 7d), which is consistent with the
TCSPC findings. In DCM, the TA from the LE transients of the
double-helical heterocycles undergo a picosecond decay ac-
companied by the growth of sharp TA bands at about l=450
to 500 nm (Figure 7a–c). The sharp bands, which decay on the
nanosecond timescale, are ascribed to the CT states of butter-
fly-shaped heterocycles 6a–c. Sharp TA bands are common for
the spectra of radical ions of polyaromatic conjugates.[70,71]
Concurrently, broad and featureless spectral bands are typical
for fluorescence from CT states comprised of such radicals and
radical ions.[72–74] These considerations are consistent with the
observed fluorescence and TA spectral features for these but-
terfly-shaped conjugates.
In DMSO, the butterfly-shaped p-expanded pyrrolopyrroles
still exhibit structureless fluorescence bands that are batho-
chromichally shifted compared with the DCM samples, which
is consistent with the expected trend for the media polarity.
The band at l=450 nm associated with the CT states, howev-
er, is not apparent in the TA spectra in DMSO. Nevertheless, in
DMSO we still observe picosecond decays and shifts in the TA
spectra that significantly exceed any subtle changes associated
with TA recorded in toluene (Figure 7d,e). These observations
suggest that although the TA associated with the LE states
manifests negligible solvatochromism, the TA of the CT states
shifts noticeably on going from DCM to DMSO. This TA solvent
dependence is consistent with the CT character of a state with
increased polarity.
In accordance with the significant interest in heteroacenes
for organic electronics, we used cyclic voltammetry to investi-
gate the propensity of prepared p-extended derivatives 5a–
c and 6a–c to engage in CT processes (see the Supporting In-
formation). All investigated compounds proved to be prone to
oxidation. Dye 5a undergoes two reversible oxidations, where-
as 5b–c and 6a–c first undergo two quasireversible oxidations
followed by a reversible one. Furthermore, dyes 5c and 6a un-
dergo one additional irreversible process (at 1.04 and 1.41 V,
respectively). No reduction of the pyrrole[3,2-b]pyrrole core
was observed in the scanned potential window (Æ2.0 V). The
HOMO energy levels were estimated from the reduction po-
tentials of the first oxidation steps (Table 1). These values are
comparable to those found for diindolo[2,3-b:2’,3’-f]pyrro-
lo[3,2-b]pyrroles reported previously by our group.[55]
Conclusions
We have demonstrated that easily available 1,2,4,5-tetraaryl-
pyrrolo[3,2-b]pyrroles constitute a perfect core for building
nonplanar aromatic systems with a double-helical nature. The
methodology is general and makes it possible to assemble
butterfly-shaped dyes comprised of 12 to 14 aromatic rings in
only three steps. As predicted, distortion from planarity has
a great positive impact on the solubility of the prepared final
compounds. Replacement of the central naphthalene unit
(present in some earlier examples of double-helical dyes) with
heterocycles comprised of two conjugated five-membered
Figure 7. Transient absorption (TA) spectra for a)–c) 6a–c in DCM; and
d,e) 6c in toluene and DMSO. Insets: TA decay recorded at the blue and red
edges of the TA spectra. (lex =400 nm, 15 mJpulseÀ1; 50 fs pulse width).
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rings has a marked effect on the interconversion barriers be-
tween various conformers. A low interconversion energy barri-
er between the folded and twisted isomers prevents separa-
tion of these forms. However, X-ray crystallography data un-
equivocally confirms their existence. As a result of the strong
distortion from planarity, these dyes absorb and emit in a spec-
tral range similar to that of their counterparts with smaller mo-
lecular sizes. Our nonplanar, double-helical aromatic systems
are solvatofluorochromic, analogously to 9,9’-bianthryl (the
fluorescence quantum yield of all these butterfly-shaped aza-
PAHs does not decrease with an increase in solvent polarity).
In contrast to 9,9’-bianthryl, however, the excited-state life-
times show little solvent dependence. TA indicates that the ex-
istence of two emitting states is responsible for dual fluores-
cence: a solvent-independent (LE) state and a solvent-depen-
dent (CT) state. The combination of photophysical properties
of these structurally unique heterocycles may present unex-
plored avenues to practical applications in organic optoelec-
tronics.
