New Carbazole-, Indole-, and Diphenylamine-Based Emissive Compounds: Synthesis, Photophysical Properties, and Formation of Nanoparticles

The electronic and optical behavior of conjugated small molecules constitutes one of the most extensively studied properties of this class of organic compounds. In particular, electron donor-acceptor compounds separated by π-conjugation have attracted much research interest because of their applications as electroactive and photoactive materials in molecular electronic areas such as fluorescent technology, chemoluminescence, and photovoltaics. This work aims to demonstrate the synthesis and some of the interesting properties of new carbazole, indole, and diphenylamine donor-based donor-acceptor compounds.

Basically, we have divided our compounds into two groups: (a) aromatic fumaronitrile core-based compounds and (b) 2,7-carbazole linker-based compounds. We designed and synthesized these compounds for possible applications in electroluminescent devices (such as OLEDs), organic sensors, organic nanoparticles, sensitizers in organic dye-sensitized solar cells (DSSCs), and other optoelectronic devices.

Compounds containing an aromatic fumaronitrile core have attracted significant attention as candidates in electroluminescent devices because of their strong emissions in the solid state. Five different compounds with a highly fluorescent and stable carbazole, indole, 2-phenylindole, diphenylamine, or 3,6-disubstituted carbazole donor with an aromatic fumaronitrile core were synthesized and characterized. They showed absorption ranging from A_max 306 nm to 450 nm and emission ranging from λ_max 360 nm to 637 nm in medium polar solvent DCM. These compounds emitted blue, green, and red light. In most of the compounds, red-shifted emission in the solid state relative to that in solution was observed. The red shift in the solid state was as high as 114 nm. These compounds showed molar extinction coefficients ranging from 20881 dm³cm^-1mol^-1 to 73266 dm³cm^-1mol^-1 in DCM, quantum yields of fluorescence (Φ_F) from 0.01 up to 0.80 in solution and 0.38-0.80 in the solid state, and lifetimes of fluorescence (τ_F) ranging from less than 0.1 ns up to 7 ns. Some of these compounds have significant potential for use in organic light-emitting diode devices since their emission covers nearly the entire visible region of the spectrum with high quantum yields both in solution and in solid state. The correlation between functional groups and optical properties of some of the compounds has been established. The ability of some of the compounds to function as colorimetric and luminescence pH sensors is demonstrated with color change and luminescence switching upon the addition of trifluoroacetic acid.

Fumaronitrile core-based compounds have a high propensity to form Fluorescent Organic Nanoparticles (FONs) in appropriate superior/inferior solvent mixtures. All of these five aromatic fumaronitrile core-based compounds formed FONs in THF/water mixtures. These FONs emit light from the visible region to the near IR region. These FONs show remarkable change in terms of intensity in both absorption and emission. Some of the nanoparticles of these compounds absorb up to 580 nm and emit from 350 nm up to the near infrared (NIR) region. This is the first example that NIR emission was achievable upon the formation of nanoparticles from pure organic compounds. In some compounds, the emission intensity of nanoparticles is increased by 19 fold, and in some of the nanoparticles the emission is red shifted by 256 nm.

A new class of 15 different organic donor-acceptor compounds with a 3,6-disubstituted carbazole or diphenylamine donor, 2,7-functionalized N-substituted carbazole linker and either (i) an aldehyde, (ii) cyanoacetic acid, (iii) malononitrile acceptors or (iv) diphenylamino or 3,6-disubstituted carbazolyl donors—all with or without phenylethynyl extenders—has been synthesized. The effect on the photophysical properties of these compounds caused by changing acceptors while keeping the donor and linker constant has been studied. These compounds, also, absorb from the UV into the visible region and emit intensely from blue to green. A study of the solvent effects of some of these compounds on their photophysical properties has shown that an increase in polarity of the solvent causes a reduction of fluorescence quantum yields. Solid state fluorescence quantum yields are generally greater than those in DCM. The compounds having a cyano group in the acceptor formed fluorescent nanoparticles in THF/water mixtures.

The properties of these 2,7-functionalized N-substituted carbazole linker-based compounds were also investigated using a combination of conventional steady-state absorption spectroscopy and tools of computational photochemistry. Time-dependent density functional theory calculations provide a deep insight into the photochemistry of these compounds in terms of molecular orbitals and the changes in electron density accompanying low-lying electronic transitions. One noticeable difference is that in the absence of phenylethynyl linker extenders, delocalization along the π-framework is generally more effective. These findings point out that less efficient electronic communication between the donor and acceptor is achieved when the phenylethynyl extender is inserted by design. Compounds having cyanoacrylic acid as an acceptor have potential as sensitizers in organic DSSCs; thus, we have fabricated them into devices and obtained the power conversion efficiency of 2.7% in the blue region of the spectrum.

All these data indicate that these compounds and their nanoparticles can be utilized in different applications such as photonics, electronics, sensors, and organic DSSCs.