Polymorphs of anthracene derivatives exhibit diverse photo-physical properties that can help to develop efficient organic-based photovoltaic devices. 10-Anthryl-9-phosphoanthracene (10-APA) shows different photo-physical behaviors for the solid state due to its variety in crystalline arrangement. Herein, we investigate the ground and excited-state properties of the monomer and two different polymorphs of 10-APA from first-principles. Calculations reveal that strong spin-orbit coupling (SOC) between first excited singlet state (S-1) and triplet manifolds at their S-1-optimized geometries enabling the reverse intersystem crossing (RISC). The electron-vibration coupling (Huang-Rhys factor) in the excited state is the most relevant factor here. For both ISC and RISC, a similarity in Huang-Rhys factors for the molecular vibration along the pi center dot center dot center dot pi stacking at low-frequency region makes the rates effective. On the other side, the nonvanishing vibronic relaxation modes provide a relatively slower RISC rate in the red crystal. However, for the red crystal, small reorganization energy (lambda) and large Huang-Rhys factor toward S-1 -> S-0 conversion reduce nonradiative decay, leading to a prompt fluorescence. As the feasibility of S-1 <-> T-1 conversion increases in the yellow dimer, it allows a delay in fluorescence emission, leading to thermally activated delayed fluorescence (TADF).