Unprecedented rearrangement of diketopyrrolopyrroles leads to structurally unique chromophores

Article abstract of DOI:10.1039/c7cc07310k

Diketopyrrolopyrroles possessing thienyl, furyl and benzofuryl substituents undergo unprecedented skeletal rearrangement in the presence of trimethylsilyl bromide resulting in the formation of thieno[2,3-f]isoindole-5,8-diones and furo[2,3-f]isoindole-5,8-diones. These relatively small dyes possess favorable photophysical properties with the emission maxima within the range of 573-624 nm, large fluorescence quantum yields, moderate sensitivity of emission to solvent polarity and a HOMO-LUMO gap of ca. 1.8 eV.

Full text of DOI:10.1039/c7cc07310k

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Gryko, Chem. Commun., 2017, DOI: 10.1039/C7CC07310K.
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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc3N@C2n (2n 68 and 80). Synthesis of the
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DOI: 10.1039/C7CC07310K
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Unprecedented rearrangement of diketopyrrolopyrroles leads to
structurally unique chromophores
Olena Vakuliuk,a Shota Ooi,a Irena Deperasińska,b Olga Staszewska-Krajewska,a Marzena
Received 00th January 20xx,
Accepted 00th January 20xx
Banasiewicz,b Bolesław Kozankiewicz,b Oksana Danylyukc and Daniel T. Gryko*,a
DOI: 10.1039/x0xx00000x
www.rsc.org/
Diketopyrrolopyrroles possessing thienyl, furyl and benzofuryl while the exact mol. mass of a new compound turned out to be
substituents undergo unprecedented skeletal rearrangement in the equal to the mol. mass of substrate, the 1H NMR data revealed
presence of trimethylsilyl bromide forming thieno[2,3-f]isoindole- the loss of symmetry in the product (Figure S2). Such findings
5,8-diones and furo[2,3-f]isoindole-5,8-diones. These relatively are consistent only with the rearrangement of the DPP 1.
small dyes possess favorable photophysical properties with the
NMR analysis provides structural information about the
emission maxima within the range of 573–624 nm, large synthesized compound. 13C NMR shows two signals in the
fluorescence quantum yields, moderate sensitivity of emission to carbonyl region (171.1 ppm and 165.6 ppm) along with two
solvents polarity and HOMO-LUMO gap ca. 1.8 eV.
signals in the aliphatic region (32.8 ppm and 28.7 ppm).
Moreover, 1H NMR spectrum shows a singlet at 3.38 ppm and a
doublet at 3.42 ppm, which we ascribe as methyl groups. The
difference in the chemical shifts as well as the multiplicity of the
signals reconfirms the formation of the asymmetric product.
The character of the proton signal at 8.82 ppm (1H NMR, broad
quartet), reveals the opening of one γ-lactam ring and the
presence of an NH-CH3 moiety in the product. Moreover, COSY
experiments support a 3+2 spin system with one uncorrelated
downfield signal. The reaction on one of the carbons adjacent
to the sulfur atom can rationalize this finding. Thorough analysis
of 1H13C HSQC and 13C13C HMBC spectra finally uncover that the
Organic electronics and fluorescence imaging place demands
for the heterocycles with π-extended conjugation.1 Rational
design, aided by combinatorial and diversity-orientated
approaches, provides invaluable routes toward novel functional
materials.2 Nonetheless, certain sets of chromophores evade
even the best designed retrosynthetic analysis and serendipity
factor tends to play a role in the discovery of new dyes.3
Depending on the targeted application, fluorophores
possessing different combinations of properties are desirable.
Small red-emitters are pivotal for applications in live fluorescent
imaging.4
structure of
2 possesses the thieno[2,3-f]isoindole-5,8-dione
As a part of a larger project aimed at the synthesis of novel
fluorophores, we investigated oxidative aromatic coupling of
diketopyrrolopyrroles (DPPs, Figure S1).5 We focused on
iodine(III) reagents which efficiently promote such
transformations, making them often the reagents of choice.6
During this study, we observed that subjecting 2,5-dimethyl-
core, thus far never described in the literature (Scheme 1). The
final proof came from single crystal X-ray analysis (Figures S2-
S3).
1,4-diketo-3,6-di(thien-3-yl)pyrrolo[3,4-c]pyrrole
(1
)
to
hydroxy(tosyloxy)iodobenzene (Koser’s reagent, HTIB) and
TMSBr led to the formation of purple compound as the sole
isolable product (Scheme 1, Table 1, Entry 1).7 Surprisingly,
Scheme 1. The rearrangement of DPP
1 into thieno[2,3-f]isoindole-5,8-dione 2.
a.Dr O. Vakuliuk, S. Ooi, Dr O. Staszewska-Krajewska, Prof. D. T. Gryko Institute of
Organic Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224
Warsaw (Poland). E-mail:dtgryko@icho.edu.pl.
