Synthesis and crystal structure of 21,23-dithiaporphyrins and their nonlinear optical activities

Article abstract of DOI:10.1016/j.tetlet.2007.06.002

A series of novel 21,23-dithiaporphyrins have been synthesized and determination of their optical nonlinearities demonstrated that they have much larger nonlinear refractive cross section than normal porphyrins and exhibit reverse saturable absorption.

Full text of DOI:10.1016/j.tetlet.2007.06.002

Tetrahedron Letters 48 (2007) 5687–5691
Synthesis and crystal structure of 21,23-dithiaporphyrins
and their nonlinear optical activities
Yan Zhu,a, Yi-Zhou Zhu,a, Hai-Bin Song,a Jian-Yu Zheng,a,*
Zhi-Bo Liub and Jian-Guo Tianb,*
aState Key Laboratory and Institute of Elemento-Organic Chemistry, Nankai University, Tianjin, PR China
bThe Key Laboratory of Advanced Technique and Fabrication for Weak-Light Nonlinear Photonics Materials,
Ministry of Education, and Teda Applied Physical School, Nankai University, Tianjin, PR China
Received 9 March 2007; revised 29 May 2007; accepted 1 June 2007
Available online 5 June 2007
Abstract—A series of novel 21,23-dithiaporphyrins have been synthesized and determination of their optical nonlinearities demon-
strated that they have much larger nonlinear refractive cross section than normal porphyrins and exhibit reverse saturable
absorption.
ꢀ 2007 Elsevier Ltd. All rights reserved.
There has been a great deal of interest in the nonlinear
optical (NLO) responses of conjugated organic mole-
cules during the past 20 years because of their potential
utilities in optical switching, optical memory, optical
power limiting and 3D microfabrication.1,2 The non-
linear optical responses of organic molecules depend
strongly on the molecular structure. Among the numer-
ous molecules, porphyrins and their derivatives have
exhibited great potential application due to their large
and rigid p-conjugated structures and versatile modifica-
tion.3–7 Changing substituent groups on the meso- or
b-positions of the porphyrin can result in different non-
linear optical properties;8–10 however, outer rim modifi-
cation can only affect the electronic structure of
porphyrin slightly, core alteration will change the elec-
tronic structure of macroring more directly and remark-
ably.11–13 Core-modified porphyrins, in which one or
more internal nitrogen atoms are replaced with other
heteroatoms or carbon, have shown some novel proper-
ties such as stabilizing unusual metal oxidation states,
catalysis, and photodynamic therapy (PDT),14–17 and
are expected to be promising nonlinear optical materi-
als. Up to now the optical nonlinear property of core-
modified porphyrin is seldom reported.
To systematically exploit the distinctive properties of
core-modified porphyrins requires the ability to locate
not only the heteroatom in the core of the porphyrin,
but also substituents arranged about the porphyrin
perimeter.18–22 We herein describe a rational synthetic
strategy for 21,23-dithiaporphyrins (DSPs), bearing
one kind of substituent groups at 5,15 positions and an-
other kind of substituent groups at the 10,20 positions
(Scheme 1). Such compounds could not be readily avail-
able through normal synthetic strategies or easily sepa-
rated from their analogues.22 This approach offers
more feasibility and convenience to the porphyrin chem-
ist to design and construct the unique units. We also
report the crystal structure of 1a and compare the non-
linear refractive cross section of compound DSPs with
normal porphyrins.
*
Corresponding authors. Tel./fax: +86 22 23505572 (J.-Y.Z.); e-mail:
jyzheng@nankai.edu.cn
Scheme 1. Structures of compounds 1 and 8.
These authors have contributed equally to this work.
0040-4039/$ - see front matter ꢀ 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.06.002

