Making Chemical Bonds with Visible Light

Photo-initiated reactions that form chemical bonds are important for the conversion and storage of solar energy.  Iodide photo-oxidation yields the I-I bonds present in I₂^-• and I₃^- but the detailed mechanism(s) remain speculative.  Here we present evidence that metal-to-ligand charge transfer (MLCT) excited states can be used to facilitate the formation and breaking of I-I bonds.   Ru(II) compounds based on 2,2’-bipyrazine (bpz) were efficiently quenched by iodide in acetonitrile.  Evidence that this quenching results in iodide oxidation to the iodine atom has recently been reported utilizing Ru(bpz)₂(deeb)^2+* as the photooxidant.   In these studies, the MLCT excited state of Ru(bpz)₂(deeb)²⁺  (E^o(Ru^2+*/+) = 1.6 V vs. NHE), where deeb is 4,4'-(CO₂CH₂CH₃)₂-2,2'-bipyridine,  was quenched by iodide in acetonitrile by a purely dynamic process with a rate constant very close to that expected for a diffusion controlled reaction, 6.6 x 10¹⁰ M^-1 s^-1.  The growth of reduced ruthenium compound, Ru(bpz^-)(bpz)(deeb)⁺, monitored by transient absorption was quantified with an equal rate constant, indicating it  was a primary photoproduct.  Diiodide was observed secondary to the growth of Ru(bpz^-)(bpz)(deeb)⁺ with a rate constant three times slower, k = 2.4 x 10¹⁰ M^-1 s^-1, indicating that it was not a primary photoproduct.  This observation was in accord with the initial formation of an iodine atom that subsequently reacted with iodide to form the I-I bond of I₂^·-.  We note also that the mechanism for I-I bond formation discovered here is distinctly different from the observed in our previous work using ion-pairs in low dielectric solvents.        

      Reactivity of both I₂^·- and I₃^- by the reduced ruthenium compound, Ru(deeb^-)(deeb)₂⁺, generated by the conventional flash-quench technique were quantified.  In the flash-quench experiment, iodide oxidation of Ru(deeb)₃^2+* generated Ru(deeb^-)(deeb)₂⁺, where reaction with I₂^·- or  I₃^- could be quantified.  Titration of I₃^- into solution allowed its reduction to be the predominate pathway.  Transient absorption data confirmed that I₂^•- was a product of triiodide reduction.  Kinetic analysis for the loss of Ru(deeb^-)(deeb)₂⁺ and I₃^-, and the growth of I₂^•- revealed a linear dependence of the observed rate constant with triiodide concentration and yielded a self consistent second-order rate constant of 5.1 x 10⁹ M^-1 s^-1 for the reduction of I₃^-.  Sutin’s description of Marcus theory in the context of diffusional bimolecular reactions allowed the reduction potential to be calculated, E^o(I₃^‑/( I₂^•-, I^-)) = -0.34 V vs. NHE.  In calculating this reduction potential a reorganization energy (l = 1.0 eV) and pre-exponential factor (n_nk_el = 10¹¹ s^-1) were assumed. 

      Taken together, these studies provide a strategy for making chemical bonds with visible light while at the same time preventing back reactions that accompany bon-breaking.