Encouraged by the unique structure of the prepared dye, we
started a systematic study of the discovered transformation.
The mixture of TMSBr and HTIB provides only moderate yields
of the expected product, strongly emphasizing the need for
optimization of the reaction conditions (Table 1, Entry 1). In
b.Prof. I. Deperasińska, Dr M. Banasiewicz, Prof. B. Kozankiewicz,Institute of
Physics, Polish Academy of Sciences, Al. Lotników 32/46, 02-668 Warsaw
(Poland).
c. Dr O. Danylyuk Institute of Physical Chemistry, Polish Academy of Sciences,
Kasprzaka 44/52, 01-224 Warsaw (Poland).
Electronic Supplementary Information (ESI) available: see DOI: 10.1039/x0xx00000x
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fact, iodine(III) reagent does not support the formation of dye
Journal Name
2
Diketopyrrolopyrroles bearing 2-furyl and 3-furyl groups (9
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DOI: 10.1039/C7CC07310K
product which suggests the non-oxidative character of this and 11) display lower reactivity comparing to
1 and 7,
transformation. Rather, the HTIB causes the decomposition of respectively. The desired product was obtained in low yield only
the desired product when added in excess (Table 1, Entries 1-3). for the DPP . The attempts to improve the reaction outcome
9
Conversely, the reaction does not proceed in the absence of (HTIB, prolongation of the reaction time) did not bring about
TMSBr (Table 1, Entry 4). We also elaborated the study of the positive results. DPP 13, possessing a peripheral benzofuran
discovered transformation to exhibit strong solvent moiety displays poor reactivity as well. Fortunately, significant
dependence, which points to the importance of an acidic stability of the rearranged product allowed for the prolongation
medium (Table 1, Entries 3, 5-7). In addition, improved solubility of the reaction time as well as utilization of the iodine(III)
of the parent DPP aids a favorable outcome.
catalyst and, in consequence, we could achieve a higher yield of
14. Under similar conditions, the rearrangement occurs for DPP
15 (Scheme 2), bearing 3,4-dimethoxyphenyl groups. N-
Alkylated DPPs bearing less electron-rich aryl substituents do
not undergo this rearrangement.
Table 1. Influence of the reaction conditions on the rearrangement of DPP 1 into dye 2.a
Entry
1
Lewis acid,
TMSBr
Additive, eq.
HTIB (2)
Solvent
HFIP
Yield,b %
36
2
3
4
5
6
7
8
9
TMSBr
TMSBr
-
HTIB (0.05)
HFIP
HFIP
HFIP
TFEc
75
73
0
-
HTIB (2)
TMSBr
TMSBr
TMSBr
BF3∙Et2O
BF3∙Et2Od
-
-
-
-
-
75
0
MeOH
DCM
HFIP
TFEc
0
53
72
aUnless otherwise noted, reactions were performed with 1 (0.15 mmol), Lewis acid
(0.15 mmol), solvent (1 ml), 50 °C, 17 h; bisolated yields are reported; creaction was
performed at 70˚C; d2 eq. of the Lewis acid was utilized.
Aiming to increase the yield of the desired product, we
tested
a
stronger Lewis acid, BF3·Et2O. Experimental
in up to 72%
investigations have proven the formation of dye
2
yield (Table 1, Entries 8-9). It is noteworthy that the use of
BF3·Et2O requires a higher loading and elevated temperature to
achieve a reasonable yield of the rearranged product. Other
Lewis acids such as POCl3, TfOH, TFA or BCl3 do not mediate this
rearrangement (Table S1).
Adopting conditions 2 and 5 (Table 1), we undertook the
synthesis of analogues of heterocycle 2. Experimental findings
uncovered a negligible effect of alkyl substituents on the
discovered transformation (Scheme 2). Notably, in this case the
reaction medium plays an important role. We observed a
significant improvement in the yield of dye
when HFIP was used in place of TFE. Variations in the solubility
of the substrate in the tested solvents can probably rationalize aUnless otherwise noted, reactions were performed with TMSBr (0.15
4 (60% versus 75%)
this finding. Surprisingly, the transformation of DPP 5, bearing mmol), solvent (1 ml), 17 h at 50 °C (for HFIP) or 70 °C (for TFE). Isolated
additional tert-butyl groups, does not proceed as expected in yields are reported; breaction was conducted for 2 days; creaction was
either TFE or HFIP. Unexpectedly, the presence of HTIB in a conducted for 3 days; dreaction was performed with 2 eq. of Lewis acid.
