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Issue 1, 2019
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Insights into the excited state dynamics of Fe(ii) polypyridyl complexes from variable-temperature ultrafast spectroscopy

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Abstract

In an effort to better define the nature of the nuclear coordinate associated with excited state dynamics in first-row transition metal-based chromophores, variable-temperature ultrafast time-resolved absorption spectroscopy has been used to determine activation parameters associated with ground state recovery dynamics in a series of low-spin Fe(II) polypyridyl complexes. Our results establish that high-spin (5T2) to low-spin (1A1) conversion in complexes of the form [Fe(4,4′-di-R-2,2′-bpy′)3]2+ (R = H, CH3, or tert-butyl) is characterized by a small but nevertheless non-zero barrier in the range of 300–350 cm−1 in fluid CH3CN solution, a value that more than doubles to ∼750 cm−1 for [Fe(terpy)2]2+ (terpy = 2,2′:6′,2′′-terpyridine). The data were analyzed in the context of semi-classical Marcus theory. Changes in the ratio of the electronic coupling to reorganization energy (specifically, Hab4/λ) reveal an approximately two-fold difference between the [Fe(bpy′)3]2+ complexes (∼1/30) and [Fe(terpy)2]2+ (∼1/14), suggesting a change in the nature of the nuclear coordinate associated with ground state recovery between these two types of complexes. These experimentally-determined ratios, along with estimates for the 5T2/1A1 energy gap, yield electronic coupling values between these two states for the [Fe(bpy′)3]2+ series and [Fe(terpy)2]2+ of 4.3 ± 0.3 cm−1 and 6 ± 1 cm−1, respectively, values that are qualitatively consistent with the second-order nature of high-spin/low-spin coupling in a d6 ion. In addition to providing useful quantitative information on these prototypical Fe(II) complexes, these results underscore the utility of variable-temperature spectroscopic measurements for characterizing ultrafast excited state dynamics in this class of compounds.

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

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Supplementary files

Article information


Submitted
10 Sep 2018
Accepted
29 Nov 2018
First published
10 Dec 2018

This article is Open Access
All publication charges for this article have been paid for by the Royal Society of Chemistry

Chem. Sci., 2019,10, 134-144
Article type
Edge Article

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

M. C. Carey, Sara L. Adelman and J. K. McCusker, Chem. Sci., 2019, 10, 134 DOI: 10.1039/C8SC04025G

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