Temporal Dynamics of Solid-State Thermally Activated Delayed Fluorescence: Disorder or Ultraslow Solvation?

Abstract

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.

Thermally Activated Delayed Fluorescence: Polarity, Rigidity, and Disorder in Condensed Phases

“…Vibrational and conformational motions play a pivotal role in favoring ISC and RISC processes. − Moreover, other electronic states, typically triplet states localized on either the D or A unit, may be not too far in energy from the triplet CT state, leading to triplet states with mixed character. ,, The emerging picture, already fairly complex, is made even more problematic by environmental effects. ,, TADF is often experimentally investigated in solution, where the polarity and polarizability of the solvent nontrivially affect the dye photophysics. More dramatically, in OLED the dyes are embedded in solid matrices whose polarity, polarizability, rigidity, and disorder all enter into play. ,− …”

Ionic multiresonant thermally activated delayed fluorescence emitters for light emitting electrochemical cells

“…The photoluminescence quantum yield, Φ PL , in MeCN for DiKTa-OBuIm is 48% which decreases in air to 34%. The emission is much weaker in DiKTa-DPA-OBuIm , reflecting both the smaller oscillator strength of the transition to S 1 and the greater non-radiative decay due to the energy gap law (Φ PL = 11% and 7% under vacuum and in air, respectively) in MeCN [ 37 ]. The S 1 and T 1 levels were measured from the onsets of fluorescence (2.66 eV) and phosphorescence spectra (2.41 eV) in 2-MeTHF glass at 77 K (Figure S26, Supporting Information File 1 ).…”

Judicious Heteroatom Doping Produces High Performance Deep Blue Multiresonant Thermally Activated Delayed Fluorescence Organic Light-Emitting Diodes

“…The red-shift and spectral broadening of the emission are attributed to the polarization effect of the host matrix on the different excited state properties as CT-type TADF emitters are sensitive to the host polarity. 48,49 By contrast, MesB-DIDOBNA-N showed negligible changes in its steady-state PL spectrum in the thin film [lPL at 402 nm and a FWHM of only 19 nm (0.15 eV)] compared to that in solution [lPL= 399 nm and FWHM = 23 nm (0.17 The S1/T1 energies for DIDOBNA-N and MesB-DIDOBNA-N in 1.5 wt% doped TSPO1 films were determined from the onsets of the respective prompt fluorescence and phosphorescence spectra at 77 K, which are 3.12 eV/2.89 eV and 3.18 eV/2.94 eV, respectively. The corresponding DEST values are 0.23 eV and 0.24 eV, respectively, which are consistent with those determined in 2-MeTHF.…”