Steric hindrance modulated efficient thermally activated delayed fluorescence with non-linear optical, ferroelectric and piezoelectric properties

Multi-carbazole-based benzonitrile systems are efficient thermally activated delayed fluorescence (TADF) materials for organic light-emitting diodes (OLEDs). However, they suffer from low PLQY due to the large dihedral angle arising from steric crowding. Addressing this challenge, we demonstrate a potent strategy to engineer steric crowding in this work. To achieve our goal, we have designed three luminogens, namely, CzPHCN, tCzPHCN and Cz2CzPHCN based on phenanthrene-9,10-dicarbonitrile (PHCN), as an acceptor core to minimize the steric hindrance between the donor groups. Among the three luminogens, <i>t</i>CzPHCN exhibits a maximum PLQY of 86% and the highest RISC rate of 3.5 × 10⁵ s^-1, the underlying cause being the least dihedral angle of 45.72° and suppressed intermolecular interaction due to the presence of the bulky <i>tert</i>-butyl group. Interestingly, our QM/MM calculations and experimental evidence suggest that the RISC process of both CzPHCN and tCzPHCN takes place <i>via</i> a hot exciton channel. Unlocking a new realm of applications, the unique non-centrosymmetric space group (<i>Cmc</i>2₁) of CzPHCN offers excellent SHG with a χ⁽²⁾ value of 0.21 pm V^-1 at 1320 nm. In addition to this, the molecule depicts good ferroelectric (<i>P</i> _S = 0.32 μC cm^-2), piezoelectric energy harvesting (<i>V</i> _OC = 2.8 V) and two-photon absorption properties.