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Cationic [n]helicenes ( n = 4,5,6), based on bridged triarylmethylium skeletons, are highly stable carbenium ions, which can be stored as carbocations under air. Most of them are inert even in strongly alkaline aqueous solutions. These colorful dyes can be tuned electronically by changing their intrarim heteroatoms or by diverse regioselective additions of substituents (auxochromes). Fluorescence in the red to NIR is also afforded with moderate to excellent quantum yields and lifetimes up to 10 ns and above. Many applications, detailed herein, have been reported, thanks to their simple and flexible synthesis that yields helicenes with (very) high configurational and chemical stability and tailored optical and redox properties.

Functionalization of new sites on the triangulenium structure has been achieved by early-stage chlorination with N-chlorosuccinimide (NCS), giving rise to two new triangulenium dyes (1 and 3). By introducing the chlorine functionalities in the acridinium precursor, positions complementary to those previously obtained by electrophilic aromatic substitution on the final dyes are accessed. The chlorination is selective, giving only one regioisomer for both mono- and dichlorination products. For the monochlorinated acridinium compound, a highly selective ring-closing reaction was discovered, generating a single regioisomer of the cationic [4]helicene product. Further investigations into the mechanism of the [4]helicene formation lead to the first isolation of the previously proposed intermediate of the two-step SNAr reaction, key to all aza-bridged triangulenium and helicenium systems. Late-stage functionalization of DAOTA+ with NCS gave rise to a different dichlorinated compound (2). The fully ring closed chlorinated triangulenium dyes 1, 2, and 3 show a redshift in absorption and emission, while maintaining relatively high fluorescence quantum yields of 36%, 26%, and 41% and long fluorescence lifetimes of 15, 12.5, and 16 ns, respectively. Cyclic voltammetry shows that chlorination of the triangulenium dyes significantly lowers reduction potentials and thus allows for efficient tuning of redox and photoredox properties.

Increasing demand for detecting single molecules in challenging environments has raised the bar for the fluorophores used. To achieve better resolution and/or contrast in fluorescence microscopy, it is now essential to use bright and stable dyes with tailored photophysical properties. While long fluorescence lifetime fluorophores offer many advantages in time-resolved imaging, their inherently lower molar absorption coefficient has limited applications in single molecule imaging. Here we propose a generic approach to prepare bright, long fluorescence lifetime dyad fluorophores comprising an absorbing antenna chromophore with high absorption coefficient linked to an acceptor emitter with a long fluorescence lifetime. We introduce a dyad consisting of a perylene antenna and a triangulenium emitter with 100% energy transfer from donor to acceptor. The dyad retained the long fluorescence lifetime (∼17 ns) and high quantum yield (75%) of the triangulenium emitter, while the perylene antenna increased the molar absorption coefficient (up to 5 times) in comparison to the free triangulenium dye. These triangulenium based dyads with significantly improved brightness can now be detected at the single molecule level and easily discriminated from bright autofluorescence by time-gated and other lifetime-based detection schemes.

Fluorescent dyes are an important analytical tool. They see widespread use in biological sciences, where they help visualize anything from the dynamics of a cell to the localization of tumor tissue in patients. How the signal of a fluorophore is used can roughly be broken down into intensity-based measurements and lifetime-based measurements. In intensity-based methods, simply, the number of photons emitted by the fluorescent is recorded. In lifetime-based methods, the delay from excitation of a fluorescent dye till emission is used to gain information about the environment in which the dye is located. In this work a series of fluorescent dyes with exceptionally long fluorescence lifetimes are optimized for applications in biological sciences. Recent reports show that the fluorescent probe DAOTA-M2 gives a selective fluorescence lifetime response when bound to G-quadruplex DNA. It was investigated through analogues of the probe, what causes the selectivity over other DNA topologies. Quenching by photoinduced electron transfer from the appended morpholino-groups was found to be an important factor. From these findings a new series of morholino-substituted DAOTA+ dyes was synthesized to improve the fluorescence response in DNA binding. The large cationic triangulenium dyes are poorly soluble in water. Yet, the most interesting targets for their application are found in biological sciences, where water is almost always the solvent. Three strategies for making the DAOTA+ chromophore water-soluble were followed to synthesize the first water-soluble triangulenium dyes. Excellent photophysical properties werefound in aqueous solution of the new dyes. Based on the findings of the water-solubility study, a water-soluble long fluorescence lifetime triangulenium label for bioconjugation was prepared. The fluorescent label is expected to beuseful in the analysis of protein interactions.Lastly, it was investigated how triangulenium dyes could be synthesized to be more redshifted