Site-Selective Orbital Interactions in an Ultrathin Iron-Carbene Photosensitizer Film

We present the first experimental study of the frontier orbitals in an ultrathin film of the novel hexa-carbene photosensitizer [Fe(btz)3]3+, where btz is 3,3′-dimethyl-1,1′-bis(p-tolyl)-4,4′-bis(1,2,3-triazol-5-ylidene). Resonant photoelectron spectroscopy (RPES) was used to probe the electronic structure of films where the molecular and oxidative integrities had been confirmed with optical and X-ray spectroscopies. In combination with density functional theory calculations, RPES measurements provided direct and site-selective information about localization and interactions of occupied and unoccupied molecular orbitals. Fe 2p, N 1s, and C 1s measurements selectively probed the metal, carbene, and side-group contributions revealing strong metal–ligand orbital mixing of the frontier orbitals. This helps explain the remarkable photophysical properties of iron-carbenes in terms of unconventional electronic structure properties and favorable metal–ligand bonding interactions—important for the continued development of these type of complexes toward light-harvesting and light-emitting applications

Manipulating charge transfer excited state relaxation and spin crossover in iron coordination complexes with ligand substitution

Developing light-harvesting and photocatalytic molecules made with iron could provide a cost effective, scalable, and environmentally benign path for solar energy conversion. To date these developments have been limited by the sub-picosecond metal-to-ligand charge transfer (MLCT) electronic excited state lifetime of iron based complexes due to spin crossover-the extremely fast intersystem crossing and internal conversion to high spin metal-centered excited states. We revitalize a 30 year old synthetic strategy for extending the MLCT excited state lifetimes of iron complexes by making mixed ligand iron complexes with four cyanide (CN-;) ligands and one 2,2′-bipyridine (bpy) ligand. This enables MLCT excited state and metal-centered excited state energies to be manipulated with partial independence and provides a path to suppressing spin crossover. We have combined X-ray Free-Electron Laser (XFEL) Kβ hard X-ray fluorescence spectroscopy with femtosecond time-resolved UV-visible absorption spectroscopy to characterize the electronic excited state dynamics initiated by MLCT excitation of [Fe(CN)4(bpy)]2-. The two experimental techniques are highly complementary; the time-resolved UV-visible measurement probes allowed electronic transitions between valence states making it sensitive to ligand-centered electronic states such as MLCT states, whereas the Kβ fluorescence spectroscopy provides a sensitive measure of changes in the Fe spin state characteristic of metal-centered excited states. We conclude that the MLCT excited state of [Fe(CN)4(bpy)]2- decays with roughly a 20 ps lifetime without undergoing spin crossover, exceeding the MLCT excited state lifetime of [Fe(2,2′-bipyridine)3]2+ by more than two orders of magnitude

Influence of Triplet Surface Properties on Excited-State Deactivation of Expanded Cage Bis(tridentate)Ruthenium(II) Complexes

Calculations of excited-state potential energy surfaces (PESs) are useful to predict key properties relating to the deactivation cascade of transition-metal complexes. Here, we first perform full free optimizations of the relevant excited-state minima, followed by extensive two-dimensional PES calculations based on the minima of interest. Maps of the lowest triplet excited-state surfaces of two bistridentate RuII-complexes, [Ru(DQP)2]2+ and [Ru(DQzP)2]2+, are used to explain recent experimental findings including an unexpected order of magnitude difference in an excited-state lifetime. The calculations reveal significant differences in the fundamental shapes and spin transitions of the lowest triplet excited-state energy surfaces of the two complexes and, in particular, show that the metal-to-ligand charge transfer (MLCT) surface region of [Ru(DQzP)2]2+ with a shorter excited-state lifetime is significantly smaller than that of [Ru(DQP)2]2+. This leads to a minimum energy crossing between the triplet and singlet surfaces near the MLCT for [Ru(DQzP)2]2+ or near the MC for [Ru(DQP)2]2+. These results indicate that the experimentally observed difference in the excited-state lifetime is closely related to the set of energetically accessible 3MLCT conformations