Scheme 2. Scope of the diketopyrrolopyrroles’ rearrangement.
catalytic amount leads to significant improvement of the
reaction outcome (Scheme 2).
With the library of new dyes in hand, we conducted a series
of photophysical studies, which reveal that the absorption
bands corresponding to the lowest energy absorption are
located in the 530–606 nm region (Table 2, Figure 1), displaying
bathochromic shifts in comparison to the parent DPPs. Emission
maxima are red-shifted to the orange-red part of the visible
spectra. Compounds 2, 4 and 6 exhibit only small differences in
absorption and emission maxima. Nevertheless, the type and
position of the heteroatom influence the λabs and λem
2,5-Di(n-hexyl)-1,4-diketo-3,6-di(thien-2-yl)pyrrolo[3,4-
c]pyrrole (
even if the reaction is conducted for 3 days. The addition of
Koser’s reagent improves reactivity of DPP , however, probably
7) affords desired product 8 at a minuscule 6% yield
7
due to the oxidative character, this reagent induces the
decomposition of the product or intermediates. Luckily, the
replacement of TFE with HFIP and lowering the reaction
temperature to 50 C lead to the desired product in 32% yield.
2 | J. Name., 2012, 00, 1-3
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significantly. Different charge distribution in the molecules can second excited S2 state locates around 7000 cm-1 above the S1,
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rationalize both the red-shift of the absorption and emission meaning that the lowest excited S1 state DisOwI:e10ll.1s0e3p9a/Cra7tCeCd07fr3o10mK
maxima when regioisomeric products
and the blue-shift of λabs and λem for dyes possessing furan unit S5, Tables S3-S4). Moreover, the electronic transition S0
versus 10). Dye 14 possessing π-expanded core characterizes of the π π* character and corresponds to excitation from the
by the most bathochromically shifted emission and absorption HOMO to the LUMO level (Figure 3).
2
and
8
are superimposed the higher located, electronically excited states (Figures 2 and
→
S1 is
(2
→
in the analyzed series.
In order to get more information about the reasons for
smoothing the fluorescence spectra (lack any well resolved
vibrational structure even at 5K) and the possible depopulation
channels we simulated the vibrational structure of the
fluorescence spectrum of
2 (Figure S5). Calculation of the
Franck-Condon (FC) factors (Figure S6) shows that the
vibrational structure of the absorption and fluorescence spectra
are obscured by the three low frequency vibrations: 37, 59 and
100 cm-1. These vibrations are characterized by high shift
parameters8 and contribute to the spectrum in their common
combinations as well as in combination with other vibrations.
50000
40000
30000
20000
10000
0
Figure 1. Absorption and fluorescence spectra for 8, 10, 14 and 16 in DCM.
Table 2. Photophysical properties of novel dyes measured in dichloromethane.
200
300
400
500
600
700
800
Dye
λabs
[nm]
570
569
571
582
549
606
541
λem
[nm]
601
605
602
619
573
624
582
εmax
[M-1cm-1]
25500
17200
17300
16000
17500
59400
18200
Φ
ΔS
[cm-1]
900
1100
900
1000
800
500
τ [ns]
kr,
[107s-1]
5.9
6.3
7.3
6.7
5.6
8.9
7.9
wavelength [nm]
[%]
48a
49a
47a
15a
13b
20c
57b
2
4
6
8.21
7.78
6.45
2.25
2.33
2.24
7.25
Figure 2. Experimental absorption and fluorescence spectra of
2 in DCM.
Calculated transition energies from the ground S0 to the electronically excited
states are given by vertical solid lines with the heights proportional to the
oscillator strength of the transition (see Table S3 for the details). Vibronic
structure is smoothed by low frequency vibrations (for interpretation see Fig. S6).
8
10
14
16
FC
1300
25000
20000
15000
10000
5000
0
S2
aCalculated with respect to Rh101 in MeOH; bcalculated with respect to Rh6G in
EtOH; ccalculated with respect to SRh101 in EtOH.
T2
FC
S1
S1
Fluorescence responses for the majority of new dyes are rather
high (Table 2). Fluorescence quantum yields (Φ) and λem are
modestly influenced by the solvent polarity (Table S2). In the
exemplary case of 2, Φ changes from 56% in toluene to 28% in
methanol (Table 3). This outcome is due to their moderately
polarized electronic structure.
LUMO
T1
FC
S0
S0
HOMO
Table 3. Optical properties of 2 in various solvents.
