Boron-bridged All-ring-fused Ladders With Full-color Fluorescence: Synthesis And Applications

The advancement of organic conjugated materials contributes significantly to thedevelopment of organic optoelectronics. In this field, more attention has been attracted onthe organic ring-fused ladder-type Ï€-conjugated materials, whose unique molecularstructures are favorable for the ordered conformation, and thereby lead to the excellentthermo stability, high fluorescence efficiency as well as carrier mobility. The incorporationof main-group elements, such as B, Si, P, S and Se, into the ladder skeletons as bridgingmoieties is a powerful strategy to develop new materials with unique functions andproperties. The main-group elements have notable features, which endow unusualelectronic structures as well as suitable molecular arrangements in the solid state, givingrise to unique attractive photophysical and electronic properties. Among the reportedmain-group-doped ladders, multi-boron bridged ladder-type Ï€-conjugated compounds haveproved to be emissive organic solids with high electron affinity, which enables them to beused as both emitting and electron-transporting materials to fabricate structurally simpleand high-performance EL devices. Although ladder-type polymers have been extensivelystudied for their synthesis and application, only few boron-bridged ladder-type smallmolecules have been achieved to date probably due to the lack of efficient synthetic routes.In this regard, constructing new ladder-type multi-boron doped Ï€-conjugated skeletons bysimple synthetic routes, studying the microstructure-property relationship and finallydeveloping its practical application on OLEDs may become a very important issue.In our present work, the chemical synthesis was performed from the following twoaspects.(1) Diboron bridged ladder-type molecules were synthesized by coordinating arylboron centers with ligands of two active sites. We tuned the conjugation length from five to seven fused rings and adjusted the coordination form of boron.(2) Single boron bridgedladder-type molecules with the shortest conjugated skeleton were obtained by coordinatingboron reagents with ligands of single active-site. We were able to incorporate substituentgroups of varying electron-donating ability on fixed position and change the bridging atomin the conjugated system. Full-color emission was obtained through both these tworespects. The subsequent property characterization was performed in terms of photophysics,thermochemistry, electrochemistry, etc. This further confirmed the fact that boron-bridgedladder-type molecules were of rigid structure, efficient solution/solid fluorescence, highthermo-stability and electron transport ability. Finally, we also fabricated theelectroluminescence devices based on these molecules, and a better performance wasshown.1. In chapter II, Four ladder type π–conjugated diboron complexes have been designedand synthesized by a very simple synthetic procedure. The boron atoms in the skeletonadopt a typical tetrahedral geometry to form C,N B fused five-membered rings that bridgethe extended Ï€-conjugation structure. There is no interfacial Ï€ Ï€ interaction in the packingstructure of the single crystal due to that large steric hindrance of bulky mesityl substitutes.The decomposition temperatures measured from thermal analyses are all above340°C,which indicates that the ladders are of high thermal stability. The emission spectra of thesecomplexes are located in blue and green regions and their fluorescence quantum yields stayin a medium level in both solution and solid state. The absorption and fluorescence spectraprove that the functionalization on boron center is more effective in tuning thephotophysical properties compared to the modification of ladder skeleton for this type offluorophore. Electrochemical data of the ladders indicate that boron chelation greatlylowers the LUMO level (below–2.88eV) of the Ï€-system, and thereby endow theseÏ€-conjugated ladders with enhanced electron affinity. Photostability measurements indicatethat the mesityl boron chelated complexes rather than phenyl derivations may undergophotochemical reaction in solution. 2. In chapter III, we have designed and synthesized four novel diboron-containedÏ€-conjugated ladders. The system was prepared by a simple synthetic procedure in mildconditions and further functionalization was easily realized by introducing electronwithdrawing/donating groups into different positions of the ladder skeleton. In the rigidskeletons, all boron atoms adopt typical tetrahedral geometry to form N,O-chelatesix-membered rings which contribute to form the six-ring fused ladders. Single crystalstructures demonstrate that bulky aryl substitutes on boron centers prevent efficientÏ€-stacking. One molecule is connected with the adjacent ones by variety types ofintermolecular interactions including C–H···π, C–F···π and hydrogen bonds. Thermalanalyses indicate that the diboron-contained ladders exhibit excellent thermal stability withthe high melting points (332°C–405°C) and decomposition temperatures (341°C–378°C).In the fluorescence spectra, the four complexes show intense green or yellow fluorescence,three of which give high quantum efficiency up to around0.8in solution. Without theÏ€-stacking of the ladder skeletons, some of the complexes maintain high fluorescencequantum yields in solid state and enables them to be further used as emitting materials. TheLUMO energy levels calculated from cyclic voltammetry curves are in the low range of–2.88eV to–3.22eV, indicating high electron affinity. The electron mobilities ofnon-substituted and F-substituted ladders are determined to be3.2×10–4cm2V–1s–1and8.3×10–4cm2V–1s–1respectively, which indicates that they are good electron-transportingmaterials. Simple EL devices fabricated using two of these complexes as both emitter andelectron-transporting material exhibit the highest brightness and efficiency amongboron-contained materials reported before.3. In chapter IV, we designed two brightly fluorescent red and deep red boron-containedcomplexes by extending the π–conjugation to N,O B chelated seven-ring-fused ladders.The four side phenyl groups attached to boron atoms are effective in keeping theluminescent unit apart, and endow the compounds with high fluorescence quantum yields(0.300.41) in the solid state. Both diboron complexes are thermally stable as evidencedby their high decomposition temperatures (above400°C). p–conjugation extension canpull LUMO level down below–3.25eV and the electron mobilities are7.7×10–4cm2V–1 s^–1and2.2×10^–4cm²V–1s–1respectively, which illustrates that they are goodelectron-transporting materials. These favourable characteristics enable them to be suitablecandidates for non-doped red emitters with excellent electron-transporting ability inelectroluminescent devices.4. In chapter V, a simple modification of a boron-bridged four-ring-fused core byintroducing various amine groups allows the construction of six highly efficientcompounds with emission bands covering a wide range from deep blue to saturated red.The amine groups are attached to the para-position on the hydroxyphenyl ring, for thisposition is more effective in tuning the luminescent properties. The fluorescence ofcomplexes, substituted by carbazole, diphenylamine, dimethylamine, is red-shifted alongwith the increasing electron donor ability. The bridging atoms O and S of the ligands affectthe electronic structure of the skeleton and influence the emission spectra. The emissioncolors of these materials cover a wide range from deep blue to saturated red in bothsolution and the solid state. Crystal structure analysis discloses that two phenyl groupsattached to the boron atom effectively keep luminescent ring-fused π–conjugated skeletonsapart, making these fluorophores be highly emissive in solid forms (F=0.360.71).Organic light-emitting diodes employing these boron complexes as emitters not only keepthe full-color tunable emission feature but also show high EL performance, for instance,the greenish-blue device based on2showed the highest efficiency of7.8cd/A and theyellow light-emitting device based on4exhibited the highest brightness (31220cd/m²)among boron-contained emitters reported so far.To conclude, we successfully synthesize sixteen kinds of boron-bridged ladder-typep–conjugated compounds through two approaches, and they all demonstrate full-colorspectrum emission. Based on measurements of thermochemistry, photophysics, andelectrochemistry, the structure-property dependence is analyzed. Finally some ladders areused in organic light emitting diodes (OLEDs), with the aim to develop their potentialapplication in the field of organic optoelectronics.