Synthesis and physicochemical studies of some new quinolinoxazine pentamethine cyanine dyes

Polymethine cyanine dyes belong to a well-known class of organic compounds, which have been used in photography and as information storage in laser technology. A series of novel cyanine dyes were synthesized through the formylation of quinolinium[b,c]1,4-oxazine-chloride salt 1. Reaction of compound 2-chloro-3-formyl-quinolinium[b,c]1,4-oxazine-chloride salt 2 with different molar ratios of 2(4)-methyl substituted heterocyclic in basic catalysis afforded the corresponding 2-chloroquinolinium[b,c]1,4-oxazine-chloride salt-3[2(4)]-dimethine (3a-c), quinolinium[b,c]-1,4-oxazinechloride salt-2,3[2(4)]-pentamethine (4a-c) and quinolino[b,c]1,4-oxazine-6yl[2(4)]-monomethine-/2,3[2(4)]-pentamethine cyanine dyes (5a-c) respectively. The structure of the dyes was characterized by elemental analysis, visible absorption, fluorescence emission, IR, 1H-NMR and mass spectroscopy. The correlations between the structure and properties of these dyes have been studied. A comparison of the visible absorption maxima between compounds 3b, 4b and 5b showed that asymmetrical mono-pentamethine cyanine dye 5b reveals a bathochromic shift than both dimethine 3b and pentamethine cyanine dyes 4b. This could be attributed to the more extensive π-conjugation and increasing number of methine groups in asymmetric pentamethine. All the observations and analytical spectra in this paper support the syntheses of new di-, penta- and mono-/pentamethine cyanine dyes. The absorption spectra of dyes were investigated in organic solvents. The results indicated that the excitation for their colour is a simple charge-transfer from oxygen atom of oxazine nucleus and/or nitrogen atom of pyridine (quinoline) nucleus to N-quaternary salts in di-, penta- and mono-/pentamethine cyanine dyes respectively. These dyes showed positive solvatochromism with increased solvent polarity, which depends on the structure and the type of dye.