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Bruce R. Osterby(1) and Ronald D. McKelvey University of Wisconsin--La Crosse, WI 54601
Solvatochromism is used to describe a pronounced change in the position of a UV-vis absorption band with a change in solvent polarity. Betaine-30 (2,6-diphenyl-4(2,4,6-triphenyl-pyridinio)phenolate) has one of the largest effects ever observed. The compound is red in methanol, violet in ethanol, blue in isoamyl alcohol, green in acetone, and yellow in anisole, thus covering the whole visible range (1). This corresponds to a range for lambda sub max , of some 357 nm. The solvatochromism probably derives from solvent stabilization of the polar ground state relative to the less polar charge-transfer excited state.
Betaine-30 makes an impressive demonstration, and a picture of the dye in various solvents has recently been included in an organic textbook (2). The multistep synthesis provides students with experience working on a small scale. If only the demonstration is of interest, betaine-30 is available from Aldrich.
The syntheses and solvatochromic properties of betaine-30 and related compounds came out of the PhD theses of Reichard et al. and have been described in the primary (3-5) and secondary (1) literature. The synthesis is convergent, which offers two advantages. Each branch can be run on a modest scale, and a reasonable amount of betaine-30 can still be obtained. The synthesis can be run as a team project, with different students working on each branch. The overall synthesis of betaine-30 is shown in Figure 1. (Figure omitted)
The first branch involves the synthesis of 2,4,6-triphenylpyrilium hydrogen sulfate. This is prepared by an acid-catalyzed condensation/cyclization with chalcone and acetophenone. Chalcone is commercially available but can also be made by a crossed aldol condensation (6). The mechanism of the...