Conformational change from a twisted figure-eight to an open-extended structure in doubly fused 36π core-modified octaphyrins triggered by protonation: Implication on photodynamics and aromaticity

Article abstract of DOI:10.1002/chem.201302020

Two examples of core-modified 36π doubly fused octaphyrins that undergo a conformational change from a twisted figure-eight to an open-extended structure induced by protonation are reported. Syntheses of the two octaphyrins (in which Ar=mesityl or tolyl) were achieved by a simple acid-catalyzed condensation of dipyrrane unit containing an electron-rich, rigid dithienothiophene (DTT) core with pentafluorobenzaldehyde followed by oxidation with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). The single-crystal X-ray structure of the octaphyrin (in which Ar=mesityl) shows a figure-eight twisted conformation of the expanded porphyrin skeleton with two DTT moieties oriented in a staggered conformation with a π-cloud distance of 3.7 A. Spectroscopic and quantum mechanical calculations reveal that both octaphyrins conform to a [4n]π nonaromatic electronic structure. Protonation of the pyrrole nitrogen atoms of the octaphyrins results in dramatic structural change, which led to 1) a large redshift and sharpening of absorption bands in electronic absorption spectrum, 2) a large change in chemical shift of pyrrole β-CH and-NH protons in the ¹H NMR spectrum, 3) a small increase in singlet lifetimes, and 4) a moderate increase in two-photon absorption cross-section values. Furthermore, nucleus-independent chemical shift (NICS) values calculated at various geometrical positions show positive values and anisotropy-induced current density (AICD) plots indicate paratropic ring-currents for the diprotonated form of the octaphyrin (in which Ar=tolyl); the single-crystal X-ray structure of the diprotonated form of the octaphyrin shows an extended structure in which one of the pyrrole ring of each dipyrrin subunit undergoes a 180 ° ring-flip. Four trifluoroacetic acid (TFA) molecules are bound above and below the molecular plane defined by meso-carbon atoms and are held by Ni￡H×××O, N-H...F, and C-H...F intermolecular hydrogen-bonding interactions. The extended-open structure upon protonation allows π-delocalization and the electronic structure conforms to a [4n]π Hueckel antiaromatic in the diprotonated state. Figure-eight to open structure: A major structural change from a figure-eight conformation to an open-extended conformation upon addition of two protons to an expanded 36π fused octaphyrin cavity is reported (see figure). Such a change in conformation results in an increase in singlet lifetime, two-photon absorption cross-section values, and a change in aromaticity from a (4n)π nonaromatic to antiaromatic electronic structure.

Full text of DOI:10.1002/chem.201302020

FULL PAPER
DOI: 10.1002/chem.201302020
Conformational Change from a Twisted Figure-Eight to an Open-Extended
Structure in Doubly Fused 36p Core-Modified Octaphyrins Triggered by
Protonation: Implication on Photodynamics and Aromaticity
Ganesan Karthik,[a] Jong Min Lim,[b] A. Srinivasan,[a] C. H. Suresh,[c]
Dongho Kim,*[b] and Tavarekere K. Chandrashekar*[a]
Dedicated to Dr. T. Ramasami on the occasion of his 65th birthday
Abstract: Two examples of core-modi-
fied 36p doubly fused octaphyrins that
undergo a conformational change from
a twisted figure-eight to an open-ex-
tended structure induced by protona-
tion are reported. Syntheses of the two
octaphyrins (in which Ar=mesityl or
tolyl) were achieved by a simple acid-
catalyzed condensation of dipyrrane
unit containing an electron-rich, rigid
dithienothiophene (DTT) core with
pentafluorobenzaldehyde followed by
oxidation with 2,3-dichloro-5,6-dicya-
no-1,4-benzoquinone (DDQ). The
single-crystal X-ray structure of the oc-
taphyrin (in which Ar=mesityl) shows
a figure-eight twisted conformation of
the expanded porphyrin skeleton with
two DTT moieties oriented in a stag-
gered conformation with a p-cloud dis-
tance of 3.7 ꢁ. Spectroscopic and quan-
tum mechanical calculations reveal that
both octaphyrins conform to a [4n]p
nonaromatic electronic structure. Pro-
tonation of the pyrrole nitrogen atoms
of the octaphyrins results in dramatic
structural change, which led to 1) a
large redshift and sharpening of ab-
sorption bands in electronic absorption
spectrum, 2) a large change in chemical
ous geometrical positions show positive
values and anisotropy-induced current
density (AICD) plots indicate para-
tropic ring-currents for the diprotonat-
ed form of the octaphyrin (in which
Ar=tolyl); the single-crystal X-ray
structure of the diprotonated form of
the octaphyrin shows an extended
structure in which one of the pyrrole
ring of each dipyrrin subunit undergoes
a 1808 ring-flip. Four trifluoroacetic
acid (TFA) molecules are bound above
and below the molecular plane defined
by meso-carbon atoms and are held by
À
shift of pyrrole b-CH and NH protons
in the H NMR spectrum, 3) a small in-
1
crease in singlet lifetimes, and 4) a
moderate increase in two-photon ab-
sorption cross-section values. Further-
more, nucleus-independent chemical
shift (NICS) values calculated at vari-
À
À
À
N H···O, N H···F, and C H···F inter-
molecular hydrogen-bonding interac-
tions. The extended-open structure
upon protonation allows p-delocaliza-
tion and the electronic structure con-
forms to a [4n]p Hꢀckel antiaromatic
in the diprotonated state.
