A novel type of dye systems comprises a selection of 10H-indolo[1,2-a]indole compounds (henceforth abbreviated as IO compounds) and (5H,7H)-indolo[1,2-a]quinoline compounds (henceforth abbreviated as IQ compounds) showing a solvatochromic effect and exhibiting strong fluorescence in a variety of materials such as polypropylene, polyethylene, oils, various solvents, emulsions. Also disclosed are various methods how the IO/IQ compounds can be administered, especially how they can be produced and administered in situ from a precursor, responding to external stimuli such as enzyme activity, temperature and so forth. The response of a precursor to external stimuli can also be used to determine the presence or absence of such stimuli.