No abstract

Acridine derivatives are widely used as antiseptics, disinfectants, antibiotics, and fluorescent dyes as well as a subunit of optical sensor molecules of various analytical applications. We reported here the synthesis of several new 9‐substituted‐4,5‐dimethoxyacridine derivatives (9‐bromo‐, 9‐(p‐methylphenyl)‐, 9‐(p‐benzyloxyphenyl)‐, 9‐azido‐, 9‐phenyltriazole‐, 9‐[(trimethylsilyl)ethynyl]‐, 9‐nitrile‐) as potential multifunctionalizable fluorescent precursors of sensor molecules, indicators or dyes including optimized procedures for preparation of 9‐haloacridines and their cross‐coupling reactions. Improved stability of several new compounds could be achieved by the synthetic modification at position 9 of the acridine unit compared to their unsubstituted acridine analogs. The basic spectroscopic characteristics of these useful intermediates were also determined. The new fluorescent compounds showed promising photochemical properties for optochemical‐based applications in the future. The extremely large Stokes‐shifts (above 15500 cm−1) should especially be emphasized. Moreover, the effects of the substituents at position 9 of the acridine unit on fluorescence signaling were studied by the examples of new precursors and their macrocyclic derivatives as known optical sensor molecules. Some interesting relationships between quantum yield and structural features were reported.

Crown ethers containing an acridone or an acridine unit are successfully applied opto- and electrochemical cation sensors. The heteroaromatic unit of these macrocycles can be in different forms during the applications, which have a strong influence on the sensing behavior. Moreover, in the case of acridono-macrocycles a prototropic equilibrium takes place upon complexation, which is effected by the physicochemical characteristics. A Pb2+-selective acridono-18-crown-6 ether and its 9-phenylacridino-analogue were used as model compounds for comparing the different forms of the heterocyclic units of these sensor molecules. Since in most practical sensor applications of the fluorescent hosts a non-neutral aqueous medium is present, studies on complexation and signaling were carried out from the aspect of the relationship among protonation, coordinating ability, complex stability and tautomeric equilibrium. A strong interdependence among these factors was found and limitations of using unsubstituted acridino- and acridono-sensor molecules in comparison with their 9-substituted-acridino-analogues were discussed. This study will hopefully serve as a useful standpoint for future development of ionophore-based sensors containing an acridone or an acridine unit.

A new fluorescent bis(acridino)-macrocycle containing two allyl groups was synthesized and photophysically studied. Studies were carried out on metal ion recognition and selectivity-influencing effects including the determination of the relevant thermodynamic constants as logK and pKa. The proposed sensor molecule is recommended for the development of Zn2+-selective optochemical analyzers based on covalently immobilized ionophores as it has a unique pH-independent metal ion recognition ability, which is not influenced by anions and other potentially occurring metal ions in biological samples.

Acridino-diaza-20-crown-6 ether derivatives as new turn-on type fluorescent chemosensors with an excellent functionality and photophysical properties have been designed and synthesized for metal ion-selective optochemical sensing applications. Spectroscopic studies revealed that in an acetonitrile-based semi-aqueous medium, the sensor molecules exhibited a remarkable fluorescence enhancement with high sensitivity only toward Zn2+, Al3+ and Bi3+, among 23 different metal ions. Studies on complexation showed a great coordinating ability of logK > 4.7 with a 1:1 complex stoichiometry in each case. The detection limits were found to be from 59 nM to micromoles. The new ionophores enabled an optical response without being affected either by the pH in the range of 5.5–7.5, or the presence of various anions or competing metal ions. Varying the N-substituents of the new host-backbone provides diverse opportunities in both immobilization and practical applications without influencing the molecular recognition abilities.