3 research outputs found
Role of Free Space and Conformational Control on Photoproduct Selectivity of Optically Pure α‑Alkyldeoxybenzoins within a Water-Soluble Organic Capsule
Optically pure α-alkyl deoxybenzoins resulting
in products
of Norrish Type I and Type II reactions upon excitation has been investigated
within the octa acid (OA) capsule in water. The product distribution
was different from that in an organic solvent and was also dependent
on the length of the α-alkyl chain. Most importantly, a rearrangement
product not formed in an organic solvent arising from the triplet
radical pair generated by Norrish Type I reaction was formed, and
its yield was dependent on the alkyl chain length. In an organic solvent,
since the cage lifetime is shorter than the time required for intersystem
crossing (ISC) of the triplet radical pair to the singlet radical
pair the recombination with or without rearrangement of the primary
radical pair (phenylacetyl and benzyl) does not occur. Recombination
without rearrangement within the capsule as inferred from monitoring
the racemization of the optically pure α-alkyl deoxybenzoins
suggesting the capsule’s stability for at least 10<sup>–8</sup> s (the time required for ISC) is consistent with our previous photophysical
studies that showed partial opening and closing of the capsule in
the time range of microseconds
Kinetics of UV Irradiation Induced Chain Scission and Cross-Linking of Coumarin-Containing Polyester Ultrathin Films
Photoresponsive thin films are commonly
encountered as high performance
coatings as well as critical component, photoresists, for microelectronics
manufacture. Despite intensive investigations into the dynamics of
thin glassy polymer films, studies involving reactions of thin films
have typically been limited by difficulties in decoupling segregation
of reacting components or catalysts due to the interfaces. Here, thin
films of coumarin polyesters overcome this limitation where the polyester
undergoes predominately cross-linking upon irradiation at 350 nm,
while chain scission occurs with exposure to 254 nm light. Spectroscopic
ellipsometry is utilized to track these reactions as a function of
exposure time to elucidate the associated reaction kinetics for films
as thin as 15 nm. The cross-linking appears to follow a second order
kinetic rate law, while oxidation of the coumarin that accompanies
the chain scission and enables this reaction to be tracked spectroscopically
appears to be a first order reaction in coumarin concentration. Because
of the asymmetry in the coumarin diol monomer and the associated differences
in local structure that result during formation of the polyester,
two populations of coumarin are required to fit the reaction kinetics;
10–20% of the coumarin is significantly more reactive, but
these groups appear to undergo chain scission/oxidation at both wavelengths.
These reaction rate constants are nearly independent (within 1 order
of magnitude) of film thickness down to 15 nm. There is maximum rate
at a finite thickness for the 254 nm exposure, which we attribute
to constructive interference of the UV radiation within the polymer
film, rather than typical confinement effects; no thickness dependence
in reaction rates is observed for the 350 nm exposure. The utilization
of a single polymer with two distinct reactions enables unambiguous
investigation of thickness effects on reactions
Micropatterned Coumarin Polyester Thin Films Direct Neurite Orientation
Guidance
and migration of cells in the nervous system is imperative for proper
development, maturation, and regeneration. In the peripheral nervous
system (PNS), it is challenging for axons to bridge critical-sized
injury defects to achieve repair and the central nervous system (CNS)
has a very limited ability to regenerate after injury because of its
innate injury response. The photoreactivity of the coumarin polyester
used in this study enables efficient micropatterning using a custom
digital micromirror device (DMD) and has been previously shown to
be biodegradable, making these thin films ideal for cell guidance
substrates with potential for future in vivo applications. With DMD,
we fabricated coumarin polyester thin films into 10 × 20 μm
and 15 × 50 μm micropatterns with depths ranging from 15
to 20 nm to enhance nervous system cell alignment. Adult primary neurons,
oligodendrocytes, and astrocytes were isolated from rat brain tissue
and seeded onto the polymer surfaces. After 24 h, cell type and neurite
alignment were analyzed using phase contrast and fluorescence imaging.
There was a significant difference (<i>p</i> < 0.0001)
in cell process distribution for both emergence angle (from the body
of the cell) and orientation angle (at the tip of the growth cone)
confirming alignment on patterned surfaces compared to control substrates
(unpatterned polymer and glass surfaces). The expected frequency distribution
for parallel alignment (≤15°) is 14% and the two micropatterned
groups ranged from 42 to 49% alignment for emergence and orientation
angle measurements, where the control groups range from 12 to 22%
for parallel alignment. Despite depths being 15 to 20 nm, cell processes
could sense these topographical changes and preferred to align to
certain features of the micropatterns like the plateau/channel interface.
As a result this initial study in utilizing these new DMD micropatterned
coumarin polyester thin films has proven beneficial as an axon guidance
platform for future nervous system regenerative strategies