含时密度泛函理论研究低带隙的中性和带电的交替共聚芴的光学特性

Optical Properties of Neutral and Charged Low Band Gap Alternating Copolyfluorenes： TD-DFT Investigation

用含时的密度泛函（TD—DFT）方法研究了低带隙的中性和带电的交替共聚芴Green 1），该化合物是由烷染取代芴和（1，2，5-噻吩基-3，4-硫重氮基）喹喔啉噻吩（T—TDQ—T）单元交替重复组成，对他们的激发态特性用二维（2D）和三维（3D）实空间分析方法做了进一步分析．对于中性的Green 1，分别得到其带隙、键能、激子结合能和核驰豫能．用3D跃迁密度方法对中性和带电的Green 1的跃迁偶极矩进行比较可显示出跃迁偶极矩的取向和强度；用3D电荷差异密度方法显示出激发后的中性和带电的Green 1电荷重新分布和比较，用2D实空间分析方法（跃迁密度矩阵）来研究中性和带电的Green 1处于激发态时的电子空穴相干性．中性Green 1的激发态特性分别用TD—DFT和ZINDO两种方法进行了计算，比较得出电子-电子相互作用（在TD—DFT中）对激发态性质的重要影响．

Electronic structure and optical properties of neutral and charged low band gap alternating copolyfluorenes （Green 1, which is based on alternating repeating units consisting of alkyl-substituted fluorene and a thiophene-[1,2,5]thiadiazolo-[3,4]quinoxaline-thiophene （T-TDQ-T） unit were investigated theoretically with time-dependent density functional theory （TD-DFT） method, and their excited state properties were further analyzed with 2D site and 3D cube representations. For neutral Green 1, the band gap, binding energy, exciton binding energy, and nuclear relaxation energy were obtained. The transition dipole moments of neutral and charged Green 1 are compared using 3D transition density, which reveals the orientation and strength of transition dipole moments. The charge redistribution of neutral and charged Green 1 upon excitation are displayed and compared with 3D charge difference density. The electron-hole coherences of neutral and charged Green 1 upon excitation are investigated with 2D site representation （transition density matrix）. The excited state properties of neutral Green 1 calculated with TD-DFT method are compared with that calculated with ZINDO method, which reveals the importance of electron-electron interaction （in TD-DFT） in the excited state properties.