Modulating the Ground- and Excited-State Oxidation Potentials of Diaminonaphthalene by Sequential N-Methylation

A series of 1,5-diaminonaphthalene derivatives were synthesized and characterized to provide ground- and excited-state electron donors of similar structure but varying potential. Electrochemical and spectroscopic properties of the series are reported and together illustrate two opposing consequences of alkyl substitution on the aryl amines. Inductive effects of methylation are evident from the decrease in ground-state oxidation potential for derivatives containing monomethylamino substituents. In contrast, steric effects seem to dominate the increase in the ground-state oxidation potential of derivatives containing dimethylamino substituents since the conformational constraints created by dimethylation suppress delocalization of the nonbonding electrons. Absorption and emission properties also respond to increasing levels of N-methylation, and the excited-state oxidation potentials of the parent 1,5-diaminonaphthalene and its monomethylamine derivatives (ca. -3.2 V) are approximately 200 mV lower than the corresponding dimethylamino derivatives (-3.0 V).