5 research outputs found
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><sub>CS</sub>, was found to be very high for all the three
molecules, and it was on the order of 10<sup>10</sup>–10<sup>11</sup> s<sup>–1</sup>, while the rate of charge recombination, <i>k</i><sub>CR</sub>, was observed to be much slower, and it was
on the order of 10<sup>8</sup>–10<sup>9</sup> s<sup>–1</sup>. 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