Fluorescent materials that efficiently convert triplet excitons into singlets through reverse intersystem crossing (RISC) rival the efficiencies of current state-of-the-art organic light-emitting diodes (OLEDs) without requiring expensive heavy metals such as platinum and iridium. The efficiency with which triplet excitons are upconverted into singlet excitons, a phenomenon known as thermally activated delayed fluorescence (TADF), is dictated by the rate of RISC, a material-dependent property that is not directly measureable. In this work, an analytical model is developed to determine RISC, as well as several other important photophysical parameters such as exciton diffusion and the rate of intersystem crossing (ISC), all from simple time-resolved photoluminescence measurements. This new analytical model has been used to investigate 5 different TADF materials in order to elucidate structure-property relationships and understand how RISC can be modulated. The bromination of TADF materials results in a dramatic increase of kISC, kRISC, spin cycling, and exciton diffusion length due to the spin-orbit coupling of the heavy bromine atoms. This general methodology can be used to better understand how molecular structure affects solid state quantum yield and spin dynamics, enabling the rational design of molecules with desirable spin characteristics.