Analyzing the Effect of Substituents on the Photophysical Properties of Carbazole-Based Two-Photon Fluorescent Probes for Hypochlorite in Mitochondria

The development and utilization of two-photon fluorescent probes with large two-photon absorption (TPA) cross sections (δ_TPA) and strong TP-excited fluorescence are highly needed for monitoring the level of hypochlorite in mitochondria. In this contribution, we successfully explain the fluorescence "off-on" effect observed in experiment, and clarify the fluorescent quenching mechanism of the probe containing hydroxyl oxime from the aspect of excited-state dynamics for the first time, while exploring the influence of alkyl chain length on TPA cross section and fluorescent quantum yield. It is proposed that the alkyl chain length affects significantly the fluorescent efficiency by the vibronic coupling. The proper alkyl chain length should also be one of the smart strategies for designing highly efficiency fluorescent two-photon probes applied in biological systems. We demonstrate that the designed compound HCB-CNO has exceptional optical properties, such as the larger TPA cross section (467 GM) and higher fluorescence quantum yield (0.485), compared to the experimental molecule. Moreover, the TPA tensor elements are increased remarkably by introducing strong electron-donors at the R1 position, which promote to intramolecular charge transfer, and improve significantly two-photon absorption cross sections.