SUPERCRITICAL-FLUID TUNING OF REACTIONS RATES - THE CIS-TRANS ISOMERIZATION OF 4-4'-DISUBSTITUTED AZOBENZENES

Remarkable tuning of reaction rates has been achieved for the isomeriz ation of several dye molecules in supercritical fluid solvents using b oth small pressure changes and small additions of cosolvents. Rates of the thermal cis-trans relaxation were measured spectroscopically foll owing irradiation for three dyes in supercritical carbon dioxide and e thane, pure and with several polar and protic cosolvents. The results are explained in terms of local density and composition enhancements, as well as by a change of mechanism due to strong hydrogen bonding. Th is study demonstrates the versatility of supercritical fluid solvents both to examine reaction mechanisms and as a means to tune rates.