RESUMO
We describe the photochemical characteristics of two phosphorescent palladium diselenolenes [Pd(2)(Se(2)C(8)H(12))(2)(PBu(3))(2)] (1) and [Pd(2)(Se(2)C(8)H(12))(2)(PPh(3))(2)] (2) which, to the best of our knowledge, are the first reported examples of luminescent Pd-Se compounds. Both compounds exhibit broadband near-infrared phosphorescence in the solid state, with lambda(max) of 717 nm for 1 and 792 nm for 2 at 298 K, and 752 nm for 1 and 785 nm for 2 at 77 K. No phosphorescence was detected for either compound when they were dissolved in nitrogen-purged acetonitrile or toluene solution at 298 K but they do phosphoresce at 77 K in organic glasses with emission quantum yields of 0.12 (+/-0.01) for 1 and 0.13 (+/-0.01) for 2 in an ethanol/diethylether/toluene (1:2:1) (EDT) glass. Emission lifetimes at 77 K are the same whether in the solid state or in an organic glass with first order fit lifetimes of tau = 18.8 (+/-0.7) micros and 11.5 (+/-0.3) micros for 1 and 2, respectively. Combination of these lifetimes with quantum yields gives radiative lifetimes of 151 (+/-13) micros and 86 (+/-7) micros for compounds 1 and 2, respectively, at 77 K in EDT glass. At 77 K solid state quantum yields are estimated to be of the same order of magnitude as those in glasses, and these decrease by a factor of about 3-5 in going from 77 to 298 K. In the solid state at 298 K emission lifetimes are 1.83 (+/-0.02) micros and 7.0 (+/-0.3) micros for 1 and 2, respectively. We could detect no transients by nanosecond flash photolysis which could be assigned to the triplet state in room temperature solution, and no emission assignable to singlet oxygen across the wavelength range 1200-1350 nm upon 550 nm excitation of either 1 or 2 in acetonitrile solution. We estimate the quantum yield of singlet oxygen formation to be less than about 5 x 10(-4), which is also an upper limit for the yield of triplet states of any significant lifetime in fluid solution. Density functional theory (DFT) calculations of the S(0) to S(1) and T(1) transitions show a shift in molecular orbital character from one with significant -ene pi involvement but very little P involvement in the ground-state to one with less -ene pi but greater P involvement in the excited states; there is also a significant shift in the distribution of involvement of atomic orbitals on the four Se atoms.
