Calibration of local chemical pressure by optical probe

Chemical stabilization of a high-pressure metastable state is a major challenge for the development of advanced materials. Although chemical pressure (Pchem) can effectively simulate the effect of physical pressure (Pphy), experimental calibration of the pressure passed to local structural motifs, denoted as local chemical pressure (Pchem-Δ) which significantly governs the function of solid materials, remains absent due to the challenge of probing techniques. Here we establish an innovative methodology to experimentally calibrate the Pchem-Δ and build a bridge between Pchem and Pphy via an optical probe strategy. Site-selective Bi3+-traced REVO4 (RE = Y, Gd) is adopted as a prototype to introduce Bi3+ optical probes and on-site sense of the Pchem-Δ experienced by the REO8 motif. The cell compression of RE0.98Bi0.02VO4 under Pphy is chemically simulated by smaller-ion substitution (Sc3+ → RE3+) in RE0.98-xScxBi0.02VO4. The consistent red shift (Δλ) of the emission spectra of Bi3+, which is dominated by locally pressure-induced REO8 dodecahedral variation in RE0.98Bi0.02VO4 (Pphy) and RE0.98-xScxBi0.02VO4 (Pchem-Δ), respectively, is evidence of their similar pressure-dependent local structure evolution. This innovative Δλ-based experimental calibration of Pchem-Δ in the crystal-field dimension portrays the anisotropic transmission of Pchem to the local structure and builds a bridge between Pchem-Δ and Pphy to guide a new perspective for affordable and practical interception of metastable states.

Polarity-Tunable Host Materials and Their Applications in Thermally Activated Delayed Fluorescence Organic Light-Emitting Diodes

A series of polarity-tunable host materials were developed based on oligocarbazoles and diphenylphosphine oxide, and their polarities can be tuned through increasing distance of acceptor and donor units. Density functional theory calculations were employed, and photoluminescence spectra in different polar solvents were measured to illustrate different polarities of these host materials. As CZPO has relatively stronger polarity, electroluminescence (EL) spectrum of solution-processed device based on 6 wt % PXZDSO2:CZPO is 7 nm red-shifted relative to that of other host materials based devices. Besides, a comparable impressive external quantum efficiency (EQE) value of 18.7% is achieved for an evaporation-processed yellow device consisting of FCZBn, which is superior to that of the device based on CBP (4,4'-dicarbazolyl-1,1'-biphenyl) (17.0%), and its efficiency roll-off is also obviously reduced, giving an EQE value as high as 16.3% at the luminance of 1000 cd/m². In addition, from CZPO to FCZBn as the polarities of host materials decrease, EL spectra of solution-processed devices based on DMAC-DPS emitter blue-shift constantly from 496 to 470 nm. The current work gives a constructive approach to control EL spectra of organic light-emitting diodes with a fixed thermally activated delayed fluorescence emitter by tuning the polarities of host materials.