Structural and physical properties of triarylamines bearing alkyl chains introduced by strontium-mediated alkylation

Introducing alkyl chains into π-conjugated organic molecules is a well-established strategy for tuning solubility, crystallinity, and photophysical properties without significantly perturbing their electronic structures. However, conventional alkylation methods, such as the Grignard reaction, often fail to achieve efficient functionalization when applied to sterically hindered aromatic systems, such as triarylamines, which are widely used as building blocks in electronic materials. In this study, a strontium-mediated alkylation reaction was employed to introduce linear and branched alkyl chains at the para-positions of triarylamine derivatives bearing methoxycarbonyl groups. This method enables efficient alkylation under mild conditions, even at sterically congested benzylic positions. A series of alkylated triarylamines was synthesized using various alkyl iodides and metallic strontium. The resulting compounds were fully characterized by NMR, IR, and high-resolution mass spectrometry. Their electronic and luminescent properties were investigated by UV-Vis absorption and fluorescence spectroscopy in various solvents. Although the absorption maxima shifted only slightly depending on the alkyl substituent, indicating minimal perturbation of the π-conjugated backbone, fluorescence measurements revealed a pronounced dependence of the quantum yield on the length and branching of the alkyl chains. In particular, the introduction of bulky substituents such as neopentyl groups led to a significant decrease in emission efficiency, likely due to steric hindrance and enhanced nonradiative decay pathways. These findings demonstrate that long-chain and branched alkyl substituents serve as subtle yet effective modulators in modulating the photophysical behavior of triarylamines. Overall, strontium-mediated alkylation offers a robust functionalization strategy for modifying the periphery of triarylamine frameworks while preserving their core conjugation. This approach, which enables the introduction of bulky alkyl groups inaccessible via conventional methods, opens new avenues for designing functional organic materials with tunable electronic properties.