Keywords: aromaticity · conforma-
tion analysis · fused-ring systems ·
photodynamics · X-ray diffraction
Introduction
Expanded porphyrins continue to attract the attention of re-
searchers because of their diverse applications as new-gener-
ation nonlinear optical (NLO) materials,[1] photosensitizers
for photodynamic therapy (PDT),[2] cation and anion recep-
tors,[3] magnetic resonance imaging (MRI) contrasting
agents,[4] and to address fundamental properties of aromatic-
ity in larger cyclic p-systems.[5] The reasons for their involve-
ment in such diverse applications are their rich structural di-
versity.[6] Expanded porphyrins exhibit normal,[7] inverted,[8]
fused,[9] confused,[10] figure-eight,[11] Mçbius,[12] and
bridged[13] conformations depending on the number and
nature of the heterocyclic rings and the way these rings are
connected to each other through meso-carbon bridges. Fur-
thermore, it is possible to convert one into another by
simple chemical stimuli such as protonation,[14] change of
solvent and temperature,[15] ring fusion,[9] metal coordina-
tion,[16] and so on. For example, recently it has been shown
that di-p-benzi[28]hexaphyrin 1 can act as a Hꢀckel–Mçbius
[a] G. Karthik, Dr. A. Srinivasan, Prof. Dr. T. K. Chandrashekar
School of Chemical Sciences, National Institute of Science
Education and Research (NISER), IOP Campus, Bhubaneshwar
751005, Odisha (India)
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)
E-mail: dongho@yonsei.ac.kr
[c] Dr. C. H. Suresh
Inorganic Chemistry Section
Chemical Sciences and Technology Division
National Institute for Interdisciplinary Science
and Technology (CSIR), Thiruvananthapuram
695019, Kerala (India)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201302020.
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topology switch upon changing the solvent from nonpolar to
polar and a change of temperature.[17] Hexaphyrins, a class
of expanded porphyrins are conformationally flexible and
depending upon the solvent of crystallization both Mçbius
aromatic and Hꢀckel antiaromatic systems have been isolat-
ed in 28p hexaphyrin systems 2.[18] The 24p fused-expanded
porphyrin 3, which exhibits a Hꢀckel antiaromatic confor-
mation, can be converted into Mçbius aromatic conforma-
tion by RhI coordination.[19] Chandrashekar and co-workers
have reported that the RhI coordination in a oxasmaragdyrin
cavity increases the two-photon absorption (TPA) cross-sec-
tion values by more than ten times, which has been attribut-
ed to restoration of p-electron conjugation upon RhI coordi-
nation.[1e] More recently, it has been shown that in 26p hexa-
phyrin 4, upon introduction of a thiophene bridge between
two opposite meso-carbon atoms, changes the electronic de-
localization pathway where the bridging thiophene group
takes part in electron conjugation of the macrocycle.[13a]
Such changes in the electron conjugation pathway leads to
an increase in TPA cross-section values relative to its non-
bridged congener. Thus, it is important to study such subtle
structural changes in expanded porphyrin systems upon
simple chemical stimulation to design molecules for a specif-
ic application.
leads to a normal rubyrin structure, whereas the use of TFA
results in an inverted rubyrin structure in which two pyrrole
rings show a 1808 flip. More recently, Osuka, Kim, and co-
workers have used protonation to study conformational
changes from Hꢀckel antiaromatic and nonaromatic systems
to aromatic Mçbius conformation in 32p heptaphyrins[14]
and 36p octaphyrins,[20] respectively. Specifically, 32p hepta-
phyrins, which exhibit Hꢀckel antiaromatic conformation,
changes its conformation to a Mçbius aromatic one upon
protonation by TFA. On the other hand, the 36p octaphyrin
with a twisted figure-eight conformation in the free-base
form, changes into an open-extended structure upon proto-
nation. The driving force for such a structural change is at-
tributed to the breaking of intramolecular hydrogen bonds
in the free-base form and formation of intermolecular hy-
drogen bonds with TFA in the dicationic form. Detailed
photophysical studies revealed an increase in excited singlet-
and triplet-state lifetimes, which are attributable to the ri-
gidity of the macrocycle in the dicationic form. The TPA
values increases more than four times in the dicationic form
and this has been explained in terms of attaining Mçbius ar-
omatic character of the macrocycle in the extended struc-
ture.
In this article, we wish to report the two examples of
core-modified doubly fused 36p
octaphyrins 7 and 8, which ex-
hibit a dramatic change from
a twisted figure-eight conforma-
tion in the free-base form to an
open-extended
conformation
upon protonation. However,
unlike the 36p octaphyrins re-
ported by Osuka, Kim, and co-
workers, this structural change
does not accompany changes of
Hꢀckel–Mçbius
aromaticity
switching.[20] Thus, fused octa-
phyrins, which are nonaromatic
in the free-base form, change to
a [4n]p Hꢀckel antiaromatic in
the diprotonated state due to
an open-extended structure.
