Theoretical Investigations on the Photophysical Properties for a Series of Symmetrical and Asymmetrical Carbazole-Based Cationic Two-Photon Fluorescent Probes: The Magic of Methyl Groups

Abstract

The biological fluorescence imaging for nucleic acid has attracted attention because of the essential role of nucleic acid in a living system. Two-photon (TP) fluorescent probes are important molecular tools for biological imaging because of their high resolution and low photodamage to tissues. However, the practically applicable TP fluorescent probes are still limited because of an indistinct fluorescent mechanism and ambiguous relationship between molecular structures and TP-excited fluorescent properties. In this contribution, we researched the photophysical properties for a series of symmetrical and asymmetrical carbazole-based cationic TP fluorescent probes for nucleic acid and explained the fluorescent mechanism of the nucleic acid probes from the aspect of excited-state dynamics. It is first proposed that the fluorescence quenching for the cationic probes with terminal methyl chains is derived from methyl rotating motion in a low-frequency regime rather than vinyl rotating motion in a water solvent. We illuminated the origin of better TP absorption (TPA) properties for cationic compounds compared with corresponding neutral compounds. The TPA efficiency is much higher for a symmetrical compound than for an asymmetrical compound, and the symmetrical compound is more stable because of restricted molecular bending vibrations. Besides, the electron-donating/-withdrawing ability of substituent groups and the position of methyl at the acceptor also have an important influence on TPA properties of cationic compounds.