Diketopyrrolopyrroles disubstituted with alkylated thiophenes: effect of the donor unit size and solubilizing substituents on their redox, photo- and electroluminescence properties

Abstract

Systematic studies of redox, spectroscopic and electroluminescent properties of donor–acceptor–donor (DAD) electroactive compounds consisting of a diketopyrrolopyrrole central accepting unit symmetrically disubstituted with mono-, bi- or terthiophenes (T1, T2 and T3 series) are presented. The potential of the diketopyrrolopyrrole unit reduction is influenced neither by the type of alkyl substituent at the pyrrole nitrogen atoms, nor by the position of alkyl substituents in the thiophene rings of the D segment. Being in the range of −1.66 to −1.67 V vs. Fc/Fc⁺ in the compounds of the T1 series it is, however, raised by 100–120 mV for the T3 series of compounds. The oxidation potential of the studied compounds is even more strongly affected by the D segment, decreasing from 0.51–0.52 V in T1 compounds to 0.25–0.26 V vs. Fc/Fc⁺ in T3 ones. It is also dependent on the position of the alkyl solubilizing substituent in the thiophene ring, being raised by ca. 40 mV by changing the alkyl substituent position from 5 to 3. The electrochemical data are in perfect agreement with the spectroscopic results, as judged from the close similarity of the optically- and electrochemically-determined band gaps. The trends observed experimentally are reproduced by DFT calculations. The calculated values of ionization potentials and electron affinities are very close to the experimental ones. The highest photoluminescence quantum yields, approaching 80%, were measured for T1 compounds, whereas for T3 ones this value dropped below 20%. Time resolved photoluminescence studies consistently showed shorter emission lifetimes and larger non-radiative rate constants for compounds of lower photoluminescence quantum yields. Guest/host-type single layer light emitting diodes were fabricated from the most luminescent compounds (T1 series), molecularly dispersed (1 wt%) in a matrix consisting of 70 wt% poly(N-vinylcarbazole) and 30 wt% 2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazole. Appropriate alignment of the matrix components energy levels and those of the T1 compounds resulted in effective electroluminescence of the guest molecules. The fabricated diodes showed luminance exceeding 2600 cd m⁻² with a luminous efficiency of 0.7 cd A⁻¹.