Photochromic spiro[indoline-pyridobenzopyrans]: fluorescent metal-ion sensors

By coupling 5-bromo-(chloro)-7-formyl-8-hydroxyquinoline with 2-methylene-1,3,3-trimethyl-indoline, the 6'-halogeno-spiro[indoline-2,2'-[3,2-h ]-pyrido[2 H -1]benzopyrans] 3 have been prepared and their photochromic behaviors studied. The photoinduced merocyanine forms 4 of the spiropyrans form fluorescent complexes with divalent metal ions (Mg 2+ , Mn 2+ , Co 2+ , Ni 2+ , Cu 2+ , Zn 2+ , Cd 2+ ).


Synthesis
The previously described methods for the synthesis of spiropyrans 3a 11 and 3b 12 based on coupling Fisher's base 1 with 5-halogeno-7-formyl-8-hydroxyquinolines 2 (Scheme 1) have been modified slightly to obtain the products in better yields.The parent 7-formyl-8-hydroxyquinoline was prepared previously in low yield using a multistep method 9 because direct formylation of 8hydroxyquinoline leads to substitution not at the 7-, but at the 5-position. 13In the present work, for the formylation of 5-halogeno-8-hydroxyquinolines we employed the Duff reaction, affording the aldehydes 2 in one step, although in relatively low yields.

Photochromic and spectral properties
The long-wavelength absorption of spiropyrans 3 is observed at λ max 342-358 nm (ε = 4200-4600 mol L -1 cm -1 ), which is the spectral region characteristic of the ring-closed forms.In polar solvents, equilibrium is established between the ring-closed spirocyclic, 3, and ring-opened merocyanine, 4, forms (Scheme 2).The existence of small amounts of the latter is detected by the appearance of low intensity

Scheme 2
absorption bands in the spectral region of 522-605 nm, witnessing to the appearance of the merocyanine isomers of spiropyrans (Table 1). 1,2In polar solvents, the equilibrium amounts of more polar merocyanine forms of spiropyrans increase compared with that in toluene (Table 2).Under UV-irradiation of solutions of 3 (at λ max ), the photoinitiated electrocyclic ring-opening reaction occurs (Scheme 2), giving rise to the formation of the colored merocyanine forms 4 with the long-wavelength absorption at λ max = 561 nm (R=Cl) and 598 nm (R=Br) and weak fluorescence (Φ ≅ 0.001 in acetone) at 620-630 nm (Figure 1).The kinetics of the dark reactions 3→4 and 4→3 are well described by mono-exponential functions.Activation energies of the thermal reactions were determined from the Arrhenius relationship.Table 3 contains the data on kinetic properties of the dark reactions.

Complexation properties
The addition of salts of Mg 2+ , Mn 2+ , Co 2+ , Ni 2+ , Cu 2+ , Zn 2+ , Cd 2+ to the colorless or weakly colored solutions of spiropyrans 3 results in the appearance in the absorption spectra of new intense bands at 533-573 nm, which are assigned to the absorption of metal complexes of merocyanine forms of spiropyrans 5 (Scheme 2). 9The complexes 5 with metal ions Mg 2+ , Zn 2+ and, Cd 2+ exhibit sufficiently intense fluorescence with maxima at 610-625 nm (Figure 2).The quantum efficiency of the fluorescence (Φ = 0.007-0.011) is an order of magnitude higher than that for the merocyanine ligands.Figure 3 depicts the metal-ion influence on the fluorescent contrast as occurs on complexation of merocyanines with metal ions.Table 4 contains some data on the spectral properties of the spiropyran complexes 5.The composition of the complexes of the spiropyrans with metal ions was determined using the spectrophotometric methods of isomolar solutions (Job method) and molar ratio absorbance, which were previously employed for the analysis of the ionic equilibria of spiropyrans and chromenes. 14,15For all complexes 5, a distinct maximum appears in the plot of absorption vs. composition of the isomolar solutions at the point where the metal-ion to merocyanine ratio is 1:2 (Figure 4).This finding is in accord with the results obtained for a complex of spiropyran 3 (R = NO 2 ) with Zn 2+ . 16

Experimental Section
General Procedures.NMR spectra were recorded on a Varian Unity-300 spectrometer (300 MHz) using CDCl 3 as solvent.Electronic absorption spectra were recorded on a Varian Cary-100 spectrophotometer, and fluorescence was measured using a Shimadzu RF 5001 PC spectrofluorimeter.The irradiation of solutions was carried out with the light of a high-pressure mercury lamp DRSh-250 equipped with a set of interference filters.The highest grade solvents (Fluka) were used for spectroscopic measurements.Metal perchlorates were used as ion salts.
ISSN 1424-6376 Page 23 © ARKAT USA, Inc General procedure for the preparation of formylquinolinols 2a,b A mixture of 5-halogeno-8-hydroxyquinoline (0.015 mol), hexamethylenetetramine (4.2 g, 0.030 mol) and trifluoroacetic acid (25 ml) was heated under reflux for 6 h under an argon atmosphere.After cooling, the reaction mixture was poured onto a solution of conc.hydrochloric acid (20 ml) in water (220 ml), stirred for 30 min and neutralized with NaOH solution to pH ~ 5.
The precipitate was filtered off, washed with water, dried, and extracted with boiling benzene (3x20 ml).After evaporation of the solvent, the resulting crude product was recrystallized from ethanol.

Figure 1 .
Figure 1.Absorption spectra of spiropyran 3b in toluene solution (c=7.9⋅10 -5 M; T=293K): before irradiation (1) and after irradiation with the filtered light of a mercury lamp 365 nm (the absorption at the longest wavelength band gradually increases with increase in duration of the irradiation).

Figure 3 .
Figure 3. Metal-ion influence on the fluorescent contrast (Φ compl /Φ Mc ) at complexation of a merocyanine with metal ions.

Table 1 .
Spectral properties of the isomeric forms of spiropyrans 3 * Molar extinctions of the merocyanine forms were calculated on the basis of 1 H NMR determinations of the contents of the latter in chloroform solutions.

Table 2 .
Thermodynamic parameters of the thermal equilibria (Scheme 2) in solutions of spiropyrans 3 at 292 K* * Calculated from data on the temperature-dependence of the equilibrium constants K e in the temperature interval of 283-323 K. ** The temperature dependence of K e does not follow the van't Hoff relationship.

Table 3 .
Kinetic parameters of the dark 3 → 4 and 4 → 3 reactions of spiropyrans at 292 K The temperature-dependence of rate constants does not follow the Arrhenius relationship. *