27 research outputs found
Dictating Photoreactivity through Restricted Bond Rotations: Cross-Photoaddition of Atropisomeric Acrylimide Derivatives under UV/Visible-Light Irradiation
Nonbiaryl atropisomeric acrylimides
underwent facile [2 + 2] photocycloaddition
leading to cross-cyclobutane adducts with very high stereospecificity
(enantiomeric excess (ee): 99% and diastereomeric excess (de): 99%).
The photoreactions proceeded smoothly in isotropic media for both
direct and triplet sensitized irradiations. The reactions were also
found to be very efficient in the solid state where the same cross-cyclobutane
adduct was observed. Photophysical studies enabled us to understand
the excited-state photochemistry of acrylimides. The triplet energy
was found to be ∼63 kcal/mol. The reactions proceeded predominantly
via a singlet excited state upon direct irradiation with very poor
intersystem crossing that was ascertained by quantification of the
generated singlet oxygen. The reactions progressed smoothly with triplet
sensitization with UV or visible-light irradiations. Laser flash photolysis
experiments established the triplet transient of atropisomeric acrylimides
with a triplet lifetime at room temperature of ∼40 ns
Photochemical Reactivity of dTPT3: A Crucial Nucleobase Derivative in the Development of Semisynthetic Organisms
In 2017, the Romesberg group successfully
developed the dTPT3·dNaM
unnatural base pair to create a semisynthetic organism with enhanced
genetic fidelity and the ability to store additional genetic information
indefinitely. It is also desirable that the newly developed genetic
material remains stable upon exposure to radiation. However, the photochemical
properties of dTPT3 are presently unknown. In this contribution, excitation
of dTPT3 with near-visible radiation is shown to efficiently populate
a reactive triplet state in high yield and on a sub-1 ps time scale;
a state that is able to survive for up to a few microseconds in aqueous
solution. The triplet state can also generate singlet oxygen in ca.
30% yield, suggesting that dTPT3 can act as a pervasive photosensitizer
to accelerate oxidatively generated damage within DNA and to other
biological molecules within cells
2,4-Dithiothymine as a Potent UVA Chemotherapeutic Agent
Substitution of both oxygen atoms
in the exocyclic carbonyl groups
of the thymine chromophore by sulfur atoms results in a remarkable
redshift of its absorption spectrum from an absorption maximum at
267 nm in thymidine to 363 nm in 2,4-dithiothymine (Δ<i>E</i> = 9905 cm<sup>–1</sup>). A single sulfur substitution
of a carbonyl group in the thymine chromophore at position 2 or 4
results in a significantly smaller redshift in the absorption maximum,
which depends sensitively on the position at which the sulfur atom
is substituted, varying from 275 nm in 2-thiothymine to 335 nm in
4-thiothymidine. Femtosecond transient absorption spectroscopy reveals
that excitation of 2,4-dithiothymine at 335 or 360 nm leads to the
ultrafast population of the triplet state, with an intersystem crossing
lifetime of 180 ± 40 fsthe shortest intersystem crossing
lifetime of any DNA base derivative studied so far in aqueous solution.
Surprisingly, the degree and position at which the sulfur atom is
substituted have important effects on the magnitude of the intersystem
crossing rate constant, showing a 1.2-, 3.2-, and 4.2-fold rate increases
for 2-thiothymine, 4-thiothymidine, and 2,4-dithiothymine, respectively,
relative to that of thymidine, whereas the triplet yield increases
60-fold to near unity, independent of the site of sulfur atom substitution.
