Synthesis and optical properties of pyrrolo[3,2-b ]pyrrole-2,5(1 H,4 H)-dione (iDPP)-based molecules

Article abstract of DOI:10.1021/jp400256s

We describe the synthesis and photophysical properties of a series of derivatives of pyrrolo[3,2-b]pyrrole-2,5(1H,4H)-dione-3,6-diyl (iDPP) linked to two oligothiophenes of variable length (nT). The iso-DPP-oligothiophenes (iDPPnTs) differ from the common

Full text of DOI:10.1021/jp400256s

Article
pubs.acs.org/JPCA
Synthesis and Optical Properties of Pyrrolo[3,2‑b]pyrrole-
2,5(1H,4H)‑dione (iDPP)-Based Molecules
Mindaugas Kirkus,† Stefan Knippenberg,‡ David Beljonne,‡ Jerome Cornil,‡ Rene A. J. Janssen,†
́
̂
́
and Stefan C. J. Meskers*,†
†Molecular Materials and Nanosystems, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands
‡Laboratory for Chemistry of Novel Materials, University of Mons, Place du Parc 20, B-7000 Mons, Belgium
S
* Supporting Information
ABSTRACT: We describe the synthesis and photophysical
properties of a series of derivatives of pyrrolo[3,2-b]pyrrole-
2,5(1H,4H)-dione-3,6-diyl (iDPP) linked to two oligothio-
phenes of variable length (nT). The iso-DPP-oligothiophenes
(iDPPnTs) differ from the common pyrrolo[3,4-c]pyrrole-
1,4(2H,5H)-dione-3,6-diyl-oligothiophene analogues
(DPPnTs) by a different orientation of the two lactam rings
in the bicyclic iDPP unit compared to DPP. In contrast to the
highly fluorescent DPPnTs, the new isomeric iDPPnTs exhibit
only very weak fluorescence. We demonstrate with the help of
quantum-chemical calculations that this can be attributed to a
different symmetry of the lowest excited state in iDPPnT (A in C2 symmetry) compared to DPPnTs (B) and the corresponding
loss in oscillator strength of the lowest energy transition. Upon extending the oligothiophene moiety in the iDPPnTs molecules,
the charge transfer character of the lowest A excited state becomes more pronounced. This tends to preclude high fluorescence
quantum yields even in extended iDPPnTs systems.
The first synthesis of the symmetrical and asymmetrical
pyrrolo[3,2-b]pyrrole-2,5-diones (iDPPs, Ar1 = Ar2 = aryl and
INTRODUCTION
■
The pyrrolo[3,4-c]pyrrole-1,4-dione-3,6-diyl (DPP, Figure 1)
R1 = R2 = H, Figure 1) has been reported by Furstenwerth,8
chromophore, first reported in 1974,1 has recently emerged as
̈
starting from pulvinic acid derivatives using harsh reaction
conditions in low reaction yields due to formation of side
products. A number of symmetrical N-arylated iDPPs (R1 = R2
= Ar and Ar1 = Ar2, Figure 1) have been synthesized by reacting
aromatic ester carbanions with aromatic oxalic acid−bis-
(imidoyl)dichlorides9 that can be obtained in a one-pot
reaction from aromatic amines, oxalyl chloride, and PCl5.10,11
Asymmetrical N-arylated iDPPs (R1, R2 = aryl with R1 ≠ R2 and
Ar1 = Ar2) have been prepared by the same synthetic strategy12
but using asymmetrical oxalic acid−bis(imidoyl)dichlorides
which can be prepared in a few steps starting from commercial
ethyl-2-chloro-2-oxoacetate and different derivatives of ani-
line.13
Figure 1. Chemical structures of 3,6-diaryl-pyrrolo[3,4-c]pyrrole-1,4-
dione (DPP), 3,6-diaryl-pyrrolo[3,2-b]pyrrole-2,5-dione (iDPP), and
pulvinic acid.
The nature of the lowest excited state in DPP and iDPP
derivatives is of importance for application in opto-electronics.
