Conformational and spectroscopic properties of π-extended, bipyrrole-fused rubyrin and sapphyrin derivatives

Article abstract of DOI:10.1039/c1cc11733e

Two new expanded porphyrins, naphthorubyrin and naphthosapphyrin, were synthesized. The π-extended rubyrin was isolated and structurally characterized in its monoprotonated form. The sapphyrin congener undergoes pyrrole inversion as a function of the protonation state. These conformational effects are reflected in the spectroscopic features, including the excited singlet state lifetimes.

Full text of DOI:10.1039/c1cc11733e

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COMMUNICATION
Conformational and spectroscopic properties of p-extended,
bipyrrole-fused rubyrin and sapphyrin derivativesw
Se-Young Kee,a Jong Min Lim,b Soo-Jin Kim,a Jaeduk Yoo,a Jung-Su Park,c Tridib Sarma,d
Vincent M. Lynch,c Pradeepta K. Panda,*d Jonathan L. Sessler,*bc Dongho Kim*b and
Chang-Hee Lee*a
Received 26th March 2011, Accepted 4th May 2011
DOI: 10.1039/c1cc11733e
Two new expanded porphyrins, naphthorubyrin and naphtho-
sapphyrin, were synthesized. The p-extended rubyrin was
isolated and structurally characterized in its monoprotonated
form. The sapphyrin congener undergoes pyrrole inversion as a
function of the protonation state. These conformational effects
are reflected in the spectroscopic features, including the excited
singlet state lifetimes.
show conformational motion, undergoing in particular ‘‘pyrrole
inversion’’ such that one or more of the constituent pyrrole
NH protons points ‘‘in towards’’ or ‘‘out from’’ from the
center of the ring. In an effort to understand further the nature
of this inversion process we have designed and wish to report
here a new set of sapphyrin and rubyrin derivatives derived
from the fused bipyrrole precursor 1 (1,10-dihydrobenzo[e]-
pyrrolo[3,2-g]indole). Although several rubyrins and sapphyrins
have been reported in the literature and a variety of protonation
and structure-based effects on conformational motion have
been noted,6b to the best of our knowledge, this is the first time
that bipyrrole rigidification has been used in an effort to
impose an intrinsic conformational restriction on this set of
oligopyrrolic cores. As detailed below, we have found that this
creates a rubyrin (naphthorubyrin; 2) that retains its basic
structure upon protonation, whereas it gives rise to an inverted
sapphyrin (naphthosapphyrin; 3) that undergoes two successive
pyrrole-based ring inversions upon protonation.
Expanded porphyrins are pyrrole-containing macrocyclic
compounds that contain more than 16 non-hydrogen atoms
in their innermost periphery. This class of compounds includes
a variety of cyclic systems with more than four pyrrole rings
linked in a conjugated fashion. The interest in these latter
systems lies in their potential applications in various fields
including anion recognition,1 photodynamic therapy (PDT),2
and MRI contrast agent development.3 Expanded porphyrins
have also attracted attention as nonlinear optical materials
and as systems with which to explore the subtleties of
aromaticity.4 In recent years, many expanded porphyrins
and their analogs have been reported in the literature.5 Among
them, two systems, namely the penta- and hexapyrrolic
expanded porphyrins, sapphyrin and rubyrin, are of particular
interest because of the historic role they played in the initial
development of porphyrin analogue chemistry. These systems
and their heterocyclic analogues (where one or more pyrroles
is replaced by a different heterocycle) exhibit a considerable
degree of structural diversity, as well as unique spectroscopic
features.6 Certain sapphyrins and rubyrins are also known to
Naphthorubyrin 2 and naphthosapphyrin 3 were synthe-
sized via the mixed pyrrole–aldehyde condensation shown in
Scheme 1. The key precursor, naphthobipyrrole 1, along with
pentafluorobenzaldehyde and pyrrole (molar ratio: 1 : 1 : 2)
were reacted in the presence of an acid catalyst (trifluoroacetic
acid) at room temperature. Following DDQ oxidation, the
pyrrole-inverted rubyrin 2 and sapphyrin 3 were each obtained
in ca. 2% yield. Although the yields are low, the two compounds
were easily separated by column chromatography.
Naphthorubyrin 2 was characterized as its HCl salt by
standard spectroscopic methods as well as via single crystal
X-ray diffraction analysis.z The resulting structure revealed
a Department of Chemistry and Institute of Molecular Science and
Fusion Technology, Kangwon National University,
Chun-Chon 200-701, Korea. E-mail: chhlee@kangwon.ac.kr;
Fax: +82 33 2537582; Tel: +82 33 2508490
b Department of Chemistry, Yonsei University, Seoul 120-749, Korea.
