Saulius Jursenas

Impact of intramolecular twisting and exciton migration on emission efficiency of multifunctional fluorene-benzothiadiazole-carbazole compounds

Novel donor-acceptor compounds consisting of singly bonded fluorene (Fl), benzothiadiazole (BT), and carbazole (Cz) functional units in the same molecule were investigated. Analysis of the optical spectra and fluorescence transients of the compounds revealed the domination of intramolecular charge transfer (ICT) states with high fluorescence quantum yield (72%-85%). A similar Cz-Fl-Cz compound exhibiting 100% fluorescence quantum yield and no ICT character was also studied as a reference to reveal the impact of electron-accepting BT groups. Thorough examination of the optical properties of the compounds in different media, i.e., dilute solution and polymer matrix, indicated their twisted conformations due to steric hindrance in the ground state and flattened geometry in the excited state for both reference and ICT compounds. Remarkable fluorescence efficiency losses (amounting to 70%) observed upon casting the molecular solutions into neat films were determined to originate from the low-fluorescent twisted conformers and migration-facilitated exciton quenching. The majority of emission efficiency losses (over 70%) were caused by the twisted conformers, whereas only less than 30% by exciton-migration-induced nonradiative deactivation.

Luminescence decay in highly excited GaN grown by hydride vapor-phase epitaxy

Carrier recombination dynamics in GaN grown by hydride vapor-phase epitaxy has been studied by means of transient photoluminescence under high photoexcitation conditions that are close to stimulated emission regime. The luminescence transient featured an exponential decay with the time constant of 205 ps at room temperature. The transient was shown to be in good agreement with a model of saturated centers of nonradiative recombination with the trap density of ∼1017 cm−3 and carrier recombination coefficients of ∼10−8 cm3/s. In such a regime, the lifetimes of electrons and holes have a common value of 410 ps.

Phenylethenyl-Substituted Triphenylamines: Efficient, Easily Obtainable, and Inexpensive Hole-Transporting Materials

Star-shaped charge-transporting materials with a triphenylamine (TPA) core and various phenylethenyl side arm(s) were obtained in a one-step synthetic procedure from commercially available and relatively inexpensive starting materials. Crystallinity, glass-transition temperature, size of the π-conjugated system, energy levels, and the way molecules pack in the solid state can be significantly influenced by variation of the structure of these side arm(s). An increase in the number of phenylethenyl side arms was found to hinder intramolecular motions of the TPA core, and thereby provide significant enhancement of the fluorescence quantum yield of the TPA derivatives in solution. On the other hand, a larger number of side arms facilitated exciton migration through the dense side-arm network formed in the solid state and, thus, considerably reduces fluorescence efficiency by migration-assisted nonradiative relaxation. This dense network enables charges to move more rapidly through the hole-transport material layer, which results in very good charge drift mobility (μ up to 0.017 cm(2) V (-1) s(-1)).

Spectroscopy of self-trapped charge-transfer excitons in polar films and crystals of N,N-dimethylaminobenzylidene 1,3-indandione (DMABI)

The origin of exciton states in crystals built up by dipolar N,N-dimethylaminobenzylidene 1,3-indandione (DMABI) molecules has been investigated by means of absorption and luminescence spectroscopy. The exciton spectral and dynamical properties in the solid state are compared to those of the excitation of separated DMABI molecules embedded in a rigid polymer matrix. The luminescence excitation spectra and the luminescence decay kinetics are discussed on the basis of the specific adiabatic potential-energy scheme of the self-trapped excitons.

Pulsed Atomic Layer Epitaxy of Quaternary AlInGaN Layers for Ultraviolet Light Emitters

We report on a pulsed atomic layer epitaxy (PALE) technique for quaternary AlInGaN growth. PALE allows for the deposition of high-quality AlInGaN layers at much lower temperatures than those required for conventional low-pressure metalorganic chemical vapor deposition (MOCVD). The low growth temperature leads to an efficient incorporation of indium and, as a result, to a dramatic improvement in emission properties of the material. A deep ultraviolet optical gain of 300 cm -1 peaked at 330 nm is obtained in quaternary AlInGaN MQWs. Also, in PALE AlInGaN material, the optical emission properties do not degrade with increasing Al mole fraction. These results establish PALE as a promising technique for deep UV emitters.