Stille coupling: Synthesis of 3c
1,6-Dibromobenzaldehyde (20 mmol), 2-(tributylstannyl)thiophene
(20 mmol), and [Pd(PPh3)4] (0.26 mmol) were placed in a 100 mL
Schlenk flask, which was flushed with argon prior to use. Toluene
(70 mL) was added and the resulting mixture was stirred at 1108C
for 16 h, then water (20 mL) was added and the resulting mixture
was stirred for further 15 min. The two phases were separated and
the water phase was extracted with ethyl acetate (3ꢂ20 mL). The
organic phases were combined and dried, the solvent was evapo-
rated, and the crude product was purified by using flash column
chromatography.
Data for 3c
1,6-Dibromobenzaldehyde (5.24 g, 20 mmol), 2-(tributylstannyl)-
thiophene (6.35 mL, 20 mmol), and [Pd(PPh3)4] (300 mg, 0.26 mmol)
gave 3c as a yellow oil (yield: 63%, (3.35 g). Rf =0.48 (SiO2, DCM/
1
hexanes, 1:2); H NMR (500 MHz, CDCl3): d=10.03 (s, 1H), 7.68 (d,
J=7.9 Hz, 1H), 7.49 (dd, J=7.6, 0.8 Hz, 1H), 7.44 (dd, J=5.1,
1.1 Hz, 1H), 7.37 (t, J=7.9 Hz, 1H), 7.11 (dd, J=5.1, 3.6 Hz, 1H),
7.03 ppm (dd, J=3.6, 1.1 Hz, 1H); 13C NMR (125 MHz, CDCl3): d=
191.6, 138.5, 133.9, 132.5, 130.6, 129.9, 127.8, 127.6, 122.1 ppm;
HRMS (EI): m/z calcd for C11H7OSBr: 265.9401 [M]+; found:
265.9402.
Experimental Section
General procedures for the synthesis of aldehydes 3a–c
Suzuki–Miyaura coupling: Synthesis of 3a and b
General procedure for the synthesis of TAPPs 4a–c
Glacial acetic acid (6 mL) was placed in a 25 mL round-bottom
flask equipped with a reflux condenser and magnetic stir bar, then
4-tert-butylaniline (8 mmol), adequate aldehyde 3 (8 mmol), and
TsOH (0.8 mmol) were added. The mixture was stirred at 908C for
30 min, then butane-2,3-dione (4 mmol) was slowly added by
using a syringe and the resulting mixture was stirred at 908C for
3 h. The reaction mixture was then cooled to RT and the precipi-
tate of the obtained dye was filtered off and washed with cooled
glacial acetic acid. Recrystallization from AcOEt and drying under
vacuum afforded the pure product.
1,6-Dibromobenzaldehyde (20 mmol), phenylboronic acid (3a) or
benzo[b]thien-3-ylboronic acid (3b; 20 mmol), K2CO3 (40 mmol),
PPh3 (20% mol), and Pd(OAc)2 (10% mol) were placed in a 100 mL
Schlenk flask, which was flushed with argon prior to use. A tolu-
ene/water mixture (50 mL, 1:1 v/v) was added and the resulting
mixture was stirred at 808C for 16 h. After cooling, two phases
were separated and the water phase was extracted with ethyl ace-
tate (3ꢂ20 mL). The organic phases were combined and dried, the
solvent was evaporated, and the crude product was purified by
using flash column chromatography.
Data for 4a
Data for 3a
4-tert-Butylaniline (1.27 mL, 8 mmol), aldehyde 3a (2.08 g, 8 mmol),
TsOH (140 mg, 0.8 mmol), and butane-2,3-dione (350 mL, 4 mmol)
gave 4a as a beige solid (yield: 45%, 1.50 g). Rf =0.42 (SiO2, DCM/
1,6-Dibromobenzaldehyde (5.24 g, 20 mmol), phenylboronic acid
(2.44 g, 20 mmol), K2CO3 (5.52 g, 40 mmol), PPh3 (1.05 g, 20 mol%),
Pd(OAc)2 (450 mg, 10 mol%) gave 3a as a yellow oil (yield: 60%,
3.12 g). Rf =0.38 (SiO2, DCM/hexanes, 1:2); 1H NMR (500 MHz,
CDCl3): d=10.00 (s, 1H), 7.69 (d, J=7.7 Hz, 1H), 7.46–7.41 (m, 3H),
7.39 (d, J=7.7 Hz, 1H), 7.36 (d, J=7.5 Hz, 1H), 7.31–7.27 ppm (m,
2H); 13C NMR (125 MHz, CDCl3): d=192.0, 146.6, 138.0, 133.5,
133.3, 132.7, 130.3, 129.6, 128.4, 128.3, 122.8 ppm; HRMS (EI): m/z
calcd for C13H9OBr: 259.9837 [M]+; found: 259.9825.
