1-（苯并咪唑-2-甲基）-1,4,8,11-四氮杂环十四烷（L1H）分子是一种可用于检测Zn2+离子的高效金属离子荧光探针，为了探究其检测机理，更好地解释其与不同的金属离子结合出现的不同荧光现象，本课题采用CASPT2//CASSCF的计算方法探究该探针及其与Zn2+，Cd2+和Cu2+不同的过渡金属离子结合时的辐射或非辐射跃迁路径。对于荧光探针分子来说，非辐射跃迁路径SCT(1nπ*)与辐射跃迁路径ππ*的竞争共存是导致其弱荧光的主要原因。S0SCT(1nπ*)的跃迁是由连接N原子的孤对电子跃迁至信号单元的π*轨道上形成的，是一个典型的nπ*激发模式，并具有明显的电荷迁移特征，在该态的Franck-Condon点处，分子是以两性离子自由基的构型存在的。对于Zn2+(d10)，由于N-Zn配位键的形成，使得非辐射跃迁的路径关闭，因此，恢复较强的荧光。但是随着离子半径的增大（如，Cd2+(d10)），Cd-N金属配位键减弱和适宜的空间构型使S0SCT(1nπ*)出现，因此，该金属络合物具有较弱的荧光。一旦荧光探针与Cu2+ (d9)螯合，由于较低能量dd的跃迁可伴随着ππ*态同时出现，导致一个混合的三态的激发模式出现。通过计算模拟发现，[Cu(L1H)]2+螯合物的荧光淬灭是由两个锥形交叉和一个单三态交叉形成的阶梯状非辐射跃迁路径导致的。本文的计算结果对金属离子荧光探针的设计与合成提供了合理的理论依据。

1-(benzimidazol-2-ylmethyl)-1,4,8,11-tetraazacyclotetradecane is a highly sensitive and selective fluorescent sensor. ab initio multiconfigurational (CASPT2//CASSCF) approach has been employed to map radiative and non-radiative relaxation paths for fluorescent sensor of cyclam-methylbenzimidazole and its Zn2+, Cd2+ and Cu2+ complexes in order to provide deep understanding on ON-OFF fluorescent mechanisms of metal ion probing. The competitive coexistence between radiative decay of ππ* state localized in signaling unit and C-N bond fission chemical reaction in SCT(1nπ*) state was demonstrated to account for weaker emission performance of fluorescent sensor in comparison with that of isolated signaling molecule. The S0SCT(1nπ*) transition was determined, for first time, to originate from electron communication between lone pair of linking atom N and π* of signaling unit, which is a typical nπ* excitation pattern but exhibits a significant charge transfer character and zwitterionic radical configuration. In the case of Zn2+ (d10) detection, non-radiative relaxation channel is significantly closed due to the formation of strong N-Zn coordinate bond. With the increase of ion radius [i.e. Cd2+(d10)], a weak N-Cd bond leaves enough spatial and structural allowance to let the occurrence of S0SCT(1nπ*) excitation, thereby giving rise to the weak fluorescence emission. Once Cu2+ (d9) is bound with fluorescent sensor, a low energy dd* transition is appended to ππ* state leading to a combined excitation pattern in triplet state. The fluorescence quenching is quantitatively characterized by regulation of non-adiabatic relies of two conical intersections and one singlet/triplet crossing along a downhill ladder relaxation pathway. These computational results will provide the useful strategy for the molecular design of fluorescent sensor detecting metal ions.