Six indole derivatives B1-B6 obtained through Fischer reaction employing substituted phenyl hydrazine and butanone or cyclohexanone and a commercial available indole B7 were evaluated as herbicide by electron transport inhibition on photosynthetic apparatus. The electron flow damage can be measured by the change in the kinetics of chlorophyll a (Chl a) fluorescence that provides detailed information about the structure and function of the photosynthetic apparatus, especially the photosystem II (PSII). The activity of indole derivatives (150 μM and 450 μM) on Chl a fluorescence measurement were archived by semi-vivo assay using spinach leaf discs. The radar plots show the results of 11 photosynthetic parameters evaluated. The compound B2 (150 μM) decreased by 40% the index performance absortion PI(abs) and increased by 64% the approximated initial slope of the fluorescence transient (Mo = dV/dto) showing a slight stress at PSII. The parameters ABS/RC, TRo/RC e ETo/RC increased by approximately 20% demonstrating that the absorbed energy on fluorescence process was trapped at reaction centres therefore unable to electron transfer in the process. The compound B7 (450 μM) decreased by 62% and 27% the parameters PI(abs) and ETo/RC, respectively, and increased by 65% Mo = dV/dto. Accordingly, these results indicate that B7 inhibits the electron transfer process at reaction centre and behaves as photosynthesis inhibitor probe. Indoles B1, B3, B4, B5 and B6 do not show significant activity to Chl a fluorescence, therefore do not cause any damage to photosynthesis process.