Photorefractivity in nematic liquid crystals using a donor-acceptor dyad with a low-lying excited singlet state for charge generation

Article abstract of DOI:10.1021/jp011561v

The photorefractive effect was observed in room-temperature nematic liquid crystal composites consisting of a eutectic mixture of 65% 4a?2-(n-pentyl)-4-cyanobiphenyl and 35% 4a?2-(n-octyloxy)-4-cyanobiphenyl doped with a donor-acceptor dyad in which 9-(N-

Full text of DOI:10.1021/jp011561v

7216
J. Phys. Chem. B 2001, 105, 7216-7219
Photorefractivity in Nematic Liquid Crystals Using a Donor-Acceptor Dyad with a
Low-Lying Excited Singlet State for Charge Generation
Michael J. Fuller and Michael R. Wasielewski*
Department of Chemistry and Center for Nanofabrication and Molecular Self-Assembly,
Northwestern UniVersity, EVanston, Illinois 60208-3113
ReceiVed: April 25, 2001
The photorefractive effect was observed in room-temperature nematic liquid crystal composites consisting of
a eutectic mixture of 65% 4′-(n-pentyl)-4-cyanobiphenyl and 35% 4′-(n-octyloxy)-4-cyanobiphenyl doped
with a donor-acceptor dyad in which 9-(N-pyrrolidinyl)perylene-3,4-dicarboximide is directly linked to 1,8:
4,5-naphthalenetetracarboxydiimide. This dyad has a broad, intense absorption band that extends from 500 to
750 nm. Measurements of two-beam asymmetric energy exchange (beam-coupling) and the diffraction
efficiency of the refractive index grating produced in these liquid crystal thin films were carried out. A
comparison is made between the covalent donor-acceptor dyad and the corresponding noncovalent donor-
acceptor pair. In each case the covalent dyad outperforms the noncovalent donor-acceptor pair. The results
show that aminoaryleneimides are promising new charge donating chromophores that are capable of extending
photorefractive performance of nematic liquid crystal composites into the near-infrared region of the spectrum.
Introduction
piperidinyl)naphthalene-1,8-dicarboximide (ANI), which we
have used previously in charge generation and transport dopants
in photorefractive LC composites.15
Organic photorefractive materials display significant potential
for use in optical data storage, image amplification, phase
conjugate mirrors, and dynamic holography.1-7 Films of aligned
nematic liquid crystals (LCs) doped with chromophores capable
of charge separation and transport over macroscopic distances
exhibit significant photorefractive effects.8-12 Two coherent cw
laser beams that cross within an aligned LC sample produce a
light intensity grating in the material. Absorption of a small
fraction of the light by a chromophoric electron donor or
acceptor within the illuminated regions of the grating results in
photoinduced electron transfer leading to the formation of radical
ion pairs. A difference in diffusion coefficients for the two
oppositely charged ions coupled with a mechanism for trapping
each of the ions separately in the light and dark regions of the
grating results in a space charge field. This periodic electric
field produces a refractive index grating in the LC due
principally to the quadratic electrooptic effect. The refractive
index grating and the light intensity grating within the material
are phase shifted relative to one another, which results in
asymmetric energy exchange (two-beam coupling) between the
two laser beams that produce the grating. Two-beam coupling
is diagnostic for the photorefractive effect. In general, the
wavelength of light that produces the charges which generate
the space charge field determines the wavelength at which a
particular photorefractive material can be used for optical image
processing and storage. Therefore, it is important to develop
photorefractive materials that are active at near-infrared wave-
lengths because these wavelengths are of special interest to
optical communications technology.13,14
Charge generation reactions that use intramolecular electron
transfer within a covalently linked donor-acceptor pair have
been studied both in doped and functionalized photorefractive
polymers.16,17 Donor-acceptor dyad dopants, D-A in photo-
refractive LC composites undergo quantitative intramolecular
charge separation to yield D+-A-. These ion pairs recombine
much more slowly in the nematic LC, due to control of the
recombination rate by the relatively slow reorientation of the
LC dipole.18 When the long-lived D+-A- pairs encounter a
neutral D-A pair, intermolecular electron-transfer yields D+-A
and D-A- pairs, which separate with different diffusion
coefficients. Covalent dyads offer the distinct advantage that
their quantum efficiencies of charge generation are close to
unity. It has been shown that this high yield of charges translates
directly into enhanced photorefractivity.15
Experimental Methods
The syntheses of 5PMI, NI, and 5PMI-NI are presented in
the Supporting Information. Their structures, as well as the
structure of ANI, are shown below.
Toward this goal we have developed a push-pull perylene,
5PMI, which acts as good electron donor, and can be covalently
attached to electron acceptors. This donor is easy to oxidize,
displays a high optical extinction coefficient, and absorbs
significantly to the red of the corresponding push-pull 4-(N-
* To whom correspondence should be addressed. E-mail: wasielew@
chem.northwestern.edu.
10.1021/jp011561v CCC: $20.00 © 2001 American Chemical Society
Published on Web 06/28/2001

