5 research outputs found
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
Tunneling Rearrangement of 1âAzulenylcarbene
1-Azulenylcarbene was synthesized by photolysis of 1-azulenyldiazomethane
in argon or neon matrices at 3â10 K. The highly polar singlet
carbene is only metastable and undergoes a tunneling rearrangement
to 8-methylene-bicyclo[5.3.0]Âdeca-1,3,5,6,9-pentaene. After substitution
of the 4 and 8 positions with deuterium, the rearrangement is completely
inhibited. This indicates a very large kinetic isotope effect, as
expected for a tunneling reaction
CâH Bond Amination by Photochemically Generated Transient Borylnitrenes at Room Temperature: A Combined Experimental and Theoretical Investigation of the Insertion Mechanism and Influence of Substituents
A number of azidoboranes having substitution patterns
that are
derived from catechol (<b>3</b>), pinacol (<b>4a</b>),
1,2-diaminoethane (<b>4b</b>,<b>c</b>), 1,2-ethanedithiol
(<b>4d</b>), and 1,2,4,5-tetrahydroxybenzene as well as acyclic
dialkoxy species (<b>5</b><b></b>) were synthesized and,
in the case of <b>4c</b> (<i>N</i>,<i>N</i>â˛-ditosyl-2-azido-1,3,2-diazaborolane), also structurally
characterized. The azidoboranes were photolyzed in cyclohexane solvent
in order to investigate the tendency of the generated borylnitrenes
to undergo intermolecular CâH insertion reactions. The yields
of intermolecular insertion products ranged from very good (<b>4a</b>) to vanishingly small, depending on the substitution of
the azidoborane. For a number of borylnitrenes the zero-field splitting
parameter <i>D</i> was measured in organic glasses at 4
K. The small primary kinetic isotope effect (<i>k</i><sub>H</sub>/<i>k</i><sub>D</sub> = 1.35) measured for <b>4a</b> in mixtures of [H<sub>12</sub>]Âcyclohexane and [D<sub>12</sub>]Âcyclohexane suggests that the insertion reaction is concerted and
involves the singlet state of the borylnitrene. Computations at the
CBS-QB3 and CCSDÂ(T)/TZ2P levels of theory show that the relative energies
of singlet and triplet states of a wide variety of borylnitrenes and
even their nature as minima or saddle points depend strongly on the
substituents. Photolysis of the most reactive azidoborane, <b>4a</b>, in methane in a flow reactor at atmospheric pressure produces an
intermolecular insertion product in low yields, in agreement with
the expectation of intersystem crossing to the less reactive triplet
state of the borylnitrene
Electronic Delocalization in the Radical Cations of Porphyrin Oligomer Molecular Wires
The
radical cations of a family of Ď-conjugated porphyrin arrays
have been investigated: linear chains of <i>N</i> = 1â6
porphyrins, a 6-porphyrin nanoring and a 12-porphyrin nanotube. The
radical cations were generated in solution by chemical and electrochemical
oxidation, and probed by visâNIRâIR and EPR spectroscopies.
The cations exhibit strong NIR bands at âź1000 nm and 2000â5000
nm, which shift to longer wavelength with increasing oligomer length.
Analysis of the NIR and IR spectra indicates that the polaron is delocalized
over 2â3 porphyrin units in the linear oligomers. Some of the
IR vibrational bands are strongly intensified on oxidation, and Fano-type
antiresonances are observed when activated vibrations overlap with
electronic transitions. The solution-phase EPR spectra of the radical
cations have Gaussian lineshapes with linewidths proportional to <i>N</i><sup>â0.5</sup>, demonstrating that at room temperature
the spin hops rapidly over the whole chain on the time scale of the
hyperfine coupling (ca. 100 ns). Direct measurement of the hyperfine
couplings through electronânuclear double resonance (ENDOR)
in frozen solution (80 K) indicates distribution of the spin over
2â3 porphyrin units for all the oligomers, except the 12-porphyrin
nanotube, in which the spin is spread over about 4â6 porphyrins.
These experimental studies of linear and cyclic cations give a consistent
picture, which is supported by DFT calculations and multiparabolic
modeling with a reorganization energy of 1400â2000 cm<sup>â1</sup> and coupling of 2000 cm<sup>â1</sup> for charge transfer
between neighboring sites, placing the system in the RobinâDay
class III
Electronic Delocalization in the Radical Cations of Porphyrin Oligomer Molecular Wires
The
radical cations of a family of Ď-conjugated porphyrin arrays
have been investigated: linear chains of <i>N</i> = 1â6
porphyrins, a 6-porphyrin nanoring and a 12-porphyrin nanotube. The
radical cations were generated in solution by chemical and electrochemical
oxidation, and probed by visâNIRâIR and EPR spectroscopies.
The cations exhibit strong NIR bands at âź1000 nm and 2000â5000
nm, which shift to longer wavelength with increasing oligomer length.
Analysis of the NIR and IR spectra indicates that the polaron is delocalized
over 2â3 porphyrin units in the linear oligomers. Some of the
IR vibrational bands are strongly intensified on oxidation, and Fano-type
antiresonances are observed when activated vibrations overlap with
electronic transitions. The solution-phase EPR spectra of the radical
cations have Gaussian lineshapes with linewidths proportional to <i>N</i><sup>â0.5</sup>, demonstrating that at room temperature
the spin hops rapidly over the whole chain on the time scale of the
hyperfine coupling (ca. 100 ns). Direct measurement of the hyperfine
couplings through electronânuclear double resonance (ENDOR)
in frozen solution (80 K) indicates distribution of the spin over
2â3 porphyrin units for all the oligomers, except the 12-porphyrin
nanotube, in which the spin is spread over about 4â6 porphyrins.
These experimental studies of linear and cyclic cations give a consistent
picture, which is supported by DFT calculations and multiparabolic
modeling with a reorganization energy of 1400â2000 cm<sup>â1</sup> and coupling of 2000 cm<sup>â1</sup> for charge transfer
between neighboring sites, placing the system in the RobinâDay
class III