Photochemical synthesis of oligothiophene thin films and nano-patterns in condensed multilayer films of 2,5-diiodothiophene-Effects of surface chemistry of substrates

This paper reviews photochemical reactions of 2,5-diiodothiophene multilayers on various solid substrates that lead to production of oligothiophene thin films and micro-patterns with a thickness relevant to nanotechnology applications. Upon UV absorption, the C{single bond}I bond of 2,5-diiodothiophene dissociates generating a thienyl radical and iodine atom. The radicals generated in multilayers can react with other radicals or intact monomers to form dimers. Since the C{single bond}I bonds are present at the ends of the coupling reaction product, further photodissociation and coupling reactions can take place forming oligomeric species. On inert substrates, the average conjugation length of the product is about 3-4 thienyl units. Various pattern generation schemes can be incorporated with this photochemical reaction. Examples of masked irradiation, wettability pre-patterning, and controlled clustering of thermal desorption process are demonstrated. On copper surfaces, the average conjugation length of the produced oligothiophene is increased to 6-7 units, which is long enough for technical applications. This increase is due to Ullmann coupling reactions at the buried interface between the copper and adsorbed film. The mechanism for this buried surface reaction is elucidated from the thickness dependence of the conjugation length and Ullmann coupling side product.