Quadrupolar, emission-tunable π-expanded 1,4-dihydropyrrolo[3,2-b] pyrroles-synthesis and optical properties

Article abstract of DOI:10.1039/c4ob00143e

The synthesis and optical characterization of six novel heteroaromatic-based chromophores is described. The new dyes present mostly an A-D-A general framework, where A is an electron-deficient aromatic ring and D is an electron-rich pyrrolo[3,2-b]pyrrole moiety, linked via triple bonds. It was demonstrated that the increase in the molecular length of the chromophore effectively extends π-conjugation. The effect of structural variations on photophysical properties was studied in detail for these compounds and the relationship between the structure and photophysical properties was thoroughly elucidated by comparison with simpler tetraaryl-analogues. The strong charge-transfer characteristic of these functional dyes can be illustrated by large Stokes shifts (4100-7100 cm^-1) for A-D-A architectures. The replacement of phenyl rings at positions 2 and 5 with the arylethynylaryl substituents bathochromically shifts both absorption and emission at ca. 50-150 nm. The clear dependence of fluorescence maxima on the electron-accepting property of the peripheral arylethynyl substituent emphasizes strong π-conjugation in these molecules. The donor-acceptor interactions were also found to influence the two-photon absorption properties. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4ob00143e

Volume 12 Number 18 14 May 2014 Pages 2813–2980
Organic &
Biomolecular
Chemistry
www.rsc.org/obc
ISSN 1477-0520
PAPER
Daniel T. Gryko et al.
Quadrupolar, emission-tunable π-expanded 1,4-dihydropyrrolo[3,2-b]-
pyrroles – synthesis and optical properties

Organic &
Biomolecular Chemistry
View Article Online
View Journal | View Issue
PAPER
Quadrupolar, emission-tunable π-expanded
1,4-dihydropyrrolo[3,2-b]pyrroles – synthesis
and optical properties†
Cite this: Org. Biomol. Chem., 2014,
12, 2874
Anita Janiga,a Dominika Bednarska,a Bjarne Thorsted,b Jonathan Brewerb and
Daniel T. Gryko*a,c
The synthesis and optical characterization of six novel heteroaromatic-based chromophores is described.
The new dyes present mostly an A–D–A general framework, where A is an electron-deficient aromatic
ring and D is an electron-rich pyrrolo[3,2-b]pyrrole moiety, linked via triple bonds. It was demonstrated
that the increase in the molecular length of the chromophore effectively extends π-conjugation. The
effect of structural variations on photophysical properties was studied in detail for these compounds
and the relationship between the structure and photophysical properties was thoroughly elucidated
by comparison with simpler tetraaryl-analogues. The strong charge-transfer characteristic of these
functional dyes can be illustrated by large Stokes shifts (4100–7100 cm−1) for A–D–A architectures. The
replacement of phenyl rings at positions 2 and 5 with the arylethynylaryl substituents bathochromically
shifts both absorption and emission at ca. 50–150 nm. The clear dependence of fluorescence maxima on
the electron-accepting property of the peripheral arylethynyl substituent emphasizes strong π-conju-
gation in these molecules. The donor–acceptor interactions were also found to influence the two-
photon absorption properties.
Received 19th January 2014,
Accepted 18th February 2014
DOI: 10.1039/c4ob00143e
www.rsc.org/obc
particular in fluorescence microscopy. Although classical fluo-
rescent microscopy still plays a critical role, two-photon excited
class of 10π-electron aromatic fluorescence (TPEF) microscopy can provide better contrast,
Introduction
Heteropentalenes1 are
a
compounds and 1,4-dihydropyrrolo[3,2-b]pyrrole is the least brighter images and greater detail compared to classical fluo-
studied member of this family.2 In contrast to the well rescent microscopy.4,5 This originates from the fact that the
explored thieno[3,2-b]thiophene, the 1,4-dihydropyrrolo[3,2-b]- two-photon absorption (2PA) scales quadratically with the
pyrrole core is the strongest electron-donor among 10π-elec- intensity of the incident laser radiation, which leads to much
tron systems. The lack of an efficient synthetic method for the higher spatial resolution than could be achieved by one-
preparation of this skeleton has recently been overcome by the photon absorption.
discovery of one-pot domino reaction between aldehydes,
Very recently, it has become clear that, for the two-photon
primary amines and butane-2,3-dione.3 The superb optical technology to realize its full potential, the development of
properties, including intrinsically high fluorescence quantum more two-photon-active chromophores that also possess other
yields, combined with the straightforward character of the useful optical or chemical properties, such as high fluo-
synthesis, make 1,4-dihydropyrrolo[3,2-b]pyrrole the core unit rescence quantum yields, easy processability, good photo-
of choice for application in various areas of photonics, in stability, and durability, will play
a
vital role.4–8 The
breakthrough study by Brédas, Marder, Perry and co-workers9
has revealed that molecules possessing the quadrupolar struc-
ture (D–A–D, A–D–A) usually possess higher 2PA cross-sections
(σ2) than their dipolar analogs. This is due to the fact that the
presence of a donor as the core induces charge transfer from
the center to the periphery of the molecule, and hence enables
the molecule to display large σ2 values. Needless to say, the
stronger the donor/acceptor ability of the corresponding moi-
eties, the higher the values of σ2.4,5 We envisioned that the
conjugated π-system could be enlarged by two arylethynyl
aInstitute of Organic Chemistry, Polish Academy of Sciences, Kasprzaka, 44/52
01-224, Warsaw, Poland. E-mail: dtgryko@icho.edu.pl; Fax: +48 22 632 66 81;
Tel: +48 22 343 30 63
bDepartment of Biochemistry and Molecular Biology, University of Southern Denmark,
Odense 200-701, Denmark. Tel: +45 6550 3505
cWarsaw University of Technology, Faculty of Chemistry, Noakowskiego 3, 00-664
Warsaw, Poland. Fax: +48 22 628 27 41; Tel: +48 22 234 58 01
†Electronic supplementary information (ESI) available: 1H NMR, 13C
NMR, absorption and emission spectra of compounds 4–9. See DOI:
10.1039/c4ob00143e
2874 | Org. Biomol. Chem., 2014, 12, 2874–2881
This journal is © The Royal Society of Chemistry 2014

View Article Online
Organic & Biomolecular Chemistry
Paper
units at each side of the 1,4-dihydropyrrolo[3,2-b]pyrrole core.
