A2B2-type push-pull porphyrins as reverse saturable and saturable absorbers

Article abstract of DOI:10.1039/b618996b

A₂B₂-type push-pull porphyrins with a strong intramolecular dipole moment have been prepared via Heck and Suzuki coupling reactions as novel materials for use in nonlinear optics (NLO); they display saturable (SA) and reverse saturab

Full text of DOI:10.1039/b618996b

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A2B2-type push–pull porphyrins as reverse saturable and saturable
absorbers{
Eleni G. A. Notaras,a Marijana Fazekas,a James J. Doyle,b Werner J. Blaub and Mathias O. Senge*a
Received (in Cambridge, UK) 2nd January 2007, Accepted 2nd February 2007
First published as an Advance Article on the web 22nd February 2007
DOI: 10.1039/b618996b
core modified or p-extended macrocyclic pyrrole compounds.7
A2B2-type push–pull porphyrins with a strong intramolecular
Nevertheless, one design principle for optimized NLO materials is
the presence of a strong intramolecular dipole moment as is
realized in so-called push–pull porphyrins.8,9 meso-Substituted
porphyrins are easy to prepare and, in light of the ongoing
progress in synthetic porphyrin chemistry, offer a convenient entry
into industrially usable materials. A typical and well studied
example are unsymmetrically substituted A2BC porphyrins where
the polarizability of the porphyrin system is modulated by
appropriate donor and acceptor groups (Fig. 1).10
dipole moment have been prepared via Heck and Suzuki
coupling reactions as novel materials for use in nonlinear optics
(NLO); they display saturable (SA) and reverse saturable
absorption (RSA) properties at 532 nm and their nonlinear
optical response is characterized by RSA occuring at lower
intensity levels whereas the onset of SA prevails at higher levels.
Understanding the optical nonlinearities of dyes is of considerable
interest due to their widespread application potential. These include
SA for use of dyes for mode locking, laser pulse compression, and
laser amplification. RSA has potential for use in pulse sensors and
in optical limiting devices that protects sensitive optical compo-
nents or the human eye from laser-induced damage.1
A conceptionally related class of compounds that offer the
potential to construct systems with stronger intramolecular dipole
moment are the A2B2-type porphyrins. Here, two donor and
acceptor groups can be attached to the p-system, respectively.
However, these systems remained elusive due to the difficulty to
prepare porphyrins with a 5,10-disubstitution pattern.11 However,
our recent success in developing syntheses for meso mono- and
disubstituted porphyrins gave a possible entry into this class of
dyes.12 Here, we report the synthesis and nonlinear characteriza-
tion of a new series of optical materials that exhibit both reverse
saturable and saturable absorption at 532 nm using the Z-scan
technique under ns irradiation.
Porphyrins and their macrocycle expanded and contracted
relatives displaying large nonlinear optical responses are of major
interest in the development of photonic and optoelectronic
technologies due to their fast response time, large nonlinear
susceptibilities and also comparatively low fabrication costs.1,2
Porphyrins have outstanding chemical and thermal stability. Their
macrocyclic structure and chemical reactivity offers architectural
flexibility and facilitates the tailoring of chemical, physical and
optoelectronic parameters. They are versatile organic nanomate-
rials with a rich photochemistry and their excited state properties
are easily modulated through conformational design, molecular
symmetry, metal complexation, orientation and strength of the
molecular dipole moment, size and degree of conjugation of the
p-systems, and appropriate donor–acceptor substituents.3,4
Much research has focussed on the synthesis and NLO
characterization of novel porphyrin compounds since the first
publication on enhanced RSA.5 Porphyrin molecules display
enhanced RSA over a large region of the visible waveband. This
RSA process exhibited in porphyrins and other dyes is often
simplified to a three-level model.6 An optimized RSA material
should display a rapid intersystem crossing rate, a high intersystem
crossing quantum yield, and a long triplet lifetime. Despite the
many studies on third-order NLO properties of porphyrins no
evidence of their optical limiting properties exhibiting reverse
saturable and saturable absorption for nanosecond (ns) pulses at
532 nm has been reported to date.
The first step of the synthesis of these novel systems involved the
preparation of 5,10-disubstituted porphyrins 1 bearing electron
donating groups (phenyl, tolyl, 3-methoxyphenyl) by condensation
of tripyrrane with different alkyl or aryl aldehydes (Scheme 1).
Subsequent introduction of the electron withdrawing substituents
(‘‘B’’) was achieved via bromination to 2 followed by Heck or
Suzuki couplings or LiR chemistry.13 The bromination proceeded
smoothly in 74–83% yield and was followed by the introduction of
an electron withdrawing group (i.e. 4-cyanophenyl, 4b 47%, 4c
47%; 3-nitrophenyl, 4g 30%; 4-nitrophenyl, 4d 50%; 4-methoxy-
carbonylphenyl, 4a 52%, 4e 30%, 4f 32%).
Many NLO studies on porphyrins have focussed on simple
symmetric systems and contemporary studies increasingly target
aSchool of Chemistry, SFI Tetrapyrrole Laboratory, Trinity College
Dublin, Dublin 2, Ireland. E-mail: sengem@tcd.ie; Fax: +353 1 896 8536;
Tel: +353 1 896 8537
bSchool of Physics, Trinity College Dublin, Dublin 2, Ireland
{ Electronic supplementary information (ESI) available: Details of
synthetic procedures and spectroscopic data. See DOI: 10.1039/b618996b
