Put very simply, they do not have to obey microscopic reversibility. This has allowed the development of elegant molecular motors by Feringa, 48 and others. To take a simple example, in the E/Z isomerization reaction of hydrazone photoswitch 17 (Figure ), ignoring any thermal isomerization processes, 51 the apparent rate constant for conversion of E-17 to E-17, +, upon irradiation with light of wavelength  depends on the extinction coefficient () of Z-17, the intensity of the incident light (I) and the quantum yield of isomerization (). 52 A similar argument applies to the conversion of E-17 to Z-17.