Solvent-Induced Luminescence Quenching: Static and Time-Resolved X-Ray Absorption Spectroscopy of a Copper(I) Phenanthroline Complex

Abstract

We present a static and picosecond X-ray absorption study at the Cu K-edge of bis(2,9-dimethyl-1,10-phenanthroline)copper(I) ([Cu(dmp)2](+); dmp = 2,9-dimethyl-1,10-phenanthroline) dissolved in acetonitrile and dichloromethane. The steady-state photoluminescence spectra in dichloromethane and acetonitrile are also presented and show a shift to longer wavelengths for the latter, which points to a stronger stabilization of the excited complex. The fine structure features of the static and transient X-ray spectra allow an unambiguous assignment of the electronic and geometric structure of the molecule in both its ground and excited (3)MLCT states. Importantly, the transient spectra are remarkably similar for both solvents, and the spectral changes can be rationalized using the optimized ground- and excited-state structures of the complex. The proposed assignment of the lifetime shortening of the excited state in donor solvents (acetonitrile) to a metal-centered exciplex is not corroborated here. Molecular dynamics simulations confirm the lack of complexation; however, in both solvents the molecules come close to the metal but undergo rapid exchange with the bulk. The shortening of the lifetime of the title complex and nine additional related complexes can be rationalized by the decrease in the (3)MLCT energy. Deviations from this trend may be explained by means of the effects of the dihedral angle between the ligand planes, the solvent, and the (3)MLCT-(1)MLCT energy gap.

Photoinduced Structural Dynamics of Molecular Systems Mapped by Time-Resolved X-ray Methods

“…In particular, the new (positive) A′ feature arises, as expected, from a transition into the electron hole at the HOMO created by photoexcitation. Since the HOMO is dominated by the Cu 3d-character, 182 this feature has previously been attributed to a 1s→3d quadrupole transition. 186 The lack of solvent coordination was supported by calculations of the radial distribution functions (RDFs) from a classical molecular dynamics simulation for both the ground and lowest 3 MLCT states of the complex in both solvents.…”

“…182 , the A and B features of the ground state spectrum were analyzed using TD-DFT. The A feature is dominated by transitions into the lowest unoccupied molecular orbital (LUMO) and LUMO+1 orbitals.…”

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

“…Time-resolved X-ray absorption experiments on photosensitizers can be performed at synchrotrons using the pump-probe method [28,39,[52][53][54][55][56]. The time resolution of such measurements is ∼100 ps, limited by the duration of the X-ray pulse.…”

“…Regarding the interpretation of time-resolved XAS data of the Cu-complexes there is an ongoing discussion as to why the transient X-ray absorption spectra of [Cu(dmp) 2 ] + in coordinating and non-coordinating solvents are similar [81]. Experimental time-resolved XANES spectra of very good quality have been obtained independently by two groups [53,54,56]. The discussion is mainly based on the results obtained using complementary techniques rather than on quantitative calculations of the transient XANES spectra.…”

Long-Lived LMCT in a d0 Vanadium(V) Complex by Internal Conversion to a State of 3dxy Character

“…The lack of luminescence in this compound supports identifying the long-lived component to electronic, rather than spin, contributions and differentiates this compound from the luminescent MLCT's discussed above. 19,20 Importantly, the results show that a charge transfer state, nearby to the main LMCT absorption and of different 3d character, that has poor optical and vibrational coupling to the ground state can collect excited 3d electrons and thereby significantly lengthen the charge transfer lifetime in a 3d metal-centered LMCT chromophore. The long lifetime, in conjunction with the strong visible absorption, suggests its use as a chromophore for energy conversion applications; particularly applications that exploit the LMCT character where reduction occurs from a strongly reducing metal center.…”