1
hexanes, 1:2); m.p. 292–2948C; H NMR (500 MHz, CDCl3): d=7.68
(d, J=7.9 Hz, 2H), 7.21 (t, J=7.8 Hz, 2H), 7.15 (d, J=7.6 Hz, 2H),
7.10–7.04 (m, 6H), 6.96 (t, J=7.7 Hz, 4H), 6.62 (t, J=8.0 Hz, 8H),
6.25 (s, 2H), 1.28 ppm (s, 18H); 13C NMR (125 MHz, CDCl3): d=
147.0, 144.8, 140.5, 136.8, 133.6, 131.8, 130.7, 129.08, 129.04,
128.47, 128.42, 127.5, 127.0, 126.5, 125.1, 122.2, 98.3, 34.3,
31.4 ppm; HRMS (ESI-TOF): m/z calcd for C50H45N2Br2: 831.1950
[M+H]+; found: 831.1932.
Data for 3b
Data for 4b
1,6-Dibromobenzaldehyde (5.24 g, 20 mmol), benzo[b]thien-3-yl-
boronic acid (3.56 g, 20 mmol), K2CO3 (5.52 g, 40 mmol), PPh3
(1.05 g, 20 mol%), Pd(OAc)2 (450 mg, 10 mol%) gave 3b as
a yellow oil (yield: 50%, 3.16 g). Rf =0.42 (SiO2, DCM/hexanes, 1:2);
1H NMR (500 MHz, CDCl3): d=9.96(s, 1H), 7.92 (d, J=7.8 Hz, 1H),
7.77 (dd, J=6.2, 2.8 Hz, 1H), 7.49–7.43 (m, 3H), 7.40 (t, J=6.6 Hz,
1H), 7.38–7.34 ppm (m, 2H); 13C NMR (125 MHz, CDCl3): d=191.4,
140.0, 139.9, 138.6, 134.2, 134.1, 133.4, 133.1, 130.9, 126.4, 124.9,
124.8, 123.2, 122.9, 122.5 ppm; HRMS (EI): m/z calcd for C15H9OSBr:
315.9557 [M]+; found: 315.9548.
4-tert-Butylaniline (1.27 mL, 8 mmol), aldehyde 3b (2.53 g, 8 mmol),
TsOH (140 mg, 0.8 mmol), and butane-2,3-dione (350 mL, 4 mmol)
gave 4b as an off-white solid (yield: 36%, 1.36 g). Rf =0.44 (SiO2,
DCM/hexanes, 1:2); m.p. 309–3118C; 1H NMR (500 MHz, CD2Cl2):
d=7.75 (t, J=7.7 Hz, 4H), 7.34–7.26 (m, 4H), 7.24–7.19 (m, 2H),
7.12–7.08 (m, 4H), 6.90 (d, J=8.4 Hz, 4H), 6.71 (s, 2H), 6.60 (d, J=
8.4 Hz, 4H), 6.20 (s, 2H), 1.20 ppm (s, 18H); 13C NMR (125 MHz,
CD2Cl2): d=148.1, 139.7, 139.1, 138.9, 136.8, 135.15, 135.10, 132.3,
131.5, 129.4, 129.3, 129.0, 127.7, 125.6, 125.5, 124.4, 124.1, 122.9,
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122.60, 122.56, 122.4, 98.1, 34.5, 31.4 ppm; HRMS (ESI-TOF): m/z
calcd for C54H45Br2N2S2: 943.1391 [M+H]+; found: 943.1382.
