We present a joint experimental and theoretical study
of the through-space
charge transfer (CT) TADF moleculeTpAT-tFFO. The measured
fluorescence has a singular Gaussian line shape but two decay components,
coming from two distinct molecular CT conformers, energetically only
20 meV apart. We determined the intersystem crossing rate (1 ×
107s–1) to be 1 order of magnitude faster
than radiative decay, and prompt emission (PF) is therefore quenched
within 30 ns, leaving delayed fluorescence (DF) observable from 30
ns onward as the measured reverse intersystem crossing (rISC) rate
is >1 × 106s–1, yielding a DF/PF
ratio >98%. Time-resolved emission spectra measured between 30
ns
and 900 ms in films show no change in the spectral band shape, but
between 50 and 400 ms, we observe aca.65 meV red
shift of the emission, ascribed to the DF to phosphorescence transition,
with the phosphorescence (lifetime >1 s) emanating from the lowest3CT state. A host-independent thermal activation energy of
16 meV is found, indicating that small-amplitude vibrational motions
(∼140 cm–1) of the donor with respect to
the acceptor dominate rISC.TpAT-tFFOphotophysics is
dynamic, and these vibrational motions drive the molecule between
maximal rISC rate and high radiative decay configurations so that
the molecule can be thought to be “self-optimizing”
for the best TADF performance.