25 research outputs found
On the Influence of the Bridge on Triplet State Delocalization in Linear Porphyrin Oligomers
The extent of triplet state delocalization
is investigated in rigid
linear zinc porphyrin oligomers as a function of interporphyrin bonding
characteristics, specifically in <i>meso</i>-<i>meso</i> singly linked and β,<i>meso</i>,β fused structures,
using electron paramagnetic resonance techniques. The results are
compared with those of earlier measurements on porphyrin oligomers
with alkyne linkers exhibiting different preferred conformations.
It is shown that dihedral angles near 90° between the porphyrin
planes in directly <i>meso</i>-to-<i>meso</i> linked
porphyrin oligomers lead to localization of the photoexcited triplet
state on a single porphyrin unit, whereas previous work demonstrated
even delocalization over two units in <i>meso</i>-to-<i>meso</i> ethyne or butadiyne-bridged oligomers, where the preferred
dihedral angles amount to roughly 30° and 0°, respectively.
The triplet states of fused porphyrin oligomers (i.e., porphyrin tapes)
exhibit extended conjugation and even delocalization over more than
two porphyrin macrocycles, in contrast to <i>meso</i>-to-<i>meso</i> ethyne or butadiyne-bridged oligomers, where the spin
density distribution in molecules composed of more than two porphyrin
units is not evenly spread across the oligomer chain
Size-Independent Energy Transfer in Biomimetic Nanoring Complexes
Supramolecular
antenna-ring complexes are of great interest due
to their presence in natural light-harvesting complexes. While such
systems are known to provide benefits through robust and efficient
energy funneling, the relationship between molecular structure, strain
(governed by nuclear coordinates and motion), and energy dynamics
(arising from electronic behavior) is highly complex. We present a
synthetic antenna-nanoring system based on a series of conjugated
porphyrin chromophores ideally suited to explore such effects. By
systematically varying the size of the acceptor nanoring, we reveal
the interplay between antenna-nanoring binding, local strain, and
energy dynamics on the picosecond time scale. Binding of the antenna
unit creates a local strain in the nanoring, and this strain was measured
as a function of the size of the nanoring, by UVâvis-NIR titration,
providing information on the conformational flexibility of the system.
Strikingly, the energy-transfer rate is independent of nanoring size,
indicating the existence of strain-localized acceptor states, spread
over about six porphyrin units, arising from the noncovalent antenna-nanoring
association
PorphyrinâPolyyne [3]- and [5]Rotaxanes
Porphyrinâpolyyne
[3]- and [5]Ârotaxanes have been synthesized
by condensing aldehydeârotaxanes with pyrrole or dipyrromethane.
The crystal structure of a [3]Ârotaxane shows that the macrocycles
adopt compact conformations, holding the hexaynes near the porphyrin
core, and that the phenanthroline units form intermolecular Ď-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
Triplet State Delocalization in a Conjugated Porphyrin Dimer Probed by Transient Electron Paramagnetic Resonance Techniques
The
delocalization of the photoexcited triplet state in a linear
butadiyne-linked porphyrin dimer is investigated by time-resolved
and pulse electron paramagnetic resonance (EPR) with laser excitation.