E-mail: dongho@yonsei.ac.kr; Fax: +82-2-2123-2434;
Tel: +82-2-2123-2436
c Department of Chemistry and Biochemistry University of Texas at
Austin, Austin TX 78712, USA. E-mail: sessler@mail.utexas.edu;
Fax: +1 512 471 7550; Tel: +1 512 471 5009
d School of Chemistry, University of Hyderabad, Hyderabad-500046,
India. E-mail: pkpsc@uohyd.ernet.in
w Electronic supplementary information (ESI) available: Synthesis,
X-ray crystal data and spectroscopic data are available. CCDC
812383. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c1cc11733e
Scheme 1 Synthesis of rubyrin 2 and the pyrrole-inverted sapphyrin 3.
Chem. Commun., 2011, 47, 6813–6815 6813
c
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by molecular rigidity, molecules with distorted conformations
typically exhibit shorter excited state lifetimes than their
corresponding planar analogues.8 In the specific case of expanded
porphyrins, considerable effort has been made to correlate
conformational changes due to electronic effects or environ-
mental factors, such as aromaticity, acidity, solvent polarity,
and temperature, with various excited state properties, including
singlet lifetimes, nonlinear properties, and aromaticity.4
Moreover, structural modifications, such as pyrrole ring
confusion and fusion, as well as internal (e.g., intramacrocycle)
linkages, have been analyzed in terms of photophysical
features.9 The excited state lifetimes recorded for the various
forms of 2 are thus fully consistent with the lack of significant
conformational perturbation taking place upon protonation
that was inferred from the 1H NMR spectroscopic analyses
detailed above (ESIw).
Fig. 1 Top and side views of the structure of the mono-HCl salt of
naphthorubyrin 2 (2ꢀHCl), as deduced from a single crystal X-ray
diffraction analysis. Note that the chloride counter anion is bound within,
albeit slightly above, the central core. The NH protons were seen in the
difference map. H atoms are omitted for clarity in the side view. The
displacement ellipsoids are scaled to the 50% probability level.
that the protonated form of this annulated rubyrin exists
in a nearly planar conformation in the solid state (Fig. 1).
In contrast to what might be inferred from studies of
rubyrins lacking a fused bipyrrolic subunit, the HCl salt of
naphthorubyrin 2 (2ꢀHCl) exists as single conformation in the
solid state.6a The central macrocyclic structure is essentially
planar, and the chloride counter anion is found to be
complexed within the hexaaza core. The nitrogen–chloride
distances range from 3.163 A for the four nitrogen atoms of
the bipyrrole units to 3.753 A for the two individual pyrrolic
subunits. The chloride anion resides ca. 0.889 A above the
mean plane of the molecule.
The second product isolated from the condensation reaction
shown in Scheme 1 was the pyrrole-inverted sapphyrin 3. The
existence of the inverted pyrrole subunit was inferred from the
1H NMR spectrum. In particular, pyrrole NH signals at
11.04 ppm (in addition to a ꢁ0.72 ppm peak for the inner
pyrrole NH protons) were observed (Fig. 2).
While the dominance of this inverted conformation
stands in contrast with what was seen in the case of the
naphthorubyrin 2 (vide supra), it is consistent with what was
seen in the case of the free-base form of tetraphenyl sapphyrin.
This latter species, unlike b-alkyl substituted sapphyrins,
adopts a conformation wherein one pyrrole is inverted such
that one constituent pyrrole NH proton faces outward from
the center of the core.10 In the case of tetraphenyl sapphyrin,
protonation induces a conformational change and leads to
production of a ‘‘normal’’ conformation wherein all three NH
protons face inwards.10 The apparent stability of this species
differs from that of tetra(pentafluorophenyl)sapphyrin, where
the pyrrole-inverted form has been reported as being unstable.11
In the case of sapphyrin 3, two distinct sets of proton-
induced changes in conformation are observed. Firstly, treat-
ment with ca. 10 equiv. of trifluoroacetic acid (TFA) in CDCl3
leads to changes in the 1H NMR spectrum consistent with the
formation of an all-in conformer (Fig. 2). In particular, the
b-pyrrolic CH proton signal initially observed at ꢁ0.73 ppm
was seen to shift to 8.93 ppm upon addition of 10 equiv. of
1
The H NMR spectra of the free-base, mono- and diproto-
nated forms of naphthorubyrin 2 were recorded in DMSO-d6.
This particular solvent was chosen because it allowed for a
clean separation between all the signals. In the case of the
free-base form, the b-pyrrolic proton resonances are split and
appear as two singlets, whereas three distinct NH proton
signals were observed upon protonation (cf. ESIw). Treatment
with ca. 200 equiv. trifluoroacetic acid led to complete
conversion to the diprotonated form; this species displays two
NH signals in the 1H NMR spectrum that are broadened
compared to what is seen in the case of the monoprotonated form.
