Ona Adomėnienė

Fluorene- and benzofluorene-cored oligomers as low threshold and high gain amplifying media

Deliberate control of intermolecular interactions in fluorene- and benzofluorene-cored oligomers was attempted via introduction of different-length alkyl moieties to attain high emission amplification and low amplified spontaneous emission (ASE) threshold at high oligomer concentrations. Containing fluorenyl peripheral groups decorated with different-length alkyl moieties, the oligomers were found to express weak concentration quenching of emission, yet excellent carrier drift mobilities (close to 10−2 cm2/V/s) in the amorphous films. Owing to the larger radiative decay rates (>1.0 × 109 s−1) and smaller concentration quenching, fluorene-cored oligomers exhibited down to one order of magnitude lower ASE thresholds at higher concentrations as compared to those of benzofluorene counterparts. The lowest threshold (300 W/cm2) obtained for the fluorene-cored oligomers at the concentration of 50 wt % in polymer matrix is among the lowest reported for solution-processed amorphous films in ambient conditions, wha...

Influence of Achiral So Host on the Values of Spontaneous Polarization and Rotational Viscosity of Ferroelectric Mixtures

Abstract Spontaneous polarization and rotational viscosity of Sc guest host mixtures was measured. Three different hosts were compared.

Exciton diffusion in bifluorene single crystals studied by light induced transient grating technique

Exciton diffusion is considered to be of prime importance for controlling the recombination zone in organic light emitting devices. This is particularly relevant for organic lasers based on single crystals, where undesirable exciton-exciton annihilation at high concentrations is inevitable. Here, exciton diffusion is studied in bifluorene single crystals specifically designed for organic laser applications, therefore featuring exceptionally low threshold of light amplification. The non-destructive light induced transient grating (LITG) technique capable of simultaneously evaluating the exciton lifetime and diffusion coefficient along different directions of the crystal was employed for this study. Highly anisotropic singlet exciton diffusion with the diffusion coefficient varying from <0.05 up to 0.96 cm2/s (in a perpendicular direction) and the corresponding diffusion lengths changing from <60 up to 300 nm, respectively, was revealed in ethylene-bridged bifluorene crystals. The LITG results on the directionality of exciton diffusion are anticipated to be essential for devising single crystal based organic lasers.Exciton diffusion is considered to be of prime importance for controlling the recombination zone in organic light emitting devices. This is particularly relevant for organic lasers based on single crystals, where undesirable exciton-exciton annihilation at high concentrations is inevitable. Here, exciton diffusion is studied in bifluorene single crystals specifically designed for organic laser applications, therefore featuring exceptionally low threshold of light amplification. The non-destructive light induced transient grating (LITG) technique capable of simultaneously evaluating the exciton lifetime and diffusion coefficient along different directions of the crystal was employed for this study. Highly anisotropic singlet exciton diffusion with the diffusion coefficient varying from <0.05 up to 0.96 cm2/s (in a perpendicular direction) and the corresponding diffusion lengths changing from <60 up to 300 nm, respectively, was revealed in ethylene-bridged bifluorene crystals. The LITG results on the direct...

Low-Threshold Light Amplification in Bifluorene Single Crystals: Role of the Trap States

Organic single crystals (SCs) expressing long-range periodicity and dense molecular packing are an attractive amplifying medium for the realization of electrically driven organic lasers. However, the amplified spontaneous emission (ASE) threshold (1-10 kW/cm2) of SCs is still significantly higher compared to those of amorphous neat or doped films. The current study addresses this issue by investigating ASE properties of rigid bridging group-containing bifluorene SCs. Introduction of the rigid bridges in bifluorenes enables considerable reduction of nonradiative decay, which, along with enhanced fluorescence quantum yield (72-82%) and short excited state lifetime (1.5-2.5 ns), results in high radiative decay rates (∼0.5 × 109 s-1) of the SCs, making them highly attractive for lasing applications. The revealed ASE threshold of 400 W/cm2 in acetylene-bridged bifluorene SCs is found to be among the lowest ever reported for organic crystals. Ultrafast transient absorption spectroscopy enabled one to disclose pronounced differences in the excited state dynamics of the studied SCs, pointing out the essential role of radiative traps in achieving a record low ASE threshold. Although the origin of the trap states was not completely unveiled, the obtained results clearly evidence that the crystal doping approach can be successful in achieving extremely low ASE thresholds required for electrically pumped organic laser.

Biphenyl and 1,2-Diphenylethane Derivatives Containing Two Terminal Alkynyl Groups

Abstract Synthesis and mesomorphic properties of biphenyl and 1,2- diphenylethane derivatives, containing two terminal alkynyl groups, are presented.

Synthesis and Properties of Ferroelectric 4-[4-(S-1-Methylheptyloxy)Benzoyloxy]-4′-n-Alkyloxycarbonylbiphenyls

Abstract Synthesis, mesomorphic properties and spontaneous polarization values of 4-[4-(S-1-methyl-heptyloxy)benzoyloxy]-4′-alkyloxyoarbonylbiphenyls are given.

Mesomorphic Properties of New 5-Alkyl-2-Pyrimidinols Esters

Abstract A synthesis of a number of 5-alkyl-2-pyrimidinol esters was conducted, their mesomorphic properties were examined. Two ring esters of said pyrimidinols are nonmesomorphic, while three ring esters are typical mesogens, forming nematic either nematic-smectic mesophases.