Ryu Tomohiro, Ruseckas Arvydas, Saigo Masaki, Miyata Kiyoshi, Tsuchiya Youichi, Nakanotani Hajime, Adachi Chihaya, Samuel Ifor D W, Onda Ken
Department of Chemistry, Faculty of Science, Kyushu University, 744 Motooka, Nishi, Fukuoka 819-0395, Japan.
Organic Semiconductor Centre, School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews, Fife, KY16 9SS, UK.
Phys Chem Chem Phys. 2025 Jun 25;27(25):13735-13744. doi: 10.1039/d5cp01226k.
Linearly-linked donor-acceptor-type (D-A) thermally activated delayed fluorescence molecules have been expected to be used as efficient emitters in organic light emitting diodes. Despite their simple molecular structures, some of these molecules exhibit a complex dual emission mechanism due to their two conformers: quasi-coplanar (q-copl.) and perpendicular (perp.) conformers. We have investigated three molecules of this type: phenothiazine-triphenyltriazine, 9,9-dimethyl-9,10-dihydroacridine-triphenyltriazine, and phenoxazine-triphenyltriazine using picosecond time-resolved photoluminescence and femtosecond transient absorption spectroscopy measurements. We have revealed the dual emission mechanism common to the three molecules: after photoexcitation, in the q-copl. conformer, the second singlet excited state with local excitation character emits strong fluorescence, which decays in 3-7 ps as it relaxes to the lowest singlet excited state with charge transfer (CT) character. The CT state exhibits relatively weak fluorescence with a lifetime of tens to hundreds of picoseconds. In the perp. conformer, the excited state shows a pronounced CT character with a weaker oscillator strength reduced by two orders of magnitude, structural relaxation in about 4 ps and a slow decay in >1 ns. The dual emission intensity ratio is determined by the population ratio between the q-copl. and perp. conformers in the ground state. The difference in this intensity ratio between the three molecules is ascribed to the difference in relative energetic stability between the two conformers in the ground state. The emission mechanism common to the linearly-linked D-A molecules deepens the understanding of their photophysical properties and opens new pathways for the development of advanced photofunctional materials.
线性连接的供体-受体型（D-A）热激活延迟荧光分子有望用作有机发光二极管中的高效发光体。尽管这些分子的结构简单，但其中一些分子由于其两种构象异构体：准共平面（q-copl.）和垂直（perp.）构象异构体，表现出复杂的双发射机制。我们使用皮秒时间分辨光致发光和飞秒瞬态吸收光谱测量研究了三种这种类型的分子：吩噻嗪-三苯基三嗪、9,9-二甲基-9,10-二氢吖啶-三苯基三嗪和吩恶嗪-三苯基三嗪。我们揭示了这三种分子共有的双发射机制：光激发后，在q-copl.构象异构体中，具有局域激发特征的第二单线态激发态发射强荧光，当其弛豫到具有电荷转移（CT）特征的最低单线态激发态时，在3-7皮秒内衰减。CT态表现出相对较弱的荧光，寿命为几十到几百皮秒。在perp.构象异构体中，激发态表现出明显的CT特征，振子强度较弱，降低了两个数量级，在约4皮秒内发生结构弛豫，在>1纳秒内缓慢衰减。双发射强度比由基态中q-copl.和perp.构象异构体之间的布居比决定。这三种分子之间这种强度比的差异归因于基态中两种构象异构体之间相对能量稳定性的差异。线性连接的D-A分子共有的发射机制加深了对其光物理性质的理解，并为先进光功能材料的开发开辟了新途径。
