Intra- vs Intermolecular Photoinduced Electron Transfer Reactions of a Macrocyclic Donor−Acceptor Dyad

The synthesis, structural characterization, and photophysical behavior of a 14-membered tetraazamacrocycle
with pendant 4-dimethylaminobenzyl (DMAB) and 9-anthracenylmethyl groups is reported (L3, 6-((9-anthracenylmethyl)amino)-trans-6,13-dimethyl-13-((4-dimethylaminobenzyl)amino)-1,4,8,11-tetraazacyclotetradecane). In its free base form, this compound displays rapid intramolecular photoinduced electron
transfer (PET) quenching of the anthracene emission, with both the secondary amines and the DMAB group
capable of acting as electron donors. When complexed with Zn(II), the characteristic fluorescence of the
anthracene chromophore is restored as the former of these pathways is deactivated by coordination. Importantly,
it is shown that the DMAB group, which remains uncoordinated and PET active, acts only very weakly to
quench emission, by comparison to the behavior of a model Zn complex lacking the pendant DMAB group,
[ZnL2]2+ (Chart ). By contrast, Stern−Volmer analysis of intermolecular quenching of [ZnL2]2+ by N,N-dimethylaniline (DMA) has shown that this reaction is diffusion limited. Hence, the pivotal role of the bridge
in influencing intramolecular PET is highlighted.