While normal DPPs are renowned for their bright fluorescence
in solution,14,15 many structurally related chromophores show
hardly any photoluminescence activity. For example, recent
studies on benzodipyrrolidones revealed that these molecules
show essentially no fluorescence.16 Similarly low fluorescence
quantum yields (from 0.3% to 2%) were reported for
an important and versatile building block for organic π-
conjugated polymers and extended π-systems with interesting
opto-electronic properties that find application in organic solar
cells and thin-film transistors.2 DPP-based molecules have
several desirable optical properties such as high extinction
coefficients, readily tunable absorption wavelength, and bright
fluorescence.3,4 The usefulness and versatility of the DPP unit
has inspired the search for related chromophores. In this
research, we focus on the isomeric pyrrolo[3,2-b]pyrrole-2,5-
dione-3,6-diyl (iDPP) which is a natural derivative of the
pulvinic acid (Figure 1)5,6 and used as a pigment.7
Received: January 9, 2013
Revised: March 4, 2013
© XXXX American Chemical Society
A
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a
Scheme 1. Synthesis of Pyrrolo[3,2-b]pyrrole-2,5-dione-Based N-Arylated Derivatives T1, T2, and T3
a
(a) K2CO3, 2-ethylhexyl bromide, 2-butanone, reflux, 12 h; (b) ethanol, HCl, reflux, 12 h; (c) oxalyl chloride, toluene, rt, 30 min; PCl5, reflux, 1 h;
(d) diisopropylamine, 2.5 M BuLi in n-hexane, THF, −78 °C; then ethyl-2-thiopheneacetate, rt, 12 h; (e) 2.5 M BuLi in n-hexane, THF, −78 °C, 1.
5 h, −78 °C, rt, C12H25Br, reflux, 2 days; (f) NBS, AcOH/chloroform, 0 °C; then rt, 12 h; (g) Mg, 2-bromothiophene, Et2O, rt; then 50 °C, 4 h; (6),
Pd(dppf)Cl2, Et2O, 0−5 °C, rt, overnight; (h) 2.5 M BuLi in n-hexane, THF, −78 °C, 30 min; then rt, 30 min; then −78 °C, trimethyltin chloride, rt,
overnight; (i) NBS, 50 °C, 12 h; (j) 2-(tributylstannyl)thiophene or 8, Pd[PPh3]4, toluene, 100 °C, 36 h.
benzodifuranone derivatives17 as well as for iDPP polymers
(<1%) .18 For 3,6-diphenyl-2,5-dihydropyrrolo[3,2-b]pyrrole-
2,5-dione (iDPP, R1 = R2 = H, Ar1 = Ar2 = Ph) fluorescence
could not be observed, which was related to the forbidden
nature of the lowest electronic transition.19
from the lowest, A-symmetry, singlet excited state in these
oligothiophene-substituted iDPPnTs. The energy gap between
A and B excited states decreases with the number of thiophene
rings in the oligothiophene substituent. We observe, however,
only a very moderate improvement in the transition dipole
strength for the S1 → S0 transition in the extended systems.
Development of substantial intramolecular charge transfer
character in the lowest excited state of the larger systems
appears to preclude a strong increase in the fluorescence
quantum yield.
Quantum-chemical calculations based on density functional
theory (DFT) performed on the isomeric linear benzodifur-
anones, benzodipyrrolinones, and their homologues20 indicate
the presence of low-energy n−π* states in these π-conjugated
systems. From the experimental studies on azoaromatics,21,22
e.g., acridine, it is well known that if the lowest excited state is
of n−π* nature, the fluorescence quantum yield is negatively
affected. In this case, the low quantum yield is attributed to
efficient intersystem crossing to the triplet states. Even if the
lowest excited state in a π-conjugated molecule is of the desired
orbital nature (π−π*), the fluorescence yield can still be very
low. This is the case for, e.g., polyenes,23−25 polydiacety-
lenes,26,27 polythienylenevinylenes,28 and retinal.29,30 In con-
trast, π-conjugated polymers and oligomers of phenyleneviny-
lene, phenyleneethynylene, thiophene, and fluorene show very
bright fluorescence. This is generally attributed to the
symmetry of the lowest excited state. Assuming the C2 point
group for the π-systems, the lowest π−π* excited singlet state
in the nonfluorescent molecules is assigned to the A symmetry
while for the fluorescent materials the lowest excited singlet
state has B character. The energy difference between A and B
excited singlet states can be small; in such cases, small
alterations in chemical structure or the environment of the
molecule can have a drastic influence on the fluorescence
properties.
