Organic semiconductors are promising materials due to their tunable optical properties, facile processing, and mechanical flexibility. Advancing their applications requires a deeper understanding of organic semiconductors, particularly how intermolecular interactions can be manipulated to achieve selective complexation. Improving our understanding of organic semiconductor phosphorescence, an uncommon photophysical process of organic semiconductors, is also essential. Multi-component organic semiconductor systems are useful for such investigations. In exploring fullerene-buckybowl solution complexation, we identify extending buckybowl π-surfaces strengthens buckybowl-fullerene complexation. We find stronger complexation with fullerenes when the buckybowl dipole moment is increased through heteroatom inclusion.

We investigate second-scale room temperature phosphorescence of organic semiconductors, a phenomenon known as ultralong room temperature phosphorescence (URTP). URTP is generally rare due to quenching by nonradiative recombination processes. We enable URTP in various organic semiconductors by embedding them in rigid polymer hosts and thermally annealing to induce sub-micron aggregation. We deduce sub-micron organic semiconductor aggregates suppress nonradiative recombination and reduce diffusional exciton quenching, and we propose URTP is more ubiquitous than previously thought.

We find contorted hexabenzocoronene (cHBC) exhibits uniquely efficient red URTP. This stems from the proximity of a higher-lying triplet to the lowest-lying singlet enhancing intersystem crossing, and comparatively slow fluorescence decay. Eliminating C-H stretching modes, which
disproportionately contribute to nonradiative recombination, by perdeuterating cHBC significantly prolongs its URTP lifetime, generating the longest-lived organic red-emitter to our knowledge. We developed a melt-processable perdeuterated cHBC and rubbery polymer composite that is compatible with 3D printing, enabling the development of customizable phosphorescent objects.  We demonstrate ubiquitous access to URTP in nanoparticles comprising an organic semiconductor, homopolymer, and surfactant stabilizer made by flash nanoprecipitation (FNP). FNP offers precise control over nanoparticle size and composition. The URTP lifetime of the nanoparticle dispersions is stable to drying and redispersion. Moreover, this nanoparticle form factor is extendable to formulating anti-counterfeiting inks, or bioimaging.

This thesis highlights the use multi-component organic semiconductor systems to better understand the intermolecular interactions and photophysical phenomena governing their behavior. Using this insight, we realize unique properties of organic semiconductors that contribute to generating materials that can fit new applications.