Design, Synthesis, Photophysical And Theoretical Investigation Of Iridium (Iii) Complexes And Organic Luminophores For Leds/Oleds And Chemosensors

Abstract

The present thesis works deals with molecular designing and synthesis of novel class of different cyclometallated ligands, organic luminophores and Donor-Acceptor organic dyes for Ir(III) complexes and explore the possibility of using the same in white light emitting diodes (LEDs), chemosensors and OLED applications.
In chapter 1, a general overview of the development of new-generation optical transition metal based complexes - introduction, literature survey of recent trends and brief objectives of the thesis was discussed. In the introduction section, the basic concepts of the OLED, materials used in OLED, transitional metal complexes (particularly Iridium complexes) and organic fluorophores using in white LEDs, chemosensors were discussed in details. The main aim and importance of the proposed work of the thesis was summarized in this chapter.
In chapter 2, design and synthesis of four homo and heteroleptic NIR emitting Ir(III) complexes with two different cyclometallted ligands (Acenaphthene-imidazole) and one ancillary ligand (acetyleacetone (acac)) were reported. The photophysical properties of synthesized Ir(III) complexes (Ir-1 to Ir-4) were systematically investigated by using different experimental techniques and confirmed by theoretical investigations. All the synthesized Ir(III) complexes were showing NIR emission in the range of 600 – 800 nm with moderate quantum yield (0.10-0.17). The photophysical and electrochemical properties have been investigated experimentally and theoretically. Compare to homoleptic complexes heteroleptic complexes were shown red shifted emission and more quantum yields. Singlet and triplet energy levels were calculated by using time dependent Density Functional Theory (TD-DFT) calculations. The excited triplet energy was estimated and correlated to structural and energetic characteristics of the reported host materials. Further, investigations have been performed to explore the optical, electronic, charge transport, and stability properties of Ir(III) complexes as charge transport and emissive materials for organic light emitting devices (OLEDs). Furthermore, all the complexes are promising luminescent and its hole transporting performance is more favorable than electron transport performances. According to theoretical calculations, it is clearly indicating that, Ir-4 complex is a more potential emitter for NIR-OLED applications.
In chapter-3, a series of new phosphorescent heteroleptic Ir(III) complexes (Ir-Ben-ppy, Ir-ben-pic and Ir-Ben-acac) with 2-phenylpyridine, acetylacetone and 2-picolinic acid as ancillary ligands have been designed and synthesized. The photophysical and electrochemical properties have been investigated experimentally and theoretically. The UV-Visible absorption and photoluminescence (PL) emission spectra of the complexes were carried out in solution as well as thin film. The PL study indicates that the Iridium complexes show broadband emission ranging from green to yellowish-green in solution and greenish-yellow to yellow emission in PMMA films with appropriate Commission Internationale de I’Eclairage (CIE) color gamut. The PLQY of the complexes were found to be in the range of 0.12 – 0.29. Singlet and triplet energy levels were calculated by using TD-DFT calculations. The excited triplet energy was estimated and correlated to structural and energetic characteristics of the reported host materials. Further, investigations have been performed to explore the optical, electronic, charge transport, and stability properties of Ir(III) complexes as charge transport and emissive materials for OLEDs. In addition, all the complexes are shown potential luminescent compoenent and its hole transporting characteristics are more favorable than electron transport performances. According to theoretical calculations, it is clearly indicating that, Ben-Ir-acac complex is a more potential emitter for OLED applications.
In chapter-4, three Near-Infrared emitting Ir(III) Complexes (Ir(ppy)2(iqbt), Ir(ppz)2(iqbt) and Ir(thpy)2(iqbt)) were successfully designed and synthesized. All the complexes show remarkably intense NIR emission in the range 600 – 800 nm. The photophysical and electrochemical properties have been investigated experimentally and theoretically. The UV-Visible absorption and PL emission spectra of the complexes were carried out in dichlormethane (CH2Cl2) solution at room temperature. The PL study indicates that the all Iridium complexes show broadband emission ranging at NIR emission with appropriate CIE color coordinates. All the complexes have shown broad emission with PLQYs in the range of 0.13-0.14 in CH2Cl2 at 298 K. The HOMO-LUMO energy levels were calculated by using cyclic voltammetry and further they were correlated with DFT analysis. The electron densities of the frontier molecular orbital distributions are also influenced by different cyclometalated ligand substitutions of these complexes and hence their effect on the phosphorescent properties and OLED performance was studied through theoretical calculations. Singlet and triplet energy levels were calculated by using TD-DFT calculations. The excited triplet energy was estimated and correlated to structural and energetic characteristics of the reported host materials. The OLED device fabricated for the complex Ir(thpy)2iqbt and the device turns on at ca. 5 V and has an external quantum efficiency (EQE) of 0.25.
