Synthesis, photophysical properties and application in organic light emitting devices of rhenium( i ) carbonyls incorporating functionalized 2,2′:6′,2′′-terpyridines

Tomasz Klemens; Anna Świtlicka-Olszewska; Barbara Machura; Marzena Grucela; Henryk Janeczek; Ewa Schab-Balcerzak; Agata Szlapa; Slawomir Kula; Stanisław Krompiec; Karolina Smolarek; Dorota Kowalska; Sebastian Mackowski; Karol Erfurt; Piotr Lodowski

doi:10.1039/C6RA08981J

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DOI: 10.1039/C6RA08981J (Paper) RSC Adv., 2016, 6, 56335-56352

Synthesis, photophysical properties and application in organic light emitting devices of rhenium(I) carbonyls incorporating functionalized 2,2′:6′,2′′-terpyridines†

Tomasz Klemens^a, Anna Świtlicka-Olszewska^a, Barbara Machura*^a, Marzena Grucela^b, Henryk Janeczek^b, Ewa Schab-Balcerzak*^bc, Agata Szlapa^d, Slawomir Kula^d, Stanisław Krompiec^d, Karolina Smolarek^e, Dorota Kowalska^e, Sebastian Mackowski^e, Karol Erfurt^f and Piotr Lodowski^g
^aDepartment of Crystallography, Institute of Chemistry, University of Silesia, 9th Szkolna St., 40-006 Katowice, Poland. E-mail: bmachura@poczta.onet.pl
^bCentre of Polymer and Carbon Materials, Polish Academy of Sciences, 34M. Curie-Sklodowska Str., 41-819 Zabrze, Poland
^cDeparment of Polymer Chemistry, University of Silesia, 9th Szkolna St., 40-006 Katowice, Poland. E-mail: ewa.schab-balcerzak@us.edu.pl
^dDepartment of Inorganic, Organometallic Chemistry and Catalysis, Institute of Chemistry, University of Silesia, 9th Szkolna St., 40-006 Katowice, Poland
^eInstitute of Physics, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University, 5 Grudziadzka Str., 87-100 Torun, Poland
^fDepartment of Chemical Organic Technology and Petrochemistry, Silesian University of Technology, Krzywoustego 4, 44-100 Gliwice, Poland
^gDepartment of Theoretical Chemistry, Institute of Chemistry, University of Silesia, 9th Szkolna St., 40-006 Katowice, Poland

Received 7th April 2016 , Accepted 31st May 2016

First published on 3rd June 2016

Abstract

Several new rhenium(I) complexes [ReCl(CO)₃(4′-R-terpy-κ²N)] incorporating 2,2′:6′,2′′-terpyridine-based ligands were successfully synthetized and characterized by IR, NMR (¹H and ¹³C), UV-vis spectroscopy and single crystal X-ray analysis. The luminescent properties of [ReCl(CO)₃(4′-R-terpy-κ²N)] were studied in solution and solid state, at 298 and 77 K. To better understand the photophysical properties of [ReCl(CO)₃(4′-R-terpy-κ²N)], density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations were performed. Preliminary studies towards application of these complexes in organic light emitting diodes (OLEDs) were carried out, including testing the possibility of electroluminescence intensity increase by including metallic nanowires in the structure design.

Introduction

Since the first observation of electronically excited luminescence in [ReCl(CO)₃(4,7-(Ph)₂phen)] and [ReCl(CO)₃(5-R-phen)] (R – H, CH₃, Cl, Br, NO₂),¹ rhenium(I) tricarbonyl complexes based on 1,10-phenanthroline, 2,2′-bipyridine or related bidentate diimine derivatives [Re(CO)₃L_a(N–N)]ⁿ⁺ (L_a – axial ligand; n = 0 or 1) have attracted significant attention due to their photochemical and photophysical properties as well as potential applicability.² The studies revealed that the axial ligand L and structural variations of N,N ligand strongly affect the nature and energetic order of low-lying excited states and, thus, the photophysics, photochemistry and electrochemistry of [Re(CO)₃L_a(N–N)]ⁿ⁺ complexes. Depending on the relative energy levels of the metal and ligand orbitals, the occurrence of various excited states (metal-to-ligand charge transfer (MLCT), ligand-to-ligand charge transfer (LLCT), σ-bond-to-ligand charge transfer (σ → π*) and intraligand (IL)) is possible, as well as tuning of interaction between them.³ Importantly, most of [Re(CO)₃L_a(N–N)]ⁿ⁺ exhibit good phosphorescence properties thanks to the strong spin–orbit coupling induced by the Re ion that enhances the singlet–triplet mixing, affording a fairly long-lived excited state and appreciable quantum efficiency of emission. Intersystem crossing processes (ISC) in this class of compounds were interpreted on the basis of density functional theory (DFT) calculations including spin–orbit coupling (SOC) effects.⁴ Richness and variability of the photophysical and photochemical behaviours make these compounds attractive for solar energy conversion,⁵ luminescence sensing,⁶ for photosensitization of organic substrates,⁷ organic light-emitting diodes (OLEDs)⁸ and in other fields such as catalysis⁹ and medicinal chemistry.¹⁰

