Porphyrin-Polyyne [3]- and [5]Rotaxanes

Article abstract of DOI:10.1021/acs.orglett.6b03528

Porphyrin-polyyne [3]- and [5]rotaxanes have been synthesized by condensing aldehyde-rotaxanes with pyrrole or dipyrromethane. The crystal structure of a [3]rotaxane shows that the macrocycles adopt compact conformations, holding the hexaynes near the por

Full text of DOI:10.1021/acs.orglett.6b03528

Letter
pubs.acs.org/OrgLett
Porphyrin−Polyyne [3]- and [5]Rotaxanes
Daniel R. Kohn, Levon D. Movsisyan, Amber L. Thompson, and Harry L. Anderson*
University of Oxford, Department of Chemistry, Chemistry Research Laboratory, Oxford OX1 3TA, United Kingdom
S
* Supporting Information
ABSTRACT: Porphyrin−polyyne [3]- and [5]rotaxanes have
been synthesized by condensing aldehyde−rotaxanes with
pyrrole or dipyrromethane. The crystal structure of a
[3]rotaxane shows that the macrocycles adopt compact
conformations, holding the hexaynes near the porphyrin
core, and that the phenanthroline units form intermolecular
π-stacked dimers in the solid. Fluorescence spectra reveal
singlet excited-state energy transfer from the threaded hexayne
to the porphyrin, from the phenanthroline to the porphyrin,
and from the phenanthroline to the hexayne.
acrocycles consisting entirely of sp-hydridized carbon are
1−3
Mintriguing synthetic targets.
Cyclocarbons have been
studied in the gas phase,1,2,4 but they have not yet been isolated
or characterized in solution, despite attempts by Diederich,
Tobe, Rees, and co-workers.1−3 Advances in active metal
template synthesis5 provide access to an expanding diversity of
interlocked molecules, including threaded polyynes,6−11 and it
has been found that linear chains of sp-hybridized carbon atoms
(both polyynes and cumulenes) can be stabilized by threading
them through macrocycles to form rotaxanes.11−13 As part of a
project directed toward the synthesis of cyclocarbon catenanes,
we are exploring strategies for positioning several polyyne
rotaxane units around a central molecular hub, so that the
polyynes can then be linked together to form a threaded
cyclocarbon. Here, we present the first step in this endeavor: the
synthesis of a [5]rotaxane in which four hexaynes are positioned
around a porphyrin core. While the primary motivation for this
work was to explore approaches to cyclocarbon catenanes, this
study also provides insights into the interactions between the
singlet excited states of polyyne and porphyrin chromophores.
The initial target of this work was the porphyrin [5]rotaxane
P5Ra shown in Figure 1. All attempts at the synthesis of this
Figure 1. Structure of target [5]rotaxane P5Ra.
[5]rotaxane failed, presumably as a consequence of the steric
bulk of the four [2]rotaxane substituents. However, we
successfully synthesized the analogous porphyrin [5]rotaxane
P5Rb with p-phenylene spacers at the meso-positions and the
wicz coupling7,11 of supertrityl triyne Tr*C6H and bromotriyne
Tr*C6Br [where Tr* is tris(3,5-di-tert-butylphenyl)methyl16] in
the presence of the copper(I) complex of Ma gave the
[2]rotaxane aldehyde 2Ra in 58% yield. A wide variety of
conditions were tested for the reaction of 2Ra with pyrrole in
attempts to prepare P5Ra, but they all failed to give even a trace
of this target [5]rotaxane, whereas control reactions of aldehyde
Ma produced porphyrin PM4a in good yield. In contrast,
reaction of the rotaxane aldehyde 2Ra with dipyrromethane gave
the porphyrin [3]rotaxane P3Ra in 62% yield. These results
corresponding porphyrin [3]rotaxanes P3Ra and P3Rb (Scheme
1). Here, we present the synthesis of these rotaxanes and an
analysis of their fluorescence behavior, together with the crystal
structure of a porphyrin [3]rotaxane P3Ra. This structure shows
that the macrocycles adopt compact conformations due to the
predominantly gauche geometries of the O−CH2CH2−O links,
which increases the effective steric bulk of the rotaxane
substituents and explains why the [5]rotaxane P5Ra could not
be synthesized.