Spectroscopic and theoretical
studies on the free-base and di-
protonated forms support the
presence of paratropic ring-cur-
rent and positive NICS values
justifying such
a conclusion.
The excited-state photodynam-
ics reveal a small increase in ex-
Protonation of pyrrole nitrogen atoms has been often
used to interconvert one structure into another. For example
cited singlet-state lifetime in the mono- and diprotonated
state and the TPA values also show a moderate increase in
the protonated derivatives, presumably due to the open
structure and the local aromaticity of the fused-dithienothio-
phene (DTT) moieties.
´
Latos-Graz˙ynski and co-workers have shown that it is possi-
ble to convert an inverted structure into a normal structure
by protonation in sapphyrins.[8a,b] Osuka and co-workers
have shown that the structure of rubyrins depends on the
nature of the acid used for protonation.[8c] The use of HCl
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Conformational Change in Octaphyrins
FULL PAPER
Results and Discussion
Synthesis: Expanded porphyrins, by virtue of their rich
structural diversity,[6] are well-suited for the creation of
Mçbius/Hꢀckel switching and several reports have already
appeared in the literature that exhibit such changes in topol-
ogy under various physical or chemical stimuli.[12,14,20] Our
synthetic strategy involved incorporation of half-rigid and
half-flexible subunits in an expanded porphyrin skeleton
with appropriate linking groups. The rigid precursor we
have chosen was DTT, which has an electron-rich core.[9b,21]
The key precursors, fused DTT-diol (9 or 10) was treated
with pyrrole in presence of 0.2 equivalents of trifluoroacetic
acid to obtain the fused DTT-dipyrrane (11 or 12) in 85%
yield (Scheme 1). A further acid-catalyzed condensation of
11 or 12 with an equal amount of pentafluorobenzaldehyde
in presence of 0.3 equivalents of p-toluenesulfonic acid fol-
lowed by oxidation with 2,3-dichloro-5,6-dicyano-1,4-benzo-
quinone (DDQ) gave the crude product. Repeated purifica-
tion by column chromatography over basic alumina followed
by silica gel (100–200 mesh) with CH2Cl2/hexane (30:70)
gave a pink fraction, which was identified as the doubly
fused octaphyrins 7 or 8. After evaporation of the solvent,
a green-colored solid was isolated in 7–10% yield. The com-
position of 7 and 8 were confirmed by mass spectral analysis.
Octaphyrin 7 shows the parent ion peak at m/z 1417.1589
[M+H+], whereas 8 was found at 1529.287 [M+H+] (see
the Supporting Information).
Scheme 1. Syntheses of doubly fused octaphyrins.
tion peaks (+0.421, +0.204 V) against the Ag/AgNO3 refer-
ence electrode in dichloromethane (see the Supporting In-
formation). This voltammetric behavior encouraged us to
try chemical oxidation and reduction of 7. Oxidation with
MnO2 in dichloromethane resulted in the ring opening of
the macrocycle giving two products with m/z 741.2 and
1049.8 (see the Supporting Information). Chemical reduc-
tion with NaBH4 results in decomposition of the macrocycle.
1
1H NMR characterization: The H NMR spectra of 7 and its
diprotonated derivative 7·2H+ along with the assignments
are shown in Figure 1. The two inner-NH protons of pyrrole
ring in 7 resonate as a singlet in the deshielded region at d=
In cyclic voltammetry, octaphyrin 7 shows two reversible
oxidation peaks (+1.11, +1.25 V) and two reversible reduc-
1
Figure 1. 1H NMR spectra of 7 in CDCl3 (1 mm): a) before and b) after protonation by TFA; inset: H–1H COSY correlations.
Chem. Eur. J. 2013, 19, 17011 – 17020
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D. Kim, T. K. Chandrashekar et al.
15.48 ppm and this assignment was confirmed by using
a D2O-exchange experiment. Two multiplets centered at d=
6.32 and 5.87 ppm are assigned to four pyrrole b-CH pro-
ways of interpretation of the ring-current effect (paratropic
vs. diatropic) in the macrocycle.
If the protonated derivatives are aromatic ([4n]p
Mçbius), one would expect a large diatropic ring-current,
1
tons and this assignment was confirmed by using a H–1H
À
correlation spectroscopy (COSY) experiment (Figure 1,
inset).
which is reflected in the chemical shifts of b-CH and NH
protons for normal and inverted pyrrole rings. In such a sce-
nario, the following assignment holds: normal pyrrole ring:
The meso-tolyl-CH protons appear as a doublet of dou-
1
blet at d=7.25 and 7.41 ppm (confirmed by H–1H COSY
(b-CH: d=15.91 and NH: À0.1 ppm); inverted pyrrole
À
À
spectrum, Figure 1, inset). Four DTT ring protons appear as
two sharp singlets at d=9.38 and 6.85 ppm and the NOESY
experiment confirms the presence of long-range correlation
between these protons. meso-Tolyl-CH3 protons resonate as
two singlets at d=2.48 and 2.45 ppm (see the Supporting In-
formation). Furthermore, the 1H NMR spectrum of 7 did
not show any major changes in the chemical shift on varying
the temperature from 298–178 K, except that the resolution
of the spectrum was better at 178 K (see the Supporting In-
formation). This observation suggests that there is no
change in the structure of the macrocycle upon temperature
variation. However, at 343 K the spectrum became broad.
ring: (b-CH: d=4.36 and NH: 18.7 ppm). Hence, the cor-
responding Dd values are 16.1 and 14.34 ppm for normal
and inverted pyrrole rings, respectively.
