Strong two-photon absorption enhancement in a unique bis-porphyrin bearing a diketopyrrolopyrrole unit

Article abstract of DOI:10.1039/c3cc44728f

A π-extended bis-porphyrin bridged via a diketopyrrolopyrrole unit was prepared in 5 steps. This fully conjugated π-system displays strongly distorted linear absorption, while its two-photon absorption cross-section reaches 2500-3000 GM at 940 nm. LC-like behaviour, easy orientation and low viscosity are, according to XRD, POM and DSC measurements, due to formation of plastic rather than a liquid crystal.

Full text of DOI:10.1039/c3cc44728f

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Strong two-photon absorption enhancement in a
unique bis-porphyrin bearing a diketopyrrolopyrrole
Cite this: DOI: 10.1039/c3cc44728f
unit†
Received 24th June 2013,
Accepted 17th July 2013
a
a
b
´
Agnieszka Nowak-Krol, Marek Grzybowski, Jerzy Romiszewski,
Mikhail Drobizhev,c Geoffrey Wicks,c Maciej Chotkowski,b Aleksander Rebane,*cd
DOI: 10.1039/c3cc44728f
b
a
´
Ewa Gorecka* and Daniel T. Gryko*
www.rsc.org/chemcomm
A p-extended bis-porphyrin bridged via a diketopyrrolopyrrole (DPP) bridge.5 Diketopyrrolopyrroles due to their superb physico-
unit was prepared in 5 steps. This fully conjugated p-system dis- chemical properties are currently some of the most popular
plays strongly distorted linear absorption, while its two-photon building blocks in molecular electronics and in luminescence-
absorption cross-section reaches 2500–3000 GM at 940 nm. LC-like based studies.6 We envisioned that an electron-poor DPP core
behaviour, easy orientation and low viscosity are, according to can serve as an acceptor unit in quadruple donor–acceptor–
XRD, POM and DSC measurements, due to formation of plastic donor (D–A–D) type dyes exhibiting high two-photon absorption
rather than a liquid crystal.
cross-sections. To provide a reasonable solubility of the final
compound, we decorated our molecule with long alkyl and glycol
The desired optical properties of p-extended porphyrins are chains.
often achieved via preparation of dyes comprised of multiple
The hybrid molecule Por2–DPP was prepared by using a
porphyrin units.1 The bridging groups enhance direct conjuga- convergent strategy. DPP and porphyrin building blocks were
tion between essentially coplanar chromophores, often result- prepared independently and then combined in the ultimate
ing in split Soret bands characteristic of extensive excitonic stage of the synthetic pathway. Syntheses of both components
interactions and strongly red shifted Q bands.2 The magnitude involved incorporation of triethyleneglycol chains and it is shown
of the splitting depends however on the type of aromatic unit in detail in ESI.† DPP was prepared from 4-bromobenzonitrile
introduced between porphyrin rings.3 Remarkably, 9,10-diethynyl- following standard procedures.7 The synthesis of porphyrin coun-
anthracene spacer enhances electronic communication between terpart Por-Br from 3,4,5-trihydroxybenzaldehyde was achieved in
meso-linked porphyrins better than butadiyne, 1,4-diethynyl- 4 steps.
benzene or 2,5-diethynylthiophene bridges,4 giving extremely
The last step of the synthesis involves the coupling between
high value of two-photon absorption cross-sections. Therefore, porphyrin Por-Br and DPP-alkyne partner DPP to give Por2–DPP
it is clear that the intervening aromatic groups are put between in 34% yield (Scheme 1). The target molecule proved to be
two porphyrin rings not only as linkers but are often carefully highly soluble in most common organic solvents, such as DCM,
selected to confer specific physical and photophysical proper- chloroform, acetone, ethyl acetate, and hexane.
ties on the final systems.
The analysis of electrochemical data obtained by cyclic
Herein, we would like to present a new molecular design voltammetry reveals that the characteristic oxidation and
when two porphyrin units are linked by a diketopyrrolopyrrole reduction potentials of diketopyrrolopyrroles8 are not visible
in the CV of the dye Por2–DPP. As a matter of fact neither Por
a Institute of Organic Chemistry of the Polish Academy of Sciences, 01-224 Warsaw,
Poland. E-mail: dtgryko@icho.edu.pl; Fax: +48 22 6326681; Tel: +48 22 3433063
b Warsaw University, Faculty of Chemistry, 00-273 Warsaw, Poland.
Fax: +48 22 6282741; Tel: +48 22 2345801
c Department of Physics, Montana State University, Bozeman, MT 59717, USA.
E-mail: rebane@physics.montana.edu; Fax: +1 406 994 4452;
Tel: +1 406 994 7831
d National Institute of Chemical Physics and Biophysics, Tallinn 12618, Estonia.
Fax: +372 670 3662; Tel: +372 639 8300
nor Por2–DPP has any distinctive redox potentials (Fig. 1). This
strongly suggests that the p-expanded bis-porphyrin Por2–DPP
is fully conjugated and the DPP core does not maintain its
individual character.
The 1PA spectra of Por, DPP, and Por2–DPP are presented in