Our synthetic approach starts with the macrocyclic aldehyde
Received: November 26, 2016
Ma (Scheme 1).14,15 Active metal template Cadiot−Chodkie-
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b03528
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Organic Letters
Letter
a
Scheme 1. Polyrotaxane and Porphyrin Synthesis
a
Porphyrins PM4a, PM2a, P3Ra, P5Ra, PM4b, PM2b, P3Rb, and P5Rb were isolated as zinc complexes after treatment with Zn(OAc)2·2H2O in
methanol/chloroform.
imply that formation of P5Ra is blocked by the steric bulk of the
rotaxane units, so we investigated the analogous compounds
derived from macrocycle Mb, which has a p-phenylene spacer
between the aldehyde functionality and the macrocycle.
Increasing the distance between the bulky rotaxane unit and
the porphyrin hub made it possible to synthesize [5]rotaxane
P5Rb (10% yield isolated).
The porphyrin [3]- and [5]rotaxanes, and reference
porphyrins, were fully characterized as their zinc complexes
Zn−P3Ra, Zn−P3Rb, Zn−P5Rb, Zn−PM2a, Zn−PM4a, Zn−
PM2b, and Zn−PM4b (see the Supporting Information).
Crystals of Zn−P3Ra were grown by diffusion of methanol
vapor into a solution in chloroform and analyzed by single-crystal
X-ray diffraction.17 The crystal structure has two molecules of
B
DOI: 10.1021/acs.orglett.6b03528
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Organic Letters
Letter
Zn−P3Ra in the asymmetric unit; both rotaxane molecules have
similar conformations with methanol coordinated to the zinc
centers. The phenanthroline units of each rotaxane form
intermolecular π-stacked dimers with crystallographically equiv-
alent rotaxanes, forming infinite strands of interactions, as
illustrated in Figure 2. The oligoethylene glycol chains of the
demonstrates that there is no significant energy migration from
the porphyrin to the polyyne or to the macrocycle.
Fluorescence excitation spectra of the rotaxanes, measured by
recording emission from the porphyrin Q-band at around 640
nm, show clear features from excitation into the polyyne and
phenanthroline macrocycle components, as illustrated for Zn−
P3Ra and Zn−PM2a in Figure 3, indicating that both of these
Figure 3. (a) Absorption spectra (continuous line) and excitation
spectra (dashed line) of Zn−P3Ra (red) and Zn-PM2a (blue); the
spectral intensities are normalized to 1.0 at the Q-band, and the
excitation spectra are recorded for emission at 639 nm (porphyrin Q-
band). Solvent: CH2Cl2. (b) Jablonski diagram showing the energy-
transfer processes.
Figure 2. Projection of the structure of the [3]rotaxane Zn−P3Ra from
X-ray crystallography. This figure only shows one of the two molecules
in the asymmetric unit.
chromophores transfer energy to the porphyrin. A detailed
examination of the absorption spectra, fluorescence spectra, and
excitation spectra of the whole family of compounds allowed us
to determine the quantum yields of excited-state energy transfer
(EET) from the polyyne to the porphyrin and from the
phenanthroline macrocycle to the porphyrin as summarized in
Table 1 (see the SI for full details).
macrocycles adopt compact conformations and are mostly
gauche (O−CH2CH2−O torsional angle 40−80°) with the
polyyne chains held close to the porphyrin units. Thus, all four
polyyne chains in the asymmetric unit make van der Waals
contacts with the β-pyrrole protons of the porphyrins (sp-C···H
distance: 2.9−3.1 Å). The hexayne chains have slightly curved
geometries, similar to those reported in crystal structures of other
polyynes.9,11,16,18−20
Table 1. Energy-Transfer Efficiencies ϕEET
compd
ϕEET(macrocycle → porphyrin) ϕEET(polyyne → porphyrin)
The family of rotaxanes synthesized during this study provides
a unique opportunity to investigate photophysical interactions
between the excited states of polyyne and porphyrin
chromophores. The photophysical behavior of polyynes is
particularly interesting because their absorption spectra are
dominated by strong transitions to higher excited states at 280−
320 nm (S0−Sn, ε ≈ 3 × 105 M−1 cm−1); absorption into the first
excited state is dipole-forbidden and can occasionally be
observed as a very weak band at 350−450 nm (S0−S1, ε ≈ 500
M−1 cm−1).16,18 Polyynes also exhibit remarkably fast inter-
system crossing (S1−T1), which usually prevents observation of
fluorescence.
