Origin of a counterintuitive yellow light-emitting electrochemical cell based on a blue-emitting heteroleptic copper(i) complex. Issue 21 (10th May 2016)

• Record Type: Journal Article
• Authors: Weber, Michael D.; Garino, Claudio; Volpi, Giorgio; Casamassa, Enrico; Milanesio, Marco; Barolo, Claudia; Costa, Rubén D.
• Is Part Of: Dalton transactions. Volume 45:Issue 21(2016)
• Page Start: 8984
• Page End: 8993
• Publication Date: 2016-05-10
• Subjects: Chemistry, Inorganic -- Periodicals; Chemistry, Physical and theoretical -- Periodicals; Chemistry, Inorganic -- Periodicals; 546.05
• DOI: 10.1039/c6dt00970k
• Languages: English
• ISSNs: 1477-9226

Abstract

Abstract : A new copper(i ) complex, which lacks of charge transfer character in the excited state, features a blue fluorescence and yellow phosphorescence photo- and electro-responses, respectively. Abstract : This work provides the synthesis, structural characterization, electrochemical and photophysical features, as well as the application in light-emitting electrochemical cells (LECs) of a novel heteroleptic copper(i ) complex – [Cu(impy)(POP)][PF6 ], where impy is 3-(2-methoxyphenyl)-1-(pyridine-2-yl)imidazo[1, 5- a ]pyridine and POP is bis{2-(diphenylphosphanyl)phenyl}ether. This compound shows blue photoluminescence (PL, λ = 450 nm) in solution and solid-state and excellent redox stability. Despite these excellent features, the electroluminescence (EL) response is located at ∼550 nm. Although the EL spectrum of LECs is typically red-shifted compared to the PL of the electroluminescent material, a shift of ca . 100 nm represents the largest one reported in LECs. To date, the large shift phenomena have been attributed to (i) a change in the nature of the lowest emitting state due to a concentration effect of the films, (ii) a reversible substitution of the ligands due to the weak coordination to the Cu(i ), and (iii) a change in the distribution of the excited states due to polarization effects. After having discarded these along with others like the irreversible degradation of the emitter during device fabrication and/or under operation conditions, driving conditions, active layer composition, and changes in the excited states under different external electrical stimuli, we attribute the origin of this unexpected shift to a lack of a thermally activated delayed fluorescence (TADF) process due to the solely ligand-centered character of the excited states. As such, the lack of a charge transfer character in the excited states leads to a blue-fluorescence and yellow-phosphorescence photo- and electro-responses, respectively. This corroborates recent studies focused on the design of TADF for heteroleptic copper(i ) complexes. Overall, this work is a clear insight into the design of new copper(i ) complexes towards the preparation of blue LECs, which are still unexplored.