Insights into the excited state dynamics of Fe(ii) polypyridyl complexes from variable-temperature ultrafast spectroscopy

Abstract

In order to better define the nature of the nuclear coordinate associated with excited state dynamics in first-row transition metal chromophores, variable-temperature ultrafast time-resolved absorption spectroscopy has been used to determine activation parameters associated with ground state recovery in a series of low-spin Fe(ii) polypyridyl complexes.

Using Ultrafast X-ray Spectroscopy To Address Questions in Ligand-Field Theory: The Excited State Spin and Structure of [Fe(dcpp)2]2+

“…28 However, it has recently been determined that ground-state recovery for [Fe(bpy)3] 2+ in fluid solution has a small but nevertheless measurable barrier of ~300 cm -1 , 67 whereas corresponding studies on [Fe(dcpp)2] 2+ under identical conditions reveal an essentially barrierless process with measured rate constants at 295 K and 215 K that are within experimental error of each other. 68 Interestingly, the pre-exponential term in an Arrhenius treatment of the data on [Fe(bpy)3] 2+ yields a value of ca. 250 ps -1 , i.e., very close to the observed ground-state recovery time for [Fe(dcpp)2] 2+ .…”

Is Iron the New Ruthenium?

“…The normalc oordinate on the x axis often represents an essentially totally symmetrical( a 1g )d istortion, but the types of relevant nuclear coordinatesc an be different from one compound to another. [10] Each of the following six sections discusses one of six key concepts to achievel ong-lived MLCT excited states in Fe II complexes. This includes the creation of highly symmetric ligand fields (Figure 2a), the use of push-pull ligand sets ( Figure 2b), highly strained complexes giving access to 5 ( Figure 2e), and the use of mesoionic carbenes as combined s donor, p acceptor ligands (Figure2f).…”

Long-Lived, Strongly Emissive, and Highly Reducing Excited States in Mo(0) Complexes with Chelating Isocyanides

“…However, such 3d 6 complexes have energetically low-lying metal-centered (MC) excited states that deactivate the MLCT states very efficiently. [16][17][18] Different concepts have been explored to lengthen 3 MLCT lifetimes in Fe(II) complexes, including the optimization of metal coordination geometries, [19][20][21] the use of push-pull ligand environments, [22][23] and the enhancement of ligand fields with N-heterocyclic [24][25][26][27] or mesoionic carbenes. 28 Recently, these efforts culminated in the discovery of two Fe(III) complexes that luminesce from a ligand-to-metal charge transfer (LMCT) excited state [29][30] and an Fe(II) complex with a 3 MLCT lifetime of 528 ps in solution at room temperature.…”