Theoretical study on the intramolecular charge transfer induced twisting of the nitro and dimethylamino groups

• The photophysical properties of DMANF have been theoretically investigated.
• The calculated absorption and fluorescence spectrum agree well with the experiment.
• TICT process is responsible for the excited state fluorescence in DMANF molecule.
• The nitrogen lone pair electrons play an important role in the TICT process.

The ground-state and excited-state electronic structures, relaxation dynamics as well as photophysical properties of the donor–π–acceptor compound 2-dimethylamino-7-nitrofluorene (DMANF) have been theoretically investigated using density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods. The calculated absorption and fluorescence spectrum agree well with the experimental results. The qualitative potential energy curves of twisting either donor or acceptor group favor the excited state intramolecular charge-transfer (ICT) relaxation process. However, the frontier molecular orbital picture supports that the twisting along the donor part is responsible for the fluorescence of the excited state. Further investigation of the strong dipolar stabilization with different structures indicates that two processes are involved in the whole twist and charge transfer dynamics: first, charge transfer is rapidly caused by the excitation to the first singlet state; second, charge transfer goes on slowly together with the twist motion. For strong dipolar stabilization, induced by twisting of the dimethylamino group, the charge transfer excited state corresponds to a more stable state with the twisted structure. Which results in these kinds of D–π–A compounds do not twist in nonpolar solvent. In the twisting charge transfer process, the nitrogen lone pair electrons play an important role.