Core-modified meso-aryl hexaphyrins with an internal thiophene bridge: Structure, aromaticity, and photodynamics

Article abstract of DOI:10.1002/chem.201203737

Take the shortcut: The synthesis of core-modified meso aryl hexaphyrins with an internal thiophene bridge is reported. Introduction of the thiophene bridge alters the electronic structure as well as the π-electron circuit, resulting in increases in singlet lifetime (τ_s) and the two-photon absorption (TPA) cross-section. Furthermore, for the sulfur derivative, the internal bridging thiophene participates in a π-electron conjugation pathway. Copyright

Full text of DOI:10.1002/chem.201203737

DOI: 10.1002/chem.201203737
Core-Modified meso-Aryl Hexaphyrins with an Internal Thiophene Bridge:
Structure, Aromaticity, and Photodynamics
Ganesan Karthik,[a] Mahima Sneha,[a] V. Prabhu Raja,[a] Jong Min Lim,[b]
Dongho Kim,*[b] A. Srinivasan,[a] and Tavarekere K. Chandrashekar*[a]
Expanded porphyrins continue to attract the attention of
researchers because of their diverse applications.[1–4] De-
pending on the number of heterocycle/pyrrole rings, the
nature of connectivity and the number of p electrons in con-
jugation, they can be aromatic, antiaromatic and Mçbius ar-
omatic in nature.[5] An increase in number of pyrrole rings
and meso carbon atoms result in conformational flexibility,
leading to nonplanar twisted structures.[6] Various synthetic
approaches have been followed in the literature to avoid
twisting of the structures, leading to planar aromatic 30p,
34p and 42p systems.[7–9] One approach to avoid the twisting
of structure is to introduce an internal bridging group link-
ing the meso carbon atoms. Using this approach, Osuka and
co-workers reported the synthesis of near-planar decaphyrin
1 (Scheme 1) using an internally bridging 1,4-phenylene
group.[10] Later, same group also synthesized planar hexa-
phyrin 2 using an internal vinylene bridge, and these mole-
cules have an interesting electronic structure.[11,5b]
To the best of our knowledge, there are no reports on
core-modified analogues of internally bridged expanded por-
phyrins. This report describes synthesis of core-modified
hexaphyrins with an internal bridging thiophene following a
simple and efficient synthetic approach. Spectroscopic and
structural studies revealed that hexaphyrin macrocycle was
planar and aromatic and that the internal bridging thio-
phene ring deviates from the mean plane of meso carbon
atoms. For 3, the deviation is 498, while it is 738 for 4. Com-
pound 4 exhibits the expected 26p hexaphyrin electron con-
jugation, while the conjugation pathway for 3 has a hybrid
character with contributions from both the 18p porphyrin-
like skeleton and 26p hexaphyrin skeleton in the free-base
form. Preliminary photophysical studies indicate an increase
Scheme 1. Examples of bridged expanded porphyrins.
in singlet lifetime and two-photon absorption cross-section
for the bridged systems.
For the synthesis of the thiophene-bridged hexaphyrins,
we adopted the method similar to that used for the synthesis
of 1. The required 2,5-thienylbis(dipyrromethane) 7 was syn-
thesized by condensing 2,5-thiophenedicarboxaldehyde 6a
with pyrrole in the presence of trifluoroacetic acid (TFA) as
catalyst to afford 7 in 45% yield. 7 was further condensed
with 2,5-bis(mesitylhydroxymethyl) thiophene 8 in the pres-
ence of 0.5 equiv of TFA and later oxidized with p-chloranil
to afford 3 in 4% yield along with two side products, mono-
condensed porphyrin (5,10-dimesityl-15,20-dipyrrolyl dithia-
porphyrin) and tetramesityldithiaporphyrin (Supporting In-
formation). Increasing the acid concentration led to more
acidolysis product and reduced yield of 3, while decreasing
the acid concentration led to more mono-condensed prod-
uct. By changing the acid catalyst to p-toluenesulphonic acid
(p-TSA), no drastic change in the yield of 3 was observed;
however, the amount of acidolysis product was reduced. By
using this synthetic route, and varying the acid and its con-
centration, a maximum 6% yield of 3 was observed at
[a] G. Karthik, M. Sneha, V. P. Raja, Dr. A. Srinivasan,
Prof. Dr. T. K. Chandrashekar
School of Chemical Sciences
National Institute of Science Education and Research (NISER)
Bhubaneswar 751 005, Odisha (India)
Fax : (+91)674-2302436
E-mail: tkc@niser.ac.in
[b] Dr. J. M. Lim, Prof. Dr. D. Kim
Department of Chemistry, Yonsei University
Shinchon-dong 134, Seodaemoon-gu, Seoul 120-749 (Korea)
Fax : (+82)2-2123-2434
E-mail: dongho@yonsei.ac.kr
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201203737.
