New near-infrared absorbing donor-acceptor chromophores have been investigated by varying the electron donating and accepting strength of the two halves of the molecule within wide limits. The dihydroperimidine, perimidine, Michler's ethylene and 1-decyl-2(1H)- methyl-benz[c, d]indolium iodide residues were examined as donor residues, and these were coupled to 4-nitrobenzenediazonium chloride to give monoazo dyes. The λmax values of these gave a qualitative
indication of relative electron donor strengths, and the 1-ethyl-2-methylperimidine azo dyes proved to be the most bathochromic, being blue in colour. The dyes were amongst the most bathochromic monoazo dyes yet prepared containing the 4-nitrophenylazo residue.
The N-alkyl-3-cyano-6-hydroxy-4-methyl-2-pyridone system was
investigated as a potentially powerful electron acceptor system, and the 5-formyl and 5-nitroso derivatives were condensed with Michler's ethylene and 1-decyl-2(1H)-methyl-benz[c, d]indolium iodide to give new donor-acceptor dyes. The aza dyes prepared from the nitroso compounds proved to be the most bathochromic in accord with PMO theory and many
were near-infrared absorbing.
A series of near-infrared absorbing squarylium dyes with narrow, intense absorption bands at about 800nm were obtained by reacting squaric acid with 2,2-disubstituted dihydroperimidines. The first dyes of this type possessed poor organic solvent solubilities but, through modification of the 2,2-substituents of the dihydroperimidines it was possible to obtain squarylium dyes with good organic solvent
solubility, this being a much sought after property of infrared dyes. Other squarylium dyes were obtained by the reaction of squaric acid with 1-ethyl-2-methylperimidine, Michler's ethylene and 1-decyl-2(1H)-methyl-benz[c, djindolium iodide. The latter two dyes absorbed in the
infrared region at 809 and 900nm respectively in toluene.
A modified procedure for the synthesis of croconic acid was
developed, which enables the acid to be obtained in the anhydrous form readily. Reaction of croconic acid with 3-hydroxy-N, N-dialkylanilines afforded highly bathochromic dyes (λ max ca. 830nm). Reaction with 1- decyl-2(1H)-methyl benz[c, d]indolium iodide gave a croconium dye that absorbed beyond 1000nm. The reaction of 8-hydroxyjulolidine with croconic acid was particularly interesting as it occurred readily at room temperature. Thus it was possible to undertake a kinetic study of mechanistic aspects of the condensation reaction between croconic acid and arylamines. The results indicated that the optimum reaction conditions involved using a low proportion of an alcohol in a nonpolar
aprotic solvent in the presence of a weak acid catalyst.
Dyes were also obtained from the reaction of various electrophilic chlorine-substituted compounds with electron-donor aromatic residues, thus giving new donor-acceptor dyes, several of which were nearinfrared absorbing with low molecular masses and good organic solvent solubilities. The dyes were, however, strongly coloured due to their broad absorption bands which extended well into the visible region.
The thermal and photochemical stabilities of representative
examples of all the infrared dye classes prepared in this work have been examined, using standard procedures.