Extending Metal‐to‐Polyoxometalate Charge Transfer Lifetimes: The Effect of Heterometal Location

Abstract

In an effort to develop robust molecular sensitizers for solar fuel production, we examine the electronic structure and photodynamics of transition-metal-substituted polyoxometalates (POMs), a novel class of compound in this context. (4a), 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 biexponential 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 1a and 3a are considered to result from metal-topolyoxometalate charge transfer (MPCT) from Co II to W, while the longer-lived excited state of 1a is tentatively assigned to a localized intermediate MPCT state. The excited state formed by the tetrahedral cobalt(II) centered heteropolyanion (1a) is far longer-lived ( = 420 ps in H2O;  = 1700 ps in MeCN) than that of 3a ( = 1.3 ps), where the single Co II atom is located in a pseudo-octahedral addendum site. Short-lived states are observed for the two Co III containing heteropolyanions 2a ( = 4.4 ps) and 4a ( = 6.3 ps) and assigned solely to OCo III charge transfer. The dramatically extended lifetime for 1a vs 3a 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.

A Keggin Polyoxometalate Shows Water Oxidation Activity at Neutral pH: POM@ZIF‐8, an Efficient and Robust Electrocatalyst

“…showed K 6 [CoW 12 O 40 ] to be inactive to photocatalytic WO, while K 7 [Co III Co II (H 2 O)W 11 O 39 ], was found to be active owing to a peripheral Co III –aqua coordination complex. We thus made an attempt to prepare WOC from [CoW 12 O 40 ] 6− Keggin, because unlike other Keggins having a main‐group element at the center in perfect tetrahedral geometry, [CoW 12 O 40 ] 6− is established to have slightly distorted tetrahedral geometry around the central cobalt, as all of the Co−O bond lengths are not equal in this case . This special structural feature has introduced new scope to explore [CoW 12 O 40 ] 6− further as a functional material.…”

“…AK ubelka-Munk-converted solidstate electronic (diffused reflectance) spectrum ( Figure 1b)of POM@ZIF-8 was found to carry maximum similarity with that of K 6 [CoW 12 O 40 ]P OM. Earlier, Hill and co-workers [40] and CasaÇ-Pastor and co-workers [51] in different works have highlighted the fact that POM (K 6 [CoW 12 O 40 ]) has low HOMO-LUMO energy separation and comparable e-t 2 (Co II )e nergy gap.T hus,s everal transitions occur in the lowenergy region of electronic diffused reflectance spectra of POM itself and POM@ZIF-8 (for ad etailed discussion, see the Supporting Information, Section S6). Most of the peaks were shifted in POM@ZIF-8 (for example,the d-d transition peak of Co II is blue-shifted by ca.…”

A Keggin Polyoxometalate Shows Water Oxidation Activity at Neutral pH: POM@ZIF‐8, an Efficient and Robust Electrocatalyst

“…Thec omposite was found to be stable to mechanical disturbance and ah arsh chemical environment (Supporting Information, Section S3). Earlier, Hill and co-workers [40] and CasaÇ-Pastor and co-workers [51] in different works have highlighted the fact that POM (K 6 [CoW 12 O 40 ]) has low HOMO-LUMO energy separation and comparable e-t 2 (Co II )e nergy gap.T hus,s everal transitions occur in the lowenergy region of electronic diffused reflectance spectra of POM itself and POM@ZIF-8 (for ad etailed discussion, see the Supporting Information, Section S6). AFTIR spectrum of POM@ZIF-8 was found to have extra peaks as well as shifted peaks in comparison to POM and ZIF-8 separately (Supporting Information, Section S5).…”

Oxygen Adsorption and Activation on Cobalt Center in Modified Keggin Anion-DFT Calculations

“…The theoretical calculations concerning this topic are rather poor [46][47][48]. Zhang and Maestre [46,47] studied the thermal stability and redox properties of the CoW 12 O 40 5− alpha/beta Keggin anion.…”