Simultaneous External and Internal Heavy-Atom Effects in Binary Adducts of 1-Halonaphthalenes with Trinuclear Perfluoro-ortho-phenylene Mercury(II): A Structural and Photophysical Study

The 1-halonaphthalenes series has often been used to demonstrate the internal heavy-atom effect provided by the halide. In a continuation of our work on the phosphorescence of arenes induced by π-complexation to trimeric perfluoro-ortho-phenylene mercury (1), we now present a structural and photophysical study of the halonaphthalene adducts [1·1-chloronaphthalene] (2), [1·1-bromonaphthalene] (3), and [1·1-iodonaphthalene] (4). The triplet lifetimes in these adducts are considerably shorter than those for the free 1-halonaphthalenes. Analysis of lifetime data versus temperature affords room-temperature phosphorescence quantum yields of 70%, 64%, and 7% for the solid adducts 2, 3, and 4, respectively, compared to 54% for [1·naphthalene]. The photophysical data suggest that the synergy of the internal and external heavy-atom effects has a sensitizing effect for adducts 2 and 3 but a quenching effect for adduct 4 compared to [1·naphthalene]. The luminescence excitation spectra of the solid binary adducts show intense bands that are significantly red-shifted from the absorptions of the individual molecular components, and thus assigned to charge transfer (CT) states. Excitation bands corresponding to the S0 → T1 direct absorption of the 1-halonaphthalene are also detected, albeit much less intense than the CT absorption. The spectral analyses suggest that CT is the major excitation route that leads to the green phosphorescence of adducts 2−4.