If the protonated derivatives are antiaromatic ([4n]p
Hꢀckel), one would expect a large paratropic ring-current.
In such a case, the following assignment holds: Normal pyr-
À
role ring: (b-CH: d=4.36 and NH: 18.7 ppm); inverted
À
pyrrole ring: (b-CH: d=15.91 and NH: À0.1 ppm).
However, other spectroscopic data and theoretical calcu-
lations support the later assignment. Thus, octaphyrin 7 ex-
hibits the antiaromatic character with extension of p-conju-
gation upon protonation. The observed upfield shift of DTT
protons upon protonation also supports such a conclusion.
1
The H NMR data, on comparison with the data reported
for other expanded porphyrins suggest that 7 in its free-base
form conforms to the 36p twisted figure-eight structure and
is nonaromatic.[14,20]
Structural characterization: The explicit structure of 8 was
unambiguously confirmed by single-crystal X-ray diffraction
analysis, in which the macrocycle crystallizes in a monoclinic
system with P21/n space group (Figure 2). As predicted
Major changes were observed in 1H NMR spectrum on
protonation of 7 by TFA in CDCl3 solution at 298 K. The
changes are: 1) two broad NH signals were observed at d=
18.7 and À0.1 ppm (the peak at d=À0.1 ppm was observed
only at 178 K); 2) two sets of pyrrole b-CH resonances;
a doublet of doublet centered at d=15.91 ppm (3J=5.2 Hz)
and other at d=4.36 ppm (3J=5.2 Hz; assignment confirmed
1
by H–1H COSY spectrum, inset in Figure 1); 3) the tolyl-
CH protons are shielded and appear as four doublets in the
region d=5.97–6.78 ppm and 4) the DTT protons are shield-
ed relative to free-base form and appear as two sharp sin-
glets at d=4.17 and 4.08 ppm. These results suggest a major
structural change upon protonation.
The appearance of two sets of pyrrolic b-CH protons in
the shielded and deshielded region is attributed to ring in-
versions in which one pyrrole ring of each dipyrrin subunits
undergoes an 1808 rotation, which are common in core-
modified expanded porphyrins (Scheme 2).[6] This interpre-
tation also accounts for observation of two separate NH sig-
nals in the shielded and deshielded region. Pyrrole-ring in-
version upon protonation of 7 led us to have two possible
Figure 2. Single-crystal X-ray structure of 8.
from the NMR spectral analysis, the structure adopts figure-
eight conformation in the solid state with 36p-electron cir-
cuit. There are two dipyrrin moieties and two DTT units
that are connected through the six meso-carbon bridges by
mesityl and pentafluorophenyl units, in which all the pyrrol-
ic nitrogen atoms point inwards. The amino (N2-H2A and
N3-H3) and imino (N1 and N4) nitrogen atoms that are in
the dipyrrin moieties are in strong intramolecular hydrogen-
bonding interactions (N2-H2A···N1 and N3-H3···N4) with
distances of 2.13 and 2.20 ꢁ and angles of 1248, respectively.
Both the DTT moieties are staggered and appended one
over the other with the p-cloud distance of 3.70 ꢁ. This ob-
Scheme 2. Ring-inversion of pyrrole rings on protonation for 7.
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Conformational Change in Octaphyrins
FULL PAPER
servation is consistent with the large chemical-shift differ-
ence for the DTT protons found in the H NMR spectrum
(N2-H2) and one of its b-CH protons (C49-H49) are in a in-
termolecular interaction with TFA fluorine atoms (F14 and
F13) with the distance and angles of N2-H2···F14 and C49-
H49···F13 are 2.85, 2.83 ꢁ and 164, 1628, respectively. Out
of the four TFA molecules, two TFA units are above and
two are below the mean macrocyclic plane defined by six
meso-carbons (C1, C13, C24, C40, C52, C63) with distances
of 3.02, 2.21, 2.04, and 2.44 ꢁ, respectively (Figure 3). On
the other hand, the inverted pyrrolic units (N4, N2) are
more deviated from the plane with respect to the normal
(N3, N1) counterparts with the dihedral angles of 31.15,
24.16 and 13.28, 20.608. However, the comparison of dihe-
dral angles of meso-tolyl groups in free-base form (76 to
888) and protonated derivative (50.34 to 39.78) indicates
a decrease in the protonated derivative suggesting the exten-
sion of conjugation. The observed torsion angles of 29.8
1
of 8. The macrocyclic conjugation is nicely preserved be-
cause the pyrrole rings and DTT rings are in the same plane
as that of the six meso-carbon atoms making dihedral angle
almost zero in conformation with twisted double-sided
Hꢀckel topology.[20]
The octaphyrin 7·2H+ (with the TFA anion) crystallizes in
¯
the triclinic system with P1 space group (Figure 3). The
doubly twisted figure-eight conformation of the free-base
was dramatically changed to an open conformation upon
protonation. All the intramolecular hydrogen bonds ob-
(C39-C40) and 24.38ACTHNUGRTENUNG(C63-C71) are within 308 and this
allows p-electron conjugation in the extended structure of
protonated derivative.[20,22]
Electronic absorption spectral studies: The electronic ab-
sorption spectra of 7 in CH2Cl2 shows broad and ill-defined
peaks, with three major bands at 368, 524 and 724 nm with-
out any distinct Q-bands (Figure 4). This is in contrast with
Figure 3. Single-crystal X-ray structure of 7·2H+. Top view (above) side
view (below). For clarity, the meso-aryl groups are omitted in the top and
side views.