The intrinsically high electron-donating ability of the pyrrolo-
[3,2-b]pyrrole core prompted us to perform the study oriented
towards the synthesis of A–D–A molecules based on this
skeleton.
Table 1 Optimisation of the Sonogashira reaction leading to com-
pound 4
Entry Reaction conditions
% Yield
1
2
Pd(PPh3)4, CuI, Et3N THF, 70 °C, 16 h
Pd(PhCN)2Cl2, P(t-Bu)3, CuI, HN(i-Pr)2,
dioxane, rt, 16 h
24
0
3
4
5
6
Pd2dba3, AsPh3, Et3N, toluene, rt/80 °C, 24 h
Cs2CO3, CuI, DABCO, dioxane, 135 °C, 16 h
Pd2(dba)3, P(t-Bu)3, Et3N, THF, rt, 16 h
20
0
9
Results and discussion
Cs2CO3, CuI, phenanthroline, dioxane, 135 °C, 16 h 10
Our design implied decoration of the pyrrolo[3,2-b]pyrrole
skeleton with electron-withdrawing groups linked through C–C
triple bonds. Moreover, synthesis of a derivative with terminal
methoxy groups was planned as to allow for the direct com-
parison between the influence of electron-rich and electron-
poor substituents on optical properties of the chromophore.
We reasoned that due to the small dihedral angles between
the phenyl ring and the core in analogous to 2-phenylindoles
(∼30°)10 the conjugation between the three parts of the desired
dyes will be strong. 1,4-Di(4-methylphenyl)-2,5-di(alkynyl-
phenyl)-1,4-dihydropyrrolo[3,2-b]pyrrole (3) was prepared in
two steps (Scheme 1). Since the Sonogashira coupling is the
preferred method leading to derivatives of diarylacetylenes11 to
obtain the designed compounds we decided to synthesize an
electron-rich heterocycle bearing two terminal acetylenes and
subsequently react it with a range of aryl bromides and aryl
iodides. The three component condensation of 4-trimethylsilyl-
ethynylbenzaldehyde (1),12 4-methylaniline and butane-2,3-
dione in boiling glacial acetic acid produced the derivative 1,4-
dihydropyrrolo[3,2-b]pyrrole (2) with 15% yield. Deprotection
of the TMS group by reaction with tetrabutylammonium
fluoride (TBAF) gave product 3, which served as a platform for
planned modifications. The Sonogashira coupling between
alkyne 3 and 4-bromobenzonitrile or 4-iodobenzonitrile has
been chosen as a model system for the optimization studies
(Scheme 1, Table 1). In spite of using diverse and modern pro-
tocols13 we could not reach a higher yield of product 4 than
24%.
Looking for new strategies we directed our research efforts
towards the sila-Sonogashira coupling.14 The combination of
both deprotection and coupling in one step, under conditions
described by Henze et al.15 allowed us to obtain compound 4
in higher yield with concomitant shortening of the whole pro-
cedure (Table 2). Encouraged by this finding, we subsequently
investigated whether this synthetic route is applicable for
other iodoarenes and bromoarenes.
Subjecting the mixture of 2,6-di(4-(trimethylsilylethynyl)-
phenyl)-1,5-di(4-methylphenyl)-1,4-dihydropyrrolo[3,2-b]pyrrole
(2) and aryl halides bearing various substituents to the sila-
Sonogashira coupling conditions gave products 4–8 in yields
ranging from 15 to 56%. It is noteworthy that although reac-
tion proceeds for both electron-poor and electron-rich halides,
4-bromobenzonitrile was the most efficient substrate in
this reaction. Still, compound 8 could only be obtained
when the process was performed with 4-iodoanisole rather
Scheme 1
This journal is © The Royal Society of Chemistry 2014
Org. Biomol. Chem., 2014, 12, 2874–2881 | 2875

View Article Online
Paper
Organic & Biomolecular Chemistry
Table 2 Results of the one-pot sila-Sonogashira coupling reaction
Fig. 1 Structures of pyrrolo[3,2-b]pyrroles 10 and 11.
Entry
1
Aryl halide
Product no.
Yield (%)
56
4
2
3
4
5
6
7
21
15
33
Fig. 2 Absorption (—) and normalized fluorescence (- - -) spectra of 4
in CH2Cl2.
than 4-bromoanisole (entries 5 and 6, Table 2). Interestingly,
reaction of 4-bromobenzaldehyde with compound 2 led to a
broad range of side products which impeded the purification
process and caused a loss in the overall yield of derivative 9.
This unsatisfactory result prompted us to change the reaction
conditions and employ the standard Sonogashira coupling
(see Table 1, entry 1); this increased the yield of the product 9
to 56%. Spectral characteristics of products 4–9 were then
examined and compared to those of parent compounds 10 and
113 (Fig. 1–3, Table 3). The absorption spectra of these dyes
have broad bands located between 350 and 500 nm (Fig. 2 and
3). The molar absorption coefficients are in the range
50 000–70 000 regardless of the electronic character of the substitu-
ents, i.e. they are significantly higher than for previously reported
tetraaryl-1,4-dihydropyrrolo[3,2-b]pyrroles.3 The characteristic feature
5
6
7
8
8
9
0
30
Tracesa
a Due to the unsatisfactory result we decided to change reaction
conditions and employ the standard Sonogashira reaction conditions
(see Table 1, entry 1) which increased the yield of the product 9 to
56%.
2876 | Org. Biomol. Chem., 2014, 12, 2874–2881
This journal is © The Royal Society of Chemistry 2014

View Article Online
Organic & Biomolecular Chemistry
Paper
Fig. 3 Absorption spectra of 5 (—), 7 (- - -), and 8 (- · -) in CH2Cl2.
Fig. 4 Two-photon absorption spectra of 4 (black dots), 6 (black dia-
monds), 7 (red squares), and 10 (green triangles) in CH2Cl2.
Table 3 Photophysical data for 4–11 in CH2Cl2
a
Product
no.