Fig. 1 Illustration of A2BC- and A2B2-type porphyrins with electron
donating and accepting substituents to yield systems with strong
intramolecular dipole moments.
2166 | Chem. Commun., 2007, 2166–2168
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Scheme 1 Synthesis of A2B2-type porphyrins via Heck and Suzuki coupling reactions. Reagents and conditions: (i) NBS, CHCl3, acetone; (ii) K3PO4,
Pd(PPh3)4, arylboronic acids and esters; (iii) ZnO, TFA; (iv) Pd(OAc)2, Na(OAc)2, PPh3, 3-nitrostyrene, DMF.
Herein, we choose eight compounds illustrating this approach
which gave the strongest nonlinear optical response. The zinc(II)
porphyrin 3 is an example prepared via Heck coupling reaction in
good yield. For the synthesis of porphyrins 4a–g, Suzuki coupling
reaction conditions have been employed, and, in this case the
coupling occurred between the free base of the respective 5,10-
dibromo-15,20-disubstituted porphyrins 2 with different aryl
boronic acids or esters under tetrakis(triphenylphosphine)
palladium catalysis in moderate yields due to the low solubility
of 5,10-disubstituted porphyrins.
A very convenient and fast experimental method to characterize
materials for optical limiting is the open aperture Z-scan
technique.14 Z-scan measurements of the target compounds with
irradiation at 532 nm revealed a novel NLO feature inherent to the
A2B2-type compounds.{ The nonlinear absorption behavior
observed in these porphyrins at nanosecond timescale switches
from RSA to SA and then again to RSA as the intensity increases.
An RSA/SA/RSA switch is exhibited in the tight focal intensity
regime shown in Fig. 3. It represents typical experimental results of
Z-scan measurements and a plot of normalized transmission
against z-distance at varying focal intensities. At an intensity of I0–1
these materials exhibited a characteristic response of RSA. As the
focal intensity is increased a sharp peak of SA is noted at the tight
intensity focal point superimposed on the RSA curve. As the
Fig. 2 shows the linear optical spectrum of A2B2-type
porphyrins in DMF. The main absorption bands remain constant
for the majority of the porphyrins and shift bathochromically
for the metallated porphyrin as is characteristic for tetra-meso-
substituted porphyrins. The effect of peripheral substitution on the
linear absorption properties is negligible. All porphyrins from this
series exhibit low linear absorption at 532 nm.
Fig. 3 Open aperture Z-scan data for 3 in DMF at laser intensities of
I0–1 = y0.07 GW cm21, I0–2 = y0.2 GW cm21 and I0–3 = y0.5 GW cm21
with indication of the tight focal regime.
Fig. 2 Characteristic electronic absorption spectra of A2B2-type por-
phyrins 3 and 4a at y3 6 1025 mol L21 solution in DMF.
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spectroscopic grade DMF. The porphyrins were placed into a low power
(60 W) sonic bath for about 1 h in order to obtain homogenous and
complete dissolution.
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increases symmetrically. At low fluence, y0.07–0.13 GW cm21, a
three-level model was successfully used to mathematically describe
the resulting NLO behavior.
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At very high inputs, (0.5 GW cm21) a high intensity nonlinear
response was observed which may be connected to a photo-
chemical reaction of the excited state of the molecule, leading to
the formation and accumulation of a new product within the
experimental lifetime. As the intensity increased the behavior
switched to SA from RSA, most likely due to two-photon
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behavior was dominated by SA. In these A2B2-type porphyrins the
RSA/SA/RSA switch at the focal point regime is fully reversible.
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function of incident energy per pulse for 1 are depicted in Fig. 4
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A new type of push–pull porphyrins has been synthesized using
Suzuki and Heck coupling reaction conditions. Our Z-scan
experiments have revealed interesting features of nonlinear
absorption properties for this series of A2B2-type porphyrins. At
532 nm the nonlinear processes are studied under ns irradiation, a
non-resonant absorption. RSA was observed at low fluence
intensities and at higher intensities SA followed by RSA again.
This tight focal switch regime was shown to occur at high
intensities symmetrically. Hence, porphyrins of this type offer a
significant degree of design flexibility and thus potential to
fabricate practical optical materials.
We thank Science Foundation Ireland for financial support
through grants SFI 04/RP1/B482 and SFI 06/RFP/CHO044 and
Frontier Scientific Inc. for generously supplying boronic acids and
esters.
Notes and references
{ The experiments were performed using y6 ns Gaussian pulses from a
Q-switched Nd:YAG laser. The beam was spatially filtered to remove the
higher modes and tightly focussed with a 9 cm focal length lens. The laser
was operated at its second harmonic, 532 nm, with a pulse repetition rate of
10 Hz. The irradiation at 532 nm is a no resonant excitation. The
measurements were carried out in solution in a quartz cuvette of 1 mm path
length containing porphyrins in a concentration of y4 6 1024 mol L21 in
14 (a) M. Sheik-Bahae, A. A. Said, T.-H. Wei, D. J. Hagan and E. W.
Van Stryland, IEEE J. Quantum Electron., 1990, 26, 760; (b)
S. O’Flaherty, S. V. Hold, M. J. Cook, T. Torres, Y. Chen,
M. Hanack and W. J. Blau, Adv. Mater., 2003, 15, 19.
2168 | Chem. Commun., 2007, 2166–2168
This journal is ß The Royal Society of Chemistry 2007