Delayed fluorescence based phenanthroimidazoles as OLED emitters having electron transporting properties

229-240Three new phenanthroimidazole derivatives have been synthesised and their optical, electrochemical, and thermal properties evaluated with the purpose of using them for making of possible OLED materials. Synthesised compounds have fluorescence quantum yields up to 73%, robust thermal properties with high decomposition temperatures between 400 to 430°C and glass transition temperature of between 115 to 122°C. The delayed fluorescence observed is due to intra-molecular triplet to singlet energy transfer. The electrochemical properties together with optical properties and in particular the delayed fluorescence properties make them potential electron transporting cum blue emitting materials for OLEDs

Synthesis and Ultrafast Dynamics of a Donor–Acceptor–Donor Molecule Having Optoelectronic Properties

The use of push–pull molecules having donor (D) and acceptor (A) parts arranged in different shapes are being widely studied for application in various optoelectronic devices. In this study three covalently linked D–A–D molecules containing three different carbazole derivatives as donor, anthracene as acceptor, and thiophene as spacer have been synthesized and characterized. A detailed stepwise study has been carried out using anthracene, thiophene–anthracene, and carbazole–thiophene–anthracene derivatives so as to indicate the role of each moiety in the molecule. Steady state fluorescence, time-resolved fluorescence, transient absorption, and cyclic voltammetric methods have been employed to understand the intramolecular charge separation (CS) and charge recombination (CR) dynamics in solvents of different polarity. The thermodynamic free-energy obtained by measuring the redox potential and singlet state energy suggested the possibility of electron transfer from the excited singlet state of carbazole moiety to the anthracene entity. Steady state and time-resolved fluorescence studies showed fluorescence quenching of anthracene moiety upon addition of thiophene while highly efficient fluorescence quenching of anthracene moiety was observed on addition of carbazole derivatives. Femtosecond transient absorption studies confirmed the electron transfer to be the mechanism of fluorescence quenching, in which formation and recombination dynamics of electron-transfer products, anthracene radical anion and carbazole radical cation, were analyzed. The rate of charge separation, <i>k</i>_CS, was found to be very high for all the three molecules, and it was on the order of 10¹⁰–10¹¹ s^–1, while the rate of charge recombination, <i>k</i>_CR, was observed to be much slower, and it was on the order of 10⁸–10⁹ s^–1. The stepwise structure–property relationship leading to the efficient charge separated state established in the systems studied would help in the improved design of optoelectronic materials that use these moieties

Pyrene–Oxadiazoles for Organic Light-Emitting Diodes: Triplet to Singlet Energy Transfer and Role of Hole-Injection/Hole-Blocking Materials

Three pyrene–oxadiazole derivatives were synthesized and characterized by optical, electrochemical, thermal, and theoretical investigations to obtain efficient multifunctional organic light emitting diode (OLED) materials. Synthesized molecules were used as emitters and electron transporters in three different device configurations, involving hole-injection/hole-blocking materials that showed good current and power efficiencies. To understand the underlying mechanisms involved in the application of these molecules as emitters and transporters, a detailed photophysical characterization of molecules <b>4</b>–<b>6</b> was carried out. The absorption, steady-state fluorescence, phosphorescence, fluorescence lifetime, and phosphorescence lifetime measurements were carried out. The high quantum yield and efficient reverse intersystem crossing leading to delayed fluorescence emission makes the molecule a good emitter, and the charge delocalization properties leading to excimer formation make them efficient electron transporters. Isoenergetic singlet and triplet states of the molecules make the reverse intersystem crossing feasible at room temperature even in the absence of thermal activation