Quantum chemical calculations were employed to construct Jablonski diagrams for a series of phenolic carbonyls, including vanillin, iso-vanillin, 4-hydroxybenzaldehyde, syringaldehyde, and coniferyl aldehyde. These molecules can enter the Earth's atmosphere from forest fire emissions and participate in photochemical reactions within the atmospheric condensed phase, including cloud and fog droplets and aqueous aerosol particles. This photochemistry alters the composition of light-absorbing organic content, or brown carbon, in droplets and particles through the formation and destruction of key chromophores. This study demonstrates that following photon absorption, phenolic carbonyls efficiently transition to triplet states via intersystem crossings (ISC), with rate coefficients ranging from 10⁹ to 10¹⁰ s⁻¹. Despite the presence of multiple potential ISC pathways due to several lower-lying triplet states, a single channel is found to dominate for each system. We further investigated the dependence of the ISC rate constant (k_ISC) on the vibrational excitation energy of the first accessible (ππ*) singlet excited state (S₁ or S₂, depending on the molecule), and compared it with the measured wavelength dependence of the photochemical quantum yield (Φ_loss). Although our model only accounts for intramolecular nonradiative electronic transitions, it successfully captures the overall trends. All studied molecules, except coniferyl aldehyde, exhibit saturation in the dependence of both k_ISC and Φ_loss on the wavelength (or vibrational excitation energy). In contrast, coniferyl aldehyde displays a single maximum, followed by a monotonic decrease as the excitation energy increases (wavelength decreases). This distinct behavior in coniferyl aldehyde may be attributed to the presence of a double-bonded substituent, which enhances π-electron conjugation, and reduces the exchange energy and thus the adiabatic energy gap between the S₁(ππ*) state and the target triplet state. For small energy gaps, the classical acceptor modes of the ISC process are less effective, leading to a low effective density of final states. Larger gaps enhance the effective density of states, making the wavelength dependence of the ISC more pronounced. Our calculations show that while all the studied phenolic carbonyls have similar acceptor modes, coniferyl aldehyde has a substantially smaller adiabatic gap (1700 cm⁻¹) than the other molecules. The magnitude of the adiabatic energy gap is identified as the primary factor determining the energy/wavelength dependence of the ISC rate and thus Φ_loss.

中文翻译：

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野火排放中酚羰基化合物的波长依赖性系统间穿越动力学


采用量子化学计算构建一系列酚羰基化合物的 Jablonski 图，包括香兰素、异香兰素、4-羟基苯甲醛、丁香醛和松香醛。这些分子可以从森林火灾排放物进入地球大气层，并参与大气凝聚相内的光化学反应，包括云和雾滴以及水性气溶胶颗粒。这种光化学通过形成和破坏关键发色团来改变液滴和颗粒中吸收光的有机物或棕色碳的组成。本研究表明，光子吸收后，酚羰基化合物通过系统间交叉 （ISC） 有效地转变为三重态，速率系数范围为 10 ^{9 10} 至 10 s ⁻¹ 。尽管由于几个较低的三重态而存在多个潜在的 ISC 通路，但发现每个系统都有一个通道占主导地位。我们进一步研究了 ISC 速率常数 （k _ISC ） 对第一个可及 （ππ*） 单重态激发态（S ₁ 或 S ₂ ，取决于分子）的振动激发能的依赖性，并将其与光化学量子产率 （Φ） 的测量波长依赖性进行了比较 _loss 。虽然我们的模型只考虑了分子内非辐射电子跃迁，但它成功地捕捉了总体趋势。除针叶醛外，所有研究的分子都表现出 k _ISC 和 Φ _loss 对波长（或振动激发能）的依赖性饱和。相比之下，针叶醛显示单个最大值，然后随着激发能量的增加（波长减小）呈单调降低。 针叶醛中的这种独特行为可能归因于双键取代基的存在，它增强了π电子共轭，降低了交换能，从而降低了 S ₁ （ππ*） 状态和目标三重态之间的绝热能隙。对于小能隙，ISC 过程的经典受体模式效果较差，导致最终态的有效密度较低。较大的间隙增强了状态的有效密度，使 ISC 的波长依赖性更加明显。我们的计算表明，虽然所有研究的酚羰基化合物都具有相似的受体模式，但针叶醛的绝热间隙 （1700 cm ⁻¹ ） 比其他分子小得多。绝热能量间隙的大小被确定为决定 ISC 速率的能量/波长依赖性的主要因素，因此 Φ _loss .