No abstract

As part of our efforts to discover simple routes to room-temperature phosphors, we have investigated the interaction of bis(pentafluorophenyl)mercury (1) or trimeric perfluoro-o-phenylene mercury (2) with selected arenes (naphthalene, biphenyl, and fluorene). Solution studies indicate that 2, unlike 1, quenches the fluorescence of naphthalene. When compared to 1, the high quenching efficiency of 2 may be correlated to the higher affinity that 2 displays for arenes as well as to more acute external heavy-atom effects caused by the three mercury atoms. In the crystal, the adducts [1.naphthalene], [1.biphenyl], [1.fluorene], and [2.fluorene] form supramolecular binary stacks in which the arene approaches the mercury centers of 1 or 2 to form Hg-C pi-interactions. Analysis of the electrostatic potential surfaces of the individual components supports the involvement of electrostatic interactions. The luminescence spectra of the adducts show complete quenching of the fluorescence and display heavy-atom-induced emission whose energies and vibronic progressions correspond to the phosphorescence of the respective pure arene. The phosphorescence lifetimes are shortened by 3 or 4 orders of magnitude when compared with those of the free arenes. Taken collectively, the structural, photophysical, and computational results herein suggest that the proximity of the three mercury centers serves to enhance the Lewis acidity of 2, which becomes a better acceptor and a more effective heavy-atom effect inducer than 1.

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.