Excited-state deactivation mechanisms of the M2(dba)3 complexes (M = palladium, platinum; dba = dibenzylideneacetone)

Abstract

The flash photolysis transients (30 ps-10 [mu]s) of the M[sub 2](dba)[sub 3] complexes (M = Pd, Pt; dba = dibenzylideneacetone) have been investigated in toluene solutions at 298 K. Due to the biphasic decay kinetic results observed in the Pd[sub 2](dba)[sub 3] case, the flash photolysis experiments were also performed on the free dba ligand and two substituted derivatives (bis(2-methylbenzylidene)acetone (Medba) and bis(2-methoxybenzylidene)acetone (MeOdba)). For the free ligands, two transient species are observed, a short-lived one (8.3<[tau]<13.3 ns; broad band located at [approximately]470 nm) corresponding to the intraligand triplet excited state and a longer-lived one (2.5-6.4 [mu]s; narrower band located at [approximately]450 nm) associated with the geometric isomers responsible for the cis,trans-isomerization reactions. On the other hand, M[sub 2](dba)[sub 3] difference absorption spectra (M = Pd, Pt) exhibit intense transient signals located at [approximately]600 and [approximately]660 nm for M = Pd and Pt, respectively, which decay via a double exponential for M = Pd (1.0 ns and 0.50 [mu]s) and a single exponential for M = Pt (0.26 [mu]s). The [mu]s values are the metal-to-ligand charge-transfer (MLCT) triplet excited-state lifetimes, and the ns value is assigned to a new species associated with a geometric isomer photoinduced by a labile dbamore » ligand. Finally, the Medba has been characterized by single-crystal X-ray diffraction (for EHMO calculation purposes) and adopts the s-cis,s-cis configuration, where the methyl groups (2-positions) are oriented parallel with the carbonyl group. 32 refs., 7 figs., 3 tabs.« less