title: Transient infrared spectroscopy: a new approach to investigate valence tautomerism.
abstract: In this work we present, to our knowledge for the first time, the results of a transient infrared spectroscopic study of the photoinduced valence tautomerism process in cobalt-dioxolene complexes with sub-picosecond time resolution. The molecular systems investigated were [Co(tpa)(diox)]PF(6) (1) and [Co(Me(3)tpa)(diox)]PF(6) (2), where diox = 3,5-di-tert-butyl-1,2-dioxolene; tpa = tris(2-pyridylmethyl)amine and Me(3)tpa its 6-methylated analogue. Complex (1) is present in solution as ls-Co(III)(catecholate) (1-CAT), while (2) as hs-Co(II)(semiquinonate) (2-SQ). DFT calculation of the harmonic frequencies for (1) and (2) allowed us to identify the vibrational markers of catecholate and semiquinonate redox isomers. Irradiation with 405 and 810 nm pulses (~35 fs) of (1-CAT) induces the formation of an intermediate excited species from which the ground state population is recovered with a time constant of 1.5 ± 0.3 ns. Comparing the 1 ns transient infrared spectrum with the experimental difference spectrum FTIR(2-SQ)-FTIR(1-CAT) and with the calculated difference spectrum IR(c)(1-SQ)-IR(c)(1-CAT) we are able to unequivocally identify the long lived species as the semiquinonate redox isomer of (1). On the other hand, no evidence of photoconversion is observed upon irradiation of (2) with 405 nm. Temporal evolution of transient spectra was analyzed with the combined approach consisting of singular values decomposition and global fitting (global analysis). After 405 and 810 nm excitation of (1-CAT), the semiquinonate excited species is formed on an ultrafast time scale (<200 fs) and cools down within the first 50 ps. Excitation of (2-SQ) with 405 nm wavelength produces a short lived excited state in which the semiquinonate nature of dioxolene is preserved and the ground state recovery is completed within 30 ps.
authors.affiliation: European Laboratory for Non Linear Spectroscopy (LENS), Firenze, Italy.
authors.forename: Patricia Tourón
authors.initials: PT
authors.lastname: Touceda
authors.forename: Tourón Touceda
authors.initials: TT
authors.lastname: Patricia
authors.forename: Sandra Mosquera
authors.initials: SM
authors.lastname: Vázquez
authors.forename: Mosquera Vázquez
authors.initials: MV
authors.lastname: Sandra
authors.forename: Manuela
authors.initials: M
authors.lastname: Lima
authors.forename: Lima
authors.initials: L
authors.lastname: Manuela
authors.forename: Andrea
authors.initials: A
authors.lastname: Lapini
authors.forename: Lapini
authors.initials: L
authors.lastname: Andrea
authors.forename: Paolo
authors.initials: P
authors.lastname: Foggi
authors.forename: Foggi
authors.initials: F
authors.lastname: Paolo
authors.forename: Andrea
authors.initials: A
authors.lastname: Dei
authors.forename: Dei
authors.initials: D
authors.lastname: Andrea
authors.forename: Roberto
authors.initials: R
authors.lastname: Righini
authors.forename: Righini
authors.initials: R
authors.lastname: Roberto