anes, 1:2); m.p. >4008C; 1H NMR (500 MHz, [D8]toluene): d=8.36
(d, J=1.9 Hz, 2H), 8.28 (d, J=7.7 Hz, 2H), 7.89 (d, J=8.7 Hz, 2H),
7.65 (dd, J=8.6, 1.9 Hz, 2H), 7.42 (dd, J=7.3, 1.2 Hz, 2H), 7.22 (dd,
J=4.9, 1.2 Hz, 2H), 7.17–7.13 (m, 2H), 7.07–7.05 (m, 2H), 7.05–7.03
(m, 2H), 6.29 (s, 2H), 1.43 ppm (s, 18H); 13C NMR (125 MHz,
[D8]toluene): d=144.8, 132.9, 132.0, 130.4, 127.05, 127.01, 126.3,
125.9, 125.7, 124.2, 122.9, 120.6, 119.9, 115.2, 110.0, 93.5, 34.4,
31.3 ppm; HRMS (EI): m/z calcd for C46H38N2S2: 682.2476 [M]+;
found: 682.2496.
Data for 4c
4-tert-Butylaniline (1.27 mL, 8 mmol), aldehyde 3c (2.13 g, 8 mmol),
TsOH (140 mg, 0.8 mmol), and butane-2,3-dione (350 mL, 4 mmol)
gave 4c as a yellowish solid (yield: 35%, 1.18 g). Rf =0.45 (SiO2,
DCM/hexanes, 1:2); m.p. 288–2908C; 1H NMR (500 MHz, CD2Cl2):
d=7.62 (d, J=7.9 Hz, 2H), 7.47 (d, J=7.7 Hz, 2H), 7.23 (dd, J=7.9,
7.7 Hz, 2H), 7.18–7.14 (m, 6H), 6.89 (d, J=8.4 Hz, 4H), 6.83 (dd, J=
3.9, 3.9 Hz, 2H), 6.64 (d, J=2.9 Hz, 2H), 6.31 (s, 2H), 1.26 ppm (s,
18H); 13C NMR (125 MHz, CD2Cl2): d=148.2, 142.4, 138.7, 137.0,
133.3, 131.9, 131.8, 130.0, 129.6, 128.6, 127.1, 127.0, 126.93, 126.88,
126.4, 125.8, 125.7, 122.9, 98.0, 34.6, 31.4 ppm; HRMS (EI): m/z
calcd for C46H40N2S2Br2: 842.1000 [M]+; found: 842.0975.
General procedure for the synthesis of butterfly-shaped p-
expanded PPs 6a–c
Dry methylene chloride (6 mL) and adequate 5 (0.5 mmol) were
placed in a 25 mL round-bottom flask that was flushed with argon
and equipped with magnetic stir bar and septum prior to use.
Iron(III) chloride (10 mmol) dissolved in dry nitromethane (6 mL)
was added to the suspended substrate by using a syringe. The re-
action was conducted at RT for 30 min, then water (12 mL) was
added and the resulting mixture was stirred for further 15 min. The
two phases were separated and the water phase was extracted
with methylene chloride (3ꢂ10 mL). The organic phases were com-
bined and dried, the solvent was evaporated, and the crude prod-
uct was purified by using flash column chromatography.
General procedure for the synthesis of p-expanded TAPPs
5a–c
Dry toluene (8 mL) and adequate TAPP 4 (1 mmol) were placed in
a 20 mL sealed tube that had been flushed with argon and
equipped with magnetic stir bar prior to use. Pd(OAc)2 (22 mg,
0.1 mmol), Cs2CO3 782 mg (2.4 mmol), and PPh3 60 mg (0.22 mmol)
were added to the dissolved substrate and the reaction was con-
ducted at 1208C for 3 h. After cooling, water (8 mL) was added
and resulting mixture was stirred for another 15 min. The two
phases were separated and the water phase was extracted with
ethyl acetate (3ꢂ10 mL). The organic phases were combined and
dried, the solvent was evaporated, and the crude product was
washed with copious amounts of diethyl ether.
