title: Deactivating unproductive pathways in multichromophoric sensitizers.
abstract: The effects of solvent and substituents on a multichromophoric complex containing a boron-dipyrromethene (Bodipy) chromophore and Pt(bpy)(bdt) (bpy = 2,2'-bipyridine, bdt =1,2-benzenedithiolate) were studied using steady-state absorption, emission, and ultrafast transient absorption spectroscopy. When the Bodipy molecule is connected to either the bpy or bdt in acetonitrile, excitation ultimately leads to the dyad undergoing triplet energy transfer (TEnT) from the redox-active Pt triplet mixed-metal-ligand-to-ligand' charge transfer ((3)MMLL'CT) state to the Bodipy (3)ππ* state in 8 and 160 ps, respectively. This is disadvantageous for solar energy applications. Here, we investigate two methods to lower the energy of the (3)MMLL'CT state, thereby making TEnT unfavorable. By switching to a low dielectric constant solvent, we are able to extend the lifetime of the (3)MMLL'CT state to over 1 ns, the time frame of our experiment. Additionally, electron-withdrawing groups, such as carboxylate and phosphonate esters, on the bpy lower the energy of the (3)MMLL'CT state such that the photoexcited dyad remains in that state and avoids TEnT to the Bodipy (3)ππ* state. It is also shown that a single methylene spacer between the bpy and phosphonate ester is sufficient to eliminate this effect, raising the energy of the (3)MMLL'CT state and inducing relaxation to the (3)ππ*.
authors.affiliation: Department of Chemistry, University of Rochester , Rochester, New York 14627, United States.
authors.forename: Randy Pat
authors.initials: RP
authors.lastname: Sabatini
authors.forename: Bo
authors.initials: B
authors.lastname: Zheng
authors.forename: Wen-Fu
authors.initials: WF
authors.lastname: Fu
authors.forename: Daniel J
authors.initials: DJ
authors.lastname: Mark
authors.forename: Michael F
authors.initials: MF
authors.lastname: Mark
authors.forename: Emily Anne
authors.initials: EA
authors.lastname: Hillenbrand
authors.forename: Richard
authors.initials: R
authors.lastname: Eisenberg
authors.forename: David W
authors.initials: DW
authors.lastname: McCamant