The transient EPR spectra of the photoexcited triplet states of the
porphyrin monomer and dimer are characterized by significantly different
spin polarizations and an increase of the zero-field splitting parameter <i>D</i> from monomer to dimer. The proton and nitrogen hyperfine
couplings, determined using electron nuclear double resonance (ENDOR)
and X- and Q-band HYSCORE, are reduced to about half in the porphyrin
dimer. These data unequivocally prove the delocalization of the triplet
state over both porphyrin units, in contrast to the conclusions from
previous studies on the triplet states of closely related porphyrin
dimers. The results presented here demonstrate that the most accurate
estimate of the extent of triplet state delocalization can be obtained
from the hyperfine couplings, while interpretation of the zero-field
splitting parameter <i>D</i> can lead to underestimation
of the delocalization length, unless combined with quantum chemical
calculations. Furthermore, orientation-selective ENDOR and HYSCORE
results, in combination with the results of density functional theory
(DFT) calculations, allowed determination of the orientations of the
zero-field splitting tensors with respect to the molecular frame in
both porphyrin monomer and dimer. The results provide evidence for
a reorientation of the zero-field splitting tensor and a change in
the sign of the zero-field splitting <i>D</i> value. The
direction of maximum dipolar coupling shifts from the out-of-plane
direction in the porphyrin monomer to the vector connecting the two
porphyrin units in the dimer. This reorientation, leading to an alignment
of the principal optical transition moment and the axis of maximum
dipolar coupling, is also confirmed by magnetophotoselection experiments
Self-Assembly of Linear Porphyrin Oligomers into Well-Defined Aggregates
Conjugated zinc porphyrin oligomers of various lengths
are shown
to form well-defined planar aggregates at low temperatures. The aggregation
occurs over a narrow temperature interval (170â150 K) and is
accompanied by dramatic changes in the electronic absorption and emission
spectra. Similar changes are found in J-aggregates in which the transition
dipole moments of aggregated chromophores couple to form a new and
intense transition in the absorption spectrum, red shifted from the
monomeric chromophore band. For the present porphyrin oligomers, the
dramatic absorption changes are not associated with the formation
of large aggregates, but rather with the dimerization accompanied
by planarization of the oligomers. Free oligomers have a broad distribution
of porphyrinâporphyrin dihedral angles and show a broad and
unstructured absorption spectrum. As the oligomers stack to form aggregates,
they planarize and the width of the conformational distribution is
reduced to include virtually only the planar conformers, resulting
in the observed change of the absorption spectrum. No experimental
evidence for the formation of large aggregates was found, while a
small aggregate, probably only dimer, is supported by the minor changes
of the fluorescence rate constant upon aggregation and the fact that
pyridine has no significant effect on the formation of this aggregate,
which otherwise is very effective for inhibiting aggregation of zinc
porphyrin oligomers. Compared to most porphyrin aggregates, which
show broad absorption spectra and quenched fluorescence, these aggregates
give sharp absorption and emission spectra with little change in the
fluorescence quantum yield. Similar aggregates were also observed
for oligomers substituted with both a fullerene electron acceptor
and a ferrocene donor. The results presented here will be potentially
useful as tools to understand how electron transfer and delocalization
processes are influenced by molecular order/disorder transitions
Synthesis of Polyynes Using Dicobalt Masking Groups
Extended triisopropylsilyl
end-capped polyynes have been prepared
from the corresponding tetracobalt complexes by removing the complexed
dicobalt tetracarbonyldiphenylphosphinomethane (Co<sub>2</sub>(CO)<sub>4</sub>dppm) moieties. Unmasking of this âmasked alkyne equivalentâ
was achieved under mild conditions with elemental iodine at room temperature,
making it possible to obtain fragile polyynes with up to 20 contiguous
sp-hybridized carbon atoms. The Co<sub>2</sub>(CO)<sub>4</sub>dppm
moiety has a strong geometric and steric effect on the polyyne but
does not have a marked electronic effect on the terminal alkyne, as
indicated by NMR and IR spectroscopy, density functional theory calculations,
and X-ray crystallography. An unusual âalkyne hoppingâ
