Molecular Conformational Isomerization:An Efficient Way to Design Novel Emitters with Dual-Thermally Activated Delayed Fluorescence Characteristics and Their Optoelectronic and Bioimaging Applications

Single-molecule luminophores with dual-thermally activated delayed fluorescence(TADF)properties are receiving increasing attention.However,how to achieve these goals requires more in-depth studies.Herein,we demonstrate a novel example emitter,10-(5-(2-(pyridin-3-yl)-[4,5′-bipyrimidin]-6-yl)pyridin-2-yl)-10Hphenoxazine(PmPy-PXZ),enabling dual-TADF properties due to its key feature of conformational isomerization.Introducing a pyridine bridge can greatly reduce the steric hindrance and facilitate dual-stable conformations in the ground state,where the quasi-axial(QA)forms predominate.Moreover,unlike previously reported TADF molecules with dual confirmations,both theoretical and experimental measurements show that not only the quasi-equatorial(QE)forms but also the QA forms exhibit distinct TADF characteristics,which can be attributed to an additional higher reverse intersystem crossing pathway.This is the first time that dual-TADF properties of single molecules have been achieved based on conformational isomerism.Its applications in“self-doping”organic light-emitting diode and biomedical imaging have further been investigated.All these results show the good potential of such dual-band TADF emitters based on molecular conformational isomerization.

Conformational isomerization:A novel mechanism to realize the AIE-TADF behaviors

Thermally activated delayed fluorescence(TADF)materials with aggregationinduced emission(AIE)features can overcome aggregation-caused quenching(ACQ)and emit intensely in aggregate states and thus have attracted enormous attention in the fields of high-efficiency organic light-emitting diodes,bioimaging,photodynamic therapy,photocatalysis,etc.However,their corresponding exact working mechanisms at the microscopic level are still far from clear.Herein,by carefully investigating the physical properties of our newly designed TADF material 6-(10H-spiro[acridine-9,9′-fluoren]-10-yl)nicotinonitrile in various states,we concluded that conformational isomerization plays an important role in realizing high photoluminescence quantum yields in its amorphous neat film state,in which the high-lying quasi-axial conformations with non-TADF features and low-lying quasiequal conformations with TADF characteristics serve as the host matrix and dopant,respectively,thus suppressing ACQ in disordered aggregate states.Our work not only offers a new possible microscopic mechanism by using conformational isomerization for the AIE-TADF phenomenon but also provides a novel method for designing high-efficiency AIE-TADF emitters.

Synthesis of cyclic peptide reniochalistatin E and conformational isomers

Here, we report a convenient and efficient synthesis strategy for the total synthesis of cyclic peptide reniochalistatin E and its conformational isomers with 32% overall yield. We found the linear peptide precursor without side chain gave better cyclization yield.

A Proposed Stereochemical Mechanism for the Improved Preparation of Maleic Anhydride Cycloadduct of CLA

The fatty acid derivatives, prepared from renewable natural oils, can be used as highly promising and potential substitutes for petrochemicals. The study of process improvement and stereochemical mechanism for preparing these derivatives would be beneficial for their industrial production. Conjugated linoleic acid (CLA) containing 9cis-11trans (9c, 11t) and 10trans-12cis (10t, 12c) isomers was prepared from Salicornia herbacea seed oil. Maleic anhydride cycloadduct of CLA (MAC) was prepared by an improved process, and it was characterized by FTIR, 1H and 13C NMR, etc. A new method to calculate conformers-ratio of CLA or MAC was also developed. Furthermore, the stereochemical mechanism for the improved preparation of MAC was proposed primarily by the calculation method above. The following observations were made: 1) The yield of MAC could reach as high as 96.7% under mild reaction conditions and with an easy and efficient product separation;2) The trans-trans CLA in the s-cis conformation acted as a predominant reactant to Diels-Alder [4 + 2] cycloaddition of maleic anhydride, which was the main reaction occurred simultaneously with catalytic configurational isomerizations of CLA in one step;3) From all studied CLA conformers, the most stable conformation was the s-trans conformation of trans-trans CLA, while the s-cis conformation of trans-trans CLA had the most favorable structural parameters for cyclohexenyl ring formation;4) Four MAC conformers derived from 9c, 11t- and 10t, 12c-CLA, were obtained as final main products that were determined to be cis-cycloadducts;5) The endo/exo ratios of the cis- cycloadducts derived from 9c, 11t- and 10t, 12c-CLA, were 2.14:1 and 1.99:1, respectively;and 6) The results obtained from the calculation method above were in excellent accordance with those from our experiments.

