Exploration of New Eu(III) Complexes and their Applications in Light Emitting Diodes and Sensors: Combined Experimental and Theoretical Investigations

Abstract

This thesis work mainly going to describe about the designing and synthesis of the ancillary ligand (antenna) for the Eu(III) based complexes and to investigate their application in red/white LEDs, sensors (temperature and vapoluminescence sensor) Chapter 1, represents a general introduction of lanthanides and its unique spectral properties and it application in various fields. The importance of Eu(III) complexes, antenna effect (DBM and ancillary ligand), the lanthanides (specially Eu(III)) application in Red/white LED, temperature sensor and vapoluminescence were systematically discussed. In addition, the recent progress of Eu(III) complexes for LEDs and sensors were also been discussed. The main objective and significance of the present work of the thesis were documented. Chapter 2, Narrow band red emitting phosphor plays vital role in high performance smart white LEDs. In this context, a series of new Eu(III) complexes have been synthesized with neutral ligand (C1functionalised Phenanthro-imidazole-based ancillary ligands substituted with functionalized phenyl moieties and DBM as an anionic ligand. All the newly synthesized Eu-complexes showed extremely narrow band red emission due to ED transition (5D0-7F2, both in solid and thin film) with high quantum efficiency. Combined experimental and theoretical study indicates that the energy transfer from ligand to metal ion is complete. Temperature dependent PL study in solution reveals that the red emission is retained (only the ED (5D0-7F2) intensity decreases with increasing temperature). Red LED and hybrid white LEDs were fabricated by using near UV LED chip with Eu-complexes and near UV/blue LED conjugated with yellow dye and Eu-complex mixture, respectively. Hybrid white LED (Eu(DBM)3PhenpCN-pCF3 + yellow dye) showed superior CRI (84%), CIE (x = 0.36, y = 0.39) (near UV based) and CRI (82%), CIE (x = 0.34, y = 0.36) (blue LED based). In addition, presently studied Eu-complexes showed excellent reversible on-off-on luminescence behavior with exposure of acid-base vapours. Detailed spectroscopic investigation reveals that the protonation of the Eu-complexes (ligands) plays key role in the on-off-on luminescence. Chapter 3, a series of luminescent ternary Eu(III) complexes were synthesized and studied their photophysical properties. PL emission spectra of ligands showed blue emission, whereas Eu(III) complex showed red emission in solid while in solution phase Eu(DBM)3Phen-pCH3-mCF3 are showing multiple emission (near white emission). Fabrication of LED has been done by conjugating the Eu-complex with near UV LED the obtained results are very close to National Television System Committee (NTSC) standard for pure red emission. LED fabrication of Eu(DBM)3Phen-pCH3-mCF3 with 1:50 ratio show near white emission with CIE, x = 0.35, y = 0.20. In addition, presently studied Eu(III) complex shows excellent sensing behavior in the thermo-sensor (in the range 303K-453K) and vapoluminescent sensor.
Chapter 4, Eu(III) complexes coordinated with phenanthroimidazole ligands based on fluorinated moieties (ancillary ligand) and DBM (anionic ligand) were designed and synthesized successfully. Eucomplexes in solid phase shown pure red emission (due to ED transitions) and solution slightly deviated (observed multiple emission) due to ligand emission in the EuIII complex. Solvatochromism study reveals that the Eu-complexes showed red emission along with the ligand emission peak (comparatively intense) in methanol and DMSO solution, which leads to obtain white emission. In addition, the lifetime of the fluorinated mCF3 ligand revealed high. Remarkable linear decrease in luminescence intensity with increasing the temperature, open up the new window to use this material as sensitive temperature sensor in the temperature range of 30−80oC. Fabricated red LEDs (InGaN) showed very close to the NTSC standard values for bright red emission. EuIII complexes exhibits on−off switching of photoluminescence via vapoluminescence process and observed best response to the acid-base (HCl-NH3) vapours. Chapter 5, a series of soft UV excitable new narrow-band red-emitting trivalent europium complexes were designed, synthesized and their photophysical properties were studied. The newly synthesized ancillary ligands showed deep blue emission with CIE y < 0.1. In contrast, the corresponding Eu(III) complexes shown extremely narrow-band red emission (in solid, solution [except pCF3 functionalized Eu(III) complex, it shown both ligand and Eu emission leads to nearly white emission] and thin-film) with