Abstract:
Organic materials exhibiting thermally activated delayed fluorescence (TADF) are now recognized as next-generation materials for organic light-emitting diodes. TADF is generally known
to originate from reverse intersystem crossing (RISC) from the lowest excited triplet state (T1)
to the lowest excited singlet state (S1). However, the details of TADF emission mechanisms are
not fully understood.
Here we study excited-state dynamics of 2,3-di(9H-carbazole-9- yl)benzonitrile (2CzBN)
in toluene by means of transient spectroscopies. Previously, we found that the 2CzBN system
is TADF non-emissive system at room temperature; however, it becomes TADF emissive when
the solution is frozen. To understand this phenomenon, we here determined the temperature
dependent lifetime of T1 of the system.
By decreasing temperature, we found that a lifetime of T1 increases according to Arrhenius
law. Especially, we observed that the lifetime suddenly increased when the sample was frozen,
and TADF was observed at that temperature. However, further decreasing the temperature
resulted in no TADF emission, and near liquid nitrogen temperature only phosphorescence was
observed.
In general, the lifetime of T1 is determined by the following competitive processes; RISC,
radiative decay to ground state (S0), and nonradiative decay to S0 . Our results indicated that
when the solution is frozen, the nonradiative decay of T 1 to S0 is strongly suppressed because of
suppression of molecular collisions to surrounding solvent molecules. In that condition, while
RISC also be weakened, molecules can still vibrate effectively to promote thermal activation of
T1 to S1 for subsequent TADF emission. Further decreasing of the temperature switches off the
RISC due to further suppression of the thermal vibration and only the radiative decay path is
left for phosphorescence emission. Consequently, we can propose that not only RISC but also
suppression of the nonradiative decay from T1 to S0 are key factors for the TADF activation and
emission efficiency.
