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Issue 2, 2012
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Transient infrared spectroscopy: a new approach to investigate valence tautomerism

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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(Me3tpa)(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-CoIII(catecholate) (1-CAT), while (2) as hs-CoII(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-SQ2-SQ)–FTIR(1-CAT1-CAT) and with the calculated difference spectrum IRc(1-SQ1-SQ)–IRc(1-CAT1-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.

Graphical abstract: Transient infrared spectroscopy: a new approach to investigate valence tautomerism

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Publication details

The article was received on 08 Aug 2011, accepted on 20 Oct 2011 and first published on 16 Nov 2011

Article type: Paper
DOI: 10.1039/C1CP22557J
Citation: Phys. Chem. Chem. Phys., 2012,14, 1038-1047
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    Transient infrared spectroscopy: a new approach to investigate valence tautomerism

    T. T. Patricia, M. V. Sandra, L. Manuela, L. Andrea, F. Paolo, D. Andrea and R. Roberto, Phys. Chem. Chem. Phys., 2012, 14, 1038
    DOI: 10.1039/C1CP22557J

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