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Transient absorption dynamics of sterically congested Cu(I) MLCT excited states
Sofia Garakyaraghi1, Evgeny O Danilov1, Catherine E McCusker1
1Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204, United States.
The Journal of Physical Chemistry. A
|March 10, 2015
Summary
Investigating copper(I) metal-to-ligand charge transfer (MLCT) photosensitizers, this study reveals that impeding structural distortion in the excited state accelerates deactivation dynamics. Faster excited-state decay and longer lifetimes were observed in molecules with inhibited distortion, like [Cu(dsbtmp)2](+).
Area of Science:
- Photochemistry and Photophysics
- Inorganic Chemistry
- Materials Science
Background:
- Homoleptic Cu(I) metal-to-ligand charge transfer (MLCT) photosensitizers are crucial for various light-driven applications.
- Understanding the excited-state dynamics of these complexes is essential for optimizing their performance.
- Previous studies have investigated related copper complexes, providing a foundation for this research.
Purpose of the Study:
- To investigate the subpicosecond to supra-nanosecond transient absorption dynamics of homoleptic Cu(I) MLCT photosensitizers.
- To elucidate the relationship between excited-state structural distortion and deactivation pathways.
- To identify redox-specific spectroscopic signatures and their influence on excited-state dynamics.
Main Methods:
- Transient absorption spectroscopy utilizing four excitation wavelengths (418-530 nm) and broad probe wavelengths (350-1450 nm).
- Spectroelectrochemical measurements of singly reduced species in tetrahydrofuran.
- Investigation of complexes including [Cu(dmp)2](+), [Cu(dsbp)2](+), and [Cu(dsbtmp)2](+) in dichloromethane and tetrahydrofuran.
Main Results:
- Excited-state decay was found to be independent of excitation wavelength and largely unaffected by solvent in the initial stages.
- Two time constants were observed, with both components lengthening as a function of structural distortion: [Cu(dsbtmp)2](+) < [Cu(dsbp)2](+) < [Cu(dmp)2](+).
- The complex with the most inhibited distortion, [Cu(dsbtmp)2](+), exhibited the fastest ultrafast dynamics and longest excited-state lifetimes.
Conclusions:
- Strongly impeded structural distortion in the Cu(I) MLCT excited state enables more rapid surface crossings in deactivation dynamics.
- The observed dynamics suggest a small degree of excited-state distortion, rapid intersystem crossing, and weak vibronic coupling.
- Initial time constants assigned to pseudo-Jahn-Teller distortion and intersystem crossing are intimately coupled and vary systematically with molecular structure.
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