Extending metal-to-polyoxometalate charge transfer lifetimes: the effect of heterometal location

Glass, E. N., Fielden, J., Kaledin, A. L., Musaev, D. G., Lian, T. and Hill, C. L. (2014) Extending metal-to-polyoxometalate charge transfer lifetimes: the effect of heterometal location. Chemistry: A European Journal, 20(15), pp. 4297-4307. (doi: 10.1002/chem.201304119) (PMID:24604763)

Abstract

In an effort to develop robust molecular sensitizers for solar fuel production, the electronic structure and photodynamics of transition-metal-substituted polyoxometalates (POMs), a novel class of compound in this context, was examined. Experimental and computational techniques including femtosecond (fs) transient absorption spectroscopy have been used to study the cobalt-containing Keggin POMs, [CoIIW12O40]6− (1 a), [CoIIIW12O40]5− (2 a), [SiCoII(H2O)W11O39]6− (3 a), and [SiCoIII(H2O)W11O39]5− (4 a), finding the longest lived charge transfer excited state so far observed in a POM and elucidating the electronic structures and excited-state dynamics of these compounds at an unprecedented level. All species exhibit a bi-exponential decay in which early dynamic processes with time constants in the fs domain yield longer lived excited states which decay with time constants in the ps to ns domain. The initially formed states of 1 a and 3 a are considered to result from metal-to-polyoxometalate charge transfer (MPCT) from CoII to W, while the longer-lived excited state of 1 a is tentatively assigned to a localized intermediate MPCT state. The excited state formed by the tetrahedral cobalt(II) centered heteropolyanion (1 a) is far longer-lived (τ=420 ps in H2O; τ=1700 ps in MeCN) than that of 3 a (τ=1.3 ps), in which the single CoII atom is located in a pseudo-octahedral addendum site. Short-lived states are observed for the two CoIII-containing heteropolyanions 2 a (τ=4.4 ps) and 4 a (τ=6.3 ps) and assigned solely to OCoIII charge transfer. The dramatically extended lifetime for 1 a versus 3 a is ascribed to a structural change permitted by the coordinatively flexible central site, weak orbital overlap of the central Co with the polytungstate framework, and putative transient valence trapping of the excited electron on a single W atom, a phenomenon not noted previously in POMs.

• Item Type: Articles
• Status: Published
• Refereed: Yes
• Glasgow Author(s) Enlighten ID: Fielden, Dr John
• Authors: Glass, E. N., Fielden, J., Kaledin, A. L., Musaev, D. G., Lian, T., and Hill, C. L.
• College/School: College of Science and Engineering > School of Chemistry
• Journal Name: Chemistry: A European Journal
• Publisher: Wiley
• ISSN: 0947-6539
• ISSN (Online): 1521-3765