Does the diiodide(1−) anion exist?
I found a paper$\ce{^{[1]}}$ regarding the formation of diiodide anion species($\ce{I2^-}$). It is assumed to be an unstable species which forms during the formation of triiodide anion (excess of iodide in solution which makes it brown). This is a part of the paper which describes the formation of the species. For more information, read the full paper. Also, you can check the abstract which @andselisk linked.

The $\ce{I2^-}$ species is formed by the 248-nm laser photolysis of
iodide through the following reactions:
$$\ce{I- + hν -&gt; I + e_s-}$$
$$\ce{I + I- -&gt;[k2]I2^-}$$
$$\ce{I2- + I2- -&gt;[k3]I3- + I-}$$
As discussed later, we found that photodetachment of electrons from
$\ce{I-}$ ions produces solvated electrons ($\ce{e_s-}$) in ionic
liquids as well as in molecular solvents. After photodetachment,
iodine atoms react with $\ce{I-}$ to form diiodide anion radicals. In
aqueous solution, the transient absorption maxima of $\ce{I2-}$ are
located around 400 and 720 nm. The extinction coefficients of
$\ce{I2-}$ in aqueous solution at 385 and 725 nm are 10000 and 2560
M-1cm-1, respectively.[...]

$\ce{^{[1]}}$: J. Phys. Chem. B, 2007, 111 (18), pp 4807–4811