Theoretical characterization of two-photon fluorescent probes for nitric oxide detection: sensing mechanism, photophysical properties and protonation effects.

Nitric oxide (NO) is an important signal molecule in biological systems and is correlated with many physiological processes and pathological diseases. To date, numerous fluorescent probes based on o-diamino aromatics have been designed and synthesized for NO detection utilizing the principle of photoinduced electron transfer (PET). However, the underlying PET mechanism has rarely been validated, and a systematic computational study on the photophysical properties is urgently desired. In this study, we used a theoretical protocol to comparatively investigate the sensing mechanism, photophysical properties and protonation effects of two emblematic probes NINO and PYSNO in aqueous solution, which combines a polarizable continuum model (PCM), time-dependent density functional theory (TD-DFT) and thermal vibration correlation function formalism (TVCF). Our findings reveal that the weak emission of NINO is due to activated PET with negative driving energy and blocked fluorescence with significant charge separation. In contrast, the poor luminescence of PYSNO is caused by the facilitated non-radiative dissipation, even though the fluorescence emission remains unobstructed. Although NINO has been successfully used in two-photon microscopy for detecting NO, we suggest that PYSNO possesses a superior two-photon absorption (TPA) cross section in the near-infrared region. The protonation effects suggest that both probes can function effectively in practical acidic lysosomal environments. Our study opens a new avenue for understanding the mechanism and predicting the properties of two-photon fluorescent probes for NO detection, thus aiding the rational design of efficient fluorescent sensors.

Referential modification strategy based on phenolic hydroxyl-containing KSA luminogens for ER-targeting probe construction.

The endoplasmic reticulum (ER) plays essential roles in various physiological processes and is intimately connected to kinds of diseases. The development of ER-targeting theranostic agents is highly demanded for precise treatments, however, the effective and referential strategies for the construction of ER-targeting probes are limited. Herein, we developed series of ER-targeting luminogens based on keto-salicylaldehyde azine (KSA) framework by introducing phenolic hydroxyl group, which present good theranostic performance with selective enrichment in ER. Under systematical structure modulation, the key role of phenolic hydroxyl group at K-terminal in ER-targeting was experimentally confirmed. Besides, the cyanobenzyl moiety at S-terminal can enhance the luminous efficiency and improve cellular uptake ability. Moreover, the generated reactive oxygen species (ROS) of these KSA derivatives can efficiently trigger ER stress to induce the apoptosis of cancer cells, resulting in the effective inhibition of tumor cells both in vitro and in vivo. Therefore, this feasible modification strategy of inserting phenolic hydroxyl group to common multi-aryl-based luminogens provides a reliable and referential approach for ER-targeting probe establishment.

New ß-diketone-boron difluoride based near-infrared fluorescent probes for polarity detection.

Two new ß-diketone-boron difluoride based near-infrared fluorescent probes 1 and 2 which exhibit polarity sensitivity have been designed and synthesized. Probes 1 and 2 are composed of a ß-diketone-boron difluoride moiety as an acceptor unit, and a diethylamino group and a phenolic hydroxyl group as donor units. The long conjugate structures form a "donor-acceptor-donor" configuration, induce intramolecular charge transfer (ICT), and confer near-infrared fluorescence emission and excellent polarity sensitivity. The photophysical properties of these two probes were investigated in detail. Experimental data demonstrated that as the environmental polarity decreased, the fluorescence intensity of the probes increased obviously, accompanied by a blue-shift of the maximum emission wavelength. In addition, these two probes were photostable and solely sensitive to polarity without interference from viscosity, pH and common active species. Theoretical calculations indicated that probes 1 and 2 displayed lower energy gaps and faster non-radiative decay in polar solvents. Furthermore, probes 1 and 2 were utilized to quantitatively detect the polarity of a binary mixture through the satisfactory linear relationship between the fluorescence emission intensity ratios and the orientation polarizability of the mixed solvent. Additionally, probe 1 was successfully utilized to visualize the polarity distribution of live cells. Both of these probes are perfect candidates for studying polarity in vitro and even in live systems.