migration of the dicobalt group was noticed as a minor side reaction
during copper-catalyzed Eglinton coupling
A Discrete Three-Layer Stack Aggregate of a Linear Porphyrin Tetramer: Solution-Phase Structure Elucidation by NMR and Xâray Scattering
Formation
of stacked aggregates can dramatically alter the properties
of aromatic Ď-systems, yet the solution-phase structure elucidation
of these aggregates is often impossible because broad distributions
of species are formed, giving uninformative spectroscopic data. Here,
we show that a butadiyne-linked zinc porphyrin tetramer forms a remarkably
well-defined aggregate, consisting of exactly three molecules, in
a parallel stacked arrangement (in chloroform at room temperature;
concentration 1 mMâ0.1 ÎźM). The aggregate has a mass
of 14.7 kDa. Unlike most previously reported aggregates, it gives
sharp NMR resonances and aggregation is in slow exchange on the NMR
time scale. The structure was elucidated using a range of NMR techniques,
including diffusion-editing, <sup>1</sup>Hâ<sup>29</sup>Si
HMBC, <sup>1</sup>Hâ<sup>1</sup>H COSY, TOCSY and NOESY, and <sup>1</sup>Hâ<sup>13</sup>C edited HSQC spectroscopy. Surprisingly,
the <sup>1</sup>Hâ<sup>1</sup>H COSY spectrum revealed many
long-range residual dipolar couplings (RDCs), and detailed analysis
of magnetic field-induced <sup>1</sup>Hâ<sup>13</sup>C RDCs
provided further evidence for the structural model. The size and shape
of the aggregate is supported by small-angle X-ray scattering (SAXS)
data. It adopts a geometry that maximizes van der Waals contact between
the porphyrins, while avoiding clashes between side chains. The need
for interdigitation of the side chains prevents formation of stacks
consisting of more than three layers. Although a detailed analysis
has only been carried out for one compound (the tetramer), comparison
with the NMR spectra of other oligomers indicates that they form similar
three-layer stacks. In all cases, aggregation can be prevented by
addition of pyridine, although at low pyridine concentrations, disaggregation
takes many hours to reach equilibrium
Photoswitchable Spiropyran Dyads for Biological Imaging
The synthesis of
a small-molecule dyad consisting of a far-red-emitting
silicon rhodamine dye that is covalently linked to a photochromic
spironaphthothiopyran unit, which serves as a photoswitchable quencher,
is reported. This system can be switched reversibly between the fluorescent
and nonfluorescent states using visible light at wavelengths of 405
and 630 nm, respectively, and it works effectively in aqueous solution.
Live-cell imaging demonstrates that this dyad has several desirable
features, including excellent membrane permeability, fast and reversible
modulation of fluorescence by visible light, and good contrast between
the bright and dark states
Polyyne Rotaxanes: Stabilization by Encapsulation
Active metal template Glaser coupling
has been used to synthesize
a series of rotaxanes consisting of a polyyne, with up to 24 contiguous <i>sp-</i>hybridized carbon atoms, threaded through a variety of
macrocycles. CadiotâChodkiewicz cross-coupling affords higher
yields of rotaxanes than homocoupling. This methodology has been used
to prepare [3]Ârotaxanes with two polyyne chains locked through the
same macrocycle. The crystal structure of one of these [3]Ârotaxanes
shows that there is extremely close contact between the central carbon
atoms of the threaded hexayne chains (C¡¡¡C distance
3.29 Ă
vs 3.4 Ă
for the sum of van der Waals radii) and
that the bond-length-alternation is perturbed in the vicinity of this
contact. However, despite the close interaction between the hexayne
chains, the [3]Ârotaxane is remarkably stable under ambient conditions,
probably because the two polyynes adopt a crossed geometry. In the
solid state, the angle between the two polyyne chains is 74°,
and this crossed geometry appears to be dictated by the bulk of the
âsupertritylâ end groups. Several rotaxanes have been
synthesized to explore gem-dibromoethene moieties as âmaskedâ
polyynes. However, the reductive FritschâButtenbergâWiechell
rearrangement to form the desired polyyne rotaxanes has not yet been
achieved. X-ray crystallographic analysis on six [2]Ârotaxanes and
two [3]Ârotaxanes provides insight into the noncovalent interactions
in these systems. Differential scanning calorimetry (DSC) reveals
that the longer polyyne rotaxanes (C16, C18, and C24) decompose at
higher temperatures than the corresponding unthreaded polyyne axles.
The stability enhancement increases as the polyyne becomes longer,
reaching 60 °C in the C24 rotaxane