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COMMUNICATION
0.3 equiv of p-TSA (Scheme 2, method I) with trace amount
of side products. To avoid the side-product formation, we
adopted the modified synthetic method shown in Scheme 2
(method II). Precursor 9a or 9b was condensed with 6a or
Figure 1. Electronic absorption spectra of 3, 4 and their protonated forms
formed by treatment with TFA.
shifted to 609 nm with a shift value of 42 nm, indicating an
extension of the conjugation.
1
The H NMR spectrum of 3 in CD2Cl2 at 298 K is shown
in Figure 2. In the downfield region, two doublets and three
singlets with equal intensity are observed. The doublets (b
Scheme 2. The synthesis of internally thiophene-bridged meso aryl hexa-
phyrins.
6b in the presence of 0.5 equiv of p-TSA followed by oxida-
tion with p-chloranil to afford the hexaphyrins (3–5) in 8–
15% yield, while the rest are inseparable polymeric materi-
als. As tripyrranes are less prone to acidolysis, no other side
products were detected. All of the macrocycles are soluble
in most polar organic solvents.
Figure 2. 1H NMR spectrum of 3 in CD2Cl2.
and c) at 9.8 and 8.6 ppm are assigned to four pyrrolic b-CH
protons. This was further confirmed by 2D homonuclear cor-
relation spectroscopy (COSY) (Supporting Information).
The b-CH protons of the two thiophene rings in the core
resonate as a singlet (a) at 9.17 ppm. The remaining two sin-
glets, (Mes-CH) at 7.4 and 7.2 ppm, are assigned to the
phenyl protons of the mesityl group. The methyl protons of
mesityl resonate as three singlets (Mes-methyl) at 2.6, 2.4
and 1.49 ppm, respectively. The spectrum also contains a sin-
glet (d) at 3.95 ppm, which is assigned to the bridging thio-
phene b-CH protons. The Dd value (the shift difference be-
tween the bridged thiophene and the macrocyclic core thio-
phene b-CH protons) of 5.22 ppm shows that the bridging
thiophene unit is experiencing an aromatic ring current
effect from core porphyrin ring, suggesting that the bridging
thiophene unit was not completely orthogonal with respect
to the plane of macrocycle. Protonation of 3 using TFA in
Mass spectrometry shows parent ion peaks at m/z
1051.7947 [M; H+] for 3 and 1146.8234 [M+] for 4, thus con-
firming a composition that is consistent with the expected
macrocycles (Supporting Information). The electronic ab-
sorption spectra of 3 and 4 were characterized by typical
strong Soret-like band and weak low-energy Q bands
(Figure 1). In the Soret region, 3 exhibits two bands (433 nm
and 531 nm). The position of higher-energy band was similar
to that found in dithiaporphyrin,[12] while the band at
531 nm has similar features to hexaphyrin.[13] However, 4
has only one Soret band at 567 nm and was found to be 1.6
times more intense than observed for 3. The position of low-
energy Q bands were typical of hexaphyrins. Upon protona-
tion, the band at 531 nm in 3 is red-shifted to 570 nm with a
shift value of 39 nm, while the band at 567 nm in 4 is red-
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D. Kim, T. K. Chandrashekar et al.
