Synthesis and Photophysical Properties of N-Arylcarbazolophanes

Abstract

Two carbazolophanes bearing benzene or naphthalene rings on the nitrogen atoms of carbazole were successfully synthesized by the intramolecular etherification of the precursor tetraols, and their photophysical properties were investigated in detail based on the absorption spectra, fluorescence spectra, fluorescence lifetimes, quantum yields, phosphorescence spectra, and transient absorption spectra. It was concluded that the carbazolophane bearing benzene rings forms carbazole excimer in both excited singlet and triplet states, while the carbazolophane bearing naphthalene rings forms carbazole excimer only in the excited singlet state and has a locally excited state of naphthalene in the excited triplet state. These results are reasonably explained by the relationship of excitation energies between carbazole and benzene (or naphthalene) and their arrangement such as the extent of overlap. The introduction of aromatic rings on the carbazole nitrogen atoms was found to affect the electronic and photophysical properties of carbazolophanes sensitively.

Triplet Excimer Emission in a Series of 4,4′-Bis(N-carbazolyl)-2,2′-biphenyl Derivatives

“…We have already shown that the strong torsion in the biphenyl unit in compounds 1 − 6 electronically decouples the two halves of the molecule to some degree, resulting in absorption and phosphorescence spectra that are identical to those of N -phenylcarbazole. N -Arylcarbazoles and N -alkylcarbazoles have been studied extensively in the past by Klöpffer, Johnson, and many others, ,− and it is well-known that they are prone to form excimers. Given the similar electronic structure, it is instructive to compare the excimer emission we find for compounds 1 − 6 with the existing literature on excimers in N -arylcarbazoles and N -alkylcarbazoles.…”

“…With a view to the ubiquitous use of CBP derivatives as hosts for WOLED applications we note that the well-known propensity of this general class of materials to form excimer states (and the polarity dependence of triplet excimer formation) deserves more attention. 11,[18][19][20][21][22][23][24][25][26][27][28] The presence of poorly emissive triplet excimers by the host can easily go unnoticed, in particular when combined with efficient phosphorescent emitters that cover a broad spectral range as is the case in WOLEDs. Nevertheless, long-lived triplet excimers may present nonradiative decay channels, and they can reduce device efficiency and device lifetime if not suitably managed.…”

“…While this intermolecular triplet-state emission is present for both types of compounds, the radiative excimer emission rate is higher for the CH 3 -substituted compounds than for the compounds with the more polar CF 3 group, where the intermolecular excited state seems to acquire a stronger charge-transfer component. With a view to the ubiquitous use of CBP derivatives as hosts for WOLED applications we note that the well-known propensity of this general class of materials to form excimer states (and the polarity dependence of triplet excimer formation) deserves more attention. ,− The presence of poorly emissive triplet excimers by the host can easily go unnoticed, in particular when combined with efficient phosphorescent emitters that cover a broad spectral range as is the case in WOLEDs. Nevertheless, long-lived triplet excimers may present nonradiative decay channels, and they can reduce device efficiency and device lifetime if not suitably managed.…”