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Y. Zhu et al. / Tetrahedron Letters 48 (2007) 5687–5691
Scheme 2. Synthesis of 21,23-dithiaporphyrins 1.
The synthetic strategy of DSPs is shown in Scheme 2.
Compound 3 was obtained by treating thiophene with
benzoyl chloride, and then protected by glycol. Treat-
ment of 3 with n-butyllithium followed by an aromatic
aldehyde afforded the corresponding compounds 4.18
To get compound 5, the mild Lewis acid catalyst magne-
sium bromide was selected against trifluoroacetic acid
and trifluoroborane, since the strong acid catalyst would
destroy the protective group. As 5 was not stable
enough, the hydrolyzation would also react under mild
condition, and carbon tetrabromide was selected as
the appropriate catalyst.23 The ketone of 6 was easily
reduced to alcohol by sodium borohydride,16 then
10 equiv of ammonium chloride and 0.1 equiv of boron
trifluoride etherate were added,18 the reaction solution
was stirred for 0.5–1 h under nitrogen atmosphere, then
2 equiv of DDQ was added. The crude compounds
were purified by silica gel column chromatography and
1 was obtained as purple solids in 15–23% yields. The
target compounds were characterized by 1H NMR,
13C NMR, HR-MS and ESI-MS. So it is a general route
to synthesize such kind of 21,23-dithiaporphyrins
whose 10,20 positions were phenyl and 5,15 positions
contained electron-withdrawing or electron-donating
groups.
Figure 1. Absorption spectra and fluorescence spectra (inset, at
ex = 435 nm) of compound 1 (10ꢀ6 mol/l) (all in chloroform).
k
The structure of 1a was elucidated by a single crystal
X-ray diffraction (Fig. 2), and the structure is C2 sym-
metrical. The deviations from planarity of the core,
expressed as dihedral angles between the plane of the
four meso carbons and the planes of the five-membered
rings, are thiophene 0.2, pyrrole 7.5 of 1a. The thio-
phene rings are almost in the plane of the core, so the
substituents at meso carbons induce more planarity in
our compounds compared to the parent dithiaporphy-
rins.24 Meanwhile, distances between N,N0 and S,S0
are0 shortened. The nonbonded distances of N,N0 and
The absorption spectra (Fig. 1 and Table 1) of DSPs
show typical porphyrin character, with different substit-
uents changing the observed wavelengths and deeply
affecting the absorbency. The emission bands (Fig. 1
and Table 1) of DSPs are also red shifted relative to nor-
mal porphyrin. Fitting parameters of fluorescence decay
curves (s) for compounds 1 and 8 are shown in Table 2.
They indicate that fluorescence lifetimes of 1 are shorter
than normal porphyrins and they are not affected deeply
by different substituents.
˚
˚
S,S are 4.581 A and 3.056 A, respectively, in 1a.
The optical nonlinear absorption and nonlinear ref-
raction of the new 21,23-dithiaporphyrins have been