λabs
nm
/
λem
nm
/
Фfl
σ2 (720 nm)
(GM)
Stokes
(%)
shift (cm−1
)
4
5
6
7
428
418
414
421
401
434
406
368
549
523
511
522
479
629
461
462
22
16
42
37
53
2
250
5
5200
4800
4600
4600
4100
7100
2900
5500
800
500
100
300
260
25
8
9
10b
11b
88
17
a Determined with quinine sulphate in H2SO4 (0.5 M) as a standard.
b Ref. 3.
was the bathochromic shift of absorption when going from
compounds 10–11 to dyes 4–9 (∼50 nm). This strong batho-
chromic shift proves the existence of electronic communi-
cation in the ground state throughout the whole molecule. In
typical low viscosity solvents such as CH2Cl2, the rotation of
the benzene ring along the linker is not disturbed, and the
coplanarity between the substituted phenyl ring and the
pyrrolo[3,2-b]pyrrole core is attained.
Fig. 5 Two-photon absorption spectra of
squares), 5 (open triangles) and 11 (black triangles) in CH2Cl2.
8 (black dots), 9 (red
bathochromic shift of absorption and emission as expected for
push–pull chromophores.16 The large Stokes shift corresponds
to a significant change of geometry between ground and
excited states. Dyes 4–9 compare very favorably with quadru-
polar dyes of similar complexity.17
All of the π-expanded pyrrolo[3,2-b]pyrroles are fluorescent.
The color of fluorescence of 4–9 ranges from blue, turquoise to
orange. Fluorescence maxima, except for compound 8, are
fl
max
shifted above 500 nm. The fluorescence maxima (λ ) of the
Two-photon absorption has been measured using TPEF
and results are shown in Table 3 and in Fig. 4 and 5. The
measurements were conducted in the 700–1020 nm range
(corresponding to the relevant biological spectral window).
In the spectral range of interest, two-photon absorption
cross-sections are not strongly dependent on the magnitude
of charge-transfer. Indeed, the strongly electron-donating
4-methoxyphenyl substituent gave relatively intense 2PA signal.
Interestingly two-photon absorption of these quadrupolar
molecules is typically within 100–800 GM at 720 nm.
series 4–9 are gradually red-shifted upon increasing the
strength of the electron-withdrawing group. The strong
relationship between the electron-withdrawing strength of the
fl
max
substituent and the λ
supports the existence of strong elec-
tronic communication in these molecules in the excited state.
Fluorescence quantum yields of products 4–8 were found to be
moderate to good (Фfl = 16–53%) and the highest fluorescence
quantum yield (53%) was measured for 8. Product 9 had the
lowest quantum yield (2%), which is quite typical for alde-
hydes. The fluorescence quantum yield and Stokes shift of 6
are similar to those of compound 7, while dialdehyde 9
possesses very high Stokes shift (7000 cm−1). The stronger
electron-withdrawing substituents in 9 (CHO) and 4 (CN)
compared to those in 6 and 7 (CF3) confer a further
Indeed the highest value in the NIR region of the spectrum
can be attributed to compound 6 possessing two 4-trifluoro-
methyl substituents while strongly electron-withdrawing 4 and
9 have lower σ2 (Table 3, Fig. 4). On the other hand, it is clear
This journal is © The Royal Society of Chemistry 2014
Org. Biomol. Chem., 2014, 12, 2874–2881 | 2877

View Article Online
Paper
Organic & Biomolecular Chemistry
that tetraaryl-derivative 10 possessing two 4-cyanophenyl sub- Synthesis
stituents at positions 2 and 5 also behaves like the A–D–A
1,4-Di-(4-methylphenyl)-2,5-bis(4((trimethylsilyl)ethynyl)-
phenyl)-1,4-dihydropyrrolo[3,2-b]pyrrole (2). 4-Methylaniline
(1.27 g, 12 mmol) and 4-(trimethylsilylethynyl)benzaldehyde
(2.4 g, 12 mmol) were stirred in glacial acetic acid (10 mL) at
100 °C for 30 min. Then butane-2,3-dione (519 μl, 6 mmol)
was added and the resulting mixture was stirred at 100 °C for
3 h. After cooling the precipitate was filtered off and washed
with glacial acetic acid. Recrystallization from EtOAc afforded
the pure product as a yellow solid in 15% yield (567 mg).
Rf = 0.78 (SiO2, hexane–CH2Cl2, 1 : 1). Mp 314.0–314.3 °C
(decomposition). 1H NMR (500 MHz, CDCl3) δ 7.26 (AA′XX′,
4H), 7.14 (m, 12H), 6.36 (s, 2H), 2.37 (s, 6H), 0.23 (s, 18H); 13C
NMR (125 MHz, CDCl3) δ 137.0, 135.9, 132.0 (2), 130.0, 129.4,
129.1, 128.2, 127.8 (2), 125.3, 106.6, 94.9, 21.2, 0.14. HRMS
(EI+) calcd for C42H42N2Si2: 630.2905 [M+], found: 630.2905.
λabs (CH2Cl2, ε × 10−3) 393 (60) nm.
system (σ2 ∼300 GM). Eventually, the critical figure of merit
i.e. two-photon brightness is the highest for dye 6 (∼340 GM).
As illustrated in Fig. 4 and 5 for compounds 4–11, the lowest
(strongly one-photon allowed) excited state is moderately inten-
sive, and the higher (only weakly one-photon allowed) state,
responsible for the large 2PA response, is located below
700 nm. This feature is reminiscent of the behavior of
symmetrical quadrupolar derivatives.18
Conclusions
In summary, the first examples of π-expanded pyrrolo[3,2-b]-
pyrroles have been described.22 It has been proven that only
‘sila version’ of the Sonogashira coupling is an efficient
method to assemble these molecules. The proposed synthetic
method is operationally simple and leads to new functional
dyes possessing strong absorption in the violet-blue region
combined with reasonably intensive blue-green fluorescence.
