Molecules capable of thermally activated delayed fluorescence (TADF) are promising as emitters in organic light-emitting devices. Processes leading to and competing with TADF in 4,5-di(9H-carbazol-9-yl)-phthalonitrile are analyzed in detail. It is demonstrated that the key features of an efficient TADF emitter include the presence of two triplet states of different natures with potential energy surfaces crossing between the T-1 and S-1 minima and a noticeable dependence of the S-1 -> S-0 oscillator strength on molecular deformations from low-frequency antisymmetric vibrational modes. These conclusions can be useful in the targeted design of efficient TADF emitters.