Photoisomerization of DODCI at solid/liquid interfaces studied by steady-state and time-resolved total-internal-reflection fluorescence spectroscopy

Photoisomerization of 3,3'-diethyloxadicarbocyanine iodide (DODCI) between the id and P isomers at bare borosilicate and octadecyl chain-modified C-18 glass/ethanol interfaces was studied by steady-state and time-resolved tot al-internal-reflection (TIR) fluorescence spectroscopies. Both the fluoresc ence spectrum and the lifetime of DODCI determined at the glass/ethanol int erface were different from those in bulk ethanol. Simulations of the photoi somerization kinetics on the basis of a three-valley S-1 potential surface diagram revealed that the kinetics of P --> N isomerization in the ground s tate was largely affected by the presence of the octadecyl groups on the gl ass surface, while that of N --> P conversion in the excited state was simi lar but slightly decreased compared to that in a bulk phase. The large decr ease in the P --> N isomerization rate was explained in terms of an increas e in local friction (viscosity) at the interface and strong structural inte ractions between DODCI and the octadecyl chains bound to the glass surface.