Changes in Emission Properties by π-Stacking and Conformation Adjustment of an Imidazole-Tethered Naphthalimide Derivative

摘要

Solid state self-assemblies of 2,4-dinitrophenolate, 2,4,6-trinitro-phenolate salts of N-(3-imidazol-1-yl-propyl)-1,8-naphthalimide (abbreviated as imidanap) have distinguishable it-stacking patterns, each stack influences fluorescence emission causing fluorescence quenching. It is found that the solid sample of nitrate or bromide salt of imidanap retains fluorescence properties of the parent compound. Photo-induced electron transfer process in solution gets prominence upon interactions of imidanap with hydrobromic as well as nitric acid but in the nitrophenolate salts the effect of it-interactions over-ride the effect of protonation of imidazole to change intensity of fluorescence emission. DFT calculations using B3LYP functional with 6-31 +G (d,p) as basis set have revealed LUMO in the imidanap and in the respective nitrophenolate salts are localized on the naphthalimide unit whereas HOMO in each case is localization differently over the rings other than the naphthalimide ring. Studies based on ~1HNMR suggest that water causes conformation adjustments of imidanap by forming hydrogen bonds. This enhances photo-induced electron transfer (PET) emission of the imidanap until a limiting concentration of water is reached, beyond which increase in amount of water causes quenching of emission. The PET-ON state of imidanap generated by addition of water to an acetonitrile solution is quenched by 2,4,6-trinitrophenol due to charge-transfer interactions.