Theoretical study of the structures and electronic properties of the dimers of fluorene and carbazole and their derivatives

2,2'-Bi(9,9-di-ethyl-fluorene)

The title compound, C₃₄H₃₄, systematic name 9,9,9′,9′-tetra­ethyl-2,2′-bi(9H-fluorene), crystallized with two crystallographically independent mol­ecules (A and B) in the asymmetric unit. These differ mainly in the orientation of the lateral ethyl chains: in mol­ecule A, they are both on the same side of the mol­ecule whereas in mol­ecule B, one di­ethyl­fluorene moiety has undergone a 180° rotation such that the two pairs of ethyl residues appear on opposite sides of the mol­ecule. The fluorene ring systems subtend dihedral angles of 31.37 (4) and 43.18 (3)° in mol­ecules A and B, respectively. Hence the two fluorene moieties are tilted slightly toward one another. This may be due to the presence of inter­molecular C—H⋯π inter­actions between neighboring mol­ecules. The lateral ethyl chains (excluding H atoms) are also almost planar, with each pair almost perpendicular to the plane of the fluorene system to which they are attached with dihedral angles between the ethyl and fluorene planes in the range 86.04 (8)–89.5 (1)°.

Excited state properties, fluorescence energies, and lifetimes of a poly(fluorene-phenylene), based on TD-DFT investigation

The structural and electronic properties of fluorene-phenylene copolymer (FP)(n), n = 1-4 were studied by means of quantum chemical calculations based on density functional theory (DFT) and time dependent density functional theory (TD-DFT) using B3LYP functional. Geometry optimizations of these oligomers were performed for the ground state and the lowest singlet excited state. It was found that (FP)(n) is nonplanar in its ground state while the electronic excitations lead to planarity in its S(1) state. Absorption and fluorescence energies were calculated using TD-B3LYP/SVP and TD-B3LYP/SVP+ methods. Vertical excitation energies and fluorescence energies were obtained by extrapolating these values to infinite chain length, resulting in extrapolated values for vertical excitation energy of 2.89 and 2.87 eV, respectively. The S(1) <-- S(0) electronic excitation is characterized as a highest occupied molecular orbital to lowest unoccupied molecular orbital transition and is distinguishing in terms of oscillator strength. Fluorescence energies of (FP)(n) calculated from TD-B3LYP/SVP and TD-B3LYP/SVP+ methods are 2.27 and 2.26 eV, respectively. Radiative lifetimes are predicted to be 0.55 and 0.51 ns for TD-B3LYP/SVP and TD-B3LYP/SVP+ calculations, respectively. These fundamental information are valuable data in designing and making of promising materials for LED materials.