AI 中文总结
研究通过多方法解析4a,4b-氮杂硼菲单体和二聚体光物理，发现单体S$_1$态长寿命，还有源于二聚体解离电离的皮秒成分，计算表明中性二聚体中约15皮秒时间常数归因于准分子形成，揭示了硼氮掺杂菲气相光动力学中的隐藏准分子形成。
AI 中文摘要
用等电子硼氮基序取代碳碳单元是调节多环芳烃（PAHs）电子结构和激发态化学的有力策略。本文结合多光子电离光谱、时间分辨光电子成像、离子速度映射成像和量子化学计算，来解析4a,4b-氮杂硼菲的单体和二聚体光物理过程。单体呈现出结构化的S$_1 \leftarrow$ S$_0$光谱，绝热电离能为$7.18 \pm 0.02\,\mathrm{eV}$。时间分辨光电子图像揭示出一个额外的皮秒成分，离子成像表明该成分源于分子二聚体的解离电离。计算确定最初激发的二聚体状态为明亮的H-聚集体状激子，随后超快S$_2 \rightarrow$ S$_1$内转换，接着向准分子S$_1$最小值进行结构弛豫。实验观察到的约15皮秒时间常数归因于中性二聚体中的准分子形成。
英文摘要
Replacing CC units by isoelectronic BN motifs provides a powerful strategy to tune the electronic structure and excited-state chemistry of polycyclic aromatic hydrocarbons (PAHs). Here, we combine multiphoton ionization spectroscopy, time-resolved photoelectron imaging, ion velocity-map imaging, and quantum-chemical calculations to disentangle the monomer and dimer photophysics of 4a,4b-azaboraphenanthrene. The monomer exhibits a structured S$_1 \leftarrow$ S$_0$ spectrum with an origin at $22880 \pm 15\,\mathrm{cm}^{-1}$, corresponding to $2.837 \,\mathrm{eV}$, and pronounced activity in low-wavenumber deformation modes. Photoelectron spectroscopy yields an adiabatic ionization energy of $7.18 \pm 0.02\,\mathrm{eV}$. While the structured spectrum, high fluorescence quantum yield, small computed geometry changes, and weak spin-orbit couplings all point to a long-lived monomer S$_1$ state, time-resolved photoelectron images reveal an additional picosecond component. Ion imaging shows that this component originates from dissociative ionization of the molecular dimer, which projects dimer excited-state dynamics into the monomer mass channel. Computations identify the initially excited dimer state as a bright H-aggregate-like exciton, followed by ultrafast S$_2 \rightarrow$ S$_1$ internal conversion and subsequent structural relaxation toward an excimeric S$_1$ minimum. The experimentally observed $\approx 15\,\mathrm{ps}$ time constant is therefore assigned to excimer formation in the neutral dimer.
CommentsPaper: 11 pages, 13 figures; SI: 28 pages, 25 figures
