Addition/Correction pubs.acs.org/JACS
Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Correction to “A Synthetically Tunable System To Control MLCT Excited-State Lifetimes and Spin States in Iron(II) Polypyridines” Steven M. Fatur, Samuel G. Shepard, Robert F. Higgins, Matthew P. Shores, and Niels H. Damrauer* J. Am. Chem. Soc. 2017, 139, 4493−4505. DOI: 10.1021/jacs.7b00700 surfaces in [Fe(dftpy)2]2+, the temperature dependence of bleach recovery was measured over the range of 279 to 316 K. However, in analyzing these lifetimes, simple Arrhenius analysis will not suffice. Due to the fact that [Fe(dftpy)2]2+ is a spincrossover system, the forward rate constant (in this case, 5MC → 1A, denoted kHL) is not substantially larger than the backward rate constant (1A → 5MC, denoted kLH), so the apparent rate constant, kapp, is the sum of kHL and kLH.72 Furthermore, the equilibrium constant for the spin-crossover will change as a function of temperature, which will also alter the temperature dependence of kapp. Because the spin-crossover equilibrium constant can be expressed as a ratio of kLH to kHL, a modified Arrhenius expression for fitting the observed kinetics can be expressed as shown below:
Page 4500. There is an error in the Arrhenius analysis of the nanosecond transient absorption data. In this Addition/ Correction, we reanalyze the data and report lower values of the pre-exponential factor and the activation barrier to 5MC → 1 A interconversion. However, the correction does not alter the overall conclusions of the paper. The second paragraph of the second column of the original publication should be amended to include new calculations, a corrected version of Figure 8, and an additional reference (72),1 and should read as follows: A nanosecond transient absorption experiment was used to determine the lifetime of the bleach feature. At room temperature, the lifetime is 22 ns as can be seen in Figure 8, which represents a substantial elongation compared to the reported value of 5.35 ns for [Fe(tpy)2]2+ under comparable conditions.31 In order to better understand this lifetime increase via the interplay between low-energy singlet and quintet
kapp = kHL + kLH = kHL + KeqkHL kapp = kHL(1 + Keq) ⎡ ΔE 1 ⎤ kapp = A exp⎢ − act ⎥(1 + Keq) ⎣ R T⎦ ⎛ kapp ⎞ ΔEact 1 ⎟⎟ = ln(A) − ln⎜⎜ R T ⎝ 1 + Keq ⎠
Here, kapp/(1 + Keq) has replaced the familiar rate constant in a traditional Arrhenius expression, and Keq values are calculated using the thermodynamic parameters determined by SQUID (vide supra). The fitting reveals an activation barrier of 10 kJ/ mol for the interconversion between 5MC and 1A states. This barrier is larger than the literature value of 6.4 kJ/mol measured for [Fe(tpy)2]2+ under similar conditions.66 The increase in barrier relative to [Fe(tpy)2]2+ is likely the result of two contributing factors. First, interligand steric interactions exacerbated by the halogen substituents in [Fe(dftpy)2]2+ may be expected to increase the reorganization energy in going from a geometry relevant for the 5MC to one relevant for the 1A. Second, the 5MC/1A interconversion driving force is reduced (the origin of the near-room-temperature spin crossover behavior) and this would lead to an increase in the activation barrier even under circumstances where the reorganization energy is comparable. It is also noted that the pre-exponential factor determined from the Arrhenius analysis (A = 7.4 × 107 s−1) is low in comparison to the values measured for other Fe2+ spin-crossover systems,72 although in those systems a trigonal torsional mode was invoked in the relaxation process, allowing it to be mediated by the 3MC state. We believe this is not possible in [Fe(dftpy)2]2+ due to the
Figure 8. Top: Arrhenius plots of kHL (red) and kLH (blue) of [Fe(dftpy)2]2+. kHL is fit to an activation barrier of 10 ± 3 kJ/mol and a natural log of the pre-exponential of 18 ± 1 (A = 7.4 × 107 s−1). kLH is fit to an activation barrier of 35 ± 3 kJ/mol and a natural log of the pre-exponential of 31 ± 1 (A = 2.9 × 1013 s−1). Bottom: Singlewavelength kinetics at 510 nm following 520 nm excitation. Lines indicate fits to Gaussian-convoluted single exponential decay. © XXXX American Chemical Society
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DOI: 10.1021/jacs.7b13746 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Journal of the American Chemical Society
Addition/Correction
geometric constraints imposed by meridional tridentate ligands, lowering our intrinsic rate constant.
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ACKNOWLEDGMENTS We thank Dr. Gerald Hörner (Technische Univ. Berlin) for alerting us to this error in our original analysis. This work was supported by the NSF and EPA through CHE-1339674.
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REFERENCES
(1) The following should be added to the original paper as ref 72: (72) Stock, P.; Deck, E.; Hohnstein, S.; Korzekwa, J.; Meyer, K.; Heinemann, F. W.; Breher, F.; Hörner, G. Inorg. Chem. 2016, 55, 5254−5265.
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DOI: 10.1021/jacs.7b13746 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