ΔS [cm-1]
500
Φa, %
-
Figure 3. Energy diagram with the transition energies for absorption from the
ground S0 state to the S1FC and fluorescence from the relaxed S1 state to S0FC for 2.
On the right - the orbitals HOMO and LUMO of 2. Electronic configuration
Solvent
hexane
λabs [nm]
566
λem [nm]
581
HOMO
→LUMO describes the transition S0→S1.
toluene
DCM
573
570
566
565
599
601
603
617
800
900
56
To elucidate the influence of the solvent polarity on the
absorption and fluorescence spectra, we calculated dipole
moments for
comparison of HOMO and LUMO orbitals clearly depicts the
charge shift in the excited S1 state (Figure 3). This shift causes
the increase of the dipole moment (for ca. 1.5-2.2 D) and the
bathochromic shift of both absorption and emission maxima in
the polar solvents (Table S5).
48
42
28
ACN
1100
1500
2 in the isolated form as well as in solution. The
MeOH
aCalculated with respect to Rh101 in MeOH.
To rationalize these spectra, we performed additional
quantum chemistry calculations for dye , which concurred with
2
experimental data (Figure 2). According to our findings, the
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The performed calculations also provide information on the initiated by mild Lewis acids. Strong emissionVioewf Arreticdle Olinglihnet
DOI: 10.1039/C7CC07310K
origin of the remaining non-radiative depopulation channel. We combined with straightforward synthesis, relatively small
found the excited S1 state is located about 9500 cm-1 above the molecular mass, and potential chemical reactivity may open
triplet T1 and 2500 cm-1 below the T2 states. Such an energetic doors for these dyes in applications in fluorescence microscopy
situation precludes efficient intersystem crossing pathway for (bioimaging).
the S1
→
T1 depopulation. Thus, the main non-radiative
The financial support of the Polish National Science Centre
depopulation channel of the S1 state should proceed via internal (MAESTRO-2012/06/A/ST5/00216), Foundation for Polish
conversion. This result concurs with our unsuccessful effort to Science and and by the Global Research Laboratory Program
detect phosphorescence emission at 5 K.
(2014K1A1A2064569) through the National Research
Our calculations also indicate that possesses a planar core Foundation (NRF) funded by the Ministry of Science, ICT &
2
with the peripheral thiophene ring rotated around a single bond Future Planning (Korea) is cordially acknowledged.
by around 25˚ with respect to the molecular plane (Table S4).
Thus, new heterocycles are planar, structurally rigid
fluorophores, the structure of which does not change
Notes and references
substantially upon electronic excitation. As a consequence of
the small conformational changes following the transition
between S0 and S1 states in these compounds (additional
computational data in Table S6) the Stokes’ shifts are small.
To verify the potential use of discovered dyes for optoelectronic
applications we performed a series of cyclic voltammetry (CV)
measurements, which depict the influence of structural
changes on their susceptibility towards oxidation and reduction.
In the majority of cases, the investigated molecules show
reversible reduction and quasi-reversible oxidation waves
(Table 4, Figure S5). The exception to the rule is observed only
for 10 (irreversible oxidation). It is noteworthy that no changes
were perceived with increasing number of redox cycles and the
direction of the CV-measurement.
1
2
3
4
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dichloromethane.a
Eoxpa, V
0.90
IP, eV
-5.1
Ered1/2, V Eredonset, V
EA, eV
-3.3
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2
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-1.10
-1.15
-1.17
-1.15
-1.20b
-0.93
-1.28
-1.02
-1.06
-1.07
-1.03
-1.05
-0.81
-1.19
4
6
0.96
0.86
0.92
0.94
0.89
0.80
-5.1
-5.1
-5.1
-5.2
-5.1
-5.0
-3.3
-3.3
-3.3
-3.3
-3.5
-3.2
8
10
14
16
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sweep rate: 100 mVs-1, working electrode: glassy carbon (GC); auxiliary electrode:
Pt wire; reference electrode: Ag/AgCl; all measurements were conducted at room
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We calculated both ionization potential (IP) and electron
affinity (EA) values from the corresponding onset potentials.
The results clearly indicate that peripheral substituents, as well
as the electronic nature of the aromatic rings participating in
the rearrangement, have significant influence neither on the
ionic potential (-5.1±0.1 eV) nor on the electron affinity (-
3.3±0.2 eV).
7
8
Herein, we have introduced structurally unique dyes
possessing an isoindolodione core, linearly fused with a
thiophene, furan, or benzene ring. We have demonstrated that
diketopyrrolopyrroles possessing moderately electron-rich aryl
substituents undergo heretofore unknown rearrangements
4 | J. Name., 2012, 00, 1-3
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