Uncovering the Mechanism of Thermally Activated Delayed Fluorescence in Coplanar Emitters Using Potential Energy Surface Analysis

Abstract

Planarized emitters exhibiting thermally activated delayed fluorescence (TADF) have attracted attention due to their narrow emission spectra, improved photostability, and high quantum yields, but with large singlet–triplet energy gaps (ΔE ST ) and no heavy atoms, the origin of their TADF remains a subject of debate. Here we prepare two isomeric, coplanar donor–acceptor compounds, with HMAT-2PYM performing dual TADF and room-temperature phosphorescence but with HMAT-4PYM exhibiting only prompt fluorescence. Although conventional TADF design principles suggest that neither isomer should exhibit TADF, we reveal differences in the excited state potential energy surfaces that enable spin-flip processes in only one isomer. We also find that hydrogen bonding is absent between the planar units of these emitters, despite earlier claims of intramolecular hydrogen bonding in similar compounds. Overall, this work demonstrates that potential energy surface analysis is a practical strategy for designing coplanar TADF materials that might otherwise be overlooked by conventional TADF design metrics.

Realizing High-Efficiency Orange-Red Thermally Activated Delayed Fluorescence Materials through the Construction of Intramolecular Noncovalent Interactions

“…As displayed in Figure , the intramolecular C–H···N distances between the pteridine core and the phenyl linkers range from 2.28 to 2.67 Å, which are shorter than the sum of the van der Waals radii of the hydrogen (H) and nitrogen (N) atoms. − Additionally, the C–H–N angles fall between 100 and 124°. These observations suggest the presence of weak intramolecular C–H···N interactions, including H-bonding and van der Waals interactions (vide infra, Theoretical Calculations section), which can rigidify the molecular conformation and restrict the rotational and vibrational motions, thus being conducive to reducing the nonradiative decays of these materials. , Meanwhile, these intramolecular noncovalent interactions, together with the lack of steric hindrance, , result in relatively small torsion angles between the pteridine electron-acceptor and the bridging phenyl rings, which would extend the π-conjugation of the acceptor fragment and be in favor of realizing efficient intramolecular charge transfer. , In contrast to the non-planar diphenylamine donor units of TPA-PT, the rigid dimethylacridine donor units of DMAC-PT ensure large D–A twist angles (θ 3 = 86.24°, θ 4 = 88.19°) via the steric repulsion between the α-position H atoms. The nearly orthogonal D–A configuration is expected to afford an effective separation of the frontier molecular orbital (FMO) distribution and hence a relatively small Δ E ST .…”

“…These observations suggest the presence of weak intramolecular C−H•••N interactions, including H-bonding 38 and van der Waals interactions (vide infra, Theoretical Calculations section), which can rigidify the molecular conformation and restrict the rotational and vibrational motions, thus being conducive to reducing the nonradiative decays of these materials. 39,40 Meanwhile, these intramolecular noncovalent interactions, together with the lack of steric hindrance, 33,34 result in relatively small torsion angles between the pteridine electronacceptor and the bridging phenyl rings, which would extend the π-conjugation of the acceptor fragment and be in favor of realizing efficient intramolecular charge transfer. 41,42 In contrast to the non-planar diphenylamine donor units of TPA-PT, the rigid dimethylacridine donor units of DMAC-PT ensure large D−A twist angles (θ 3 = 86.24°, θ 4 = 88.19°) via the steric repulsion between the α-position H atoms.…”

“…27 Apart from chemical bonds, intramolecular interactions play crucial roles in modulating molecular configuration and photophysical properties. 28−32 It is essential to verify the presence of intramolecular interactions 33,34 and understand their effects on the molecular structure and properties. Herein, we design and synthesize two orange and orange-red TADF materials, namely TPA-PT and DMAC-PT, based on a new pteridine (PT) acceptor and two triphenylamine (TPA)/ dimethylacridine (DMAC) donors.…”

“…To design high-efficiency orange/red TADF materials, the trade-offs among the nonradiative transition rate constant ( k nr ), k r and Δ E ST should be elaborately balanced by rational regulating the molecular structures, including chemical modification of the donors (D), acceptors (A), and π-linkers . Apart from chemical bonds, intramolecular interactions play crucial roles in modulating molecular configuration and photophysical properties. − It is essential to verify the presence of intramolecular interactions , and understand their effects on the molecular structure and properties. Herein, we design and synthesize two orange and orange-red TADF materials, namely TPA-PT and DMAC-PT, based on a new pteridine (PT) acceptor and two triphenylamine (TPA)/dimethylacridine (DMAC) donors.…”

Phosphorescence or Delayed Fluorescence? Fate of Excitons in Core-Substituted Naphthalimide-Derived Emitters and Ratiometric Optical Oxygen Sensors

“… − Suppression of unwanted nonradiative pathways would lead to efficient room-temperature phosphorescence (RTP) or TADF, depending on these specific molecular features mentioned above. − Both these phenomena have vital significance in optical devices. To achieve a high RISC rate ( k RISC ) in conventional TADF emitters, a well-known strategy for lowering the HOMO–LUMO overlap integral and, therefore, a low Δ E S–T has been commonly followed. − Fine-tuning the k RISC by utilizing the hybrid electronic states of suitable energy has also been successful in achieving efficient TADF emitters. − The singlet–triplet energy offset (Δ E S–T ) and the SOC matrix element of these states drive the forward and backward spin-flip processes (ISC and RISC). − Rates of various nonradiative deactivation pathways of triplet excitons are also crucial for deciding the overall quantum yields (QYs) of the TADF or phosphorescence. , The functional features of RTP and TADF span from efficient lighting devices to metal-free photocatalysis for organic transformations that warrant precise engineering of the energy levels. , The spin-forbidden nature of the T 1 → S 0 transitions results in longer emission lifetimes of the order of ms that can be further extended to the order of seconds to exhibit ultralong RTP. , Likewise, the rate of delayed fluorescence ( k d ) is controlled by the energy offset of triplet states and singlet manifold and consequently is a competing process to the RTP ( k r T ) . A systematic experimental approach and design strategy are necessary to attain molecules of selective emission pathways from the triplet states. , …”