| Carbon-based nanomaterials,represented by fullerene,carbon nanotubes,graphene and their derivatives have been well explored in the field of nonlinear optics due to their good?electron conjugation and thermal stability.However,the pristine graphene is nonpolar.Doping heteroatoms,adding electron donor and acceptor and introducing non-hexagonal defects could significantly enhance the second-order nonlinear optical(NLO)properties of graphene.Based on this,polar azulenes are introduced into coronene-centered nanographene to design materials with optimal second-order NLO properties,and the correlation between the second-order NLO properties and structure of azulene-defect carbon nanomaterials is investigated with quantum chemical calculations.The specific content is as follows:First,the electronic properties of all investigated materials were calculated through density functional theory based method PBE0 with the 6-31G(d,p)basis set.The results showed that the ground state of most of azulene-defect buckybowls and nanographenes was not closed-shell singlet state except for Out-all and Out-2c.For Out-all,the sum-over-states(SOS)model based calculations demonstrate that the dipolar contributions(?J=1)play a more important role in enhancing the second-order NLO properties of those azulene-defect nanomaterials,while it has small ground state dipole moment.Nano-graphenes with two parallel orientating azulenes(i.e.,Out-P and Out-Ps)have large dipole moments,while their ground state is triplet state.Introducing B atoms,N atoms,B and N atoms to eliminate non-bonding orbitals transfers the ground state of Out-P and Out-Ps to closed-shell singlet sate,and the Out-Ps-2N has large static first hyperpolarizability(<?0>)of 1621.67×10-30 esu(29.00×10-30 esu per heavy atom).Addition of an electron donor(NH2)at the pentagon end of the Out-Ps-2N further enhances the<?0>to 1906.22×10-30esu(33.44×10-30 esu per heavy atom).The two-dimensional second-order NLO spectra of the those nanographenes find rich information on the second-order NLO responses under external fields,particularly the strong sum frequency generations and difference frequency generations in near infrared-visible region(e.g.,700 nm?1000 nm)shed light on the possible applications in biological nonlinear optics.In summary,introduction of azulene into carbon nanomaterials would effectively improve the second-order NLO properties of systems.This work hopes to provide useful information for the design of novel NLO materials. |
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