In this contribution, we utilize
multiple time-resolved and nonlinear
optical measurements and quantum chemical simulations to investigate
the excited-state dynamics of organic chromophores with thermally
activated delayed fluorescent (TADF) characteristics. We are most
interested in probing the influence of a phenylene linker on the photophysical
properties of emitters composed of carbazole-based donors linked to
either a phthalonitrile (PN) or diphenyltriazine (Trz) acceptor. The
PN-acceptor compounds display a near double-fluorescence quantum yield
(ΦF) enhancement in oxygen-free conditions. The fluorescent
lifetime measurements indicate that the Trz-acceptor compounds are
more efficient fluorescence emitters and quickly go from a delocalized
to localized state. They also reveal that only the PN-acceptor compounds
display a long-lived emissive lifetime component associated with TADF
activity. Analysis of the nanosecond transient absorption spectra
and kinetics reveals long-lived excited-state absorption (ESA) bands
associated with triplet states for the PN-acceptor compounds. No ESA
bands were observed for the Trz-acceptor compounds, despite observing
a quantum yield enhancement for the Trz-acceptor compounds after oxygen
purging. From the transient absorption measurements, it was determined
that the PN-acceptor compounds have reverse intersystem crossing rates
(k
rISC) that are able to compete with
other triplet decay pathways. From quantum chemical calculations,
it is proposed that inclusion of the phenylene linker prevents sufficient
highest occupied molecular orbital/lowest unoccupied molecular orbital
separation and suppresses TADF activity and that directly linking
multiple donors to the acceptor will aid in achieving TADF activity.
This study investigates the use of benzobisoxazole-based (BBO) materials with a donor-acceptor-donor structure towards the production of materials with thermally-activated delayed fluorescent (TADF) properties for use in organic light emitting...
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