Photorefractivity in Nematic Liquid Crystals
J. Phys. Chem. B, Vol. 105, No. 30, 2001 7217
Figure 2. Ground-state absorption spectra of 5PMI-NI, 5PMI, and
NI in benzonitrile.
Figure 1. A schematic of the experimental geometry for two-beam
coupling and diffraction efficiency measurements is illustrated. The
laser beams are p-polarized in the plane of the page. EA is the applied
electric field.
Electrochemical measurements were performed with a CH
Instruments model 660 system using Pt working and counter
electrodes as well as a Ag/AgCl reference electrode. Potentials
were measured in benzonitrile containing 0.1 M tetra-n-
butylammonium perchlorate. Ferrocene was used as an internal
reference. The nematic LC solvent used in these studies is a
mixture of 35% 4′-(n-octyloxy)-4-cyanobiphenyl (8OCB) (Al-
drich) and 65% 4′-(n-pentyl)-4-cyanobiphenyl (5CB) (Aldrich)
by weight. This specific LC mixture was chosen because the
temperature range of its nematic phase is broad, and its viscosity
at room temperature is lower than that of 5CB alone. Indium
tin oxide (ITO)-coated glass slides were functionalized with the
surfactant octadecyltrichlorosilane to induce homeotropic align-
ment of the LC mixture. The path length of the cells was 26
µm thick, as determined by a cellulose triacetate spacer. The
doped 5CB/8OCB mixture was introduced into the cell by
capillary action, and alignment of the LC mixture occurred in
a few minutes yielding cells of high optical quality.
Two-beam coupling and diffraction efficiency measurements
were obtained using the experimental geometry shown in Figure
1, where two coherent beams of equal energy from a tunable
Ar+ laser operating at 514.5 nm cross in the sample cell, the
normal of which is tilted â ) 30° relative to the bisector of the
two intersecting laser beams. The crossing angle θ ) 0.53° fixes
the grating spacing at Λ ) 37 µm, which was employed for all
the measurements described here. As the diffraction pattern is
present only when a small voltage (<2 V) is applied to the cell
and p-polarized laser beams are used, the grating is an
orientational photorefractive grating and not a thermal or
absorption grating.5,8,11,19 Beam coupling ratios rather than gain
values are reported because the photorefractive gain is poorly
defined for a thin, Raman-Nath grating.20 Photoconductivity
measurements were made by illuminating the entire cell, which
has approximately a 4 cm2 surface area with a 50 mW, 514 nm
Ar+ laser beam. The cell was biased at voltages comparable to
those used for the two-beam coupling and diffraction efficiency
measurements. Both photoconductivity and dark conductivity
measurements were made with a Keithley model 485 picoam-
meter.
Figure 3. The asymmetric two-beam coupling data for 5PMI-NI (8.0
× 10-4) and for PMI (1.0 × 10-3)/NI (3.0 × 10-3) in a 5CB/8OCB
mixture (as calculated by the ratio of intensity of the beam that gains
(loses) intensity relative to its intensity incident upon the sample) is
illustrated.
Results and Discussion
The ground-state absorption spectrum of each molecule in
benzonitrile is illustrated in Figure 2. The broad absorption band
of 5PMI-NI extends from 500 to 750 nm with a maximum at
650 nm, while its fluorescence spectrum (not shown) has a
maximum at 755 nm. The energy of the lowest excited singlet
state of 5PMI-NI ES, calculated from the average of the energies
of the absorption and fluorescence maxima, is 1.77 eV. The
oxidation potential EDOX of the 5PMI donor is 0.61 V vs SCE,
which is significantly lower than the corresponding 1.2 V
oxidation potential for the ANI donor previously studied as a
dopant in photorefractive LC composites.15 The reduction
potential EARED of NI is -0.53 V, so that the free energy of the
reaction 1*5PMI-NI f 5PMI+-NI- in the polar LC is ap-
proximately ∆GCS ) ED - ERAED - ES ) -0.63 eV.
OX
Figure 3 compares the beam coupling ratio data for 5PMI-
NI and 5PMI/NI mixtures. The 5PMI-NI donor-acceptor dyad
exhibits higher beam coupling ratios, and therefore enhanced
photorefractivity, at all applied voltages compared to the 5PMI/
NI mixtures. It is also noteworthy that the data presented in
Figure 3 show that lower concentrations of 5PMI-NI are
necessary to achieve efficient asymmetric energy exchange
between interfering beams than 5PMI/NI mixtures.
Diffraction efficiency η measurements were carried out to
compare the performance of the two composites. The diffraction

