UCLA

Rational design of bright near and shortwave infrared (NIR: 700-1000 SWIR: 1000-2000 nm) molecular and nanoscale emitters is fundamental for applications ranging from deep tissue imaging to telecommunications. However, currently all reported organic chromophores with energy gaps in the SWIR have suboptimal properties due to their especially low fluorescent quantum yields. This thesis introduces the main questions pertaining to molecular absorption and emission in Chapter 1, and in Chapter 2 demonstrates the fundamental principles that govern photophysical properties of SWIR dyes, explicating the precipitous decline of quantum yield with longer wavelengths. I demonstrate that for polymethine dyes such declines are consistent with the combination of decreased radiative rates due to shrinking singlet energy gaps and increased nonradiative deactivation via high frequency vibrations. Through simplifying and combining earlier energy gap laws, I develop an energy gap independent parameter that enables comparison of quantum yields among NIR/SWIR chromophores independent of the differences between each molecule’s energy gaps. Applying the energy gap independent parameter, I provide predictions on whether specific synthetic modifications of a dye would lead to improvements in photophysical properties. In Chapter 3, I recontextualize my work in terms of brightness, which is the quantum yield weighted by the dye’s absorptivity, a paramount chromophore property for imaging applications. Here the analysis expands to a greater survey of other dye scaffolds. In Chapter 4, I show preliminary experimental work that tests temperature dependence, heavy atom effects, and other dye scaffolds. In Chapter 5, I discuss potential synthetic and photophysical experiments to further understand the molecular parameters important to bright SWIR chromophores. Modelling both molecular and solvation impacts of fluorescence lifetimes and quantum yield will give both synthetic and physical chemists a handle on key variables to help develop better organic SWIR imaging systems.