In contrast to traditional short-lived fluorescent probes, long-lived phosphorescent probes based on transition-metal complexes can effectively eliminate unwanted background interference by using time-resolved luminescence imaging techniques, such as photoluminescence lifetime imaging microscopy. Hence, phosphorescent probes have become one of the most attractive candidates for investigating biological events in living systems. However, most of them are based on single emission intensity changes, which might be affected by a variety of intracellular environmental factors. Ratiometric measurement allows simultaneous recording of two separated wavelengths instead of measuring mere intensity changes and thus offers built-in correction for environmental effects. Herein, for the first time, a soft salt based phosphorescent probe has been developed for ratiometric and lifetime imaging of intracellular pH variations in real time. Specifically, a pH sensitive cationic complex (C1) and a pH insensitive anionic complex (A1) are directly connected through electrostatic interaction to form a soft salt based probe (S1), which exhibits a ratiometric phosphorescent response to pH with two well-resolved emission peaks separated by about 150 nm (from 475 to 625 nm). This novel probe was then successfully applied for ratiometric and lifetime imaging of intracellular pH variations. Moreover, quantitative measurements of intracellular pH fluctuations caused by oxidative stress have been performed for S1 based on the pH-dependent calibration curve.

A few of excited state intramolecular proton transfer (ESIPT) compounds have been discovered for their aggregation induced emission enhancement (AIEE). To understand the AIEE mechanism, an ESIPT compound BTHPB (N-(4-(benzo[d]thiazol-2-yl)-3-hydroxyphenyl)benzamide) with simple structure was designed and synthesized. BTHPB showed apparent AIEE property and the emission efficiency was observed as high as 0.27 in the aggregates. On the basis of viscochromism experiments and calculations employing the linear coupling model, the restriction of the rotation between the two subunits taken place in ESIPT was considered as the main factor for the AIEE. The micro- and femtosecond transient absorption experiments offered evidence for the considerations. Additionally, we also observed a negative effect of aggregation on the fluorescence emission in the system. So the AIEE of ESIPT compound BTHPB originated from the combination effects of positive and negative factors induced by the aggregation.

In this study, we have grafted poly(3-hexylthiophene) (P3HT) brushes on GO sheets via a silylation reaction of the surfaces of GO sheets and a succeeding “click” reaction (GO(C)/P3HT composite). Compared with pure P3HT and the blend of P3HT and GO, the GO(C)/P3HT composite shows a red-shifted optical absorption maximum because of increased conjugation length of the grafted P3HT, which might be due to the crowding of the P3HT chains grafted on GO sheets. For comparison, P3HT chains were also grafted on GO sheets via the amidation reaction of the carboxylic acid groups (GO(A)/P3HT composite). However, the GO(A)/P3HT composite doesn't show a red shift in the UV-visible spectrum. The overall fluorescence quenching in the GO(C)/P3HT composite includes both dynamic quenching and forming a non-fluorescent ground-state complex. Additionally, thermogravimetric analysis and X-ray photoelectron spectroscopy results demonstrate that the efficiency of grafting P3HT on GO sheets via the “click” reaction approach is higher than via the amidation reaction approach. Atomic force microscopy and transmission electron microscopy observations show that the GO(C)/P3HT composite sheets are much thicker than the GO(A)/P3HT composite sheets, due to the higher grafting efficiency of the “click” reaction approach. Therefore, this work opens a novel avenue to tuning the optical properties of conjugated polymers via surface grafting chemistry.