Abstract
Room-temperature dual emission consisting of spectrally separated fluorescence and phosphorescence is highly attractive as a design principle for ratiometric sensing materials, for example, for detection of dioxygen. Compounds susceptible to emission quenching by dioxygen, producing dioxygen in electronically excited states, are also used as photosensitizers for singlet oxygen generation. ...
Abstract
Room-temperature dual emission consisting of spectrally separated fluorescence and phosphorescence is highly attractive as a design principle for ratiometric sensing materials, for example, for detection of dioxygen. Compounds susceptible to emission quenching by dioxygen, producing dioxygen in electronically excited states, are also used as photosensitizers for singlet oxygen generation. Combination of the dual emission behavior and efficient energy transfer from one of the emitting states (triplet state) of the dual emissive compound to molecular dioxygen can result in potent photosensitizers easily traceable by fluorescence spectroscopy, which may be advantageous for instance in biology studies. Herein, we present two Pt(II) complexes 1 and 2 of dinuclear structure which exhibit green fluorescence with sub-nanosecond lifetimes and near infrared (NIR) phosphorescence with microsecond lifetimes. Such properties are achieved via the design of a strongly pi-excessive ditopic ligand with a N<^>C-C<^>N coordinating mode that bridges the metal centers. The ligand centered character of the lowest excited singlet (S-1) and triplet (T-1) states leads to strong exchange interaction of the unpaired electrons and hence to large energy separation Delta E(S-1-T-1) amounting to 0.6 eV for 1 and 0.7 eV for 2, respectively. The large energy gap Delta E(S-1-T-1) and weak metal contribution to the states S-1 and T-1 results in unusually long intersystem crossing (ISC) times tau(ISC)(S-1 -> T-1) of 27.5 ps (1) and 65.2 ps (2), respectively, as determined by transient absorption spectroscopy. Owing to the slow ISC, the T-1 -> S-0 phosphorescence of both 1 and 2 is accompanied by S-1 -> S-0 fluorescence of comparable intensity. The large gap Delta E(S-1-T-1) provides also a good optical separation of the two emissions. The phosphorescence signal is efficiently quenched in the presence of dioxygen, which is manifested in both the lower relative intensity and shorter decay time of phosphorescence. Thus, the compounds show high potential as ratiometric dioxygen sensing materials. The singlet oxygen photogeneration efficiencies of complexes 1 and 2, measured in air saturated dichloromethane, are as high as phi(Delta) approximate to 0.77 +/- 0.1 and 0.57 +/- 0.1, respectively. Thus, the compounds represent efficient singlet oxygen photosensitizers.
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