Herein, we present the synthesis, characterization and photophysical properties of six new Re(I) complexes incorporating 2,2′:6′,2′′-terpyridine ligand framework (Scheme 1). In particular, we aim to elucidate the possibility to tune the emission performance of Re(I)-terpyridine carbonyls by structural modifications of terpy ligand. 2,2′:6′,2′′-Terpyridine derivatives are noticeable ligands in coordination chemistry, but investigations of Re(I)-terpyridine carbonyls are scarce and the structure–property relationship remains unexplored.¹¹ The paucity of data on Re(I)-terpyridine carbonyls can be attributed to the fact that [Re(CO)₃(terpy)Cl], synthesized in 1988 by Juris and co-workers,¹² was found to be nonluminescent in solution at room temperature. Recent studies, however, revealed the possibility of improving the emission performance of [ReCl(CO)₃(4′-R-terpy-κ²N)] by structural modifications of terpy ligand and proved that Re(I) complexes incorporating 2,2′:6′,2′′-terpyridine ligand framework are suitable candidates for OLED materials.^11c,13


	Scheme 1 2,2′:6′,2′′-Terpyridine derivatives employed in this study.

The luminescent properties of [ReCl(CO)₃(4′-R-terpy-κ²N)] were studied in solution and solid state, at 298 and 77 K. To get detailed insight into their electronic structure and spectroscopic properties, the density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations were performed.

Experimental section

Materials

Re(CO)₅Cl was commercially available (Sigma Aldrich) and was used without further purification. Poly(9-vinylcarbazole) PVK (M_n = 25 [thin space (1/6-em)]

000–50

000) was purchased from Sigma Aldrich and used without additional purification. Poly(3,4-(ethylenedioxy)thiophene) [thin space (1/6-em)]

poly-(styrenesulfonate) (PEDOT [thin space (1/6-em)]

PSS) (0.1–1.0 S cm⁻¹) and substrates with pixilated ITO anodes were supplied by Ossila. 2,2′:6′,2′′-Terpyridine derivatives were obtained as described in our previous work.¹⁴ NMR spectra of those derivatives were also reported previously.¹⁴ All solvents for synthesis were of reagent grade and were used as received. For spectroscopy studies HPLC grade solvents were used.

Silver nanowires (AgNWs) were synthesized using the polyol process, in which ethylene glycol (EG) served as the reducing and solvent reagent.¹⁵ The product was purified by centrifugation process and the mixture was diluted with isopropyl alcohol and centrifuged. The supernatant containing silver particles and unreacted substrates was removed. Finally the product was redispersed in 2 mL of pure water. Scanning electron microscopy yields diameters of the nanowires in the range from 50 to 150 nm, while their lengths range from 4 to even 30 microns.¹⁶ The samples for investigating the influence of silver nanowires on the electroluminescence properties of the devices were fabricated by placing the nanowires into PEDOT [thin space (1/6-em)] :PSS layer. The morphology of the samples were not substantially affected by the presence of the metallic nanoparticles.

General synthesis route for [ReCl(CO)₃(4′-R-terpy-κ²N)] complexes (1–6)

Complexes 1–5. Re(CO)₅Cl (0.10 g, 0.27 mmol) and suitable 4′-R-terpy ligand (0.27 mmol) were dissolved in acetonitrile (80 mL) and refluxed under argon for 6 hours. The resulting reaction solution was reduced in volume to 10 mL and allowed to cool to room temperature. The resulting yellow (1, 2, 3 and 4) or orange (5) solid was collected by filtration washed with diethyl ether, and dried. X-ray quality crystals were obtained by slow recrystallization from acetonitrile.

Complex 6. Re(CO)₅Cl (0.10 g, 0.27 mmol) and suitable 4′-R-terpy ligand (0.27 mmol) were dissolved in argon-saturated acetonitrile (80 mL). Resulting solution was heated in autoclave for 24 hours to 150 °C, remained in that temperature for 30 hours and was cooled to room temperature for another 30 hours. X-ray quality brown crystals were obtained by filtration and washed with diethyl ether.

[ReCl(CO)₃(4′-R¹-terpy-κ²N)] (1). Yield: 75%. IR (KBr, cm⁻¹): 2021(vs), 1899(vs) and 1887(vs) ν(C [triple bond, length as m-dash]