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

Abstract The absorption 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 (1 H -pyrrolo[2,3 b ]pyridine-3-acetic acid) are compared. Two to four absorbance maxima in the 260–310-nm range are interpreted as overlapping vibronic lines of the 1 L a ← 1 A and 1 L b ← 1 A transitions. Two further maxima in the 200–230-nm region are assigned to the 1 B a ← 1 A and 1 B b ← 1 A transitions. 4- and 7-Fluoroindole-3-acetic acid 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 1 B b ← 1 A transition band.