Synthesis, photo-and ionochromic properties of indolyl(thienyl)maleimides with phenanthroline receptor

Photochromic indolyl(thienyl)maleimides containing phenanthroline receptor in the bridge moiety were synthesized. Ring-opened maleimides possess fluorescence with quantum yields of 0.027-0.037. Irradiation of their solutions with light of 436 nm results in the formation of non-fluorescent ring-closed isomers. Re-opening of the cycle occurs by exposure to visible light ( λ > 500 nm). The obtained compounds demonstrate selective chemosensor activity to Fe 2+ ions.


Introduction
Organic molecular switches represent molecules capable of reversible transformation between two stable states under the influence of external factors: changes in pH, temperature, magnetic field, ionic strength of solution and action of electric current.2][3][4][5][6][7] Spirocyclic compounds, [8][9][10] fulgides and fulgimides, [11][12][13] dihetarylethenes, 14,15 photochromic heterocyclic ketoenamines 16,17 and norbornadienes 18,19 are of particular interest due to their potential industrial applications, including design of materials for molecular electronics, rewritable optical memory, photo optical commutation, organic displays, photo-pharmacology, biological data visualization, chemo-and biosensors.Chemical modification of photochromic molecules with ionochromic substituents opens new possibilities to create of photo controllable chemosensors in which the coordination and release of ions is modulated with light of different wavelengths. 6,20In previous studies, we described the synthesis and investigation of a series of photochromic heterocyclic maleimides, including those capable of detecting cations and anions. 16,21,22Herein, we report the synthesis of maleimides containing phenanthroline a receptor group in the bridged pyrrole-2,5-dione moiety and investigation of their photochromic, fluorescent and chemosensor properties.[25][26]
The structures of synthesized compounds were supported by the data of 1 H NMR, IR, mass-spectra and elemental analysis.Spectral-luminescent, photochromic and chemosensor properties of maleimides 2a-c were studied in toluene and acetonitrile solutions at ambient temperature (Table 1).Electronic absorption spectra of maleimides 2a-c are characterized by long-wave absorption bands with the maxima in the region 466-471 nm and the molar extinction coefficients of ring-opened O forms (5.67-8.60)•10 3 L mol -1 cm -1 .Maleimides 2a-c exhibit fluorescent properties in solutions: the maxima of the fluorescence bands are in the spectral region at 571-574 nm.The fluorescence excitation spectra are in good agreement with the absorption spectra, which confirms the correctness of attributing the observed emission to the initial ring-opened form O. Irradiation of solutions of compounds 2a-c in toluene with the filtered light of mercury lamp 436 nm led to their photocoloration which was attributed to the appearance of new long wavelength absorption bands in the region at 591-604 nm, the intensity of which increased upon irradiation, while the intensity of the initial bands decreased (Fig. 1a).Three distinct isosbestic points were also observed and the intensity of the fluorescence bands decreased without any change of their maxima (Fig. 1b).Prolonged irradiation, however, did not lead to complete quenching of the emission properties.Selective chromogenic activity of maleimides 2a-c to cations of iron(II) was revealed upon addition of dmetal perchlorates (Ni 2+ , Co 2+ , Cd 2+ , Cu 2+ , Pb 2+ , Zn 2+ , Fe 2+ and Hg 2+ ) to their solutions in acetonitrile.Fe 2+ ions induce a significant increase in the intensity of the long-wave absorption bands simultaneously with bathochromic shift of 12 nm that causes a distinct "naked-eye" effect -visually distinguishable color change of the solutions from orange to red (Fig. 2).Among the other cations only Co 2+ demonstrated a measurable effect (Fig. 3).According to the data of spectrophotometric titration and the isomolar series method, the most sensitive compound 2a forms with iron(II) cations a complex with 1:1 composition with a detection limit of 5.7 µМ.Complexes of ring-opened forms 2a-c O with Fe 2+ in acetonitrile solutions are non-fluorescent, their irradiation with filtered light of the mercury lamp 436 nm did not lead to formation of the corresponding ringclosed forms C, but did lead to thermally reversible dissociation (Scheme 3). 29heme 3. Photoinitiated dissociation of maleimide complexes 2a-c O .Fe 2+ in acetonitrile.

Conclusions
New indolyl(thienyl)maleimides containing phenanthroline receptor in the bridge moiety were synthesized.The compounds exhibited photochromic and fluorescence properties in toluene solutions and "naked-eye" selective chemosensor activity to Fe 2+ cations in acetonitrile solution.

Experimental Section
General.The IR spectra were recorded on a Varian Excalibur 3100 FT-IR instrument using the attenuated total internal reflection technique (ZnSe crystal).The 1 H NMR spectra in CDCl3 were recorded on a Varian Unity 300 spectrometer (300 MHz), the signals were referred with respect to the signals of residual protons of deuterosolvent (7.24 ppm), δ values were measured with precision 0.01 ppm.Mass spectra were recorded on a Shimadzu GCMS-QP2010SE instrument with direct sample entry into the ion source (EI, 70 eV).Toluene and acetonitrile of the spectroscopic grade and d-metal perchlorates (Aldrich) were used to prepare solutions.Elemental analysis was performed on a KOVO CHN-analyzer.Melting points were determined on a PTP (M) instrument.

Figure 2 .
Figure 2. Visual color change in acetonitrile with 2a and Fe 2+ .