New extended methine dyes for application in laser technology
Based on the known reverse saturable absorption properties of certain coumarin and long chain polymethine dyes, the synthesis of new related structures was undertaken in order to examine the relationships between structure and RSA activity. It was hoped that in this way, more effective RSA chromophores could be synthesised. In addition to RSA activity (at 532nm) the general light absorption and fluorescence characteristics of the new dye materials were examined. In the first series of dyes, the 7-diethylaminocoumarin system with extensively conjugated acceptor groups in the 3-position were examined. Several dyes with absorption maxima in the range 500-700nm were prepared, but none showed RSA properties at 532nm. The poor photostability properties of these dyes prompted the synthesis of related fused ring systems derived from 4-chloro-7-diethylamino-3-formyl-coumarin and in at least one example a marked improvement in photostability was achieved. In an attempt to move the absorption maxima of the coumarin dyes into the near infrared, chromophores with extended chains containing an isophorone bridge were introduced into the 3-position of the coumarin system. Although this gave greater conjugation and an increase in structural rigidity, Amax values could not be obtained at wavelengths greater than about 570nm, and photostability properties of the dyes were poor. Condensation of active methylene compounds with 2-chloro-l-formyl-3-hydroxymethylenecyclohexene led to the synthesis of a series of extended donoracceptor chromophores containing a chlorocyclohexene bridge, where the donor was the 1,3,3-trimethylindoline ring system. Absorption maxima of the dyes ranged from 620-860nm. However, none showed RSA properties and their photostability in cellulose acetate film was generally poor. In an attempt to improve photostability the intermediate formed from 2-methylene-l,3,3-trimethylindoline (Fischer's base) and 2-chloro-l-formyl-3-hydromethylenecyclohexene was condensed with active methylene compounds containing groups capable of intramolecular nucleophilic displacement of the chlorine atom. Thus under suitably forcing reaction conditions intramolecular cyclisation occurred, giving a series of extended donor-acceptor chromophores containing fused heterocyclic residues, the absorption maxima of these new chromophores ranging from 540-730nm. However, resultant increase in molecular rigidity did not give the expected improvement in photostability. In a further attempt to prepare RSA active chromophores, selected symmetrical polymethine dyes were synthesised and evaluated. All showed a measurable RSA effect at 532nm but in particular, near-infrared absorbing squarylium dye containing 2,3-dihydroperimidine end groups exhibited the greatest degree of RSA thus far observed. SCF -CI PPP-MO theory was applied to the theoretical qualitative prediction of the RSA properties of dye chromophores. Theoretical predictions were in good agreement with experimental observations, enabling a distinction to be made between dyes with high or zero RSA activity.