Photophysical studies of hetero-bischelated complexes of rhodium(III) containing 2,2'-dipyridylamine

Four mixed-ligand complexes, cis-Rh[(bipy)(HDPA)Cl-2]Cl (1), cis-[Rh(phen)(HDPA)Cl-2]Cl (2), cis-[Rh(bipy)(DPA)Cl-2](3), and cis-[Rh(phen)(DPA)Cl-2] (4) (where bipy = 2,2'-bipyridine, phen = 1,10-phenantroline, HDPA = 2,2'-dipyridylaminc, and DPA = the deprotonated form of 2,2'-dipyridylamine) have been synthesized and characterized. In slightly acidic solution and at low temperature (77 K), both complexes 1 and 2 show a broad, symmetric and structureless red emission with microsecond lifetime identified as dd* phosphorescence. In slightly basic solution, the deprotonated complexes (3 and 4) exhibit a broad and asymmetric blue emission, showing no vibrational structure with a lifetime in the order of microseconds. Emission of complex 3 reveals a blue shift of 0.81 mu m(-1) compared to the emission of complex 1 and that of complex 4 shows a blue shift of 0.77 mu m(-1) with respect to complex 2. Electrochemical data have also been obtained for the four complexes in CH3CN. There are two reduction peaks observed for both complexes 1 and 2. Each peak is followed by a one-electron reduction at the metal, with an elimination of chloride during each reduction step, which is in consistent with the dd* phosphorescence assignment for the two complexes. For complexes 3 and 4, only a one-electron reduction process occurs at the metal with an elimination of chloride. Based on the luminescence and electrochernical data, the emission of complexes 3 and 4 are assigned as pi d* phosphorescence. Results from density functional theory (DFT) calculations provide theoretical evidence in support of this pi d* assignments. (c) 2005 Elsevier B.V. All rights reserved.