Fig. 2. The spectrum of Por is typical for a porphyrin, showing
an intense Soret band at shorter wavelengths and a relatively
weak lowest-energy Q band. The absorption spectrum of DPP
shows a relatively intense broad band (lmax = 484 nm). Both
Por and DPP are fluorescent with Stokes shifts of 618 and
2446 cmÀ1 for Por, and DPP, respectively. The Por2–DPP dye
† Electronic supplementary information (ESI) available: Full synthetic and analy-
tical data of compounds DPP, Por-Br, Por, Por2–DPP and intermediates and their
synthesis as well as copies of 1H NMR and 1C NMR spectra. See DOI: 10.1039/
c3cc44728f
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Fig. 2 Absorption spectra of the conjugate Por2–DPP (solid line), aminoporphyrin
Por (dashed line) and the DPP dye (dash-dot line) recorded in CHCl3. Normalized
absorption of Por and Por2–DPP in the Q-band region is shown in the inset.
compared to the monomer Por and an increase in oscillator
strength of the Q band relative to the B band. A characteristic,
broad band originating from the DPP fragment is not visible in
the spectrum of Por2–DPP, which further proves that the new
p-conjugated system formed includes the whole molecule. The
comparison with related mono-porphyrin–mono-DPPs5 reveals
that Por2–DPP possesses appreciably stronger and bathochro-
mically shifted absorption.
Fig. 3 shows the 2PA spectrum of Por2–DPP measured in
CDCl3 solution using the fluorescence excitation method,9 and
Rhodamine B and Styryl 9M as 2PA reference standards.10
A
linear absorption spectrum in CHCl3 (solid line) is shown for
comparison. Deuterated chloroform was used for 2PA measure-
ments to avoid artifacts caused by absorption of the solvent in
the near IR range of wavelengths. As is typical for most
tetrapyrroles, the two-photon spectra may be divided into a
Q-band region (1100–1500 nm) and a Soret region (o1100 nm).
In the Q-band region, the two-photon transitions coincide
with the corresponding bands observed in the 1PA spectrum.
However, peak 2PA cross section values remain relatively low,
s2 o 103 GM, which is typical for most porphyrinoids including
Scheme 1 Synthesis of the target molecule.
Fig. 1 Cyclic voltammetry of dye DPP (red), aminoporphyrin Por (green) and the
conjugate Por2–DPP (black).
also displays fluorescence at lmax = 710 nm. The spectrum of
Por2–DPP indicates interactions between the corresponding
moieties resulting in red-shift of both Soret and Q bands
Fig. 3 2PA spectrum of Por2–DPP in CDCl3 (symbols). Linear absorption spec-
trum in CHCl3 (solid line) is shown for comparison. The shaded area at l o 850
corresponds to wavelengths with increasing contribution from linear absorption.
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extended conjugated porphyrins. The strong 2PA band in the Research Grant No. FA9550-09-1-0219. J.R. acknowledges support
Q-band region with a peak value of s2 B 3000 GM appears at of the Foundation of Polish Science MPD Program cofinanced by
940 nm. The fact that this peak has no obvious counterpart in the EU European Regional Development Fund.
the 1PA spectrum may be explained by spectral broadening that
masks weaker transitions. In the high energy end of the Soret
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Por2–DPP exhibits properties that place it at the border
between crystalline and liquid-crystalline materials. The crystal
melts to the isotropic phase at 168 1C with a relatively low
transition enthalpy of 5.58 J gÀ1 (22.5 kJ molÀ1), indicating
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enthalpy, the iso-crystal phase transition is characterised by
large hysteresis. Upon cooling the crystal phase appears from
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tions (Fig. S1, ESI†), similar to that observed for highly ordered
columnar liquid crystals. The crystal formed is very soft and can
be easily deformed; upon weak shearing nearly uniform bire-
fringent texture is obtained (Fig. S2, ESI†). The XRD measure-
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stretching vibration (1670 cmÀ1) with largest absorption along
the shear direction. Vibration at 1600 cmÀ1 ascribed to CQC
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maximum absorption perpendicular to the shear direction
(Fig. S4, ESI†). Optical retardation measurements show that a
slow optical axis is oriented perpendicular to the shear direc-
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In conclusion, we have demonstrated that a novel porphyrin–
diketopyrrolopyrrole dye with D–A–D structure possesses large
intrinsic 2PA in the near-IR range of wavelengths, with maxi-
mum values of s2 B 3000 GM. The introduction of peripheral
alkyl chains proved to be an excellent strategy not only to achieve
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c
This journal is The Royal Society of Chemistry 2013
Chem. Commun.