The absorption spectra of rotaxanes Zn−P3Ra, Zn−P3Rb,
and Zn−P5Rb demonstrate that threading causes very little
perturbation to the electronic structure of the component
chromophores. Thus, the absorption bands of the polyyne
(280−320 nm), the phenanthroline macrocycle (250−380 nm),
and porphyrin (400−450 nm Soret band and 530−560 nm Q-
band) are readily identified in the rotaxanes. Comparison with
simple reference porphyrin derivatives (meso-tetraphenyl
porphyrin for Zn−P5Rb and Zn−PM4a; 5,15-diaryl porphyrin
for Zn−P3Ra, Zn−P3Rb, Zn−PM2a, and Zn−PM2b) shows
that the polyyne and phenanthroline macrocycle units have no
effect on the fluorescence quantum yield, measured on excitation
of the porphyrin Soret band at around 415 nm. This
Zn−PM2a
Zn−P3Ra
Zn−PM2b
Zn−P3Rb
Zn−PM4b
Zn−P5Rb
0.67 0.08
0.32 0.03
0.88 0.11
0.22 0.03
0.68 0.06
0.25 0.02
0.16 0.02
0.10 0.03
0.09 0.02
The comparison of macrocycle → porphyrin EET efficiencies
in the presence and absence of polyyne show that the polyyne
acts as a sink for singlet excited state energy from the
phenanthroline macrocycle, even though it does not quench
the porphyrin.18 The excited-state energy of the macrocycle is
split between the porphyrin and the polyyne. The higher
efficiency of polyyne → porphyrin EET in [3]rotaxane Zn−
P3Ra, compared with Zn−P3Rb and Zn−P5Rb, reflects the
shorter polyyne−porphyrin distance. The polyyne → porphyrin
EET must be very rapid to compete with intersystem crossing (τ
≈ 0.5 ns).18 Furthermore, the S1 of the polyyne is a dark state
with negligible oscillator strength; therefore, Forster-type energy
transfer is expected to be very inefficient, implying that the
observed EET occurs via a Dexter mechanism.
In conclusion, we have prepared a series of porphyrin−
polyyne [3]- and [5]rotaxanes that comprise multiple
chromophores in a unique interlocked architecture. An analysis
̈
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DOI: 10.1021/acs.orglett.6b03528
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Organic Letters
Letter
(9) Movsisyan, L. D.; Kondratuk, D. V.; Franz, M.; Thompson, A. L.;
Tykwinski, R. R.; Anderson, H. L. Org. Lett. 2012, 14, 3424.
(10) (a) Weisbach, N.; Baranova, Z.; Gauthier, S.; Reibenspies, J. H.;
of the absorption, fluorescence, and excitation spectra of the
rotaxanes reveals three types of energy-transfer processes (a)
from the phenanthroline macrocycle to the porphyrin, (b) from
the phenanthroline macrocycle to the polyyne, and (c) from the
polyyne to the porphyrin. This work paves the way for a
template-directed synthesis of polycatenane cyclo[n]carbons.
́
Gladysz, J. A. Chem. Commun. 2012, 48, 7562. (b) Baranova, Z.; Amini,
H.; Bhuvanesh, N.; Gladysz, J. A. Organometallics 2014, 33, 6746.
(11) Movsisyan, L. D.; Franz, M.; Hampel, F.; Thompson, A. L.;
Tykwinski, R. R.; Anderson, H. L. J. Am. Chem. Soc. 2016, 138, 1366.
