FLUORESCENCE OF CROWNED BUTADIENYL DYE AND ITS METAL COMPLEXES
S.I. Druzhinin1), M.V. Rusalov1), B.M. Uzhinov1), S.P. Gromov2), S.A. Sergeev2), and M.V. Alfimov2)
1)
Chemical Dept., M.V. Lomonosov Moscow State University, Vorob'evy gory, 119899, Moscow, Russia (e-mail: druzh@light.chem.msu.su)2)
N.N. Semenov Institute of Chemical Physics, Russian Academy of Science, Novatorov 7a, 117421, Moscow, RussiaJournal of Fluorescence (in print)
ABSTRACT. The absorption and fluorescence spectra of complexes of styryl dye (1) with lithium, sodium, magnesium, and calcium cations in MeCN have been investigated. The addition of Li, Na, Mg, and Ca perchlorates to the solution of dye 1 in acetonitrile results in the significant (up to 5900 cm-1) shortwavelength shift of absorption spectrum and a small (about 200 cm-1) shortwavelength shift of fluorescence spectrum. The recoordination reaction in metal complexes of 1 takes place by intramolecular mechanism. The fluorescence quantum yield of 1Li+, 1Na+, and 1Ca2+ is approximately two times higher than that for 1. It was supposed that coordinates predominantly with oxygen atoms of macrocycle and, hence, influences weakly on macrocycle nitrogen atom conjugated with a molecule p-system.
INTRODUCTION
Crowned dyes can change ion-binding ability at the irradiation [1]. It was shown recently that in excited state of crowned dyes adiabatic recoordination takes place. [2-7]. The reaction was observed in crowned stilbene [2], merocyanine dyes [3, 4] and styryl dyes [5-7]. The mechanism of the reaction consists in excited state breaking of the bond of metal cation with macrocycle heteroatom linked with p -system of the dye [2‚ 3, 5]. However, systematic investigation of cation influence on photorecoordination reaction in metal complexes of crown-ethers was not carried out. In present work the photorecoordination reaction in complexes of styryl dye (1) with lithium, sodium, magnesium and calcium cations has been investigated
RESULTS AND DISCUSSION
The analog of crowned styryl dye (2) whose molecule does not contain a macrocycle fluoresces weakly in acetonitrile (Table). The absorption spectrum of dye 2 is not practically changed by the introduction of a crown-ether moiety to dye molecule. The fluorescence spectrum is also not shifted but the fluorescence quantum yield (j) increases substantially. The absorption and fluorescence spectra of crowned dye 1 in acetonitrile are given in Fig. 1
Fig. 1. Absorption and fluorescence spectra of styryl dye 1, complexes of 1 with trifluoroacetic acid (1H+), lithium, sodium, magnesium, and calcium cations in acetonitrile at room temperature. The squares under fluorescence spectrum of 1 and its complexes are proportional to their fluorescence quantum yields.
The addition of alkali (Li, Na) and alkaline-earth (Mg, Ca) metal perchlorates to the solution of dye 1 in acetonitrile results in the significant (up to 5900 cm-1) shortwavelength shift of dye absorption spectrum and a small (about 200 cm-1, excluding Mg(ClO4)2) shortwavelength shift of dye fluorescence spectrum. The absorption and fluorescence spectra of 1 in the presence of different concentrations of calcium perchlorate are shown in Fig. 2.
Fig. 2. Absorption (a, b, c, d, e, and f) and fluorescence (a’, b’, c’, d’, and e’) spectra of styryl dye 1 in acetonitrile at room temperature. The concentration of Ca(ClO4)2 is equal to 0 (a and a’), 0.00218 (b and b’), 0.00739 (c), 0.0169 (d and c’), 0.00328 M (e and d’), and Ґ (f and e’).