Figure 4. Titration of a dilute solution of TFA (0.1–100 mm) with
(0.01 mm) in CH2Cl2.
7
served in the free-base are replaced by intermolecular hy-
drogen bonds with TFA counteranion. One pyrrole ring of
each dipyrrin unit is inverted as reflected in the H NMR
the absorption spectra of doubly fused rubyrin 6, which ex-
hibits an intense split Soret band in the region of 500–
550 nm and weak Q-bands in the region of 600–900 nm (see
the Supporting Information). Doubly fused rubyrins are
known to be highly aromatic.[9b] Furthermore, the absorption
spectra of 7 remain unaltered upon changing the polarity of
the solvent (see the Supporting Information). Taken togeth-
er, these observations suggest a nonaromatic nature of 7 and
that the conformation of 7 remains intact upon changing the
solvent polarity. The absorption spectra of 7 upon stepwise
protonation using a dilute solution of TFA reveals definite
spectral changes (Figure 4). Interestingly, broad absorption
bands change to intense sharp ones with an increase in
molar absorption coefficient accompanied by significant red-
1
spectroscopic analysis upon protonation. The distance be-
tween the two heterocyclic rings S6 and N3, and S3 and N1
are 7.28 and 7.36 ꢁ, respectively, which is sufficient enough
to accommodate two pyrrole b-CH protons inside the mac-
rocyclic ring. There are four TFA molecules (O6, O4, O2,
and O3), which are directly bound with the four pyrrolic
NHs (N1-H1, N2-H2, N3-H3A, and N4-H4A) through inter-
molecular hydrogen-bonding interactions (N1-H1···O6;
N2-H2···O4; N3-H3A···O2, and N4-H4A···O3) with distan-
ces of 2.33, 2.51, 2.21, and 2.10 ꢁ, and angles of 135, 106,
140, and 1528, respectively. In addition, the N2 pyrrolic NH
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D. Kim, T. K. Chandrashekar et al.
shifts. Specifically, monoprotonated 7 shows a Soret-like
band appearing at 568 and 696 nm with additional bands at
816 and 1136 nm. Further protonation leads to diprotona-
tion resulting in a further shift of the bands to 416, 632, 701,
and 992 nm. These observations reveal that a major structur-
al change has taken place upon protonation.
the frontier orbitals. These observations clearly suggest the
antiaromatic nature of macrocycle in the diprotonated state
(Figure 6).
The first look at changes in the absorption characteristics
suggests that upon protonation, octaphyrins 7 and 8 might
be aromatic in nature.[7b] However, theoretical calculations
and the MO diagram does not support the presence of aro-
maticity in the diprotonated form (see below).
We have performed time-dependent (TD)-DFT calcula-
tions on the doubly fused rubyrin 6, which is known to be
aromatic. Keeping this as the reference, calculations were
performed on both free-base and diprotonated forms of 7
and 8. For 6, from the MO diagram (see the Supporting In-
formation), we confirmed the aromatic porphyrin-like fea-
tures following typical Goutermanꢃs four-orbital model.[23]
These calculations revealed nearly degenerate HOMO/
HOMOÀ1 and LUMO/LUMOÀ1 for 6, which are charac-
teristic of aromatic expanded porphyrins. A calculation of
oscillator strength for the major transition of 6 reveals
96.58% for the transition from HOMOÀ1 to LUMO. On
the other hand, the MO energy-level diagram of 7 shows
a
broken degeneracy for HOMO/HOMOÀ1, LUMO/
LUMO+1 orbitals (Figure 5). The oscillator strength for
the major transition is only 14% from HOMOÀ1 to
LUMO+1 confirming the nonaromatic nature of the
doubly fused octaphyrins in the free-base form.
Figure 6. Calculated MO energy diagram of 7·2H+.
The MO diagram calculated for the diprotonated deriva-
tive 7·2H+ reveals: 1) the perturbed degeneracy of HOMO/
HOMOÀ1 and LUMO/LUMO+1; 2) a relatively small
HOMO–LUMO gap, and 3) the same number of nodes for
Theoretical calculations: We have calculated the optimized
structures (B3LYP/6-311GACHTNUGRTNEUNG
(d,p))[24] for neutral and diproto-
nated octaphyrins by using the coordinates obtained from
crystal structures of 8 and 7·2H+. Compared with the highly
distorted conformation of the neutral form, the diprotonated
octaphyrin reveals quite planar structures with distortions of
pyrrole rings. Based on both the X-ray crystal and optimized
structures, we have estimated the harmonic oscillator model
of aromaticity (HOMA) and the bond-length alternation
(BLA) values (Table 1).[25] Whereas the HOMA value in-
creases, the BLA value decreases upon protonation. These
features indicate that not only conformational changes but
also an extension of p-conjugation is achieved upon proto-
nation.
