Single-wall carbon nanotubes (SWCNTs) exist in a variety of discrete structures, giving distinct optical and electronic properties. Purification of as-grown SWCNTs and sorting of SWCNT mixtures into pure (n,m) fractions are required for enabling basic research progress and developing advanced applications. A remarkably effective method for SWCNT sorting is based on structure-selective wrapping interactions of single-stranded DNA (ssDNA) with nanotubes. In this thesis, optical spectroscopy has been used to investigate the ssDNA-SWCNT system for aspects of photophysics and photochemistry. A simple way to reveal the wrapping ability of ssDNA sequences with SWCNT structural forms is studying fluorescence spectral kinetics during displacement of ssDNA coating by sodium deoxycholate (SDC). It is found that the displacement rate dramatically decreases with increasing nanotube diameter for (GT)20-SWCNT samples. Moreover, the structure-specific interactions of (ATT)4-(7,5) hybrids show strong enough enantiomeric differences to cause resolvable shifts in fluorescence spectra and dramatic kinetic differences in coating displacement. The quenching of ssDNA-SWCNT fluorescence by dissolved oxygen was observed and the effect was quickly and fully reversed at room temperature by displacing the O2. The extent of quenching depends on structure-selective interactions of ssDNA with SWCNTs, allowing their specific affinities to be uncovered and studied through quick and nondestructive optical measurements. In addition, methylene blue (MB), a common dye, has been found to significantly quench ssDNA-SWCNT fluorescence through a charge-transfer process. The distinct MB quenching for different (n,m) species also shows the specific affinities of ssDNA-SWCNT hybrids. Structure-selective photochemistry of SWCNTs has been achieved using near-infrared (NIR) monochromatic irradiation in the presence of dissolved oxygen. The covalent functionalization of (8,3), (6,5), and (7,6) species in unsorted SWCNT dispersions was induced by irradiation with wavelengths of 955, 985, and 1130 nm, respectively. After the photoinduced reaction, fluorescence spectrum was significantly and selectively quenched and the absorption spectrum was heavily and selectively distorted. So the optical properties of SWCNT samples can be easily tailored by such nanotube structure-selective photochemistry. Furthermore, the spatial pattern of covalent functionalization sites on SWCNT sidewalls and the tunable and smooth modification of SWCNT energy levels have been readily obtained using ssDNA as a template to covalently functionalize SWCNTs. A room temperature chemical reaction between the nanotube surface and guanine nucleotides in the ssDNA coating proceeds quickly on exposure to singlet oxygen (1O2). This reaction preserves the SWCNT characteristic fluorescence but causes spectral red-shifts that depend on the spatial guanine density of the ssDNA oligo. Customized spatial patterns and depths of band gap modulation can thus be inscribed on the nanotube by selecting the templating ssDNA sequence. In addition, previously studied sparse chemical doping and DNA-templated covalent functionalization show distinct treated emission features, allowing flexible strategies for modifying SWCNTs.