Photoinduced structural distortions and singlet–triplet intersystem crossing in Cu(I) MLCT excited states monitored by optically gated fluorescence spectroscopy

Transition metals are interesting for a number of aspects with regards to solar energy conversion applications. They have large spin-orbit coupling constants, which provide for a mechanism to induce intersystem crossing to long-lived triplet states. This makes them suitable for the manufacture of photosensitizers in applications that demand for bimolecular energy or electron transfer. High density of states and spin-orbit coupling are necessary for the convoluted and fast excited state dynamics.

Although a successful conversion of solar energy into chemical potential is dependent on the cascade of events that are subsequent to the preliminary absorptive event, an in-depth understanding of the photo physical processes that occur is important for ideal implementation and design of any photosensitizer. In fact, ultrafast spectroscopic measurements are necessary since they provide the time resolution needed for observation of the primary events subsequent to photoexcitation.

Copper(I) bis-phenanthroline complexes have been focused as appropriate alternatives to the ubiquitous Ru(II) tri-bipyridine derivatives considering that they possess features that are highly suitable for photosensitizers and they contain earth-abundant transition metal. These Cu(I) phenanthroline complexes have strong metal-to-ligand charge transfer absorptions spanning the visible spectral region. They as well feature triplet metal-to-ligand charge transfer states with microsecond lifetimes as well as temperature photoluminescence, all of which are necessary for supporting bimolecular energy and electron transfer. Therefore, these copper complexes have been identified as appropriate in the pursuit for designing sustainable sensitizers with a capacity for broad implementation in the real applications.

Copper(I) phenanthroline metal-to-ligand charge transfer complexes experience considerable photo-induced structural rearrangements.  The excited state geometric distortions result in excited states that are susceptible to exciplex formation. The exciplex formation is responsible for nonradiative decay channels, which deactivate the excited state while shortening the lifetime. Therefore, this limits the effectiveness of Cu(I) complexes as sensitizers.

Researchers led by Professor Felix Castellano at the North Carolina State University performed a detailed analysis of the photo-induced structural distortions as well as singlet-triplet intersystem crossing dynamics of a number of sterically encumbered Cu(I) phenanthroline chromophores. Their research work is published in journal, Physical Chemistry Chemical Physics.

The authors investigated for a string of sterically encumbered Cu(I) metal-to-ligand charge transfer complexes, the sub-picosecond time resolved emission study of the photo-induced rearrangement as well as subsequent singlet-triplet intersystem crossings. By monitoring the singlet photoluminescence decay, the authors were able to obtain a direct measure of the singlet-triplet intersystem crossing as well as an insight to the additional dynamic pegged to the changes in the singlet state surface.

The experimental results indicated strong wavelength dependence of the singlet emission, with fast sub-pico-second decay dominating at higher energies. The research team also observed that at lower emission energies, raising the contribution of a longer decay component was evident. The aspect of wavelength dependence was a footprint of the excited state structural rearrangement of the phenanthroline ligands that lowered the excited state energy.

The sub-picosecond component was linked to the photo-induced structural rearrangement that reduced the energy of the singlet excited state. The longer decay component represented the lifetime of the S₁ excited state, and therefore the time-scale of the single-triplet intersystem crossing.