Effect of Complexation with LanthanideMetal Ions on the Photochromism of ( 1 , 3 , 3-Trimethyl-5 ′-Hydroxy-6 ′-Formyl-Indoline-Spiro 2 , 2 ′-[ 2 H ] chromene ) in Different Media

Complexation of lanthanide ions {Ln(III) ions [Tb(III), Eu(III), or Sm(III)]} with the spiropyran-derived merocyanine obtained in dark and under steady irradiation of indoline spiropyran (1,3,3-trimethyl-5′-hydroxy-6′-formyl-indoline-spiro-2,2′[2H]chromene) induces a noticeable hypsochromic shift of about 10–110 nm of its visible absorption band concomitant with hypochromic effect and influences its thermal bleaching in the dark. The effect of lanthanide ions and medium on photochromic, spectral-and-kinetic, and luminescence properties of the spiropyran and its complexes in solution and polymer matrix of polymethylmethacrylate (PMMA) is studied. Efficient energy transfer from the spiropyran moiety results in efficient typical luminescence from the Ln(III) ion that becomes more pronounced in polar nonalcoholic solvents and PMMA solid matrix. Moreover, luminescence mappings for pattern recognition analysis have been obtained from which the nature of the solvent and/or the ligand is clearly identified.


INTRODUCTION
Photochromism is the process of inducing color changes in a medium by incident electromagnetic radiation [1].While many different compounds have been shown to exhibit photochromism, the class of spiropyrans molecules has been the most extensively studied over the past several decades.Since the discovery of the photochromic reactions of spiropyrans in 1952 by Fisher and Hirshberg [2], researchers in many laboratories have investigated spiropyrans for various profitable applications.Even in the early work, Hirshberg [2] suggested using the phenomena for a "photochemical erasable memory" and efforts in this direction continue today.Many other applications for spiropyran-based photochromic reactions have been investigated including use as filters in displays, in eye-protective laser goggles, and in self-developing photography [3].More recently, these molecules have been proposed for use in photoswitching of protein activity [3,4], as chromophores in 3D optical data storage [5], in information processing [6][7][8], and as a means to photocontrol ion permeation through biological membranes [9].This paper reports on the studies of the photochemical and kinetic behavior of (1,3,3-trimethyl-5 -hydroxy-6formyl-indoline-spiro-2,2 -[2H]chromene) (SP), containing chelating functional group and its lanthanide metal ion complexation ability in stabilizing the colored merocyanine (MC) form.Furthermore, the process of the energy transfer from the spiropyran moiety to the ion is studied.Factors affecting energy transfer in these systems as well as thermal equilibrium between the closed SP and opened MC forms will be investigated to explore the possibility to stabilize the colored MC form towards thermal reversion by complexation with selected lanthanide metal ions in appropriate solvents.International Journal of Photoenergy Scheme 2

