Abstract:
A series of zwitterionic materials of
general formula R-D*-CH=C(CN)-
CÖH4-C(CN)2', where D* is a pyridirium or quinolinium acceptor and R is a
hydrophobic alkyl chain or an aryl group, were deposited using the Langmuir-
Blodgett (LB) technique and their photochromic and nonlinear optical properties
characterised. The materials are
highly solvatochromic, exhibiting a broad
photochromic charge transfer band in the visible region which bleaches when
irradiated.
LB films of the zwitterions, Z-ß-(1-hexadecyl-4-pyridinium)-a-cyano-4-
styryldicyanomethanide (CMH33-P3CNQ) and the quinolinium analogue, Z-ß-(1-
hexadecyl-4-quinolinium)-a-cyano-4-styryldicyanomethanide (CIÖH33-Q3CNQ), are
non-centrosymmetric (Z-type). They exhibit sharp charge transfer bands at 495 n
ana ses n
respecivey with haf widths ai half maximum of 27 and zz mi.
Unusually, mixed LB films of CIGH33-P3CNQ and C161-133-Q3CNQ exhibit a single
sharp charge transfer band whose position is dependent upon the mole fraction and
is
finely tunable in the range 495 to 565 nm. These films are photobleached
when
irradiated at
wavelengths which overlap the absorption bands, and may find
potential applications a components of a multifrequency optical memory. Also, the
unique peak wavelength tuning of the heteromolecular films enabled the effect of
the
position of the absorption band on second harmonic generation to be
investigated for the first time.
The zwitterionic materials have
exciting norlinear optical properties and the
strongest second harmonic intensity from any LB film has been obtained. LB films
of the
quinolinium zwitterion (CMH33-Q3CNQ) are non-centrosymmetric and the
second harmonic
intensity increases quadratically with the number of layers
deposited to thicknesses of ca. 1 m. It is one of only five known materials to show
such behaviour and the second-order
susceptibility (xa) = 180 pm V* at 1064 nm)
is the
highest value obtained for a multilayer structure.