The absorption spectra of free-base rubyrin 2 recorded in
CH2Cl2 was characterized by a split Soret band (lmax = 528
and 562 nm), as well as by two Q-bands appearing at 841 and
875 nm. The mono-protonated form displayed a similar
UV-vis spectrum, albeit with slightly red-shifted absorption
bands. In this case, the Soret bands appeared at 532 and
590 nm while a single Q-band at 877 nm was observed. On the
other hand, the diprotonated form was characterized by a
relatively strong Soret-like band (lmax = 557 nm), as well as
by multiple Q-bands at 733, 823 and 922 nm (ESIw). The
neutral and protonated forms of 2 all gave rise to appreciable
fluorescence emission bands, which shift to the red on
protonation (lmax = 888, 908, and 948 nm for the free-base,
mono- and diprotonated forms, respectively).
The free-base form of 2 (studied in toluene) shows single
exponential decay dynamics with a time component of 537 ps
in analogy to what is observed for normal rubyrin.7 In
contrast, the singlet excited state lifetimes of the mono- and
diprotonated forms are reduced to 137 and 289 ps, respectively
(ESIw). Since nonradiative deactivation processes are influenced
Fig. 2 1H NMR spectral changes seen for 3 upon addition of
trifluoroacetic acid in CDCl3. Note the proton-induced changes in
the chemical shifts of the CH (Hc) and NH (Ha) protons of the pyrrole
subunit undergoing inversion.
c
6814 Chem. Commun., 2011, 47, 6813–6815
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The spectral features assigned to the longer time constants are
thus thought to reflect the extended conjugation provided by
the fused bipyrrole linkage (ESIw).
In conclusion, we have synthesized a distortion-resistant,
p-extended rubyrin and a congeneric annulated sapphyrin.
Naphthorubyrin 2 exhibited spectroscopic properties consistent
with aromaticity and displayed a lack of conformational motion.
Naphthosapphyrin 3 also displayed full aromatic character.
However, in this case, one pyrrole adopts an inverted geometry.
Reinversion of the inverted pyrrole was observed upon mono-
protonation, while treatment with excess acid led to formation of
a diprotonated form with spectral features consistent with
an ‘‘NH out’’ arrangement of the pyrrolic subunit. These
proton-triggered conformational interconversions go beyond
what has been observed in other expanded porphyrin systems,
including simple meso-tetraaryl substituted sapphyrins, and lead
us to suggest that compound 3 could have a role to play as a
proton-triggered molecular switching entity.
Scheme 2 Proposed conformational changes that accompany the
sequential protonation of 3 with trifluoroacetic acid.
TFA, while the NH resonance originally seen at 11.04 ppm
was seen to shift to ꢁ2.29 ppm. This all-in species was assigned
to the mono-protonated form of 3, [3ꢀH]+.
The further addition of TFA led to increasing production of
the diprotonated form of 3. In the presence of ca. 1500 equiv.
of TFA, the diprotonated form dominates. This form was
characterized by features analogous to the original free-base
form (Fig. 2). In particular, one set of b-pyrrolic CH resonances
was seen at ꢁ0.44 ppm, while three sets of NH signals were
observed (at ca. 14.51, ꢁ5.80, and ꢁ2.07 ppm, respectively).
We thus conclude that double protonation causes re-inversion
of the pyrrolic subunit and production of a ‘‘pyrrole NH out’’
conformation (cf. Scheme 2).
This work was supported by Basic Science Research
Program through the NRF (2009-008713), the National
Science Foundation (CHE 0749571 to J.L.S. and CHE
0741973 for the X-ray diffractometer), the Robert A. Welch
Foundation (F-1018 to J.L.S.), and the Korean WCU
program (grant R32-2010-10217-0). MEST and NRF support
for the Central Instrumentation Facility at KNU and the
‘Human Resource Development Center for Economic Region
Leading Industry’ is also acknowledged.
Proton-induced effects were also observed in the UV-vis
spectrum of 3. For instance, the free-base form of naphtho-
sapphyrin 3 exhibits split Soret-like bands at 508 and 533 nm,
along with three Q-like bands at 643, 715, and 781 nm (ESIw).
In the presence of 10 equiv. of TFA where the monoprotonated
form predominates, the longer wavelength absorption of the
Soret-like band is shifted to 546 nm and featureless, broad
Q-like bands are observed. On the other hand, the diprotonated
form (obtained in the presence of excess TFA) is characterized
by a reduced splitting between the two Soret-like bands and a
significant increase in the molar absorptivity. Relatively clear
Q-like bands are also observed at 657 and 716 nm with a rather
strong feature being seen at 784 nm. There is thus a spectral
congruence between the free-base and diprotonated forms of
naphthosapphyrin (3 and [3ꢀ2H]2+) that leads us to propose
that both species have common electronic and geometric
features.
