Effect of positional isomerism on electron-transfer photochromism and photoluminescence of two pyromellitic diimide-based organic molecules

Two positional isomeric organic molecules, 2-PMPMD (1) and 4-PMPMD (2) (2/4-PMPMD = N, N'-bis(2/4-pyridylmethyl)-pyromellitic diimide) have been constructed by anchoring two pyridylmethyl groups at both sides of nitrogen positions in pyromellitic diimide (PMD), which are assembled into different supramolecular architectures through lone pair-pi interactions. Noteworthy, 1 and 2 exhibit unique electron-transfer photochromic behaviors with completely different photoresponsive rate and coloration contrast, which are contributed to the distinct interfacial contacts of electron donors and acceptors. Interestingly, photoluminescence properties of two compounds show different changes during the coloration-decoloration process (the slightly red-shift and weakening of the fluorescence for 1 vs. the almost completely quenching of the fluorescence for 2). The present study illustrates the influence of positional isomerism on the resultant supramolecular networks and photochromic behaviors accompanying adjustable photoluminescence properties.