Density Functional Studies On Photophysical Properties And Chemical Reactivities Of Small Organoboron Compounds

In recent years, luminescent organoboron compounds are of great importance inapplications as diverse as organic light-emitting diodes (OLEDs), nonlinear optics,fluorescent molecular probes, and intracellular fluorescence imaging, owing to theirfascinating photoelectronic properties. Upon incorporation of the boron atom into theπ-conjugated systems, the lowest unoccupational molecular orbitals (LUMOs) will bedramatically lowered, resulting from the interaction between the empty p orbital of the boronatom and the conjugation electrons. Thus the photophysical properties were facilely tuned.Therefore, it is important to investigate the relationship between the structures and propertiesfor design and development of novel optoelectronic materials with ideal properties.In this paper, the photophysical properties of the luminescent organoboron compounds,such as BODIPY derivatives, were investigated in detail with an aim to help researchersunderstand the structure-property relation of this kind of luminescent organoboron materials,and hence orientate the synthesis efforts. The main content of this paper are as follows:Part1: Monomeric BODIPY molecules1and Ph1, corniform dimers (bisBODIPY)2and Ph2, and their packing systems were taken as calculation models to investigate therelationship between monomeric structures and spectral properties of packing systems. Ourresults reveal that2and Ph2show clearly bathochromic shift and exhibit a clear excitonsplitting in the absorption spectrum compared with those of1and Ph1. The intermolecularaggregation slightly affects the excitation energy of2and Ph2since the unique corniformstructures can greatly reduce self-quenching effect induced by the formation of excimers.2and Ph2could act as promising ambipolar materials and efficient luminescent materials forelectroluminescent devices.Part2: The photophysical properties of four BODIPY derivatives were investigated toshed light on the origin of their large Stokes shifts. The results show that the pronouncedgeometric distortion due to the rotation of unlocked phenyl groups and intramolecular chargetransfer are responsible for the large Stokes shift of1and2, while3shows a relativelyblue-shifted emission wavelength due to its mild geometric distortion upon photoemission,although it has a comparable energy gap to1. Finally, the designed compound4showsdesirable and expected properties, such as high Stokes shift (4847cm-1), red emission at660nm, and reasonable fluorescence efficiency. These properties give it great potential as an idealemitter in organic light-emitting diodes.Part3: Three-coordinate organoboron compounds1-2and twisted reference compound Mes3B were investigated by employing using density functional theory (DFT) and conceptualDFT methods to shed light on the planarity effects on the photophysical properties and thechemical reactivity. Our results show that due to the strong electronic induction effect ofboron centers, the HOMO levels of1-2were significantly lowered. Importantly, the reactivitystrength of the boron atoms in1-2is much lower than that in Mes3B, owing to the strong p-πelectronic interactions, that is, the empty p-orbital of boron center is partly filled by π-electronof the neighbor carbon atoms.