STUDIES OF SPIN RELAXATION AND MOLECULAR-DYNAMICS IN LIQUID-CRYSTALS BY 2-DIMENSIONAL FOURIER-TRANSFORM ELECTRON-SPIN-RESONANCE .1. CHOLESTANE IN BUTOXY BENZYLIDENE-OCTYLANILINE AND DYNAMIC CAGE EFFECTS
Vss. Sastry et al., STUDIES OF SPIN RELAXATION AND MOLECULAR-DYNAMICS IN LIQUID-CRYSTALS BY 2-DIMENSIONAL FOURIER-TRANSFORM ELECTRON-SPIN-RESONANCE .1. CHOLESTANE IN BUTOXY BENZYLIDENE-OCTYLANILINE AND DYNAMIC CAGE EFFECTS, The Journal of chemical physics, 105(14), 1996, pp. 5753-5772
Two-dimensional Fourier transform (2D-FT) electron spin resonance (ESR
) studies on the rigid rodlike cholestane (CSL) spin-label in the liqu
id crystal solvent 4O,8 (butoxy benzylidene octylaniline) are reported
. These experiments were performed over a wide temperature range: 96 d
egrees C to 25 degrees C covering the isotropic (I), nematic (N), smec
tic A (S-A) smectic B (S-B), and crystal (C) phases. It is shown that
2D-FT-ESR, especially in the form of 2D-ELDOR (two-dimensional electro
n-electron double resonance) provides greatly enhanced sensitivity to
rotational dynamics than previous cw-ESR studies on this and related s
ystems. This sensitivity is enhanced by obtaining a series of 2D-ELDOR
spectra as a function of mixing time, T-m, yielding essentially a thr
ee-dimensional experiment. Advantage is taken of this sensitivity to s
tudy the applicability of the model of a slowly relaxing local structu
re (SRLS). In this model, a dynamic cage of solvent molecules, which r
elaxes on a slower time scale than the CSL solute, provides a local or
ienting potential in addition to that of the macroscopic aligning pote
ntial in the liquid crystalline phase. The theory of Polimeno and Free
d for SRLS in the ESR slow motional regime is extended by utilizing th
e theory of Lee et al. to include 2D-FT-ESR experiments, and it serves
as the basis for the analysis of the 2D-ELDOR experiments. It is show
n that the SRLS model leads to significantly improved non-linear least
squares fits to experiment over those obtained with the standard mode
l of Brownian reorientation in a macroscopic aligning potential. This
is most evident for the S-A phase, and the use of the SRLS model also
removes the necessity of fitting with the unreasonably large CSL rotat
ional asymmetries in the smectic phases that are required in both the
cw-ESR and 2D-ELDOR fits with the standard model. The cage potential i
s found to vary from about k(B)T in the isotropic phase to greater tha
n 2k(B)T in the N and S-A phases, with an abrupt drop to about 0.2k(B)
T in the S-B and C phases. Concomitant with this drop at the S-A-S-B t
ransition is an almost comparable increase in the orienting potential
associated with the macroscopic alignment. This is consistent with a f
reezing in of the smectic structure at this transition. The cage relax
ation rate given by R(c), its ''rotational diffusion coefficient,'' is
of order of 10(7) s-(1) in the I and N phases. It drops somewhat in t
he S-A phase, but there is a greater than order of magnitude drop in R
(c) for the S-B and C phases to about 10(5) s(-1). This drop is also c
onsistent with the freezing in of the smectic structure. The rotationa
l diffusion tensor of the CSL probe is significantly larger than R(c)
which is consistent with the basic physical premise of the SRLS model.
In particular, R(perpendicular to)(0) and R(parallel to)(0) are of or
der 10(8) s(-1) and 10(9) s(-1) respectively. (C) 1996 American Instit
ute of Physics.