Fast and Tunable Phosphorescence from Organic Ionic Crystals

Crystalline diphosphonium iodides [MeR(2)Pspacer-R2Me]I with phenylene (1, 2), naphthalene (3, 4), biphenyl (5) and anthracene (6) as aromatic spacers, are photoemissive under ambient conditions. The emission colors (lambda(em) values from 550 to 880 nm) and intensities (Phi(em) reaching 0.75) are defined by the composition and substitution geometry of the central conjugated chromophore motif, and the anion-p interactions. Time-resolved and variable-temperature luminescence studies suggest phosphorescence for all the titled compounds, which demonstrate observed lifetimes of 0.46-92.23 mu s at 297 K. Radiative rate constants k(r) as high as 2.8x10(5) s(-1) deduced for salts 1-3 were assigned to strong spin-orbit coupling enhanced by an external heavy atom effect arising from the anion-p charge-transfer character of the triplet excited state. These rates of anomalously fast metal-free phosphorescence are comparable to those of transition metal complexes and organic luminophores that utilize triplet excitons via a thermally activated delayed fluorescence mechanism, making such ionic luminophores a new paradigm for the design

Automated summary

Crystalline diphosphonium iodides with different aromatic spacers exhibit photoemissive properties under ambient conditions. The emission colors and intensities are determined by the structure and substitution geometry of the chromophore motif, as well as the anion-p interactions. Time-resolved and variable-temperature luminescence studies confirm the phosphorescent nature of these compounds, with observed lifetimes ranging from 0.46 to 92.23 μs at 297 K. The fast radiative rate constants for salts 1-3 suggest the involvement of spin-orbit coupling and charge transfer in the triplet excited state. This metal-free phosphorescence mechanism is comparable to that of transition metal complexes and organic luminophores utilizing thermally activated delayed fluorescence.