Iodine-catalysed synthesis of a novel meso -substituted dipyrromethane and its evaluation as a cation chemosensor

: The synthesis and characterization of a dipyrromethane functionalised with an arylthiophene bridge was achieved by using an iodine catalytic method. Evaluation of the chemosensory ability in the presence of several cations showed both chromogenic and fluorescent response, with Fe 3+ , Cu 2+ , Hg 2+ and Cr 3+ among the most easily detected ions. The results suggest that the novel dipyrromethane behaves as a dual channel chemosensor, with higher selectivity and sensitivity for Cu 2+ and Fe 3+ .


Introduction
In recent years, meso-substituted dipyrromethanes have been frequently explored as potential ion sensors, providing valuable qualitative and quantitative information on molecular recognition processes.Some of these compounds exhibit very interesting photophysical properties, which can be triggered in the presence of some cations and anions and display as chromogenic and/or fluorescent changes [1].Due to the acidic character of NH protons, these compounds are also able to interact with several anions.Also, the inclusion of strong charge-transfer substituents often enhances their fluorescence properties.In this regard, chromophores based on (oligo)thiophene and arylthiophene derivatives are known to display fluoroionophoric properties and can also be used for various optical applications [2].
However, most common methods for the synthesis of these molecules can be laborious and time-consuming, requiring a great excess of pyrrole and a tightly controlled atmosphere [3].Recently, a new method based on molecular iodine-catalysis has been proposed as a fast, inexpensive and good-yielding procedure with reduced reaction times.Using molecular iodine as a catalyst withdraws the requirement of strong and expensive acid-catalysts, such as TFA [4].
Bearing in mind the above facts and our research interests, we now report the synthesis and characterization of a dipyrromethane functionalized with an aryltiophene bridge.The evaluation of the chemosensory ability in the presence of several ions was also carried out in acetonitrile solution.

General procedure for the synthesis of compound 1
Formyl-arylthiophene (1 mmol), pyrrole (10 mmol) and iodine (0.1 mmol) were added to dichloromethane (10 mL).The mixture was stirred for 1h at room temperature and reaction progress was monitored by TLC.After total conversion of the aldehyde, the reaction was stopped by addition of NaOH 0.1 M (2 mL).The mixture was washed several times with dichloromethane and water and the organic phase was dried with anhydrous MgSO 4 .After filtration, the solvent was evaporated and the crude residue was purified by chromatography using a gradient of diethyl ether/petroleum ether as eluent.

Spectrophotometric and spectrofluorimetric titrations
All spectrophotometric titrations were performed as follows: a stock solution of compound 1 (1 x 10 -4 M) was prepared by dissolving an appropriate amount of the solid in CH 3 CN in a 100 mL volumetric flask.Titration solution (1 x 10 -5 M) was prepared by appropriate dilution of the stock solution.Titrations of compound 1 were carried out by addition of microliter amounts of standard solutions of the metal cations (in the form of hexahydratated tetrafluorborate or perchlorate salts) in CH 3 CN.All measurements were performed at 298 K. Fluorimetric titrations were performed under the same conditions described for absorption spectra.

Synthesis
Dipyrromethane 1 was synthesized by condensation of pyrrole with formylarylthiophene in the presence of iodine, in dry dichloromethane, by stirring for 1 h at room temperature (Scheme).The product was obtained as a solid with high purity and a yield of 60% after column chromatography (Table 1).

Spectrophotometric and spectrofluorimetric titrations of compound 1 with cations in acetonitrile
To explore the sensing ability of dipyrromethane 1 in solution in the presence of Cd 2+ , Co 2+ , Cr 3+ , Cu 2+ , Fe 2+ , Fe 3+ , Hg 2+ , Ni 2+ , and Zn 2+ , a preliminary sensing study was performed with addition of 50 equivalents of metal ion to a 100 μM acetonitrile solution of compound 1.For Cd 2+ , Co 2+ , Fe 3+ , Hg 2+ , and Zn 2+ , no visible colour changes were seen, whereas the remaining cations showed a change from colourless to pale yellow (Ni 2+ and Cr 3+ ) or orange (Cu 2+ and Fe 3+ ) (Figure 1).shifted band appeared at about 490 nm (Figure 2).Chromogenic selective detection was possible for Cu 2+ and Fe 3+ , which required only 2-3 equivalents to reach a plateau.and Fe 3+ (Figure 3).The number of metal equivalents necessary for complete quenching of the initial fluorescence intensity is in agreement with the values obtained in the spectrophotometric titrations: once again, compound 1 is more sensitive to the presence of Cu 2+ and Fe 3+ , whereas Ni 2+ and Cr 3+ required a much larger amount of metal (250 and 300 equivalents, respectively).

Conclusions
Dipyrromethane 1 was synthesized in good yield (60%) using a simple experimental procedure.The evaluation of the chemosensory ability showed that compound 1 exhibited both chromogenic and fluorogenic response for Cu 2+ , Fe 3+ , Ni 2+ and Cr 3+ behaving as a dual channel chemosensor.Moreover, higher selectivity and sensitivity was possible for Cu 2+ and Fe 3+ .Further experiments are expected to bring more data on analogous compounds, which will hopefully display identical chemical and photophysical behaviour.