Unraveling the Mechanism of Near-Infrared Thermally Activated Delayed Fluorescence of TPA-Based Molecules: Effect of Hydrogen Bond Steric Hindrance

Abstract

A recently synthesized novel molecule (named CAT-1) exhibits intriguing near-infrared (NIR) thermally activated delayed fluorescence (TADF) close to 1000 nm wavelength; however, the mechanism behind these intrinsic properties is not fully understood. Herein, we unravel that the fluorescence emission spectrum with a broad wavelength range (770–950 nm) of CAT-1 is primarily induced by hydrogen bond steric hindrance based on density functional theory and Marcus theory. It is found that the hydrogen bond steric hindrance plays a critical role in inhibiting the twist of the configuration of different excited states, which leads to the minor driving force for fast electron trapping between the excited states, as well as small internal reorganization energy caused by less changed geometric configuration. Furthermore, such steric hindrance will cause a more distorted plane, resulting in a less favorable electron delocalization. A faster reverse intersystem crossing (RISC) rate is then obtained due to the nearly unchanged conformation between excited states caused by steric hindrance, although the spin–orbit coupling is small. Consequently, the NIR TADF with a longer wavelength can be emitted in CAT-1. This work shows that the hydrogen bond steric hindrance can fine-tune the electronic interactions of the donor and acceptor units to control the TADF.

Photophysical Properties of Boron‐Based Chromophores as Effective Moieties in TADF Devices: A Computational Study**

“…This implies that as the strength of donating and accepting moieties increases, leading to more effective π electron transfer in conjugated systems, the HOMO–LUMO gaps (E g ) decreases [80–82] . Thus, reducing ΔE ST is achieved by using strong donor and acceptor units [83,84] . Furthermore, the electron donating and accepting ability of D and A groups do not only affect ΔE ST , but they also have an important role in tuning the emission color of TADF emitters [85] .…”

“…[80][81][82] Thus, reducing ΔE ST is achieved by using strong donor and acceptor units. [83,84] Furthermore, the electron donating and accepting ability of D and A groups do not only affect ΔE ST , but they also have an important role in tuning the emission color of TADF emitters. [85] The experimental emission wavelengths are reported in SI showing that the molecules covers efficiently the blue and green portion of the electromagnetic spectrum.…”

Boron-Based Chromophores As Effective Moieties In Optoelectronic Devices

“…[71][72][73] Thus, reducing ΔEST is achieved by using strong donor and acceptor units. 74,75 Furthermore, the electron donating and accepting ability of D and A groups do not only affect ΔEST but they also have an important role in tuning the emission color of TADF emitters. 76 In our molecular set, we benefited from the electron accepting ability of boron atom and we have analyzed its effect on hole orbitals.…”

On donor–acceptor-bridging intramolecular hydrogen bonds in NIR-TADF molecules

“…2 ) has recently been reported to lie in the use of intramolecular hydrogen bonds to fixate the relative positions and rotations of charge-transfer (CT) donor and acceptor units. 6 In comparing CAT-1 with its non hydrogen-bonded counterpart TPAAP, 7 (3-(4-(diphenylamino)phenyl)acenaphtho[1,2- b ]pyrazine-8,9-dicarbonitrile) Leng et al found a lowered λ int that may be responsible for its more efficient NIR-TADF process compared to TPAAP.…”