Study On The Regulation Mechanism Of The Luminescence Performance Of Deep Red And Near-infrared Thermally Activated Delayed Fluorescence Molecular Systems

The long-wavelength emission characteristics of deep red(DR)or near-infrared(NIR)luminescent molecular systems make them highly promising for applications in organic light-emitting diodes(OLEDs),organic photovoltaics,organic sensors,fiber-optic communication,biomedical imaging,medical diagnosis,and phototherapy devices,among others,which have enriched the research and rapid development of organic semiconductors.Emerging thermally activated delayed fluorescence(TADF)molecules possess efficient reverse intersystem crossing rates and can fully utilize triplet excitons.Therefore,TADF-based OLEDs can achieve high exciton utilization efficiency and low-cost fabrication.However,achieving high luminescence efficiency and low efficiency roll-off in DR-and NIR-TADF OLEDs is a significant challenge.As the core component of OLEDs,the luminescence efficiency and charge carrier transport characteristics of organic luminescent molecular systems directly determine the device efficiency and efficiency roll-off degree.Therefore,the design of organic molecules with high luminescence efficiency and low balanced carrier mobility is a critical prerequisite for achieving the widespread application of OLEDs.Based on this consideration,we employ the Polarizable Continuum Model(PCM),molecular dynamics(MD),molecular mechanics,and quantum mechanics(QM/MM),in conjunction with the thermal vibration correlation function(TVCF)method,to comparatively study the excited-state dynamics of DR-and NIR-TADF molecules in liquid and aggregation states to reveal the relationship between DR-or NIR-TADF molecular structures and luminescence efficiency.The luminescence mechanism of organic molecules is also explored.Some novel efficient DR-and NIR-TADF molecules are theoretically designed and studied.Additionally,we investigate the carrier tranport mechanism of DR-or NIR-TADF molecules in solid state.The influence of organic molecular structures and packing modes on the charge carrier tranport properties are explored,and some novel TADF molecules with bipolar transport are theoretically designed.Our work would provide important theoretical reference for the design of DR or NIR-TADF emitters with high efficiency.(1)Mechanistic study of delayed fluorescence process of DR-or NIR-TADF molecules based on phase controlA difluoroboron organic molecule M1 was reported that it exhibits delayed fluorescence in the amorphous state,but not in the toluene or crystal states.Using a multiscale approach combined with the TVCF method,we theoretically investigated the photophysical properties of this molecule in the toluene,crystal,and amorphous states.We explored the effects of intramolecular p-hydrogen bonding and disordered molecular stacking on the molecular structure,and revealed the intrinsic photophysical mechanism of the phase-transition-induced delayed fluorescence.Based on these findings,we further designed NIR-delayed fluorescence molecules theoretically.This study elucidates the mechanism of delayed fluorescence induced by phase-dependent conformational changes,and provides guidance for the design of new NIR-TADF molecules.(2)Mechanistic study of controlling the non-radiative processes of DR-or NIR-TADF molecules based on the modulation of excited state planarity in donorDue to that the internal conversion rate would exponentially increase with decreasing energy gap,DR-or NIR-TADF molecules usually have high internal conversion rates,which makes it difficult to achieve high luminescence efficiency.To address this issue,we designed a new DR-TADF molecule,DBPz-2sp Ac,using a rigid donor spirocyclic acridine(sp Ac).We used a multi-scale and TVCF method to theoretically investigate its photophysical properties in toluene and aggregate states,explored the inherent relationship between the sp Ac excited state planarity and the reorganization energy as well as the nonradiative rate of the molecule,and revealed the effect of the donor excited state planarity on the luminescence efficiency.Our collaborators demonstrated a reduction in the nonradiative rate and an improvement in the luminescence efficiency in the aggregate state experimentally.Furthermore,we designed five TADF molecules based on spiroacridine derivatives,and our results showed that the reduction of the excited state planarity in the spiroacridine derivatives resulted in a decrease in the reorganization energy and nonradiative rate.This study elucidates the mechanism of controlling the nonradiative rate through excited state planarity in donor and improves our understanding ofthe luminescent behavior of TADF molecules based on spiroacridine derivatives in solution and aggregate states,paving the way for the design of efficient DR-and NIR-TADF molecules.