Stilbenoid Dendrimers in Their Neat Phases
A R T I C L E S
enthalpy equals the sum of the three transitions detected during
the heating cycle.
molecular motions and different segmental motions, 1 was
selectively deuterated at various positions as shown in Figure
1. Dendrimer 1b is deuterated at the central benzene ring; thus,
the results obtained for this sample reflect the molecular motion
of the whole mesogen. Additional dynamics of the dendrimer
arms can subsequently be investigated by analyzing the spectra
of the molecules selectively deuterated at the olefinic and the
outer aromatic positions, 1c and 1d. The sample 1e deuterated
at the R-CH2 groups of the alkoxy chains finally provides
information about the restricted mobility of the side chain in
Dendrimers 2 show a similar behavior. In the heating curves,
three transitions Cr/Colhd (5.8 ( 4.6 °C/53.0 ( 10.5 kJ/mol),
Colhd/Colob (29.7 ( 2.3 °C/11.8 ( 8.2 kJ/mol) and Colob/I (97.3
( 2.4 °C/16.0 ( 4.2 kJ/mol) take place. Thereby, transitions
between the hexagonal phase and the oblique phase appear very
broad and can be supercooled; consequently, the cooling curves
exhibit only two transitions at 92.3 ( 2.1 °C (-15.5 ( 4.0 kJ/
mol) and 6.3 ( 2.1 °C (-44.5 ( 5.5 kJ/mol). Interestingly,
the phase sequence of these dendritic mesogens is inverted
(Colhd at lower temperatures than Colob) compared to other
discotic molecules forming columnar phases.24 This may be
attributed to the presence of several conformation isomers,
whose distribution changes with temperature and, thus, might
lead to the observed distortion of the hexagonal phase at elevated
temperatures.
2
the vicinity of the aromatic core. For the H NMR studies of
the second generation dendrimer 2, two different isotopomers
were prepared, which are shown in Figure 1. Molecules 2b and
2c were selectively deuterated at the second shell of the
dendrimer, the outer olefinic positions and the R-methylene units
of the side chains, respectively. A deuteration of the outer parts
of the molecule was preferred over a deuteration of the core
NMR Background. In solid-state 2H NMR, the NMR
frequency ωQ (in the rotating frame) depends on the orientation
of the external static magnetic field B0 with respect to the
principal axes system of the electric-field gradient (EFG)
tensor.16 ωQ is described by the following equation:
moieties, since the very low overall content of H in the case
2
of core deuteration would have presumably led to severe signal-
to-noise problems.
A. First Generation Dendrimers 1. In the following section,
we focus on the sample selectively deuterated at the central
benzene ring, 1b. The temperature-dependent 2H NMR spectra
are presented in Figure 3. At 22 °C 1b is in its crystalline Cr3
phase. Hence, no significant mobility of the core of the molecule
is expected, and correspondingly, the spectrum of 1b has a Pake
pattern with a quadrupole splitting of 130 kHz, which is the
typical splitting constant of a rigid C-D bond. Therefore, the
core of the molecule is immobile on the fast time scale (e10-4
s). Heating to 67 °C and 72 °C into the liquid crystalline phase
leads to spectra with a strongly reduced quadrupolar splitting
of 46 kHz. The spectra also exhibit a comparably strong signal
at ω0, which is due to isotropic parts in the sample and will be
discussed in detail below. The observed reduction of the
quadrupolar splitting is due to the onset of the rotation of the
mesogens. It should be noted that for symmetry reasons in the
fast motional limit a free rotation cannot be distinguished from
δ
2
ωQ ) ( (3 cos2 θ - 1 - η sin2 θ cos 2æ)
(1)
where θ and æ are the polar coordinates of the magnetic field
in the principal axes system of the EFG tensor. The anisotropy
parameter δ of the quadrupolar interaction is on the order of
2π × 125 kHz for C-D deuterons. η is the asymmetry
parameter of the electric field gradient (EFG) tensor and is
usually negligibly small for C-D bonds. Since the deuteron is
a spin-1 nucleus, two transitions are allowed between the
corresponding three energy levels, which is reflected in the two
2
signs in eq 1. One-dimensional H NMR spectra are usually
recorded by applying the solid-echo pulse sequence, which
consists of two π/2 pulses separated by an echo delay tp and
shifted in phase by π/2. The solid-echo sequence leads to the
formation of an echo at t ) 2tp. In absence of molecular motions,
the line shape of the 1D spectrum is independent of the length
of the applied echo delay tP. In this static case, the signal of
isotropic media results from summing over all values of θ and
æ and is commonly referred to as the Pake spectrum (assuming
an axially symmetric EFG tensor (η ) 0)), which features two
singularities with a separation δ (θ ) 90°) and feet with a total
width of 2δ (θ ) 0).
In contrast, if molecular reorientations take place during the
pulse sequence, a time-dependent signal ωQ(t) results.25 In this
case, the line shape of the 1D spectra is very sensitive to both
the kind of motion and the interpulse delay tP. Therefore, detailed
information about the mechanism of molecular reorientation can
be extracted by carrying out a line shape analysis.
2H NMR Investigations. From various discotic systems, it
is well-known that different segments in the molecule might
exhibit large differences in the mobility.26,27 To distinguish
2
three-site (or higher) jumps by means of H NMR line shape
analysis. Discrete three-site jumps (120°) around the column
axis are known from other discotic molecules with C3 sym-
metry.27 In the following, we refer to a rotation around the
column axis as the motional process, keeping in mind that as
well in-plane 120° jumps might take place. In the case of an
ideal rotation around the C3 symmetry axis of the molecule, a
reduction of the quadrupolar splitting by a factor 0.5 to ∼65
kHz is expected. Further reduction of the quadrupolar splitting
is due to less well-defined rotational motion. In the case of
nonaligned, isotropic samples, the order parameter for the disk
rotation can be defined as
2δobserved
S )
(2)
δ
The measured reduction to 46 kHz therefore corresponds to
an order parameter of S ) 0.74. Usually in discotic systems
order parameters S > 0.8 are observed.28,29 The reduced order
parameter obtained in this system can be explained by the
dynamics of the dendritic arms as will be shown later with
(24) Cammidge, A. N.; Bushby, R. J. In Handbook of Liquid Crystals; Demus,
D., Gray, G. W., Spiess, H.-W., Vill, V., Eds.; VCH: 1998; Vol. 2B, pp
781-798.
(25) Spiess, H. W.; Sillescu, H. J. Magn. Res. 1981, 42, 381.
(26) Spielberg, N.; Sarkar, M.; Luz, Z.; Poupko, R.; Billards, J.; Zimmermann,
H. Liq. Cryst. 1993, 15 (3), 311.
(28) Goldfarb, D.; Luz, Z. J. Physique 1981, 42, 1303-1311.
(29) Herwig, P.; Kayser, C. W.; Mu¨llen, K.; Spiess, H. W. AdV. Mater. 1996,
8, 510-513.
(27) Leisen, J.; Werth, M.; Boeffel, C.; Spiess, H. W. J. Chem. Phys. 1992, 97
(5), 3749.
9
J. AM. CHEM. SOC. VOL. 126, NO. 3, 2004 775