Novel Photochromic Zwitterions For Multifrequency Data Storage
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NOVEL PHOTOCHROMIC SWITTERIONS FOR MULTIFREQUENCY DATA STORAGE
G. J. ASHWELL*, E. J. C. DAWNAY, A. P. KUCZYNSKI AND M. SZABLEWSKI Centre for Molecular Electronics, Cranfield Institute of Technology, Cranfield, Bedford MK43 OAL, UK ABSTRACT Z-P-(N-alkyl-4-pyridinium)-a-cyano-4-styryldicyanomethanide (R-P3CNQ) and its quinolinium analogue (R-Q3CNQ) are photochromic. Their LB films show sharp charge transfer bands at 495 nm (C16H33P3CNQ), 565 nm (C16 H33 -Q3CNQ) and 614 nm (C8H17-Q3CNQ) with half widths at half maximum of 27, 22 and 37 nm respectively. They show interesting nonlinear optical properties and have the potential to store three bits per pixel when addressed at these wavelengths. INTRODUCTION 7,7,8,8-Tetracyano-p-quinodimethane undergoes substitution reactions with (i) primary and secondary amines to give materials of general formula R'R"N-C(CN)=C 6H4=C(CN) 2 and (R'R"N) 2C=C 6H4=C(CN) 2 [1] and (ii) picolinium, quinaldinium and lepidinium salts to give the zwitterionic derivatives D÷-CH=C(CN)-C 6H4 -C(CN) 2- where D+ is a heterocyclic cation (see Figure 1) [2-7). The zwitterionic ground state is substantiated by the bond lengths and segmented molecular geometry: the N-methyl-2-pyridinium analogue has dihedral angles of 36.1 and 5.00 between the plane through the -CH=C(CN)- bridge and the donor and acceptor planes respectively [8]. It has a large second order hyperpolarisability but intermolecular dipole-dipole interactions cause the crystal structure to be centrosymmetric. The molecules overlap in a head-to-tail manner with a spacing of 3.54 A between the pyridinium ring and -C(CN) 2 group. For second harmonic generation (SHG) it is necessary to suppress this arrangement and the LB deposition of three amphiphilic congeners has provided non-centrosymmetric Z-type films. In this work the photochromic switching and nonlinear optical properties of the zwitterionic LB films are reported. Our main interest was aroused by the narrow photochromic absorption bands and their conceivable use in a multifrequency memory device. To store several bits per pixel demands insignificant overlap of the bands in order that they may be independently addressed. There are few suitable materials: the spiropyrans which have narrow J-bands at =600 nm [9,10] and the D÷-CH=C(CN)-C 6H4-C(CN)2- zwitterions [3] which show sharp charge transfer bands at 495, 565 and 614 nm. *To whom correspondence should be addressed.
Mat. Res. Soc. Symp. Proc. Vol. 173. ©1990 Materials Research Society
508
H
N
HN
C /tc N
C\\ N
(a)
cC
/tc N
N
(b)
Figure 1. Molecular structures of (a) Z-A-(N-alkyl-4-pyridinium)a-cyano-4-styryldicyanomethanide (R-P3CNQ) and (b) Z-P-(N-alkyl4-quinolinium)-a-cyano-4-styryldicyanomethanide (R-Q3CNQ).
PHOTOCHROMISM Photochromism was reported as early as in the 19th century [11] and it results from reversible changes in the molecular structure [12] or charge distribution [13,14) during irradiation. The zwitterions bleach when irradiated at the frequency of the charge transfer band [3,4] and, in solution, the blea
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