The design and synthesis of new π-conjugated organic semiconductors are currently at the forefront of research in order
to realize stable and efficient organic electronic devices. Linear
polycyclic aromatic hydrocarbons are π-conjugated organic systems
widely studied for electronic applications. Pentacenes, for example, are a
current choice in Organic Field Effect Transistors (OFETs). But practical
applications of this class of compounds have been limited by their air
(O2) and light sensitivity and poor solubility in most common organic
compounds.
Structural modification increases the stability, improve the
solubility, and improve the thin film packing and mobility over the parent
hydrocarbon. The electronic, physical, and photophysical properties
of pentacenes, can be easily modified by changing the substitution
pattern on the main aromatic skeleton.
A comparative study of suitably
functionalized, highly soluble tetraceno[2,3-b]thiophenes (3.1-3.3) and
pentacenes (3.4-3.6) that show higher photoxidative stability than that
of unfunctionalized corresponding acenes is reported. The absorption and
emission of 3.1-3.3 (Amax = 624-656 nm,
λmax = 634-672 nm, ΦF
≈ 10%) and 3.4-3.6 (Amax = 672-704 nm,
λmax = 682-718 nm, ΦF
≈ 10%) were found to be systematically red-shifted by the substitution
in the order of the tert-butylethynyl < triisopropylsilylethynyl <
phenylethynyl groups. The oxidation potentials of these compounds were
similar (E1/2 ≈ 0.70 V), except for 3.4, which showed lower oxidation
potential (E1/2 ≈ 0.63 V).
Stability and emission in the solid state are important features
of organic compounds to be used as Organic Light Emitting Diodes
(OLEDs). The substantial emission of compounds in solution usually
becomes weak in the solid state due to both intermolecular energy and
electron transfer. Compounds containing an aromatic fumaronitrile core
have attracted significant attention as candidates in electroluminescent
devices because of their strong emissions in the solid state. Changing
the substituents on fumaronitrile core helps to change the emission
properties in the solid state. These compounds have higher propensity to
form Fluorescent Organic Nanoparticles in appropriate inferior/superior
solvent mixtures.
Compounds 5.1 and 5.2 show negative solvatochromic absorption
behavior, but show both positive and negative solvatochromic behavior
in the fluorescence spectra. In a water/THF mixture, 5.1 as well as 5.2
aggregate into 50-150 nm nanoparticles. The emission of nanoparticles
of the new types of fluorescent organic nanoparticles 5.1 and 5.2 is
much higher than that of either 5.1 or 5.2 in solution.
The emission spectra and morphologies of 5.3, 5.4, and 5.5 are affected
by the water/THF ratio. At a high water/THF ratio (7:1), nanorods (1-3
μm x 80 nm) of 5.4 and nanofibers (0.8-1.5 μm × 100 nm) of
5.5 are observed. Nanoparticles of 5.3 retain a spherical structure. The
chemical effects of the electron-donating or electron-withdrawing groups
at the para positions are believed to play a major role in the formation
of such nanostructures.