Direct thermal patterning of pi-conjugated polymers

The focus of this work is to investigate a novel thermal patterning methodology involving near-infrared (NIR) 830 nm laser irradiation, by taking advantage of computer-to-plate imaging processes employed in the graphic arts industry and applying that methodology to functionalized derivatives of polythiophene and polyfluorene. Poly(3-(2-(2-tetrahydropyranyloxy)ethyl)thiophene) (PTHPET) was employed as the primary material in this proof-of-concept study, which involved the interaction of NIR laser light with a NIR absorbing dye doped in a pi-conjugated polymer film. Thermal imaging of PTHPET using 830 nm laser irradiation was based on the observation that thermolytic cleavage and removal of the bulky THP group can be accomplished in the absence of acid. Exposure of NIR dye-doped polymer films to a NIR laser liberated the THP group, resulting in an insoluble, negative-tone pattern that was characterized using UV-vis spectroscopy, fluorescence spectroscopy and microscopy, TGA, variable temperature FT-IR and surface profilometry. PTHPET was patterned with 10 micron resolution. A second method for obtaining pi-conjugated polymer patterns using NIR laser light was demonstrated, wherein, following deposition of a precursor pi-conjugated polymer layer, a second, non-conjugated polymer film was deposited that contained the NIR absorbing dye. The top layer served as a sacrificial light-to-heat conversion layer that absorbed the laser energy and heated the underlying pi-conjugated polymer film. This method of patterning was demonstrated using a THP-functionalized polyfluorene-co-thiophene copolymer, which was found to retain its photoluminescent properties, ~ 86% of the pristine quantum yield, and could be patterned with < 20 micron resolution. The generality of the technique was demonstrated with THP-functionalized polythiophene which was used to fabricate patterns of polymer with electrical conductivities similar to that of the parent polymers, and with a novel cross-linkable oxetane-functionalized polyfluorene.