Data for 6a
Compound 5a (335 mg, 0.5 mmol) was treated with FeCl3 (1.62 g,
10 mmol) to give 6a as a yellow solid (yield: 90%, 300 mg). Rf =
1
0.38 (SiO2, DCM/hexanes, 1:2); m.p. 384–3868C; H NMR (500 MHz,
[D8]toluene): d=8.74 (d, J=7.9 Hz, 2H), 8.59 (d, J=8.1 Hz, 2H),
8.57 (dd, J=8.1, 1.0 Hz, 2H), 8.55 (d, J=2.0 Hz, 2H), 8.39 (d, J=
7.6 Hz, 2H), 8.24 (d, J=8.5 Hz, 2H), 7.70 (t, J=7.9 Hz, 2H), 7.58 (dt,
J=6.9, 1.0 Hz, 2H), 7.51 (dt, J=6.9, 1.0 Hz, 2H), 7.43 (dd, J=8.5,
2.1 Hz, 2H), 1.44 ppm (s, 18H); 13C NMR (125 MHz, [D8]toluene): d=
146.1, 133.1, 131.4, 129.8, 129.0, 127.9, 127.2, 126.0, 125.2, 125.0,
124.2, 123.8, 123.4, 123.0, 121.6, 121.5, 121.3, 120.8, 118.2, 104.6,
34.5, 31.3 ppm; HRMS (EI): m/z calcd for C50H38N2: 666.3035 [M]+;
found: 666.3057.
Data for 5a
Compound 4a (831 mg, 1 mmol), Pd(OAc)2 (22 mg, 0.1 mmol),
Cs2CO3 (782 mg, 2.4 mmol), and PPh3 (60 mg (0.22 mmol) gave 5a
as a yellow solid (yield: 81%, 543 mg). Rf =0.40 (SiO2, DCM/hex-
anes, 1:2); m.p. >4008C; 1H NMR (500 MHz, C6D6, 608C): d=7.52
(d, J=8.1 Hz, 2H), 6.98 (dd, J=8.9, 1.5 Hz, 2H), 6.93–6.83 (m, 12H),
6.79–6.74 (m, 6H), 6.39 (s, 2H), 1.15 ppm (s, 18H); 13C NMR
(125 MHz, C6D6, 608C): d=147.6, 145.9, 141.2, 137.8, 134.5, 132.3,
131.9, 129.62, 129.56, 129.47, 129.40, 129.1, 127.1, 126.8, 125.7,
125.3, 123.3, 123.1, 99.1, 34.5, 31.6 ppm; HRMS (EI): m/z calcd for
C50H42N2: 670.3348 [M]+; found: 670.3340.
Data for 6b
Compound 5b (391 mg, 0.5 mmol) was treated with FeCl3 (1.62 g,
10 mmol) to give 6b as a yellow solid (yield: 88%, 342 mg). Rf =
0.40 (SiO2, DCM/hexanes, 1:2); m.p. >4008C; 1H NMR (500 MHz,
[D8]toluene): d=9.08 (d, J=8.2 Hz, 2H), 8.88 (d, J=8.5 Hz, 2H),
8.81 (d, J=8.3 Hz, 2H), 8.63 (d, J=2.0 Hz, 2H), 8.39 (d, J=7.6 Hz,
2H), 7.84 (t, J=7.9 Hz, 2H), 7.78 (dd, J=8.4, 2.0 Hz, 2H), 7.71 (d,
J=7.7 Hz, 2H), 7.47 (dt, J=7.6, 1.0 Hz, 2H), 7.23 (t, J=7.6 Hz, 2H),
1.51 ppm (s, 18H); 13C NMR (125 MHz, [D8]toluene): d=147.0,
139.3, 137.4, 132.5, 132.3, 132.2, 127.1, 126.0, 125.6, 125.1, 124.2,
123.8, 123.6, 123.4, 123.1, 123.0, 122.6, 121.5, 121.1, 121.0, 120.2,
115.9, 110.0, 104.0, 34.7, 31.4 ppm; HRMS (EI): m/z calcd for
C54H38N2S2: 778.2476 [M]+; found: 778.2463.
