In recent years, derivatives of carbazole have been studied intensively with focus on their optical properties to scry for applicability as materials in OLEDs and OFETs. Fine tuning of the electronic properties of the materials was achieved by a large variety of substitution patterns both on the central nitrogen atom and the carbon periphery.
Recently, the group of Dr. Hinz (Karlsruhe Institute of Technology, Germany) has investigated a series of alkali metal carbazolides with bulky arenes in positions 1 and 8 of the carbazole scaffold (see Figure 1). The alkali metal complexes of the type [(Cbz)M] show visible fluorescence with emissions maxima in the range of 520 and 460 nm in dependence of the metal ion. Quantum yields of up to 29% in the solid state were measured for the rubidium derivative.
The electronic transitions were rationalised with the aid of TD-DFT calculations. Upon HOMO→LUMO+1 excitation, density shifts from the carbazole scaffold to the arenes in positions 1 and 8 and thus are intraligand transitions. The geometry of the complexes only change slightly upon excitation which enables the highly efficient fluorescence.
Coordination of additional toluene molecules to the alkali metal shifts the emission maximum to lower energy and enables a second emission band arising from an interligand excitation from the carbazole to a toluene molecule. The quantum yields for the complexes in toluene solution are even higher than in the solid state and reach 100% for the lithium complex.
Alexander Hinz studied chemistry in Rostock, Germany, and is currently a junior group leader at the Karlsruhe Institute of Technology. The work of the Hinz group is focused on molecular main group chemistry and devoted to the investigation of low-coordinated, but highly reactive compounds.