In sharp contrast to previously described tetraaryl-pyrrolo[3,2-b]-
pyrroles both absorption and emission are bathochromically
shifted. Thus, it can be reasonably concluded that the connec-
tion with arylethynylaryl groups at the 2,5-positions of pyrrolo-
[3,2-b]pyrroles extends π-conjugation effectively relative to that
of 1,2,4,5-tetraarylpyrrolo[3,2-b]pyrroles. The most promising
dye bearing two 4-(CF3)C6H4 groups possesses appreciable
two-photon brightness (∼340 GM) while its calculated Stokes
shift remains relatively large (4600 cm−1). A new and efficient
turquoise fluorescence emitter, that is a compound possessing
two 4-methoxyphenylethynylphenyl substituents, was identi-
fied. Our studies illustrate the effect of different substituents
on the electronic properties of the pyrrolo[3,2-b]pyrroles and
reveal that both absorption and emission can be tuned by
straightforward structural manipulations. These results are not
2,5-Bis(4-ethynylphenyl)-1,4-di-(4-methylphenyl)-1,4-dihydro-
pyrrolo[3,2-b]pyrrole (3). TMS protected derivative 2 (567 mg,
0.90 mmol) was dissolved in THF (5 ml) and TBAF (588 mg,
2.25 mmol) was added. The reaction mixture was stirred for
3 h at rt. The solvent was evaporated and the crude product
was recrystallized from EtOAc. The pure product was obtained
as a yellow solid in 96% yield (420 mg). Rf = 0.67 (SiO2,
1
hexane–CH2Cl2, 1 : 1). Mp 290.0–290.3 °C (decomposition). H
NMR (500 MHz, CDCl3) δ 7.33 (AA′XX′, 4H), 7.16 (m, 8H), 7.14
(AA′XX′, 4H), 6.39 (s, 2H), 3.07 (s, 2H), 2.38 (s, 6H); 13C NMR
(125 MHz, CDCl3) δ 137.4, 135.9, 135.6, 134.2, 132.6, 132.1,
130.0, 127.8, 125.3, 119.5, 95.0, 84.0, 77.6, 21.2. HRMS (EI+)
calcd for C36H26N2: 486.2096 [M+], found: 486.2100. Anal.
calcd for C36H26N2: C, 88.86; H, 5.39; N, 5.76; found: C, 88.70;
H, 5.31; N, 5.77. λabs (CH2Cl2, ε × 10−3) 387 (53) nm.
Representative procedure for the sila-Sonogashira coupling
2,5-Di(4-(4-cyanophenylethynyl)phenyl)-1,4-di(4-methylphenyl)-
only of theoretical significance in that they provide insight 1,4-dihydropyrrolo[3,2-b]pyrrole (4). The mixture of 1,4-di-(4-
into factors influencing the electronic structure of extended methylphenyl)-2,5-bis(4-((trimethylsilyl)ethynyl)phenyl)-1,4-
π-systems, but they may also open doors to practical appli- dihydropyrrolo[3,2-b]pyrrole (2, 1.20 mg, 3.17 × 10−5 mol),
cations in such diverse areas as molecular electronics and fluo- PdCl2(PPh3)2 (2.2 mg, 3.17 × 10−6 mol), CuI (0.6 mg, 3.17 ×
rescence imaging.
10−6 mol) and 4-bromobenzonitrile (12 mg, 6.317 × 10−5 mol)
in dry THF (0.5 ml) with Et3N (0.5 ml, 3.6 mmol) was deoxy-
genated by freeze–pump–thaw cycles and purged with argon gas
in an oven dried Schlenk flask. TBAF (21 mg, 7.92 × 10−6 mol)
was added, and the reaction mixture was stirred for 16 h at rt
under an argon atmosphere. The crude mixture was filtered
Experimental section
All chemicals were used as received unless otherwise noted. through celite and the solvent was distilled off. Purification
Reagent grade solvents (CH2Cl2, hexanes) were distilled prior using the DCVC method (SiO2, hexane–CH2Cl2, 4 : 1) afforded
to use. All reported NMR spectra were recorded on 400 MHz, the pure product as a yellow solid in 56% yield (12 mg). Rf =
500 MHz and 600 MHz spectrometers. UV-vis absorption and 0.27 (SiO2, hexane–CH2Cl2, 1 : 1). Mp 313.2–313.5 °C
fluorescent spectra were recorded in CH2Cl2. Chromatography (decomposition). 1H NMR (500 MHz, CDCl3) δ 7.61 (AA′XX′,
was performed on silica (200–400 mesh) and dry column 4H), 7.57 (AA′XX′, 4H), 7.38 (AA′XX′, 4H), 7.21 (AA′XX′, 4H),
vacuum chromatography (DCVC) was performed on prepara- 7.19 (m, 8H), 6.42 (s, 2H), 2.39 (s, 6H); 13C NMR (125 MHz,
tive thin layer chromatography silica (Merck 107747). Mass CDCl3) δ 137.5, 136.1, 135.7, 134.5, 132.9, 132.2, 132.1, 131.8,
spectra were obtained via EI-MS and FD-MS.
130.1, 128.5, 127.9, 125.4, 119.6, 118.6, 111.5, 95.2, 88.5, 21.2.
2878 | Org. Biomol. Chem., 2014, 12, 2874–2881
This journal is © The Royal Society of Chemistry 2014

View Article Online
Organic & Biomolecular Chemistry
Paper
HRMS (EI+) calcd for C50H32N4: 688.2627 [M+], found: crude mixture was filtered through celite and the solvent was
688.2596. λabs (CH2Cl2, ε × 10−3) 428 (70) nm.
distilled off. Purification using the DCVC method (SiO2,
2,5-Di(4-(4-pentafluorothiophenyl)ethynylphenyl)-1,4-di- hexane–CH2Cl2, 1 : 1) afforded the pure product as an orange
(4-methylphenyl)-1,4-dihydropyrrolo[3,2-b]pyrrole (5). Purifi- solid in 56% yield (31 mg). Rf = 0.16 (SiO2, hexane–CH2Cl2,
1
cation using the DCVC method (SiO2, hexane–CH2Cl2, 4 : 1) 1 : 1). Mp 283.3–283.6 °C (decomposition). H NMR (500 MHz,
afforded the pure product as a yellow solid in 21% yield CDCl3) δ 10.01 (s, 2H), 7.85 (AA′XX′, 4H), 7.64 (AA′XX′, 4H),
(6 mg). Rf
=
0.72 (SiO2, hexane–CH2Cl2, 1 : 1). Mp 7.40 (AA′XX′, 4H), 7.19 (s, 12H), 6.43 (s, 2H), 2.39 (s, 6H); 13C
207.0–207.3 °C. 1H NMR (500 MHz, CDCl3) δ 7.65 (AA′XX′, 4H), NMR (125 MHz, CDCl3) δ 191.5, 137.4, 136.0, 135.5, 132.1,
7.48 (AA′XX′, 4H), 7.33 (AA′XX′, 4H), 7.14 (AA′XX′, 4H), 131.8 (2), 130.1, 129.7, 127.8 (2), 125.3, 120.0, 110.1, 94.9, 93.9,
7.13–7.08 (m, 8H), 6.36 (s, 2H), 2.32 (s, 6H); 13C NMR 89.5, 29.7, 21.2. HRMS (EI+) calcd for C50H34N2O2: 694.2580
(125 MHz, CDCl3) δ 137.3, 135.9, 135.5, 134.2, 132.6, 131.6, [M+], found: 694.2585. λabs (CHCl3, ε × 103) 434 (30) nm.
