In-Depth Ab Initio Study of Thermally Activated Delayed Fluorescence in 4,5-Di(9H-carbazol-9-yl)-phthalonitrile

Abstract

Molecules capable of thermally activated delayed fluorescence (TADF) are promising as emitters in organic light-emitting devices. Processes leading to and competing with TADF in 4,5-di­(9H-carbazol-9-yl)-phthalonitrile are analyzed in detail. It is demonstrated that the key features of an efficient TADF emitter include the presence of two triplet states of different natures with potential energy surfaces crossing between the T1 and S1 minima and a noticeable dependence of the S1 → S0 oscillator strength on molecular deformations from low-frequency antisymmetric vibrational modes. These conclusions can be useful in the targeted design of efficient TADF emitters.

The role of excited-state character, structural relaxation, and symmetry breaking in enabling delayed fluorescence activity in push–pull chromophores

“…8 Furthermore, the importance of a second, locally excited (LE), triplet state has been emphasised. 6,9,10 Finally, multipolar systems are of interest where one acceptor unit is connected to two or more donor units and vice versa. 4,11 Aside from the above considerations, the possibility of symmetry breaking in the excited state is particularly intriguing in such multipolar systems.…”

“…8,17 Furthermore, state-specific excited-state solvation effects have to be described at a high level considering that the common linear-response polarizable continuum model (LR-PCM) is inadequate for CT states with large electron-hole separation 18,19 and does not even provide any correction beyond zeroth order to triplet states due to their vanishing transition densities. Finally, the importance of dynamic mix-ing effects, 20 structural deformations, 10 and vibronic coupling between the LE and CT triplet states 9,21 has been emphasised.…”

The role of excited-state character, structural relaxation, and symmetry breaking in enabling delayed fluorescence activity in push-pull chromophores

“…The difference is even more apparent in terms of the exciton sizes (bottom panels of Fig. [9][10][11] showing that the triplet states (red dots) are consistently below the singlet states (black dots).…”

“…8,17 Furthermore, state-specific excited-state solvation effects have to be described at a high level considering that the common linear-response polarizable continuum model (LR-PCM) is inadequate for CT states with large electron-hole separation 18,19 and does not even provide any correction beyond zeroth order to triplet states due to their vanishing transition densities. Finally, the importance of dynamic mixing effects, 20 structural deformations, 10 and vibronic coupling between the LE and CT triplet states 9,21 has been emphasised.…”

Constructing high-performance TADF polymers from non-TADF monomers: a computational investigation

“…Theoretical calculation is becoming a powerful tool to study the geometric and electronic properties of TADF molecules and provides useful guidance on molecular design. Bre ´das et al evaluated k RISC and discussed how the factors including DE ST values and spin-orbit coupling (SOC) matrix elements influence the RISC rate; 30 Monkman and Freidzon et al demonstrated that the vibronic coupling mechanism is the driving force for efficient RISC, and thus affects the TADF performance; [31][32][33][34] Shuai et al proposed a molecular descriptor of the activation energy (DG), and a linear relationship is observed between DG and the RISC rate; 35 Shao et al employed four electronic structure analysis tools via orbital interaction, excitation energy components, charge transfer numbers and vibrational reorganization energies to reveal the underlying molecular design principles of TADF materials. 36 And Ma et al revealed that enhancing the low frequency vibronic coupling strength is favourable for realizing narrowband emission of organic fluorescene emitters.…”