depending on the nature of solvent. The protic solvents tend to localize the triiodide anion's excess charge, resulting in the triiodide anion's asymmetric structure. For example, the triiodide anion in methanol has an asymmetric bent structure with a charge localized on the longer end of the anion. The dimensions of the triiodide [Ia−Ib−Ic]− bonds in a few sample compounds are shown below: {| class="wikitable" |- ! compound ! Ia−Ib (pm) ! Ib−Ic (pm) ! angle (°) |- | TlI3 | 306.3 | 282.6 | 177.9 |- | RbI3 | 305.1 | 283.3 | 178.11 |- | CsI3 | 303.8 | 284.2 | 178.00 |- | NH4I3 | 311.4 | 279.7 | 178.55 |- | I3− (in methanol) | 309.0 | 296.0 | 152.0 |} Properties The triiodide ion is the simplest polyiodide; several higher polyiodides exist. In solution, it appears yellow in low concentrations, and brown at higher concentrations. The triiodide ion is responsible for the well-known blue-black color which arises when iodine solutions interact with starch. Iodide does not react with starch; nor do solutions of iodine in nonpolar solvents. Lugol's iodine contains potassium iodide and a stoichiometric amount of elemental iodine, so that significant amounts of triiodide ion exist in this solution. Tincture of iodine, although nominally a solution of elemental iodine in ethanol, also contains significant amounts of triiodide, due to its content of both iodide and water. Photochemistry Triiodide is a model system in photochemistry. Its reaction mechanism has been studied in gas phase, solution and the solid state. In gas phase, the reaction proceeds in multiple pathways that include iodine molecule, metastable ions and iodine radicals as photoproducts, which are formed by two-body and three-body dissociation. In condensed phases, due to confinement, geminate recombination is more common. In solution, only two-body dissociation of triiodide has been observed. In the protic solvents, an iodine atom at the shorter end of the triiodide anion dissociates upon photoexcitation showing two-body dissociation. In the solid state, the triiodide photochemistry has been studied in compounds involving quaternary ammonium cations, such as tetrabutylammonium triiodide. It has been shown that the solid state photoreaction mechanism depends on the light wavelength, yielding fast recovery in a few picoseconds or going through a two-stage process that involves the formation and break-up of a tetraiodide intermediate on longer timescales. Besides, triiodide photochemistry is an important contributor in the environmental cycle of iodine. Because of the