Thiol maleimide units

The ability of the maleimide unit to switch off emission is also exemplified by 86, due to Verhoeven s coworkers [161] at the University of Amsterdam and Akzo Nobel in The Netherlands. Again the Michael reaction of the maleimide with thiols produces nicely emissive material. Solvent-sensitive emission, characteristic of these donor-acceptor systems with strongly coupling bridges, is a special feature of 86 after thiolation. An added interest of 86 stems from the occurrence of PET to the maleimide unit from the through-bond charge-transfer excited state [162], an unusual combination of photophenomena. 

The reaction with thiols takes place in meta-cresol or in acetic acid containing DMF. The slightly acidic medium is needed to favor the protonation of the intermediate species (pathway a) and minimize the reaction with other maleimide units (pathway b) giving crosslinked polymers (Fig. 10). 

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