Luminescence of the Conjugate Bases of [2-(2-Hydroxyphenyl)phenyl]phosphinic Acid and Single-Crystal X-Ray Structure Determination of Sodium [2-(2-Hydroxyphenyl)phenyl]phosphinate

The commercial flame-retardant 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) was almost quantitatively converted in sodium [2-(2-hydroxyphenyl)phenyl]phosphinate Na[OH-Ph-Ph-PHO2] and disodium 2-(2-phosphinatophenyl)benzen-1-olate Na2[O-Ph-Ph-PHO2] under mild reaction conditions and without the use of toxic reactants. The structure of Na[OH-Ph-Ph-PHO2] was determined by means of single-crystal X-ray diffraction. The inter- and intramolecular Na-O interactions generate a stair-like framework where the sodium cations are five-coordinated and exhibit a highly distorted coordination sphere. The two compounds are characterized by appreciable blue luminescence at the solid state upon excitation with UV light, attributed to S1→S0 decays on the basis of time-resolved measurements and computational calculations. The photoluminescence quantum yield is higher for Na2[O-Ph-Ph-PHO2], and the emission and excitation bands are shifted at longer wavelengths. The disodium salt showed affinity towards cellulose, and doped Na2[O-Ph-Ph-PHO2]@cellulose samples maintained emission features comparable to those of the pure compound. The nature of the interaction between cellulose and the emitting species was studied by means of periodic density functional theory calculations, that highlighted the role of the sodium cations.