RESULTS AND DISCUSSION
■
Synthesis. The structure and synthesis of iDPPnTs T1−T3
with a different number of thiophene rings in the backbone are
shown in Scheme 1. For preparation of T1−T3 we used a
synthetic strategy reported by Langer et al.11,13 To increase the
solubility of the compounds we included 2-ethylhexyloxy
substituents on the N-phenyls. The corresponding aniline
derivative 2 was synthesized from commercial 4-acetoamido-
phenol using Williamson ether synthesis,31 followed by
deacylation of the amide using concentrated hydrochloric
acid in ethanol resulting in 2. Synthesis of the symmetric oxalic
acid−bis(imidoyl)dichloride was performed in a one-pot
reaction of oxalyl chloride with 2, followed by treatment with
anhydrous PCl5 in boiling toluene. Although many oxalic acid
bis-alkyl derivatives can be purified by recrystallization from
propylencarbonate, the resulting oily intermediate 3 did not
crystallize and was used in the next step without purification.
Reaction of 3 with the enolate of ethyl-2-thiopheneacetate
prepared by treatment with lithium diisopropylamide (LDA)
resulted in the iDPP derivative T1 in high (68%) yield.
Derivatives T2 and T3 could be obtained by further
functionalization of T1. To this end, T1 was first brominated
with NBS in chloroform at 50 °C and the resulting dibromo
compound 8 was used in Stille couplings with the organotin
derivatives of either a thiophene or a 2,2′-bithiophene 7.
Here, we describe the synthesis of three 3,6-bis-
(oligothiophene)pyrrolo[3,2-b]pyrrole-2,5-dione (iDPPnT, n
= 1−3) derivatives and investigate their optical properties in
detail using experimental and theoretical methods. We find that
these molecules fluoresce but with very low quantum yield
(10−3−10−4). We attribute the low fluorescence to emission
B
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Derivative 7 was prepared by alkylation of the thiophene using
n-BuLi and the corresponding alkyl halide, followed by
bromination and Kumada coupling, resulting in monoalkylated
2,2′-bithiophene derivative 6. Usually bis(diphenylphosphino)-
propane nickel(II) chloride (Ni[dppp]Cl2) is used in Kumada
couplings as a catalyst.32,33 In our synthesis we used [1,1′-
bis(diphenylphosphino)ferrocene]dichloropalladium (Pd-
[dppf]Cl2) as a catalyst because it is more active compared
to Ni[dppp]Cl2, giving higher yields, shorter reaction times,
and higher purity of the target derivative 6.34 Stannylation
reaction of 6 with trimethyltin chloride in anhydrous THF gave
analytically pure trimethylstannane derivative 7 in high (89%)
yield. Finally, Stille coupling35 was used for synthesis of the
target compounds T2 and T3 using Pd[PPh3]4 as a catalyst. T2
and T3 could be obtained in low yields. The Stille reaction is
often an efficient method for C−C coupling between DPPs and
the electron-rich units. Stille coupling has, for instance, been
used to synthesize small DPP molecules36−39 and high
molecular weight DPP polymers.40−44 In contrast, the final
C−C coupling reactions to get T1 and T2 proceeded with only
low yields, illustrating the need for further optimization.
