Simulation and Analysis of the Transient Absorption Spectrum of 4‑(N,N‑Dimethylamino)benzonitrile (DMABN) in Acetonitrile

4-(N,N-Dimethylamino)­benzonitrile (DMABN) is a well-known model compound for dual fluorescencein sufficiently polar solvents, it exhibits two distinct fluorescence emission bands. The interpretation of its transient absorption (TA) spectrum in the visible range is the subject of a long-standing controversy. In the present study, we resolve this issue by calculating the TA spectrum on the basis of nonadiabatic molecular dynamics simulations. An unambiguous assignment of spectral signals to specific excited-state structures is achieved by breaking down the calculated spectrum into contributions from twisted and nontwisted molecular geometries. In particular, the much-discussed excited-state absorption band near 1.7 eV (ca. 700 nm) is attributed to the near-planar locally excited (LE) minimum on the S1 state. On the technical side, our study demonstrates that the second-order approximate coupled cluster singles and doubles (CC2) method can be used successfully to calculate the TA spectra of moderately large organic molecules, provided that the system in question does not approach a crossing between the lowest excited state and the singlet ground state within the time frame of the simulation.4-(N,N-二甲基氨基)苯甲腈（4-(N,N-Dimethylamino)benzonitrile，简称DMABN）是一种经典的双荧光模型化合物：在极性足够的溶剂中，它会呈现两条截然不同的荧光发射谱带。其可见光范围瞬态吸收（transient absorption，TA）光谱的解析长期以来存在争议。本研究通过基于非绝热分子动力学（nonadiabatic molecular dynamics）模拟计算TA光谱，解决了这一争议。通过将计算得到的光谱拆解为扭转与非扭转分子几何构型的贡献，我们实现了将光谱信号明确归属至特定激发态结构。其中，学界广泛讨论的1.7 eV（约700 nm）附近的激发态吸收带，被归属为S1态上近似平面的局域激发（locally excited，LE）极小值。技术层面上，本研究证明，只要体系在模拟时间范围内未出现最低激发态与单重态基态的交叉，二阶近似耦合簇单双（second-order approximate coupled cluster singles and doubles，CC2）方法便可成功用于计算中等尺寸有机分子的TA光谱。