Data for 5b
Compound 4b (943 mg, 1 mmol), Pd(OAc)2 (22 mg, 0.1 mmol),
Cs2CO3 (782 mg, 2.4 mmol), and PPh3 (60 mg, 0.22 mmol) gave 5b
as a yellow solid (yield: 60%, 469 mg). Rf =0.42 (SiO2, DCM/hex-
anes, 1:2); m.p. >4008C; 1H NMR (500 MHz, [D8]toluene): d=7.50
(dd, J=8.0, 1.1 Hz, 2H), 6.94 (dd, J=7.7, 1.0 Hz, 2H), 6.90–6.83 (m,
8H), 6.79 (d, J=8.5 Hz, 4H), 6.76 (t, J=7.9 Hz, 2H), 6.71 (d, J=
7.1 Hz, 4H), 6.36 (s, 2H), 1.16 ppm (s, 18H); 13C NMR (125 MHz,
[D8]toluene): d=147.2, 145.5, 141.0, 134.2, 132.2, 131.5, 129.4,
129.2, 128.1, 127.9, 127.53, 127.47, 126.8, 125.5,124.7, 123.0, 98.8,
34.3, 31.4 ppm; HRMS (EI): m/z calcd for C54H42N2S2: 782.2789 [M]+;
found: 782.2781.
Data for 6c
Compound 5c (341 mg, 0.5 mmol) was treated with FeCl3 (1.62 g,
10 mmol) to give 6c as a yellow solid (yield: 85%, 288 mg). Rf =
0.41 (SiO2, DCM/hexanes, 1:2); m.p. 3608C (decomp.); 1H NMR
(500 MHz, [D8]toluene): d=8.51 (d, J=1.7 Hz, 2H), 8.44 (d, J=
8.2 Hz, 2H), 8.23–8.18 (m, 4H), 8.16 (d, J=7.8 Hz, 2H), 7.58 (t, J=
7.8 Hz, 2H), 7.54 (dd, J=8.5, 1.9 Hz, 2H), 7.27 (d, J=5.6 Hz, 2H),
1.44 ppm (s, 18H); 13C NMR (125 MHz, [D8]toluene): d=146.4,
Data for 5c
Compound 4c (842 mg, 1 mmol), Pd(OAc)2 (22 mg, 0.1 mmol),
Cs2CO3 (782 mg, 2.4 mmol), and PPh3 (60 mg, 0.22 mmol) gave 5c
as a yellow solid (yield: 80%, 546 mg). Rf =0.43 (SiO2, DCM/hex-
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133.1, 131.5, 131.3, 131.0, 126.5, 125.2, 124.4, 122.97, 122.90, 121.2,
121.0, 119.8, 119.4, 116.3, 110.0, 103.7, 34.6, 31.4 ppm; HRMS (EI):
m/z calcd for C46H34N2S2: 678.2163 [M]+; found: 678.2178.
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We thank the Foundation for Polish Science (MISTRZ pro-
gramme 4./2013); the National Science Centre (Poland) (PREL-
UDIUM 2015/19/N/ST5/00826); the Global Research Laboratory
Program (2014K1A1A2064569) through the National Research
Foundation (NRF) funded by the Ministry of Science, ICT &
Future Planning, Korea; and the U.S.A. National Science Foun-
dation (grants CHE 1465284 and CBET 0923408). T.K. thanks
the Ministry of Education, Culture, Sports, Science, and Tech-
nology (15 H01 086).
Keywords: chromophores
·
computer
chemistry
·
fluorescence · fused-ring systems · solvatochromism
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Received: July 11, 2016
Published online on && &&, 0000
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These are not the final page numbers! ÞÞ

Full Paper
FULL PAPER
Let’s twist again! In two steps, simple
pyrrolo[3,2-b]pyrroles can be trans-
formed into double helical polycyclic ar-
omatic heteroaromatics with low
energy barriers between the conforma-
tional isomers and fluorescence solvato-
chromism (see figure). The existence of
folded and twisted isomers was un-
equivocally confirmed by using X-ray
crystallography.
&
Fused-Ring Systems
M. Krzeszewski, T. Kodama,
E. M. Espinoza, V. I. Vullev,* T. Kubo,*
D. T. Gryko*
&& – &&
Nonplanar Butterfly-Shaped p-
Expanded Pyrrolopyrroles
&
&
Chem. Eur. J. 2016, 22, 1 – 12
www.chemeurj.org
12
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!