Peri-N-amine-perylenes, with and without phenyl bridge: Photophysical studies and their OLED applications

Abstract

Donor-bridge-acceptor framework has been studied widely for their detailed photophysical properties with the aim of efficient long range charge transfer. Carbazole attached with electron acceptors has been used for charge transfer properties, thermally activated delayed fluorescence, exciton dynamics etc. In this study, we synthesized donor–acceptor dyads of seconday amine (carbazole and diphenyl amine) attached to perylene through nitrogen, with phenyl (P-Ph-N-CBZ and P-Ph-N-BP) and without phenyl (P-N-CBZ and P-N-BP). Further their detailed photophysical and electroluminescent properties are discussed. P-N-BP is known and is used as standard in this work. DFT studies show nearly orthogonal geometry of P-N-CBZ with the dihedral angle of ∼ 71° between the planes of perylene and carbazole. For P-Ph-N-BP and P-Ph-N-CBZ, the perylene and amines are adopting almost co-planar geometries. Emission spectra of P-Ph-N-CBZ and P-N-CBZ in dichloromethane are structural with clear vibronic features while for P-Ph-N-BP and P-N-BP, the emission is structureless and broad indicating prominent electronic coupling between molecular orbitals of donor and acceptor in the latter case. Fluorescence quantum yields of phenyl-linked dyads are found to be higher than directly linked perylene-amine derivatives. High quantum yield (0.79) in non-polar hexane and polar DMSO (0.66) is observed for P-Ph-N-CBZ. It is interesting to note that the fluorescence lifetimes of these dyads are increasing while fluorescence quantum yields are decreasing with increasing polar solvents, suggesting that the emission has a significant contribution from the 1CT state. Dipole moments estimated from DFT studies and photophysical methods are higher for phenyl-linked dyads than those directly attached which is also consistent with their solvatochromism properties. Weaker electronic coupling in phenyl-linked dyads resulted due to the long distance between donor and acceptor in phenyl-linked dyads. Furthermore application in OLEDs was shown for P-Ph-N-CBZ and P-Ph-N-BP. OLEDs of P-Ph-N-BP showed a peak emission at 568 nm resulting in an amber device. A high luminance of 4.3 × 103 Cd/m2 at a current density of 100 mA/cm2 and a maximum EQE of 4.2% with a low turn on volatage of ∼ 4 V was obtained for these devices. Thus, our results show that weaker charge transfer takes place in perylene based donor–acceptor dyads resulting in bright OLED devices.