Gobeze Habtom B, Jagadesan Pradeepkumar, Schanze Kirk S
University of Texas at San Antonio, Department of Chemistry, One UTSA Circle, San Antonio, TX 78249, USA.
Phys Chem Chem Phys. 2023 Sep 13;25(35):23685-23695. doi: 10.1039/d3cp03067a.
This paper reports an investigation of the electronic structure and photophysical properties of two "diblock" π-conjugated oligomers (T4-TBT and T8-TBT) that feature electron rich tetra(thiophene) (T4) or octa(thiophene) (T8) segments linked to an electron poor 4,7-bis(2-thienyl)-2,1,3-benzothiadiazole (TBT) moiety. Electrochemistry and UV-visible absorption spectroscopy reveals that the diblock oligomers display redox and absorption features that can be attributed to the Tn and TBT units. Density functional theory (DFT) and time-dependent DFT (TDDFT) calculations support the experimental electrochemistry and optical spectroscopy results, suggesting that the frontier orbitals on the diblock oligomers retain characteristics of the individual π-conjugated segments. However, low energy optical transitions are anticipated to arise from Tn to TBT charge transfer. Fluorescence spectroscopy on the diblock oligomers reveals that the oligomers feature strongly solvent dependent fluorescence. In non-polar solvents (hexane, toluene), the emission is structured with a moderate Stokes shift; however, in more polar solvents the emission becomes broader, and red-shifts significantly. Transient absorption spectroscopy on timescales from femtoseconds (fs) to microseconds (μs) reveals that in non-polar solvents excitation produces a singlet excited state (LE) that decays uniformly to the ground state in parallel with intersystem crossing to a triplet state. By contrast, in more polar solvents, excitation produces a very short-lived excited state (1-3 ps) which evolves rapidly into a second excited state that is attributed to the charge transfer (CT) state. The fast dynamics are associated with crossing from the LE state, which is populated initially by photoexcitation, into the CT state, which then decays to the ground state. The photophysical properties and dynamics of the LE and CT excited states are very similar for T4-TBT and T8-TBT, suggesting that the length of the oligo(thiophene) segment does not have a strong influence on the energy, structure or dynamics of the LE and CT excited states.
本文报道了对两种“二嵌段”π共轭低聚物（T4-TBT和T8-TBT）的电子结构和光物理性质的研究，这些低聚物具有富电子的四（噻吩）（T4）或八（噻吩）（T8）链段，与缺电子的4,7-双（2-噻吩基）-2,1,3-苯并噻二唑（TBT）部分相连。电化学和紫外可见吸收光谱表明，二嵌段低聚物表现出可归因于Tn和TBT单元的氧化还原和吸收特征。密度泛函理论（DFT）和含时密度泛函理论（TDDFT）计算支持了实验电化学和光谱学结果，表明二嵌段低聚物上的前沿轨道保留了各个π共轭链段的特征。然而，预计低能光学跃迁将由Tn到TBT的电荷转移引起。对二嵌段低聚物的荧光光谱研究表明，这些低聚物具有强烈的溶剂依赖性荧光。在非极性溶剂（己烷、甲苯）中，发射光谱具有一定结构，斯托克斯位移适中；然而，在极性更强的溶剂中，发射光谱变宽，并发生显著红移。从飞秒（fs）到微秒（μs）时间尺度的瞬态吸收光谱表明,在非极性溶剂中，激发产生一个单重激发态（LE），它与系间窜越到三重态同时均匀地衰减到基态。相比之下，在极性更强的溶剂中，激发产生一个非常短寿命的激发态（1-3 ps），它迅速演变成第二个激发态，该激发态归因于电荷转移（CT）态。快速动力学与从最初由光激发填充的LE态跃迁到CT态有关，然后CT态衰减到基态。T-4-TBT和T8-TBT的LE和CT激发态的光物理性质和动力学非常相似，这表明聚（噻吩）链段的长度对LE和CT激发态的能量、结构或动力学没有强烈影响。