In chapter-5, designed and synthesized donor (D) and acceptor (A) phosphors/dyes (D and A refer to the electron-donating and electron-withdrawing moieties, respectively) as organic dyes. The photophysical and electrochemical properties of the phosphors have been studied in details. The UV-Vis spectra of the organic phosphors show multiple absorption bands (UV to near UV region, due to π - π* transitions of conjugated chain) and all the phosphors are showing yellowish-green to orange emission with appropriate CIE color coordinates values. In contrast, bathochromic shift were observed in the solid-state emission spectra for all the organic dyes. By using TD-DFT calculations singlet and triplet energy levels also calculated. In addition, solvatochromism studies also been carried out and the results are interpreted by using Lippert−Mataga equation. White light can be realized from the currently synthesized organic yellow phosphors integrated with blue LEDs. Among all hybrid white LED devices TPA-2 has shown bright white emission with CIE color coordinates x = 0.32, y = 0.33. Theoretical calculations have been performed to explore the optical, electronic, charge transport, and stability properties of TPA derivatives as charge transport and emissive materials for OLEDs. The results show that, all the derivatives are highly luminescent and their hole transporting performances are more favourable than their electron transport performances. So these materials can be used as hole transporting materials for OLEDs.
In chapter-6, a series of acenaphthene based luminophores (comprise of acenaphthene-imidazole unit as the electron transporting moiety and carbazole as the hole transporting moiety) were designed and synthesized. The synthesized luminophores were structurally characterized through, single crystal XRD, NMR and Mass Spectrometry. To understand their photophysical properties, their UV-Vis absorption, PL emission spectra were recorded and analyzed. All the luminophores shown an absorption in the range of 250 nm and 430 nm (assigned to the π→π* and n→π* transitions, respectively). The emission spectra show an emission maximum in the range of 547-571 nm and all the luminophores give a broad yellowish-orange emission. All the luminophores were used for the detection of picric acid (PA) on the basis of fluorescence quenching. All the luminophores were showing highest quenching efficiency with PA, compared with other explosives like, 2, 4-dinitrophenol (2, 4-DNP),4-nitrophenol (NP), benzoic acid (BA), and phenol (PH). The induced photo electron transfer (PET) from luminophore to PA was confirmed by DFT analysis. Additionally, photographic detection of PA has been successfully demonstrated in solution and solid state (TLC plate). Electrochemical analyses were performed and verified using DFT calculations. Further, to get a clear picture of the Frontier Molecular Orbital (FMO) Energy levels, theoretical calculations were done by using DFT method. The singlet and triplet energy states of the excited molecules were calculated through TD-DFT calculations.
In chapter-7, benzil-imidazole derivatives (TPI-1-TPI-4) and Imidazo-pyridine (ImPy) luminophores were synthesized, characterized by NMR, Mass, UV-Visible and fluorescence spectroscopy. All the synthesized imidazole derivatives were used for the detection of picric acid (PA) on the basis of fluorescence quenching. All the luminophore’s luminescence significantly quenched by addition of very small amount of PA were compared with other explosives like, 2, 4-DNP, NP, BA, and PH. The induced photo electron transfer (PET) from luminophore to PA was confirmed by DFT analysis. Additionally, photographic detection of PA has been successfully demonstrated in solution and solid state (TLC plate). Electrochemical analyses were performed and verified using DFT calculations. Theoretical investigation like optimization, singlet-triplet energy level calculations of luminophores were calculated by using DFT and TD-DFT analysis. The excited singlet-triplet energy gap of TPI derivatives were estimated and correlated to structural and energetic characteristics of the reported host materials. Further, examinations carried out to know the optical, electronic, charge transport and stability properties of TPI derivatives as emissive and charge transport materials for OLEDs. Experimental and theoretical investigations demonstrated, all the luminophores currently synthesised can be used as potential PA detector in trace PA explosive in real samples. In addition, the fluorophores are shown potential PL properties in the blue region and further explored for OLED device fabrication. Device fabricated for TPI luminophores with ITO/PEDOT:PSS/CBP:TPI-1, TPI-2, TPI-3, and TPA-4 /TPBI/LiF/Al and ITO/PEDOT:PSS/TPI-1, TPI-2, TPI-3, and TPA-4/TPBI/LiF/Al for doped (Device II) and non-doped (Device I) devices, respectively. Compare to the entire devices TPI-4 based device shows better performance with EQE 1.4 %.
In chapter 8, deals the summary and conclusion as well as future perspective of the work. The present thesis works deals with molecular designing and synthesis of novel class of cyclometallated ligands, oranic luminophores and D-A organic dyes for Ir(III) complexes and explore the possibility of using the same in white LEDs, chemosensor and OLED applications. The observations and the conclusions derived from the present investigations are summarized in this chapter.