Design, Synthesis, and Fluorescence Property Tuning of Methyl p-Ethynylbenzoate-Based [10]Cycloparaphenylenes

Abstract

This study designs a series of [10]Cycloparaphenylenes ([10]CPP) derivatives modified with carboxymethyl ester groups, which exhibit solvatochromism and intramolecular charge transfer behavior, as revealed by computational analysis. The most notable derivative is DP[10]CPP, modified with double methyl 4-ethynylbenzoate (DP), synthesized via the Suzuki-Miyaura method. Compared to [10]CPP, DP[10]CPP shows a blueshift in the main absorption peak from 338 nm to 330 nm, a redshift in the weak absorption band from 390 nm to 398 nm, and a redshifted fluorescence spectrum. With increasing solvent polarity, the UV-Vis absorption spectra show a slight redshift, while fluorescence spectra exhibit a more pronounced redshift, attributed to charge transfer states, as confirmed by Lippert-Mataga fitting analysis. Computational studies suggest that while the DP fragment reduces π-conjugation within the [10]CPP moiety, it extended structure enhances the overall conjugation of DP[10]CPP, thereby modulating π-π* transitions. Molecular orbital analysis further indicates that the DP fragment disrupts the symmetry of the [10]CPP core, lifting Laporte-forbidden transitions, which enhances and redshifts the weak absorption band. As a result, DP[10]CPP achieves a photoluminescence quantum yield of 0.77 in tetrahydrofuran. The study also discusses the solvent and intramolecular charge transfer behaviors, providing insights for tuning the electronic properties of such molecules.