Stimuli-Responsive Aggregation-Induced Delayed Fluorescence Emitters Featuring the Asymmetric D-A Structure with a Novel Diarylketone Acceptor Toward Efficient OLEDs with Negligible Efficiency Roll-Off

A Series of D-A-D Structured Disilane-Bridged Triads: Structure and Stimuli-Responsive Luminescence Studies

A series of σ-π extended octamethyltetrasilanes, which have phenothiazine, 9,9-dimethyl-9,10-dihydroacridine, or phenoxazine (1, 2, and 3) groups as donor moieties and thienopyrazine or benzothiadiazole (a and b) groups as acceptor fragments, has been prepared, and their optical properties have been studied as an extension of our work. All six compounds exhibited fluorescence in the solid state with maximum wavelengths centered in the range of 400 and 650 nm upon excitation by a UV lamp. Compound 2b showed apparent dual emission behavior in solution, which depends on solvent polarity, and a reversible photoluminescent change under mechanical and thermal stimuli in the solid state. Quantum chemical calculations suggest the contribution of a quasi-axial conformer of the 9,9-dimethyl-9,10-dihydroacridine moiety in 2b to the dual emission in solution and the mechanofluoroluminescence in the solid state, similarly to 1a. These studies provide new insight into the preparation of disilane-bridged triads capable of responding to multiple stimuli.

Effects of Secondary Acceptors on Excited-State Properties of Sky-Blue Thermally Activated Delayed Fluorescence Molecules: Luminescence Mechanism and Molecular Design

The development of efficient sky-blue thermally activated delayed fluorescence (TADF) emitters is highly desired. However, the types and amounts of sky-blue TADF are far from meeting the requirements, and effective molecular design strategies are expected. Herein, the photophysical properties and excited-state dynamics of 12 molecules are theoretically studied based on the thermal vibration correlation function method. Distributions of holes and electrons are analyzed by the heat maps. The frontier molecular orbital distribution, adiabatic singlet-triplet energy gap, and reorganization energy are analyzed in detail. Furthermore, the radiative and non-radiative as well as the intersystem crossing (ISC) and reverse intersystem crossing (RISC) processes are studied, and the up-conversion process is illustrated. Our results indicate that different substitution positions and numbers play an important role in the luminescence properties of TADF molecules. The meta-position substitutions restrict the geometry variations, hinder the non-radiative energy consumption process, and promote the radiative process of TADF molecules. Meanwhile, molecules with ortho-position substitutions possess the smallest energy gaps (ΔE_st) and the largest RISC rates. Moreover, molecules with the substitutions of one tBCz group and two PO groups have the smallest ΔE_st and the largest spin orbital coupling. Thus, a wise molecular design strategy, namely, ortho-position substitutions as well as substitutions with one tBCz group and two PO groups, is proposed to facilitate the RISC process. Based on this rule, new efficient TADF molecules are theoretically designed and proposed. Our work reasonably elucidates the experimental measurements, and the effects of different substitution numbers and positions of secondary acceptors on TADF properties are highlighted, which could provide a theoretical perspective for designing efficient sky-blue TADF molecules.