Theoretical Studies Of The Bonding And Nonlinear Optical Properties Of Phenalenyl Dimer And Derivatives

Phenalenyl?PLY?is a well-konwn neutral radicals with high stability,which plays an important role in organic chemistry.So far,PLY and its derivatives have been widely studied in electronic,magnetic,optical fields.Because of unique electronic structure of PLY,an unpaired electron is equivalently delocalized over the six?-carbon atoms,its dimer can form interesting 12-center 2-electron bonding?12c/2e?through effective?-?overlap.PLY and its derivatives with novel structures,highly delocalized?-electrons,unconventional bonding and good chemical stability,which makes promising candidate as high-performance nonlinear optical?NLO?materials.With the development of NLO materials,it has been a hot topic to design and synthesize the second-order NLO materials with adjustable and high performance.Finding an effective way to improve NLO performance is still one of the core issues.Our research interest is to explore the novel chemical bonds of PLY and its derivatives,further evaluate the second-order NLO response?the first hyperpolarizability?by appropriate quantum chemical methods,and screen out compounds with large first hyperpolarizabilities.Theoretically,exploring the relationship between the electronic structure and the nonlinear properties and revealing the microscopic mechanism of the second-order NLO response of such PLY compounds,which provides a reliable theoretical foundation for guiding experimental synthesis of high performance second-order NLO materials.Herein,the structures,intermolecular interactions and nonlinear optical properties of PLY and its derivatives are systematically investigated by using density functional theory?DFT?and time-dependent density functional theory?TD-DFT?.The main content includes two parts:1.In order to construct strong?-?bonding and induce large charge transfer,the unique PLY dimer derivatives are designed by using appropriate strategies,which further enhances the stabilities of the dimers;2.The second-order NLO responses of PLY dimer derivatives are improved by effective strategies,and the essential reasons for regulating and improving NLO properties are discussed in detail.The specific work includes the following five aspects:?1?Inspired by the unique molecular structure and the?-?bonding of PLY,we designed a novel biphenalenyl biradicaloid.It is noteworthy that a stable dimer can be formed by combining with conventional biradical compounds.From the analysis of molecular structure,the dimer contains two staggered PLY dimers,the distance between two layers is shorter than van der Waals carbon-carbon distance?3.40??.From the analysis of electronic properties,the calculated frontier molecular orbitals show that the dimer possesses two bonding orbitals,which is defined as two parallel pancake?-?bonds.In addition,dimers have small diradical and tetraradical characters,large interaction energies and strong aromaticities,which confirms the high stability of dimers.?2?The external environment affects the stability of the molecule by changing the electronic properties.We investigated the effect of an applied electric field on the molecular structure and 12c/2e bonding of PLY dimer.With increasing of the external electric field,the interlayer distance between the two layers is gradually elongated to above 3.50??greater than 3.40??,which shows that 12c/2e bonding gradually weakens and until breaking under the external electric field.At the same time,the charge transfer between the two layers is induced by the external electric field,which enhances the electrostatic interaction.Significatly,investigation of frontier molecular orbital confirms the above views.With increasing of the external electric field,orbital overlap between two layers is disconnected,indicating that12c/2e breaking;the electron density gradually shifts to the upper layer,indicating that the electrostatic interaction between the two layers increases gradually.Therefore,the conversion between the 12c/2e bonding and the electrostatic interactions can be realized under the external electric field.?3?In above investigation,we found that an applied electric field can induce charge transfer?CT?.Therefore,the applied electric field affects NLO response.Based on previous designed PLY dimer derivative with polar 12c/2e bonding,we applied an external electric field to the bond axis of dimer?z-axis?.When the external electric field along the negative direction of z-axis,it is beneficial to improve the stability of?-dimer due to the increase of bonding and electrostatic interactions.Interestingly,the highest occupied molecular orbital?HOMO?and HOMO-1 undergo orbital interchange under the external electric field along the positive direction of z-axis.It is worth noting that the external electric field effectively regulates the first hyperpolarizability??tot?.When the external electric field increases from the negative to the positive direction,the?tot values gradually decrease and until drop to zero,then increase sharply.Further,the evolution of the absorption spectra well explain the trend of the?_totot values.?4?Based on the above investigation,we explore that the applied electric field regulates intra-and intermolecular charge transfer and the nonlinear optical properties of dimers.We investigate the changes of the geometric structure and electronic properties of biradical?IDPL?dimer under the applied electric field along horizontal direction?F_x?or vertical direction?F_z?.With increasing of Fx,IDPL dimer produces intramolecular CT,and larger intramolecular CT induces intermolecular CT.Interestingly,Fz induces larger intermolecular CT,further induces intramolecular CT.More importantly,the external electric field effectively regulates and controls the first hyperpolarizability of IDPL dimer.Therefore,the applied electric field effect induces intra-and intermolecular charge transfer,further effectively regulates nonlinear optical properties.?5?Based on the above investigation,we discuss the effect of intra-and intermolecular CT of systems by the boron/nitrogen substitution on the nonlinear optical properties.The results show that designed dimer?2₂?presents intra-and intermolecular CT.Significantly,2₂has large first hyperpolarizability.This is mainly attributed to the horizontal and the vertical components of the first hyperpolarizability,and the horizontal component of the first hyperpolarizability is larger than the vertical component of the first hyperpolarizability.Correspondingly,68%is derived from the highest occupied molecular orbital?HOMO??the lowest unoccupied molecular orbital?LUMO?transition,and 18%is originated from HOMO?LUMO+1 transition.These results indicate that the intramolecular CT contributes to the first hyperpolarizability is greater than that of intermolecular CT.Therefore,the boron/nitrogen substitution effect can induce intra-and intermolecular CT,further significantly strengthens nonlinear optical properties.