Photoluminescence of donor–acceptor carbazole-based molecules in amorphous and powder forms

Abstract

We present absorption and photoluminescence features of four samples of carbazole molecules substituted with various electron–acceptor groups. These molecules named 1-(N-ethylcarbazolyl)-2-substituted-2-cyanovinylene contain in their structure the electron–donor carbazole nucleus and cyanovinylene bearing either another nitrile function, an ethylester, a phenyl, or a para-nitrophenyl groups. It is shown that depending on the strength of the donor–acceptor internal charge transfer, both the absorption and emission spectra are more or less redshifted. It is found that the ethyl-ester derivative displays the best relative photoluminescence efficiency among all the samples and its peak is measured at 490 nm when taking amorphous thin film. The microcrystalline powder form of the same material exhibits spectral narrowing and shift of the peak emission. We obtain further narrowing of the emission band and further redshifting of the emission when we illuminate, transversely, a glass capillary containing the crystalline sample by an ultraviolet light-emitting diode.

Doped and non-doped organic light-emitting diodes based on a yellow carbazole emitter into a blue-emitting matrix

“…As already reported for several donor-acceptor carbazole-based molecules, substitution in 3-position of the electron-donor heterocycle by cyanovinylene electron-withdrawing groups leads to an internal charge transfer which shows up in the visible domain of the optical spectrum [23]. In the case of (OcCz2CN) 2 , i.e 3,3'-dimer of OcCz2CN, the charge-transfer band is peaking at λ max = 450 nm.…”

“…From the onset of the absorption threshold of the spectrum the energy gap value Eg was found to be close to 2.4 eV. Furthermore, it is noteworthy that π-conjugation within the bicarbazyl core provokes, as expected, a red-shift of the visible band, since the corresponding signal appears at λ max = 428 nm for films of OcCz2CN monomer (Eg=2.7 eV) [23]. Hence, a yellow-light emission is observed for (OcCz2CN) 2 with a maximum at λ max = 590 nm ( Figure 4) whereas a green one is observed for the monomer derivative (λ max = 528 nm).…”

Synthesis of Poly(2,7‐carbazole)s and Derivatives

“…Only a few studies have focused on the synthesis of yellow, green, and red light-emitting polymers because of the difficulty to functionalize at different positions. Still, electron-withdrawing units were copolymerized to lower the bandgap of the polymers and therefore to redshift the light-emitting properties of the resulting materials [60]. Lately, some research groups have focused on the synthesis of new materials for WOLEDs because of the need for lightweight illumination devices in aircrafts, space shuttles, and liquid crystal displays [61,62].…”

Microcavités à polymères du carbazole : vers des lasers plastiques bleus

“…Dans cette optique, la molécule de carbazole est un excellent candidat puisque dans sa conformation la plus simple, elle produit une émission bleue-violette intense. De plus, il a été montré [1] que par fonctionnalisation du carbazole par des substituants spécifiques, cette émission peut alors se décaler vers le rouge, offrant ainsi la possibilité d'obtenir les trois couleurs nécessaires aux afficheurs multicouleurs.…”