Unexpected Drastic Decrease in the Excited-State Electronic Communication between Porphyrin Chromophores Covalently Linked by a Palladium(II) Bridge

• Published in: Chemistry : a European journal 2014-09, Vol.20 (40), p.12988-13001
• Main Authors: Abdelhameed, Mohammed, Karsenti, Paul-Ludovic, Langlois, Adam, Lefebvre, Jean-François, Richeter, Sébastien, Ruppert, Romain, Harvey, Pierre D.
• Format: Article
• Language: English
• Subjects: Acceptors (electronic); Bridges (structures); Carbenes; Chemical Sciences; Chemistry; Coordination chemistry; Donors (electronic); Electronics; Electrons; Energy Transfer; Förster theory; Heterocyclic Compounds - chemistry; Metalloporphyrins - chemistry; Methane - analogs & derivatives; Methane - chemistry; Models, Molecular; or physical chemistry; palladium; Palladium - chemistry; Porphyrins; Quantum Theory; singlet energy transfer; Theoretical and; Zinc; Zinc - chemistry
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201403146

A dyad built up of a zinc(II) porphyrin and the corresponding free base, [Zn‐Fb], fused to N‐heterocyclic carbene (NHCs) ligands, respectively acting as singlet energy donor and acceptor, and a bridging trans‐PdI2 unit, along with the corresponding [Zn‐Zn] and [Fb‐Fb] dimers were prepared and investigated by absorption and emission spectroscopy and density functional computations. Despite favorable structural and spectroscopic parameters, unexpectedly slow singlet energy transfer rates are measured in comparison with the predicted values by the Förster theory and those observed for other structurally related dyads. This observation is rationalized by the lack of large molecular orbital (MO) overlaps between the frontier MOs of the donor and acceptor, thus preventing a double electron exchange through the trans‐PdI2 bridge, and by an electronic shielding induced by the presence of this same linker preventing the two chromophores to fully interact via their transition dipoles. Slow down! The photophysical properties of a dyad built upon a trans PdI2 complex ligated to N‐heterocyclic carbene ligands fused to zinc(II) and free‐base porphyrins (see figure) show unexpectedly slow singlet energy transfer rates in comparison with the predicted values by the Förster theory and those observed for other structurally related dyads. Computational studies bring key information to explain this phenomenon.