Optical absorption and photoconductivity in iodine-excess ionic liquids: the case of 1-alkyl-3-methyl imidazolium iodides

Abstract

We investigated the optical absorption and photoconductivity of iodine-excess ionic liquids (ILs) based on 1-alkyl-3-methyl imidazolium iodide ([C_nmim][I]; n = 3, 4, and 6). The iodide concentration m was 2 ≦ m ≦ 8, which was determined by the molar fraction [C_nmim]⁺ : [I_m]⁻ = 1 : m. By adding iodine, an absorption edge shifted from 282 nm in the UV region to around 600 nm in the visible-light region. The optical bandgaps E_o decreased gradually from 2.3 eV to 1.9 eV with increasing m from 2 to 8. The alkyl-side chain lengths of the cations have little effect on the E_o. This experimental result was confirmed by ab initio molecular orbital calculations. The effects were reflected in the photoconductivity of the ILs, as expected. [C₄mim][I_m] exhibited greater photo-induced electron generation compared with [C₃mim][I_m] and [C₆mim][I_m]. The photoconductivity in both [C₃mim][I_m] and [C₆mim][I_m] increased slightly with increasing m. The trend of photoconductivity in [C₄mim][I_m] exhibited an N-shaped form. The highest photoconductivity 1.6 was observed in [C₄mim][I₈].