Increase of free carrier lifetime in nonpolar a-plane GaN grown by epitaxial lateral overgrowth

Carrier recombination dynamics in epitaxial a-plane GaN and fully coalesced epitaxial laterally overgrown (ELOG) a-plane GaN films has been studied by means of time-resolved photoluminescence under high photoexcitation. The results were compared with conventional c-plane GaN films grown under similar conditions. In a-plane GaN epilayers, the total efficiency of electron-hole plasma spontaneous luminescence decreases 20 times, whereas the luminescence decay time reduces from τLU=42 to τLU⩽10ps in comparison with c-plane GaN films. Meanwhile, an essential increase in total emission efficiency (by more than two orders of magnitude) and an increase of the decay time up to τLU=430ps have been observed for an ELOG a-plane sample in comparison with a-plane GaN films. This confirms a significant reduction of the nonradiative recombination rate for nonequilibrium carriers. Assuming a saturation of the nonradiative deep-level transitions, the room-temperature free-carrier lifetime of τ=910ps for ELOG a-plane GaN sa...

Exciton diffusion enhancement in triphenylamines via incorporation of phenylethenyl sidearms

Exciton diffusion length strongly impacts the performance of organic photovoltaic cells and light emitting diodes, and therefore ways of manipulating the diffusion length must be sought out. Here, we present an approach for controlling singlet exciton diffusion in triphenylamine (TPA)-based amorphous films via incorporation of phenylethenyl sidearms. Exciton diffusion of the TPA derivatives possessing different number (one, two and three) and different type (2-methyl-2-phenylethenyl, 2,2-diphenylethenyl and 2,2-di(4-methoxyphenyl)ethenyl) of sidearms was investigated by employing the volume quenching method in combination with Monte Carlo simulation of 3D exciton diffusion. A nearly three-fold enhancement of the exciton diffusion length by increasing the number of peripheral phenylethenyl groups from one to three was achieved mainly due to the enhanced overlap of the emission and absorption spectra. The dominance of the spectral overlap integral in determining energy transfer rates was confirmed by calculations based on Forster energy transfer theory, which also proved a key role of phenylethenyl sidearms in facilitating exciton diffusion.

2,4-Bis(4-aryl-1,2,3-triazol-1-yl)pyrrolo[2,3-d]pyrimidines: synthesis and tuning of optical properties by polar substituents

2,4-Bis(4-aryl-1,2,3-triazol-1-yl)pyrrolo[2,3-d]pyrimidines as D–π–A–π–D chromophores were successfully prepared by CuAAC reaction of 2,4-diazido-7-methylpyrrolo[2,3-d]pyrimidine with ethynylarenes in dichloromethane in the presence CuI/DIPEA/AcOH as a catalyst system. The incorporation of small polar substituents enabled tuning of the energy of frontier orbitals and thus the FMOs energy gap by up to 0.9 eV, while the incorporation of bulky steric substituents resulted in narrowing of the energy gap by up to 0.4 eV. Owing to electron-accepting properties of pyrrolo[2,3-d]pyrimidine core extending to triazole moieties the compounds with electron-donating groups showed expressed intramolecular charge transfer character (ICT) of the excited states which was proved by solvatochromic dynamics and supported by DFT calculations. The optimization of ICT reduced radiative and non-radiative deactivation pathways resulted in enhancement of fluorescence quantum yield up to 73%.

Spectroscopy of excitons in the polar molecular crystal DMABI

This paper considers the origin of the excited states and spectroscopic features of polar molecular compounds of N,N-dimethylaminobenzylidene 1,3-indandione (DMABI). Evidence of a strong exciton-phonon interaction and its effects on the exciton absorption and luminescence spectra was discovered. The spectral features were related to the crystal structure and were ascribed to self-trapped charge transfer excitons. A weak, narrow luminescence line was also observed in resonance with the lowest absorption band and was attributed to free exciton radiative recombination.

AlGaN-based deep-UV LEDs for fluorescence sensing

Recent progress in wide-bandgap semiconductor optoelectronics resulted in an appearance of deep-UV light-emitting diodes (LEDs), which can be used for fluorescence excitation in a variety of chemical and biological compounds. We used two generations of AlGaN-based UVTOP series deep ultraviolet LEDs developed by Sensor Electronic Technology, Inc. The peak wavelength of these fully packaged devices is 340 nm and 280 nm, line width at half maximum approximately 10 nm, wall-plug efficiency up to 0.9% and output power in the milliwatt range. The second-generation emitters are shown to have an extremely low level of unwanted long-wavelength emission what is important for fluorescence measurements. The UV LEDs were tested for fluorescence excitation in standard fluorophores (organic dyes), autofluorescent biological compounds (riboflavin, NADH, tryptophan, and tyrosine) and medical specimens (fluid secreted by prostate gland). Fluorescence lifetime measurements in the frequency domain were demonstrated using UVTOP-340 and -280 devices. The output of the LEDs was modulated at frequencies up to 200 MHz by high-frequency current drivers and the phase angle of the fluorescence signal was resolved using a radio-frequency lock-in amplifier. Nanosecond-scaled measurements of fluorescence lifetimes, which are the “fingerprints” of chemical and biological compounds, were demonstrated.