Benzimidazole derivatives: synthesis, physical properties, and n-type semiconducting properties.

A series of new benzimidazole derivatives were synthesized by the solid-state condensation and direct sublimation (SSC-DS) method and their physical properties were investigated. The reaction yields and product stability were significantly affected by the identity of the diamine and anhydride substituents. On the other hand, the substituents of the benzimidazole ring allowed fine tuning of the emission maxima, fluorescence quantum yields, and redox potentials. The HOMO-LUMO levels were estimated by cyclic voltammetry in film on indium tin oxide (ITO) and compared with values obtained by other methods. The described benzimidazoles showed high crystallinity, which is attributed to a high planarity and interactions between carbon and heteroatoms. These compounds showed n-type semiconducting behavior in organic field-effect transistors (OFETs). Optimized devices for fluorinated NTCBI (naphthalene tetracarboxylic bisbenzimidazole) showed respectable electron mobilities of ∼10(-2)  cm(2) V(-1) s(-1) .

Green synthesis of polycyclic benzimidazole derivatives and organic semiconductors.

Polycyclic benzimidazole derivatives, an important class of compounds in organic electronics and photovoltaics, were prepared using a solvent-free "green" process based on heating carboxylic acid anhydrides and arylene diamines in the presence of zinc acetate in the solid state. Products were isolated and purified directly by train sublimation of the crude reaction mixtures. The reaction conditions were optimized using various carboxylic acid anhydrides. Optical and electrochemical properties of these materials are also described.

High-efficiency tris(8-hydroxyquinoline)aluminum (Alq3) complexes for organic white-light-emitting diodes and solid-state lighting.

Combinations of electron-withdrawing and -donating substituents on the 8-hydroxyquinoline ligand of the tris(8-hydroxyquinoline)aluminum (Alq(3)) complexes allow for control of the HOMO and LUMO energies and the HOMO-LUMO gap responsible for emission from the complexes. Here, we present a systematic study on tuning the emission and electroluminescence (EL) from Alq(3) complexes from the green to blue region. In this study, we explored the combination of electron-donating substituents on C4 and C6. Compounds 1-6 displayed the emission tuning between 478 and 526 nm, and fluorescence quantum yield between 0.15 and 0.57. The compounds 2-6 were used as emitters and hosts in organic light-emitting diodes (OLEDs). The highest OLED external quantum efficiency (EQE) observed was 4.6%, which is among the highest observed for Alq(3) complexes. Also, the compounds 3-5 were used as hosts for red phosphorescent dopants to obtain white light-emitting diodes (WOLED). The WOLEDs displayed high efficiency (EQE up to 19%) and high white color purity (color rendering index (CRI≈85).

Encapsulated energy-transfer cassettes with extremely well resolved fluorescent outputs.

This paper concerns the development of water-compatible fluorescent imaging probes with tunable photonic properties that can be excited at a single wavelength. Bichromophoric cassettes 1a-1c consisting of a BODIPY donor and a cyanine acceptor were prepared using a simple synthetic route, and their photophysical properties were investigated. Upon excitation of the BODIPY moiety at 488 nm the excitation energy is transferred through an acetylene bridge to the cyanine dye acceptor, which emits light at approximately 600, 700, and 800 nm, i.e., with remarkable dispersions. This effect is facilitated by efficient energy transfer that gives a "quasi-Stokes" shift between 86 and 290 nm, opening a huge spectral window for imaging. The emissive properties of the cassettes depend on the energy-transfer (ET) mechanism: the faster the transfer, the more efficient it is. Measurements of rates of ET indicate that a through-bond ET takes place in the cassettes 1a and 1b that is 2 orders of magnitude faster than the classical through-space, Förster ET. In the case of cassette 1c, however, both mechanisms are possible, and the rate measurements do not allow us to discern between them. Thus, the cassettes 1a-1c are well suited for multiplexing experiments in biotechnological methods that involve a single laser excitation source. However, for widespread application of these probes, their solubility in aqueous media must be improved. Consequently, the probes were encapsulated in calcium phosphate/silicate nanoparticles (diameter ca. 22 nm) that are freely dispersible in water. This encapsulation process resulted in only minor changes in the photophysical properties of the cassettes. The system based on cassette 1a was chosen to probe how effectively these nanoparticles could be used to deliver the dyes into cells. Encapsulated cassette 1a permeated Clone 9 rat liver cells, where it localized in the mitochondria and fluoresced through the acceptor part, i.e., red. Overall, this paper reports readily accessible, cyanine-based through-bond ET cassettes that are lypophilic but can be encapsulated to form nanoparticles that disperse freely in water. These particles can be used to enter cells and to label organelles.

True blue: blue-emitting aluminum(III) quinolinolate complexes.

Blue-emitting heteroleptic aluminum(III) bis(2-methyl-8-quinolinolate)phenolate complexes were synthesized. A tunable, blue-to-green emission is achieved by attaching electron-withdrawing modulators to the emisssive quinaldinate ligand. The electronic nature of modulator substituents attached to the position of the highest HOMO (highest occupied molecular orbital) density is used to modulate ligand HOMO levels to achieve effective emission tuning to obtain blue-emitting materials. Optical and electrochemical properties of the resulting complexes were investigated and compared to the results of density functional theory (DFT/B3LYP/6-31G*) studies. The resulting materials may find application as organic light-emitting device materials.

Molecular-wire behavior of OLED materials: exciton dynamics in multichromophoric Alq3-oligofluorene-Pt(II)porphyrin triads.

Donor-bridge-acceptor triads consisting of the Alq3 complex, oligofluorene bridge, and PtII tetraphenylporphyrin (PtTPP) were synthesized. The triads were designed to study the energy level/distance-dependence in energy transfer both in a solution and in solid state. The materials show effective singlet transfer from the Alq3-fluorene fluorophore to the porphyrin, while the triplet energy transfer, owing to the shorter delocalization of triplet excitons, appears to take place via a triplet energy cascade. Using femtosecond transient spectroscopy, the rate of the singlet-singlet energy transfer was determined. The exponential dependence of the donor-acceptor distance and the respective energy transfer rates of 7.1 x 1010 to 1.0 x 109 s-1 with the attenuation factor â of 0.21 +/- 0.02 A-1 suggest that the energy transfer proceeds via a mixed incohererent wire/superexchange mechanism. In the OLEDs fabricated using the Alq3-oligofluorene-PtTPP triads with better triplet level alignment, the order of a magnitude increase in efficacy appears to be due to facile triplet energy transfer. The devices, where the triplet-triplet energy transfer is of paramount importance, showed high color purity emission (CIE X,Y: 0.706, 0.277), which is almost identical to the emission from thin films. Most importantly, we believe that the design principles demonstrated above are general and may be used to prepare OLED materials with enhanced quantum efficacy at lowered operational potentials, being crucial for improved lifespan of OLEDs.