Luminescent EuIII complexes based on phenanthro-imidazole ligands for white LEDs/OLEDs and temperature sensors: Combined experimental and theoretical investigations

Abstract

The present theis works deals with molecular designing and synthesis of novel class bipolar or ancillary ligand for europium complexes and explore the possibility of using the same in white LEDs, temperature sensor and OLED applications

In chapter 1, a general overview of the development of new-generation optical lanthanide based complexes - introduction, literature survey of recent trends and brief objectives of the thesis was discus. In introduction, the basic concepts of the lanthanides, OLED device materials, LEDs applications and ratiometric thermal sensors were discussed. The main aim and importance of the proposed work of the thesis was summarized in this chapter.

In chapter 2, four europium complexes (Eu(TTA)3Phen-Ph-Ph, Eu(TTA)3Phen-mCF3-Ph Eu(TTA)3Phen-pCF3-Ph and Eu(TTA)3Phen-Fl-Ph) were designed and synthesized. The N1-functionalization of the phenanthro-imidazole ring by phenyl, substituted phenyl moiety (CF3, electron withdrawing group), fluorene and their influence on photophysical and electrochemical properties of EuIII complexes were determined by experimental and theoretical analyses. Among all the ligands, fluorene functionalized ligand shows white emission in the solid state. All the complexes (in solid and solution) showed the distinctive emission of EuIII ion at 612 nm, due to electric dipole transition (5D0→7F2). The absence of ligand emissions (solution, thin film and solid) in the PL emission spectra of EuIII complexes indicate that the efficient energy transfer from ligand to central metal ion (antenna effect), confirmed by DFT, TD-DFT. The HOMO-LUMO levels were determined by CV studies. Eu-complex was doped in PMMA matrix to fabricate the composite film devices (Eu(TTA)3Phen-pCF3-Ph shown highest quantum yield 78.7 %). The fluorene functionalized ligand integrated with InGaN LED chip (395 nm, forward bias 20 mA) show the potentiality of the ligand and shown white emission. The obtained efficient red emission from the fabricated LEDs (EuIII complexes coated on InGaN-based near UV LED) shown that the currently synthesized complexes could be a potential red component for warm white LEDs.

In chapter 3, A new class of bipolar phenanthroimidazole based (N1 functionalization with Ph, mCF3, pCF3 and Fl) ligands and their efficient -diketonate EuIII complexes have been designed, synthesized, characterized successfully and their photophysical, electrochemical properties have also been investigated. All the ligands and complexes show similar UV-Visible absorption behaviour ( - *, at ~270, ~360 nm). Photoluminescence emission spectra of Eu-complexes and its ligands were carried out in solution form as well as in solid and thin film. The PL study indicates that the Eu-complex emits tunable emission due to incomplete/partial energy transfer (white (solution), red (solid)); whereas fluorene decorated Eu-complex shows narrow band red emission with appropriate CIE color gamut. The obtained PL emission clearly indicates that the efficient energy transfer encountered in case of fluorene based complex. The energy transfer mechanism for all the Eu-complexes was proposed based on combined experimental and theoretical study (DFT, TD-DFT). The PL lifetime of the EuIII complexes also supports the PL emission behaviour. The Judd–Ofelt spectral intensity parameters, electrochemical study and absolute QY (mCF3 based Eu-complex shows better QY of 75.9 %) of the Eu-complexes were also been investigated. White and red LED was fabricated using these complexes with near UV InGaN based LEDs (395 nm).

In chapter 4, the efficient -diketonate red emitting carbazole-based EuIII complexes were synthesized and their photophysical, electrochemical properties were also been investigated. The PL study indicating that the efficient energy transfer from ligand to EuIII metal ion (dominant pathway) with appropriate CIE color gamut and time-dependent density functional theory (TD-DFT) also confirms the identical. The Judd-Ofelt theory to the emissive properties of EuIII complexes was investigated. The Eu(TTA)3Phen-Fl-CBZ complex shown better lifetime was found to be 0.64 ms. The absolute PL quantum yield (QY) of the complexes in solid is found to be 77.3 % and it possesses high thermal decomposition temperature (235C). The Judd-Ofelt intensity and related parameters were calculated for two complexes. The electrochemical analysis was shown narrow band gap energy (HOMO and LUMO). The PMMA film study of the complexes showed enhanced results than the solution. The fabricated Eu complexes with 395 nm emitted LED (InGaN) chips under 20 mA forward-bias current shown pure red emission and the corresponding CIE color coordinates are x = 0.66, y = 0.33. The obtained pure red emission is superior as compare to that of the solution and solid form of the complexes and the results are shown the presently investigated complexes find potential application in warm white LEDs.

