Transient infrared spectroscopy: a new approach to investigate valence tautomerism

Year: 2012

Authors: Tourón Touceda P., Mosquera Vázquez S., Lima M., Lapini A., Foggi P., Dei A., Righini R.

Autors Affiliation: European Laboratory for Non Linear Spectroscopy (LENS), via nello Carrara 1, 50019 Sesto Fiorentino, Firenze, Italy;
Dipartimento di Chimica, Universita` di Perugia, via Elce di Sotto 8, 06123 Perugia, Italy;
Dipartimento di Chimica, Universita` di Firenze, via della Lastruccia 13, 50019 Sesto Fiorentino, Firenze, Italy

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)] PF6 (1) and [Co(Me(3)tpa)(diox)]PF6 (2), where diox = 3,5-di-tert-butyl-1,2-dioxolene; tpa = tris(2-pyridylmethyl)amine and Me3tpa 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 (similar to 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(1-SQ)c-IR(1-CAT)c 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. Journal/Review:

Volume: 14 (2)      Pages from: 1038  to: 1047

More Information: L. A. acknowledges the financial support of the Regione Toscana through the fund PORFSE 2007-2013 obiettivo 2 asse IV, project EPHODS. The financial support of the Cassa di Risparmio di Firenze is also gratefully acknowledged.
DOI: 10.1039/c1cp22557j

Citations: 38
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