Curiously, there are very few examples in the literature of
C−C coupling reactions between dibromo derivatives of
pyrrolo[3,2-b]pyrrole-2,5-diones and stannyl derivatives or
bispinacolboronic esters of electron-rich aromatic units. For
synthesis of the π-conjugated polymers containing pyrrolo[3,2-
b]pyrrole-2,5-diones, Suzuki coupling was used as described by
Welerlich et al.18 However, the indices of polydispersity (PDI)
are around 5.6, thus showing that the Suzuki reaction
conditions are not ideal for this chromophore. The iDPP
polymers reported in a recent patent were prepared by Stille
couplings using Pd2(dba)3/(o-tol)3P as a catalytic system.45
The structure and purity of the intermediate and final
compounds were determined by MALDI-TOF, 13C NMR,
only 0.6 kcal/mol higher in energy than the C2 isomer. A
similar result was recently found for a iDPP-type molecule
where the thiophenes are replaced by hydrogen.19
To investigate the nature of the lowest two excited states,
approximate second-order coupled cluster (CC2),47−49 equa-
tion of motion-coupled cluster singles and doubles (EOM-
CCSD),50 and time-dependent density functional theory
calculations (TD-DFT) along with the long-range-corrected
ωB97xd51 and ωPBE52 functionals have been performed using
Turbomole 5.10,53 Molpro 2009,54 and Q-Chem 4.0,55
consistently relying on the 6-31G* basis set. The reliability of
the excitation energies obtained at the TD-DFT level is
substantiated by the so-called Λ overlap parameter proposed by
Peach et al.,56 which measures the degree of CT character and
hence the probability of a CT-related failure of TDDFT. For
the lowest two single excited states of both conformers of iDPP,
Λ is comfortably large and supports therefore the analysis given
here.
For the C2 and Ci conformers, the lowest singlet excited state,
S1, is very similar in energy and orbital nature (Table 1). The
same holds for the second singlet excited state, S2. TD-DFT as
well as ab initio methods yield π−π* character for S1 and S2 of
both conformers. For the C2 conformer, the Hartree−Fock
orbitals are illustrated in Figure 3. Focusing on the central
butadiene part of the pyrrolopyrrole fused ring system, the
HOMO−1 corresponds to the lowest occupied π-orbital for a
butadiene segment. The HOMO has the expected bonding
interaction over the double bonds, whereas the LUMO has
nodes over the double bonds and a bonding character over the
central C−C bond (Figure 3). The S1 state is built on the
HOMO−1→ LUMO excitation, while S2 has HOMO →
LUMO character. The S1 (Ag) ← S0 (Ag) transition in the Ci
conformer is dipole forbidden because both the initial and the
final states have Ag character; S2 (Au) ← S0 (Ag) is dipole
allowed and predicted to have an oscillator strength close to
unity (Table 1). For the C2 conformer, both transitions are
formally dipole allowed, though an extremely low oscillator
strength is predicted for S1(A) ← S0(A) (Table 1). The
transition dipole moment for this transition is perpendicular to
the plane of the central fused ring unit and therefore small in
magnitude. For the S2(B) ← S0(A) transition, a large oscillator
strength is predicted.
Optical Properties. Room-temperature (298 K) absorp-
tion, fluorescence, and fluorescence excitation spectra of T1,
T2, and T3 in solvents of increasing polarity (toluene (Tol), o-
dichlorobenzene (o-DCB), and benzonitrile (BZN)) are shown
in Figure 4. Corresponding data can be found in Table 2.
The absorption spectrum of T1 in toluene features an
intense broad absorption band in the visible range with a
maximum around 2.95 eV and molar decadic absorption
coefficient εmax of 22 × 103 M−1 cm−1. The magnitude of εmax
points to an optically allowed π−π* transition. Therefore, we
assign the 2.95 eV absorption band to a transition from the 11A
ground state to an excited singlet state of B symmetry (1B*),
adopting the C2 point group for the chromophore. Notably,
below 2.6 eV, the absorption spectrum of T1 shows a low-
intensity shoulder at the low-energy side of the allowed 1B* ←
11A transition. The absorption coefficient for this low-energy
shoulder of T1 at 2.43 eV is only 1.8 × 103 M−1 cm−1.
A possible explanation for the weak intensity shoulder below
2.6 eV in the absorption spectrum of T1 could be a
conformational equilibrium of the molecule in the room-
temperature solution. For instance, thiophene-substituted DPP
1
and H NMR techniques.
Quantum-Chemical Calculations. Quantum-chemical
calculations were performed on an analogue of T1 to find
the low-energy conformations of the electronic ground state of
the iDPP moiety. To simplify the calculations, the two alkoxy
substituents on the phenyl moieties were replaced with
hydrogen atoms. The ground state geometry of the T1
analogue was optimized at the second-order Møller−Plesset
(MP2) level along with the Pople 6-31G* basis set with 520
basis functions using the Gaussian09 quantum-chemistry
program.46 Two conformers have been identified (Figure 2).