CDCl3 leads to small downfield shift in the pyrrole and thio-
phene b-CH protons in the H NMR spectrum of 3. On the
we could assume that the hybrid character of bridged hexa-
phyrin cause two transitions; one is porphyrin-like transi-
tion, while the other is a hexaphyrin-like transition. Based
on a comparison of the absorption spectra with the calculat-
ed results, we could assume that the peak at 433 nm is origi-
nated from porphyrin, that around at 531 nm region is
mixed with hexaphyrin and porphyin, and one at 917 nm is
caused by hexaphyrin. This hybrid character of 3 is also sup-
ported by the AICD plots, which show the 3D image of de-
localized electron densities with a scalar field and illustrates
the paramagnetic term of the induced current density; aro-
matic molecules show clockwise current density and antiaro-
matic species show counter-clockwise current density. The
AICD plot shows that the ring currents flow along not only
the hexaphyrin frame but also the bridged thiophene (Sup-
porting Information). From this result, we could assume that
the overall ring current density consist of the ring current
flows, which are mainly localized on hexaphyrin and partial-
ly localized on porphyrin. On the other hand, the MO pic-
ture for 4 (Supporting Information) indicates that the elec-
tron density distribution is mainly localized on the 26p hexa-
phyrin skeleton and an intense Soret-like band observed at
567 nm is consistent with this conclusion.
For the evaluation of aromaticity, we calculated the
NICS(0) values both within the inner porphyrin cavities and
outside (Supporting Information). To avoid the local aro-
matic effect from the bridging thiophene ring, we chose to
calculate the NICS values at the centers of two porphyrin-
like cavities rather than at the center of the hexaphyrin skel-
eton. The highly negative NICS(0) values observed (for 3,
À14.0 and À13.3 ppm; for 4, À18.3 and À17.9 ppm) at the
inner cavities and positive values outside strongly suggest
the aromatic nature of the bridging hexaphyrins. The rever-
sible/quasi reversible redox waves from the cyclic voltam-
metric experiments further confirm such a conclusion.
The final confirmation of structures of 3 and 4 came from
the single-crystal X-ray structural analyses (Supporting In-
formation). In both 3 and 4, the molecule is located on a
crystallographic two-fold axis. As predicted from the spec-
tral analyses, 3 contains two thiatripyrrin units, while 4 con-
sists of two selenatripyrrin units and both the units are indi-
vidually bridged by a thiophene moiety and the remaining
four meso positions are occupied by the mesityl groups (Fig-
ure 4a). The bridged thiophene in 3 and 4 is in positional
disorder, where two of the thiophene units overlap each
other. Analysis of crystal structure reveals that the thiatri-
pyrrin units in 3 (N1-S1-N2) are slightly deviated from the
mean plane with deviations of 10.28, 3.38 and 5.78, where the
selenatripyrrin in 4 (N1-Se1-N2) units are hardly deviated
from the plane. However, the bridged thiophene unit in 3
and 4 are deviated by 498 and 738, respectively (Figure 4b),
while the meso-mesityl rings in 3 and 4 are almost perpen-
dicular (89.38; 81.58 for 3 and 83.48; 78.18 for 4) to the mean
macrocyclic plane. Further, one of the meso-mesityl CH
groups is in intermolecular hydrogen bonding interaction
with meso-mesityl p-cloud (C23-H23c···Mes(p)) of the adja-
cent molecule that was used to generate the one-dimension-
1
contrary, the b-CH protons of bridging thiophene experi-
ence shielding of 1.2 ppm upon addition of 2 equiv of TFA.
However, excess addition of TFA (4 equiv) leads to down-
field of the b-CH protons, and they appear at 4.2 ppm (Sup-
porting Information, Figure S14). These observations clearly
suggest that the conformation of the bridging thiophene
group is sensitive to protonation, and significant changes
occur in the conformation upon protonation.
The protonated inner NH peaks were observed only at
low temperature (183 K) (Supporting Information). On
adding 2 equiv of TFA, five different NH peaks were ob-
served in the range of À1 to À7 ppm, suggesting the pres-
ence of different species that are chemically and magnetical-
ly inequivalent. On adding 4 equiv of TFA, a single peak at
À4.7 ppm was observed, which indicates the saturation of
protonation centers. Furthermore, the increase in the
number of meso-mesityl methyl and pyrrole peaks (Support-
ing Information) is attributed to 1) a lack of rotational free-
dom owing to the internal bridging thiophene; and 2) a low-
ering of the symmetry, making the meso-mesityl rings ineq-
uivalent. The 1H NMR spectrum of 4 at 298 K shows the
similar pattern as 3. The b-CH protons of the bridging thio-
phene in 4 are more shielded relative to 3 and resonate at
1.35 ppm.
