Substituted tetrapyrazolylporphyrins: application in organic light-emitting diodes

Abstract

Six substituted tetrapyrazolylporphyrins ( TAPyPH 2), meso-tetrakis(1-phenylpyrazol-4-yl)porphyrin ( TPPyPH 2), meso-tetrakis[1-(p-tolyl)pyrazol-4-yl] porphyrin ((p- CH 3) TPPyPH 2), meso-tetrakis[1-(p-methoxyphenyl)pyrazol-4-yl]porphyrin ((p- CH 3 O ) TPPyPH 2), meso-tetrakis[1-(p-chlorophenyl) pyrazol-4-yl]porphyrin ((p- Cl ) TPPyPH 2), meso-tetrakis[1-(p-bromophenyl)pyrazol-4-yl]porphyrin ((p- Br ) TPPyPH 2) and meso-tetrakis[1-(m-tolyl)pyrazol-4-yl]porphyrin ((m- CH 3) TPPyPH 2), were synthesized by the condensation of pyrrole with the corresponding substituted formylpyrazole and used as the doping materials in the Organic Light-Emitting Diodes (OLEDs) with hole-transport materials of N , N '-diphenyl- N , N '-bis (4-methylphenyl)-[1,1'-biphenyl]-4,4'-diamine (TPD) within a tris(8-hydroxyquinoline) aluminum ( Alq 3) host layer. Both the electroluminescence properties of these OLEDs and effects of the peripheral substituents of the tetrapyrazolylporphyrins on EL properties of the OLEDs were studied. The tests found that the OLEDs using substituted tetrapyrazolylporphyrins ( TAPyPH 2) as the dopants had the saturated red emission at a concentration of 1.5 wt.%. The electron-donating groups of tetrapyrazolylporphyrins (for example, p- CH 3 and p- CH 3 O ) increased the luminance of OLEDs, while the electron-drawing groups (for example, p- Cl and p- Br ) decreased the luminance. The OLEDs using meso-tetrakis(1-phenylpyrazol-4-yl)porphyrin ( TPPyPH 2) as a dopant showed saturated red emission at 680 nm (CIE coordinates of x = 0.65, y = 0.31) with a luminance of 305 cd/m2 at a driving voltage of 25 V at concentration of 1.5 wt.%. Compared with OLEDs doped by the free-base tetraphenylporphyrin (TPPH2), the OLEDs doped by the tetrapyrazolylporphyrins had better luminance and higher emission efficiency. An explanation for these results was given based on the molecular structure and spectral properties of the porphyrin compounds.