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

Abstract

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-Bu2 bpy)(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.

Photoisomerization of PtII Complexes Containing Two Different Photochromic Chromophores: Boron Chromophore versus Dithienylethene Chromophore

“…The room temperature emission spectrum was also collected for the diboryl Pt II complex 4 , which displays a dual‐emission phenomenon. Both the lifetime measurement and oxygen sensitive tests confirm that the high‐energy emission peak ( λ em =455 nm) is fluorescent in nature, due to emission from the LC state on the boron acetylide ligands, whilst the low‐energy band at about 514 nm, with a shoulder band at ≈539 nm, is due to phosphorescence, which may be assigned to the alkyne‐chelate backbone by comparison to the previously reported analogues …”

“…The Pt atoms in compounds 1 and 2 have nearly ideal square planar geometry, as evidenced by the P1‐Pt1‐P2 and C1‐Pt1‐C19 angles (174.94(4) and 176.8(2)°, respectively) in 1 ; and the P1‐Pt1‐C59 and P2‐Pt1‐C1 angles (178.26(14) and 175.70(13)°, respectively) in 2 . The Pt−P and Pt−C bond lengths in 1 and 2 are similar to related compounds . The S1 thienyl plane and the ppy plane are nearly orthogonal.…”

“…Moreover, the geometry of the linker in the boron system can also effectively influence the isomerization quantum efficiency in compounds that contain multiple B(ppy)Mes 2 units . When two or more chromophores are linked by a non‐conjugated backbone, the quantum efficiency of the active boryl unit can be significantly increased by the non‐active boryl units through the antenna effect, whereas the conjugated linker usually results in a pronounced inhibition of the photoactivity, due to the efficient energy dissipation through the localized chelate backbone emission …”

Strongly Emissive and Photostable Four‐Coordinate Organoboron N,C Chelates and Their Use in Fluorescence Microscopy

“…Recently, the photochromic properties of structurally related B(ppy)Mes2 N,C-chelates were reported by the Wang group. [21][22][23][24] In oxygen-free solutions these compounds can be transformed by light irradiation into borabicyclo[4.1.0]hepta-2,4-diene derivatives. These are then re-converted into the starting material by a thermally activated process.…”

“…[20] Interestingly, some of the latter were shown to feature reversible photochromic behaviour. [21][22][23][24] On the one hand, this switching characteristic is interesting by itself for applications that require the light-induced control of optical properties. [22] On the other hand, such photoreactivity constitutes a severe drawback for applications [22] which require strong light sources or prolonged irradiation times and where photodecomposition should be dimished.…”