Photoexcitation mechanisms of centrosymmetric and asymmetric fluorene derivatives in two-photon absorption

With the two dimensional (2D) site and the three dimensional (3D) cube representations in two-photon absorption [M.T. Sun, J.N. Chen, H.X. Xu, J. Chem. Phys. 128(1-6) (2008) 064106], the charge transfer and electron-hole coherence for centrosymmetric and asymmetric fluorene derivatives have been investigated theoretically. For centrosymmetric fluorene derivative, the excitation from the ground state to intermediate excited state by the first photon is a Frenkel excitation; while the excitation by the second photon from the intermediate excited state to the final excited state is an intraband excitation, and charge transfers from one substituent to the other substituent. For asymmetric fluorene derivative in TPA, the excitations by the first and second photons are all charge transfer excited states. Theoretical visual evidence has been provided that diphenylamino-end groups do not strongly participate in pure 2PA, which confirmed the experimental assumption. We also visualize the orientation and strength of transition polarizability density (TPD) in TPA with 3D cube representation. The transition probability in TPA also was visualized with 2D site representation.