Bioapplications Manipulated by AIEgens with Nonlinear Optical Effect

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.

The inspiration and challenge for through-space charge transfer architecture: from thermally activated delayed fluorescence to non-linear optical properties

Organic molecules consisting of electron donor (D) and electron acceptor (A) subunits linked by π-conjugated bridges are promising building blocks for thermally activated delayed fluorescence (TADF) and non-linear optics (NLO) materials due to their intramolecular charge transfer (CT) processes in response to external stimuli. According to the electron interaction pattern, the CT process in D-π-A architectures can be divided into two categories, through-bond/-space charge transfer (TB/TSCT). To date, research into the TADF properties of TSCT characteristic molecules has since seen significant growth. In fact, TSCT characteristic materials show great advantages in such NLO responses. In this perspective, we first briefly introduced the basic principles of NLO and TADF effects. Successively, we discuss the influence of TBCT and TSCT patterns on NLO and TADF properties, especially for TSCT characteristic. In the final part, we address the diversity and potential advantages of TSCT characteristic molecules as high-performance NLO materials. With these, it is expected that the greater structural flexibility of spatial conjugation can bring more functionality to NLO materials in the future.