Dual Fluorescence Spectra of 1-Hydroxyindoles with Various C-C-Nb Side Chains at 3-Position in Methanol

Abstract

Almost the same ultraviolet absorption spectra with a broad peak at 292-294 nm and a shoulder peak at 275 nm were observed for methanol solutions of 1-hydroxyindoles with various C-C-N b side chains at 3-position such as 1 -hydroxy-N b -methoxycarbonyltryptamine (1), N b -acetyl-11-hydroxytryptophan methyl ester (2), N, N-dimethyl-l-hydroxyindole-3-acetamide (3), and l-hydroxy-N b -pivaloyltryptamine (4). Dual fluorescence excitation and emission spectra resulting from 1 L a - 1 A and 1 L b - 1 A fluorescence emissions were observed for these 1-hydroxyindoles (1 - 4). Excitation spectra were found to be dependent on observing wavelength of broad fluorescence emission spectra (λ max = 346-350 nm for 1, 2, and 3; λ max = 366 nm for 4). Dual fluorescence behaviors can be explained by the fact that 1 L a state locates slight below 1 L b state, and it is supported by time-dependent density functional theory calculations of 1 - 4 in methanol. It is noteworthy that dual fluorescence behaviors are delicately modified by C-C-N b side chain at 3-position of indole ring.