EXPERIMENTAL
The spiropyran (SP) (1,3,3-trimethyl-  C with vigorous stirring for 15 minutes then Eu-SP or Tb-SP complexes were incorporated into PMMA matrix at 30 • C under vigorous stirring for 15 minutes, then PMMA thin film was prepared.The thickness of the thin film was measured by a micrometer and it was found to be 0.4 mm. UV and visible absorption spectra were measured at room temperature in the UV/visible range (200-650 nm) using λ-Helios SP pye-Unicam spectrophotometer.
Apparent association constants of Sm 3+ , Eu 3+ , and Tb 3+ -SP complexes in acetonitrile were calculated by using Benesi-Hildebrand-type plot (1) [11]: where A o , A c , A obs , K app , and [M] are the absorbance of the ligand, the absorbance of the complex, the absorbance of the ligand at various concentrations of the metal ion, the formation constant, and the concentration of the metal ion, respectively.
Luminescence spectra were measured in the range (290-750 nm) using Shimadzu RF5301 (PC) spectrofluorophotometer. Luminescence quantum yield values (Φ L ) of the complexes in different solvents were obtained [12]  The irradiation source was a 150 W Xenon arc lamp (PTI-LPS-220 Photon Technology International) operated at 70 W with a filter of 437 nm.The light intensity was measured using radiometer (IL 1700 International Light).
The kinetic rate parameters were determined where the rate of transformation of SP to MC obeys first-order kinetics and were calculated from (2) [13]: where A t is the absorbance at λ max of the merocyanine form at time t, A e is the equilibrium absorbance at the λ max attained under dark conditions (taking the absorbance after at least 15 hours in dark) and A o is the absorbance at λ max at time zero.
The absorption spectrum of the compound in acetonitrile has two broad bands in the UV at λ SP = 255 nm (ε = 34100 M −1 cm −1 ) and at λ MC = 520 nm (ε = 5131 M −1 cm −1 ) attributed to π-π* transitions, Figure 1.The equilibrium between both forms is strongly displaced upon irradiation to the side of open-chain colored photomerocyanine and spontaneously converts to the colorless spiroform to reach thermally equilibrated state [20] immediately after removing the light.
The SP-MC equilibrium can be described by the ratio of absorbances at λ MC and λ SP , which is small in all solvents  except for DMF (Table 1) [21].This is due to thermal equilibrium between the closed and opened forms which is affected by the change in solvent polarity, since polar solvents promote the formation of the colored form at room temperature in the absence of light, Scheme 2 and Table 1. Figure 1 shows that upon addition of lanthanides (Tb(III), Eu(III), or Sm(III)), the absorption band of MC form is blue shifted by (10-110) nm with respect to λ MC indicating that the complex is formed in the ground state (Table 2).This could be due to stabilization of the ground state of MC relative to its excited state, which is noticed to be more pronounced in polar solvents [13,17].The blue shift noticed in the presence of rare-earth ions, Figure 1, is larger for the ion with a smallest ionic radius [22].The formation of the complex is suggested to be due to an equilibrium described in Scheme 3 between the observed solvated complex (termed MC-M n+ and S denotes solvent molecules and m = number of molecules) and the MC form.The equilibrium between MC and MC-M n+ can be also described by the ratio of absorbances at the two maxima, λ com and λ SP (Table 2).It can be seen from Table 2 that for 3 × 10 −5 M of SP and 8 × 10 −5 M of metal ions, the A com /A SP ratio ranges between 0.3 and 0.93 for (Sm 3+ , Eu 3+ , and Tb 3+ ) in methanol and ethanol but ranges between 0.15 and 0.4 in DMSO and DMF.In acetonitrile, the values are higher than 1.0 at λ max com indicating that a high concentration of the complex in the ground state is favored in polar alcoholic solvents and in acetonitrile.The results clearly show that complexation results in dramatic shift towards MC form [23][24][25][26][27].
More quantitative estimate for complex formation is obtained by applying Benesi-Hildebrand (1).Absorption at 475 nm, as a measure of the complex concentration, increases with the metal ion concentration as shown in Figures 2  and 3.The formation constants of 58 × 10 3 , 56 × 10 3 , and 55 × 10 3 M −1 were obtained for Tb 3+ , Eu 3+ , and Sm 3+ , respectively.

Effect of irradiation time
Upon UV irradiation, as the irradiation time increased the absorbance of both trans MC and MC-M n+ forms is enhanced as well as the luminescence intensity of the complexes, whereas the absorbance of SP form decreases and the thermal relaxation is observed in dark after irradiation.It is well known that the formation and relaxation of the colored form after preirradiation of SP, follows first-order decay kinetics [6,7,28,29].The life time of the open form  is generally believed to be shorter in lower polarity solvents [30][31][32].Moreover, indoline spiropyrans and their colored merocyanines forms are separated by the energy barrier [30][31][32].
In case of reactions that proceed to equilibrium and do not vastly favor the reactant, the concentration (absorbance in this case due to Beer's Law) of reactant follows (2).
k obs in the equation is the sum of the rate constants for the forward and reverse reactions, however, since the thermal ring-opening process is much slower than the ring-closing process [13], The observed rate constants (k obs ) for MC and its complexes with (Sm 3+ , Eu 3+ , and Tb 3+ ) are calculated from the straight lines of ln(A t − A e ) versus time (t). Figure 4 gives an example for k obs calculations.
The calculated k obs in s −1 increases in the order Sm(1.19)< Eu(2.01) < Tb(3.49) for the MC-Ln complexes in DMSO.

Thermal relaxation
It is well known that the MC concentration can be significantly increased upon UV irradiation of SP.After removal of the 70 W light with a filter = 436 nm, the enhancement coloration slowly disappears approaching steady state A e value at λ MC .The decay is first order and the relaxation time of   τ SP-MC = 3 S.The rate constants k 1 and k −1 were obtained from τ SP-MC and K 1 (Tables 3 and 4).One relaxation time of the complex τ SP-com is obtained (for a fixed [M n+ ] concentration) from the first-order build up kinetics, i.e. the plots of the linear dependences versus time of ln(A t − A e )/(A o − A e ), where A o and A e denote the initial and final (maximum) values, respectively.The complex concentration under equilibrium conditions and for [M n+ ] [MC-M n+ ] is given by (4) [33], see Table 4 [ Here, subscripts e and o refer to the equilibrium and initial concentrations, respectively.
Relaxation time decreases, relative to free MC, in case of Tb and Eu complexes due to strong complex formation.Complexation with samarium induced retardation in relaxation, probably due to a pronounced increase in the formation of MC form as reflected in higher intensity ratios listed in Table 2.

