A process for altering the absorption of electromagnetic radiation by one or more compounds of the general formulae (I) wherein these compounds are irradiated with electromagnetic radiation of wavelength from 300 to 750 nm. The use of compounds of the general formula (I) or (II) for marking materials, for example paper or mineral oil, and use of compounds of the general formula (I) or (II) for causing a color change. The use of compounds of the general formula (I) or (II) for laser welding, heat management, as a photoinitiator, as a free-radical scavenger or for detection of oxygen. A process for regulating the absorption or transmission of electromagnetic radiation by a material wherein one or more compounds of the general formula (I) or (II) are contacted with this material and these compounds are irradiated with electromagnetic radiation of wavelength from 300 to 750 nm. Specific compounds of the general formula (I).