7218 J. Phys. Chem. B, Vol. 105, No. 30, 2001
Fuller and Wasielewski
Figure 4. The first-order diffraction efficiency (η) of the photorefrac-
tive grating for a variety of LC composites is illustrated.
Figure 5. Dark conductivity as a function of applied voltage for 5PMI-
NI (8.0 × 10-4) in a 5CB/8OCB mixture.
efficiency in these thin (Raman-Nath) gratings is calculated
as the ratio of the intensity of the first diffracted beam to the
intensity of one of the writing beams incident upon the sample.
Several concentrations of both 5PMI-NI and 5PMI/NI were
examined, and Figure 4 plots the diffraction efficiency η against
applied voltage. It is apparent that the composite achieving the
best diffraction efficiency at the lowest voltage is the one doped
with 5PMI-NI (8.0 × 10-4 M). The composite containing 5PMI
(1.0 × 10-3 M)/NI (3.0 × 10-3 M) requires an additional 0.4
V applied to the film to achieve a comparable diffraction
efficiency. Figure 4 also shows that the diffraction efficiency
of the composite using a 5PMI/NI mixture with a 3x molar
excess of NI is better than that of one doped with equimolar
amounts of 5PMI and NI. This behavior is consistent with the
expected increased efficiency of the reaction: 1*5PMI + NI
f 5PMI+ + NI-, when higher concentrations of either reactant
are present.
processes involving oxidation of the donor and/or reduction of
the acceptor molecules at the ITO electrodes. The rates of these
redox processes are retarded by the octadecylsiloxane layer on
the ITO, but these thin layers do not completely inhibit the redox
chemistry. By comparison the photoconductivity at low voltages
is about 10-11 ohm-1 cm-1, and is weakly dependent on voltage
for voltages <2 V. The data in Figure 5 suggest that voltages
below 1.5 V are necessary to main a reasonable ratio of σph/
(σph + σd). Using the values K ) 7 × 10-7 dyne,21 ne ) 1.5, ꢀ∞
) 2.25, ꢀs ) 10.5, T ) 298 K, L ) 26 µm, m ) 1, λ ) 514
nm, q ) 3.7 × 103 cm-1, EA ) 1.2 V, σph ) 10-11 ohm-1
cm-1, and σd ) 10-10 ohm-1 cm-1, along with the measured
value of η ) 0.11 for the composite containing 5PMI-NI at
1.2 V, eq 1 yields ν ) 0.25. This value is comparable to ν )
0.29 obtained for the corresponding ANI-NI system at 1.5 V
reported earlier.15 Since the diffraction efficiency for the 5PMI/
NI sample is 0.013 at 1.2 V, eq 1 yields ν ) 0.08. Given the
definition of ν in eq 2, the larger value of ν for 5PMI-NI shows
that the difference in diffusion coefficients between the 5PMI+-
NI and 5PMI-NI- ions is greater than that between 5PMI+
and NI-. This results in a more efficient photorefractive grating
using the 5PMI-NI dyad.
The diffraction efficiency η of a Raman-Nath orientational
grating is given by8
2
LmkbT EAꢀsꢀ∞sin â
σph
η )
ν
(1)
(
)
(
)
ꢀ∞E2A
2πKq2
λneKqe
σph + σd
1 +
(
(
)
)
Conclusions
We have demonstrated that an aminoaryleneimide with
extended conjugation, 5PMI, which possesses a low energy
absorption band extending into the near-infrared region of the
spectrum, exhibits significant photorefractive effects in nematic
LC composites. Moreover, using a covalently linked 5PMI-NI
donor-acceptor pair for charge generation rather than the
corresponding noncovalent mixture of 5PMI and NI signifi-
cantly improves performance. This work indicates that molecular
designs based on extended aminoaryleneimides should allow
us to produce new charge generation materials for photorefrac-
tive LC composites to access the near-infrared spectral region.
D+ - D-
D+ + D-
where
ν )
(2)
D+ and D- are the diffusion constants for the cations and anions,
respectively; L is the thickness of the sample, m is the
modulation index, which equals 1 if equal intensity beams
generate the grating; kB is the Boltzmann constant, T is the
temperature, q is the wavevector of the grating, λ is the
wavelength; ne is the index of refraction along the extraordinary
axis; K is the single constant approximation of the Frank elastic
constant;8 e is the electronic charge; EA is the voltage applied
to the cell; ꢀs is the average static dielectric constant of the LC;
ꢀ∞ is the high-frequency dielectric constant; σph is the photo-
conductivity; and σd is the dark conductivity.
Acknowledgment. We gratefully acknowledge support from
the Office of Naval Research under Grant N00014-99-1-0411.
It is clear from eq 1 that two important factors that determine
the diffraction efficiency are the difference between the
photoconductivity and the dark conductivity and the difference
between the diffusion coefficients of the cations and anions.
The data in Figure 5 show that the dark conductivity of the
sample rises significantly when the applied voltage is greater
than about 1.5 V. This increase in dark conductivity with
increasing applied voltage is most likely due to faradaic redox
Supporting Information Available: Syntheses of 5PMI,
NI, and 5PMI-NI. This material is available free of charge via
the Internet at http://pubs.acs.org.
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