Ultrafast Spectroscopic Investigation of the Aggregation Induced TADF from High-Level Reversed Intersystem Crossing

The thermally activated delayed fluorescence (TADF) originating
from high-level intersystem crossing (hRISC) presents great potential
in realizing a more full utilization of triplet excitons. In this
study, DPA-FBP and TPA-FBP were doped in a PMMA film with different
weight fractions to study the effect of aggregation on the luminescence
properties. As a result, the TADF feature from hRISC was only found
in the 50 wt % doped film, whereas the 1 wt % doped film only shows
prompt fluorescence. The fs-TA spectroscopy results reveal that the
50 wt % film will generate charge transfer species to lower the energy
gap, so that the high-lying triplet exciton can transition back to
the singlet state, whereas that of the 1 wt % film will quickly transition
to the lowest triplet state due to the unfavorable energy splitting.
This study provides a new insight into aggregation effects on the
excited-state properties of hot exciton materials and the solid-state
photodynamic.