UCLA

Optical imaging has been a vital tool in chemical biology for elucidating biological processes. Traditionally, light in the visible (VIS; 400–700 nm) and near-infrared (NIR; 700–1000 nm) regions of the electromagnetic spectrum has been harnessed for various imaging applications. Recently, it has been shown that light in the shortwave infrared (SWIR; 1000–2000 nm) possesses numerous advantages over the aforementioned regions, such as enhanced contrast and resolution. Numerous classes of SWIR contrast agents have been developed in the past couple of years. These range from inorganic materials such as single-walled carbon nanotubes, quantum dots, conjugated polymers, and rare earth doped nanoparticles, as well as organic, small molecule fluorophores. A small molecule scaffold of choice comes in the form of polymethine dyes. Presently, several SWIR-emissive polymethine dyes have been designed and used for real-time in vivo imaging applications. However, only a handful of SWIR-absorbing polymethines with sufficient molecular brightness have been developed. The work in this dissertation outlines efforts towards better understanding design principles for chromenylium-based polymethine dyes, in order to design improved SWIR-absorbing contrast agents.Chapter One is a perspective on relevant polymethine design principles and introduces early flavylium-based polymethine dyes and their SWIR imaging potential. In Chapter Two, we elucidated flavylium and chromenylium structure-property relationships that allowed us to modulate λmax and ΦF through heterocycle modifications. This insight led to a handful of fluorophores matched to commercial NIR and SWIR laser lines, allowing us to perform real-time, excitation-multiplexed SWIR in vivo imaging. Through this work, up to 4 dyes could be orthogonally excited and imaged with video framerates. In Chapter Two, we noted that modifications at the 2-position influenced ΦF but sought to gain further insight. In Chapter Three, we sought to understand these effects with additional 2-position modifications, considering molecular parameters such as steric effects and substituent electronics. These efforts allowed us to identify additional 2-position substituents that could red-shift λmax or directly improve ΦF. Previously, we dealt only with pentamethine and heptamethine chromenylium dyes for imaging. However, fundamental polymethine design principles led us to investigate longer chain derivatives. In Chapter Four, we developed red-shifted nonamethine chromenylium dyes. These dyes are the brightest fluorophores at their bandgaps to date. For the first time, we demonstrated excitation-multiplexed, 2-color imaging with small molecule fluorophores using only SWIR excitation. Finally in Chapter Five, we embarked on another flavylium structure-property investigation, this time combining 7-position modifications seen in Chapter Two and 2-position modifications seen in Chapter Three. Additionally, we synthesized and characterized pentamethine analogues of these fusion flavylium dyes. This work helped us understand the additive effects of these modifications.