Photoactivity and pH sensitivity of methyl orange functionalized poly(propyleneamine) dendrimers

For the first time a pH indicator that responds to two different external stimuli, i.e. pH and light, namely methyl orange, has been implemented in a dendrimer. Six generations (G0-G5) of methyl orange-functionalized poly(propyleneamine) dendrimers ("MO dendrimers") have been synthesized and characterized using 1H NMR, 13C NMR, and MALDI-TOF mass spectrometry. Subsequently, the response of the MO dendrimers toward pH changes has been monitored using UV-vis spectroscopy and the photophysical properties have been investigated. Furthermore, the photoisomerization from E to Z and the thermal recovery from Z to E, have been studied using UV-vis and transient-absorption spectroscopy. Interestingly, the response of the MO units toward pH changes is found to be generation dependent. On the other hand, the photophysical properties are found to be similar for all generations, except for the ε value, which deviates significantly from the expected value in the case of G4 and G5. Also the isomerization processes show no generation dependence within the same solvent, but different rate constants for the Z/E isomerization have been observed for different solvents.