A photochromic porphyrin-perinaphthothioindigo conjugate and its two-photon absorption properties

Article abstract of DOI:10.1039/b713520c

A porphyrin-perinaphthothioindigo conjugate having two-photon absorption cross-sections of ～2000 GM and ～700 GM for trans- and cis-isomers, respectively, was synthesized and exhibited clear photochromic behavior upon one-photon and two-photon excitation.

Full text of DOI:10.1039/b713520c

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A photochromic porphyrin–perinaphthothioindigo conjugate and its
two-photon absorption properties{
Joanne T. Dy,a Rena Maeda,a Yasunori Nagatsuka,a Kazuya Ogawa,*a Kenji Kamada,b Koji Ohtab and
Yoshiaki Kobuke*a
Received (in Cambridge, UK) 4th September 2007, Accepted 25th September 2007
First published as an Advance Article on the web 10th October 2007
DOI: 10.1039/b713520c
a lone perinaphthothioindigo (PNT),10 which has a HOMO–
A porphyrin–perinaphthothioindigo conjugate having two-
LUMO absorption band around 630 nm (2.0 eV), is lower than
photon absorption cross-sections of y2000 GM and
that of the zinc porphyrin monomer S1 state (y600 nm). This
unique combination permits the switching of the PNT moiety after
two-photon excitation of the zinc porphyrin, followed by the
intramolecular energy transfer from the zinc porphyrin to the PNT
moiety. In this work, in order to produce a more efficient two-
photon-absorbing photochromic compound, a perinaphthothio-
indigo was attached to a zinc porphyrin using an acetylene bond to
give compound 1 (Scheme 1). There have been several reports on
porphyrins linked to a photochromic moiety for switching.11 In
these cases, the switching was still performed under one-photon
irradiation conditions at the absorption wavelengths of the
photochromic component.
y700 GM for trans- and cis-isomers, respectively, was
synthesized and exhibited clear photochromic behavior upon
one-photon and two-photon excitation.
The search for high density storage devices with photonic
capabilities has rapidly gained momentum as top-down-oriented
semiconductor technology approaches its limits. So far, the most
promising candidates for this purpose are photochromic com-
pounds, which have the ability to reversibly switch structure
between two different states upon photoirradiation.1
One of the promising approaches to producing high density
memory is two-photon absorption (2PA), which has also attracted
much attention because of the diverse possibilities for its
application in the fields of microscopy,2 3D microfabrication,3
photodynamic therapy4 and 3D optical memory.5 A focused laser
source is used in 2PA so that a specific and minute volume can be
spatially targeted, and thus provide 3D resolution. Potential
candidates, such as diarylethenes,6a,b indolylfulgides6c and azoben-
zenes,6d have been investigated as two-photon photochromic
storage media. However, these compounds still lack a high two-
photon absorption efficiency. Diarylethene derivatives have a
maximum 2PA cross-section value, s(2), of y44 GM6a at 770 nm,
while indolylfulgide has a maximum value of 1030 GM6c at a
shorter wavelength of 775 nm.
Scheme 2 shows a synthetic route to compound 1. Ethynyl-
porphyrin 2 was synthesized according to a previous report.12
Diiodo-PNT 3 was prepared according to the literature.13
A
palladium-catalyzed Sonogashira coupling of 3 with 2 gave
compound 1 as the main product (ESI{). One of the iodines in
3 was replaced by hydrogen due to reduction, and a small amount
of bisporphyrin-substituted PNT was obtained. This is currently
being prepared by a different route on a larger scale, and details
We have previously found that acetylene-linked bis(pyridyl-
ethynylphenyl)7 and bisporphyrins8 are attractive for 2PA applica-
tions because they exhibit large s(2) values. This excellent 2PA
property is attributed to the combination of the acetylene p-bridge
and the large p-conjugation of the porphyrin, which provides the
pathway for electronic communication between the porphyrins.8,9
However, after two-photon excitation to the zinc porphyrin’s S2
state (around 2.9 eV), the molecule rapidly relaxes to its S1 state
(2.1 eV), which is insufficient to induce the isomerization of
photochromic compounds, such as diarylethenes, azobenzenes and
fulgides. On the other hand, the excited state of the trans-isomer of
aGraduate School of Materials Science, Nara Institute of Science and
Technology, 8916-5 Takayama, Ikoma, Nara 630-0101, Japan.
E-mail: oga@ms.naist.jp; kobuke@ms.naist.jp; Fax: +81 743 726119;
Tel: +81 743 726112
bPhotonics Research Institute, National Institute of Advanced Industrial
Science and Technology, 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577,
Japan
{ Electronic Supplementary Information (ESI) available: Detailed experi-
mental procedures and associated spectrometric data. See DOI: 10.1039/
b713520c
Scheme 1 Photochromic properties of compound 1 via one-photon
absorption (1PA) and 2PA.
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Scheme 2 Synthesis of compound 1.
Fig. 3 2PA spectra of 1trans
(
