Phenol-Pyridine Boron Compounds: Polymorphs And Luminescent Properties

The design and synthesis of organic optical and electronic materials haveattracted much attention because of their possible applications in organicelectroluminescent devices (OLEDs), thin film transistors (OTFTs) andphotovoltaic diodes (OPVDs). For organic solid materials, the constituentmolecules may form strong intermolecular interactions and assembly packingstructures resulting in that the properties of these materials are governed by thewhole collective rather than by individual molecules. The performance of organicmolecule-based devices strongly depend on the molecular assembly structures.Therefore, understanding and controlling molecular arrangement in solid state arefundamental issues for obtaining the desired chemical and physical properties of amaterial. The increasing demand for new molecular organic functional materialsrequires elucidating the relationship between molecular packing characteristicsand optical and electronic properties, which enable the development of newstrategies towards high performance organic materials. Non-covalentintermolecular interactions such as hydrogen bonding, π…π interactions aswell as the presence of solvent molecules are all able to strongly influence thefinal packing structure, make polymorphism more probable in organic materials.Different polymorphs may have very different physical and chemical properties,and therefore represent special situations for the study of structure-propertyrelationships with a minimum number of varibles, since differences arise fromcrystal packing effects but not because of the presence of different chemicalspecies. In order to get more insight into the structure-property relationship, wehave designed and synthesized a series of phenol-pyridine boron-containingcomplexes 1-12, used these organic molecules as building blocks to constructluminescent single crystals based on hydrogen bonding and π…π stackinginteractions. We also demonstrated the relationship between the molecularpacking characteristics and the optical properties of the complexes.In view of the important potential applications of boron-containing compoundsin organic light emitting devices (OLEDs), we have explored the synthesis ofnovel four-coordinate boron complexes that contain the mixed phenol-pyridylfunctional group. To improve the performance of the boron compounds, wemodified the boron molecules by attaching aromatic groups to the boron center.The structures of the boron compounds 1, 3, 4 and 11 were determined bysingle-crystal X-ray diffraction and the molecular packing is characterized byintermolecular π … π and hydrogen-bonding interactions. The molecularpacking motif feature in crystal 11 is that the phenol-pyridine moieties are welloutside their van der Waals distance from each other, attributing to the largeststeric hindrance of bis(4-n-butyl-phenyl)phenyleneamine attached to the boroncenter. While for the crystals 3 and 4, close face-to-face contacts betweenphenol-pyridine moieties exist in their crystals. The crystal 1 with naphthylsubstitute are characterized of loose π…π interactions between phenol-pyridinemoieties. We have demonstrated that molecular structure can affect molecularpacking structures in crystals.The methoxyl substituted boron complexes 5-8 show polymorphiccharacteristic and each compound has two or three kinds of polymorphs. Forexample, we have already characterized the independent molecules of 5 in threecrystal structures (A-C) that may be classified as conformational polymorphs (Aand C: cis-isomer, B: trans-isomer). Furthermore, forms A and B are concomitantpolymorphs because they crystallize from the same test tube, and forms A and Care supramolecular isomerism because both cis-isomers present differentmolecular packing modes. All the polymorphs are richly hydrogen bonded and itis possible to define fundamental subunits by which each crystal form isconstructed. In the crystal structure of form A, there are one-dimensionalhydrogen bonded molecular chains that can be divided into the four-pointhead-head (HH) hydrogen-bonding subunits (I) and two-point head-tail (HT)hydrogen-bonding subunits (II);For B, the molecules are tied intoone-dimensional zig-zag like chains that are constructed by the two-pointhead-head (HH) hydrogen-bonding subunits (III) and two-point head-tail (HT)hydrogen-bonding subunits (IV);In C, the molecules are tied in one-dimensionalchains that contain the four-point head-head (HH) hydrogen-bonding subunits (V)and two-point head-tail (HT) hydrogen-bonding subunits (VI).The boron complexes 1-4 and 11 have been used to fabricate organicelectroluminescent devices, which exhibited high EL performance. Thecompounds 1-4 show strong blue photoluminescence in solution and solid state,and the luminescence of the boron compounds is caused by π→π* electronictransitions centered on the phenol-pyridine ligands (L1 and L2). The EL spectra of1-4 are broad, ranging from 400 nm to 680 nm, which contains a blue emissionpeak and an orange one, and completely cover the total wavelength region ofvisible spectrum, that is, white color emission was obtained. Thenaphthyl-substituted compound 1 appears to be the most promising as an emitterfor EL displays. Considering the possibility of exciplex emission between 1 layerand NPB layer, the second hole-transporting layer of CBP was inserted betweenNPB and 1, the exciplex emission disappeared and the device give an EL that isdominated by emission of 1 at about 460 nm. A novel multifunctional1,6-bis(2-hydroxyphenyl)pyridine boron bis(4-n-butyl-phenyl)phenyleneaminecompound 11 in which the hole-transporting (HT), electron-transporting (ET), andemitting (EM) components are integrated into a single molecule is used as anemitting material to fabricate an efficient single-layer electroluminescent device.This single-layer device reached a maximum efficiency of 5.2 cd A-1 (3.6 lm W-1)and a maximum brightness of 2654 cd m-2. To our knowledge, the efficiency is thehighest value for small molecule single-layer EL devices that have been achieveduntil now.We have presented the comparison of EL preference and molecular structures ofboron-containing compounds 1,3,4 and 11. It is clear that the luminescentproperties of 1,3,4 and 11 are strictly related to their molecular structures.Especially, the steric hindrances of the substitutes attached to the boron centerhave curial effect on the luminescent of these boron compounds. The compounds3 and 4 with small steric hindrances exhibit strong π…πinteractions and thedevices show poor brightness and efficiency. The boron compound 1 with largersteric hindrance exhibit weak π…πinteraction and the device shows higherbrightness and efficiency. Compound 11 with the largest steric hindrance andshow almost noπ…πinteraction and the single-layer device shows the highestbrightness and efficiency. Three polymorphs (5α,5βand 5γ) of compound[2,6-bis(2-hydroxyphenyl)pyridyl boron p-methoxylbenzene] (5) exhibitconformational polymorphism, concomitant polymorphism, and supramolecularisomerism and show dramatically different photoluminescent properties. Thecis-isomers exhibit strong blue emission, while the trans-isomer shows very weakemission. The characteristics of molecular conformation and packing arrangementin 5α,5βand 5γ may be responsible for the dramatically different luminescentproperties.In conclusions, we have designed and synthesized a series of boron-containingcompounds 1-12 and obtained single crystals of these compounds. We alsocharacterized the PL and EL preference based on compounds 1,2,3,4,5 and 11and elucidated the relationships between molecular structures/molecular packingstructures and luminescent properties.