131.5, 129.9, 127.7, 127.1, 126.0, 125.2, 119.5, 95.0, 92.7, 87.9,
Two photon fluorescence intensity measurements
29.7, 21.0. HRMS (EI+) calcd for C48H32F10N2S2: 890.1847 [M+],
found: 890.1843. λabs (CH2Cl2, ε × 10−3) 418 (45) nm.
Dichloromethane (CHROMASOLV®, for HPLC, ≥99.9%),
2,5-Di(4-(4-trifluoromethyl)ethynylphenyl)-1,4-di(4-methyl- methanol (CHROMASOLV®, for HPLC, ≥99.9), 5-carboxyfluore-
phenyl)-1,4-dihydropyrrolo[3,2-b]pyrrole (6). Purification using scein (99% (HPLC)), Rhodamine B and Rhodamine 6G were
the DCVC method (SiO2, hexane–CH2Cl2, 6 : 1) afforded the purchased from Sigma-Aldrich, Denmark. The samples were
pure product as an orange solid in 15% yield (4 mg). Rf = 0.76 dissolved in dichloromethane; Rh B and Rh 6G were dissolved
(SiO2, hexane–CH2Cl2, 1 : 1). Mp 346.9–347.2 °C (decompo- in methanol. The fluorescein was dissolved in a CAPS buffer
1
sition). H NMR (400 MHz, CDCl3) δ 7.59 (s, 8H), 7.39 (AA′XX′, (N-cyclohexyl-3-aminopropanesulfonic acid) of pH 11. The con-
4H), 7.19 (m, 12H), 6.42 (s, 2H), 2.38 (s, 6H); 13C NMR centrations of the samples and references were determined by
(125 MHz, CDCl3) δ 137.5, 136.0, 131.9 (2), 131.7, 130.1, 129.8, a dilution series in a spectrophotometer (Perkin Elmer lambda
127.9 (2), 127.3 (2), 125.5 (2), 125.4, 125.4, 123.1, 112.6, 95.1, 35) using quartz cuvettes. The single photon fluorescence
21.2. HRMS (EI+) calcd for C50H32F6N2: 774.2470 [M+], found: emission spectra were measured using a spectrofluorometer
774.2461. λabs (CH2Cl2, ε × 10−3) 414 (62) nm.
(ChronosFD from ISS, Champaign, IL, USA). The two photon
2,5-Di(4-(3,5-di(trifluoromethyl))ethynylphenyl)-1,4-di- excited emission spectra were collected using a custom built
(4-methylphenyl)-1,4-dihydropyrrolo[3,2-b]pyrrole (7). Purifi- multiphoton excitation spectrofluorometer similar to the pre-
cation using the DCVC method (SiO2, hexane–CH2Cl2, 8 : 1) viously used one.19 Briefly, the excitation source was a Ti:Sa
afforded the pure product as a yellow solid in 33% yield laser (HPeMaiTai DeepSee, Spectra Physics, Mountain View,
(10 mg). Rf
= 0.78 (SiO2, hexane–CH2Cl2, 1 : 1). Mp CA). The laser power was controlled using a motorized half-
1
316.6–316.9 °C. H NMR (500 MHz, CDCl3) δ 7.92 (s, 4H), 7.80 wave plate together with a polarizer. The excitation light was
(s, 2H), 7.40 (AA′XX′, 4H), 7.23 (AA′XX′, 4H), 7.20 (m, 8H), 6.45 focused onto the sample using a 60× super long working
(s, 2H), 2.39 (s, 6H); 13C NMR (125 MHz, CDCl3) δ 137.4, 136.1, objective (Nikon). The emission was collected through the
134.6, 132.9, 132.2, 132.0, 131.8, 131.4, 130.1, 127.9, 125.9, objective and passed through a Multiphoton-Emitter HC 680/
125.3, 124.2, 122.0, 119.1, 95.2, 93.3, 87.0, 21.2. HRMS (EI+) SP (AHF analysentechnik AG, Tuebingen, Germany) to a multi-
calcd for C52H30F12N2: 910.2215 [M+], found: 910.2188. λabs mode optical fiber (M200L02S-A, Thorlabs Sweden AB
(CH2Cl2, ε × 10−3) 421 (67) nm.
Goteborg, Sweden). The emission was then sent through a
2,5-Di(4-(methoxy)ethynylphenyl)-1,4-di(4-methylphenyl)-1,4- monochromator (ARC-SP2155, BFi OPTiLAS, Sweden) and the
dihydropyrrolo[3,2-b]pyrrole (8). Purification using the DCVC spectra were imaged using a cooled CCD camera (PIXIS 400B,
method (SiO2, hexane–CH2Cl2, 3 : 1) afforded the pure product Princeton Instruments, New Jersey, USA). A motorized XY
as a yellow solid in 30% yield (7 mg). Rf = 0.44 (silica, hexane– microscope stage (Nikon) was used as a sample holder. The
1
CH2Cl2, 1 : 1). Mp 315.3–315.6 °C. H NMR (500 MHz, CDCl3) laser, laser power, camera and XY stage were controlled using
δ 7.44 (AA′XX′, 4H), 7.35 (AA′XX′, 4H), 7.18 (s, 12H), 6.87 (AA′ ImageJ20 and custom scripts. The relative laser power was
XX′, 4H), 6.40 (s, 2H), 3.82 (s, 6H), 2.38 (s, 6H); 13C NMR monitored using a power meter (PM100D with a S142C head,
(125 MHz, CDCl3) δ 159.7, 137.5, 135.8, 133.1, 133.0, 131.3 (2), Thorlabs Sweden AB Goteborg, Sweden). Calculations were
130.0, 129.2, 128.4, 127.8 (2), 125.8, 125.3, 114.1, 95.0, 55.5, done using the custom Matlab code (MathWorks, Natick,
21.6, 21.2. HRMS (EI+) calcd for C50H38N2O2: 698.2933 [M+], USA). The 2-photon absorption spectra were measured relative
found: 698.2955. λabs (CH2Cl2, ε × 10−3) 401 (71) nm.