Computational characterization of competing energy and electron transfer states in bimetallic donor-acceptor systems for photocatalytic conversion

The rapidly growing interest in photocatalytic systems for direct solar fuel production such as hydrogen generation from water splitting is grounded in the unique opportunity to achieve charge separation in molecular systems provided by electron transfer processes. In general, both photoinduced and catalytic processes involve complicated dynamics that depend on both structural and electronic effects. Here the excited state landscape of metal centered light harvester-catalyst pairs is explored using density functional theory calculations. In weakly bound systems, the interplay between structural and electronic factors involved can be constructed from the various mononuclear relaxed excited states. For this study, supramolecular states of electron transfer and excitation energy transfer character have been constructed from constituent full optimizations of multiple charge/spin states for a set of three Ru-based light harvesters and nine transition metal catalysts (based on Ru, Rh, Re, Pd, and Co) in terms of energy, structure, and electronic properties. The complete set of combined charge-spin states for each donor-acceptor system provides information about the competition of excited state energy transfer states with the catalytically active electron transfer states, enabling the identification of the most promising candidates for photocatalytic applications from this perspective

Predicting Structures of Ru-Centered Dyes: A Computational Screening Tool

Dye-sensitized solar cells (DSCs) represent a means for harvesting solar energy to produce electrical power. Though a number of light harvesting dyes are in use, the search continues for more efficient and effective compounds to make commercially viable DSCs a reality. Computational methods have been increasingly applied to understand the dyes currently in use and to aid in the search for improved light harvesting compounds. Semiempirical quantum chemistry methods have a well-deserved reputation for giving good quality results in a very short amount of computer time. The most recent semiempirical models such as PM6 and PM7 are parametrized for a wide variety of molecule types, including organometallic complexes similar to DSC chromophores. In this article, the performance of PM6 is tested against a set of 20 molecules whose geometries were optimized using a density functional theory (DFT) method. It is found that PM6 gives geometries that are in good agreement with the optimized DFT structures. In order to reduce the differences between geometries optimized using PM6 and geometries optimized using DFT, the PM6 basis set parameters have been optimized for a subset of the molecules. It is found that it is sufficient to optimize the basis set for Ru alone to improve the agreement between the PM6 results and the DFT results. When this optimized Ru basis set is used, the mean unsigned error in Ru–ligand bond lengths is reduced from 0.043 to 0.017 Å in the set of 20 test molecules. Though the magnitude of these differences is small, the effect on the calculated UV/vis spectra is significant. These results clearly demonstrate the value of using PM6 to screen DSC chromophores as well as the value of optimizing PM6 basis set parameters for a specific set of molecules

Supramolecular Hemicage Cobalt Mediators for Dye-Sensitized Solar Cells

A new class of dye-sensitized solar cells (DSSCs) using the hemicage cobalt-based mediator [Co(ttb)](2+/3+) with the highly preorganized hexadentate ligand 5,5 '', 5 ''''-((2,4,6-triethyl benzene-1,3,5-triyl)tris(ethane-2,1-diyl))tri-2,2'-bipyridine (ttb) has been fully investigated. The performances of DSSCs sensitized with organic D-p-A dyes utilizing either [Co(ttb)](2+/3+) or the conventional [Co(bpy)(3)](2+/3+) (bpy = 2,2'-bipyridine) redox mediator are comparable under 1000 Wm(-2) AM 1.5 G illumination. However, the hemicage complexes exhibit exceptional stability under thermal and light stress. In particular, a 120-hour continuous light illumination stability test for DSSCs using [Co(ttb)](2+/3+) resulted in a 10% increase in the performance, whereas a 40% decrease in performance was found for [Co(bpy)(3)](2+/3+) electrolyte-based DSSCs under the same conditions. These results demonstrate the great promise of [Co(ttb)](2+/3+) complexes as redox mediators for efficient, cost-effective, large-scale DSSC devices