The first conformer (Figure 2a) has C2 symmetry, while the
other conformer has Ci symmetry (Figure 2b). The Ci isomer is
Figure 2. Two low-energy ground-state conformers of the T1
derivative, as identified at the MP2/6-31G* level of theory: (a) C2
and (b) Ci symmetry structures.
C
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Table 1. Vertical Excitation Energies from the Ground State to the Lowest Two Excited States of the Two Conformers of T1
a
Shown in Figure 2 (with C2 and Ci symmetry), As Computed at Different Levels of Theory
C2
b
c
method
ωB97xd
E[S1(A)] (eV)
f
E[S2(B)] (eV)
f
E[S3(A)] (eV)
f
E[S4(B)] (eV)
f
d
e
2.70
0.004
0.004
3.18
0.607
0.623
3.87 (nπ*)
3.92 (nπ*)
3.75 (nπ*)
0.002
0.002
4.20
4.27
0.102
0.104
ωPBE
2.76
2.75
3.13
3.22
3.36
3.72
CC2
EOM-CCSD
Ci
b
c
method
E[S1(Ag)] (eV)
f
E[S2(Au)] (eV)
f
E[S3(Au)] (eV)
f
E[S4(Au)] (eV)
f
f
g
ωB97xd
ωPBE
2.68
0
0
3.18
0.586
0.604
3.90 (nπ*)
3.95 (nπ*)
3.78 (nπ*)
0.046
0.046
4.34
4.41
0.106
0.110
2.73
2.73
3.23
3.37
CC2
EOM-CCSD
3.11
3.74
a
b
c
d
e
f
g
f is the oscillator strength. H−1 → L (π−π*) excitation. H → L (π−π*) excitation. Λ = 0.698. Λ = 0.850. Λ = 0.561. Λ = 0.654. Excited states
are of ππ* nature, except where indicated.
the molecules, leading to increased π-conjugation, which is
known for various thiophene derivatives.59
The absorption spectrum for T1 was investigated at variable
temperatures, and results are shown in Figure 5 (top). As can
1
be seen, upon lowering the temperature the main B* ← 11A
UV band near 2.95 eV sharpens somewhat and shows some
vibronic fine structure at 80 K. The low-energy shoulder below
2.6 eV persists at low temperature. We thus conclude that the
low-energy absorption tail in the room-temperature absorption
spectrum of T1 is not the result of a conformational
equilibrium in the molecule. For T2 and T3 similar changes
occur when reducing the temperature. The absorption bands
remain broad and do not show clear vibronic structure.
Figure 3. Hartree−Fock orbitals involved in the S1 and S2 excitations
of the T1 analogue with C2 symmetry.
In toluene, T1 shows a weak fluorescence featuring a broad
fluorescence band centered around 1.8 eV (Figure 4). The
fluorescence quantum yield ϕflu is very low. The low yield
implies efficient nonradiative decay pathways of the excited
state, which are common for carbonyl-containing chromo-
phores.60−64 Using Rhodamine 101 as a standard,65 we
determine ϕflu to be on the order of 10−4 (Table 2). In more
polar solvents like o-DCB and BZN the fluorescence quantum
(DPP1T) shows a narrowing of the widths of the peaks upon
cooling (Figure 5, bottom) due to planarization of the
molecules with the appearance of the distinct vibronic peaks
separated by ∼0.18 eV, which can be attributed to the change
in the C−C bond length alternation of the π-conjugated system
after excitation.58,59 The red shift in the absorption of DPP1T
upon cooling can be explained by the more planar structure of
Figure 4. Room-temperature absorption (black line), excitation (red line), and emission (blue line) spectra of T1, T2, and T3 in dilute (OD < 0.1)
toluene, o-DCB, and benzonitrile solutions. Excitation photon energies: T1 2.8 (toluene, o-DCB) and 3.02 eV (benzonitrile); T2 2.5 eV; T3 2.3 eV.
