Asymmetric Photopolymerization Induced By Chiral Electromagnetic Fields

Chirality is a ubiquitous and fascinating characteristic in nature and has been extensively discussed in physics,chemistry,biology,and materials science.Chiral conjugated polymers,particularly those with strong circularly polarized luminescence,have recently gained increasing interest as they promise many potential applications in quantum computing,biosensing with chiral recognition,optical metamaterials.In the past decades,many strategies have been developed to synthesize chiral conjugated polymers,typically using chiral monomer,dopants or catalysts or asymmetric external influences including circularly polarized light(CPL),vortex motion,stirring and other physical forces,as the symmetry breaker.Ideally,symmetry breaking in photopolymerization reactions from achiral monomers could be in principle induced by irradiation with CPL.Unfortunately,only small enantiomeric excess can be obtained in typical cases,as evident by their very small asymmetry factors(gabs<10-3).The optical activity,enantioselective recognition ability and the circularly polarized luminescence of the prepared chiral PDA films all were much lower,limited their practical applications.In this sense,there is good motivation to find generic mechanisms to further enhance dissymmetry in CPL-triggered enantioselective reactions,which may open the door to promote absolute asymmetric synthesis in which light provides the chiral bias.In this paper,the main contents and results are as follows:1.Far-field superchiral light(SCL),generated by the interference of two counter-propagating 325nm CPL with same frequency,opposite handedness and slightly different intensity,exhibits enhanced dissymmetry in its interaction with chiral molecules.It is anticipated that the enhanced optical dissymmetry in SCL may be utilized to promote the efficiency of asymmetric photochemical reactions.We demonstrated experimentally that the enantioselective photopolymerization of achiral diacetylene monomer can be greatly enhanced by application of SCL,and a maximum about 6-fold enhancement in gabs of PDA film over conventional CPL can be achieved.The enhancement in enantioselective synthesis of helical PDA chains can be rigorously controlled with the relative light intensity ratio of the two counter-propagating CPL waves.An explanation based on the chiral transfer and amplification of chiral bias from SCL during the polymerization process has been proposed.Moreover,thus formed chiral PDA films polymerized by SCL exhibited enhanced enantioselective recognition ability,and can serve as a direct visual probe for the discrimination of some specific enantiomers.2.The enantioselective photopolymerization of achiral diacetylene monomers can also be achieved using cysteine-modified AgNPs(Cys@AgNPs),a chiral molecule-NP complex,as the symmetry breaker in the photopolymerization of PDA.Importantly,the direction of symmetry breaking(the preferred handedness)could be reversed by varying either the chiral configuration of chiral ligands on nanoparticles or the excitation wavelength.This allows us to pattern PDA with a desired helical sense(left or right handed)onto a film through photopatterning with unpolarized UV only.Therefore,this allowed the chirality and chirality distribution of PDA films to be rigorously controlled by manipulating the relevant physical parameters(wavelength,intensity,and their spacial distributions)of the UV irradiation.Programmable patterns of PDA including chirality gradients and microarrays with tailorable circularly polarized luminescence could be readily fabricated.Such patterned films promise a wide range of potential applications,such as high density optical storage,where information can be recoded in a four dimensional space.A programmable array with a 2D distribution of circularly polarized fluorescence,which could be used in 3D display or quantum communications.An enantioselective colorimetric sensor,which reports chirality of analyte with color changes visible to naked eyes,and gradient chiral surfaces,which would be uniquely useful in chiral separations and biomimetics.3.CPL-triggered thiol-ene click chemistry is particularly interesting as it combines absolute asymmetric synthesis with click reactions,allowing enantioselective synthesis of chiral poly(thioether)from racemic monomers.Importantly,poly(thioether),naturally containing electron-rich sulphur(S)atoms,often exhibits high luminescence in aggregated states,known as clusterization-triggered emission(CTE).We expect chiral poly(thioether)to emit circularly polarized luminescence in CTE and to serve as a promising optically active material.Unfortunately,the final product produced by CPL directly showed rather weak chiral preference and the involved enantioselective reaction mechanism is unclear.We believe that the low enantioselectivity of thiol-ene chemistry is an undesirable side-effect in its unique reaction kinetics.The key intermediate in thiol-ene reactions is thiyl radicals,which are ubiquitous in biochemistry.Strategic introduction of an achiral hydrogen atom donor(HAD)into the reaction system should quench the carbon-centered radical directly and suppress the chain transfer step,breaking the conventional cycle of thiol-ene reaction.Moreover,HAD could also donate a hydrogen atom to quench the racemic thiyl radical and the regenerated thiol molecule could be re-triggered by CPL with chiral preference.It is anticipated that this new approach should ensure most thiyl radicals that attack the ene are triggered by CPL and therefore improve overall enantioselectivity of such thiol-ene reactions via a competitive inhibition mechanism.We demonstrated experimentally that enantioselectivity in CPL-triggered thiol-ene reactions could be improved upon addition of achiral HAD,such as achiral 1,4-cyclohexadiene.As a result,a maximum excess of enantiomeric units(-20%)in final poly(thioether)chains could be obtained,much higher than those without HAD.This study exemplifies a novel approach to improve enantioselectivity of an asymmetric reaction by simply manipulating its reaction kinetics without introducing any new or stronger chiral stimuli.Interestingly,the obtained chiral poly(thioether)not only possess inborn high luminescence in aggregated states,but also display distinct circularly polarized luminescence under certain solvation conditions.