Interligand electron transfer as a reason of very weak red luminescence of Eu((i-Bu)2PS2)3Phen and Eu(C4H8NCS2)3Phen complexes

Nanosecond laser flash photolysis and time resolved luminescence were used to study the photophysical processes for Eu((i-Bu)2PS2)3Phen (1) and Eu(C4H8NCS2)3Phen (2) complexes in acetonitrile. These complexes show a very weak red Eu3+ luminescence in spite of the fact that the phenanthroline molecule in triplet state is a good antenna to excite the red luminescence of many Eu(III)-Phen complexes. To determine the reasons of this effect the photoprocesses in solutions, containing the (i-Bu)2PS2− or C4H8NCS2− ions and free phenanthroline molecule, have been studied with the use of laser flash photolysis (266 nm). It was shown that the phenanthroline in triplet excited state (TPhen*) deprives the electron from these dithiolate ions with a high rate constants close to 109 M−1 s−1. The transient spectra of phenanthroline anion-radical and dithiolate radicals were recorded which are in a good agreement with literature data. Since the effective concentration of dithiolate ions (L−) in the coordination sphere of 1 and 2 complexes is close to 10 M the time of electron transfer between L− and TPhen* is in the range of 100 ps or less. As the laser flash photolysis of solutions of 1 and 2 complexes with a 10 ns time resolution failed to detect the spectra of phenanthroline anion-radical and dithiolate radicals, it indicates that the time of back electron transfer is less than 10−8 s. Thus, the very weak red luminescence of 1 and 2 complexes is due to the electron transfer between ligands in the coordination sphere which successfully suppresses the energy transfer from the phenanthroline triplet state to Eu3+ ion.