title: Enhancing the efficiency of two-photon absorption by metal coordination.
abstract: The intensity of the two-photon absorption (TPA) spectrum of a terpyridine ligand acting as a D-pi-A chromophore (D = donor and A = acceptor) is enhanced by a factor of about 2 upon coordination to ZnCl(2). Based on an analysis of linear absorption and fluorescence spectra of both the ligand and its Zn(II) complex, we have defined essential-state models for the two species. Linear and TPA spectra of the ligand are well reproduced in terms of a two-state model accounting for the D-pi-A <--> D(+)-pi-A(-) charge resonance. However, the enhancement of the TPA response of its Zn(II) complex can only be understood by extending the model to account for the active role of the "ZnCl(2)" moiety acting as a virtual A(v) acceptor group of a D-pi-AA(v) structure. The virtual D(+)AA(v)(-) state of the relevant three-state model has negligible weight in the ground state but contributes to the first excited state. The resulting increase of the excited-state dipole moment is responsible for the enhancement of the TPA cross section, and also explains the increase of the second order nonlinear optical response upon coordination.
authors.affiliation: Dipartimento di Chimica GIAF, Università di Parma & INSTM UdR Parma, Parco Area delle Scienze 17/A, 43100 Parma, Italy.
authors.forename: Luca
authors.initials: L
authors.lastname: Grisanti
authors.forename: Cristina
authors.initials: C
authors.lastname: Sissa
authors.forename: Francesca
authors.initials: F
authors.lastname: Terenziani
authors.forename: Anna
authors.initials: A
authors.lastname: Painelli
authors.forename: Dominique
authors.initials: D
authors.lastname: Roberto
authors.forename: Francesca
authors.initials: F
authors.lastname: Tessore
authors.forename: Renato
authors.initials: R
authors.lastname: Ugo
authors.forename: Silvio
authors.initials: S
authors.lastname: Quici
authors.forename: Ilaria
authors.initials: I
authors.lastname: Fortunati
authors.forename: Eleonora
authors.initials: E
authors.lastname: Garbin
authors.forename: Camilla
authors.initials: C
authors.lastname: Ferrante
authors.forename: Renato
authors.initials: R
authors.lastname: Bozio
mesh_terms: D000042:Absorption; D002712:Chlorides / Q000737:chemistry*; D004579:Electron Transport; D008956:Models, Chemical; D017785:Photons; D011725:Pyridines / Q000737:chemistry; D013057:Spectrum Analysis; D017967:Zinc Compounds / Q000737:chemistry*
chemical_list: D002712:Chlorides; D011725:Pyridines; D017967:Zinc Compounds; C016837:zinc chloride; C023666:pyridine