Arylallylidene-Imidazolones and Their Heterocyclic Analogues: Synthesis and Investigation of Fluorescent Properties

Fluorescence microscopy remains one of the most convenient methods for observing living objects in experimental biology. It involves the application of various fluorescent and fluorogenic dyes that stain colorless biological objects in a specific way. Recently, fluorogenic labels have gained much more popularity compared to classical dyes. Their fluorescence intensity increases when they bound to biological objects. Some of the most promising labels are arylidene-imidazolones and their derivatives. In particular, the compounds with an expanded system of conjugated bonds and improved optical properties are in high demand. Such substances have absorption and emission maxima in the long-wavelength region of the spectrum. The most widespread strategy for constructing such systems is based on modification of the imidazolone ring by introducing an additional styrene fragment. The least explored approach involves adding multiple bonds in the arylidene part of the molecule. Moreover, some other interesting objects for investigation are arylidene-rhodanines and their derivatives that may be considered as heterocyclic analogues of arylidene-imidazolones. They are widely known in the scientific community due to special optical and biologically active properties. In this work, we proposed a new series of fluorogenic dyes with an increased π-system, i.e., arylallylidene-imidazolones and arylallylidene-rhodanines. We found an approach for obtaining these compounds and synthesized a representative group of them. Their structures were confirmed by various analytical methods. It was demonstrated that predominantly new arylallylidene-imidazolones and their heterocyclic analogues existed as mixtures of (Z/E)- and (Z/Z)-isomers. In addition, their optical properties in a set of solvents with different characteristics were studied. It was observed that new fluorogenic dyes had a noticeable shift of absorption and emission maxima to the long-wavelength region of the spectrum, an impressive Stokes shift, as well as a significant variation of the fluorescence quantum yield depending on the medium. Despite the low photostability of arylallylidene-imidazolones and arylallylidene-rhodanines, it was shown that one compound from the series was capable to stain lipid droplets and membranes in living cells. As a result, it was discovered that the proposed library of the new compounds with an increased system of conjugated bonds can act as potential fluorogenic dyes and polarity sensors.