Achieving intersystem crossing (ISC) through structural tuning in biological systems is an evolving area for therapeutic and materials research. Eumelanin, a natural pigment, offers huge potential for bio-inspired material design, yet remains underexplored in this regard. Herein, we report the ultrafast intersystem crossing in di-brominated (DMICE-Br₂) and tri-brominated (DMICE-Br₃) eumelanin model monomers through transient absorption spectroscopy and surface hopping dynamics. Femtosecond and nanosecond transient absorption experiments suggest triplet excited state populations in DMICE-Br₂ and DMICE-Br₃ with triplet quantum yields and rates of ISC as , and , respectively. Theoretical insights into ISC were obtained with nonadiabatic dynamics simulations using the surface hopping including arbitrary couplings method coupled to potential energy surfaces, modelled by linear vibronic coupling (SHARC/LVC). The results show that for both DMICE-Br₂ and DMICE-Br₃, the initial S₁ population decays to the T₂ and T₃ states in the picosecond timescale to further undergo internal conversion to T₁ within sub-ns for DMICE-Br₂ and sub-ps for DMICE-Br₃. The simulated and corroborate to the assignment of the ultrafast triplet excited state population observed in the experiments. The increased triplet yields and ISC rates in DMICE-Br₂ and DMICE-Br₃ are attributed to the enhanced heavy atom effect from additional bromine atoms. This work presents the experimental and computational evidence for ultrafast ISC in multi-brominated eumelanin monomers, with promising implications for eumelanin-inspired material design and photodynamic applications.

中文翻译：

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通过瞬时吸收和表面跳跃动力学探测多溴真黑素的系统间交叉


通过生物系统中的结构调整实现系统间交叉 （ISC） 是治疗和材料研究的一个不断发展的领域。真黑素是一种天然色素，为生物启发的材料设计提供了巨大的潜力，但在这方面仍未得到充分开发。在此，我们通过瞬态吸收光谱和表面跳跃动力学报道了二溴 （DMICE-Br₂） 和三溴 （DMICE-Br₃） 真黑素模型单体中的超快系统间交叉。飞秒和纳秒瞬态吸收实验表明，DMICE-Br₂ 和 DMICE-Br₃ 中存在三重态激发态群，三重态量子产率和 ISC 速率分别为 和 。 通过使用表面跳跃的非绝热动力学仿真获得了对 ISC 的理论见解，包括通过线性振动耦合 （SHARC/LVC） 建模的耦合到势能表面的任意耦合方法。结果表明，对于 DMICE-Br₂ 和 DMICE-Br₃，初始 S₁ 群体在皮秒时间尺度上衰减到 T₂ 和 T₃ 状态，进一步在 DMICE-Br₂ 的亚纳秒内内内转化为 T_{1，DMICE-Br 3} 的亚 ps 内。模拟 并 证实了实验中观察到的超快三重态激发态种群的分配。 DMICE-Br₂ 和 DMICE-Br₃ 中增加的三重态产率和 ISC 速率归因于额外的溴原子增强的重原子效应。这项工作提出了多溴真黑素单体中超快 ISC 的实验和计算证据，对真黑素启发的材料设计和光动力学应用具有广阔的意义。