Quantum Simulations of Thermally Activated Delayed Fluorescence in an All-organic Emitter

We investigate the prototypical thermally activated delayed fluorescence (TADF) emitter NAI-DMAC^[1] [C₃₇H₃₂N₂O₂], in the dilute- and high-packing fraction limits at finite temperature (T= 300 K), by combining first principles molecular dynamics^[2]with a quantum thermostat^[3,4] to account for nuclear quantum effects (NQE). We find a weak dependence of the singlet-triplet energy gap (ΔE_ST) on temperature in both the solid and the molecule, and a substantial effect of packing. While the ΔE_STvanishes in the perfect crystal, it is of the order of ~ 0.3 eV in the molecule, with fluctuations ranging from 0.1 to 0.4 eV at 300 K. The transition probability between HOMO and LUMO molecular orbitals has a stronger dependence on temperature than the singlet-triplet gap, with a desirable effect for thermally activated fluorescence; such temperature effect is weaker in the condensed phase than in the molecule. Our results ^[5] show that optimization of packing and geometrical conformation is critical to increase the efficiency of TADF compounds and highlight the importance of considering thermal fluctuations and NQE to obtain robust predictions of the electronic properties of NAI-DMAC. 

[1] W. Zeng, et al., Adv. Mat., 2018, 30(5), 1704961

[2] F. Gygi, IBM J. Res. Dev., 2008, 52, p.137; code Qbox: http://qboxcode.org.

[3] M. Ceriotti et al., PRL 2009, 103 (3), 030603

[4] A. Kundu et al., PRM 2021, 5 (7).

[5] T.Francese et al. 2021 (submitted)

 

We acknowledge support by MURI Project N00014-19-1-2453