Long-Lived Charge-Transfer State from B-N Frustrated Lewis Pairs Enchained in Supramolecular Copolymers

Abstract

The field of supramolecular polymers is rapidly expanding; however, the exploitation of these systems as functional materials is still elusive. To become competitive, supramolecular polymers must display microstructural order and the emergence of new properties upon copolymerization. To tackle this, a greater understanding of the relationship between monomers' design and polymer microstructure is required as well as a set of functional monomers that efficiently interact with one another to synergistically generate new properties upon copolymerization. Here, we present the first implementation of frustrated Lewis pairs into supramolecular copolymers. Two supramolecular copolymers based on π-conjugated O-bridged triphenylborane and two different triphenylamines display the formation of B-N pairs within the supramolecular chain. The remarkably long lifetime and the circularly polarized nature of the resulting photoluminescence emission highlight the possibility to obtain an intermolecular B-N charge transfer. These results are proposed to be the consequences of the enchainment of B-N frustrated Lewis pairs within 1D supramolecular aggregates. Although it is challenging to obtain a precise molecular picture of the copolymer microstructure, the formation of random blocklike copolymers could be deduced from a combination of optical spectroscopic techniques and theoretical simulation.

Scheme 1. Chemical Structures of B and N Supramolecular Monomers

S-B1 and S-N1 have an O-bridged core that imparts higher rigidity and electron density to the core’s atom compared to those of the triphenylamine of S-N2 and a-N2. All the monomers’ cores are functionalized with three peripheral gallic amides functionalized with either (S)-3,7-dimethyloctyl chains (S-B1, S-N1, and S-N2) or n-octyl chains (a-N2).

Figure 1

Supramolecular homopolymerization. VT-CD spectra of (a) poly(S-B1), (c) poly(S-N1), and (e) poly(S-N2) obtained via slow cooling from 100 or 90 °C (red lines) to 20 °C (blue lines) with 5 °C interval (gray lines). CD cooling curves of (b) poly(S-B1), (d) poly(S-N1), and (f) poly(S-N2). Measurements were performed in decalin at c = 30 μM with a cooling rate of 0.3 °C min^–1.

Figure 2

Photoluminescence properties. (a) A view of the emission of supramolecular homopolymers and copolymers under a long-wavelength UV lamp and (b) their emission spectra (λ_exc = 387 nm). Homopolymers poly(S-N2) (purple line), poly(S-B1) (light blue line), and poly(S-N1) (blue line) display violet-blue emission, while the copolymers poly[(S-B1)-co-(S-N1)] (orange line) and poly[(S-B1)-co-(S-N1)] (green line) display red-shifted emissions. Measurements were performed at 20 °C in decalin at c_tot = 30 μM (equimolar ratio of each monomer was used for copolymers).

Figure 3

Photophysical study of B–N supramolecular copolymers. (a) Schematic representation of the mechanism of CT emission in the copolymers. Light absorption from the ground state converts each species to its molecular excited state, followed by CT with the ground state of another species. (b) Lifetime measurement of poly[(S-B1)-co-(S-N1)] for degassed (orange line) and air-equilibrated (brown line) decalin (λ_em = 550 nm, λ_exc = 400 nm, 20 °C). (c) Circularly polarized luminescence spectrum of poly[(S-B1)-co-(S-N1)] (λ_exc = 365 nm, 20 °C). (d) VT-photoluminescence cooling curves (λ_ex = 387 nm) for poly[(S-B1)-co-(S-N2)] (green dots; λ_em = 530 nm) and poly[(S-B1)-co-(S-N1)] (orange dots; λ_em = 600 nm).

Figure 4

B–N supramolecular copolymerization. CD spectra at 20 °C of (a) poly[(S-B1)-co-(S-N2)] (dark green line) and poly[(S-B1)-co-(a-N2)] (light green line), and of (e) poly[(S-B1)-co-(S-N1)] (orange line). Both systems are compared with the linear sum of the related homopolymers’ CD spectra (black lines). (b) CD cooling curves of poly[(S-B1)-co-(S-N2)] (green line), poly(S–B1) (purple line), poly(S-N2) (blue line), and the linear sum of the related homopolymers curves (black line). Simulation of hypothetical poly(A-co-B) showing (c) an average block length for homocontacts (A–A, purple line; B–B, blue line) and heterocontacts (A–B, green line) (an average block length of 2 is defined as a block containing 2 monomers and 1 bond); and (d) equivalent monomer concentrations in poly(A-co-B) (solid lines) and in the related homopolymers (dotted lines). (f) CD cooling curves of poly[(S-B1)-co-(S-N1)] (orange line), poly(S-B1) (purple line), and poly(S-N1) (blue line). Measurements were performed in decalin with each monomer at c = 15 μM and a cooling rate of 0.3 °C min^–1. The linear sum was calculated as [poly(S-B1) + poly(S-N2)] (and analogous for S-N1) with each monomer at c = 15 μM.

Figure 5

Proposed mechanism of copolymerization. Schematic representation of copolymerization via slow cooling from the monomerically dissolved state.

Figure 6

Bulk properties. (a) Picture of drop-casted samples under photoexcitation using a long-wavelength UV lamp and b) their photoluminescence emission spectra (λ_exc = 400 nm, 20 °C) poly(S-B1) (purple line), poly(S-N1) (blue line), and poly[(S-B1)-co-(S-N1)]) (orange line). (c) Heating (top) and cooling (bottom) DSC traces of poly(S-B1), poly(S-N1), and poly[(S-B1)-co-(S-N1)]. A second DSC run of the mixture prepared by dissolving both compounds in dichloromethane and drying on a DSC pan prior to the measurement. Cooling/heating rate = 10 °C min^–1, endothermic process is down.