D
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Table 2. UV−Vis Absorption and Fluorescence Data in Organic Solvents of Different Polarity for T1, T2, and T3 at 298 K
abs
flu
a
b
b
c
d
e
f
1
compound solvent Eons (eV) Emax (eV) Emax (eV) εmax (M−1cm−1
)
τPL (ns)
ϕflu
krad (μs−1
)
knrad (ns−1
)
1B* → A krad (μs−1
)
T1
T2
T3
Tol
2.18
2.17
2.17
2.09
2.07
2.10
2.00
1.92
1.92
2.97
2.94
2.95
2.59
2.54
2.56
2.48
2.38
2.67
1.80
1.81
1.80
1.79
1.74
1.75
1.78
1.70
1.66
0.64
0.45
0.27
1.0
7 × 10−4
4 × 10−4
2 × 10−4
8 × 10−3
5 × 10−3
3 × 10−3
2 × 10−2
9 × 10−3
3 × 10−3
1.1
0.9
0.7
8
1.6
2.2
3.7
1.0
1.3
1.6
1.0
1.3
1.7
o-DCB
BZN
Tol
21 845
22 232
1.1 × 102
o-DCB
BZN
Tol
0.77
0.64
1.0
6
1.3 × 102
5
20
11
o-DCB
BZN
0.80
0.60
5
a
b
c
Onset of absorption spectra measured at 298 K. Absorption and emission maxima measured at 298 K. Extinction coefficient calculated from the
d
e
maximum of the absorption at 298 K. Lifetime of the emission approximated with a single-exponential function at 298 K. Relative to Rhodamine
101, excitation photon energy = 2.79 eV. Calculated using the Strickler−Berg equation.57
f
value of 0.9 μs (Table 2). This small value of krad is consistent
with an optically forbidden but spin-allowed S1 → S0 transition.
The experimentally determined radiative rate may be
compared to the radiative rate predicted from the absorption
spectrum in o-DCB using the Strickler−Berg relation,57
assuming that all the oscillator strength in the visible absorption
band below 3.2 eV is also available for the emissive transition.
For T1 we integrate the absorption spectrum over the 3.2−2.2
eV energy range and predict a radiative rate constant 0.1 ns−1.
This is more than 2 orders of magnitude higher than the
experimental value (9 × 10−4 ns−1) and clearly shows that the
main absorption band with maximum at 2.95 eV generates an
excited state that is different from the excited state, giving rise
to the fluorescence.
On the basis of the small rate for radiative decay, we assign
the fluorescence to a 21A* → 11A transition. The low-energy,
low-intensity shoulder in the absorption at 2.5 eV is then
assigned to the reverse transition (21A* ← 11A) and the main
absorption band at 2.95 eV to 11B* ← 11A. In this assignment,
the S1 excited state is assumed to be of π−π* orbital nature, as
supported by theory. Alternatively, a lowest excited singlet state
of n−π* character could also explain the small radiative decay
rate for the lowest excited state. However, 1n−π* excited states
Figure 5. Variable-temperature UV−vis absorption of T1, T2, T3, and
DPP1T in dilute 2-methyltetrahydrofuran solutions.
with energy below 2.0 eV are uncommon in organic molecules.
For linear polyenal molecules, 1n−π * states occur with energy
about 3.0 eV above the ground state.66,67 For retinal, the 1n−π
1
* and Ag*state are close in energy and both these states are
below the 1Bu* state.29,68−70 The weak fluorescence from
2,4,6,8,10-dodecapentaenal has been assigned to 21Ag* →
11Ag.71 For longer polyenals, it has been argued that the singlet
yield is even lower. To ensure that the weak fluorescence
signals observed are indeed due to the T1, T2, and T3
molecules, fluorescence excitation spectra were measured at low
optical densities for all compounds and compared to the
absorption spectra in the same solvent (Figure 4). We find that
absorption and excitation traces overlap nicely, which firmly
supports the assignment of the weak fluorescence signals to the
iDPPnT molecules.
1
π−π* excited state of Ag* symmetry is lower in energy than
1
the n−π* state. It should be noted that upon lowering the
symmetry from C2h to, e.g., C2, extensive mixing of n−π* and
π−π* states could occur and it may no longer be meaningful to
distinguish n−π* and π−π* states. At the B97xd/6-31G* level
of theory for both the C2 and Ci conformers, the first state with
a solid n−π* character is the S3 state (S4 is π−π*).
