We have judiciously constructed a novel ICT-based ratiometric OCl(-) probe capable of ratiometric imaging in the live cells based on the new OCl(-)-promoted de-diaminomaleonitrile reaction.

Small-molecule organic dyes have been widely used in fluorescent probes, [1] labels, [2] logic gates, [3] light-emitting materials, [4] and light-harvesting systems. [5] However, the undesirable photophysical properties of various fluorophores still constrain the full potential of their applications. For instance, many bright organic dyes including rhodamine, fluorescein, boron dipyrromethane (BODIPY), and cyanine derivatives have the serious disadvantage of very small Stokes shifts (typically less than 25 nm), which can lead to serious self-quenching and fluorescence detection errors because of excitation backscattering effects.[6] Therefore, there is a need to develop dyes with improved properties.Since it is still difficult to judiciously design single organic dyes with desirable photophysical properties, considerable attention has recently been paid to the exploration of multifluorophores with energy-donor-acceptor architectures. [1m, 6b, 7-9] In this regard, some energy-donor-acceptor systems based on fluorescence resonance energy transfer (FRET) have been constructed. [6b, 8] FRET dyads are usually linked by a nonconjugated spacer, and the energy transfer occurs through space. Although the pseudo-Stokes shifts (the wavelength discrepancy between the donor absorption and the acceptor emission in an energy transfer system with almost 100 % energy transfer efficiency [7] ) of FRET-based energy cassettes are larger than the Stokes shifts of either the donor or acceptor dyes, FRET-based cassettes are still limited by the requirement that the donor emission must have strong overlap with the acceptor absorption.[10] This requirement essentially restricts the pseudo-Stokes shifts as well as the emission shifts (the emission wavelength shift between the donor and acceptor) of FRET-based systems. Like the pseudo-Stokes shift, the emission shift is also an important parameter in energy-transfer dyads. A large emission shift in energy transfer systems should result in two well-separated emission peaks, which is favorable for the precise measurement of the peak intensities and ratios.[6b, 11] Thus, energytransfer dyads with large pseudo-Stokes shifts and emission shifts are desirable.By contrast, through-bond energy transfer (TBET) is theoretically not subjected to the constraint of intense spectral overlap between the donor emission and the acceptor absorption.[9] Thus, TBET cassettes may have large pseudoStokes shifts and emission shifts. Unlike through-space energy-transfer cassettes, in TBET cassettes, the donor and the acceptor units are joined by a conjugated spacer. Burgess and co-workers have developed elegant TBET systems based on the conjugated fluorescein-rhodamine system. [9a,b] However, the fluorescein (donor) emission overlaps significantly with the rhodamine (acceptor) absorption and the advantage of TBET, that is, no requirement of strong spectral overlap between the donor emission and the acceptor absorption, was not really capitalized upon in these conjugated fluoresceinrhodamine energy transfer ...

Hypochlorous acid (HOCl), a reactive oxygen species (ROS), plays a significant biological role in living systems. However, abnormal levels of HOCl are implicated in many inflammation-associated diseases. Therefore, the detection of HOCl is of great importance. In this work, we describe the HOCl-promoted cyclization of rhodamine-thiosemicarbazides to rhodamine-oxadiazoles, which is then exploited as a novel design strategy for the development of a new fluorescence turn-on HOCl probe 2. On the basis of the fluorescence resonance energy transfer (FRET) signaling mechanism, 2 was further converted into 1a and 1b, which represent the first paradigm of FRET-based ratiometric fluorescent HOCl probes. The outstanding features of 1a and 1b include well-resolved emission peaks, high sensitivity, high selectivity, good functionality at physiological pH, rapid response, low cytotoxicity, and good cell-membrane permeability. Furthermore, these excellent attributes enable us to demonstrate, for the first time, the ratiometric imaging of endogenously produced HOCl in living cells by using these novel ratiometric probes. We expect that 1a and 1b will be useful molecular tools for studies of HOCl biology. In addition, the HOCl-promoted cyclization reaction of rhodamine-thiosemicarbazides to rhodamine-oxadiazoles should be widely applicable for the development of different types of fluorescent HOCl probes.

The first ratiometric fluorescent probe for hydroxyl radical ratiometric imaging in living cells was rationally designed based on a hybrid coumarin-cyanine platform.

A novel highly reactive ratiometric fluorescent cyanide probe was judiciously designed based on 2-formylacrylonitrile moiety as a new cyanide reaction site. A DFT study was conducted to rationalize the extremely high reactivity nature of the ratiometric fluorescent cyanide probe.