Study of the electroluminescence of highly stereoregular poly(N-pentenyl-carbazole) for blue and white OLEDs

Abstract

The electroluminescence (EL) of isotactic and syndiotactic poly(N-pentenyl-carbazole) (PPK), achieved by coordination polymerization, is studied in order to investigate the interrelation between thepolymer tacticity and theirphysical-chemical properties. The use of these polymers in organic light-emitting diode (OLED) fabrication is also explored. Thermal and x-ray diffraction analyses of PPKs show that the isotactic stereoisomer is semicrystalline, whereas the syndiotactic one is amorphous. Optical analysis of both stereoisomers, carried out on film samples, reveals the presence of two different excimers:'sandwich-like' and 'partially overlapping'. Nevertheless, the emission intensity ratio between 'sandwich-like' and 'partially overlapping' excimers is higher in the isotactic than in the syndiotactic stereoisomer. Using the synthesized polymers as OLED emitting layers, the influence of the polymer tacticity on theEL properties of the device is highlighted. In detail, while blue OLEDs are obtained by using the syndiotactic stereoisomer, OLEDs witha multilayer structure fabricated with the isotactic stereoisomer emit white light. The contribution of three different emissions (fluorescence, phosphorescence and electromer emissions) with comparable intensities to the detected white light is discussed.

Optoeletronic properties of poly(N‐alkenyl‐carbazole)s driven by polymer stereoregularity

“…Conversely, we found that all our synthesized stereoregular polymers in the solid state as thin films, show both sandwich‐like and “partially overlapped” excimer emissions . These results were explained assuming that polymer chain conformational freedom in solution prevents excimer formation, while on the other side, in the solid state and at temperatures lower than glass transition temperature ( T g ), a reduced conformational freedom allows the excimer formation . It is worth noting that the most interesting result of our previous experimental studies is that the excimer formation is essentially driven by the polymer tacticity.…”

“…Consistently with these observations, the fluorescence spectra of diluted solutions of our synthesized stereoregular polymers collected at room temperature do not exhibit excimer emission, but only the monomer fluorescence is observed. Conversely, we found that all our synthesized stereoregular polymers in the solid state as thin films, show both sandwich‐like and “partially overlapped” excimer emissions . These results were explained assuming that polymer chain conformational freedom in solution prevents excimer formation, while on the other side, in the solid state and at temperatures lower than glass transition temperature ( T g ), a reduced conformational freedom allows the excimer formation .…”

“…Recently, some of us reported the synthesis of several higher homologues of PVK with high stereoregularity degree, that is, isotactic poly( N ‐butenyl‐carbazole) ( i ‐PBK), isotactic and syndiotactic poly( N ‐pentenyl‐carbazole) ( i ‐PPK and s ‐PPK, respectively), and isotactic and syndiotactic poly(4‐(N‐carbazolyl)methyl styrene) ( i ‐PSK and s ‐PSK, respectively) . All stereoregular polymers were obtained by Ziegler‐Natta (Z‐N) homogeneous polymerization, that is, a polymerization process failing for the synthesis of PVK due to the conjugation of the vinyl group with the carbazole aromatic system of the monomer (9‐vinylcarbazole) …”

“…It is worth noting that the most interesting result of our previous experimental studies is that the excimer formation is essentially driven by the polymer tacticity. In detail, in the solid state, highly isotactic polymers favor the formation of sandwich‐like excimer, while highly syndiotactic polymers give rise to partially overlapping excimers . Of course, the different optical properties of two different stereoisomers of the same polymer can also induce a different electroluminescent behavior.…”

“…Of course, the different optical properties of two different stereoisomers of the same polymer can also induce a different electroluminescent behavior. We achieved interesting results by comparing the electroluminescent behavior of the PPK stereoisomers ( i ‐PPK and s ‐PPK) . Specifically, white OLEDs were obtained by i ‐PPK samples as emissive layer, whereas from s ‐PPK, blue light emitting OLEDs were achieved …”