Table 1. HOMA and BLA values.
Structure
HOMA
BLA
neutral
neutral
protonated
protonated
X-ray
optimized
X-ray
0.454
0.570
0.557
0.587
0.150
0.068
0.133
0.058
optimized
To verify aromatic/antiaromatic characteristics in the ex-
tended protonated octaphyrins, we have adopted the nu-
cleus-independent chemical shift (NICS)[26] and anisotropy-
induced current density (AICD)[27] methods that are known
Figure 5. Molecular orbital diagram of 7.
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Conformational Change in Octaphyrins
FULL PAPER
to be a good index for the aromaticity. Large positive NICS
values were calculated at various geometrical positions (see
the Supporting Information) of the diprotonated form (e.g.,
NICS(0)=9.94 and NICS(1)=9.34). And the NICS values
at the central positions of pyrrole (d=À1.54 to
ꢀ
+1.40 ppm) and thiophene rings of DTT (d=À12.60 ppm)
indicate its main conjugation pathway of 36p-electronic
system. In addition, we have also attempted the direct visu-
alization of the induced ring-current by using the AICD
method, which describes the 3D image of delocalized elec-
tron densities with a scalar field. Since the AICD method il-
lustrates the paramagnetic term of the induced current den-
sity, the aromatic molecules show clockwise current density
and the antiaromatic species show counter-clockwise current
density.
The AICD plot of diprotonated octaphyrin (see the Sup-
porting Information) reveals clear counter-clockwise cur-
rent-density vectors, thus indicating a paramagnetic ring-cur-
rent. These results demonstrate that protonated octaphyrin
has a Hꢀckel antiaromatic character with continuous and
paramagnetic current-density in its extended conformation.
Excited-state dynamics and optical nonlinear properties: We
have investigated the excited-state dynamics of the neutral
and protonated forms of octaphyrins by utilizing femtosec-
ond transient absorption measurements (Figure 7). The tran-
sient species of neutral and protonated octaphyrins exhibit
prominent excited-state absorption (ESA) and ground-state
bleaching (GSB) signals corresponding to their steady-state
absorption features[28] (Figure 4). The highly distorted neu-
tral octaphyrin shows broad GSB and ESA signals in the
entire spectral region. And the excited-state lifetime is esti-
mated to be 0.6 (83%) and 8.8 ps (17%). We can assign the
decay-time component of shorter than 1 ps as an energy re-
laxation processes from highest to lowest excited state such
as a relaxation process from the S2 to S1 state.[29,30] However,
despite spectral changes upon protonation, the transient spe-
cies of protonated octaphyrins exhibit a reasonably fast ex-
cited-state lifetime. Also the small increase in lifetime ob-
served for protonated derivatives reveals the rigidity of the
macrocycle arising from the intermolecular hydrogen bond-
ing interactions.[20,29]
We have also measured the TPA cross-section (s2) values
for 7 and its protonated derivative by using a femtosecond
Z-scan technique (Figure 8). The normal figure-eight octa-
phyrin 5 without any fused rings shows a TPA value of
870 GM.[16b] Compared with this, octaphyrin 7 reveals an in-
creased TPA cross-section value of 1600 GM upon excita-
tion at 1400 nm exhibiting the rigid fused moieties. Further-
more, upon protonation, the TPA cross-section values in-
creased to 3200 and 2700 GM for mono and diprotonated
forms, respectively. The TPA values in expanded porphyrin
systems are known to depend on planarity, aromaticity, and
the number of p-electrons in conjugation.[1,29] We and others
have recently shown that an increase in aromaticity of the
macrocycle results in a significant increase in the TPA
values.[1] TPA values are also known to increase upon chang-
Figure 7. Femtosecond transient absorption spectra and representative
decay profiles of a) 7, b) 7·H+, and c) 7·2H+ in toluene.
ing the topology from Hꢀckel antiaromatic to the Mçbius
aromatic one.[14,20] The moderate increase in TPA values
upon protonation in doubly fused octaphyrin is attributed to
the structural change from figure-eight to an open-extended
conjugated structure. The planarity of the DTT rings in the
extended structure allows an extension of the p-conjugation.
Changes in the polarization of p-electrons upon interaction
with intense laser light are effective in the extended struc-
ture relative to the twisted figure-eight conformation leading
to a moderate increase in the TPA values.[1]
Chem. Eur. J. 2013, 19, 17011 – 17020
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17017

D. Kim, T. K. Chandrashekar et al.
to the twisted figure-eight structure.[1c,14,20] Further studies
are in progress on a variety of expanded porphyrin systems
to understand the structure–function correlation in this labo-
ratory.
Experimental Section
Representative procedure for synthesis of DTT-dipyrrane: Pyrrole
(3.66 mL) was added to 5,5’-bis-(mesitylhydroxylmethyl)dithienothio-
phene (DTT-mes-diol; 0.5 g) and the mixture was degassed by bubbling
nitrogen gas. TFA (0.03 mL) was added to this solution and the reaction
mixture was stirred for about 30 min at room temperature. After comple-
tion of the reaction, dichloromethane (100 mL) was added and the reac-
tion mixture was neutralized with 0.1m NaOH solution. The organic
layer was separated and washed twice with water (50 mL) then dried
over sodium sulfate. The solvent and excess pyrrole was removed by
vacuum. The obtained product was purified by column chromatography
over silica gel (100–200 mesh) with ethyl acetate/hexane (8:92, v/v).