Furthermore, especially in polar solvents, the picture may be
further complicated by the presence of low-lying charge transfer
(CT) like excited states which have been found in
carotenoids.72,73 The decrease in radiative and increase in
nonradiative decay rate for T1 upon increasing the polarity of
the solvents (see Table 2) provides an indication for
involvement of the CT state in the photophysical behavior.
The photon energy corresponding to the maximum of the
fluorescence intensity, however, does not shift appreciably with
The weak fluorescence band of T1 overlaps with the weak
low-energy shoulder in the absorption spectrum and appears to
1
be unrelated to the allowed B* ← 11A transition at 2.95 eV.
This suggests that the fluorescence and low-energy absorption
shoulder arise from an electronic transition different from the
1B ← 11A transition band, as suggested by the theoretical
calculations. Fluorescence of T1 decays approximately
exponentially in time after pulsed excitation at 4.0 eV with an
excited state lifetime of 0.64 ns (Table 2). From the measured
fluorescence lifetime and the fluorescence quantum yield we
calculate the rate of radiative decay (krad) in o-DCB and find a
E
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increasing solvent polarity. The absence of such a shift suggests
a limited CT character in the lowest excited state; still the CT
state could be involved in the decay from S1 to S0 as a transition
state.
charge transfer character. The solvatochromic shift for the
corresponding transition in absorption (21A* ← 11A) is
difficult to evaluate experimentally because of the small dipole
strength and the spectral overlap with the allowed (1B* ← 11A)
transition. Intramolecular excited state charge transfer is well
known for organic compounds containing carbonyl groups, e.g.,
coumarin dyes74 and carotenoids.73,75
Upon extending the conjugation length in these molecules by
adding more electron-rich thiophene rings (compounds T2 and
T3), the main absorption maximum in the visible range shifts to
lower energy and is located at 2.6 and 2.5 eV, respectively
(Figure 4). For T2 and T3, we also assign the main absorption
band in the visible range to the 11B* ← 11A transition. For T2
the low-energy absorption shoulder below the 11B* ← 11A
band is more pronounced compared to T1. For T3, the
shoulder is almost invisible. The fluorescence quantum yield for
T2 and T3 remains low (<0.1) but increases approximately by
1 order of magnitude for each pair of additional thiophene rings
in the oligothiophene substituents (Table 2). The fluorescence
lifetimes for T2 and T3 are in the nanosecond time domain.
Rates for radiative decay remain small considering the size of
the conjugated system and indicate a forbidden nature for the
fluorescent transition. Therefore, we assign the lowest excited
state S1 in all three oligomers to the 21A* state. The increase in
radiative decay rate with increasing number of thiophene rings
may then be ascribed to more efficient vibronic mixing of the
11B* and 21A* states facilitated by their smaller energy
difference. The assignments of the lowest excited states of T1,
T2, and T3 are summarized in Figure 6. Comparing the
Electrochemical characterization in o-DCB shows a reduction
of oligomers T1 and T2 near −1.11 and −1.05 V vs Ag/AgCl,
respectively. Oxidation of the T2 molecule occurs at +0.75 V vs
Ag/AgCl, resulting an in electrochemical band gap of 1.8 eV for
the T2 molecule. For T1, no oxidation peak could be detected
in the electrochemically accessible voltage range. Electro-
chemical measurements indicate for T2 that an intramolecular
charge transfer (CT) state with an oligothiophene substituent
acting as donor and the central iDPP unit as acceptor may be
expected with an energy around 2.0 eV after dielectric
relaxation of the solvent around the photoexcited molecule.
The presence of a CT state close in energy to the lowest excited
state may account for the reduction in the rate for radiative
decay from the 21A* level, see Table 2. In solvents of high
polarity, the CT character of the relaxed lowest excited state
1
would increase at the expense of the vibronically admixed B
character. Since the transition probability mainly derives from
1
the admixture of the B* state, the radiative rate is expected to
go down. The increase in the rate of nonradiative decay from
the lowest excited state with increasing solvent polarity may
also be accounted for by the admixture of CT character. It is
well known that in electron-donor acceptor dyads intersystem
crossing to the triplet state can be enhanced by the energetic
proximity of CT states,76 providing a channel for nonradiative
decay.
