The Effect of Ligand Constraints on the Metal-to-Ligand Charge-Tranfer Relaxation Dynamics of Copper(I)−Phenanthroline Complexes:  A Comparative Study by Femtosecond Time-Resolved Spectroscopy

Abstract

Two geometrically constrained Cu(I)-monophenanthroline complexes were investigated spectroscopically to study the effects that ligand constraints have on the excited-state dynamics of copper-phenanthroline complexes. Room temperature steady-state absorption and emission spectra were obtained for the two complexes and time-resolved fluorescence spectra of the Cu(I) complexes in dichloromethane were collected using time-correlated single photon counting. The effect of the ligands on the excited-state dynamics of the two complexes was monitored by femtosecond pump-probe spectroscopy. Transient absorption spectroscopy with femtosecond resolution enabled the direct observation of the relaxation in the photoinduced metal-toligand charge-transfer (MLCT) state of the complexes, where the tetrahedral Franck-Condon Cu(II) complex undergoes vibrational relaxation to form the energetically favorable square planar conformation. The flattening relaxation was found to have two lifetime components of 8-10 and 40-52 ps for the rearrangement of the 1:1 copper-phenanthroline complexes, which is likely related to the flattening around the metal center and the subsequent rearrangement of the methylphenyl-pyrazole backbone according to the more planar conformation, respectively. The excited-state dynamics correlate with the measured UV-vis absorption and emission properties of the complexes and are discussed in the light of steric constraints on the photophysics of the Cu(I)-monophenanthroline complexes. The higher quantum yield, longer emission lifetime, and faster relaxation dynamics of the dimethyl-substituted pyrazole-based complex compared with the unmethylated analogue agree with the steric arguments that in a 1:1 ligand framework, addition of alkyl substituents in the ligand hinders the flattening relaxation. This investigation is complementing photophysical studies performed over the past two decades on the effect of substitutents on the photophysics of the structurally related 1:2 Cu(I)-phenanthroline complexes by providing a time-resolved spectroscopic observation of the excited-state relaxation in geometrically constrained Cu(I)-phenanthroline complexes following the photoinduced MLCT.

Strong Steric Hindrance Effect on Excited State Structural Dynamics of Cu(I) Diimine Complexes

“…One important aspect of this research involves determining the relationship between the structural factors and energy differences as well as the interconversion rates of the singlet, triplet, and ground states in this family of Cu(I) diimine complexes. Some less sterically hindered Cu phenanthrolines have already been extensively studied, such as [Cu I (phen) 2 ] + and [Cu I (dmp) 2 ] + . ,,, [Cu I (phen) 2 ] + has an excited state lifetime of 1.5 ps; due to the lack of bulky substituents at the 2,9 positions of the phenanthroline, this complex is subject to quick exciplex quenching. In [Cu I (dmp) 2 ] + , however, the methyl groups at the 2,9 positions provide some protection against ligand attack to the Cu center, and therefore has a longer excited state lifetime of ∼100 ns in dichloromethane .…”

Kinetics of Fluorophore Formation in Bovine Serum Albumin–Gold Complexes

“…Identification of the mechanism of fluorophore formation in the BSA–Au complex is a critical step toward understanding the mechanism for the large Stokes shift, the energy transfer, and potentially, the local charge transfer. − …”

Recoverable fluorescence chemosensors for Ni2+ ions based on hydrogen-bonded side-chain copolymers presenting pendent benzoic acid and pyridyl receptor units

“…Heavy metal ions tend to quench the luminescence of conjugated receptors, including polymers and monomers, through electrontransfer and energy-transfer processes. [21][22][23][24] Comparing the fluorescence quenching spectra titrated with Ni 2+ ions in Fig. 4 and 6, emergence of the s 1 decay component in the P1 + Ni 2+ system clearly indicates that its TRF traces featured two contributing factors: one (s 2 ) from free P1 and the other (s 1 ) from its complex.…”

“…The observed fluorescence quenching phenomena of the ligands presumably reflect excitation energy transfer from the ligands to the metal ion's d orbital. 23,24 Therefore, with the addition of CDs, the disrupted hydrogen bonding between the pyridyl receptor moieties (of the proton-acceptor M1 units) and the benzoic acid moieties (of the proton-donor M2 units) in the copolymers enhanced the fluorescence quenching efficiencies of the Ni 2+ ions in the fluorescence decay experiments, thereby facilitating the use of supramolecular side-chain copolymers as recoverable and/or adjustable chemosensors.…”