Multiple ultrafast spectroscopic techniques and quantum chemical simulations (QCS) were used
to investigate the excited state dynamics of BCC-TPTA. This organic chromophore is believed to
possess excited state dynamics governed by a thermally activated delayed fluorescence (TADF)
mechanism with a reported internal quantum efficiency of 84 %. In addition, a significant
enhancement in its quantum yield in solution after purging oxygen has been reported. This enhancement has been widely accepted as due to a delayed fluorescence process occurring in the
μs time-scale. The spectroscopic measurements were carried out both in solution and blended
films, and from fs to μs time-scales. The excited state dynamics of Rhodamine B and Ir(BT)2(acac)
were also probed for comparison. Investigations in the absence of oxygen were also carried out.
Our time-correlated single photon counting (TCSPC) measurements revealed a lack of long-lived
emissive lifetime for BCC-TPTA in any of the medium tested. Our ns transient absorption
spectroscopy (ns TAS) experiments revealed that BCC-TPTA does not possess triplet transient
states that could be linked to a delayed fluorescence process. Instead, the evidence obtained from
our spectroscopic tools suggests that BCC-TPTA has excited state dynamics that of a typical
fluorescence chromophore and that just comparing the quantum yield difference before and after purging
oxygen from the solution is not an accurate method to claim excited state dynamics governed by
a delayed fluorescence mechanism. Consequently, we believe that previous studies, in which the
photo-physics of organic chromophores with TADF characteristics are reported, may have
overlooked the influence of the host materials on the obtained optical properties in blended films. |
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