Advancements in understanding dynamic processes within purely organic molecular systems have spurred new paradigms for controlling excitons in the realm of organic electronics. One promising strategy involves the utilization of organic luminescent materials that exhibit thermally activated delayed fluorescence (TADF). This phenomenon relies on spin-flip mechanisms, including both forward and reverse intersystem crossings. Notably, TADF molecule facilitates spin up-conversion driven by thermal energy, even at room temperature, resulting in the emission of delayed fluorescence (Figure 1). These remarkable photophysical properties have propelled TADF molecules into the spotlight, attracting significant attention from industry, academia, and research institutes since its initial discovery in 2012.¹

To delve deeper into the critical spin conversion processes between the excited singlet state (S) and triplet state (T) in TADF molecules, we can utilize the following general relation: λ∝H_SO/ΔE_ST. Here, λ represents the first-order mixing coefficient, H_SO denotes the spin-orbit coupling matrix element (SOCME) between the T and S states, and ΔE_ST signifies the energy splitting between the S and T states. As expected, enhancing λ—essentially increasing the mixing extent of singlet and triplet wave functions—can be accomplished by minimizing ΔE_ST. This approach has been widely embraced in the synthetic design of efficient TADF materials over the past decade. In essence, researchers have established design principles by focusing on spatial charge separation, often manifested as twisted intramolecular charge-transfer (TICT), to minimize ΔE_ST. This is achieved by introducing a twisted dihedral angle, approaching orthogonality, between the electron-rich donor moieties and the electron-deficient acceptor groups. Through this strategy, the efficiency of spin-up conversion, particularly in CT-type TADF molecules reported to date, has approached the unity.

In CT-type TADF systems, minimizing the energy difference (ΔE_ST) between the lowest excited singlet state (S₁) and triplet state (T₁) is important, as it reflects the need to reduce electron exchange energy (J), a quantum mechanical effect. This suggests that these systems typically exhibit significant CT excitation characteristics in both S₁ and T₁ states. However, direct spin-flip between S₁ and T₁ states, which share the same CT-type molecular orbital (MO) configuration, is theoretically expected to be inefficient or even forbidden according to the conservation of total angular momentum, where the spin-orbit coupling (H_SO) ideally equals 0.² This raises the question of how such efficient spin-flip occurs in CT-type TADF molecules.

To address this, researchers are exploring high-lying T states, particularly those with locally excited triplet states (³LE). These states have significantly different excitation characteristics compared to states with purely CT excitation nature. For instance, molecule like 10-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-10H-phenoxazine (PXZ-TRZ) exhibits a similar CT-type MO configuration between S₁ state (¹CT) and T₁ state (³CT). However, the T₂ state features a LE excitation character (³LE), resulting in a large SOCME value between the S₁ state and T₂ state (Figure 2).³ In this context, several research groups have proposed that RISC, specifically the spin up-conversion process, operates from ³LE (T₂) to ¹CT (S₁) after the completion of fast triplet equilibrium between ³LE (T₂) and ³CT (T₁) via non-adiabatic coupling under the same spin multiplicity.⁴

It is important to note that not all CT-type molecular systems adhere to the previously mentioned spin-vibronic coupling (SVC) mechanism. For example, this mechanism falls short in explaining the spin-flip processes in representative multiple donor-acceptor (D-A) type carbazole-cyano (CzCN) molecular systems like 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN). Unlike D-A type TADF molecules such as PXZ-TRZ, the spin-conversion process in these CzCN derivatives characterized by multiple carbazole (Cz) units remains incompletely understood, despite numerous studies on the topic. Moreover, accurately defining the high-lying T states of CT-type TADF molecular systems spectroscopically poses a challenge due to Kasha's rule.⁵ According to this rule, photon emission predominantly occurs from the most stable excited state of a given spin multiplicity, complicating the elucidation of spin-flip processes in detail. Therefore, researchers often rely on quantum-chemical calculations to investigate the mechanism, yet these calculations still lack precision.

In a previous report,⁶ it was shown that spin-flip processes in CT-type molecules occur through a specific intermediate high-lying T state, which is associated with the electronic structure of a "partial" molecular framework within the CT-type molecule. Despite the recognition of the importance of this high-lying T state in spin-flip processes, the underlying principle of this state remains unclear. Therefore, the objective of this research was to revisit and investigate the spin-flip processes in these CzCN-derivative TADF materials. We specifically focused on the 4CzIPN molecule and its partial molecular structures detached to Cz-units: 4,5,6-tri(9H-carbazol-9-yl)isophthalonitrile (o-3CzIPN) and 2,4,6-tri(9H-carbazol-9-yl)isophthalonitrile (m-3CzIPN) (Figure 4).

In our initial assessment for this research, we observed that both 4CzIPN and o-3CzIPN exhibit the same energetic order, with ¹CT > ³LE > ³CT. However, m-3CzIPN shows an inverted triplet character with the order of ¹CT > ³CT > ³LE (Figure. 5). Despite these differences, these CzCN-derivative display similar rate constants of RISC in the range of approximately 10⁶ s^-1. This was determined experimentally using a three-level kinetic model, which only considers the S₁, T₁, and ground (S₀) states. Given these findings, a natural question arises regarding the necessity of a comprehensive dynamic model to fully understand the spin-flip processes, including the involvement of high-lying T states such as the T₂ state, in these molecular systems.

Driven by this, our study offers valuable insights into the intricate spin-flip processes within typical CT-type molecular systems. The comprehensive analytical spin-flips (COMPASS) model, which incorporates a four-level electronic structure involving the S₁, T₂, T₁, and S₀ states, has proven indispensable in deciphering the complex pathways governing exciton behavior. Our combined theoretical and experimental findings highlight the significance of the high-lying T state arising from a partial molecular framework in CT-type molecules, particularly in CzCN derivatives. This partial electronic structure leads to the difference in the spin-flip pathway, especially when an energetic inversion occurs within triplet manifolds. We further explored the correlation between the spin-flip route and its implications on device performance, particularly in terms of roll-off characteristics and operational stability. The COMPASS model emerges as a unified and robust framework for fully understanding exciton dynamics in CT-type molecules. We anticipate that this model will serve as a guiding compass for addressing the pressing issues that have been discussed over the last decade in this field.

 References

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