  Engineering a low singlet-triplet energy gap ({\Delta}EST) is necessary for
efficient reverse intersystem crossing (rISC) in delayed fluorescence (DF)
organic semiconductors, but results in a small radiative rate that limits
performance in LEDs. Here, we study a model DF material, BF2, that exhibits a
strong optical absorption (absorption coefficient =3.8x10^5 cm^-1) and a
relatively large {\Delta}EST of 0.2 eV. In isolated BF2 molecules,
intramolecular rISC is slow (260 {\mu}s), but in aggregated films, BF2
generates intermolecular CT (inter-CT) states on picosecond timescales. In
contrast to the microsecond intramolecular rISC that is promoted by spin-orbit
interactions in most isolated DF molecules, photoluminescence-detected magnetic
resonance shows that these inter-CT states undergo rISC mediated by hyperfine
interactions on a ~24 ns timescale and have an average electron-hole separation
of >1.5 nm. Transfer back to the emissive singlet exciton then enables
efficient DF and LED operation. Thus, access to these inter-CT states resolves
the conflicting requirements of fast radiative emission and low {\Delta}EST.