(12) Franz, M.; Januszewski, J. A.; Wendinger, D.; Neiss, C.;
ASSOCIATED CONTENT
* Supporting Information
Movsisyan, L. D.; Hampel, F.; Anderson, H. L.; Gorling, A.;
̈
■
Tykwinski, R. R. Angew. Chem., Int. Ed. 2015, 54, 6645.
S
(13) Schrettl, S.; Contal, E.; Hoheisel, T.; Fritzsche, M.; Balog, S.;
Szilluweit, R.; Frauenrath, H. Chem. Sci. 2015, 6, 564.
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.6b03528.
(14) (a) Roche, C.; Sour, A.; Niess, F.; Heitz, V.; Sauvage, J. P. Eur. J.
Org. Chem. 2009, 2009, 2795. (b) Roche, C.; Sauvage, J.-P.; Sour, A.;
Strutt, N. L. New J. Chem. 2011, 35, 2820.
(15) (a) Megiatto, J. D.; Schuster, D. I. Org. Lett. 2011, 13, 1808.
(b) Megiatto, J. D.; Schuster, D. I.; de Miguel, G.; Wolfrum, S.; Guldi, D.
M. Chem. Mater. 2012, 24, 2472.
Details of synthetic procedures and characterization data,
analysis of absorption, fluorescence and excitation spectra,
and crystallographic analysis (PDF)
Crystallographic data for P3Ra (CIF)
(16) Chalifoux, W.; Tykwinski, R. R. Nat. Chem. 2010, 2, 967.
(17) Single-crystal X-ray diffraction data for Zn-P3Ra were collected
with synchrotron radiation using I19 (EH1) at Diamond Light Source
(λ = 0.6889 Å): Nowell, H.; Barnett, S. A.; Christensen, K. E.; Teat, S. J.;
Allan, D. R. J. Synchrotron Radiat. 2012, 19, 435. Cell parameters were
determined and refined and raw frame data were integrated using
CrysAlisPro (Agilent Technologies, 2010). The structure was solved
with SuperFlip: Palatinus, L.; Chapuis, G. J. Appl. Crystallogr. 2007, 40,
786. It was then refined by full-matrix least-squares on F2 using
CRYSTALS: Betteridge, P. W.; Carruthers, J. R.; Cooper, R. I.; Prout,
K.; Watkin, D. J. J. Appl. Crystallogr. 2003, 36, 1487. Cooper, R. I.;
Thompson, A. L.; Watkin, D. J. J. Appl. Crystallogr. 2010, 43, 1100.
Parois, P.; Cooper, R. I.; Thompson, A. L. Chem. Cent. J. 2015, 9, 30. The
structure contains large solvent-accessible voids comprising weak,
diffuse electron density which were treated using PLATON/
SQUEEZE: Spek, A. J. Appl. Crystallogr. 2003, 36, 7. van der Sluis, P.;
Spek, A. L. Acta Crystallogr., Sect. A: Found. Crystallogr. 1990, 46, 194.
Single Crystal Data: C301H348N8O17Zn, Mr = 4415.49, monoclinic, Pc; a
= 31.7209(5) Å, b = 19.3373(2) Å, c = 50.4748(7) Å, β = 96.0619(14)°,
V = 30787.9(7) Å, data/restraints/parameters: 52640/18794/5933, Rint
= 6.99%, final R1 = 12.36%, wR2 = 31.63% (I > 2σ(I)), Δρmin,max = −0.78,
+0.88 e Å3. Crystallographic data (excluding structure factors) have been
deposited with the Cambridge Crystallographic Data Centre (CCDC
1519120); copies of these data can be obtained free of charge via www.
ccdc.cam.ac.uk/data_request/cif.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: harry.anderson@chem.ox.ac.uk.
ORCID
Harry L. Anderson: 0000-0002-1801-8132
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the EPSRC and the European Research Council
(Grant No. 320969) for support, the EPSRC UK Mass
Spectrometry Facility at Swansea University for mass spectra,
and Diamond Light Source for an award of beamtime.
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Org. Lett. XXXX, XXX, XXX−XXX