Synthesis and two-photon spectrum of a bis(porphyrin)-substituted squaraine

Article abstract of DOI:10.1021/ja901244e

(Figure Presented) A chromophore in which zinc porphyrin donors are linked through their meso positions by ethynyl bridges to a bis(indolinylidenemethyl) squaraine core has been synthesized using Sonogashira coupling. The chromophore exhibits a two-photon

Full text of DOI:10.1021/ja901244e

Published on Web 05/12/2009
Synthesis and Two-Photon Spectrum of a Bis(Porphyrin)-Substituted
Squaraine
Susan A. Odom,† Scott Webster,‡ Lazaro A. Padilha,‡ Davorin Peceli,‡ Honghua Hu,‡ Gero Nootz,‡
Sung-Jae Chung,† Shino Ohira,† Jonathan D. Matichak,† Olga V. Przhonska,§ Alexei D. Kachkovski,|
Stephen Barlow,† Jean-Luc Bre´das,† Harry L. Anderson,⊥ David J. Hagan,‡
Eric W. Van Stryland,‡ and Seth R. Marder*,†
School of Chemistry & Biochemistry and Center for Organic Photonics and Electronics, Georgia Institute of
Technology, Atlanta, Georgia 30332-0400, CREOL, The College of Optics and Photonics, UniVersity of Central
Florida, Orlando, Florida 32826, Institute of Physics, National Academy of Sciences, KieV 03028, Ukraine, Institute
of Organic Chemistry, National Academy of Sciences, KieV 03094, Ukraine, and Department of Chemistry,
UniVersity of Oxford, South Parks Road, Oxford OX1 3TA, U.K.
Received February 25, 2009; E-mail: seth.marder@chemistry.gatech.edu
Scheme 1. Synthesis of Compound 1
Organic materials with large two-photon absorption (2PA) cross
sections, δ, are of interest for applications including fluorescence
microscopy,1 microfabrication,2 and optical pulse suppression.3 One
of the design strategies that has been used to achieve large δ values
is the use of π-conjugated molecules with donor-acceptor-donor
(D-A-D) structural motifs.4 Squaraines can be considered as
examples of D-A-D chromophores and have been found to exhibit
large values of δ at wavelengths close to their one-photon absorption
(1PA) edges,5 especially when bearing substituents with extended
conjugation.6 Dimers and oligomers of conjugated zinc porphyrins
have also been shown to exhibit very strong 2PA, with δmax values
up to 500 times those of monomeric analogues.7 Moreover, linear
absorption spectra of both squaraines and porphyrins exhibit bands
characterized by large transition dipole moments, a key prerequisite
for obtaining strong 2PA. In addition, the S0-S1 transition energies
of bis(indolinylidenemethyl) squaraines closely match those of
porphyrins, suggesting the possibility of substantial excitonic
electronic coupling in a π-conjugated hybrid of these two compo-
nents. On the basis of these observations, we were interested in
investigating whether the electronic coupling between the constitu-
ent subunits of D-A-D bis(porphyrin)-substituted squaraines could
be sufficiently strong to result in enhanced 2PA. Here we report
on the synthesis and characterization of the new triad 1 (Scheme
1), in which a squaraine core is linked to zinc porphyrin donors
using alkyne bridges.
sation of squaric acid with 5-iodo-1-ethyl-3,3-dimethyl-2-methyl-
eneindoline, which was obtained from its bromo analogue5a,6b by
bromine-lithium exchange with n-butyllithium followed by treat-
ment with iodine. The noniodinated squaraine 3b5a,6b was also
synthesized as a model squaraine for comparison with 1. The
structure of compound 1 was confirmed by H, COSY, and 13C
NMR spectroscopy and mass spectrometry; the characterization data
are shown in the Supporting Information (SI) along with full
synthetic details for 1 and its precursors.
1
The 1PA spectra for 1, 2, and 3b are shown in Figure 1a. The
spectrum of 2 is typical for a porphyrin, showing a relatively weak
low-energy Q band and a more intense B (Soret) band at shorter
wavelength, the splitting of which can be attributed to the lowering
of symmetry by the ethynyl substituent. The absorption spectrum
of 3b shows a strong low-energy transition at 636 nm, which is
typical for a squaraine of this type. The lowest-energy absorption
band of 1 is intense and red-shifted by ∼100 nm relative to that of
3b, while the higher-energy features resemble the Q and B bands
of 2. All three compounds are fluorescent with small Stokes shifts;
fluorescence spectra are shown in the SI.
Compound 1 was synthesized using in situ deprotection of the
trimethylsilyl-protected meso-ethynyl-substituted porphyrin 2 fol-
lowed by Sonogashira coupling with the diiodosquaraine derivative
3a (Scheme 1). The in situ deprotection conditions were adopted
because of the tendency of terminal alkyne derivatives of zinc
porphyrins to undergo Glaser couplings [which would, in this case,
give a bis(porphyrin)diacetylene], even without the addition of
traditional alkyne-coupling reagents.8 Compound 1 is soluble in
solvents including dichloromethane, tetrahydrofuran, benzene, and
glycerols.
Using density functional theory (DFT)-optimized (B3LYP/6-
31G**) geometries, we calculated the 1PA and 2PA properties of
1, following the INDO/MRD-CI methodology we successfully used
previously in the case of large conjugated molecules6b (see the SI).
The frontier orbitals (Figure S3 in the SI) show significant
delocalization across the whole molecular backbone. The 1PA
calculations indicate the presence of three main peaks in the visible
region, with the lowest-energy feature primarily due to a mixing
of HOMO-to-LUMO and (to a lesser extent) HOMO-1-to-
LUMO+1 transitions.
Compound 2 was synthesized in a fashion similar to that for
previously reported porphyrin derivatives with alkynyl substituents
in the meso position,9 with the specific substituents being chosen
to facilitate solubility and recrystallization of the intermediates. The
diiodosquaraine building block 3a was synthesized by the conden-
† Georgia Institute of Technology.
‡ University of Central Florida.
§ Institute of Physics, Kiev.
| Institute of Organic Chemistry, Kiev.
⊥ University of Oxford.
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7510 J. AM. CHEM. SOC. 2009, 131, 7510–7511
10.1021/ja901244e CCC: $40.75  2009 American Chemical Society

