Theoretical Studies On The PET Mechanism Of BODIPY Fluorescent Probes

BODIPY (4,4-difluoro-4-bora-3a,4a-diaza-s-indacene) fluorescent probes are "switched on-off" fluorescent dyes based on photo-induced electron transfer mechanism. By introducing different groups in the recognition groups results in variable photo sensitivities. In this paper density functional theory (DFT) at the theoretical level of B3LYP/6-31G*and time-dependent density functional theory (TD-DFT) methods were used to optimize the structures of a series of probe molecules and to study the exited states.1. DFT method at the theoretical level of B3LYP/6-31G*was used to optimize the structures of six different probe molecules and TD-DFT methods at B3LYP/6-311++G**was used to calculate the exited states. The results indicated that at ground state, the HOMO and LUMO of the probe molecules are locate on Ï€ and Ï€*orbitals of BODIPY dye, and the HOMO-1is attributed to the "long pair" electron orbital of nitrogen atom in the recognition group. However, at the first exited state, the probes with the recognition group having two substituted groups on nitrogen atom showed the minimum energy transition of HOMO-1â†'LUMO, implying that the energy level of the long pair orbital of nitrogen atom rose up above the HOMO of BODIPY, which may induce the PET effect and quench the fluorescence. After the geometry optimization of the exited states, no matter one or two groups linked on nitrogen atom, the orbital symmetry of nitrogen atom changed as sp3â†'sp2, and the "long pair" electrons occupy the p orbital, which situated between the HOMO and LUMO of BODIPY, resulting in PET effect. The calculated results are in good agreement with the experiment.2. DFT and TD-DFTmethods were used to optimize the structures of BODIPY fluorescent Hg2+probes and to study the exited states. The results indicated that without Hg2+the HOMO and LUMO of the BHgl-3molecules were located on Ï€ and Ï€*orbitals of BODIPY moiety, and the HOMO-1is attributed to the HOMO of aniline moiety. However at the exitation state, the minimum energy transition was HOMO-1â†'LUMO, implying that the energy level of aniline moiety rose up above the HOMO of BODIPY, which may induce the PET effect and quench the fluorescence. When the probe molecules combined with Hg2+, the N atom in aniline donated the electrons from the p orbital to Hg2+cation, and the p orbital of N was not conjugated with benzene ring. The HOMO of aniline decreses belowe the HOMO of BODIPY dye, which may not induce the PET effect and therefore, the fluorescence will berecovered.