Restricted Photochemistry in the Molecular Solid State: Structural Changes on Photoexcitation of Cu(I) Phenanthroline Metal-to-Ligand Charge Transfer (MLCT) Complexes by Time-Resolved Diffraction

Abstract

The excited state structure of [Cu(1)[(1,10-phenanthroline-N,N’) bis(triphenylphosphine)] cations in their crystalline [BF4] salt has been determined at both 180 and 90K by single-pulse time-resolved synchrotron experiments with the modified polychromatic Laue method. The two independent molecules in the crystal show distortions on MLCT excitation which differ in magnitude and direction, a difference attributed to a pronounced difference in the molecular environment of the two complexes. As the excited states differ, the decay of the emission is bi-exponential with two strongly different lifetimes, the longer lifetime, assigned to the more restricted molecule, becoming more prevalent as the temperature increases. Standard deviations in the current Laue study are very much lower than those achieved in a previous monochromatic study of a Cu(I) 2,9 dimethyl-phenanthroline substituted complex (J. Am. Chem. Soc. 2009, 131, 6566), but the magnitude of the shifts on excitation is similar, indicating that lattice restrictions dominate over the steric effect of the methyl substitution. Above all the study illustrates emphatically that molecules in solids have physical properties different from those of isolated molecules and that their properties depend on the specific molecular environment. This conclusion is relevant for the understanding of the properties of molecular solid state devices which are increasingly used in current technology.

Synthesis, Structural Characterisation, and Spectroscopic Properties of Copper(I) Complexes and their Applications to Dye-Sensitised Solar Cells

“…[38,39] Broad absorption bands in the range of 325-450 nm with a molar extinction coefficient (e) of 4000-20000 M À1 cm À1 are attributed to dp-p* metal to ligand charge transfer (MLCT) absorptions. [40] The MLCT absorption maxima of these complexes, as reported previously, [41][42][43][44] are observed at slightly longer wavelengths. The lowest energy bands display that compounds 1 and 2 have the same absorbent peak at 345 nm, showing limited dependence on the identity of bridging ligands.…”

Photocrystallographic Observation of Halide-Bridged Intermediates in Halogen Photoeliminations

“…51 To probe the impact of restricted isocyanide ligand dissociation, the geometry of Rh 2 [I,III] structure A was evaluated as a function of M–L bond length. The results are tabulated in Table 2 and show that small perturbations of the M–L bond length are sufficient to induce nascent migration of a proximal Cl ligand toward a bridging position.…”

Perspective: On the relevance of slower-than-femtosecond time scales in chemical structural-dynamics studies

“…Such chemically equivalent, but crystallographically independent molecules can show different distortions on excitation and differences in wavelength and lifetime of their luminescence (Makal et al , 2012; Coppens et al , 2013). Comparison of results of isolated-molecule calculations with calculations in which the lattice is taken into account confirms such differences.…”

“…Other studies involve intramolecular electron transfer (MLCT, metal-to-ligand charge transfer), in a number of highly luminescent Cu(I) dimethyl-phenanthroline photosensitizer complexes (Vorontsov et al , 2009; Makal et al , 2012). Such complexes are typical for use as light-emitting diodes (Armaroli et al , 2006; Yersin et al , 2011).…”