Tuning Iron(II) Excited States with Bulky Ligands and Investigating Chromium(III) Photocatalytic Mechanisms for Earth-Abundant Photocatalysis

Photocatalysis opens up new synthetic pathways and is a potential method for storing solar energy in chemical bonds. Many transition metal photocatalysts utilize long-lived metal-to-ligand charge transfer (MLCT) states as the catalytically active state. However, earth-abundant Fe(II) analogues of successful Ru(II) photocatalysts are plagued by ultrafast relaxation. Decreased ligand-field splitting in these first-row metal complexes opens new relaxation pathways via low-lying metal-centered states. To design potential photocatalysts with Fe(II) centers we chose to exploit the decreased ligand-field splitting by destabilizing the singlet state until the lowest energy quintet state became the ground state, opening up a distinct dynamical picture and lengthening the MLCT excited-state lifetime. In general, this class of complexes displays a &gt; 100-fold improvement relative to the ~100 fs MLCT lifetime of the low-spin parent, Fe(II) bis-terpyridine. This is accomplished using a sterically demanding bis-terpyridyl ligand framework in which interligand repulsion destabilizes lower spin states to force the quintet ground state. Furthermore, this framework is easily altered synthetically by employing substituents with either steric or electronic effects, allowing for substantial tunability. We have systematically synthesized a range of these compounds and investigated them using x-ray crystallography, electrochemistry, magnetic measurements, and transient absorption spectroscopy. Through this effort, we highlight a novel approach to controlling excited-state dynamics and ground-state absorption in Fe(II) polypyridines with potential photocatalysis applications. A secondary project explores the mechanisms of Cr(III) photocatalyzed [4+2] cycloadditions. Cr(III) polypyridyl complexes have been found to serve as replacements for more expensive Ru(II) photocatalysts in these reactions. However, their application to a wider scope of reactions is limited by a lack of mechanistic understanding. With static and time-resolved photoluminescence spectroscopy and Stern-Volmer quenching studies, we show that [Cr(Ph2phen)3]3+ (Ph2phen = 4,7-diphenyl-phenanthroline) is a potent photooxidant for promoting radical cation Diels-Alder reactions. Further experiments show that atmospheric oxygen is critical for turning over the catalyst. Finally, the mechanistic study of a related reaction with the same catalyst suggests that an energy transfer mechanism is also feasible in certain cases. This broader understanding of mechanistic pathways uncovers a rich vein for further mechanistic studies and Cr(III) catalyst development.</p

Ultrafast Time-Resolved Hard X-Ray Emission Spectroscopy on a Tabletop

Experimental tools capable of monitoring both atomic and electronic structure on ultrafast (femtosecond to picosecond) time scales are needed for investigating photophysical processes fundamental to light harvesting, photocatalysis, energy and data storage, and optical display technologies. Time-resolved hard x-ray (>3 keV) spectroscopies have proven valuable for these measurements due to their elemental specificity and sensitivity to geometric and electronic structures. Here, we present the first tabletop apparatus capable of performing time-resolved x-ray emission spectroscopy. The time resolution of the apparatus is better than 6 ps. By combining a compact laser-driven plasma source with a highly efficient array of microcalorimeter x-ray detectors, we are able to observe photoinduced spin changes in an archetypal polypyridyl iron complex ½Feð2; 20 -bipyridineÞ3 2þ and accurately measure the lifetime of the quintet spin state. Our results demonstrate that ultrafast hard x-ray emission spectroscopy is no longer confined to large facilities and now can be performed in conventional laboratories with 10 times better time resolution than at synchrotrons. Our results are enabled, in part, by a 100- to 1000-fold increase in x-ray collection efficiency compared to current techniques

Highly Strained Iron(II) Polypyridines: Exploiting the Quintet Manifold To Extend the Lifetime of MLCT Excited States