DTT-dipyrrane was obtained as yellow semi-solid.Yield:85%.
Figure 8. Steady-state absorption spectra (black line) and TPA cross-sec-
tions (spheres) of 7 (top), 7·H+ (middle), and 7·2H+ (bottom).
Conclusion
Compound 11: 1H NMR (400 MHz, CDCl3, 298 K, TMS): d=7.89 (br,
2H), 7.01 (s, 2H), 7.00 (d, 4H), 6.86 (d, 4H), 6.13 (m, 6H) 2.28 ppm (s,
6H); ESI-MS: m/z calcd for C32H26N2S3 +H+: 534.1258; found: 534.1378.
Compound 12: 1H NMR (400 MHz, CDCl3, 298 K): d=8.24 (br, 2H),
7.05 (s, 2H), 6.78 (s, 4H), 6.73 (s, 2H), 6.25 (m, 6H), 2.29 (s, 12H),
2.15 ppm (s, 6H); ESI-MS: m/z calcd for C36H34N2S3 +H+: 590.1884;
found: 590.1127.
We have successfully synthesized and characterized two
doubly fused 36p octaphyrins with the objective of designing
a system that can switch over from an antiaromatic Hꢀckel
[4n]p topology to an aromatic [4n]p Mçbius topology by an
external chemical stimulation. The reasons for introducing
fusion using rigid DTT rings, which are linked to each other
through flexible dipyrrin subunits in the same macrocyclic
framework, is to allow such a topology change. Even though
we succeeded in achieving the expected topology change
upon protonation, the switch over to Mçbius aromaticity
was not observed in the fused 36p octaphyrins. To attain
Mçbius aromatic stabilization in [4n]p systems, an appropri-
ate mix between planarity and distortion within a macrocy-
clic framework is necessary so that the p orbitals containing
the p-system would contain an odd number of nodes.[31] Ob-
viously, in the fused 36p octaphyrins this is not achieved.
Nevertheless, this study clearly demonstrates that change in
the structure from a twisted figure-eight to an extended-
open structure does not necessarily accompany the change
to Mçbius aromaticity in [4n]p systems.[14,20] We believe that
the breaking of two intramolecular hydrogen bonding net-
Representative procedure for the synthesis of doubly fused octaphyrins:
Dry dichloromethane (200 mL) was added to DTT-dipyrrane (0.250 g,
0.42 mmol) and the mixture was degassed by bubbling nitrogen gas. Pen-
tafluorobenzaldehyde (0.052 mL, 1 equiv) was added to this solution, and
the reaction mixture was stirred for about 15 min at 298 K. Then 0.024 g
(0.3 equiv) of p-TSA was added and stirring was continued for 2 h. DDQ
(0.144 g, 1.5 equiv) was added to the reaction mixture and stirring was
continued for further 2 h. The crude product obtained was purified by
column chromatography over basic alumina followed by silica gel (100–
200 mesh) with CH2Cl2/hexane (20:80, v/v). The pink color fraction was
identified as product, after evaporation by vacuum yielded (7–10%)
green-colored solid.
Compound 7: 1H NMR (400 MHz, CDCl3, 298 K, TMS): d=15.48 (brs,
2H), 9.38 (s, 2H), 6.85 (s, 2H), 7.41 (d, 8H), 7.25 (d, 8H), 6.32 (d, 2H),
6.30 (d, 2H), 5.87 (m, 4H), 2.48 (s, 6H), 2.45 ppm (s, 6H); 7·2H+:
1H NMR (400 MHz, TFA/CDCl3, 298 K, TMS): d=18.69 (brs, 2H), 15.98
(d, 2H), 15.84 (d, 2H), 6.77 (d, 4H), 6.60 (d, 4H), 6.29 (d, 4H), 5.98 (d,
4H), 4.59 (d, 2H), 4.13 (d, 2H), 4.17 (s, 2H), 4.08 (s, 2H), 2.09 (s, 6H),
2.0 ppm (s, 6H); 7: UV/Vis (CH2Cl2): lmax (e)=368 (24300), 524 (33100),
608 (22200) 724 nm (10100 molÀ1 m3 cmÀ1); 7·H+ (TFA/CH2Cl2): lmax
(e)=452 (14900), 568 (48100), 696 (63500), 816 (7840), 1136 (2500),
1232 nm (2750 molÀ1 m3 cmÀ1); 7·2H+ (TFA/CH2Cl2): lmax (e)=416
(10300), 632 (109000), 701 (93900), 992 nm (1300 molÀ1 m3 cmÀ1); 7: ESI-
MS: m/z calcd for C78H42F10N4S6 +H+: 1417.1652; found: 1417.1589; ele-
mental analysis calcd (%) for C78H42F10N4S6: C 66.09, H 2.99, N 3.95;
found: C 65.97, H 2.96, N 3.93.
