| Compared with inorganic semiconductor materials,organic optoelectronic materials have the advantages of light weight,easy processing,low-cost large-area preparation,etc.,and show bright prospects.The conjugated structures directly affect organic optoelectronic materials in many aspects,such as the energy levels,photophysical properties,chemical stability,processability,etc.It is self-evident that the adjustment of the conjugated structures is in highly importance when designing the organic optoelectronic materials.The conjugation can be roughly classified in two categories namely through-bond conjugation and through-space conjugation.The through-bond conjugation has been well studied,while the through-space conjugation is still not well-known.Different from through-bond conjugation,through-space conjugation is essentially an orbital overlap formed owing to noncovalent weak interactions between two closely ?-?-or n-?-stacked groups.When comparing with through-bond conjugated systems,through-space conjugated ones have great potential in multi-dimensional electron and energy transfer.However,there are few through-space conjugated systems available for research,and the systematically study and application developments of through-space conjugated systems are thus under huge limit.This dissertation focuses on the through-space conjugated systems such as folded tetraphenylethene(TPE)derivatives,o-phenylenes(o P)derivatives,folded phenanthrene derivatives.We systematically study their structure-property relationships and apply them to organic optoelectronic devices like single-molecule devices and electron transport layer materials in organic light-emitting diodes(OLEDs).The content includes the following five parts:1.Taking pyridine-modified TPE foldamers as the models,we explore the effect of through-space conjugation on the intramolecular charge transfer state.Experimental and theoretical results prove that,through-space conjugation can stabilize the intramolecular charge transfer state through the large-area delocalization of the electron cloud,slow down the non-radiative charge recombination processes between the electron donor and the acceptor,and improve the fluorescence emission efficiency.This work provides a new strategy for developing luminescent materials.2.By adjusting through-space conjugation,we realize not only dual fluorescence and dual phosphorescence but also dual circularly polarized luminescence emission with opposite signs in folded compounds with phenanthrene skeleton.Experimental and theoretical results prove that,introduction of groups with appropriate electron-withdrawing ability can reduce the energy levels of the through-space conjugated orbitals,leading to the formation of new emission states.Thus,the photophysical behaviors of molecules no longer follow the Kasha rule strictly,and dual fluorescence or dual phosphorescence emission can be observed.Besides,two emission states exhibit two signals with opposite signs in the circularly polarized luminescence owning to the different angles between the electric and magnetic transition dipole moments.This work provides new ideas for the development of luminescent functional materials.3.A series of TPE foldamers modified by different numbers of anchoring groups at different positions are synthesized to study the influence of through-space conjugated conductance channels on the single-molecule conductance.Theoretical calculation results demonstrate that,due to the altered number and location of the anchoring groups,different contact configurations between the molecules and gold electrodes greatly affect electron transport processes.Multiple conductance states can be observed in f-4Ph-4SMe which can contacts to gold electrodes in different configurations with four anchoring groups.Among them,when the four anchoring groups of f-4Ph-4SMe are all contacted to the gold electrodes,the through-space conjugated channels and the through-bond conjugated channels work synergistically,resulting in the largest conductance.This work provides new ideas for the designing and developing of single-molecule parallel circuits.4.The conformational distribution of helical o P derivatives with different lengths in solution and their single-molecule conductance are systematically studied.Experimental results show that,o P derivatives have a variety of conformations which can be transformed into each other spontaneously in solution.In the single-molecule conductance tests,h-5Ph(the o P of 5benzene rings in length)exhibits two conductance states,while h-6Ph(the o P of 6 benzene rings in length)exhibits only one conductance state.Theoretical analyses prove that the two conductance states of h-5Ph should be attributed to two different conformations,while one conductance state of h-6Ph arises from several conformations.This is because when h-6Ph is contacted to the gold electrodes in different conformations,the through-bond conjugated and through-space conjugated channels can work synergistically and compensate each other,contributing to electron transport processes.In that case,the single-molecule conductance of h-6Ph in different conformations tend to be close,resulting in a single conductance state.This work is of great significance for understanding the mechanism of multi-dimensional electron transport.5.By introducing the quinoline groups into phenanthrene skeleton,we synthesize molecules with folded and linear structures,respectively,and investigate their potential in OLED as the electron transport layer(ETL)materials.Experimental and theoretical results prove that,the electron mobility of folded molecules which have twisted structures and poorer through-bond conjugation are 2 orders of magnitude higher than that of linear molecules with much better through-bond conjugation.In addition,using OLED molecules as ETL to fabricate OLED devices can greatly extend the life-time while ensuring the efficiency of the devices.Theoretical analyses prove that,through-space conjugation equips materials with characteristics of high triplet energy level,high electron mobility,no glass transition temperature and so on.Therefore,through-space conjugated molecules show broad commercial prospects as universal ETL materials in OLED devices featuring high efficiency and long life-time. |
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