Trifluoromethyl enable high-performance single-emitter white organic light-emitting devices based on quinazoline acceptor

Single-emitter white organic light-emitting diode(WOLED) based on small organic molecule exhibits great potential in simplifying fabrication process of WOLEDs. However, the design and synthesis of molecule for highly efficient single-emitter WOLED still remains a challenge. Herein, two asymmetric donor-acceptor-acceptor'(D-A-A') type molecule(PTZ-PQ-F and PTZ-PQ-CF3) are developed by employing trifluoromethyl(CF_(3)) or fluorine atom as secondary acceptor, which can exhibit white lighting with dual emission bands consisting of blue traditional fluorescence from quasi-axial(ax) conformer and orange thermally activated delayed fluorescence(TADF) from quasi-equatorial(eq) conformer. The introduction of CF_(3) into PTZ-PQ-CF3 greatly enhanced the photoluminescence quantum yield(PLQY) by suppressing the nonradiative deactivation. Owing to electron-inductive-effect of CF3, the “eq” conformer of PTZ-PQCF3 exhibits a much smaller ΔESTof 0.01 e V to realize more efficient reverse intersystem crossing(RISC)process, and then enhance the exciton utilization(nearly 100%) of the whole dual emission system. Consequently, single-emitter WOLEDs based on PTZ-PQ-CF3 show nearly standard white emission with EQE of 13.0% and CIE of(0.35, 0.36) in m CP host and show warm white emission with high EQE of 25.5%and CIE of(0.40, 0.47) in 35 Dcz PPy host, which are the best performance among reported single-emitter WOLEDs.

Molecular Engineering of Naphthalimide Methylcyclohexane Luminogen:Unraveling J^(*)-Aggregation Pattern and Sensing Melamine in Aqueous Media

Naphthalimide methylcyclohexane(NMICY)luminogen that features molecular conformation-directed aggregation-induced emission was prepared,and its pure equatorial conformer was isolated and wellcharacterized via various techniques,including single-crystal X-ray diffraction(SC-XRD)analysis.In NMICY,the methyl cyclohexane substituent adapted the most stable chair conformation,preventing intermolecularπ-πinteractions among the naphthalimide cores.Careful analysis of the self-assembly behavior indicated that an uncommon J^(*)-aggregation pattern was predominant for NMICY that exhibited∼450 nm emission in crystal as well as in the aggregated state[λmax=455 nm,фPL=23.22%,fw=99.9%in water or water-dimethylformamide mixture].NMICY self-assembled into cube-shaped 2D-nanoarchitecture as characterized by fieldemission scanning electron microscopy,atomic force microscopy,and dynamic light scattering(Zavg=143 nm,∼20 nm height)studies while also displaying unique optical properties in the aggregated state.The supramolecular self-assembly and aggregated state structure-property relationship in aqueous media of NMICY were optimized,and a facile strategy for the detection of melamine in milk and an aqueous solution of milk powder is reported with a limit of detection value of 0.184 ppm(milk)and 0.101 ppm(milk powder),achieved via the rare acceptor-excited photoinduced electron transfer mechanism.Furthermore,a smartphone-based,portable,and uncomplicated sensing platform was developed that simplifies the melamine detection process.