A TD-DFT investigation of two-photon absorption of fluorene derivatives

We report time-dependent density functional theory (TD-DFT) calculations on the two-photon absorption (TPA) properties of fluorene and derivatives. The influence of donor and acceptor groups and of dimerisation is investigated. Firstly the choice of a DFT functional and of the basis set is performed by comparison of experimental and calculated excitation energies and two-photon cross sections. Then, the calculations display an enhancement of the cross section with acceptor groups or with a combination of one donor and one acceptor groups (push-pull), at some positions on the cycles. Moreover, the largest cross section is obtained for bifluorene. The replacement of carbon atoms by nitrogen atoms, giving heterocycles, is not efficient. In chloroform as solvent, the excitation energy decreases and the two-photon cross section increases, mainly with a polar molecule. Finally, a rationalization of the results is given based on the three-level model by analysis of the transition moments and of the molecular orbitals.