Visible light switching of metallosupramolecular assemblies

Abstract

A photoswitchable ligand and palladium(II) ions form a dynamic mixture of self-assembled metallosupramolecular structures. The photoswitching ligand is an ortho-fluoroazobenzene with appended pyridyl groups. The E-isomer is combined with palladium(II) salts affords a double-walled triangle with composition [Pd3L6] 6+ and a distorted tetrahedron [Pd4L8] 8+ (1:2 ratio at 298 K). Irradiation with 410 nm light generates a photostationary state with ~80% of the E-isomer of the ligand which results in the selective disassembly of the tetrahedron, the more thermodynamically stable structure, and the formation of the triangle, the kinetic product. The triangle is then slowly transformed back into the tetrahedron over 2 days at 333 K. The Z-isomer of the ligand does not form any well-defined structures and has a thermal half-life of 25 days at 298K. This approach shows how a thermodynamically preferred self-assembled structure can be reversibly pumped to a kinetic trap by small perturbations of the isomer distribution using non-destructive visible light.

Visible‐Light‐Responsive Self‐Assembled Complexes: Improved Photoswitching Properties by Metal Ion Coordination**

“…ortho‐ Tetrafluoroazobenzene [30, 31] ‐based ligands with appended pyridyl groups were assembled with palladium(II) ions to form a dynamic mixture of two species (trimer and tetramer). In that case, the most thermodynamically stable species could be selectively disassembled using light by isomerizing just 15 % to the metastable isomer [29] . Herein, we report a new photoswitchable ligand that can be assembled into a single well‐defined cage structure and reversibly switched to another discrete structure using visible light (Figure 1).…”

“…We recently reported the first example of self‐assembled cages that were switchable with visible light [29] . ortho‐ Tetrafluoroazobenzene [30, 31] ‐based ligands with appended pyridyl groups were assembled with palladium(II) ions to form a dynamic mixture of two species (trimer and tetramer).…”

Guest‐Modulated Circularly Polarized Luminescence by Ligand‐to‐Ligand Chirality Transfer in Heteroleptic PdII Coordination Cages

“…In the last decades, researchers developed numerous artificial host systems offering functions such as selective host–guest interaction, [6] catalysis, [7–9] sensing [10] and molecular transportation, [11–13] just to name a few. The choice of tailormade building blocks allows to introduce functional moieties such as dyes, [14, 15] photoswitches, [16–19] redox centers, [20–24] luminophores [25–29] or chiral groups [30–37] . This can lead to emergent properties, for instance, the introduction of photoswitches [38] has been used to trigger topological rearrangements or guest binding and release [19, 39, 40] .…”

Light‐Triggered High‐Affinity Binding of Tetramethylammonium over Potassium Ions by [18]crown‐6 in a Tetrahedral Anion Cage

“…[2] In previous reports, the photoactive group was more often seen in the guest or single host molecule, while modification of complicated assemblies with such units has been less common. [3][4][5][6][7] In the latter cases, if the phototriggers are appended on the side arms of the host, the photoactivity is normally preserved. [2a,c, 4] However, isomerization of multiple photoactive groups in the main backbone of discrete constructs may be problematic, because these groups can be locked within the architecture.…”

“…Notably, a few azo (or other photoactive groups)decorated metallo-cycles and cages have been studied in light-driven structural transformations, e.g. the interconversion of homo-or heterometallic (Pd/Re) tetranuclear and dinuclear squares bridged by azobenzene-pyridyl ligands; [10] assembly and disassembly of Pd 2 L 4 lantern-shaped cage with an azobenzene-containing bis-pyridyl ligand; [11] visible-light modulated transformations of a dynamic mixture of Pd 3 L 6 and Pd 4 L 8 complexes with ortho-fluoroazobenzene modified pyridyl ligands; [7] light-driven structural rearrangement of a Pd 3 L 6 ring based on bis(3-pyridyl)dithienylethene ligands into a rhombicuboctahedral Pd 24 L 48 sphere; [6a] among other examples. [12] However, the M 4 L 6 tetrahedron had not been accessed until very recently that the [Zn 4 L 6 ] 8 + complex with dynamic coordination of a bis(diimine) ligand incorporating an azobenzene linker was reported, yet the azo bonds were locked without structural transformation under UV light.…”