Compound 8: 1H NMR (400 MHz, CDCl3, 298 K, TMS): d=16.24 (brs,
2H), 9.69 (s, 2H), 6.606 (s, 2H), 7.34 (s, 2H), 7.14 (s, 2H), 6.93 (s, 2H),
6.81 (s, 2H), 6.17 (d, 2H), 6.00 (d, 2H), 5.81 (m, 4H), 3.68 (s, 6H), 3.03
(s, 6H), 2.54 (s, 6H), 2.38 (s, 6H), 1.71 (s, 6H), 1.53 ppm (s, 6H); 8·2H+:
1H NMR (400 MHz, TFA/CDCl3, 298 K, TMS): d=23.03 (brs, 2H), 17.84
(d, 2H), 17.67 (d, 2H), 6.38 (s, 4H), 6.25 (s, 4H), 3.79 (d, 2H), 3.55 (d,
2H), 3.09 (s, 2H), 2.90 (s, 2H), 2.07 (s, 12H), 1.96 (s, 12H), 1.90 (s, 6H),
1.81 ppm (s, 6H); 8: UV/Vis (CH2Cl2): lmax (e)=348 (70800), 528
(151300), 680 nm (27600 molÀ1 m3 cmÀ1); 8·H+ (TFA/CH2Cl2): lmax (e)=
392 (74400), 688 (295300), 816 (48800), 1188 (29100), 1176 nm
(30600 molÀ1 m3 cmÀ1); 8·2H+ (TFA/CH2Cl2): lmax (e)=388 (33800), 640
(196300), 688 (290800), 816 (17700), 1152 nm (13680 molÀ1 m3 cmÀ1); 8:
ESI-MS: m/z: calcd for C86H58F10N4S6 +H+: 1529.2904; found: 1529.287;
À
works (N H···N) in the free-base form and reformation of
the six intermolecular hydrogen bonding networks
À
À
À
(N H···O, N H···F, C H···F) with the counteranion, is the
driving force for such a structural change upon protonation.
The excited-state dynamics also give some insights into
structural change upon protonation. Flexible figure-eight
conformation of the free-base form is responsible for short
singlet excited-state lifetimes since the flexibility provides
an effective nonradiative decay pathway.[1c,29] However, in
the diprotonated state, the presence of a rigid framework
due to intermolecular hydrogen bonding and the presence
of rigid DTT moiety partially destroys the effective nonra-
diative decay pathway and is responsible for the small in-
crease in the singlet excited-state lifetime.[1c,29] The moderate
increase in the TPA cross-section value upon protonation
suggests that the perturbation of p electrons by femtosecond
laser light is more effective in an extended structure relative
17018
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Chem. Eur. J. 2013, 19, 17011 – 17020

Conformational Change in Octaphyrins
FULL PAPER
elemental analysis calcd (%) for C86H58F10N4S6: C 67.52, H 3.82, N 3.66;
found: C 67.48, H 3.81, N 3.68.
Acknowledgements
1H NMR measurements: 1H NMR spectra were recorded on a Bruker
(400 MHz) spectrometer. The NMR titration experiments were carried
out with TFA solution dissolved in deutrated solvent. Variable-tempera-
ture measurements were performed for compound 7 in the [D8]THF sol-
vent.
T.K.C. thanks the Department of Science and Technology (DST), New
Delhi, India for the J. C. Bose Fellowship. We thank Dr. Arindam Ghosh
and Dr. V. Krishnan for useful discussions on NMR spectroscopy and X-
ray structures, respectively. The work at Yonsei University was supported
by the World Class University Program (R32-10217) and Midcareer Re-
searcher Program (2010-0029668) funded by the Ministry of Education,
Science and Technology through the National Research Foundation of
Korea.
Steady-state absorption measurement: Steady-state absorption spectra
were obtained by using an UV/VIS/NIR spectrometer (Varian,
Cary5000).
Femtosecond transient absorption measurement: The femtosecond time-
resolved transient absorption (TA) spectrometer consisted of a home-
made noncollinear optical parametric amplifier (NOPA) pumped by a Ti:-
sapphire regenerative amplifier system (Quantronix, Integra-C) operating
at 1 kHz repetition rate and an optical detection system. The generated
visible NOPA pulses had a pulse width of approximately 100 fs and an
average power of 1 mW in the range 500–700 nm, which were used as
pump pulses. White-light continuum (WLC) probe pulses were generated
using a sapphire window (3 mm of thickness) by focusing of small portion
of the fundamental 800 nm pulses, which was picked off by a quartz plate
before entering to the NOPA. The time delay between the pump and
probe beams was controlled by making the pump beam travel along a var-
iable optical delay (Newport, ILS250). Intensities of the spectrally dis-
persed WLC probe pulses are monitored by miniature spectrograph
(OceanOptics, USB2000+). The polarization angle between the pump
and probe beam was set at the magic angle (54.78) to prevent polariza-
tion-dependent signals. The cross-correlation full-width at half-maximum
(fwhm) in pump-probe experiments was less than 200 fs and chirp of
WLC probe pulses was measured to be 800 fs in the 400–800 nm region.
To minimize chirp, all reflection optics in probe beam path and 2 mm
path length of quartz cell were used.
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Received: May 26, 2013
Published online: November 6, 2013
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