Metal-free room-temperature phosphorescent (RTP) materials are attracting attention in such applications as organic light-emitting diodes and bioimaging. However, the chemical structures of RTP materials reported thus far are mostly predominantly based on π-conjugated systems incorporating heavy atoms such as bromine atoms or carbonyl groups, resulting in limited structural diversity. On the other hand, triarylboranes are known for their strong Lewis acidity and deep LUMO energy levels, but few studies have reported on their RTP properties. In this study, we discovered that compounds based on a tetracyclic structure containing boron, referred to as benzo[d]dithieno[b,f]borepins, exhibit strong solid-state reddish phosphorescence even in air. Quantum chemical calculations, including those for model compounds, revealed that the loss of planarity of the tetracyclic structure increases spin-orbit coupling matrix elements, thereby accelerating the intersystem crossing process. Moreover, single-crystal X-ray structural analysis and natural energy decomposition analysis suggested that the borepin compounds without bromine or oxygen atoms, unlike typical RTP materials, exhibit red-shifted phosphorescence in the crystalline state owing to structural relaxation in the T1 state. Additionally, the borepin compounds showed potential application as bioimaging dyes.

中文翻译：

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深入了解非平面π共轭有机硼化合物晶态微红磷光的机理解释

无金属室温磷光（RTP）材料在有机发光二极管和生物成像等应用中引起了人们的关注。然而，迄今为止报道的RTP材料的化学结构大多主要基于包含重原子（例如溴原子或羰基）的π共轭体系，导致结构多样性有限。另一方面，三芳基硼烷以其强路易斯酸性和深LUMO能级而闻名，但很少有研究报道其RTP特性。在这项研究中，我们发现基于含硼四环结构的化合物（称为苯并[d]二噻吩并[b,f]硼化合物）即使在空气中也表现出强烈的固态微红色磷光。量子化学计算，包括模型化合物的计算，表明四环结构平面性的丧失增加了自旋轨道耦合矩阵元素，从而加速了系间穿越过程。此外，单晶X射线结构分析和自然能量分解分析表明，与典型的RTP材料不同，不含溴或氧原子的Borpin化合物由于T1态的结构弛豫而在晶态下表现出红移磷光。此外，钻头化合物显示出作为生物成像染料的潜在应用。