Linkage Isomers and Their Enantiomeric Siblings as CPL-TADF Emitters: Modulating Chirality and Enhancing Dissymmetry Factor Through N- and O-Linked Carbazoles

Abstract

Circularly polarized luminescence (CPL) thermally activated delayed fluorescence (TADF) materials have garnered massive attention due to their significant practical applications. Herein, we developed two linkage isomers and their optically active enantiomers (dicyanobenzene unit as the acceptor and a carbazole system as the donor) and analyzed their comparative photophysical and chiroptical properties. The linkage isomers were designed through the different linking positions of the carbazole to the acceptor core. In this D-A system, carbazole was connected through a nitrogen (N-linkage; R/S-CZ4BTXY) or oxygen (O-linkage; R/S-NBTOCZ). An axially chiral BINOL system induced the chirality in these D-A-based molecules. The impact of this linkage isomerization on the photophysical and chiroptical properties was further analyzed. UV-vis absorption studies revealed that the N-linked isomer (R-CZ4BTXY) exhibited a small intensity charge transfer (CT) band; however, this type of band was not observed in the O-linked isomer. This indicated the possibility of strong CT character in the N-linked isomer relative to the O-linked isomer. In the PL spectra, the O-linked isomer depicted emission mainly from the LE state; however, a CT band was dominant in the N-linked isomer. The CT band diminished in polar solvents, which may be attributed to increased non-radiative decays in these solvents. This CT band can be credited to the comparatively stronger electron-donating ability of nitrogen-linked carbazole. Solvatochromism studies confirmed that the N-linked isomer exhibited higher emission intensity in non-polar solvents and significant quenching in polar environments. Moreover, aggregation-induced enhanced intensity was observed in the higher percentage of water in the THF-water mixtures for the N-linked isomer. N- and O-linked isomers and their enantiomers depicted distinct circular dichroism (CD) and CPL signals in their toluene solution. Notably, the luminescence dissymmetry factor (glum) of the O-linked isomer is nearly four times greater than that of the N-linked counterpart. This might be attributed to the more spatially defined chiral environment created by O-linkage. N-linked isomer exhibits a sign reversal between CD and CPL. This suggests that linkage modifies the relationship between ground- and excited-state chirality. This work highlights the critical role of linkage variation in tuning the optoelectronic and chiroptical properties of chiral TADF materials.