Decisive Role of Heavy-Atom Orientation for Efficient Enhancement of Spin-Orbit Coupling in Organic Thermally Activated Delayed Fluorescence Emitters

A heavy-atom field effect (HAFE) is discovered in red TADF emitters bearing Br atoms in donor fragment. Depending on the position of heavy atoms, HAFE can either accelerate or inhibit reverse intersystem crossing.

. Steady-State PL spectra of investigated compounds: 6% (A, E) and 10% (C) CBP with onsets and phosphorescence spectra measured in 10K, 6% (B, F) and 10% (D) CBP.   Theoretical rate constants of rISC were calculated using Marcus-Hush equation: where V is SOC constant, ħ is reduced Planck constant, λ is sum of internal λint and external (λsolv) reorganization energies for respective transition (in our calculations we assumed λsolv, = 0.3 eV) ΔEST is the energy gap between singlet and respective triplet state, k B stands for Boltzmann constant, T is temperature.

Calculations of the 3 LE→ 1 CT rate constants.
To verify whether 3 LE→ 1 CT channel has an impact on rISC, we performed theoretical calculations of its rate constants (k3LE-1CT) using Marcus-Hush equation (S1) and experimental ΔE3LE-1CT values. Since we consider two triplet states from which rISC is potentially possible, at first, we estimated relative population of these levels using Boltzmann distribution law: where Δ denotes the energy difference between lowest triplet state (T1) and respective triplet state (Ti): = exp (− (T − T 1 ) ).
(S5) From the results included in Table S3, it can be seen, that population of triplet states is strongly dominated by 3 CT state due to large difference in energies between 3 CT and 3 LE levels.
Next, values of k3LE-1CT for each emitter were calculated just as k3CT-1CT, taking into account population of 3 LE state χ 3LE(A) . Results are presented in Table S3 and Figure 4.
Since theoretical predictions of rISC constant rates based on exclusively 3 LE-1 CT channel did not showed a good correlation with experimental values, we conclude that 3 LE-1 CT channel has not considerable impact on rISC in most of the cases except for the rotamers with very low rates of 3 CT-1 CT transition as 2Br-anti ones.  Figure S9. Natural transition orbitals for the S1-S0 and T1-S0 transitions for selected 3Br rotamers. NTO indicate almost negligible role of bromine atoms in the electronic transitions. Figure S10. Triplet spin density distribution (TSDD) maps S13 Section S3: NMR spectra of target emitters 1 H NMR spectrum of 3-(4-((2-bromo-4-methylphenyl)(p-tolyl)amino)phenyl)-dibenzo[a,c] phenazine-11,12-dicarbonitrile (1Br) in CDCl3 Section S4: Determination of photophysical parameters PL decay curves (presented in Figures 2F and S4) were fitted with the multiexponential equation: where is the pre-exponential factor, τ is the decay time and ( ) is emission intensity. Average lifetimes of prompt (τ ) and delayed fluorescence (τ ) were determined using the following formula: where is fractional contribution of -th component expressed as: The ratio of DF and PF quantum yields ⁄ was determined as follows: where ( ) and ( ) is the pre-exponential factor of delayed and prompt fluorescence component, respectively; τ ( ) and τ ( ) is the lifetime of delayed and prompt fluorescence component, respectively. The rate constants of radiative ( ) and nonradiative ( ) decay and intersystem crossing ( ) are given by equations [S2]: = τ , (S10) = φ τ , (S11) here φ is PLQY ( + ). Further, the quantum yields for ISC and rISC were calculated as = τ , (S13) S19 = 1 − /φ . (S14) Finally, the rate constant of rISC ( ) was calculated as = τ ( φ ).
Thus obtained photophysical parameters are presented in Table 2 (main text).