pH-dependent excited-state properties of N,N '-di(2-phosphonoethyl)-1,4,5,8-naphthalenediimide

The effect of the protonation state of a novel aqueous soluble N,N'-di(2-ph osphonoethyl)-1,4,5,8-naphthalene- diimide (DPN) on its spectroscopic prope rties is studied, In aqueous solution DPN depicts linear Lambert-Beer plots up to the solubility limits at pH 1.0, 7.1 and 10.1, in accordance with it s solubilization as a monomer, A small increase in the extinction coefficie nts with pH is observed. Conversely a strong decrease in the fluorescence e mission yields are observed with the increase in pH. These results are show n to originate from the distinct efficiencies of the intersystem crossing o f DPN as a function of the net charge on the phosphono moieties. Accordingl y, from the decrease in fluorescence emission, titration curves were obtain ed resulting in two pKa of similar to 1.5 and 6.7 in agreement with the pot entiometric analysis. In parallel monitoring the amount of heat deposited i n the system shows an inverse pattern, that is, increase in the thermal len sing time-resolved signal with pH, Laser flash photolysis and thermal lensi ng data in the absence and presence of oxygen show the interplay of an inte rsystem crossing process with the protonation state of the dye. The changes in protonation from neutral to two- and four-fold charged species are acco mpanied by an enhancing effect on the naphthalenediimide ring toward spin i nversion (triplet formation) and consequently in different fluorescence yie lds.