Conjugated Polymer Nanoparticles Having Modified Band Gaps Assembled into Nano- and Micropatterned Organic Light-Emitting Diodes

Nano- and micropatterns from the Langmuir-Blodgett (LB) assembly of poly(2,5-bis(3-sulfonatopropoxy)-1,4-phenylene disodium salt nanoparticles (PPP-NPs) on glass substrates exhibited a variety of structures and tunable band gaps. Tuning the band gap of optical materials used in devices based on conjugated polymers is useful for manufacturing organic light-emitting diodes and optoelectronics. The PPP-NPs are prepared by supramolecular assembly of the polymer in methanol. A highly packed, collapsed PPP-NPs monolayer assembly is obtained when the uncompressed LB film is transferred to the surface of a glass substrate. The band gap of the PPP-NPs dispersed in methanol is reduced by 0.56 eV after monolayer assembly into 2D nanostructure with an average diameter of 28 +/- 7 nm and 4.2 +/- 0.5 nm height to be 2.47 eV. This large band gap decrease is attributed to the collapse of their supramolecular assembly, the interparticle energy transfer, and change of the dielectric function from methanol to air. Because of the soft nature of the PPP-NPs, compressing the LB film resulted in a hierarchical assembly of the PPP-NPs into microdisks with a few nanometers in thickness. The single microdisk has a band gap of 1.61 eV and a broad optical spectrum composed of multiple peaks due to random energy transfer between the PPP-NPs. When the LB film of the microdisks assembly is compressed, nanopillars of an average similar to 200 nm diameter and similar to 40 nm height are obtained, which have an intense optical signal. The band gap of the individual nanopillar is 1.6 eV.