CONCLUSIONS
■
A series of 1,4-diaryl-3,6-di(oligothiophen-2-yl)pyrrolo[3,2-
b]pyrrole-2,5(1H,4H)-dione (iDPPnT) (T1−T3) derivatives
substituted with a different number of thiophene rings was
synthesized. Photophysical properties were investigated using
different solvents and temperatures. In contrast to the more
common 3,6-di(oligothiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4-
(2H,5H)-dione (DPPnT) molecules that show bright fluo-
rescence in solution, the new isomeric iDPPnTs exhibit only
very weak fluorescence. For solutions of T1−T3 we find that
the lowest excited state of iDPPnT has symmetry properties
(1A) different from that of DPPnT (1B) and that the transition
between the ground state and the lowest excited singlet state in
iDPP molecules has negligible oscillator strength. This finding
is corroborated by quantum chemical calculations. Upon
extending the oligothiophene moiety in the iDPP molecules,
the charge transfer character of the lowest excited state
becomes more pronounced. This tends to preclude high
fluorescence quantum yields even in extended oligothiophene−
iDPP systems.
Figure 6. Jablonski diagram showing the estimated energetic positions
of the singlet excited states of T1, T2, and T3. For T2 and T3 the CT
character of the relaxed lowest excited state in polar solvent is
indicated by the dashed line. Numbers in italics correspond to energies
tentatively assigned to excited singlet states associated with the
oligothiophene moiety.
1
1
energies of B* and A* excited states for T1 obtained from
experiment and quantum chemical calculations (Table 1) we
note that the experimental values are 0.5 eV lower; the energy
1
1
difference between B* and A* excited states is predicted
accurately.
For T2 and T3 an additional absorption band is observed in
the UV at 3.7 and 3.2 eV, respectively. These absorption
energies match approximately the excitation energy of the
isolated oligothiophene segment. This correspondence suggests
that the associated excited states, labeled Sn in Figure 6, are
mainly localized on the oligothiophene segment.
For molecules T2 and T3 with extended π-conjugated
systems, the emission spectra measured at room temperature
are sensitive to solvent polarity and the emission maxima of T2
and T3 are slightly red shifted going from toluene to
benzonitrile (0.04 eV for T2 and 0.12 eV for T3) (Figure 4).
In contrast, for molecule T1, the emission maximum (1.80 eV)
does not vary with solvent polarity (Table 2). The red shift of
the fluorescence maxima for molecules T2 and T3 with
increasing solvent polarity suggests that the emitting state has
ASSOCIATED CONTENT
■
S
* Supporting Information
Synthesis, experimental procedures, and fluorescence decay
curves for T1, T2, and T3 in solvents of different polarity, and
complete refs 40, 46, and 54. This material is available free of
charge via the Internet at http://pubs.acs.org.
F
dx.doi.org/10.1021/jp400256s | J. Phys. Chem. A XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry A
Article
(18) Welterlich, I.; Charov, O.; Tieke, B. Deeply Colored Polymers
Containing 1,3,4,6-Tetraarylpyrrolo[3,2-b]pyrrolo-2,5-Dione
(IsoDPP) Units in the Main Chain. Macromolecules 2012, 45,
4511−4519.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: s.c.j.meskers@tue.nl.
̌ ́ ̌
(19) Lunak, S.; Vynuchal, J.; Hrdina, R. Geometry and Absorption of
Notes
Diketo-Pyrrolo-Pyrrole Isomers and Their π-Isoelectronic Furo-
Furanone Analogues. J. Mol. Struct. 2009, 919, 239−245.
The authors declare no competing financial interest.
̌ ́ ̌
(20) Lunak, S.; Vynuchal, J.; Hrdina, R. DFT and TD DFT Study of
ACKNOWLEDGMENTS
Isomeric Linear Benzodifuranones, Benzodipyrrolinones and Their
Homologues. J. Mol. Struct. 2009, 935, 82−91.
■
This research was supported by The Netherlands Organization
for Scientific Research (NWO) through a grant in the VIDI
scheme. S.K. is grateful to the European Marie Curie
(COFUND) programme along with the Belgian science policy
office (“Belspo”) for current funding. D.B. and J.C. are both
FNRS Research Directors.
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