Interplanar interactions in the excited triplet states of carbazole dimers by means of time-resolved EPR spectroscopy

Synthesis and crystal structure of anti-10-butyl-10,11,22,23-tetrahydro-9H,21H-5,8:15,12-bis(metheno)[1,5,11]triazacyclohexadecino[1,16-a:5,6-a′]diindole

In the title compound, the mol­ecule adopts an anti-configuration, in which two partially overlapped carbazole fragments form an intra­molecular slipped π–π inter­action. In the crystal, the mol­ecules are cross-linked via inter­molecular C—H⋯N hydrogen bonds and C—H⋯π inter­actions into a three-dimensional network.

The title compound, C₃₃H₃₃N₃, is a carbazolophane, which is a cyclo­phane composed of two carbazole fragments. It has a planar chirality but crystallizes as a racemate in the space group P. The mol­ecule adopts an anti-configuration, in which two carbazole fragments are partially overlapped. Both carbazole ring systems are slightly bent, with the C atoms at 3-positions showing the largest deviations from the mean planes. The dihedral angle between two carbazole fragments is 5.19 (3)°, allowing an intra­molecular slipped π–π inter­action [Cg⋯Cg = 3.2514 (8) Å]. In the crystal, the mol­ecules are linked via inter­molecular C—H⋯N hydrogen bonds and C—H⋯π inter­actions into a network sheet parallel to the ab plane. The mol­ecules of different sheets form other C—H⋯π inter­actions, thus forming a three-dimensional network.

Electron spin resonance resolves intermediate triplet states in delayed fluorescence

Molecular organic fluorophores are currently used in organic light-emitting diodes, though non-emissive triplet excitons generated in devices incorporating conventional fluorophores limit the efficiency. This limit can be overcome in materials that have intramolecular charge-transfer excitonic states and associated small singlet-triplet energy separations; triplets can then be converted to emissive singlet excitons resulting in efficient delayed fluorescence. However, the mechanistic details of the spin interconversion have not yet been fully resolved. We report transient electron spin resonance studies that allow direct probing of the spin conversion in a series of delayed fluorescence fluorophores with varying energy gaps between local excitation and charge-transfer triplet states. The observation of distinct triplet signals, unusual in transient electron spin resonance, suggests that multiple triplet states mediate the photophysics for efficient light emission in delayed fluorescence emitters. We reveal that as the energy separation between local excitation and charge-transfer triplet states decreases, spin interconversion changes from a direct, singlet-triplet mechanism to an indirect mechanism involving intermediate states.

Persistent Dimer Emission in Thermally Activated Delayed Fluorescence Materials

We expose significant changes in the emission color of carbazole-based thermally activated delayed fluorescence (TADF) emitters that arise from the presence of persistent dimer states in thin films and organic light-emitting diodes (OLEDs). Direct photoexcitation of this dimer state in 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) reveals the significant influence of dimer species on the color purity of its photoluminescence and electroluminescence. The dimer species is sensitive to the sample preparation method, and its enduring presence contributes to the widely reported concentration-mediated red shift in the photoluminescence and electroluminescence of evaporated thin films. This discovery has implications on the usability of these, and similar, molecules for OLEDs and explains disparate electroluminescence spectra presented in the literature for these compounds. The dimerization-controlled changes observed in the TADF process and photoluminescence efficiency mean that careful consideration of dimer states is imperative in the design of future TADF emitters and the interpretation of previously reported studies of carbazole-based TADF materials.