Tunable electronic structures and optical properties of fluorenone-based molecular materials by heteroatoms

The ground-state and excited-state electronic structures as well as the tunable optical properties of a variety of newly designed fluorenone-based molecular materials have been theoretically investigated using density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods. The substitutes on the O atom in the carbonyl group of the fluorenone (FN) molecule with S (FN-C=S), Se (FN-C=Se), and Te (FN-C=Te) atoms can significantly influence their electronic structures, molecular orbitals, geometric conformations, and optical properties of fluorenone-based molecular materials. Due to the important difference of electronegativity for O, S, Se, and Te atoms in the same group, the ground-state dipole moment of these fluorenone-based molecular materials is gradually decreased in the order FN, FN-C=S, FN-C=Se, and FN-C=Te. At the same time, the ground-state bond length of the C=X (X refers O, S, Se, and Te) is gradually increased in the order of FN, FN-C=S, FN-C=Se, and FN-C=Te. Due to the different nature of the S(1) state for FN (pipi* character) and FN-C=S, FN-C=Se, and FN-C=Te (sigmapi* character), the excited-state dipole moment of FN in the S(1) state is dramatically increased in comparison with that in the ground state; however, the excited-state dipole moments of FN-C=S, FN-C=Se, and FN-C=Te are significantly diminished. In addition, the excited-state bond length of C=X (X refers O, S, Se, and Te) in the S(1) state is lengthened in comparison with that in the ground state due to the photoexcitation of the C=X bond FN, FN-C=S, FN-C=Se, and FN-C=Te. On the other hand, the energy level of the HOMO orbital is heightened and that of LUMO orbital is lowered with the introduction of heteroatoms in the order of S, Se, and Te. Consequently, the energy gap between LUMO and HOMO orbtials is gradually decreased in the order of the FN, FN-C=S, FN-C=Se, and FN-C=Te. Consequently, the calculated fluorescence wavelengths are strongly red-shifted from the visible region for FN to the near-infrared (NIR) region for FN-C=S, FN-C=Se, and FN-C=Te. These newly designed fluorenone-based molecules may be potential NIR fluorescent molecular functional materials.