| 58 |
|
\begin{doublespace} |
| 59 |
|
|
| 60 |
|
\begin{abstract} |
| 61 |
< |
Nitrile Stark shift repsonses to electric fields have been used |
| 62 |
< |
extensively in biology for the probing of local internal fields of |
| 63 |
< |
structures like proteins and DNA. Intigration of these probes into |
| 64 |
< |
different areas of interest are important for studing local structure |
| 65 |
< |
and fields within confined, nanoscopic |
| 66 |
< |
systems. 4-Cyano-4'-pentylbiphenyl (5CB) is a liquid crystal with a known |
| 67 |
< |
macroscopic structure reordering from the isotropic to nematic |
| 68 |
< |
phase with the application of an external |
| 69 |
< |
field and as the name suggests has an inherent nitrile group. Through |
| 70 |
< |
simulations of this molecule where application of |
| 71 |
< |
large, nanoscale external fields were applied, the nitrile was invenstigated |
| 72 |
< |
as a local field sensor. It was |
| 73 |
< |
found that while most computational methods for nitrile spectral |
| 74 |
< |
calculations rely on a correlation between local electric field and |
| 75 |
< |
the nitrile bond, 5CB did not have an easily obtained |
| 76 |
< |
correlation. Instead classical calculation through correlation of the |
| 77 |
< |
cyanide bond displacement in time use enabled to show a spectral |
| 78 |
< |
change in the formation of a red |
| 79 |
< |
shifted peak from the main peak as an external field was applied. This indicates |
| 80 |
< |
that local structure has a larger impact on the nitrile frequency then |
| 81 |
< |
does the local electric field. By better understanding how nitrile |
| 82 |
< |
groups respond to local and external fields it will help |
| 83 |
< |
nitrile groups branch out beyond their biological |
| 84 |
< |
applications to uses in electronics and surface sciences. |
| 61 |
> |
4-Cyano-4'-pentylbiphenyl (5CB) is a liquid-crystal-forming compound |
| 62 |
> |
with a terminal nitrile group aligned with the long axis of the |
| 63 |
> |
molecule. Simulations of condensed-phase 5CB were carried out both |
| 64 |
> |
with and without the presence of static electric fields to provide |
| 65 |
> |
an understanding of the various contributions to the Stark shift of |
| 66 |
> |
the terminal nitrile group. A field-induced isotropic-nematic phase |
| 67 |
> |
transition was observed in the simulations, and the effects of this |
| 68 |
> |
transition on the distribution of nitrile frequencies were |
| 69 |
> |
computed. Classical bond displacement correlation functions |
| 70 |
> |
exhibited a ($\approx 40 \mathrm{cm}^{-1}$ red shift of a fraction |
| 71 |
> |
of the main nitrile peak, and this shift was observed only when the |
| 72 |
> |
fields were large enough to induce orientational ordering of the |
| 73 |
> |
bulk phase. Our simulations appear to indicate that phase-induced |
| 74 |
> |
changes to the local surroundings are a larger contribution to the |
| 75 |
> |
change in the nitrile spectrum than the direct field contribution. |
| 76 |
|
\end{abstract} |
| 77 |
|
|
| 78 |
|
\newpage |
| 79 |
|
|
| 80 |
|
\section{Introduction} |
| 81 |
+ |
The Stark shift of nitrile groups in response to applied electric |
| 82 |
+ |
fields have been used extensively in biology for probing the internal |
| 83 |
+ |
fields of structures like proteins and DNA. Integration of these |
| 84 |
+ |
probes into different materials is also important for studying local |
| 85 |
+ |
structure in confined fluids. This work centers on the vibrational |
| 86 |
+ |
response of the terminal nitrile group in 4-Cyano-4'-pentylbiphenyl |
| 87 |
+ |
(5CB), a liquid crystalline molecule with a isotropic to nematic phase |
| 88 |
+ |
that can be triggered by the application of an external field. |
| 89 |
|
|
| 90 |
|
The fundamental characteristic of liquid crystal mesophases is that |
| 91 |
|
they maintain some degree of orientational order while translational |
| 124 |
|
|
| 125 |
|