) and 1cis (&) in THF.
spectrum, showing that it is relatively thermally stable. On the
other hand, 1trans exhibited no change in its UV-vis spectrum when
kept in the dark for several months. 1H NMR spectra of 1trans and
1cis exhibited completely unique peaks for the naphthalene protons
closest to the switching alkene bond of the PNT, which are the
most sensitive to stereochemical changes during isomerization (SI).
The 2PA cross-section values of 1 in THF were determined by
an open aperture Z-scan method, employing wavelength-tunable
130 fs pulses with a repetition frequency of 1 kHz.7,8 The 2PA
cross-section maxima for both isomers appeared around 850 nm,
which was almost twice as long as the Soret band wavelength of
the porphyrin part of the molecule, with values of 2000 GM for
Fig. 1 Photoisomerization of completely trans-compound, 1trans, to
completely cis-compound, 1cis, in THF by photoirradiation at .700 nm
using a Xe lamp with a cut off filter.
will be reported elsewhere. After the reaction, 1 existed as a
mixture of its trans- (1trans) and cis- (1cis) stereoisomers. The
completely trans-compound, 1trans, was initially isolated from the
mixture using preparative GPC in pyridine (20% yield). It
exhibited a strong Soret band at 435 nm and a broad trans-PNT
absorption at 655 nm. The peak maximum of the unsubstituted
trans-PNT occurred around 630 nm.2 Isomerization using one-
photon irradiation was examined to investigate the photochromic
properties of this compound. Pure 1cis can be generated from its
trans-stereoisomer by irradiation at .700 nm with a quantum
yield of 5%, as shown in Fig. 1. The broad trans-PNT band,
occurring around 655 nm, gradually reduces in intensity, and a
sharp porphyrin Q-band appears at 625 nm. Moreover, the Soret
band is bathochromically shifted to 440 nm, and absorption in the
500 nm region, which corresponds to the absorption of the cis-
isomer,10 increases in intensity. 1trans can be generated from its cis-
isomer by irradiation at 500 nm with a 78% conversion and a
quantum yield of 15%. In both photoisomerization experiments,
the absorption spectra changed, while also passing through clear-
cut isosbestic points at 432, 457, 472, 561, 615 and 630 nm. Fig. 2
shows colour images of 1trans and 1cis in THF. 1trans was found to
be more stable than 1cis. When the a 1cis solution was kept in the
dark at rt for a period of 7 d, no change was observed in its UV-vis
1trans and 700 GM for 1cis (Fig. 3). The value for 1trans is two orders
of magnitude larger than that of the ethynylporphyrin monomer
(s(2) of 2 . 20 GM). As observed in the linear absorption spectra,
strong electronic communication between the individual moieties
(the porphyrin and the PNT) led to a significant enhancement in
the 2PA of 1. Unfortunately, due to low solubility, the s(2) of 3 was
difficult to measure.
To the best of our knowledge, compound 1 bears the largest
2PA cross-section value reported for photochromic compounds at
near-infrared (NIR) wavelengths.{ In 1cis, the PNT carbonyl units
are in close proximity, resulting in a distortion of the PNT unit due
to steric hindrance, thereby disrupting the electronic communica-
tion throughout the entire molecule (Fig. 4). On the other hand, in
1trans, these carbonyl units are located on opposite sides of the
PNT, allowing an almost planar structure. Increased conjugation
enhances the s(2) value in the trans-conjugate system. Encouraged
by these results, we proceeded to investigate isomerization using
two-photon excitation by employing a Ti:sapphire laser using
200 fs pulses with a peak intensity of 0.53 GW cm22 and a beam
Fig. 4 Optimized structures of trans- and cis-PNT, obtained by AM1
Fig. 2 Colour images of 1trans (left) and 1cis (right) (both solutions
calculations.
0.04 mM in THF).
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In summary, we have successfully synthesized a porphyrin–
perinaphthothioindigo conjugate exhibiting an efficient two-
photon absorption and demonstrated the switching of a PNT
moiety via both two-photon excitation and one-photon irradiation.
It would be interesting to observe the performance of compound 1
in a solid matrix, e.g. a polymer medium, to confirm its
applicability to 3D optical data storage, and this project is now
being actively pursued.
We thank Dr A. Ishizumi and Mr Y. Okajima for the two-
photon photoisomerization experiments. This work was supported
by Grant-in-Aids for Scientific Research (A) (no. 15205020) and
for Young Scientists (B) (no. 18750118) (KO) from Ministry of
Education, Culture, Sports, Science and Technology, Japan
(Monbu Kagakusho).
Notes and references
{ Even if we consider the experimental errors, the maximum value that can
be obtained at the other wavelength of 900 nm is 1600 ¡ 560 GM (2160 y
1040 GM), which is still larger than the 1030 GM reported for
indolylfulgide.
12 K. Ogawa, J. Dy and Y. Kobuke, J. Porphyrins Phthalocyanines, 2005,
9, 735.
13 J. Leavitt, Chem. Abs., 1971, 74, 113196.
5172 | Chem. Commun., 2007, 5170–5172
This journal is ß The Royal Society of Chemistry 2007