Reversible wettability of photoresponsive pyrimidine-coated surfaces

Thin coatings of photoresponsive, pyrimidine-terminated molecules, attached to gold or quartz substrates in contact with water, undergo dimerization a nd wettability changes when irradiated with UV light at 280 and 240 nm. Sel f-assembled monolayers of long chain thymine-terminated thiols give the lar gest, reversible photoinduced contact angle changes. The latter are caused by a decrease in surface charge as the thymine monomer dimerizes upon irrad iation, a process which is accompanied by an increase in the acidity consta nt of the dimer. Uracil self-assembled monolayers photodimerize but do not photocleave; there is an irreversible change in contact angle. Spin-cast fi lms of thymines give smaller contact angle changes, the maximum values corr esponding to films which are composed of a mixture of crystalline and amorp hous states.