Halogen substitution at the 6 and 6″ positions of terpyridine (6,6″-Cl₂-2,2:6′,2″-terpyridine = dctpy) is used to produce a room-temperature high-spin iron­(II) complex [Fe­(dctpy)₂]­(BF₄)₂. Using UV−vis absorption, spectroelectrochemistry, transient absorption, and TD-DFT calculations, we present evidence that the quintet metal-to-ligand charge-transfer excited state (⁵MLCT) can be accessed via visible light absorption and that the thermalized ^5,7MLCT is long-lived at 16 ps, representing a > 100 fold increase compared to the ^1,3MLCT within species such as [Fe­(bpy)₃]²⁺. This result opens a new strategy for extending iron­(II) MLCT lifetimes for potential use in photoredox processes

A Synthetically Tunable System To Control MLCT Excited-State Lifetimes and Spin States in Iron(II) Polypyridines

2,2′:6′,2″-Terpyridyl (tpy) ligands modified by fluorine (dftpy), chlorine (dctpy), or bromine (dbtpy) substitution at the 6- and 6″-positions are used to synthesize a series of bis-homoleptic Fe­(II) complexes. Two of these species, [Fe­(dctpy)₂]²⁺ and [Fe­(dbtpy)₂]²⁺, which incorporate the larger dctpy and dbtpy ligands, assume a high-spin quintet ground state due to substituent-induced intramolecular strain. The smaller fluorine atoms in [Fe­(dftpy)₂]²⁺ enable spin crossover with a <i>T</i>_1/2 of 220 K and a mixture of low-spin (singlet) and high-spin (quintet) populations at room temperature. Taking advantage of this equilibrium, dynamics originating from either the singlet or quintet manifold can be explored using variable wavelength laser excitation. Pumping at 530 nm leads to ultrafast nonradiative relaxation from the singlet metal-to-ligand charge transfer (¹MLCT) excited state into a quintet metal centered state (⁵MC) as has been observed for prototypical low-spin Fe­(II) polypyridine complexes such as [Fe­(tpy)₂]²⁺. On the other hand, pumping at 400 nm excites the molecule into the quintet manifold (⁵MLCT ← ⁵MC) and leads to the observation of a greatly increased MLCT lifetime of 14.0 ps. Importantly, this measurement enables an exploration of how the lifetime of the ⁵MLCT (or ⁷MLCT, in the event of intersystem crossing) responds to the structural modifications of the series as a whole. We find that increasing the amount of steric strain serves to extend the lifetime of the ^5,7MLCT from 14.0 ps for [Fe­(dftpy)₂]²⁺ to the largest known value at 17.4 ps for [Fe­(dbtpy)₂]²⁺. These data support the design hypothesis wherein interligand steric interactions are employed to limit conformational dynamics and/or alter relative state energies, thereby slowing nonradiative loss of charge-transfer energy

Ultrafast Time-Resolved Hard X-Ray Emission Spectroscopy on a Tabletop

Experimental tools capable of monitoring both atomic and electronic structure on ultrafast (femtosecond to picosecond) time scales are needed for investigating photophysical processes fundamental to light harvesting, photocatalysis, energy and data storage, and optical display technologies. Time-resolved hard x-ray (>3  keV) spectroscopies have proven valuable for these measurements due to their elemental specificity and sensitivity to geometric and electronic structures. Here, we present the first tabletop apparatus capable of performing time-resolved x-ray emission spectroscopy. The time resolution of the apparatus is better than 6 ps. By combining a compact laser-driven plasma source with a highly efficient array of microcalorimeter x-ray detectors, we are able to observe photoinduced spin changes in an archetypal polypyridyl iron complex [Fe(2,2^{′}-bipyridine)_{3}]^{2+} and accurately measure the lifetime of the quintet spin state. Our results demonstrate that ultrafast hard x-ray emission spectroscopy is no longer confined to large facilities and now can be performed in conventional laboratories with 10 times better time resolution than at synchrotrons. Our results are enabled, in part, by a 100- to 1000-fold increase in x-ray collection efficiency compared to current techniques