Experimental studies by Levelut {\it et al.}~\cite{Levelut:1981eu} |
| 126 |
|
revealed that terminal cyano or nitro groups usually induce permanent |
| 127 |
< |
longitudinal dipole moments on the molecules. |
| 127 |
> |
longitudinal dipole moments on the molecules. Liquid crystalline |
| 128 |
> |
materials with dipole moments located at the ends of the molecules |
| 129 |
> |
have important applications in display technologies in addition to |
| 130 |
> |
their relevance in biological systems.\cite{LCapp} |
| 131 |
|
|
| 130 |
– |
Liquid crystalline materials with dipole moments located at the ends |
| 131 |
– |
of the molecules have important applications in display technologies |
| 132 |
– |
in addition to their relevance in biological systems.\cite{LCapp} |
| 133 |
– |
|
| 132 |
|
Many of the technological applications of the lyotropic mesogens |
| 133 |
|
manipulate the orientation and structuring of the liquid crystal |
| 134 |
|
through application of external electric fields.\cite{?} |
| 141 |
|
similar compounds) in 1973 in an effort to find a LC that had a |
| 142 |
|
melting point near room temperature.\cite{Gray:1973ca} It's known to |
| 143 |
|
have a crystalline to nematic phase transition at 18 C and a nematic |
| 144 |
< |
to isotropic transition at 35 C.\cite{Gray:1973ca} Recently it has |
| 145 |
< |
seen new life with the application of droplets of the molecule in |
| 146 |
< |
water being used to study defect sites and nanoparticle |
| 149 |
< |
strcuturing.\cite{PhysRevLett.111.227801} |
| 144 |
> |
to isotropic transition at 35 C.\cite{Gray:1973ca} Recently there has |
| 145 |
> |
been renewed interest in 5CB in nanodroplets to understand defect |
| 146 |
> |
sites and nanoparticle structuring.\cite{PhysRevLett.111.227801} |
| 147 |
|
|
| 148 |
|
Nitrile groups can serve as very precise electric field reporters via |
| 149 |
|
their distinctive Raman and IR signatures.\cite{Boxer:2009xw} The |
| 161 |
|
field,\cite{Lim:2006xq} the effect of a nanoscopic field application |
| 162 |
|
has not been probed. These previous studies have shown the nitrile |
| 163 |
|
group serves as an excellent indicator of the molecular orientation |
| 164 |
< |
within the field applied. Lee {\it et al.}~showed the 180 degree change in field |
| 165 |
< |
direction could be probed with the nitrile peak intensity as it |
| 166 |
< |
decreased and increased with molecule alignment in the |
| 164 |
> |
within the field applied. Lee {\it et al.}~showed the 180 degree |
| 165 |
> |
change in field direction could be probed with the nitrile peak |
| 166 |
> |
intensity as it decreased and increased with molecule alignment in the |
| 167 |
|
field.\cite{Lee:2006qd,Leyte:97} |
| 168 |
|
|
| 169 |
|
While these macroscopic fields worked well at showing the bulk |
| 183 |
|
bond. This should be readily visible experimentally through Raman or |
| 184 |
|
IR spectroscopy. |
| 185 |
|
|
| 186 |
< |
Herein, we show the computational investigation of these electric |
| 187 |
< |
field effects through atomistic simulations of 5CB with external |
| 188 |
< |
fields applied. These simulations are then coupled with ab intio and |
| 189 |
< |
classical spectrum calculations to predict changes. These changes are |
| 190 |
< |
easily varifiable with experiments and should be able to replicated |
| 191 |
< |
experimentally. |
| 186 |
> |
Herein, we investigate these electric field effects using atomistic |
| 187 |
> |
simulations of 5CB with applied external fields. These simulations are |
| 188 |
> |
then coupled with both {\it ab intio} calculations of CN-deformations |
| 189 |
> |
and classical correlation functions to predict spectral shifts. These |
| 190 |
> |
predictions should be easily varifiable with scanning electrochemical |
| 191 |
> |
microscopy experiments. |
| 192 |
|
|
| 193 |
|
\section{Computational Details} |
| 194 |
|
The force field was mainly taken from Guo et al.\cite{Zhang:2011hh} A |
