Newly synthesized ruthenium based photosensitizers with an extended π-system were found to exhibit long lived excited states with lifetimes of 1.7 and 24.7 µs after optical excitation. The applied biipo ligand coordinates via a 1,10 phenanthroline moiety, which is extended with a 1,8 naphthalimide unit.[1] In order to replace the rare noble metal, two copper based analogues were developed, see figure. Nanosecond transient absorption measurements revealed, that in the Cu complexes, non-emissive excited states are populated which exhibit even much longer lifetimes.

In the present work, we investigate these complexes and the plain ligands by femtosecond transient absorption spectroscopy and observe rich intramolecular relaxation dynamics. The decay associated spectra of the Cu complexes are comparable to other compounds that were previously investigated.[2][3] The shortest lifetime of ≈0.3 ps can be assigned to a flattening of the geometrical structure of the complex after optical excitation. The second exponential component of ≈3.3 ps reflects probably intersystem crossing to a triplet metal-to-ligand charge transfer state (MLCT). However, the associated spectral signatures decay again with a time constant of about 10 ps. This may point to a transfer of the excited electron from the MLCT-state to a plain ligand-centered state. A remaining long-lived absorption has the same spectral shape as the signals observed by the nanosecond experiments indicating that this state is the finally populated electronically excited state.

[1]: Yang et al., Chem. Eur. J., 2020, 10.1002/chem.202001564.
[2]: S. Tschierlei, et al., Chem. Phys. Chem., 2014, 15, 17, 3709-3713, 10.1002/cphc.201402585.
[3]: M. Heberle, et al., Chem. Eur. J., 2017, 23, 2, 312-319, 10.1002/chem.201604005.