Franca Larissa G, Danos Andrew, Saxena Rishabh, Kuila Suman, Stavrou Kleitos, Li Chunyong, Wedler Stefan, Köhler Anna, Monkman Andrew P
Department of Physics, Durham University, South Road, Durham DH13LE, United Kingdom.
Soft Matter Optoelectronics and Bavarian Polymer Institute (BPS), University of Bayreuth, Universitätsstrasse 30, Bayreuth 95440, Germany.
J Phys Chem Lett. 2024 Feb 15;15(6):1734-1740. doi: 10.1021/acs.jpclett.4c00030. Epub 2024 Feb 7.
Optical pump-probe techniques allow for an in-depth study of dark excited states. Here, we utilize them to map and gain insights into the excited states involved in the thermally activated delayed fluorescence (TADF) mechanism of a benchmark TADF emitter . The results identify different electronic excited states involved in the key TADF transitions and their nature by combining pump-probe and photoluminescence measurements. The photoinduced absorption signals are highly dependent on polarity, affecting the transition oscillator strength but not their relative energy positions. In methylcyclohexane, a strong and vibronically structured local triplet excited state absorption (LE → LE) is observed, which is quenched in higher polarity solvents as CT becomes the lowest triplet state. Furthermore, ultrafast transient absorption (fsTA) confirms the presence of two stable conformers of : (1) quasi-axial (QA) interconverting within 20 ps into (2) quasi-equatorial (QE) in the excited state. Moreover, fsTA highlights how sensitive excited state couplings are to the environment and the molecular conformation.
光泵浦 - 探测技术能够深入研究暗激发态。在此，我们利用这些技术来绘制并深入了解一种基准热活化延迟荧光（TADF）发射体的TADF机制中涉及的激发态。通过结合泵浦 - 探测和光致发光测量，结果确定了关键TADF跃迁中涉及的不同电子激发态及其性质。光致吸收信号高度依赖于极性，影响跃迁振子强度，但不影响其相对能量位置。在甲基环己烷中，观察到一个强的且具有振动结构的局域三重态激发态吸收（LE→LE），在极性更高的溶剂中，随着CT成为最低三重态，该吸收被淬灭。此外，超快瞬态吸收（fsTA）证实了存在两种稳定构象：（1）准轴向（QA）在20皮秒内相互转化为（2）激发态下的准赤道（QE）。此外，fsTA突出了激发态耦合对环境和分子构象的敏感性。
