Diverse chemical species such as ions and molecules exist within living cells and organisms undergoing dynamic changes in their local environment by a web of continuously interacting reactions. Reaction‐based fluorescent probes with a highly sought reversible feature can provide a real‐time monitor of the concentration dynamics (increases and decreases) of such chemical species, thus ideally suited to understand the physiological function, and pathogenic mechanisms of corresponding bio‐species in the regulation of cellular function and disease progression. This review summarizes the current methods for constructing reversible reaction‐based fluorescent probes. The sensing mechanisms and biological applications of these probes are also discussed. The representative examples reported recently are categorized according to the type of reversible chemical reactions utilized: nucleophilic additions (Michael additions, chromophore reactions), nucleophilic addition‐condensation reactions, and redox reactions. Finally, we present the potential challenges and suggestions for developing probes based on dynamic and reversible covalent bond formation reactions.
Photocyclization, irrespective of whether multiple steps (e.g.,N orrish-Yang cyclization) or as ingle concerted step (e.g.,6 p photocyclization) are involved, is an intramolecular photochemical processr esulting in the formation of one new single bond to afford ar ing system. In particular, visible-light-induced photocyclization offers ag reen and sustainable route to organic cyclic compounds that are difficult to access by thermalr eactions. Herein,w edescribe the ambient light-induced intramolecular photocyclization of a series of donor/acceptorc hromophores 1d-3d containing two types of photoresponsive motifs, namely an electron-deficient BF 2-chelated ketone fused with an electron-rich thiophene, and probe the solution-phase ands olid-statep hotochromic performance of these compounds. The results reveal that simple variation of Rs ubstituents on the diaryl moiety allows one to control the intramolecular photocyclization mechanism with high photochemical selectivity,e .g., under ambient light, methyl-substituted 1d and 2d undergo reversible 6p photocyclization, whereas ethyl-substituted 3d exclusively undergoes irreversible Norrish-Yang photocyclization. Single-crystal X-ray analysis of Norrish-Yang cyclization products reveals the formation of four pairs of conformationale nantiomers differing in the dihedral angle between benzothiophene and the BF 2 core, namely (AE)N-3d@688,(AE)N-3 d@-778,(AE)N-3 d@-788,a nd (AE)N-3 d@-1028. The UV/Vis absorption spectra of 1d-3d cover ab road visible-lightregion (380-572nm), whileDFT and TD-DFT calculations reveal that absorptioni nt his region is dominated by the charge-transfer (CT) transition from the thiophene-centered HOMO to the LUMO of the electron-deficient p-conjugated BF 2-chelated unit and the n!p*a nd p!p*t ransitions within the latter unit. The spatial separation of the HOMO and LUMO of these dyes promotes triplet-state generationa nd self-photosensitizes intramolecular photocyclization in the visible-light region. Three-dimensionalt ime-resolved ands teady-state emission spectrao f3d show that the Norrish-Yang photocyclization takes place within milliseconds with excellent conversion efficiency (96 %).
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