to standard fluorophores, with well characterized spectra.21
2,5-Di(4-(formylethynyl)phenyl)-1,4-di(4-methylphenyl)- The measurement of the sample and standard under the same
1,4-dihydropyrrolo[3,2-b]pyrrole (9). The mixture of 4-bromo- conditions allows for correction of changes of the temporal
benzaldehyde (50 mg, 2.71 × 10−4 mol), 2,5-bis(4-ethynyl- pulse profile and spatial beam profile of the laser at different
phenyl)-1,4-di-(4-methylphenyl)-1,4-dihydropyrrolo[3,2-b]pyrrole (3, wavelengths. To eliminate possible artifacts, due to photo-
66 mg, 1.36 × 10−4 mol), Pd(PPh3)4 (7 mg, 6.06 × 10−6 mol), bleaching or linear absorption, we checked that the fluo-
and CuI (2 mg, 1.06 × 10−5 mol) in THF (2 ml) with Et3N rescence signal increased as the square of the excitation inten-
(120 μl, 0.86 mmol) was deoxygenated by freeze–pump–thaw sity at different excitation wavelengths for the different
cycles in a Schlenk flask and stirred at 70 °C for 16 h. The samples. The absolute 2-photon absorption cross-sections
This journal is © The Royal Society of Chemistry 2014
Org. Biomol. Chem., 2014, 12, 2874–2881 | 2879

View Article Online
Paper
Organic & Biomolecular Chemistry
were calculated using a relative fluorescence intensity tech-
nique as described previously.21 Fluorescein in a CAPS buffer
(pH 11) and Rh 6G and Rh B in methanol were used to cali-
brate the system. The measurements for the two photon cross-
sections were carried out at least five times for the different
samples at excitation wavelengths from 700 nm to 1020 nm.
G. Suzukamo, Tetrahedron Lett., 1989, 30, 1655–1656;
(c) S. Lotz, M. Landman, H. Goerls, C. Crause, H. Nienaber
and A. Olivier, Z. Naturforsch., B: Chem. Sci., 2007, 62, 419–
426; (d) B. J. Stokes, H. Dong, B. E. Leslie, A. L. Pumphrey
and T. G. Driver, J. Am. Chem. Soc., 2007, 129, 7500–7501;
(e) T. Kumagai, K. Satake, K. Kidoura and T. Mukai, Tetra-
hedron Lett., 1983, 24, 2275–2278; (f) T. Mukai, A. Konno,
T. Kumagai and K. Satake, Chem. Lett., 1985, 1809–1812;
(g) K. Satake, T. Kumagai and T. Mukai, Chem. Lett., 1984,
2033–2036; (h) T. Kumagai, S. Tanaka and T. Mukai, Tetra-
hedron Lett., 1984, 25, 5669–5672; (i) K. Satake, K. Yano,
M. Fujiwara and M. Kimura, Heterocycles, 1996, 43, 2361–
2365; ( j) K. Satake, D. Nakoge and M. Kimura, Heterocycles,
1998, 48, 433–436; (k) B. P. Murphy, R. W. Glenn, M. Lim,
J. M. Gardlik, S. D. Jones, W. D. Laidig and J. D. Shaffer,
WO, 2005077324 A1 20050825, 2005; (l) M. L. R. Heffernan,
J. M. Dorsey, Q. K. Fang, R. J. Foglesong, S. C. Hopkins,
M. L. Jones, S. W. Jones, C. O. Ogbu, J. B. Perales and
M. Soukri, US, 20080058395 A1 20080306, 2008;
(m) T. A. Hembrough, G. E. Agoston, A. M. Treston and
A. D. Hanson, WO, 2007076055 A2 20070705, 2007;
(n) H. T. Dieck, U. Verfürth, K. Diblitz, J. Ehlers and
G. Fendesak, Chem. Ber., 1989, 122, 129–131;
(o) B. Waltenberger, K. Wiechmann, J. Bauer, P. Markt,
S. M. Noha, G. Wolber, J. M. Rollinger, O. Werz,
D. Schuster and H. Stuppner, J. Med. Chem., 2011, 54,
3163–3174; (p) F. Rörsch, I. Wobst, H. Zettl, M. Schubert-
Zsilavecz, S. Grösch, G. Geisslinger, G. Schneider and
E. Proschak, J. Med. Chem., 2010, 53, 911–915;
(q) S. Tanaka, T. Kumagai, T. Mukai and T. Kobayashi, Bull.
Chem. Soc. Jpn., 1987, 60, 1981–1983.
3 (a) A. Janiga, E. Głodkowska-Mrówka, T. Stokłosa and
D. T. Gryko, Asian J. Org. Chem., 2013, 2, 411–415;
(b) D. T. Gryko and A. Janiga, Novel, strongly fluorescent
heterocyclic functional dyes and the method of their prepa-
ration. Polish Pat., Appl. P.401473, 2012.
4 For recent reviews, see: (a) M. Pawlicki, H. A. Collins,
R. G. Denning and H. L. Anderson, Angew. Chem., Int. Ed.,
2009, 48, 3244–3266; (b) H. M. Kim and B. R. Cho,
Chem. Commun., 2009, 153–164; (c) H. Meier, Angew. Chem.,
Int. Ed., 2005, 44, 2482–2506; (d) G. S. He, L.-S. Tan,
Q. Zheng and P. N. Prasad, Chem. Rev., 2008, 108, 1245–
1330; (e) H. M. Kim and B. R. Cho, Acc. Chem. Res., 2009,
42, 863–872; (f) F. Terenziani, C. Katan, E. Badaeva,
S. Tretiak and M. Blanchard-Desce, Adv. Mater., 2008, 20,
4641–4678.
Acknowledgements
Financial support of our work from the Foundation for Polish
Science (grant number TEAM/2009-4/3) is gratefully acknowl-
edged. The research leading to these results has received
partial funding from the European Community’s Seventh
Framework Programme under the TOPBIO project – grant
agreement no. 264362. JB gratefully acknowledges support
from the Danish Molecular Biomedical Imaging Center
(DaMBIC), and the Carlsberg Foundation.
Notes and references
1 (a) L. S. Fuller, B. Iddon and K. A. Smith, J. Chem. Soc.,
Perkin Trans. 1, 1997, 3465–3470; (b) H. Behringer and
E. Meinetsberger, Liebigs Ann. Chem., 1981, 1729–1750;
(c) H. Behringer and E. Meinetsberger, Tetrahedron Lett.,
1973, 22, 1915–1918; (d) M. Kuhn, F. C. Falk and
J.
