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Novel fluorescent 4,5-diarylimidazolyl-phenylalanines 3a-d were prepared by reaction of N-tert-butyloxycarbonyl-4-formylphenylalanine methyl ester and appropriate (hetero)aromatic diones. The photophysical properties of these new unnatural amino acids were evaluated by UV-Vis absorption and fluorescence spectroscopy in solvents of different character and aqueous mixtures with acetonitrile. They were evaluated as novel amino acid based fluorimetric chemosensors for ions through spectrophotometric and spectrofluorimetric titrations with biologically and analytically important anions and cations such as F-, OH-, Cu 2+ and Fe 3+. The results indicate that there was a strong interaction through the donor N, O and S atoms at the side chain, especially for 4,5-di(furan-2'-yl)imidazolyl-phenylalanine 3a and 4,5-di(thiophen-2'-yl)imidazolyl-phenylalanine 3b with Cu 2+ and Fe 3+ , in a 1:1 complex stoichiometry. The photophysical and metal ion sensing properties of these amino acids suggest that they can be suitable for incorporation into chemosensory peptidic frameworks.

a b s t r a c tA family of new asparagine derivatives bearing benzothiazole and benzimidazole units, functionalised with electron donor or acceptor groups, were synthesized in good to excellent yields. The photophysical characterization of these new heterocyclic amino acids was performed by UV-visible absorption and fluorescence emission studies and revealed that the compounds displayed remarkably high fluorescence quantum yields and Stokes' shifts, making them good candidates for application as fluorescent probes by incorporation into peptidic frameworks.

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Novel phenylalanine derivatives bearing benzimidazole and crown ethers as coordinating/reporting units and thiophene as spacer unit were synthesized, and their evaluation as fluorimetric chemosensors was carried out in acetonitrile and acetonitrile/water solutions. 15-Crown-5 benzimidazolyl phenylalanine methyl ester, 15-crown-5 thienylbenzimidazolyl phenylalanine methyl ester and 18-crown-6 thienylbenzimidazolyl phenylalanine methyl ester were tested for alkaline, alkaline-earth and transition metal ions (such as Na + , Ca 2+ , Cd 2+ , Co 2+ , Cr 3+ , Cu 2+ , Fe 2+ , Fe 3+ , Hg 2+ , Ni 2+ , Pd 2+ and Zn 2+). The different crown ether binding moieties as well as the electronic nature and the length of the π-bridge linked to the benzimidazole heterocycle allowed the fine tuning of the sensory properties as seen by spectrofluorimetric titrations. Therefore, 15-crown-5 benzimidazolyl phenylalanine methyl ester is a fluorimetric chemosensor, being selective and sensitive for Cu 2+ and Pd 2+ in aqueous solutions (ACN/H2O; 80:20). On the other hand, the metal cation sensing properties displayed by 15-crown-5 thienylbenzimidazolyl phenylalanine methyl ester bearing an arylthienyl spacer showed that this is a promising candidate as fluorimetric chemosensor for Fe 3+ , Pb 2+ and Pd 2+ in acetonitrile solution.

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