In chapter 5, A new diphenylamine (DPA) and carbazole (CBZ) functionalized ancillary ligands coordinated β-diketonate EuIII complexes shown incomplete or complete energy transfer from ligand to EuIII ion. Solvatochromism study of DPA based complex leads to balancing the primary RGB colors to obtain single molecule white emission. The temperature dependent PL study indicates that the DPA based complex could be used as ratiometric temperature sensor (color changes from blue to yellowish-red via white). In addition shown white emission with 0.34, 0.33 CIE coordinates. In the case of CBZ functionalized bipolar ligand and its corresponding β-diketonate EuIII complex shown efficient energy transfers from the ligand to EuIII center metal ion and emits narrow band red emission with apt CIE color gamut. TD-DFT calculations were performed to know the energies of the singlet (1S) and triplet (3T) levels for the bipolar ligand and shown good overlap between the ligand triplet level and EuIII excited level. The PLQY is found to be 44.4 %, whereas the DPA based complex shown comparatively less QY (supports the inefficient energy transfer). HOMO and LUMO energy levels energies (redox reaction) were calculated from the electrochemical analysis for the Eu-complexes. The synthesized EuIII complex was doped in PMMA with different percentage ratio and found to be concentration variation influence on emission intensity and symmetry. The CBZ-Eu-complex conjugated with near UV LED (395 nm) shown red emission with CIE color coordinates of 0.66, 0.33 and could find potential application in white LEDs.

In chapter 6, the effect of functionalization of carbazole with spacer in C1 position and fluorine in N1 position in the phenanthroline-imidazole based bipolar ligand has been designed, synthesised, same is utilized to synthesise Eu(TTA)3Phen-Fl-O-CBZ complex and studied their photophysical properties. In addition, phenyl and fluorene functionalization in N1 position of phenanthro-imidazole ring (with alkoxy spacer) and its influence on photophysical properties of their binuclear Eu- complexes were systematically investigated. The mono and binuclear Eu-complexes emission spectra (pure red emission) clearly indicate that the complete energy transfer from ligand (L) to EuIII ion occurs, since there is no emission from ligand was encountered (confirmed by DFT and TD-DFT calculations). It is found that the spacer molecule can decrease the energy gap of HOMO-LUMO energy levels (2.6 eV) with respect to that of without spacer one and increment in the singlet and triplet energy levels was also observed, consequences efficient energy transfer (L to M). The enhanced QY observed by 1% doping with PMMA as compare with other doping concentrations (14.2%). Binuclear Eu show dominant electric dipole transition of EuIII ion (5D0→7F2, confirms the EuIII ion in the non-centrosymmetric site). The highest QY (59.5 %, for thin film) obtained for the Eu2(TTA)6(L2). The binuclear EuIII complexes were combined with InGaN near UV LED, obtained pure red emission with CIE color coordinate values x = 0.65, y = 0.34 and x = 0.66, y = 0.33 for Eu2(TTA)6(L1) and Eu2(TTA)6(L2), respectively. The obtained results indicate that the synthesized complexes are potential aspirant for light converting devices.

In chapter 7, a series of organic chromophores or ancillary ligands (based on phenanthroimidazole) conjugated with triphenylamine or carbazole moieties were designed with and without spacer and studied their excited state photophysical properties by density functional theory and time-dependent density functional theory. The UV absorption analysis shown maxima around λmax 288, which is belongs to the -* transition of the ligands. The excited state photophysical properties reveal that the location of the triplet level found among three (1a-f, 2a-f, 3a-f) series 3a-f shown better energy matching with the excited state (5D0) of EuIII ion and could facilitate the energy transfer from ligand to Eu ion very efficient. In addition, the substituted phenyl moiety (mCF3 and pCF3) at N1-position in the phenanthro-imidazole ligand give additional benefits by reducing the triplet energy comparatively with other substitution that leads to efficient energy transfer from L to Eu ion in the complex could be expected. In addition, HOMO and LUMO calculations given lead that some of the designed ligands can also serve as host materials for triplet dopant in OLEDs. The systematic theoretical study is certainly leads to synthesis of best ligand molecules for Eu complexes.

In chapter 8, the present works deals with molecular designing and synthesis of novel class bipolar or ancillary ligand for europium complexes and explore the possibility of using the same in white LEDs, temperature sensor and OLED applications. The observations and the conclusions derived from the present investigations are summarized in this chapter.