Boron-Modified [7]Helicene With Circularly Polarized Luminescence Properties

Circularly polarized luminescence（CPL）refers to the chiroptical property of the differential emission between right-and left-handed circularly polarized light from intrinsically chiral fluorophores or achiral fluorophores in chiral environments.The active-materials,which can emit CPL directly without using a circular polarization filter,have become a key channel to generate CPL.The CPL-active materials have shown potential applications in 3D displays,information encryption storage,biological probes,biological signatures,and asymmetric photosynthesis.Therefore,the research on CPLactive materials has grown rapidly in the past decade,especially in small organic molecules（SOMs）.In the research of CPL-SOMs,to achieve intense CPL with a high fluorescence quantum yield（ΦF）and a high luminescence dissymmetric factor（|glum|）and to realize CPL sign inversion without changing the absolute configuration of the chiral emitter are two important but challenging issues.Helicenes are ortho-fused polycyclic aromatic compounds with unique helical chirality.Recently,they have been recognized as good scaffolds for obtaining CPL-SOMs because of their enhanced chiroptical properties,such as high specific rotation,high absorption dissymmetric factor（|gabs|）and high |glum|.However,the ΦF of helicenes is usually very low due to the low transition probability of the first excited state,which is symmetry-forbidden.the symmetry forbidden of the frontal orbit of helicene itself.Recently,we found modification of[5]helicene with the bulky electron-accepting dimesityboryl（BMes2）is effective for increasing ΦF.In addition,by introduction of two phenyl group at the terminal phenyl rings to increase the configuration stability,we successfully obtained CPL-active[5]helicenes with ΦF up to 0.51 and |glum| up to 6.51 × 10-3.Considering the very high configuration stability of[7]heliecen,we herein designed and synthesized two triarylborane-based[7]helicences,9-dimesitylboryl[7]helicene（BHC）and 9-（2-dimesityboryl）phenyl[7]helicene（BPh-HC）,by introduction of a dimesitylboryl（BMes2）or 2-（dimesityboryl）phenyl（2-BMes2Ph）at 9-position of[7]helicene skeleton.In addition,their photophysical properties and structures were fully characterized.Although ΦFS of these two compounds are only moderate,they are still much higher relative to the parent[7]helicene（[7]HC）.Thus,the current structural modification is also effective to improve fluorescence efficiency of[7]helicene.Moreover,both compounds exhibit mirror-imaged circular dichroism（CD）and CPL spectra.It was notable that the change of peripheral substituent from dimesitylboryl to 2-（dimesityboryl）phenyl induced doubling of |glum| and sign inversion of CPL.Based on these experimental and theoretical calculations,it is safety to deduce that that the helical configuration of helicene exhibits great influence on population of propeller configurations of boron in B-HC and BPh-HC despite the interchange between them.For compound BPh-HC,in addition to helical chirality and propeller chirality,there exists axial chirality due to the very large steric effect of 2-BMes2Ph.And the axial configuration of BPh-HC should be stable and is controlled by the helical configuration.In the ground state（S0）,（MHC,PB）-B-HC（78%）is dominant for（M）-B-HC while（MHC,R,PB）-BPh-HC（91%）is dominant for（M）-BPh-HC.From S0 state to S1 state,（MHC,R,MB）-BPh-HC exhibits more efficient structural relaxation and becomes the dominant component（75%）in S1 state.In contrast to（M）-BPh-HC,the composition of（M）-B-HC is still dominated by（MHC,PB）-B-HC（65%）in S1 state.Therefore,the dominant configuration of boron retains unchanged for（M）-B-HC but is inversed for（M）-BPh-HC.It was also noted that the calculated CPL intensity of（MHC,PB）-B-HC is much stronger than that of（MHC,MB）-B-HC while that of（MHC,R,MB）-BPh-HC is stronger than that of（MHC,R,PB）-BPh-HC.In addition,the calculated CPL sign is opposite with each other for the two diastereomers with inverted propeller configuration for B-HC and BPh-HC,implying the possible significant influence of the propeller configuration of boron on both CPL intensity and CPL sign.Considering the compositions and CPL intensities of two diastereomers,the overall calculated CPL spectrum of（M）-B-HC is determined by（MHC,PB）-B-HC while that of（M）-BPh-HC is controlled by（MHC,R,MB）-BPh-HC.The CPL sign inversion from B-HC to BPhHC is closely related to the propeller configuration flipping of boron center,which exhibits remarkable influence on the stability,population,electronic nature and chiroptical properties of corresponding diastereomer.The combination of propeller chirality of boron with other type of chirality is expected to provide a new strategy to modulate the CPL sign and intensity.The above research results will lay ground basis for the design of new CPL-SOMs.