氟硼二吡咯类pH荧光探针PET光谱特性的理论计算

Theoretical Calculations on the PET Property of BODIPY Fluorescent pH Probes

氟硼二吡咯(BODIPY)类pH荧光探针分子是基于光诱导电子转移(PET)的荧光探针分子,识别基团氮原子上引入不同取代基可呈现不同的光学灵敏度.本文应用密度泛函理论(DFT)及含时密度泛函理论(TD-DFT)方法对六种含不同取代基的探针分子进行了几何构型优化及激发态计算,探讨了不同取代基对PET效应影响.计算结果表明:基态时这些探针分子的最高占有分子轨道(HOMO)和最低未占有分子轨道(LUMO)都在荧光母体BODIPY的π,π*轨道,而识别基团上氮原子孤对电子所在的轨道为HOMO-1轨道.但是在激发态,当氮原子上有两个取代基时,HOMO-1→LUMO跃迁的激发能都小于荧光团的HOMO→LUMO跃迁,这将有可能产生PET效应并导致荧光熄灭,而当氮原子上有一个取代基时不会出现这种现象.通过激发态结构优化可以发现,无论识别基团氮原子上有一个还是两个取代基,N原子的轨道对称性都发生变化,由sp3→sp2,孤对电子占据在p轨道上,其轨道能级升高至荧光团的HOMO和LUMO轨道之间,将导致不同程度的PET效应,与实验结果一致.

4,4-Difluoro-4-bora-3a,4a-diaza-s-indacene （BODIPY） fluorescent pH probes are fluorescent sensors and switchers based on photo-induced electron transfer （PET） mechanism. From experimental studies, different substituents on the amino nitrogen are known to result in variable photo sensitivities. Density functional theory （DFT） and time-dependent density functional theory （TD-DFT） calculations are used to optimize the structures of six different BODIPY probe molecules with different substituted amines, and to study their exited states. In the ground state, the highest occupied molecular orbital （HOMO） and the lowest unoccupied molecular orbital （LUMO） of the probe molecules are located on the rr and π＊ orbitals of the BODIPY dye, and the lone pair electrons from the amino nitrogen form the HOMO-1 orbital. In the first exited state, for the probes with two substituent groups on the amino nitrogen, the HOMO-1→ LUMO transition has a smaller emission energy than that of the HOMO→LUMO transition in the BODIPY host, which may induce the PET effect and quench the fluorescence. After the geometry optimization of the exited states, irrespective of whether one or two groups are linked on the amino nitrogen, the orbital symmetry of the nitrogen atom changes from sp^→sp^2, and the lone pair electrons occupy the p orbital situated between the HOMO and LUMO of BODIPY, resulting in the PET effect. The calculated results are in good agreement with the experimental results.