Comparative studies of the spectra and the second-order nonlinear polarizabilities for donor-acceptor ensembles between Zn-porphyrin/fullerene [60] and Zn-porphyrin/naphthalenediimide

A series of donor-acceptor ensembles with fullerene [60] and naphthalenediimide (NIm) as acceptors and Zn-porphyrin (ZnP) as donors have been theoretically studied. The donors and acceptors are separated by different rigid bridges (-C≡C-, -CONH and -NHCO-). Their equilibrium geometries and UV-Visible spectra were calculated by means of INDO/2 and the INDO/CI methods. The calculated results indicated that the charge transfer from the donor - Zn-porphyrin to the acceptor takes place for the ensembles with fullerene [60] as acceptors while for the molecules with NIm as acceptors, charge transfer is weak. On the basis of correct electronic spectra, calculations of the nonlinear second-order optical polarizabilites β_ijk and β _μ values were carried out using the INDO/CI method combined with a Sum-Over-States (SOS) expression. We conclude that the bridge of -NHCO- leads to comparatively large nonlinear second-order polarizability than the other two bridges; and the 3D fullerene [60] acceptor is superior to the 2D NIm acceptor due to its symmetrical shape, large size, and properties of its π-electron system. We illustrate the phenomena by means of the comparison of the frontier orbitals and net charge in the ground and main excited states.