Excited state energy and redox potentials

Electronic excited states can be considered as new chemical species with unique chemical and physical properties. In general complexes in the excited state are more powerful oxidants and reductants than the ground state. The enhanced reactivity can be explained as follows. Formation of an electronically excited state involves transfer of an electron from a molecular orbital to a higher energy one. The hole left behind in this m.o and the odd electron in the upper m.o confer enhanced redox reactivity to the electronically excited state. Thus redox reactivity will be there irrespective of the nature of electronic transition. Enhanced redox reactivity in the excited state has been observed for all three types of excited states, e.g., Ru(bpy)32+ for CT, Cr(bpy)33+ for MC and Ir(bpy)33+ for LC type. 

Establishment of the mechanism and practical applications of excited state energy- and electron transfer processes require precise information on the energies and redox potentials for oxidation and reduction processes in the ground and excited state. Onset of the luminescence is a direct way of determining the excited state energies. Due to possible solvent-induced excited state relaxation effects that can occur at ambient temperature. 

The excited state redox potentials are obtained from the redox potentials of the corresponding ground state and the excited state energy E q q- The redox potential for the excited state to act as an oxidant, E(M /M ), is obtained from the first oxidation potential E0(M -/M) and spectroscopic excited state energy E o,o- 

The inter-relationships of ground and excited state redox potentials are best illustrated in terms of Latimer-diagrams, as shown in fig.3 for Ru(bpy)32 .  

Although the doublet state is a better reducing agent than the ground state, in view of its low redox potential (E =-0.1 V), it cannot be viewed as a practical excited state reductant 

---

Whether or not a given photoredox reaction can occur spontaneously depends on the free energy change AG for the process. In simple terms, Weller s expression relates the free energy change AG with the excited state energy and redox potentials of the redox couples involved ... 

The complex Os(bpy)3 has an excited state lifetime of 19 ns in aqueous solution and excited state potentials of -0.96 V and 0.59 V for (M /M ) and E (M /M"), respectively. This excited state lifetime is considerably shorter than that of Ru(bpy)3, which is to be expected on the basis of spin-orbit quenching and from the excited state redox potentials—Os(bpy)3 is a stronger reductant but a weaker oxidant than is Ru(bpy)3. When one or more of the bpy ligands are substituted by ligands L, systematic changes in the excited state energies and lifetimes can be induced as has been done for the ruthenium analogues. ... 

The chemical association of the exciplex results from an attraction between the excited-state molecule and the ground-state molecule, brought about by a transfer of electronic charge between the molecules. Thus exciplexes are polar species, whereas excimers are nonpolar. Evidence for the charge-transfer nature of exciplexes in nonpolar solvents is provided by the strong linear correlation between the energy of the photons involved in exciplex emission and the redox potentials of the components. 

Polarography and ESR data provide important information about the energies and electron distribution of the excited states of annelated benzenes. " By incorporating rehybridization effects into the Hiickel model of electron densities, a correlation between ring strain, experimental spin densities, and redox potentials is obtained for a series of naphthalenes and naphthoquinones. These studies provide further support for ring-strain induced rehybridization. 

The change in free energy (AG) for electron-transfer (ET) reactions is given by an empirical relation (Eq. 11 Ej is the excited-state energy, Eqx and E ed are the one-electron redox potentials of donor and acceptor, respectively, and is a... 

In addition to photolabilization processes, the LF excited state can be engaged in energy and electron transfer processes. The rate of the electron transfer in the LF excited state can be much faster than that in the ground state. The acceleration is a consequence of the difference between the redox potential of the LF excited state, e°, and that of the ground state, s°. The two potentials are related by Equation 6.98. 

Fig. 6. Energy levels and redox potentials of Ru(bpy) + in its excited state and ground states...

---