Sivakumar Vaidyanathan

Structural Mimics of Phenyl Pyridine (ppy) - Substituted, Phosphorescent Cyclometalated Homo and Heteroleptic Iridium(III) Complexes for Organic Light Emitting Diodes - An Overview

Today organic light emitting diodes are a topic of significant academic and industrial research interest. OLED technology is used in commercially available displays, and efforts have been directed to improve this technology. Design and synthesis of phosphorescent based transition metals are capable of harvesting both singlet and triplet excitons and achieve 100 % internal quantum efficiency is an active area of research. Among all the transition metals, iridium is considered a prime candidate for OLEDs due to its prominent photophysical characteristics. In the present review, we have concentrated on the Iridium based homo and heteroleptic complexes that have dissimilar substitutions on phenylpyridine ligands, different ancillary ligands and the effect of substitution on HOMO/LUMO energies and a brief discussion and correlation on the photophysical, electrochemical and device performances of the different complexes have been reviewed for organic light emitting diodes.

Controlled Energy Transfer from a Ligand to an EuIII Ion: A Unique Strategy To Obtain Bright-White-Light Emission and Its Versatile Applications

A new diphenylamine-functionalized ancillary-ligand-coordinated europium(III) β-diketonate complex showed incomplete photoexcitation energy transfer from a ligand to a EuIII ion. A solvatochromism study led to a balancing of the primary colors to obtain single-molecule white-light emission. Thermal-sensing analysis of the europium complex was executed. The europium complex, conjugated with a near-UV-light-emitting diode (395 nm), showed appropriate white-light-emission CIE color coordinates (x = 0.34 and y = 0.33) with a 5152 K correlated color temperature.

Effects of electron-withdrawing groups in imidazole-phenanthroline ligands and their influence on the photophysical properties of EuIII complexes for white light-emitting diodes

Herein, three europium complexes (Eu(TTA)3Phen-Ph-Ph, Eu(TTA)3Phen-mCF3-Ph, and Eu(TTA)3Phen-pCF3-Ph) were designed and synthesized. The N1-functionalization of the phenanthro-imidazole ring by phenyl and substituted phenyl moieties (CF3, an electron-withdrawing group) and their influences on the photophysical and electrochemical properties of EuIII complexes were determined through experimental and theoretical analyses. All the complexes in the solid state and in solution showed the distinctive emission of EuIII ion at 612 nm due to the electric dipole transition (5D0 → 7F2). Absence of ligand emission (400–600 nm; solution, thin film, and solid states) in the PL emission spectra of EuIII complexes clearly indicates efficient energy transfer from the ligand to the central metal ion (antenna effect). This was confirmed by the values of the singlet and triplet energy levels of the ligands calculated via density functional theory and time-dependent density functional theory (DFT, TD-DFT) analyses. The HOMO–LUMO levels of the complexes were determined through cyclic voltammetric studies. In addition, the Eu-complex was doped in poly(methyl methacrylate) (PMMA) to fabricate composite film devices; Eu(TTA)3Phen-pCF3-Ph complex showed highest quantum yield (78.7%). Red emission from the fabricated LEDs (EuIII complexes coated on InGaN-based LED (395 nm)) was obtained with the CIE values of x = 0.65, y = 0.34.

Recent development of phenanthroimidazole-based fluorophores for blue organic light-emitting diodes (OLEDs): an overview

The imminent global energy crisis and continuing inefficient energy utilization are driving the development of smart energy-efficient devices for display and lighting applications. Energy-efficient organic light-emitting diodes (OLEDs) are considered one of the most competitive candidates for next-generation smart displays and particularly for future energy-saving lighting sources. Recently, much effort has been devoted to attempts to generate white OLEDs comprising both fluorescent and phosphorescent materials. Efficient blue-emitting materials are extremely essential for the commercialization of white OLEDs and play a vital role in energy-efficient solid-state lighting and smart display devices. The molecular designing of efficient deep-blue materials is limited due to their intrinsic wide bandgap, poor carrier charge balance, and low efficiency in the solid state. In recent decades, phenanthroimidazole (PI)-based materials have attracted tremendous interest (due to the ease of their chemical/structural modification at the N1 and C2 positions) to produce efficient deep-blue OLEDs (satisfying the color purity criteria given by the National Television System Committee (NTSC)) (CIE: 0.14, 0.08) and European Broadcasting Union (EBU) (CIE: 0.15, 0.06). This review mainly focuses on the design of PI-based blue/deep-blue emitting materials and their applications in OLEDs. Here at first, some of the PI-based blue-fluorescence emitters endowed with unipolar and bipolar-transporting abilities are comprehensively reviewed. Then attention is focused on the typical PI-based host materials for phosphorescent OLEDs. Finally, PI-based hybridized local and charge-transfer (HLCT) active fluorescent emitters are presented in brief. The rational molecular design concepts and general synthetic routes for PI-based materials are briefly discussed.