Medium effect on the luminescence spectra of MC-M n+ complexes
Upon excitation of the solution of the complexes of spiropyran with lanthanide ions at λ ex = 380 nm the emission spectra show the characteristics bands of the metal ion, Figure 5.The luminescence quantum yields Φ L for the spiropyrans complexes are low in solution; (DMSO: Tb = 0.056, Eu = 0.052, and Sm = 0.047) due to that luminescence is the only minor deactivation route.A strong increase in Φ L on going to rigid media (PMMA) (Tb = 0.108 and Eu = 0.085) indicates that other relaxation pathways, in particular rotation around a double bond, are hindered [34], Figures 6 and 7.The calculated quantum yield values of the complexes are listed in Table 5 where the higher values are observed in the less polar solvent (DMF and DMSO) due to the quenching of the emissive excited state in the polar alcoholic solvents (methanol, ethanol, n-propanol, and butanol) [11,17].Therefore, with a reduction in the polarity of the solvating medium and changing its nature there is a concomitant reduction in the energy difference between the ground and the excited states [32] and the hydrogen bond formation between the oxygen  center and solvating media with hydrogen bond donor capability [30,[35][36][37][38]. Furthermore, the luminescence intensity and the quantum yield values of terbium complexes are generally higher than those of europium or samarium complexes in all solvents.The luminescence of Ln 3+ -chelates is related to the efficiency of the intramolecular energy transfer between the triplet level of ligand and the emitting level of the ions, which depends on the energy gap between the two levels.In the organic solvents, the energy gap between the ligand triplet levels and the emitting level of the terbium favors the energy transfer process for terbium.

Luminescence mapping
Three-dimensional plots of excitation/emission matrix [39,40] are depicted in Figures 8, 10, and 12.The corresponding tomograms of the two-dimensional representation (luminescence mapping) obtained by connection of data points with the same luminescence intensity (i.e.same height) by lines.Such 2D diagrams always represent a top view [40].These diagrams seem to be useful as a qualitative tool.In particular, the location and relative intensities of peaks are suitable parameters for pattern recognition analysis as well as a useful analytical method in clinical chemistry and biochemistry [41][42][43][44].The excitation/emission matrix represents the fingerprint of metal ions (Sm, Eu, and Tb ions), and reports on its environment too (Figures 9, 11   noticeable shift of the equilibrium towards MC-Sm(III) form.The luminescence quantum yields Φ L for the spiropyrans complexes in PMMA are higher than in solution due to restricted nonradiative dynamical motion in the rigid matrix.Furthermore, luminescence mappings of emissive lanthanides were obtained and can be exploited as fingerprints for analysis.

Figure 2 :Figure 3 :
Figure 2: Effect of Tb 3+ concentration on the absorption spectra of SP in acetonitrile.

Figure 8 :
Figure 8: 3D view of the emission spectra of Eu 3+ ion in MC-Eu complex at different excitation wavelengths in DMSO.

Figure 9 :Figure 10 :Figure 11 :
Figure 9: Contour view of the emission spectra of Eu 3+ ion in MC-Eu complex at different excitation wavelengths in DMSO.

Figure 12 :Figure 13 :
Figure 12: 3D view of the emission spectra of Tb 3+ ion in MC-Tb complex at different excitation wavelengths in DMSO.

Table 1 :
Absorption maxima, ratio of absorbances of SP and trans-MC isomer in different solvents.
* λMC is the maximum wavelength of MC. † AMC and ASP are absorbance at maximum wavelength of MC and SP, respectively.

Table 2 :
Absorption maximum of the MC-M n+ complex and the ratio of absorbances of SP form and the complex in different solvents.
* Acom and ASP are absorbance at maximum wavelength for the complex, SP, respectively.† λcom is the maximum wavelength of the complex.

Table 3 :
Observed rate constant and relaxation times of MC and its complexes in acetonitrile at 25 • C.
τcom-SP = relaxation time of the SP complexes in dark condition.

Table 4 :
Values of the calculated equilibrium constants and the concentration of the complexes at equilibrium conditions at 25 • C.