2 research outputs found
Steric Hindrance of Photoswitching in Self-Assembled Monolayers of Azobenzene and Alkane Thiols
Surface-bound azobenzenes
exhibit reversible photoswitching via
trans–cis photoisomerization and have been proposed for a variety
of applications such as photowritable optical media, liquid crystal
displays, molecular electronics, and smart wetting surfaces. We report
a novel synthetic route using simple protection chemistry to form
azobenzene-functionalized SAMs on gold and present a mechanistic study
of the molecular order, orientation, and conformation in these self-assembled
monolayers (SAMs). We use vibrational sum-frequency generation (VSFG)
to characterize their vibrational modes, molecular orientation, and
photoisomerization kinetics. Trans–cis conformational change
of azobenzene leads to the change in the orientation of the nitrile
marker group detected by VSFG. Mixed SAMs of azobenzene and alkane
thiols are used to investigate the steric hindrance effects. While
100% azobenzene SAMs do not exhibit photoisomerization due to tight
packing, we observe reversible switching (>10 cycles) in mixed
SAMs
with only 34% and 50% of alkane thiol spacers
Annealing-Induced Changes in the Molecular Orientation of Poly-3-hexylthiophene at Buried Interfaces
The molecular organization at interfaces
of organic semiconducting
materials plays a crucial role in the performance of organic photovoltaics
and field effect transistors. Vibrational sum-frequency generation
(VSFG) was used to characterize the molecular orientation at interfaces
of regioregular poly-3-hexylthiophene (rrP3HT). Polarization-selected
VSFG spectra of the Cî—»C stretch of the thiophene ring yield
the orientation of the conjugated backbone of P3HT, which is directly
relevant to the electronic properties at the interface. The molecular
orientation at buried polymer–substrate interfaces was compared
for films spin-cast on SiO<sub>2</sub> and AlO<sub>X</sub> substrates,
before and after thermal annealing at 145 °C. On SiO<sub>2</sub>, annealing results in the thiophene rings adopting a more edge-on
orientation, tilting away from the surface plane by Δθ
= +(3–10)°. In contrast, an opposite change is observed
for films deposited on AlO<sub><i>x</i></sub>, Δθ
= −(3–26)°, where annealing leads to a more face-on
orientation of the thiophene rings of the polymer. Although subtle,
such orientational changes may significantly affect the electron transfer
rates across interfaces and hence the overall photovoltaic efficiency