Mono-substituted and di-substituted pyrene derivatives: photoelectric properties and HLCT mechanism

With electron-donating o-carbazole phenyl (o-CZPh) and electron-withdrawing cyanophenyl (CNPh) on 1- or 1,8-site of pyrene, two mono-substituted derivatives 1-CNPhPy and 1-oCZPy with A-D structure and three di-substituted derivatives with symmetric D-π-D (18-oCZPy) and A-π-A (18-CNPhPy) and asymmetric D-π-A (18-oCPPCN) structures are developed. Single crystals of 18-oCZPy and 18-oCPPCN indicate rich intermolecular interactions and the molecules being stacked in dimers in which pyrene planes overlap by 16% and 46% in area and separate by 3.25 Å and 3.48 Å in distance, respectively. Due to larger conjugation and intramolecular charge transfer between CNPh and pyrene moiety than those between o-CZPh and pyrene, CNPh-substituted derivatives present photoluminescent (PL) spectra at longer wavelength. Together with the influence of the compact stacking in condensed state, CNPh-substituted derivatives present more redshifted and broader PL spectra and more decline of PL quantum efficiency (PLQY) in films relative to in solutions, compared to the corresponding o-CZPh-substituted ones. Solvatochromic study presents three types of features closely related to molecular structures and hybridized local and charge transfer states in the medium polarity solvents. Nanosecond-scale PL lifetime and large energy gap of above 0.9 eV between first singlet and triplet excited states exclude the thermally activated delayed fluorescence. Theoretical calculation exhibits multiple highly-lying triplet states being positioned near to singlet states with high spin orbital coupling. Having PL peak at 427 and 449 nm, FWHM of 56 and 62 nm, PLQY of 66.8% and 47.4% in solution and in film, 18-oCPPCN is considered as potential high-performance blue emitter.