Temporal Dynamics of Solid-State Thermally Activated Delayed Fluorescence:

What is it about?

Time-resolved emission spectra of thermally activated delayed fluorescence (TADF) compounds in solid hosts demonstrate significant temporal shifts. To explain the shifts, two possible mechanisms were suggested, namely, slow solid-state solvation and conformational disorder. Here we employ solid hosts with controllable polarity for analysis of the temporal dynamics of TADF. We show that temporal fluorescence shifts are independent of the dielectric constant of the solid film; however, these shifts evidently depend on the structural parameters of both the host and the TADF dopant. A ≤50% smaller emission peak shift was observed in more rigid polymer host polystyrene than in poly(methyl methacrylate). The obtained results imply that both the host and the dopant should be as rigid as possible to minimize fluorescence instability.

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Why is it important?

Seeking to reveal the solvation role in the temporal emission dynamics, we performed the photophysical analysis of TADF compounds doped in two different polymer hosts with selectively tuned polarity. No evident dependence of the temporal shifts of prompt and delayed fluorescence on the dielectric constant of the polymer film was found. If solidstate solvation is the driving force of temporal emission dynamics, omewhat different shifts should have been observed in a more polar surrounding. However, the observed behavior was in accordance with a conformational disorder mechanism. Explicitly, a more sterically restricted molecular structure of the ACRPyr dopant favored smaller emission shifts, which were further diminished by employing a more rigid polymer host. Such enhanced spectral stability of TADF compounds is highly preferable for rapid triplet upconversion.

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