A solvatochromic study of silicates and borate containing 4‐nitrocatechol ligands

Abstract

AbstractSolvatochromism of 4‐nitrocatechol (1), tetra‐n‐butylammonium 2‐hydroxy‐4‐nitrophenolate (2), sodium tris(4‐nitrobenzene‐1,2‐diolato)silicate (3a), tetra‐n‐butylammonium tris(4‐nitrobenzene‐1,2‐diolato)silicate (3b), pyrrolidinium tris(4‐nitrobenzene‐1,2‐diolato)silicate (3c), (3‐amino‐1‐propyl)‐bis(4‐nitrobenzene‐1,2‐diolato)silicate (4a), and (N,N‐diethyl‐3‐amino‐1‐propyl)‐bis(4‐nitrobenzene‐1,2‐diolato)silicate (4b) as well as potassium bis(4‐nitrobenzene‐1,2‐diolato)borate (5) has been studied in a set of common solvents. The origin of the solvent‐induced UV/vis band shifts of 1–3c, and 5 has been determined by means of Linear Solvation Energy Relationship (LSER) using the empirical Kamlet–Taft and Catalán solvent parameter sets, respectively. The solvent‐induced UV/vis band shift of the negatively charged moiety of all solvatochromic dyes studied is mainly a function of the hydrogen‐bond donor (HBD) strength and the dipolarity/polarizability of the solvent as shown by multiple regression analyses. HBD solvents cause a hypsochromic shift of the UV/vis band due to specific solvation of the anion site. Oppositely, increasing dipolarity/polarizability of the solvent induces a bathochromic shift of the UV/vis absorption band. The donor strength of the solvent has an influence on the UV/vis band shift since ion pair formation occurs in solvents of low relative permittivity. This is shown by the impact of the counter ion of sodium tris(4‐nitrobenzene‐1,2‐diolato)silicate (3a) compared to tetra‐n‐butylammonium tris(4‐nitrobenzene‐1,2‐diolato)silicate (3b). The influence of the counter ion on the position of the UV/vis absorption band occurs in the same way as HBD solvents do. Na+ and the pyrrolidinium ion, respectively, have a stronger influence than the tetra‐n‐butylammonium ion on the solvatochromic band shift due to the stronger cation–anion interaction. Copyright © 2008 John Wiley & Sons, Ltd.