Ress Lea, Malý Pavel, Landgraf Jann B, Lindorfer Dominik, Hofer Michael, Selby Joshua, Lambert Christoph, Renger Thomas, Brixner Tobias
Institut für Physikalische und Theoretische Chemie, Universität Würzburg Am Hubland 97074 Würzburg Germany.
Faculty of Mathematics and Physics, Charles University Ke Karlovu 5 121 16 Praha 2 Czech Republic.
Chem Sci. 2023 Jul 28;14(35):9328-9349. doi: 10.1039/d3sc01674a. eCollection 2023 Sep 13.
Experimental and theoretical foundations for femtosecond time-resolved circular dichroism (TRCD) spectroscopy of excitonic systems are presented. In this method, the system is pumped with linearly polarized light and the signal is defined as the difference between the transient absorption spectrum probed with left and with right circularly polarized light. We present a new experimental setup with a polarization grating as key element to generate circularly polarized pulses. Herein the positive (negative) first order of the diffracted light is left-(right-)circularly polarized and serves as a probe pulse in a TRCD experiment. The grating is capable of transferring ultrashort broadband pulses ranging from 470 nm to 720 nm into two separate beams with opposite ellipticity. By applying a specific chopping scheme we can switch between left and right circular polarizations and detect transient absorption (TA) and TRCD spectra on a shot-to-shot basis simultaneously. We perform experiments on a squaraine polymer, investigating excitonic dynamics, and we develop a general theory for TRCD experiments of excitonically coupled systems that we then apply to describe the experimental data in this particular example. At a magic angle of 54.7° between the pump-pulse polarization and the propagation direction of the probe pulse, the TRCD and TA signals become particularly simple to analyze, since the orientational average over random orientations of complexes factorizes into that of the interaction with the pump and the probe pulse, and the intrinsic electric quadrupole contributions to the TRCD signal average to zero for isotropic samples. Application of exciton theory to linear absorption and to linear circular dichroism spectra of squaraine polymers reveals the presence of two fractions of polymer conformations, a dominant helical conformation with close interpigment distances that are suggested to lead to short-range contributions to site energy shifts and excitonic couplings of the squaraine molecules, and a fraction of unfolded random coils. Theory demonstrates that TRCD spectra of selectively excited helices can resolve state populations that are practically invisible in TA spectroscopy due to the small dipole strength of these states. A qualitative interpretation of TRCD and TA spectra in the spectral window investigated experimentally is offered. The 1 ps time component found in these spectra is related to the slow part of exciton relaxation obtained between states of the helix in the low-energy half of the exciton manifold. The dominant 140 ps time constant reflects the decay of excited states to the electronic ground state.
本文介绍了用于激子系统的飞秒时间分辨圆二色性（TRCD）光谱的实验和理论基础。在该方法中，系统用线偏振光泵浦，信号定义为用左旋和右旋圆偏振光探测的瞬态吸收光谱之间的差异。我们展示了一种以偏振光栅为关键元件的新型实验装置，用于产生圆偏振脉冲。在此，衍射光的正（负）一级是左旋（右旋）圆偏振的，并在TRCD实验中用作探测脉冲。该光栅能够将470纳米至720纳米范围内的超短宽带脉冲转换为具有相反椭圆率的两束独立光束。通过应用特定的斩波方案，我们可以在左旋和右旋圆偏振之间切换，并同时逐次检测瞬态吸收（TA）和TRCD光谱。我们对一种方酸菁聚合物进行了实验，研究激子动力学，并为激子耦合系统的TRCD实验开发了一种通用理论，然后将其应用于描述这个特定例子中的实验数据。当泵浦脉冲偏振与探测脉冲传播方向之间的夹角为54.7°的魔角时，TRCD和TA信号变得特别易于分析，因为复合物随机取向的取向平均值分解为与泵浦和探测脉冲相互作用的平均值，并且对于各向同性样品，TRCD信号的固有电四极贡献平均为零。将激子理论应用于方酸菁聚合物的线性吸收和线性圆二色性光谱，揭示了聚合物构象存在两个部分，一个占主导的螺旋构象，其色素间距离很近，这被认为会对方酸菁分子的位点能量位移和激子耦合产生短程贡献，以及一部分未折叠的无规卷曲。理论表明，选择性激发螺旋的TRCD光谱可以分辨出由于这些态的偶极强度小而在TA光谱中实际上不可见的态布居。本文对实验研究的光谱窗口中的TRCD和TA光谱进行了定性解释。在这些光谱中发现的1皮秒时间分量与激子流形低能半部分中螺旋态之间激子弛豫的慢部分有关。占主导的140皮秒时间常数反映了激发态到电子基态的衰减。
