二维对六苯分子团簇中相干激发能量转移过程研究

Coherent excitation energy transfer processes in two-dimensional para-sexiphenyl molecular clusters

激发能量转移是影响对六苯相关器件应用的重要因素之一,研究对六苯团簇在不同外场激发作用下的激子动力学过程及激子相干效应,这对于提高由对六苯及其相关衍生物所组成的分子器件的应用性能具有重要意义.本文基于密度矩阵理论,采用算符的数学平均值近似,应用不同脉冲场激发盘状对六苯分子团簇,对其激子动力学过程及激子相干行为进行了分析.研究发现,当处于强场短脉冲激发时,团簇内产生多激子效应,激子能级出现新的杂化态.在短脉冲激发下,在不同能量区域激发态分布不同,但均表现出明显对称性.由于H型团簇最高能级的激子态被优先激发,发现随脉冲场增大,激子相干增强,当脉冲场较大时,在脉冲激发过程中,激子振荡协同性消失,极短时间内大量分子产生激子相干,出现瞬态离域现象.

Excitation energy transfer is one of the most important factors affecting the applications of para-sexiphenyl devices.The study of exciton dynamics and exciton coherence effect of para-sexiphenyl clusters under external field excitation is important in order to improve the performance of molecular devices composed of parasexiphenyl and its related derivatives.In this work,the two-dimensional disc-like para-sexiphenyl molecular cluster is used as the object of study.The molecular system is simplified into a two-level model based on its structural features and energy level distribution.Within the framework of density matrix theory,the exciton dynamics and exciton coherence behavior of disk-like para-hexaphene molecular clusters excited by different pulse fields are analyzed through using the mathematical mean value approximation of the operator.The results show that when long pulses are used to excite para-sexiphenyl clusters,the single exciton state characteristic appears and is insensitive to the change of excited external field strength.When the clusters are subjected to strong pulsed fields with short pulse widths,multiple excitons are excited simultaneously in the cluster,forming multiple exciton states,with the exciton energy levels shifting toward lower energy and new hybrid states appearing.In the optical response spectrum,there appear multiple resonance peaks.And as the pulse field is enhanced,the multi-exciton effect becomes apparent and the hybridization energy level increases.Under short pulse excitation,the excited states are distributed differently in different energy regions,but all of them show obvious symmetry.As the highest-energy exciton states of H-type clusters are preferentially excited,we analyze the exciton state population and the exciton coherence evolution with time in the high-energy exciton state.With the pulse field increases,Rabi oscillations appear and the exciton coherence effect increases.When the pulsed field reaches a certain field strength,the exciton oscillation cooperativity disappears in the first 100 fs,showing the non-local characteristic.The position of the wave trough of the exciton state population corresponds to the peak in the exciton coherence size.It indicates that when the pulse field is intense enough,a large number of molecules are in the exciton coherent state during the pulsed excitation,and transient out-ofdomain phenomena occur.