Luminescence and Length Control in Nonchelated d8 -Metallosupramolecular Polymers through Metal-Metal Interactions

Abstract

Supramolecular polymers (SPs) of d⁸ transition metal complexes have received considerable attention by virtue of their rich photophysical properties arising from metal-metal interactions. However, thus far, the molecular design is restricted to complexes with chelating ligands due to their advantageous preorganization and strong ligand fields. Herein, we demonstrate unique pathway-controllable metal-metal-interactions and remarkable ³ MMLCT luminescence in SPs of a non-chelated Pt^II complex. Under kinetic control, self-complementary bisamide H-bonding motifs induce a rapid self-assembly into non-emissive H-type aggregates (1A). However, under thermodynamic conditions, a more efficient ligand coplanarization leads to superiorly stabilized SP 1B with extended Pt⋅⋅⋅Pt interactions and remarkably long ³ MMLCT luminescence (τ^{77 K} =0.26 ms). The metal-metal interactions could be subsequently exploited to control the length of the emissive SPs using the seeded-growth approach.

Keywords: hydrogen bonding; luminescent assemblies; metal-metal-interactions; pathway complexity; supramolecular polymers.

Scheme 1

a) State‐of‐the‐art ligand design using polydentate ligands to achieve luminescent MSPs based on close Pt⋅⋅⋅Pt‐contacts. b) Molecular structure and SP pathways of complex 1 investigated in this work leading to tunable MMLCT luminescence.

Figure 1

a), d) VT‐UV/Vis absorption (solid lines, c=15 μM) and emission spectra (dashed lines, c=20 μM, λ _exc=333 nm) upon cooling solutions of 1 (1 K min⁻¹, MCH:CHCl₃ 8 : 2). b), e) AFM images (HOPG). c), f) SEM images (silicon wafer) at 298 K. *Raman scattering peak.

Figure 2

a), b) Time‐resolved photoluminescence decay profiles of SP 1A (a) and SP 1B (b) at 77 K (λ _exc=325 nm, MCH:CHCl₃ 8 : 2, c=20 μM). c)–e) Phosphorescence lifetime imaging (PLIM) studies (λ _exc=375 nm, T=298 K) with micrographs of the corresponding solutions under irradiation with an LED lamp (λ _exc=365 nm). c) Photoluminescence lifetime map of SP 1A showing the absence of emission. d) Photoluminescence lifetime map of SP 1B. e) Micrograph of SP 1B depicting detection events (photons that reach the detector, which correlates with the photoluminescence intensity).

Figure 3

a) Cooling curves (1 K min⁻¹) and fits to the nucleation‐elongation model for the self‐assembly of 1A (unstirred, blue) and 1B (stirred, 800 rpm, orange). b) VT‐UV/Vis spectra recorded upon heating a solution of SP 1A (MCH:CHCl₃ 8 : 2, 1 K min⁻¹). c), d) Development of the emission intensity at λ _max in time‐dependent emission studies of the 1A→1B transformation upon stirring a solution of SP 1A at 298 K (c), shortly tempering the solutions at 313 K and subsequently continuing the measurement at 298 K (d; MCH:CHCl₃ 7 : 3, 800 rpm, λ _exc=333 nm).

Figure 4

a) Partial ¹H NMR spectra monitoring the formation of SP 1B upon increasing the MCH‐d₁₄:CDCl₃ ratio (c=1 mM, T=298 K). b) Partial thin‐film ATR‐FTIR spectra of SP 1A (blue) and SP 1B (orange). The colored areas denote the regions of free (red), intra‐ (cyan) and intermolecularly (green) H‐bonded amide groups. c), d) Proposed molecular arrangements of the complexes in the two SPs.

Figure 5

a) Possible conformations per bisamide moiety. b) Time‐dependent UV/Vis studies of 1A upon rapidly cooling a hot monomer solution from 368 K to 298 K (λ=333 nm, MCH:CHCl₃ 8 : 2, T=298 K). c) Energy landscape illustrating the SP of 1. The abbreviations “x‐Pt‐y” listed below the monomer states denote the different conformers of 1.

Figure 6

Dispersion‐corrected PM6 geometry‐optimized tetramers of 1 in a parallel (a) and a helical packing (b).

Figure 7

a) Schematic representation of the experimental procedure for the seed‐mediated LSP. b) Time‐dependent emission spectra after adding 1A to seeds of 1B (c=20 μM, T=298 K, MCH:CHCl₃ 7 : 3, λ _exc=333 nm). c) Plot of the emission intensity against time during four LSP cycles (λ=593 nm). d) Length distribution of the fibers after each cycle (lengths extracted from SEM images). AFM (e) and PLIM (f) images recorded after completion of each cycle (λ _exc=375 nm).