Paradies,
Org.
Lett.,
2011,
13,
4100–4103;
(e) V. G. Nenajdenko, V. V. Sumerin, K. Y. Chernichenko
and E. S. Balenkova, Org. Lett., 2004, 6, 3437–3439;
(f) J. T. Henssler, X. Zhang and A. J. Matzger, J. Org. Chem.,
2009, 74, 9112–9119; (g) A. Capan, H. Vaisi, A. C. Goren
and T. Ozturk, Macromolecules, 2012, 45, 8228–8236;
J. Kim, A.-R. Han, J.-H. Seo, J. H. Oh and C. Yang, Chem.
Mater., 2012, 24, 346–3472; (h) J. T. Henssler and
A. J. Matzger, Org. Lett., 2009, 11, 3144–3147; (i) K. Koh,
A. G. Wong-Foy and A. J. Matzger, J. Am. Chem. Soc., 2009,
131, 4184–4185; ( j) A. G. Wong-Foy, A. J. Matzger and
O. M. Yaghi, J. Am. Chem. Soc., 2006, 128, 3494–3495;
(k) I. McCulloch, M. Heeney, M. L. Chabinyc,
D. DeLongchamp, R. J. Kline, M. Coelle, W. Duffy,
D. Fischer, D. Cundloch, B. Hamadani, R. Hamilton,
L. Richter, A. Salleo, M. Shkunov, D. Sporrowe, S. Tierney
and W. Zhong, Adv. Mater., 2009, 21, 1091–1109;
(l) J. Roncali, Chem. Rev., 1992, 92, 711–738;
(m) C. Westerlund and A.-B. Hörnfeldt, Acta Chem. Scand.,
1976, B30, 391–396; (n) H. R. Snyder, L. A. Carpino,
J. F. Zack and J. F. Mills, J. Am. Chem. Soc., 1956, 79, 2556–
2559; (o) D. S. Matteson and H. R. Snyder, J. Org. Chem.,
1957, 22, 1500–1504; (p) S. Srinivarsan and G. B. Schuster,
Org. Lett., 2008, 10, 3657–3660; (q) I. Alkorta, F. Blanco and
J. Elguero, J. Mol. Struct., 2008, 851, 75–83; (r) M. Welch
and R. S. Phillips, Heterocycl. Commun., 1999, 5, 305–310;
(s) F. Garcia and C. Galvez, Synthesis, 1985, 143–156.
5 (a) S. Yao, H.-Y. Ahn, X. Wang, J. Fu, E. W. Van Stryland,
D. J. Hagan and K. D. Belfield, J. Org. Chem., 2010, 75,
3965–3974; (b) M. A. Ramirez, R. Custodio, A. M. Cuadro,
J. Alvarez-Builla, K. Clays, I. Assselberghs, F. Mendicuti,
O. Castaño, J. L. Andrés and J. J. Vaquuero, Org. Biomol.
Chem., 2013, 11, 7145–7154; (c) D. R. Larson, W. R. Zipfel,
R. M. Williams, S. W. Clark, M. P. Bruchez, F. W. Wise and
W. W. Webb, Science, 2003, 300, 1434–1437; (d) L. Ventelon,
S. Charier, L. Moreaux, J. Mertz and M. Blanchard-Desce,
Angew. Chem., Int. Ed., 2001, 40, 2098–2101; (e) E. Maçôas,
2 (a) H. Hemetsberger and D. Knittel, Monatsh. Chem., 1972,
103, 194–204; (b) T. Aratani, H. Yoshihara and
2880 | Org. Biomol. Chem., 2014, 12, 2874–2881
This journal is © The Royal Society of Chemistry 2014

View Article Online
Organic & Biomolecular Chemistry
Paper
G. Marcelo, S. Pinto, T. Cañeque, A. M. Cuadro, 13 (a) J.-F. Nierengarten, S. Zhang, A. Gegout, M. Urbani,
J. J. Vaquero and J. M. G. Martinho, Chem. Commun., 2011,
47, 7374–7376.
6 (a) D. A. Parthenopoulos and P. M. Renzepis, Science, 1989,
245, 843–845; (b) C. Tang, Q. Zheng, H. Zhu, L. Wang,
S.-C. Chen, E. Ma and X. Chen, J. Mater. Chem. C, 2013, 1,
1771–1780.
7 (a) S. Kim, T. Y. Ohulchanskyy, H. E. Pudavar, R. K. Pandey
and P. N. Prasad, J. Am. Chem. Soc., 2007, 129, 2669–2675;
(b) M. Tasior, V. Hugues, M. Blanchard-Desce and
D. T. Gryko, Asian J. Org. Chem., 2013, 2, 669–673;
N. Armaroli, G. Marconi and Y. Rio, J. Org. Chem., 2005, 70,
7550–7557; (b) T. Hundertmark, A. F. Littke, S. L. Buchwald
and G. C. Fu, Org. Lett., 2000, 2, 1729–1731;
(c) R. W. Wagner, Y. Ciringh, C. Clausen and J. S. Lindsey,
Chem. Mater., 1999, 11, 2974–2983; (d) J.-H. Li, J.-L. Li,
D.-P. Wang, S.-F. Pi, Y.-X. Xie, M.-B. Zhang and X.-C. Hu,
J. Org. Chem., 2007, 72, 2053–2057; (e) V. P. W. Böhm and
W. A. Herrmann, Eur. J. Org. Chem., 2000, 3679–3681;
(f) D. Chen, G. Shena and W. Bao, Org. Biomol. Chem.,
2009, 7, 4067–4073.
(c) C. B. Nielsen, J. Arnbjerg, M. Johnsen, M. Jrrgensen and 14 (a) Y. Nishihara, K. Ikegashira, A. Mori and T. Hiyama,
P. R. Ogilby, J. Org. Chem., 2009, 74, 9094–9104;
(d) J. R. Starkey, A. K. Rebane, M. A. Drobizhev, F. Q. Meng,
A. J. Gong, A. Elliott, K. McInnerney and C. W. Spangler,
Clin. Cancer Res., 2008, 14, 6564–6573; (e) H. A. Collins,
M. Khurana, E. H. Moriyama, A. Mariampillai,
E. Dahlstedt, M. Balaz, M. K. Kuimova, M. Drobizhev,
Chem. Lett., 1997, 1233–1234; (b) Y. Nishihara,
K. Ikegashira, K. Hirabayashi, J. Ando, A. Mori and
T. Hiyama, J. Org. Chem., 2000, 65, 1780–1787; (c) C. Yang
and S. P. Nolan, Organometallics, 2002, 21, 1020–1022;
(d) M. Livecchi, G. Calvet and F. Schmidt, J. Org. Chem.,
2012, 77, 5006–5016.
