Wei Yu-Chen, Shen Shin-Wei, Wu Cheng-Ham, Ho Ssu-Yu, Zhang Zhiyun, Wu Chih-I, Chou Pi-Tai
Department of Chemistry, National Taiwan University, Taipei, 10617 Taiwan, ROC.
Graduate Institute of Photonics and Optoelectronics, National Taiwan University, Taipei, 10617 Taiwan, ROC.
J Phys Chem A. 2021 Feb 4;125(4):943-953. doi: 10.1021/acs.jpca.0c09075. Epub 2021 Jan 22.
Exciton delocalization relates to many important photophysical processes such as excitation energy transfer, charge separation, and singlet fission. Here, we analyze the exciton delocalization through the photophysical measurements of the molecular crystal 2,2'-(thiazolo[5,4-]thiazole-2,5-diyl)bis(4-methylphenol) (), which is the segregated HJ-aggregate confirmed by the calculation of exciton coupling along each direction in the crystal structure. Linearly polarized steady-state absorption spectroscopy verifies that the red-shifted optical transition majorly arises from the aggregates unparalleled to the -axis. In addition, the temperature-dependent emission spectra show the increase of 0-0 versus 0-1 vibronic emission ratio as the temperature decreases with the coherence number equaling 2.2-1.0 at 140-200 K, which is the characteristic behavior of J-aggregates. To elaborate these observations, we carry out the simulation with the Holstein-type Hamiltonian considering short-range charge-transfer-mediated couplings (perturbative regime) under the two-particle approximation, showing that the 3 × 3 laminar-like aggregates in the -plane and the 3 × 3 × 2 three-dimensional aggregates fit well with the emission spectrum at 140 K. In the 3 × 3 aggregates, the coherence function in the -plane shows the in-phase correlation along (1,0,-1), elucidating how J-aggregates form in segregated HJ-aggregates with dominant positive coupling. Under the strong intralayer out-of-phase correlation, the 3 × 3 × 2 aggregates demonstrate that the vibronic coupling has a great impact on the interlayer correlation. Furthermore, the coherence function along (0,1/2,-1/2) and (-1,1/2,-1/2) exhibits the thermal-activated phase flipping. These discoveries pave the ways for further manipulations of exciton delocalization in three-dimensional molecular solids.
激子离域与许多重要的光物理过程相关，如激发能量转移、电荷分离和单线态裂变。在此，我们通过对分子晶体2,2'-(噻唑并[5,4 - ]噻唑 - 2,5 - 二基)双(4 - 甲基苯酚)（ ）进行光物理测量来分析激子离域，该晶体是通过计算晶体结构中每个方向的激子耦合而确认的分离的HJ聚集体。线性偏振稳态吸收光谱证实，红移的光学跃迁主要源于与 轴不平行的聚集体。此外，温度依赖的发射光谱表明，随着温度降低，0 - 0与0 - 1振动发射比增加，在140 - 200 K时相干数等于2.2 - 1.0，这是J聚集体的特征行为。为详细阐述这些观察结果，我们在双粒子近似下，使用考虑短程电荷转移介导耦合（微扰 regime）的霍尔斯坦型哈密顿量进行模拟，结果表明 平面中的3×3层状聚集体和3×3×2三维聚集体与140 K时的发射光谱拟合良好。在3×3聚集体中， 平面中的相干函数沿(1,0, - 1)显示同相相关，阐明了在具有主导正耦合的分离HJ聚集体中J聚集体是如何形成的。在强层内异相相关下，3×3×2聚集体表明振动耦合对层间相关性有很大影响。此外，沿(0,1/2, - 1/2)和( - 1,1/2, - 1/2)的相干函数表现出热激活的相位翻转。这些发现为进一步操纵三维分子固体中的激子离域铺平了道路。