In the case of Mg the shift of fluorescence spectrum is more noticeable (about 1000 cm-1), but it is substantially less than that for the absorption spectrum (5900 cm-1). The protonation of nitrogen atom of the molecule both of crowned dye 1 and analog 2 without macrocycle by trifluoroacetic acid results in the shortwavelength shift of absorption spectra maxima by about 7300 cm-1. The value of Kabs of 1 is approximately by one order higher than that for 2. The distinction of basicities of 1 and 2 can not be caused by breaking of dialkylaminogroup conjugation with p-system as going from 1 to 2 because of twisting of this group or pyramidalization of nitrogen atom should result in the significant distinction of absorption spectra of 1 and 2. The protonated forms of 1 and 2 do not fluoresce in acetonitrile.
The fluorescence excitation spectra of 1 are shifted to shortwavelength region at least by 1410, 2600, 5260, and 5050 cm-1 at Li, Na, Mg, and Ca perchlorate concentrations equal to 0.0573, 0.215, 0.122, and 0.0328 M, respectively. The shifts of fluorescence excitation spectra are close to those of absorption spectra. On the analogy of early studied systems [4, 7], the great difference of the values of absorption and fluorescence spectra maxima of metal complexes is explained by the photoinduced excited state recoordination reaction in metal complexes. This reaction consists in the breaking of metal cation bond with macrocycle nitrogen atom and in the displacement of cation from its equilibrium position in the ground state.
The excited state recoordination reaction proceeds by intramolecular mechanism. It is confirmed by the measurements of the fluorescence of 1 and 1Na+ in solid glassy butyronitrile matrix at 77 K where the diffusion is impeded. The shift of excitation fluorescence spectrum maxima at the complexation of 1 with Ca2+ is significantly more than that for fluorescence spectrum (Fig. 3). It points out that the conclusion on cation ejection mechanism [3] is probably not correct.
Fig. 3. Normalized to unity fluorescence (a and a’) and fluorescence excitation (b and b’) spectra of 1 in solid glassy butyronitrile at 77 K. The concentration of NaClO4 is equal to 0 (a and b) and 0.2 M (a’ and b’).
The complexation of 1 with Li+ influences weakly on the value of absorption spectrum shift in comparison with Na+. This contradicts the results obtained for complexation of azacrowned 7-aminocoumarins with Li+ and Na+ [8] and the model according to which the value of spectral shift at the formation of metal complexes of crowned dye is proportional to the ratio of cation charge to its radius [8]. This contradiction can be caused by the fact that lithium cation has a diameter approximately 1.5 times less than the size of macrocycle cavity. It coordinates predominantly with oxygen atoms of macrocycle and, hence, influences weakly on macrocycle nitrogen atom conjugated with a molecule p-system.
The complexation results in the change of j of dye 1. The change of fluorescence spectra of 1 in the presence of metal perchlorates can be described as a static fluorescence quenching owing to the formation of 1:1 complexes. The j values of pure complexes were obtained on the base of calculated limit fluorescence spectra ([Mn+] ® Ґ). The fluorescence intensity of 1Mg2+ is approximately five times less than that for 1, but the fluorescence intensity of 1Li+, 1Na+, and 1Ca2+ is approximately two times higher than that for 1. The equilibrium constants obtained from the analysis of fluorescence spectra (Kfl) are close to Kabs (Table). The similar results were obtained for azacrowned merocyanine [9]. The results obtained give an additional evidence of the fact that excited state recoordination reaction is a fast intramolecular process, which proceeds in a time domain shorter than the lifetime of excited state.
Together with the complexation the changes of absorption and fluorescence spectra can be caused also by salt effect. In order to check this possibility the absorption and fluorescence spectra of dye 2 which can not form a complex with metal salts in the presence of magnesium perchlorate (the magnesium cation has maximum charge density among considered cations). The absorption and fluorescence spectra of dye 2 are changed insignificantly at the Mg(ClO4)2 concentration equal to 0.1 M. Extinction coefficient of dye 2 increases approximately by 2%, fluorescence quantum yield increases approximately by 15%, Dnabs » 70 cm-1, Dnfl » 0 cm-1. It points out that the influence of salt on absorption and fluorescence spectra 1 can be neglected.
ACKNOLEGEMENT This work was supported by Rissian Fund for Basic Research (grant N 95-03-09482).
REFERENCES
Laser Photonics Laboratory