V. X. D. Yang, D. Phillips, A. Rebane, B. C. Wilson and 15 Z.-L. Zhou, L. Zhao, S. Zhang, K. Vincent, S. Lam and
H. L. Anderson, Nat. Photonics, 2008, 2, 420–424; D. Henze, Synth. Commun., 2012, 42, 1622–1631.
(f) M. Gary-Bobo, Y. Mir, C. Rouxel, D. Brevet, I. Basile, 16 (a) F. Würthner, S. Ahmed, C. Thalacker and
M. Maynadier, O. Mongin, M. Blanchard-Desce, A. Morere,
M. Garcia, J.-O. Durand and L. Raehm, Angew. Chem., Int.
Ed., 2011, 50, 11425–11429.
8 (a) Q. Zheng, G. S. He and P. N. Prasad, Chem. Phys. Lett.,
2009, 475, 250–255; (b) I. Kim, D. Kim, S. Sambasivan and
K. Ahn, Asian J. Org. Chem., 2012, 1, 60–64; (c) G. S. He,
G. C. Xu, P. N. Prasad, B. A. Reinhardt, J. C. Bhatt,
T. Debaerdemaeker, Chem.–Eur. J., 2002, 8, 4742–4750;
(b) J. C. Collings, S.-Y. Poon, C. L. Droumaguet, M. Charlot,
C. Katan, L. O. Palsson, A. Beeby, J. A. Mosely,
H. M. Kaiser, D. Kaufmann, W.-Y. Wong, M. Blanchard-
Desce and T. B. Marder, Chem.–Eur. J., 2009, 15, 198–208;
(c) A. J. Zucchero, P. L. McGrier and U. H. F. Bunz, Acc.
Chem. Res., 2010, 43, 397–408.
R. McKellar and A. G. Dillard, Opt. Lett., 1995, 20, 435–437; 17 (a) A. R. Morales, A. Frazer, A. W. Woodward, H.-Y. Ahn-
(d) J. E. Ehrlich, X. L. Wu, I. Y. S. Lee, Z. Y. Hu, H. Rçckel,
S. R. Marder and J. W. Perry, Opt. Lett., 1997, 22, 1843–
1845; (e) M. Charlot, N. Izard, O. Mongin, D. Riehl and
M. Blanchard-Desce, Chem. Phys. Lett., 2006, 417, 297–302;
(f) G. Lemercier, J.-C. Mulatier, C. Martineau, R. Anemian,
C. Andraud, I. Wang, O. Stephan, N. Amari and P. Baldeck,
C. R. Chim., 2005, 8, 1308–1316; (g) A. Nowak-Król,
M. Grzybowski, J. Romiszewski, M. Drobizhev, G. Wicks,
M. Chotkowski, A. Rebane, E. Górecka and D. T. Gryko,
Chem. Commun., 2013, 49, 8368–8370; (h) K. D. Belfield,
M. V. Bondar, A. R. Morales, X. Yue, G. Luchita and
O. V. Przhonska, J. Phys. Chem. C, 2012, 116, 11261–11271.
9 M. Albota, D. Beljonne, J. L. Brédas, J. E. Ehrlich, J. Y. Fu,
A. A. Heikal, S. E. Hess, T. Kogej, M. D. Levin, S. R. Marder,
White, A. Fonari, P. Tongwa, T. Timofeeva and
K. D. Belfield, J. Org. Chem., 2013, 78, 1014–1025;
(b) S. K. Das, C. S. Lim, S. Y. Yang, J. H. Han and B. R. Cho,
Chem. Commun., 2012, 48, 8395–8397; (c) S. K. Bae,
C. H. Heo, D. J. Choi, D. Sen, E.-H. Joe, B. R. Cho and
H. M. Kim, J. Am. Chem. Soc., 2013, 135, 9915–9923;
(d) D. Firmansyah, A. I. Ciuciu, V. Hugues, M. Blanchard-
Desce, L. Flamigni and D. T. Gryko, Chem.–Asian J., 2013, 8,
1279–1294; (e) O. Mongin, L. Porres, M. Charlot, C. Katan
and M. Blanchard-Desce, Chem.–Eur. J., 2007, 13, 1481–
1498.
18 (a) H. M. Kim and B. R. Cho, Chem.–Asian J., 2011, 6,
58–69; (b) M. Barzoukas and M. Blanchard-Desce, J. Chem.
Phys., 2000, 113, 3951–3959.
D. McCord-Maughon, J. W. Perry, H. Rockel, M. Rumi, 19 J. R. Brewer, M. Schiek, I. Wallmann, H.-G. Rubahn,
C. Subramaniam, W. W. Webb, X. L. Wu and C. Xu, Science,
1998, 281, 1653–1656.
M. Schiek, I. Wallmann and H.-G. Rubahn, Opt. Commun.,
2008, 281, 3892–3896.
10 T. Yin and R. Hua, Chem. Lett., 2013, 42, 836–837.
11 (a) K. Sonogashira, Y. Tohda and N. Hagihara, Tetrahedron
20 C. A. Schneider, W. S. Rasband and K. W. Eliceiri, Nat.
Methods, 2012, 9, 671–675.
Lett., 1975, 16, 4467–4470; (b) R. Chinchilla and C. Najera, 21 N. S. Makarov, M. Drobizhev and A. Rebane, Opt. Express,
Chem. Rev., 2007, 107, 874–922. 2008, 16, 4029–4047.
12 Synthesized according to the previously published pro- 22 D. T. Gryko, A. Janiga and M. Krzeszewski, New, strongly
cedure: W. J. Sommer and M. Weck, Adv. Synth. Catal.,
2006, 348, 2101–2113.
fluorescent heterocycles and a method of their preparation,
PCT/PL2013/050025, 2013.
This journal is © The Royal Society of Chemistry 2014
Org. Biomol. Chem., 2014, 12, 2874–2881 | 2881