Temperature‐ and Voltage‐Induced Ligand Rearrangement of a Dynamic Electroluminescent Metallopolymer

Abstract

A dynamic-covalent metal-containing polymer was synthesized by the condensation of linear diamine and dialdehyde subcomponents around copper(I) templates in the presence of bidentate phosphine ligands. In solution, the red polymers undergo a sol-gel transition upon heating to form a yellow gel, a process that can be either reversible or irreversible depending on the solvent used. When fabricated into a light-emitting electrochemical cell (LEC), the polymer emits infrared light at low voltage. As the voltage is increased, a blue shift in the emission wavelength is observed until yellow light is emitted, a process which is gradually reversed over time upon lowering the voltage. The mechanism underlying these apparently disparate responses is deduced to be due to loss of the copper phosphine complex from the polymer.

Origin of a counterintuitive yellow light-emitting electrochemical cell based on a blue-emitting heteroleptic copper(i) complex

“…6). Although a LC character is unusual for [Cu(N^N)(P^P)] + , in which the metal-charge transfer nature is classically observed, 2,11,12,14,17,18,[20][21][22][23][24] it is consistent with the structured shape of both PL and EL responses. Even if at the GS geometry, there are several triplet states (T 1 -T 3 ) lying between the optically populated S 1 and the ground state, at the structurally optimized S 1 and T 1 geometries, S 1 is separated from the ground state only by T 1 (Fig.…”

“…In detail, 1 shows three quasi-reversible oxidation waves at +1.02, +1.28, and +1.51 V, as well as a quasi-reversible reduction wave at −2.32 V. As a complement, the electronic structure of the ground state (GS) of 1 was optimized via DFTTables S5 and S6 and Fig. S3 2,9,11,12,14,17,18,20,21,28 the frontier molecular orbitals of 1 in the GS, i.e. HOMO (highest-occupied molecular orbital) and LUMO (lowest-unoccupied molecular orbital), are both located at the impy ligand - Fig.…”

“…9,[13][14][15][22][23][24] In general, the EL red-shifts around 20 nm compared to the PL due to the energy stabilization of the excited state under external electrical stimuli. 9,10,15 In the few cases, in which a large shift has been reported, this has been rationalized in terms of (i) a change in the nature of the lowest emitting state due to the concentration effect of the films, 54 and (ii) a reversible substitution of the ligands due to the weak coordination to the Cu(I), 12 and (iii) a change in the distribution of excited states due to the polarization effects. 55 As shown in Fig.…”

“…Even if at the GS geometry, there are several triplet states (T 1 -T 3 ) lying between the optically populated S 1 and the ground state, at the structurally optimized S 1 and T 1 geometries, S 1 is separated from the ground state only by T 1 (Fig. 6) 2,11,12,14,17,18,[20][21][22][23][24] In detail, the minimum of the potential energy surface of 1 in the S 1 excited state is located 0.44 eV above the energy of the system at the T 1 optimized geometry, leading to a ΔE S 1 -T 1 > 3000 cm −1 . This points to a lack of TADF that can be easily explained by the LC nature of the excited state that provides a large exchange integral between both excited states, as suggested by the large ΔE S 1 -T 1 .…”

“…[1][2][3][4][5][6][7][8]19 Encouraged by the aforementioned, heteroleptic copper(I) complexes based on diimine (N^N) and diphosphine (P^P) ligands, i.e., [Cu(N^N)(P^P)] + , have been recently revisited by several groups. 2,9,12,14,17,18,20,21 They have established several simple guidelines for enhancing both the synthesis protocol 20,21 and the ligand design 2,17,18 to take full advantage of the photoluminescence and electrochemical features of these materials. More striking, these guidelines have also proven to be useful for enhancing their electroluminescence features.…”