Cooperative Substituent Effects on the Excited States of Copper Phenanthrolines

Abstract

Copper phenanthrolines are attractive as potential photosensitizers because of the ready availability of the metal, but efficient nonradiative decay including a solvent-induced quenching phenomenon ordinarily limits their utility. However, the present studies show that the addition of methyl substituents in the 3,8-positions of 1,10-phenanthroline can enhance the protective effect that bulky groups in the 2,9-positions have on the reactive charge-transfer excited state of a bis-ligand copper(I) derivative. Thus, the photoexcited Cu(dbtmp)(2)(+) complex has a lifetime of 920 ns in dichloromethane, whereas the parent complex without the methyl substituents has a lifetime of only 150 ns under the same conditions (dbtmp = 2,9-di-n-butyl-3,4,7,8-tetramethyl-1,10-phenanthroline). In dichloromethane, the complex with the 2,9-diphenyl-3,4,7,8-tetramethyl-1,10-phenanthroline ligand also exhibits a long lifetime (480 ns). Even more importantly, the latter combination of substituents appears to eliminate the problem of solvent-induced exciplex quenching.

Ultrafast Structural Rearrangements in the MLCT Excited State for Copper(I) bis-Phenanthrolines in Solution

“…Copper(I) diimine coordination complexes (denoted [Cu I (NN) 2 ] + ) have a long, rich history that parallels that of their transition-metal cousins, polypyridyl ruthenium(II) systems [1][2][3][4][5][6][7][8][9][10][11] . Current interest is driven by a desire to employ transition metal coordination complexes in applications from solar energy conversion 12 to chemical sensing 13,14 and molecular devices.…”

“…In particular, cuprous diimine complexes have been considered as potential substitutes for ruthenium (II) and osmium(II) systems. 18 McMillin and coworkers 3,[19][20][21][22][23][24] performed pioneering work for elucidating the unique photophysics and photochemistry of [Cu I (NN) 2 ] + complexes and developed a fundamental picture of the photophysics occurring on the nanosecond timescale. 6 Their experimental observations led to remarkable insights into the properties of these systems in their excited states.…”

“…McMillin recognized the potential for JahnTeller distortion causing a structural rearrangement from the tetrahedral ground state to square planar or trigonal bipyramidal geometries following a metal-to-ligand-charge-transfer (MLCT) excitation in [Cu I (NN) 2 ] + complexes. 3,[23][24][25] Also, he proposed the model for exciplex quenching by Lewis bases to explain solvent-dependent luminescence and excited state lifetimes. 3,19,26 Despite the lack of any direct structural evidence, the exciplex formation hypothesis was supported by the correlation between accessibility of the cuprous center by the solvent and the luminescence lifetime, and was widely accepted.…”

“…3,[23][24][25] Also, he proposed the model for exciplex quenching by Lewis bases to explain solvent-dependent luminescence and excited state lifetimes. 3,19,26 Despite the lack of any direct structural evidence, the exciplex formation hypothesis was supported by the correlation between accessibility of the cuprous center by the solvent and the luminescence lifetime, and was widely accepted. 3,19,[27][28][29] Recent contributions from our laboratory using laser-initiated time-resolved x-ray absorption spectroscopy (LITR-XAS) have confirmed the presence of additional coordination to the copper center by a solvent molecule (or counter ion) for photoexcited [Cu I (dmp) 2 ] + in both noncoordinating toluene 30 and strongly coordinating acetonitrile.…”

“…3,19,26 Despite the lack of any direct structural evidence, the exciplex formation hypothesis was supported by the correlation between accessibility of the cuprous center by the solvent and the luminescence lifetime, and was widely accepted. 3,19,[27][28][29] Recent contributions from our laboratory using laser-initiated time-resolved x-ray absorption spectroscopy (LITR-XAS) have confirmed the presence of additional coordination to the copper center by a solvent molecule (or counter ion) for photoexcited [Cu I (dmp) 2 ] + in both noncoordinating toluene 30 and strongly coordinating acetonitrile. 31 The interaction strength with the solvent molecule is much weaker in the former and stronger in the latter and is supported by the differentiation of the average copper to ligand distance in the thermally equilibrated 3 MLCT state.…”