The intramolecular aryl embrace: from light emission to light absorption

Abstract

6-(1-Methylpyrrol-2-yl)-2,2′-bipyridine, 3, and 6-(selenophene-2-yl)-2,2′-bipyridine, 4, have been prepared and characterized in solution and by structural determinations. Copper(I) complexes [CuL₂][PF₆] in which L is 2,2′-bipyridine substituted in the 6-position by furyl, thienyl, N-methylpyrrolyl, selenopheneyl, methyl or phenyl, (L = 1–6) have been synthesized. The complexes have been characterized by electrospray mass spectrometry, and solution NMR and UV-VIS spectroscopies. The single crystal structures of [Cu(1)₂][PF₆], [Cu(2)₂][PF₆], [Cu(3)₂][PF₆], [Cu(5)₂][PF₆] and [Cu(6)₂][PF₆] have been determined. In those compounds containing an aromatic substituent attached to the bpy unit, the substituent is twisted with respect to the latter. In [Cu(3)₂][PF₆] and [Cu(5)₂][PF₆], this results in intra-cation π-stacking between ligands which is very efficient in [Cu(3)₂]⁺ despite the steric requirements of the N-methyl substituents. Face-to-face stacking between the ligands in the [Cu(2)₂]⁺ ion is achieved by complementary substituent twisting and elongation of one Cu–N bond, but there is no analogous intra-cation π-stacking in [Cu(1)₂]⁺. Ligand exchange reactions between [CuL₂][PF₆] (L = 1–6) and TiO₂-anchored ligands 7–10 (L′ = 2,2′-bipyridine-based ligands with CO₂H or PO(OH)₂ anchoring groups) have been applied to produce 24 surface-anchored heteroleptic copper(I) complexes, the formation of which has been evidenced by using MALDI-TOF mass spectrometry and thin layer solid state diffuse reflectance electronic absorption spectroscopy. The efficiencies of the complexes as dyes in DSCs have been measured, and the best efficiencies are observed for [CuLL′] with L′ = 10 which contains phosphonate anchoring groups.