Towards a Structure-Rsa Relationship for Non-Linear Optical Materials.
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(1) Defence Research Agency, Fort Halstead, Sevenoaks, Kent TN14 7BP. UK (2) University of Leeds, Department of Colour Chemistry and Dyeing, Leeds, LS2 9JT, UK (3) Defence Research Agency, St. Andrews Road, Malvem. Worcs. WR14 3PS, UK. ABSTRACT
Measurements on the reverse saturable absorption (RSA) properties of some polyene dyes are presented. For a series of linear dyes, a correlation is observed between the symmetry of a molecule and its ability to exhibit RSA. To our knowledge, this is the first structure-RSA relationship discovered to date. Certain of these materials have been studied with semi-empirical quantum chemical techniques and the observed structure-RSA relationship is rationalised in terms of the calculated electronic structures. In this paper, we report primarily on some studies we have carried out on the linear dye systems. INTRODUCTION
Reverse saturable absorption (RSA), otherwise referred to as non-linear absorption, is a phenomenon in which the excited state of a molecule absorbs more strongly than the ground state. If the absorption wavelength of the excited state matches the incident wavelength, then such molecules will have useful applications in passive optical limiting and pulse conditioning [1,2]. Only certain types of molecules exhibit RSA properties. Some macrocyclic compounds, such as phthalocyanine and naphthalocyanine [3] and porphyrin [4] have been identified as showing this effect to a
useful level. Recently, C60 and C70 have also been shown to exhibit optical limiting due to reverse saturable absorption [5,6,7]. Some non-macrocyclic type compounds, such as those with extended chromophores on linear or conjugated systems also exhibit RSA properties, but not to the same degree as the macrocyclic systems. We have examined a large number of organic dyes in an attempt to identify those molecular features which could be responsible for giving rise to reverse saturable absorption. In this paper we report on some of the studies we have carried out on a series of donor-acceptor dyes with linear structures. MATERIALS [8]
From a structural point of view, the dyes which we have examined fall into two general categories: dyes with asymmetric molecular structure, shown in figures la-g, and dyes which are symmetric about the centre of the molecule, shown in figures lh-k. The dyes I, VI, VII, VIII and X were prepared by condensing the 7-diethylamino-3 formyl coumarin with the appropriate active methylene in a small volume of ethanol at reflux. A mixture of piperidine and acetic acid in the ratio of 1:5 was used to catalyse the reaction. Except for dye I, where double condensation had occurred, the remaining four dyes reacted in equimolar quantities. The dye AM4 was prepared according to the published procedure [9] and HITCI was used as obtained from Kodak Ltd. 33 Mat. Res. Soc. Symp. Proc. Vol. 374 01995 Materials Research Society
For both the neutral and the cationic dyes, the intermediate 2-chloro-l-formyl-3-hydroxymethylenecyclohexene was used. The neutral dye was made by reacting the int
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