Photophysical properties of an extended bis-oxonol dye

A bis-oxonol dye has been prepared from reaction between N,N′-dibutylthiobarbituric acid and glutacondialdehyde dianil monohydrochloride. The dye comprises two N,N′-dibutylthiobarbiturate units connected via a pentamethine spacer, and detailed 1H NMR (2D-COSY, NOESY) experiments confirm that the polyene backbone is in the all-trans arrangement. The dye is intensely blue in colour, absorbing around 640 nm in ethanol with a molar absorption coefficient of 175,000 mol−1 dm3 cm−1. Under illumination, the first-excited all-trans singlet state is formed but suprisingly there is little or no isomerisation to a cis isomer. Quantum chemical calculations suggest that there is a high barrier to isomerisation in the ground state while the absence of photoisomerisation is attributed to ineffective population of the relevant LUMO at the S1 level. The dye, which possesses an unusually long-lived singlet excited state for a cyanine-type dye, shows modest capacity for transporting potassium cations across a membrane and functions as an excellent acceptor for electronic energy transfer from other cyanine dyes.