Phenanthroimidazole derivatives as fluorimetric chemosensors in aqueous medium

: The new 2,4,5-tri-substituted imidazole derivatives 2a - c were obtained in good yields through a Radziszewski reaction. The new compounds were characterized by the usual spectroscopic techniques and a detailed photophysical study was undertaken. The evaluation of the compounds as fluorimetric chemosensors was carried out by performing spectrofluorimetric titrations in acetonitrile and acetonitrile/water in the presence of relevant organic and inorganic anions, and of alkaline, alkaline-earth and transition metal cations.


Introduction
In recent years the research on sensors with good sensitivity and selectivity in aqueous medium has been of great interest. 1 Chemosensors soluble in aqueous media are very interesting, because of their prospective application in biomedical and environmental pollution tests. 2 Fluorescent sensor molecules for the detection of metal ions have attracted considerable interest which has led to the development of highly specific probes with a broad range of applications in environmental chemistry, biochemistry, and cell biology. 3 The analyses based on fluorescence sensors are easy to handle and less expensive. For many fluorescent chemosensors the change in emission intensity by interaction with the analyte is measured, but a shift of the emission wavelength is also a convenient means to monitor the sensing process. 4 2,4,5-Triaryl-imidazoles are versatile compounds with application in medicine, due to their biological activity, and materials sciences, for their interesting optical properties.
These properties can be tuned by careful selection of substituents at positions 2, 4 and 5: replacement of the aryl group by an heterocyclic group results in larger π-conjugated systems with improved optical properties for application in nonlinear optics, OLEDs, DNA intercalators,and chemosensors. 5 Considering our current research interests in this theme, 4,5 we now report the synthesis of new phenanthroimidazoles, substituted at position 2 with (hetero)aryl groups of different electronic character, in order to evaluate their photophysical properties and chemosensory ability. The new derivatives were characterized by the usual techniques and a detailed photophysical study was undertaken. The evaluation of the compounds as fluorimetric chemosensors was carried out by performing titrations in acetonitrile and acetonitrile/water in the presence of relevant organic and inorganic anions, and of alkaline, alkaline-earth and transition metal cations.

General procedure for the synthesis of phenanthoimidazole 2a-c
The heterocyclic aldehyde 1 (0.073 g, 1 mmol), 9,10-phenanthrenedione (0.085 g, 1 mmol) and NH 4 OAc (0.632 g, 20 mmol) were dissolved in glacial acetic acid (5 mL), followed by stirring and heating at reflux for 8 h. The mixture was then cooled to room temperature and ethyl acetate was added (15 mL) and washed with water (3 x 10 mL).

Synthesis and characterization
Phenanthroimidazoles 2a-c were synthesized in moderate to good yields (54-87 %), by Radziszewski reaction between aldehydes 1 and phenanthrenequinone (Scheme). The new compounds were completely characterized by the usual spectroscopic techniques (Table 1) (Table 1). Compounds 2a-c, differing in the substituent at position 2 of the imidazole ring, displayed shifts according to the electronic character of the substituent, e.g., imidazole 2b bearing a thiophene displayed a bathochromic shift for the maximum 5 wavelength of absorption (12 nm) and emission (6 nm) when compared to 2a (Table 1).
The relative fluorescence quantum yields were determined by using 10 -6 M solutions of DPA in ethanol as standard (Ф F = 0.95). 7 For the Ф F determination, the fluorescence standard was excited at the wavelengths of maximum absorption found for each of the compounds to be tested and in all fluorimetric measurements the absorbance of the solution did not exceed 0.1. Phenanthroimidazoles 2 exhibited high fluorescence quantum yields in ACN (Ф F = 0.31 and 0.55), the highest being for 2b (Table 1). Stokes' shifts directly relate to energy differences between the ground and excited states and all compounds showed large Stokes' shifts (from 71 nm to 90 nm) and these values are an interesting feature for biological fluorescent probes that allows an improved separation of light inherent to the matrix and the light dispersed by the sample.
Considering these photophysical properties, derivatives 2a-c would be interesting candidates as chemosensors due to the high fluorescence quantum yields, important for maximization of the response in the analysis of very dilute samples. in a preliminary study with 50 equiv of each ion revealed that compounds 2a-c displayed higher selectivity in aqueous mixtures (ACN/H 2 O (95:5)) than in ACN (Table   2).

Spectrophotometric and spectrofluorimetric titrations of phenanthroimidazoles 2a-c with metallic ions
In the spectrofluorimetric titrations with Fe 3+ , a strong decrease of the fluorescence intensity (a chelation enhancement of quenching, CHEQ effect) was observed for the all derivatives, with an almost complete fluorescence quenching (2a, 169 equiv; 2b, 245 equiv; 2c, 184 equiv) (Figure 2).

Conclusions
The synthesis of new phenanthroimidazoles 2a-c was achieved in moderate to good yields by simple experimental procedures. The photophysical properties were evaluated