Optical properties of nano-sized materials (optical nanomaterials) can either be the result of interactions of light with periodic material structures, e.g. in colloidal or block copolymer based photonic crystals, or stem from the incorporation of photoactive molecules into nano-sized, optically inactive materials, e.g. fluorescent dyes in organic-inorganic hybrid silica nanoparticles. This dissertation introduces representatives of both material classes. The first case described here are ultrasmall (sub-10 nm) amorphous silica nanoparticles (SNPs) covalently encapsulating photoactive organic moieties. Such particles, referred to as Cornell prime dots (C’ dots), have already shown tremendous success in the safe diagnosis of cancer in human clinical trials with melanoma patients. However, their full potential in the lab and clinical setting, as diagnostic as well as therapeutic probes, has not yet been fully explored. Furthermore, comprehensive understanding of particle structure-property correlations, i.e. core and surface properties, remains limited. In the first part of this dissertation, a new approach for characterizing the particles is introduced using a combination of fluorescence correlation spectroscopy (FCS), single particle bleaching, and high-performance liquid chromatography (HPLC). It is shown that the net charge of organic dyes introduced in the synthesis is a main contributor to chemical surface heterogeneities of the particles. In the second part of this thesis a new class of ultrasmall theranostic silica nanoparticles for the application in photodynamic therapy is described. It is demonstrated that high effective singlet oxygen quantum yields can be achieved, while keeping particle size below the threshold for renal clearance (sub-10 nm). Next, the concept of particle molecular photo-engineering (PMPE) is introduced as a means to tailor photophysical properties of organic dye encapsulating SNPs. By precisely engineering the chemical composition of the amorphous silica particle core network around encapsulated organic dyes using specific functional groups, i.e. mercaptopropyl or iodopropyl groups, dye transient dark states can be controlled which in turn enables super-resolution microscopy and substantially enhanced singlet oxygen quantum yields, respectively. The second class of optical nanomaterials in this dissertation is a self-assembled poly(styrene-block-tert-butyl methacrylate) (StB) diblock copolymer with a photonically active lamellar structure. Bottom-up self-assembly processes provide highly desired and cost-effective methods for the fabrication of large scale/area photonic coatings, making such materials interesting candidates for applications in architecture and design. In part three of this thesis the synthesis of ultralarge molar mass StB block copolymers and their application as iridescent and transparent thin film coatings is described. Development of a casting-lamination process to apply such coatings to window panels allowed the first architectural use of block copolymers as an iridescent façade in the form of the art installation A needle woman: Galaxy Was a Memory, Earth is a Souvenir by artist Kimsooja. Quantitative characterization of structural as well as optical properties of these coatings establishes that the block copolymer films behave as volume-phase gratings with grating periodicities close to 300 nm.