Lattice Symmetry-Guided Charge Transport in 2D Supramolecular Polymers Promotes Triplet Formation

Abstract

Singlet-to-triplet intersystem crossing (ISC) in organic molecules is intimately connected with their geometries: by modifying the molecular shape, symmetry selection rules pertaining to spin-orbit coupling can be partially relieved, leading to extra matrix elements for increased ISC. As an analog to this molecular design concept, the study finds that the lattice symmetry of supramolecular polymers also defines their triplet formation efficiencies. A supramolecular polymer self-assembled from weakly interacting molecules is considered. Its 2D oblique unit cell effectively renders it as a coplanar array of 1D molecular columns weakly bound to each other. Using momentum-resolved photoluminescence imaging in combination with Monte Carlo simulations, the study found that photogenerated charge carriers in the supramolecular polymer predominantly recombine as spin-uncorrelated carrier pairs through inter-column charge transfer states. This lattice-defined recombination pathway leads to a substantial triplet formation efficiency (≈60%) in the supramolecular polymer. These findings suggest that lattice symmetry of micro-/macroscopic structures relying on intermolecular interactions can be strategized for controlled triplet formation.

Keywords: Monte Carlo simulations; fourier imaging; lattice symmetry; spin‐uncorrelated charge carriers; supramolecular polymers; triplets.

Figure 1

a) Molecular structure of the PMI‐L5 molecules. b) Atomic force microscopy image of the supramolecular nanoribbons. c) Height profiles extracted from (b) indicate that the nanoribbons are around 2 nm thick. d) Illustration of the supramolecular nanoribbons and their unit cells. e) Absorption spectra of the PMI‐L5 molecules (gray) and nanoribbons (orange) in water. f) Schematic of the charge‐transfer/Frenkel mixing in the supramolecular nanoribbons.

Figure 2

a) A sketch illustrating the basic concept of back focal plane (BFP) imaging. b,c) Left: Numerically simulated BFP images of a linear dipole lying horizontally on (b) and perpendicularly to (c) the sample plane. Right: Line profiles along the k _x (green) and k _y (blue) directions. d) A real‐space photoluminescence image of supramolecular nanoribbons deposited on substrates. The a‐ and b‐axes of the nanoribbon used for BFP imaging are indicated. The dashed circle indicates the position on the nanoribbon where photoluminescence was collected for BFP images in (e). e,f) BFP images measured from the area indicated in (d) e) and numerically simulated using the transition dipole strength ratio of |µ_a |²: |µ_b |²: |µ _OP|² = 6.0:11.6:1.0 f). The gray circles represent that no polarization selection was performed to the emission. g,h) Line profiles extracted from the experimentally measured BFP image in (e) (circles) and the numerically simulated image in (f) (solid lines). i) An illustration of the three transition dipole components in the nanoribbons.

Figure 3

a,b) Experimentally measured a) and numerically simulated b) BFP images of a supramolecular nanoribbon with a linear polarizer aligned parallel to the b‐axis of the nanoribbon and placed in the detection beam path. c,d) Experimentally measured c) and numerically simulated d) BFP images of the same supramolecular nanoribbon with a linear polarizer aligned parallel to the a‐axis of the nanoribbon and placed in the detection beam path. e) A sketch illustrating the processes that an optically excited Frenkel exciton can undergo. ET: energy transfer; CT: charge transfer. f) Normalized absorption (dashed) and emission (solid) spectra of supramolecular nanoribbons integrated with 0–10 mol% of PMI‐L5‐PA molecules. As the concentration of PMI‐L5‐PA increases, an additional broad peak at ≈600–700 nm appears in the absorption spectra (gray area), which originates from the PMI‐L5‐PA molecules. g) Molecular structure of the PMI‐L5‐PA molecule (top) and its influence on the ratios among the three transition dipole components (bottom).

Figure 4

a) Evolutions of Frenkel excitons (“2”, yellow) and charge carriers (“1” and “−1″, green and blue) in a 220 × 60 2D lattice at various time delays after the initial optical excitation at time t = 0 s. b,c) Total number of hops made by the charge carriers b) and Frenkel excitons c) along the length (a‐axis) and width (b‐axis) of the 2D lattice till certain time after the initial excitation. d,e) Populations of charge carriers d) and Frenkel excitons e) undergoing recombination along the length (a‐axis) and width (b‐axis) of the 2D lattice. f) Schematics of the photophysical processes in the supramolecular nanoribbons upon optical excitation: within their lifetimes, the photogenerated Frenkel excitons i) can undergo energy transfer ii) or dissociate into charge carriers primarily along the 1D molecular columns, the subsequent recombination of which gives rise to the intra‐column transition dipole, µ _b iii). In parallel, spin‐uncorrelated charge carriers in adjacent molecular columns can recombine, resulting in the inter‐column transition dipole, µ _b , and triplet excitons iv).

Figure 5

a) Photoluminescence (PL) spectra of the nanoribbons measured at room temperature (blue) and 5 K (orange). b) Time‐resolved PL measurements of PMI‐L5 molecules (gray) and nanoribbons (orange) measured at 5 K. c,d) Transient spectra of nanoribbons c) and PMI‐L5 molecules d) measured at room temperature. e) Schematics of the energy levels and transition dipoles involved in the study. The solid and dashed red lines represent radiative and nonradiative recombination, respectively. f,g) Transient kinetics of nanoribbons f) and PMI‐L5 molecules g) measured at room temperature. Solid lines are exponential fits to the data.