Atomistic characterization of the active-site solvation dynamics of a model photocatalyst

Abstract

The interactions between the reactive excited state of molecular photocatalysts and surrounding solvent dictate reaction mechanisms and pathways, but are not readily accessible to conventional optical spectroscopic techniques. Here we report an investigation of the structural and solvation dynamics following excitation of a model photocatalytic molecular system [Ir2(dimen)4]2+, where dimen is para-diisocyanomenthane. The time-dependent structural changes in this model photocatalyst, as well as the changes in the solvation shell structure, have been measured with ultrafast diffuse X-ray scattering and simulated with Born-Oppenheimer Molecular Dynamics. Both methods provide direct access to the solute–solvent pair distribution function, enabling the solvation dynamics around the catalytically active iridium sites to be robustly characterized. Our results provide evidence for the coordination of the iridium atoms by the acetonitrile solvent and demonstrate the viability of using diffuse X-ray scattering at free-electron laser sources for studying the dynamics of photocatalysis.

Perspective: Opportunities for ultrafast science at SwissFEL

“…Haldrup et al 147 implemented this approach in a study of [Fe(bpy) 3 ] 2+ , this time looking at the solvent response upon the SCO. This type of study was recently extended to different classes of molecular solutes, such a bimetallic ones by van Driel et al , to look at the coordination of the excited solute with a solvent molecule 148 . It was also exploited by Canton et al 38 at SACLA to characterize the non-equilibrated electron transfer (ET) dynamics in donor–acceptor molecular assemblies consisting of a light-harvesting, ruthenium (Ru)-based chromophore linked to an optically dark cobalt (Co) electron sink by a bridge that mediates the ultrafast ET.…”

Vibrational wavepacket dynamics in Fe carbene photosensitizer determined with femtosecond X-ray emission and scattering

“…A powerful method to disentangle the electronic and nuclear motions of 3d transition metal complexes during ultrafast nonadiabatic processes is time-resolved 1s-2p (Ka) and 1s-3p (Kβ) X-ray Emission Spectroscopy (XES) [10][11][12][13][14] combined with Wide Angle X-ray Scattering (WAXS) [15][16][17][18][19][20][21][22][23]. These experiments have been utilized to assign the electronic and structural motions during excited state dynamics [24][25][26] and to project the locations of conical intersections between excited state potential energy surfaces onto critical structural coordinates [27].…”

Ultrafast Wiggling and Jiggling: Ir2(1,8-diisocyanomenthane)42+

“…The ν(C≡N) IR features of relaxed singlet and triplet dσ*pσ excited states are shifted by -22 and -16 cm -1 relative to the ground state. The 6 cm -1 ν(C≡N) difference, as well as the Ir-Ir distance ~0.02 Å longer in the triplet, 2,5 show that the bonding in singlet and triplet dσ*pσ states is similar but not identical (as often assumed).…”

“…1,2,3,4 Of particular interest is that the slow rates of excited singlet to triplet intersystem crossing (ISC) have allowed workers to detect and characterize the two spin states separately. 3,5,6 Many of these complexes also are redox-active, most especially as powerful reductants and oxidants in photoinduced electron-and atom transfer reactions.…”

“…4,8 Similarly, shortening of the Ir-Ir bond in singlet and triplet dσ*pσ states of [Ir 2 (1,8-diisocyanomenthane) 4 ] 2+ (Ir(dimen), Figure 1) was revealed by time-resolved X-ray scattering. 2,5 Femtosecond laser excitation of these complexes creates a ν(M-M) wave packet whose coherent motion is manifested by oscillations of the stimulated emission (Pt, Ir) and excited-state absorption (Pt) that persist for more than 2 ps. 3 In addition to X-ray and optical spectroscopic studies, excited states of isocyanidebridged complexes are candidates for time-resolved IR (TRIR), which monitors the temporal evolution of features attributable to C≡N-stretching vibrations (ν(C≡N)).…”