C O M M U N I C A T I O N S
The enhancement of δmax for 1 relative to those for 2 and 3b
suggests substantial electronic coupling between the porphyrin and
squaraine moieties; this idea is confirmed by the DFT frontier
orbitals(FigureS3).Thecomplexityof1andtheporphyrin-squaraine
coupling are anticipated to lead to a large number of low-lying
excited states; indeed, Figure 1a indicates the presence of several
1PA states. Therefore, the broad 2PA spectrum of 1 is likely to
arise from overlap of several transitions. The calculations support
this idea, as they reproduce the main features of the experimental
spectrum of 1, indicating peaks with δ ) ∼6.9 × 103 and 9.7 ×
103 GM at transition energies between those of the two strong 1PA
states. The observation of large δ over a 750 nm-wide wavelength
range suggests that 1 could have applications in broadband NIR
pulse suppression.
Acknowledgment. We gratefully acknowledge the support of
the National Science Foundation (through ECS 0524533, the STC
program under Agreement DMR-0120967, and a graduate research
fellowship to S.A.O.), the U.S. Army Research Laboratory
(W911NF0420012, 50372-CHMUR), the U.S. Army Research
Office (50372-CHMUR), DARPA (MORPH N00014-06-1-0897),
and EPSRC. We thank Dr. Craig Smith and Dr. Michael Frampton
for insightful discussions.
Supporting Information Available: Experimental details; 1H NMR
and COSY spectra of the aromatic region of 1; fluorescence spectra
for 1, 2, and 3b; details of quantum-chemical calculations for 1; and
complete refs 2, 4a, and 6a. This material is available free of charge
via the Internet at http://pubs.acs.org.
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