Azulene appended cellulose: Synthesis, optical and chiroptical properties, film formation by electrochemical oxidation

Abstract

Regioselective synthesis using protecting-group strategy yields azulene-cellulose conjugates 6-(azulene-2-carboxylate)-2,3-di-O-methyl cellulose AzC-1, 6-(4-azulene-1-yl-benzoate)-2,3-di-O-methyl cellulose AzC-2, and 6-(azulene-1-carboxylate)-2,3-di-O-methyl cellulose AzC-3. The spectral and electrochemical properties were investigated and compared with the azulene alkylesters 1 b, 2 b, and 3 b. The relative fluorescence quantum yield of methyl 2-azulenecarboxylate 1b exceeds that of azulene cellulose AzC-1 by an order of magnitude. Cellulose derivative AzC-3 exhibits weak dual emission around 400 nm and 520 nm at room temperature. The circular dichroism (CD) of AzC-1, AzC-2, and AzC-3 are reported. Intensity and feature of the CD signals depend on the degree of substitution (DS) as shown by AzC-1. The CD-split signals at 280 nm (3A1 transition of azulene) are likely to result from interacting azulene groups. A regular (helical) structure of the cellulose backbone is postulated, leading to the (M)-configuration of the spatial azulene units. Unlike esters 2 b and 3 b azulene appended celluloses AzC-1 and AzC-2 electropolymerize under potentiodynamic conditions. Chemically modified electrodes are formed. Dotation sates were characterized by UV-Vis-NIR-spectroelectrochemistry. Azulene appended celluloses are suitable for making conducting electrodes with chiral coating materials.