wide excitation range (200-500 nm, where the near UV and blue LED emission occurs). Solvatochromism study reveals that the Eu-complexes showed multi-color emission. The hybrid white LED was fabricated by conjugating the near UV LED with yellow organic dye and 5-Eu complex, the results showed extremely high CRI (96%), CCT (4447 K) and the CIE color coordinate (x = 0.36, y = 0.35). In addition, the complexes were used for acid-base sensing. All the complexes showed very good reversible on-off-on luminescence behavior (exposure of complexes with HCl and NH3) and these complexes can find potential applications in anti-counterfeiting. Chapter 6, a series of smart luminescent Eu(III) complexes were synthesized by using C1-functionalize phenanthro-imidazole derivatives as a neutral ligand and DBM (anionic ligand). PL emission spectra of ligands showed blue to bluish-green emission, whereas their corresponding Eu-complexes shown multicolor emission in solution and red emission in solid state (solid as well as neat thin film).Energy transfer mechanism between ligand to Eu(III) ion is explained based on both theoretical (DFT and TD-DFT) and experimental study. The red LEDs were fabricated and found all the LEDs shown excellent color purity. Hybrid white LED was fabricated by conjugating the mixed phosphor dye (ligand and complex) with near UV LED shown superior white light emission (CCT (5298), CRI (80%), CIE (x = 0.337, y = 0.343)). In addition, dual emission Eu(III) complexes were explored for thermos-sensor (in the range 303 K – 353 K) applications. Paper based acid-base vapour sensor (exposure leads to switch in the emission color (bluishgreen to red)) of these complexes were explored and can also be used for anti-counterfeiting applications.
Chapter 7, Promoting from the structural flexibility and easy solution process capability, single component white light emitting pure organo-lanthanide complexes have considered as a promising candidate for solid state lighting. A series of ancillary ligand by using imidazo-bipyridyl derivatives and DBM, as anionic ligand are employed to synthesized ternary Eu(III) complex.The photophysical and electrochemical properties of the ligand and their Eu(III) complexes were carefully investigated and the energy transfer mechanism were understood by combined experimental and theoretical calculation. Multiple-emission from both ligand and Eu(III)-ion leads to single component white light emission. White light emitting diodes (LED)was fabricated by conjugating the Eu-complex with near UVLED, the results showed superior performance (CIE (x = 0.33, y = 0.33, close to NTSC standard for pure white emission), correlated color temperature CCT (5386 K) and color rendering index CRI (63%)).The selected Eu-complex was further coated on blue LED chip and the corresponding white LED showed CIE (x = 0.36, y = 0.35), CCT (4234K) and CRI (75%). In addition, taking benefit of dual characteristic emission of the Eu-complexes, the ratiometric sensing behavior was explored. The Eu(DBM)3L-mCF3 have the relative sensitivity value of 4.9% K-1 at 323 K. Chapter 8, spectral window of dual visible light emissions of molecular europium complex leads to white light can be modulated by design strategy. A Eu-complex with butterfly structure displays dual emission, leads to single phase white light emitting molecular complex with excellent colour quality. Such behaviour is not observed with an analogous Eu-complex with triangle structure (rather shows usual red emission), suggesting that the partial lighting up of Eu-ion along with ligand emission (controlled energy transfer from ligand to EuIII ion) is more imperative for the creation of white light. The white LED assembled using butterfly Eu phosphor as white emitter yields a bright pure white light with CRI (95%), CCT (5457 K) and CIE (x = 0.33, y = 0.33),whereas the triangle Eu-phosphor displays red light with CIE value of x = 0.58, y = 0.31. The fabricated white/red LED continued to emit a bright white/red light even more than a year. The present design strategy can be widely adapted to synthesis single phase white emitting Eu-phosphor as well as explored for temperature sensing and sensor for vapoluminescence. Chapter 9, deals the summary and conclusion as well as future perspective of the work. The present thesis works deals with rational design and synthesis of new and novel class of ancillary ligand and their corresponding Eu3+ complexes for white LEDs and sensor (temperature and vapoluminescence). In addition, Eu molecular complexes also been explored for single component white light emitter based white LEDs. The observations and the conclusions derived from the present investigations are summarized in this chapter.