(3)Mechanistic study on controlling the luminescence and charge carrier transport of DR-TADF molecules by modulating the donor substitution positionsDR-TADF molecules with high efficiency and high charge carrier mobility have broad application prospects in OLEDs.In order to elucidate the relationship between molecular structure,luminescence properties,and charge carrier transport properties,we used QM/MM and TVCF methods to systematically investigate the excited-state properties and decay rates of DR-TADF molecules with meta-donor(Y-type)and para-donor(T-type)substituents in the crystal state.The charge carrier transport properties of the molecular systems were also studied through the Marcus equation and Monte Carlo simulations.The study shows that para-donor substitution can effectively improve the luminescence efficiency and obtain the balance charge carrier transport.This work reveals the influence of the substitution site of the donor group on the stacking mode,luminescence,and carrier transport properties,providing theoretical reference for designing high-efficiency non-doped TADF OLEDs with deep red emission.(4)Mechanism study of wavelength and fluorescence efficiency modulation of NIR-TADF molecules based on acceptor modificationResearch on the acceptor groups that have a significant impact on the emission color and device performance is relatively scarce,especially for NIR-TADF molecules.In this study,we first used PCM,QM/MM,and TVCF methods to reveal the emission mechanisms of two DR-TADF molecules experimentally reported.Based on this,we performed high-throughput computational screening of high-performance NIR-TADF molecules using acceptor modification strategy,and systematically investigated the photophysical properties of these molecules by studying the impact of molecular structure on their emission properties.Furthermore,using multiscale simulations,we predicted a new highly efficient NIR-TADF molecule in toluene and the amorphous state.Finally,our collaborators confirmed that this molecule is a highly efficient NIR-TADF molecule in toluene experimentally.This work not only reveals the emission mechanism of TADF molecules but also proposes an effective design strategy for NIR-TADFmolecules.(5)Mechanism study on regulation of DR/NIR-TADF molecular luminescence and charge carrier transport based on acceptor reconstructionThe development of DR-or NIR-TADF molecular systems with high luminescence efficiency and balanced charge carrier transport is still a formidable challenge,and it is an urgent requirement for new design strategies.Based on the experimentally reported TADF molecule CNQ-TPA with red emission,we theoretically design two new molecules,CNQ-b-TPA and CNQ-f-TPA,by using the design strategies of acceptor bonding and acceptor fusion,and predicted their crystal structures.Using QM/MM and TVCF methods,we studied the photophysical properties and excited-state decay rates of the three molecules in toluene and the crystal state.The results confirmed that acceptor bonding and acceptor fusion are effective strategies for achieving high-efficiency DR-and NIR-TADF molecules.In addition,Monte Carlo simulations and Marcus equations were used to study the charge carrier transport properties,and it was demonstrated that molecular bonding can effectively balance charge carrier transport.This work reveals the impact of two new design strategies on the luminescence and charge carrier transport properties and provides an important foundation for the design of high-efficiency and bipolar transport DR-and NIR-TADF molecules.The thesis consists of the following eight chapters: chapter one introduces the development process of organic light-emitting diode,the research progress of deep red and near-infrared molecule system.Chapter two introduces the theoretical methods and models used in this research: density functional method(DFT),QM/MM method,molecular dynamic(MD)simulation,crystal structure prediction,carrier transport simulation,independent gradient model(IGM)method,energy decomposition method based on molecular force field and methods for calculating radiation,non-radiation,intersystem crossing and reverse intersystem crossing rate constants.Chapter three to seven are based on the above research methods with specific work and discussion of the results.In chapter three,the molecular photophysical characteristics of toluene,crystal and amorphous states were studied theoretically by the multi-scale method combined with the TVCF method,revealing the influence of molecular configurations andphases on the DR/NIR-TADF molecular delayed fluorescence process.In chapter four,a novel DR-TADF molecule was designed by spiroacridine(sp Ac),and the emission property and photophysical mechanism of the molecule in toluene and aggregation statuses were theoretically predicted,revealing the regulation mechanism of the planarity of the donor excited state on the non-radiative process of DR/NIR-TADF molecules.In chapter five,the excited state property of the non-doped aggregation Y and T shaped DR-TADF molecules were systematically studied,revealing the influence of donor substituent positions on the accumulation mode,luminescence and carrier transpor property.In chapter six,through the acceptor modification strategy proposed,the influence law of molecular structure on its luminescence property was systematically studied and molecules with potential application were predicted.In chapter seven,novel molecules were constructed by the acceptor binding and acceptor fusion design strategies proposed,and the photophysical properties,excited state decay rates and charge transfer characteristics of the three molecules were studied in toluene and crystal states.Finally,the research work done was summarized in the last chapter,and the prospect of future research was also put forward.