Synthesis and characterization of polymethine dyes carrying thiobarbituric and carboxylic acid moieties

An efficient synthesis of polymethine dyes carrying thiobarbituric and a carboxylic acid moiety has been developed. Such compounds play a key role in many photometric detections and quantifications of enzyme activities. In such tests, the metabolite of the enzyme activities is transformed into a beta-dicarbonyl derivative. In the present study, this compound was prepared from furfural through organic synthesis. Its in situ transformation with thiobarbituric acid derivatives yields the target compounds on a gram-scale (0.4 to 0.6 g). A combined experimental and theoretical study of the photophysical properties of the synthesized compounds was carried out. Absorption and emission spectroscopy measurements highlighted a slight solvatochromism effect. The luminescence was quenched by molecular oxygen, indicating the partial triplet multiplicity character of the lowest excited state. Density Functional Theory (DFT) calculations have been applied for the evaluation of favoured conformations for these new compounds and the study of their optical properties. Within the Franck Condon principle, the vibrationally resolved electronic one-photon absorption spectrum has been simulated. This simulation shows the presence of a major band followed by a vibronic sideband, typical of organic chromophores in solution. The performed computational study revealed that the transition from the ground to the first excited electronic state has a pi-pi* character. Finally, TD-DFT energy level diagram calculations highlighted the presence of triplet states very close to the first singlet excited one, suggesting probable access to the triplet-excited state.