Y. Zhu et al. / Tetrahedron Letters 48 (2007) 5687–5691
Table 1. Absorption and fluorescence data of coumpound 1 (all in chloroform)
5689
Porphyrin
Soret band kmax (nm)
Absorption Q-bands, kmax (nm) (e · 10ꢀ3 Mꢀ1 cmꢀ1
)
Fluorenscence kmax (nm)
(kex = 435 nm)
(e · 10ꢀ3 Mꢀ1 cmꢀ1
)
IV
III
II
I
1a
1b
1c
1d
435 (612.2)
438 (396.8)
438 (506.0)
436 (607.2)
514 (34.3)
515 (26.2)
516 (24.8)
516 (24.8)
547 (8.8)
550 (9.1)
551 (9.5)
551 (9.5)
632 (2.2)
633 (1.9)
635 (1.5)
635 (1.5)
696 (5.4)
696 (4.3)
700 (5.4)
700 (5.4)
708
708
717
711
Table 2. Optical nonlinearities (r0,r1,rr1/10ꢀ17 cm2; beff/10ꢀ9 cm/W;
n2eff/10ꢀ14 cm2/W; jTjꢀ1) and fluorescence lifetimes (s/ns) of 1 and 8
DSPs have about 3–4 times enhancements of excited
state refractive cross section relative to TPP. Figure 3
depicts the Z-scan curves of 1b and 8b with the same
substituent group. Although 1b exhibits an obvious
RSA, the ratio of r1 to r0 for 1b (r1/r0 = 1.38) is much
less than that of 8b (r1/r0 = 2.02), which indicates
decreasing of RSA for 1b. The optical nonlinear refrac-
tion Z-scan curves depicted in Figure 3b are obtained
from dividing the closed-aperture Z-scan data by corre-
sponding open-aperture Z-scan data. Clearly we see that
nonlinear refraction is negative for both 1b and 8b, and
the large enhancement of nonlinear refraction can be
observed for 1b. The solid lines in Figure 3b show fits,
r0
r1
rr1
beff
n2eff
jTjꢀ1
s
8a
8b
1a
1b
1c
1d
2.63
2.96
3.38
3.23
3.02
2.89
6.85
5.98
5.92
4.45
5.74
5.52
ꢀ1.55
ꢀ1.45
ꢀ4.40
ꢀ4.60
ꢀ5.30
ꢀ4.50
3.58
2.86
2.34
0.83
1.86
2.02
ꢀ1.40
ꢀ1.28
ꢀ3.35
ꢀ3.48
ꢀ3.64
ꢀ3.38
0.037
0.042
0.135
0.394
0.184
0.157
4.885
6.410
1.039
1.088
1.044
1.072
giving
r
r1 = ꢀ4.60 · 10ꢀ17 cm2 for 1b and rr1
=
ꢀ1.45 · 10ꢀ17 cm2 for 8b.
Figure 2. Crystal structure of 1a (crystallographic data in Supplemen-
tary data).
measured by open- and closed-aperture Z-scan
experiments performed with a picosecond laser source.25
Table 2 gives the ground state S0–S1 absorption cross
section r0 and the excited state S1–Sn absorption cross
section r126 and the excited state refractive cross section
rr127 obtained by fitting the Z-scan results. We also per-
formed the Z-scan experiments of 8a and 8b to compare
the change of optical nonlinear absorption and non-
linear refraction with DSPs. TPP (8a) is well known to
exhibit RSA at the wavelength of 532 nm,3 which is
identical with our experimental results. Furthermore, a
negative nonlinear refraction was observed in TPP.
In a similar fashion to TPP, DSPs also exhibit RSA and
negative optical nonlinear refraction. However, all the
Figure 3. The Z-scan curves of 1b and 8b (top absorption and bottom
refraction).

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Y. Zhu et al. / Tetrahedron Letters 48 (2007) 5687–5691
Just as the real and imaginary components of the linear
susceptibility v(1) are associated with refraction and
absorption, nonlinear refraction (NLR) and nonlinear
absorption (NLA) can be described by the real and
imaginary parts of third-order nonlinear susceptibility
v(3). Usually, the third-order nonlinear refractive index
n2 and nonlinear absorptive coefficient b are used in
nonlinear refraction and nonlinear absorption. The
material’s suitability for device fabrication can in most
cases be estimated by considering a figure of merit
(FOM) detailing the ratio of the nonlinear refraction
to nonlinear absorption. The FOM is usually given as
jTjꢀ1 = jn2eff/(2kbeff)j.28,29 When the nonlinearity is due
to the population of excited state, what we observe is
not a true v(3) effect but is a sequential v(1):v(1) process
(effective third-order nonlinearities). Here, the first v(1)
is associated with the ground state absorption, the
second with the resulting excited state absorption or
refraction. In order to determine the FOM of DSPs,
we have also fitted the data of Z-scan experiments with
effective third-order nonlinear absorptive coefficient beff
Acknowledgments
We thank the 973 Program (2006CB932900) and
NSFC (Nos. 20572048, 20421202 and 10574075), Asia
Research Center in Nankai University (No. AS0528)
and the Preparatory Project of the National Key Funda-
mental Research Program (No. 2004CCA04400) for
their generous financial support.
Supplementary data
Supplementary data (Experimental, NMR, mass, X-ray
crystal structure data, absorption and emission spectral
data of selected compounds.) associated with this article
can be found, in the online version, at doi:10.1016/
j.tetlet.2007.06.002.
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strategy of a series of novel 21,23-dithiaporphyrins bear-
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