Ireneusz Glowacki

The nature of trapping sites and recombination centres in PVK and PVK?PBD electroluminescent matrices seen by spectrally resolved thermoluminescence

Two electroluminescent polymer matrices poly(N-vinylcarbazole) (PVK) and PVK with 40 wt% of 2-tert-butylphenyl-5-biphenyl–1,3,4-oxadiazole (PBD) were studied using spectrally resolved thermoluminescence (SRTL) in the temperature range 15–325 K. The comparison of the SRTL results with the electroluminescence (EL) spectra has allowed identification of the localized (trapping) sites and the radiative recombination centres present in the investigated matrices. In the neat PVK films deep traps with a depth about 200 meV, related to triplet excimers dominate, while in the PVK–PBD (40 wt%) blend films the traps that are related to triplet exciplexes formed by the carbazole groups and the PBD molecules dominate. Depth of the traps in the PVK–PBD blend is somewhat lower than that in the neat PVK. An analysis of the EL spectra shows that in the PVK and in the PVK–PBD blend the dominant radiative centres are singlet excimers and singlet exciplexes, respectively. However, in the neat PVK some contributions of the triplet monomer and the triplet excimer states in the EL were also detected.

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

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.

Modification of polyacetylene and polyalkylthiophene by doping with heteropolyanions

Abstract Polyacetylene and poly(3-n-butylthiophene) can be chemically and electrochemically doped with 12-molybdophosphoric anions to give conducting polymers (conductivities of 0.3 and 0.2 S/cm respectively). The doping-induced spectroscopic changes (IR, ESCA) are in both cases the same as those associated with classical monovalent anion doping. Temperature dependences of the conductivity are weak: the activation energy is 20 and 31 meV for polyacetylene and poly(3-n-butylthiophene) respectively. The doped polymers exhibit high stability of conductivity upon exposure to air.

Efficient 8-oxyquinolinato emitters based on a 9,10-dihydro-9,10-diboraanthracene scaffold for applications in optoelectronic devices

A detailed experimental characterization and theoretical evaluation of the optical as well as other relevant physicochemical properties of a series of 9,10-dihydro-9,10-diboraanthracene bis(8-oxyquinolinates) and a few other related systems is reported. The obtained compounds exhibit green luminescence with quantum yields of emission of up to 63% in CH2Cl2. Single crystal X-ray diffraction studies indicate that 9,10-dihydro-9,10-diboraanthracene complexes exist either as bent conformers (stabilized by a weak intramolecular CH⋯O interaction bringing two 8-oxyquinolinato ligands closer to each other) or symmetrical ones (bearing two ligands related by a centre of symmetry and separated from each other). Theoretical calculations revealed that the LUMO levels are lower for the bent conformers than for the symmetrical ones. This suggests that the luminescent properties of the studied compounds are affected by their specific structural properties. The obtained compounds were used as emitters for the construction of organic light emitting diodes (OLEDs). The highest luminance of ca. 2000 cd m−2 was recorded for the device containing only 2.0 wt% of the 1,6-difluoro-9,10-dihydro-9,10-diboraanthracene core in the poly(N-vinylcarbazole/2-t-butylphenyl-5-biphenyl-1,3,4-oxadiazole (PVK:PBD) matrix. The fabricated OLEDs exhibit current efficiency in the range from 0.5 to 1.1 cd A−1.

Effect of fluorine substitution of the β-ketoiminate ancillary ligand on photophysical properties and electroluminescence ability of new iridium(III) complexes

In this paper we report studies of the electronic structure as well as photophysical and emissive properties of a family of new heteroleptic iridium(III) complexes of the general formula [Ir(bzq)2(N∧O)] with N,O-donating β-ketoiminate ligands equipped with different directly fluorinated N-aryl moieties. For this purpose, a series of suitable β-ketoimines were prepared and successfully employed in the synthesis of iridium(III) phosphorescent materials. It is worth emphasizing that the developed synthetic strategy, involving the use of microwaves to accelerate the reaction course, allows reduction of the process time by up to 10 minutes. All the obtained complexes were characterized by spectroscopic methods and the structures of three of them were resolved by X-ray methods. The cyclic voltammetry measurements allowed determination of the energies of HOMO and LUMO levels of the studied complexes. Moreover, the HOMO–LUMO gaps predicted for these compounds by DFT methods showed a good correlation with the experimental values. The determined negligible influence of ligand modification on LUMO levels and only small change of HOMO levels explains the observed slight effect of different directly fluorinated N-aryl moieties on photophysical properties of the complexes. All Ir complexes were tested in host–guest type organic light emitting diodes (PhOLEDs) with an poly(N-vinylcarbazole):2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazole matrix. The PhOLEDs based on all studied iridium complexes emitted green light. The most efficient phosphorescent emitter turned out to be the iridium complex with one fluorine atom substituted in the para position in the phenyl ring of the ancillary ligand. Diodes with the emissive layer containing 1 wt% of this emitter showed the best operating parameters: luminance of about 13 000 cd m−2 and current efficiency close to 10 cd A−1. A comparison of photoluminescence, electroluminescence and spectrally resolved thermoluminescence spectra of the investigated systems revealed an important role of charge carrier trapping in the TL phenomena.

CCDC 1449071: Experimental Crystal Structure Determination

Related Article: Katarzyna Jarzembska, Radoslaw Kaminski, Krzysztof Durka, Marcin Kubsik, Krzysztof Nawara, Ewelina Witkowska, Sergiusz Lulinski, Jacek Waluk, Ireneusz Glowacki, Krzysztof Wozniak|2017|Dyes Pigm.|138|267|doi:10.1016/j.dyepig.2016.11.039