Fluorescent merocyanines: from fundamental properties to applications as molecular probes, in bioimaging and as...

Among donor-acceptor dyes, merocyanines (MCs) stand out as their absorption and emission wavelengths can be tuned over a wide visible and near-infrared (NIR) spectral range by proper choice of donor and acceptor end groups and length of the polymethine chain. Concerning emission properties, however, non-radiative relaxation channels enabled by the floppy nature of the polymethine chain and changes in bond lengths upon optical excitation are problematic but offer perspectives for environmentally sensitive fluorescent probes. In the first part of this review, we analyse the fundamental properties of MCs and point out the special case of MCs whose electronic structure is located in the cyanine limit, where structural changes upon optical excitation are minimized, thereby establishing the best starting point for emitter design. Subsequently, we show examples of highly fluorescent MC dyes and discuss the solvent dependency of emission wavelength and quantum yield. This discussion will not only provide insights into common relaxation pathways via twisted intramolecular charge transfer (TICT) states and conical intersections (CIs) but also highlight unexpected improvements in fluorescence intensity with chain elongation, providing MC emitters for the NIR spectral range. In the following, examples of the utilization of MCs as fluorescent molecular probes and for biomolecular imaging will be discussed. In the former case, probes that exhibit fluorescence turn-on upon rigidification are particularly desirable, as they enable the detection of ligand binding, protein activation, or other biochemical events, based on MCs being either covalently or non-covalently attached to nucleic acid or protein scaffolds. For the latter, a more constant luminescence intensity is preferred as a marker of a tagged biomacromolecule, e.g., in live cell imaging. In the last part, counteracting effects on fluorescence originating upon aggregation will be elucidated which consist of (desired) slow-down of non-radiative relaxation channels of photoexcited MCs due to rigidification vs. reduced radiative rates due to co-facial stacking, leading to (undesired) H-type coupling with the concomitant decrease of oscillator strength for the lowest excitonic state. Here, we will highlight examples where the first process prevails, leading to emission enhancement, as demonstrated for solid state materials as well as self-assembled aggregates and foldamers in solution.