In an effort to understand the characteristics in the ab-
sorption spectra, we calculated molecular orbitals (MO)
based on X-ray crystal structures and transition energies by
TD-DFT method using Beckeꢁs three-parameter hybrid ex-
change functional and the Lee–Yang–Parr correlation func-
tional (B3LYP) employing the 6–31G basis set for 3
(Figure 3). According to the frontier orbital diagram, the
MOs of bridged hexaphyrin consists of the summation of
the MOs in which electron density distribution is localized
on porphyrin (HOMOÀ1, HOMOÀ2) and the MOs in
which electron density distribution is localized on hexaphyr-
ins (LUMO). The MOs where electron density distribution
of 3 is localized on porphyrin give rise to porphyrin-like
transition at 433 nm. Furthermore, the MOs where electron
density distribution is localized on porphyrin moiety also
affect hexaphyrin-like transition at 917 nm. From these data,
Figure 3. Energy level diagram and molecular orbitals of 3.
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Core-Modified meso Aryl Hexaphyrins
COMMUNICATION
mation) exactly resemble that observed for 4, thus justifying
our conclusion.
The hybrid aromatic character in 3 prompted us to use
this compound for nonlinear optical applications. The pre-
liminary photophysical studies reveal an increase of singlet
lifetime by two times upon introduction of bridge, which is
presumably due to increased rigidity of the macrocycle.[9]
The two-photon absorption cross-section (TPA) for 3 is 2.6
times higher (1000 GM for dithiahexaphyrin and 2600 GM
for 3), which proves that this class of molecules are promis-
ing candidates for NLO applications.
In conclusion, we have demonstrated the syntheses of two
new internally meso thiophene-bridged core-modified hexa-
phyrins by a simple and efficient method. Spectroscopic and
structural data reveals the aromatic nature of both the hexa-
phyrins. A comparison of structures of bridged hexaphyrins
3 and 4 with the corresponding hexaphyrins without an in-
ternal bridge reveal the following:[13,14] 1) introduction of in-
ternal bridging group makes the hexaphyrin skeleton rigid,
thus preventing ring inversion of heterocyclic ring; and
2) the hexaphyrin skeleton in the bridged structure are more
planar, thus facilitating better a p conjugation pathway and
therefore increasing the aromaticity. Such structural changes
lead to changes in the electronic structure, which are impor-
tant from the point of view of their application as nonlinear
optical materials. Detailed studies are underway to probe
these properties further.
Figure 4. Single-crystal X-ray structures of
3 and 4. a),c) Top and
b),d) side views. Owing to disorder, the bridging thiophene b-CH hydro-
gen atoms are omitted in both views. C orange, H pink, N blue, S yellow,
Se red.
al array in the solid state, with a distance and angle of
3.17 ꢂ and 1448, respectively (Supporting Information).
The participation of bridging thiophene ring in the p elec-
tron conjugation pathway and the hybrid character observed
for 3 can be rationalized in terms of a smaller tilt angle of
498 of the bridging thiophene ring with respect to mean
plane of hexaphyrin skeleton. The observation of 433 nm
Soret band (typical of 18p dithiaporphyrin) also supports
such a conclusion. However, a near orthogonal tilt angle of
738 of the bridging thiophene group in 4 does not facilitate
conjugation through bridging thiophene ring, and thus 4 ex-
hibits a 26p electron conjugation pathway that is typical of
hexaphyrins.
Acknowledgements
Prof. TKC thanks the Department of Science and Technology (DST),
New Delhi, India for a J. C. Bose Fellowship. We thank Dr. V. Krishnan,
SCS, NISER, Bhubaneswar for solving the crystal structures of 3 and 4.
The work at Yonsei University was supported by the Midcareer Re-
searcher Program (2010-0029668) and World Class University (R32-2010-
000-10217-0) Programs of the Ministry of Education, Science, and Tech-
nology (MEST) of Korea.
A representation of conjugation pathway for 3 before and
after protonation is shown in Figure 5. To substantiate the
hybrid character observed in 3 further, the hexaphyrin 5
Keywords: aromaticity
·
bridged systems
·
core
modification · expanded porphyrins · macrocycles
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Received: October 19, 2012
Published online: January 4, 2013
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