A new system able to play the role of a molecular switch has been characterised. The system consists of the complexes in equilibrium. The constituent complexes differ considerably in electronic absorption (especially in the visible region) and undergo photochemical reactions proceeding in distinct spectral ranges and yielding different products. The equilibrium can be shifted by several physical and chemical stimuli changing thereby absorption and/or modifying the photochemical behaviour of the system. To the stimuli belong: pH, thiol concentration, ion strength, nature and concentration of cations, temperature and pressure. All these features make the system suitable for switching purposes and for different kinds of signal processing in digital, analogue and integrating circuits. As the system photochemi- cally produces NO-donors and/or nitric oxide, its phototherapeutic application is considered.