Molecular Design Strategies Toward Blue Thermally Activated Delayed Fluorescent Materials With Efficient Exciton Utilization Based On Excited States Modulation

Thermally activated delayed fluorescent?TADF?materials are now highly anticipated for realizing efficient organic light-emitting diodes?OLEDs?due to their capability of utilizing triplet excitons through reverse intersystem crossing?RISC?from the lowest triplet excited state?T1?to the lowest singlet excited state?S1?.Designing TADF materials with an efficient RISC process is regarded as the key to actualize efficient OLEDs with low efficiency roll-off.Achieving small singlet-triplet splitting energy(?E_ST)by forming twisted intramolecular charge-transfer?TICT?states in TADF molecules is the most popular molecular design strategy for promoting the RISC process.However,the rate of RISC processes are not only depended on the value of ?E_ST,but also related to the strength of spin-orbit coupling?SOC?.Without an strong SOC,the strategy of promoting RISC by blindly reducing ?E_ST is bound to fail.Meanwhile,the existence of TICT states will also lead to limited rate of radiative transition and redshifted emission,which is not conducive to the realization of OLED with high exciton utilization efficiency and deep-blue emission.The essence of these problems is the conventional TADF molecular design strategy which blindly reduces ?E_ST but ignores the features and natures of relevant excited states.Thus,molecular design strategies toward blue and deep-blue TADF materials with efficient exciton utilization based on excited states modulation were provided and implemented in this thesis with three key issues are stressed on:1.Firstly,through the study and analysis on the features and natures of excited states,it was explored that effective RISC process and fast radiative transition could be achieved simultaneously by adjusting the energy differences between relevant excited states of the molecule with large ?E_ST.With the novel TADF molecular design strategy,TADF materials nearly 100% exciton utilization efficiency in OLED devices despite nonobvious delayed fluorescence and a large value of ?E_ST being observed in photoluminescence measurement,which signify that photoinduced delayed fluorescence and small ?E_ST are sufficient but not necessary for TADF materials with efficient exciton utilization.Meanwhile,it provides a reference for the point of view that upper triplet excited states with different features and natures play a key role in RISC process.2.On the basis of this understanding,multiple excited states with different natures and appropriate energy levels were introduced to form efficient “multichannel” RISC process in novel TADF molecules.With the existence of “multichannel” effect,fast RISC process and were excellent device performance of OLEDs based on novel materials were observe,which also proved the importance of excited states modulation for molecular design strategy toward efficient RISC process.3.Further,the contradiction of achieving small ?E_ST and deep-blue emission simultaneously was attempted to be solved.Effective RISC process and deep-blue emission were achieved simultaneously by the novel molecular design strategy that adopting appropriate conformational relaxations to adjust the energy differences between relevant excited states.Meanwhile,with conformational relaxations,not only the energy levels,but also the nuclear coordinates,the features and the natures of relevant excited states were changed,which significantly strengthen the SOC between relevant excited states and form “multichannel” RISC process.As results,OLED devices based on novel materials showed excellent device performance with deep-blue emission.