Tuning the Photoisomerization of a N^C-Chelate Organoboron Compound with a MetalAcetylide Unit.

To examine the impact of metal moieties that have different triplet energies on the photoisomerization of B(ppy)Mes₂ compounds (ppy=2-phenyl pyridine, Mes=mesityl), three metal-functionalized B(ppy)Mes₂ compounds, Re-B, Au-B, and Pt-B, have been synthesized and fully characterized. The metal moieties in these three compounds are Re(CO)₃( tert-Bu₂bpy)(CC), Au(PPh₃)(CC), and trans-Pt(PPh₃)₂(CC)₂, respectively, which are connected to the ppy chelate through the alkyne linker. Our investigation has established that the Re^I unit completely quenches the photoisomerization of the boron unit because of a low-lying intraligand charge transfer/MLCT triplet state. The Au^I unit, albeit with a triplet energy that is much higher than that of B(ppy)Mes₂, upon conjugation with the ppy chelate unit, substantially increases the contribution of the π→π* transition, localized on the conjugated chelate backbone in the lowest triplet state, thereby leading to a decrease in the photoisomerization quantum efficiency (QE) of the boron chromophore when excited at 365 nm. At higher excitation energies, the photoisomerization QE of Au-B is comparable to that of the silyl-alkyne-functionalized B(ppy)Mes₂ ( TIPS-B), which was attributable to a triplet-state-sensitization effect by the Au^I unit. The Pt^II unit links two B(ppy)Mes₂ together in Pt-B, thereby extending the π-conjugation through both chelate backbones and leading to a very low QE of the photoisomerization. In addition, only one boron unit in Pt-B undergoes photoisomerization. The isomerization of the second boron unit is quenched by an intramolecular energy transfer of the excitation energy to the low-energy absorption band of the isomerized boron unit. TD-DFT computations and spectroscopic studies of the three metal-containing boron compounds confirm that the photoisomerization of the B(ppy)Mes₂ chromophore proceeds through a triplet photoactive state and that metal units with suitable triplet energies can be used to tune this system. [ABSTRACT FROM AUTHOR]