Notes and references
z Crystal data for 2ꢀHCl ([C64H20F20N6]1+Clꢁ,2(C5H12),2(CH2Cl2)):
Mw = 1602.45 g molꢁ1, size 0.25 ꢂ 0.07 ꢂ 0.05 mm, 233 K, tetragonal,
ꢀ
P421/m, a = 21.411(2) A, b = 21.411(2) A, c = 7.5217(8) A, a = b =
g = 901, V = 3448.2(7) A3, Z = 2, 19 139 reflections measured/1742
independent (Rint = 0.0938), R1 = 0.0526, wR2 = 0.1248 (I 4 2s(I)),
R1 = 0.0877, wR2 = 0.1409 (all data), residual density peaks 0.318 to
ꢁ0.225 e A3, CCDC 812383.
1 M. Shionoya, H. Furuta, V. Lynch, A. Harriman and J. L. Sessler,
J. Am. Chem. Soc., 1992, 114, 5714.
2 J. L. Sessler and R. A. Miller, Biochem. Pharmacol., 2000, 59, 733.
3 J. L. Sessler, T. D. Mody, G. W. Hemmi, V. M. Lynch,
S. W. Young and R. A. Miller, J. Am. Chem. Soc., 1993, 115,
10368.
Further evidence in support of the proposed protonation-
dependent conformational change came from more advanced
optical spectroscopic analyses of 3 in its various protonated
states. While the free-base form displays a fluorescent lifetime
of 1.3 ns, which is comparable to that of sapphyrin (1.2 ns;
cf. ESIw), the excited state dynamics of the mono- and
diprotonated forms show relatively reduced fluorescence life-
times of 250 and 180 ps, respectively. In addition to these key
features, short-lived components with half-lives of 33 and 14 ps
are observed for the free-base and monoprotonated forms,
respectively. Compared to naphthorubyrin 2, naphthosapphyrin
3 has an asymmetric molecular structure and shorter internal
distances among its pyrrolic nitrogens and inner hydrogens. It
was thus considered likely that the fast time components seen
for 3 and [3ꢀH]+ reflect structural perturbations originating
from the presence of the single bipyrrole-fused moiety and
various internal proton transfer processes (e.g., tautomerizations).
In fact, transient features corresponding to the shorter
time constants are seen in the case of normal sapphyrin (ESIw).
4 (a) J.-Y. Shin, K. S. Kim, M.-C. Yoon, J. M. Lim, Z. S. Yoon,
A. Osuka and D. Kim, Chem. Soc. Rev., 2010, 39, 2751;
(b) J. M. Lim, Z. S. Yoon, J.-Y. Shin, K. S. Kim, M.-C. Yoon
and D. Kim, Chem. Commun., 2009, 261.
5 J. L. Sessler and D. Seidel, Angew. Chem., Int. Ed., 2003, 42, 5134.
6 (a) S. Shimizu, W.-S. Cho, J. L. Sessler, H. Shinokubo and
A. Osuka, Chem.–Eur. J., 2008, 14, 2668; (b) R. Misra and
T. K. Chandrashekar, Acc. Chem. Res., 2008, 41, 265;
(c) D. Wu, A. B. Descalzo, F. Weik, F. Emmerling, Z. Shen,
X.-Z. You and K. Rurack, Angew. Chem., Int. Ed., 2008, 47, 193.
7 Z. S. Yoon, J. H. Kwon, M.-C. Yoon, M. K. Koh, S. B. Noh,
J. L. Sessler, J. T. Lee, D. Seidel, A. Aguilar, S. Shimizu,
M. Suzuki, A. Osuka and D. Kim, J. Am. Chem. Soc., 2006,
128, 14128.
8 H.-E. Song, J. A. Cissel, T. P. Vaid and D. Holten, J. Phys.
Chem. B, 2007, 111, 2138.
9 (a) J. M. Lim, M. Inoue, Y. M. Sung, M. Suzuki, T. Higashino,
A. Osuka and D. Kim, Chem. Commun., 2011, 47, 3960;
(b) M.-C. Yoon, S. Cho, M. Suzuki, A. Osuka and D. Kim,
J. Am. Chem. Soc., 2009, 131, 7360.
10 K. Rachlewicz, N. Sprutta, L. Latos-Grazynski, P. J. Chmielewski
and L. Szterenberg, J. Chem. Soc., Perkin Trans. 2, 1998, 959.
11 L. Simkhovich, S. Rosenberg and Z. Gross, Tetrahedron Lett.,
2001, 42, 4929.
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Chem. Commun., 2011, 47, 6813–6815 6815