Energy Dispersive XAFS: Characterization of Electronically Excited States of Copper(I) Complexes

Abstract

Energy dispersive X-ray absorption spectroscopy (ED-XAS), in which the whole XAS spectrum is acquired simultaneously, has been applied to reduce the real-time for acquisition of spectra of photoinduced excited states by using a germanium microstrip detector gated around one X-ray bunch of the ESRF (100 ps). Cu K-edge XAS was used to investigate the MLCT states of [Cu(dmp)2]+ (dmp =2,9-dimethyl-1,10-phenanthroline) and [Cu(dbtmp)2]+ (dbtmp =2,9-di-n-butyl-3,4,7,8-tetramethyl-1,10-phenanthroline) with the excited states created by excitation at 450 nm (10 Hz). The decay of the longer lived complex with bulky ligands, was monitored for up to 100 ns. DFT calculations of the longer lived MLCT excited state of [Cu(dbp)2]+ (dbp =2,9-di-n-butyl-1,10-phenanthroline) with the bulkier diimine ligands, indicated that the excited state behaves as a Jahn–Teller distorted Cu(II) site, with the interligand dihedral angle changing from 83 to 60° as the tetrahedral coordination geometry flattens and a reduction in the Cu–N distance of 0.03 Å.

The Time-resolved and Extreme-conditions XAS (TEXAS) facility at the European Synchrotron Radiation Facility: the energy-dispersive X-ray absorption spectroscopy beamline ID24

“…recorded under more challenging conditions on ID24. For example, the local structure in the initial and final state of a system undergoing a fast reaction can be elucidated from BM23 data, whereas the signature of a short-lived intermediate state can be detected from ID24 data (Tromp et al, 2013). The two beamlines BM23 and ID24 therefore complement each other, and together provide comprehensive experimental capabilities for time-resolved and extremeconditions XAS.…”

Time-resolved X-ray absorption spectroscopy for the study of molecular systems relevant for artificial photosynthesis

“…Some details of this discussion can be found in a recent review [81]. Our modeling of the transient spectra indicates that [Cu(dmp) 2 ] + in acetonitrile forms the exciplex [83] while [Cu(dbp) 2 ] + remains 4-coordinated in the same solvent [61] even if the experimental spectra appear similar from a first glance.…”

“…A similar analysis was performed for [Cu(dbp) 2 ] + (dbp = 2,9-di-n-butyl-1,10-phenanthroline) and [Cu(dbtmp) 2 ] + (dbtmp = 2,9-di-n-butyl-3,4,7,8-tetramethyl-1,10-phenanthroline) [61]. Long chains at the 2 and 9 positions (see structural models in Fig.…”

“…At high concentrations aggregation and self-quenching processes can occur and the lifetime of the excited state can change significantly. An example of such behavior can be found in [61] where measurements of X-ray absorption spectra were performed in transmission mode using an energy-dispersive setup.…”

“…The excited state lifetime of [Cu(dbtmp) 2 ] + in acetonitrile (181 ns) is much longer than that of [Cu(dmp) 2 ] + (1.7 ns), suggesting that blocking solvent access to the Cu-center is a good strategy to develop Cu-photosensitizers with longer excited state lifetime. Reproduced with the permission of authors from [61].…”