Synthesis And Properties Of BN [4]helicenes

Azaborine is an isoelectronic and isostructural of all carbon benzene.The introduction of BN units into polycyclic aromatic hydrocarbons(PAHs)is an effective way to prepare BN-PAHs with special properties,which might have potential application in high-performance organic photovoltaic materials.In recent years,the synthesis,properties,and applications of planar BN-PAHs with unique properties have been reported.However,research on non-planar BN-PAHs is rare.Helicene is a special class of non-planar PAHs,in which [4]helicene is the smallest one.The dissertation is focusing on the synthesis and properties of BN [4]helicenes.In the first part,bis-BN [4]helicenes were synthesized successfully by using Sonogashira/Suzuki coupling reaction/electrophilic borylation sequences.Five different [4]helicenes 4a-e were obtained and fully characterized by changing the substituent on the B atom.Then,we investigated their photophysical properties and solid state structure by experimental and theoretical calculations.Bis-BN [4]helicenes showed significant redshift of their absorption spectra and slightly blueshift emission spectra comparing to the all carbon [4]helicenes analogue.Up to 0.89 of the fluorescence quantum yield of the bis-BN [4]helicenes was obtained.The calculated NICS(1)values and bond length in their solid state structures demonstrated the weak aromaticity of the sub-BN ring.In the second part,we successfully synthesized mono-BN [4]helicenes 2a-c.Second series of mono-BN [4]helicenes 5a-c were obtained by switching the relative positions of the boron and nitrogen atoms,Both series of mono-BN [4]helicenes were fully characterized.Similar to bis-BN [4]helicenes counterpart,mono-BN [4]helicenes 2a-2c and 5a-5c exhibited significant redshift of their absorption spectra and slightly blueshift of their emission spectra comparing to the all carbon [4]helicene.This [4]helicene has a good fluorescence quantum yields sitting in the range between 0.42 and 0.70,but it is still lower than that of bis-BN [4]helicenes,which demonstrated that the number of BN units has a certain effect on its properties.The weak aromaticity of the sub-BN ring of mono-BN [4]helicenes was also observed.In addition,the substituents on the boron atoms play an important role on the properties of the mono-BN [4]helicenes as well.