While the natural thymine monomers owe their high degree of photostability
to ultrafast internal conversion to the ground state and low triplet
yields, the near-unity triplet yields in the thiothymine series account
for their potent photosensitization properties. Nanosecond time-resolved
luminescence spectroscopy shows that 4-thiothymidine and 2,4-dithiothymine
are efficient singlet oxygen generators, with singlet oxygen quantum
yields of 0.42 ± 0.02 and 0.46 ± 0.02, respectively, in
O<sub>2</sub>-saturated acetonitrile solution. Taken together, these
photophysical measurements strongly suggest that 2,4-dithiothymine
can act as a more effective UVA chemotherapeutic agent than the currently
used 4-thiothymidine, especially in deeper-tissue chemotherapeutic
applications
Photoinitiated Metal-Free Controlled/Living Radical Polymerization Using Polynuclear Aromatic Hydrocarbons
Photoinitiated
metal-free controlled living radical polymerization
of (meth)acrylates, and vinyl monomers was investigated using the
polynuclear aromatic compounds pyrene and anthracene. Fluorescence
spectral analyses along with nuclear magnetic resonance studies were
performed to determine the rate constants of initiator radical formation
and to investigate the mechanisms of polymerization. The obtained
polymers were analyzed by spectral and chromatographic methods. Results
show that the excited state anthracene undergoes a faster electron
transfer reaction with the alkyl halide initiator than the excited
state of pyrene. Pyrene excimers, which are formed at higher concentrations,
also react with alkyl halides to form initiator radicals. Although
pyrene monomers and excimers are acting slower, polymers with higher
control over the chain end functionalities and molecular weight characteristics
are obtained in comparison to anthracene as sensitizer
CIDEP from a Polarized Ketone Triplet State Incarcerated within a Nanocapsule to a Nitroxide in the Bulk Aqueous Solution
Thioxanthone and benzil derivatives were incarcerated into an octa acid nanocapsule. Photoexcitation of these ketones generated electronic triplet excited states, which become efficiently quenched by positively charged nitroxides adsorbed outside on the external surface of the negatively charged nanocapsule. Although the triplet excited ketone and quencher are separated by a molecular wall (nanocapsule), quenching occurs on the nanosecond time scale and generates spin-polarized nitroxides, which were observed by time-resolved EPR spectroscopy. Because opposite signs of spin polarization of nitroxides were observed for thioxanthone and benzil derivatives, it is proposed that the electron spin polarization transfer mechanism of spin-polarized triplet states to nitroxides is the major mechanism of generating nitroxide polarization
Mechanisms by which Alkynes React with CpCr(CO)<sub>3</sub>H. Application to Radical Cyclization
The reaction of CpCr(CO)<sub>3</sub>H with activated
alkynes in
benzene has been examined. The kinetics of these reactions have been
studied with various alkynes, along with the stereochemistry with
which the alkynes are hydrogenated. The hydrogenation of phenyl acetylene
and diphenyl acetylene with CpCr(CO)<sub>3</sub>H has been shown to
occur by a hydrogen atom transfer (HAT) mechanism. The reaction of
CpCr(CO)<sub>3</sub>H with dimethyl acetylenedicarboxylate (DMAD)
produced hydrogenated products as well as phenyl substitution from
reaction with solvent. On the basis of kinetic data, it is thought
that the reaction of DMAD may proceed via a single electron transfer
(SET) as the rate-determining step. The radical anion of dimethylfumarate
was observed by EPR spectroscopy during the course of the reaction,
supporting this claim. The aromatic 1,6 eneyne (<b>8</b>) gave
cyclized products in 78% yield under catalytic conditions (35 psi
H<sub>2</sub>), presumably by the 5-exo-trig cyclization
of the vinyl radical arising from H• transfer. Using a cobaloxime
catalyst (<b>12</b>) hydrogenation was completely eliminated
to yield 100% cyclized products
Mechanisms by which Alkynes React with CpCr(CO)<sub>3</sub>H. Application to Radical Cyclization
The reaction of CpCr(CO)<sub>3</sub>H with activated
alkynes in
benzene has been examined. The kinetics of these reactions have been
studied with various alkynes, along with the stereochemistry with
which the alkynes are hydrogenated. The hydrogenation of phenyl acetylene
and diphenyl acetylene with CpCr(CO)<sub>3</sub>H has been shown to
occur by a hydrogen atom transfer (HAT) mechanism. The reaction of
