  The key factors determining the emission bandwidth of thermally activated
delayed fluorescence (TADF) are investi-gated by combining computational and
experimental approaches. To achieve high internal quantum efficiencies (IQEs)
in metal-free organic light emitting diode via TADF, the first triplet (T1) to
first singlet (S1) reverse intersystem crossing (rISC) is promoted by
configuring molecules in an electron donor-acceptor (D-A) alternation with a
large di-hedral angle, which results in a small energy gap ({\Delta}EST)
between S1 and T1 levels. This allows for effective non-radiative up-conversion
of triplet excitons to singlet excitons that fluoresce. However, this
traditional molecular de-sign of TADF results in broad emission spectral bands
(full-width at half-maximum = 70-100 nm). Despite reports suggesting that
suppressing the D-A dihedral rotation narrows the emission band, the origin of
emission broadening remains elusive. Indeed, our results suggest that the
intrinsic TADF emission bandwidth is primarily determined by the charge
transfer character of the molecule, rather than its propensity for rotational
motion, which offers a renewed perspective on the rational molecular design of
organic emitters exhibiting sharp emission spectra.
