Photophysical and Photoredox Characteristics of a Novel Tricarbonyl Rhenium(I) Complex Having an Arylborane-Appended Aromatic Diimine Ligand

Abstract

We report the synthesis and photophysical/photoredox characteristics of a novel tricarbonyl rhenium(I) complex having a (dimesityl)boryldurylethynyl (DBDE) group at the 4-position of a 1,10-phenanthroline (phen) ligand, [Re(CO)(3)(4-DBDE-phen)Br] (ReB). ReB in tetrahydrofuran at 298 K showed the metal-to-ligand charge transfer (MLCT) emission at around 681 nm with the lifetime (τ(em)) of 900 ns. The relatively long emission lifetime of ReB compared with that of [Re(CO)(3)(phen)Br] (RePhen, τ(em) = 390 ns) was discussed on the basis of the temperature dependent τ(em) and Franck-Condon analysis of the emission spectra of the two complexes. Emission quenching studies of both ReB and RePhen by a series of electron donors revealed that the photoinduced electron transfer (PET) quenching rate constant of ReB was faster than that of RePhen at a given Gibbs free energy change of the PET reaction (ΔG(ET)(0) > -0.5 eV). All of the results on ReB were discussed in terms of the contribution of the CT interaction between the π-orbital(s) of the aryl group(s) and the vacant p-orbital on the boron atom in DBDE to the MLCT state of the complex.

Diimine ligand structure effects on photophysical properties of tricarbonyl rhenium(I) complexes having arylborane charge transfer units

“…ReBbpy also showed the third reduction wave at -1.80 V. Since the third reduction wave similar to that of ReBbpy has been observed for ReBphen (see Table 1) [33], we can safely ascribe the potential to the reduction of the boron atom in the DBDE group. Such electrochemical characteristics of ReBbpy should reflect on the excited-state characteristics of the complex as described in the following sections.…”

“…The second irreversible reduction wave (E(Re 0/-I ), dissociative reduction of the Br ligand) of ReBbpy was observed at -1.45 V and was also shifted to the positive direction by 250 mV compared with that of Rebpy. These positive shifts of the reduction potentials of ReBbpy can be Table 1 includes the redox potentials of ReBphen and Rephen reported earlier [33] to discuss the diimine ligand effects on the redox behaviors of the complexes. These four complexes showed comparable oxidation potentials at around E(Re II/I ) = + 1.41-1.45 V, and this demonstrated that the HOMO energies of the complexes were very similar with one another as described above.…”

“…The complex was synthesized by the Sonogashira-Hagihara cross coupling reaction between (ethynylduryl)dimesitylborane (EDDB) and fac-[Re(CO) 3 (4-Br-bpy)Br] (4-Br-bpy = 4-bromo-2,2 0 -bipyridine). EDDB [33] and 4-Br-bpy [38,39] were synthesized according to the literatures with some modifications as reported in the Supporting Information. reaction vessel was additionally evacuated and filled with an argon gas.…”

“…As seen in the figure, ReBbpy showed the broad and intense absorption bands in 270-500 nm. On the basis of the electrochemical data in Table 1 and our previous study on ReBphen [33], the absorption bands of ReBbpy at around $316 and 320-390 nm (l abs = 361 nm, e 361 = 3.39 Â 10 4 M À1 cm À1 ) can be assigned to the 1 pp* transition in the bpy ring and the ligand-centered (LC) transitions, respectively, where the latter is composed of the 1 pp* transition in the durylethynylbpy unit and the p(aryl)-p(B) CT transition. While the 1 MLCT absorption band of ReBbpy was not observed obviously due to the spectral overlap with the intense 1 LC band, we assigned the spectral shoulder at around 392 nm to the 1 MLCT transition.…”

“…In practice, we have reported that such a type of complexes show unique spectroscopic and photophysical properties: intense absorption/emission and longlived excited state [23]. To the date, Pt(II) [26,27], Ir(III) [28][29][30], Ru (II) [31,32], Re(I) [33,34] and Cu(I) complexes [35,36] bearing an arylborane unit(s) on the ligands have been reported. As a typical example, we reported previously the spectroscopic, photophysical, and photoredox properties of the tricarbonyl rhenium(I) complex having a (dimesityl)boryldurylethynyl (DBDE) group at the 4-position of a 1,10-phenanthroline (phen) ligand (fac-[Re (CO) 3 (DBDE-phen)Br] = ReBphen, see the structure in Scheme 1) [33].…”