Investigations On The Optical Properties Of Chiral Polymers

Due to the high-efficiency light absorption characteristics of organic polymers and the potential applications of chiral optical properties in chiral sensing processes,asymmetric synthesis,data storage and optical displays,the researchs on the optical properties of chiral polymers have received extensive attention.The arrangement of atoms in chiral polymers has the characteristic of asymmetry,and the arrangement of atoms with chiral conformation cannot coincide with the mirror image.Chiral materials including chiral polymers are optically active,such as naturally occurring DNA,?-helical protein,and chemically synthesized helical polyacetylene,helical polythiophene,etc.On the one hand,W.Moffit used exciton theory to study the optical properties of?-helical polypeptides,and then further improved the application of exciton theory in chiral helical molecules.I.Tonico and R.W.Woody further applied the exciton theory to helical polymers of finite length to investigate the behavior of light absorption and polarization rotation near the absorption band.On the other hand,the strong electro-acoustic coupling characteristic of organic conjugated polymers is considered.S.Phillpot and A.Bihsop obtained the absorption spectra of the amplitude breathers based on the time-dependent semiclassical method by using the SSH model which can express the strong electro-acoustic coupling characteristics.K.Fesser and A.Bishop theoretically calculated the light absorption of polaron in polyacetylene by means of continuous electro-acoustic coupling model,and obtained the characteristic absorption characteristics of polaron.These two theoretical work did not take into account the chiral helical configuration of polymer chain,but only the one-dimensional linear polymer chain was considered.Chiral molecules have unique geometrical structure(lack of structure inversion symmetry),which leads to the formation of chiral helical electrostatic potential inside the molecules.The interaction term between the electron and the chiral electrostatic potential caused by the movement of the electron can be expressed as the Rashba spin-orbit coupling term.For chiral molecules,it can be called the chiral-induced spin-orbit coupling.Chiral-induced spin-orbit coupling is commonly used in the discussion of chiral-induced spin selectivity effect,and it is determined as one of the fundamental reasons for this phenomenon.In the discussion of other physical properties of organic chiral molecules,the mechanism of chiral-induced spin-orbit coupling has not been introduced.Chiral-induced spin-orbit coupling can not only cause the change of transition frequency in the physical process of light absorption,which leads to the splitting of absorption peaks,but also extend the discussion of circular dichroism to the degree of spin freedom.The chiral-induced spin-orbit coupling is directly related to the morphology and geometric structure of chiral molecules.Therefore,it is geometrically dependent,which means that the optical physical properties of the chiral polymers are not only dependent on the spin-orbit coupling strength but also affected by the chiral molecular configuration.Based on the SSH model and the semi-classical photoinduced transition theory,we consider the inherent chiral-induced spin-orbit coupling in chiral molecules,and the effects of chiral-induced spin orbit coupling on light absorption and circular dichroism of chiral helical polymers were simulated theoretically.The specific research contents of this paper are as follows:1.Based on tight-binding SSH model and semi-classical light absorption theory,the relationship between chiral-induced spin orbit coupling and light absorption characteristics is discussed.The chiral-induced spin-orbit coupling can cause the splitting of the electron eigenenergy level and the spin mixing of the electron wave function.Chiral-induced spin-orbit coupling is derived from the coupling of linear momentum and spin in a chiral spiral electrostatic field,and chiral-induced spin-orbit coupling is directly related to the geometric configuration of chiral molecules.Therefore,it can be achieved by adjusting the molecular structure to regulate the strength of spin-orbit coupling,the degree of energy level splitting and the degree of spin mixing.The splitting of absorption peaks caused by the chiral-induced spin-orbit coupling is proportional to the strength of the spin-orbit coupling.In addition,the splitting of the absorption peak is also related to the chiral parameter of twist angle ?and lift angle ?.For the same spin-orbit coupling strength,decreasing the lift angle ?can lead to more significant splitting of energy levels as well as the splitting of the absorption peaks,but the corresponding spin mixing of the wave function will decrease.Increasing the lift angle is the opposite.The change of torsion angle ? has little effect on the splitting of the absorption peaks,which is related to the periodic structure of the helix.2.In chiral helical organic molecules,the circular dichroism is determined by the orbital magnetic moment and the spin magnetic moment.When the chiral-induced spin-orbit coupling is not considered,the circular dichroism of chiral molecules is determined only by the orbital motion of electrons,and the consideration of spin does not contribute.The chiral-induced spin-orbit coupling causes the spin mixing of the wave function,and the spin's contribution to the circular dichroism is no longer zero and cannot be ignored.Due to the normalization of the wave function and the constant intensity coefficient of the spin magnetic moment,the spin magnetic dipole transition matrix element mainly depends on the degree of spin mixing.Increasing the degree of spin mixing can be achieved by the increase of the spin-orbit coupling strength or the decrease of the lift angle ? to fulfill.The helix angle ? is not only a chiral parameter for chiral-induced spin-orbit coupling,but also a geometric configuration parameter of the helical polymer.Changing the lift angle ? can not only cause changes of the spin magnetic dipole transition matrix elements,but also change the dipole transition matrix elements and the orbital magnetic dipole transition matrix elements simultaneously.Therefore,the intensity change of circular dichroism caused by the variation of helix angle ? is the result of the synergistic effect of the dipole transition matrix element and the magnetic dipole transition matrix element.Spin-dependent circular dichroism is more obvious in oligomers with strong spin-orbit coupling,and is comparable in strength to orbital circular dichroism.3.For chiral molecules with helical structure,the change of chain length can change the energy level density and the orbital angular momentum of the molecule.The former is independent of the helical configuration and the latter is contrary.Therefore,the intensity of orbit-dependent circular dichroism tends to increase with the chain length increasing,and this trend is frequency dependent.With the increase of chain length,the distribution of spin-dependent circular dichroism becomes more intensive,which is determined by the increase of energy level density,and the intensity will not change significantly.