CpCr(CO)<sub>3</sub>H with dimethyl acetylenedicarboxylate (DMAD)
produced hydrogenated products as well as phenyl substitution from
reaction with solvent. On the basis of kinetic data, it is thought
that the reaction of DMAD may proceed via a single electron transfer
(SET) as the rate-determining step. The radical anion of dimethylfumarate
was observed by EPR spectroscopy during the course of the reaction,
supporting this claim. The aromatic 1,6 eneyne (<b>8</b>) gave
cyclized products in 78% yield under catalytic conditions (35 psi
H<sub>2</sub>), presumably by the 5-exo-trig cyclization
of the vinyl radical arising from H• transfer. Using a cobaloxime
catalyst (<b>12</b>) hydrogenation was completely eliminated
to yield 100% cyclized products
Electron Spin Polarization Transfer from a Nitroxide Incarcerated within a Nanocapsule to a Nitroxide in the Bulk Aqueous Solution
A thioxanthone derivative containing a covalently attached <sup>15</sup>N-labeled nitroxide was incarcerated into an octaacid nanocapsule. Photoexcitation of the thioxanthone chromophore generated electron spin polarization of the nitroxide. This spin polarization of the <sup>15</sup>N-labeled nitroxide was transferred through the walls of the carcerand to a <sup>14</sup>N-labeled nitroxide in external bulk solvent, a process that was directly observed by time-resolved EPR spectroscopy. The efficiency of the communication between the incarcerated guest and molecules in the bulk solvent was shown to be controlled by supramolecular factors such as Coulombic attraction and repulsion between the guest@host complex and charged molecules in the bulk solvent phase
Photoinduced Electron Transfer Reactions of Highly Conjugated Thiophenes for Initiation of Cationic Polymerization and Conjugated Polymer Formation
Photoinduced electron transfer reactions of highly conjugated
thiophene
derivatives, 4,7-di(2,3-dihydrothieno[3,4-<i>b</i>][1,4]dioxin-5-yl)benzo[1,2,5]thiadiazole
(DTDT) and 5,8-bis(2,3-dihydrothieno[3,4-<i>b</i>][1,4]dioxin-5-yl)-2,3-di(thiophen-2-yl)quinoxaline
(DTDQ), with diphenyliodonium hexafluorophosphate (Ph<sub>2</sub>I<sup>+</sup>PF<sub>6</sub><sup>–</sup>) and triphenylsulfonium
hexafluorophosphate (Ph<sub>3</sub>S<sup>+</sup>PF<sub>6</sub><sup>–</sup>) were investigated by laser flash photolysis, fluorescence
and phosphorescence spectroscopy, and polymerization studies. High
fluorescence quantum yields, long fluorescence lifetimes (∼10
ns in aprotic solvents), and absence of detectable phosphorescence
at 77 K for both compounds indicate inefficient intersystem crossing
into the triplet state and dominant role of singlet excited state.
Photolysis of DTDT or DTDQ in the presence of iodonium salt with visible
light results in the formation of radical cations of DTDT and DTDQ
as detected by laser flash photolysis. Sulfonium salts do not undergo
such electron transfer reactions due to the unfavorable redox potentials.
The importance of the described photoinduced electron transfer process
with respect to the initiation of cationic polymerization and formation
of conjugated polymers was demonstrated
Tailoring Atropisomeric Maleimides for Stereospecific [2 + 2] PhotocycloadditionPhotochemical and Photophysical Investigations Leading to Visible-Light Photocatalysis
Atropisomeric maleimides
were synthesized and employed for stereospecific
[2 + 2] photocycloaddition. Efficient reaction was observed under
direct irradiation, triplet-sensitized UV irradiation, and non-metal
catalyzed visible-light irradiation, leading to two regioisomeric
(<i>exo</i>/<i>endo</i>) photoproducts with complete
chemoselectivity (exclusive [2 + 2] photoproduct). High enantioselectivity
(ee > 98%) and diastereoselectivity (dr > 99:1%) were observed
under
the employed reaction conditions and were largely dependent on the
substituent on the maleimide double bond but minimally affected by
the substituents on the alkenyl tether. On the basis of detailed photophysical
studies, the triplet energies of the maleimides were estimated. The
triplet lifetimes appeared to be relatively short at room temperature
as a result of fast [2 + 2] photocycloaddition. For the visible-light
mediated reaction, triplet energy transfer occurred with a rate constant
close to the diffusion-limited value. The mechanism was established
by generation of singlet oxygen from the excited maleimides. The high
selectivity in the photoproduct upon reaction from the triplet excited
state was rationalized on the basis of conformational factors as well
as the type of diradical intermediate that was preferred during the
photoreaction