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Absorption and fluorescence spectra of ring-substituted indole-3-acetic acids

The absorbance and fluorescence spectra of indole-3-acetic acid (1), a plant growth regulator (auxin) and experimental cancer therapeutic, 29 ring-substituted derivatives, and the 7-aza analogue (1H-pyrrolo[2, 3b]pyridine-3-acetic acid) are compared. Two to four absorbance maxima in the 260 - 310 nm range are interpreted as vibronic lines of the 1A to 1La and 1A to 1Lb transitions. Two further maxima in the 200 - 230 nm region are assigned to the 1A to 1Ba and 1A to 1Bb transitions. 4- and 7-Fluoroindole-3-acetic acids exhibit blue shifts with respect to 1, most other derivatives show red shifts. All indole-3-acetic acids studied, with the exception of chloro-, bromo- and 4- or 7-fluoro derivatives, fluoresce at 345 - 370 nm when excited at 275 - 280 nm. 7-Azaindole-3-acetic acid emits at 411 nm. The fluorescence quantum yield of 6-fluoroindole-3-acetic acid significantly exceeds that of 1 (0.3) ; the other derivatives have lower quantum yields. The plant-growth promoting activity of the ring-substituted indole-3-acetic acids studied correlates with the position of the 1A to 1Bb transition band.

auxin ; ring-substituted indole-3-acetic acid ; UV absorbance ; fluorescence ; structure-activity relationship

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