Zinc ion–tetraphenylporphyrin interactions in the ground and excited states

Spectrochimica Acta Part A 79 (2011) 131–136

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Zinc ion–tetraphenylporphyrin interactions in the ground and excited states Tandrima Chaudhuria , Dibakar Goswamib , Manas Banerjeea,∗ a b

Department of Chemistry, University of Burdwan, Golapbag, Burdwan 713104, West Bengal, India Bio-Organic Division, Bhabha Atomic Research Centre, Mumbai 400085, India

a r t i c l e

i n f o

Article history: Received 26 July 2010 Accepted 14 February 2011 Keywords: Meso-tetraphenylporphyrin Zinc ion Ground and excited state interaction Fluorescence ratiometric sensing

a b s t r a c t In the present article, tetraphenylporphyrin a new ratiometric fluorescence sensitizer for zinc ion has been proposed. Electronic absorption, emission and 1 H NMR spectral characteristics of mesotetraphenylporphyrin (TPP) have been studied in acetonitrile medium in the presence of zinc perchlorate. Absorption spectral studies indicate the formation of a new complex between zinc ion and the porphyrin moiety in the ground state as distinguished from the characteristics of metalo(zinc) porphyrin compound. The energy of maximum fluorescence of porphyrin shifts towards blue with the addition of Zn(ClO4 )2 . Steady state emission studies point to the existence of two emitting species viz, the solvated and the complexed porphyrin in equilibrium. The fluorescence emission of tetraphenylporphyrin at 651-nm bands decreases while that at 605 nm increases upon zinc ion interaction in acetonitrile. Thus, the TPP can behave as a ratiometric fluorescent sensor. This fluorescence modulation of TPP should be applicable to dual-wavelength measurement of various biomolecules or enzyme activities. 1 H NMR spectra of the porphyrin suffered a radical change with the addition of zinc perchlorate which points to the formation of a new porphyrin complex. This change is due to the difference in the electron-donating ability of the pyrrolic nitrogens before and after complexation with Zn2+ . The values of equilibrium constant for the binding process have been determined in acetone and acetonitrile, in both ground and excited states. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Selective recognition of metal ions has attracted considerable attention for decades owing to the biological importance attributed to these metal ions [1,2]. An effective way to improve the sensitivity is to design sensors which can recognize the metal ion specifically and ratiometrically so that it can avoid the influence of the environment around (pH, polarity, temperature, and so forth). The signal (wavelength and intensity of absorption and fluorescence) would change when receptors interact with analytes. A few porphyrin based Zn2+ sensor reported yet are the cyclodextrin/porphyrin inclusion complex [3] as well as a porphyrin derivative containing two 2-(oxymethyl) pyridine unit [4] that can detect the biologically important Zn2+ through fluorescence ratiometric sensing. The photophysical properties of metaloporphyrins have been extensively studied in recent years, primarily due to their importance in biological systems [5,6]. A study of their photophysical properties points out a considerable difference in the case of incorporating different metals into the centre of the ring or in grafting various substituents at the peripheral positions [7]. The lowest energy excited states of paramagnetic Zn porphyrins and Mg porphyrins can be described

∗ Corresponding author. Tel.: +91 342 2656700; fax: +91 342 2530452; mobile: +91 9434252709. E-mail address: [email protected] (M. Banerjee). 1386-1425/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2011.02.023

to be the normal (␲, ␲*) S1 state with a very strong fluorescence [8]. But for the other first-row transition metal porphyrins, chargetransfer excited state and higher multiplet excited states have to be considered. In some metaloporphyrins like those of Fe (III), Cr (III) and Mn (II), the interaction between metal and porphyrin gives rise to charge transfer (CT) absorption bands [9]. The luminescence of metaloporphyrins, such as zinc porphyrins and magnesium porphyrins, for example, has been widely studied in model systems to understand the primary processes in photo synthesis and to establish prototypes for solar energy conversion and storage [10–14]. These complexes can serve as good photosensitizers and work as the photo reaction centre in intra molecular photo induced electron transfer processes [15–17]. Previous attempts have also been made to explain the spectral changes as due to the electric field effect of the zinc ion [18] but in many instances it has been found that the observed spectral changes are due to a change in speciation, rather than due to the non-specific electric field effect [19]. In contrast to ZnTPP, it has been observed in the present article that Zn2+ ion in aprotic solvents brings about a significant change in spectral properties of TPP. Here tetraphenylporphyrin, a new fluorescence ratiometric sensitizer for zinc ion has been proposed. Further, a complex is formed between the cation and the porphyrin moiety in the ground (S0 ) and in the excited (S1 ) states. The objective of the present work is to investigate in detail the interaction of Zn2+ with simple TPP in solution. Solvents used were acetonitrile (ACN) and acetone (AC).

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2. Experimental

4.5

2.1. Materials

4.0

The TPP has been prepared according to the reported method [20]. Purity of the prepared compound was checked by IR, absorption and fluorescence spectral data and also by thin layer chromatography. All solvents were of HPLC grade and used without further purification. Zinc perchlorate [Sigma–Aldrich] was dried in oven before use. Samples were prepared in a dry box so as to avoid contamination by air or moisture. The concentration of TPP was taken in the range 10−5 –10−6 M in all the spectral measurements. The concentration of zinc ion was of the order of ∼10−5 M. With the gradual addition of zinc ion solution to TPP, the purple colour of TPP changed to green.

3.5

-[1/d']

3.0 2.5 2.0 1.5 1.0 0.5 0.0

2.2. Steady state spectral measurements

5.0k

10.0k

15.0k

20.0k 2+ -1

Absorption (UV–Vis) spectral measurements were performed on a SHIMADZU UV 1601 PC spectrophotometer fitted with an electronic temperature controller unit (TCC – 240 A). The steady state fluorescence emission and excitation spectra were recorded on a Hitachi F-4500 spectrofluorimeter equipped with a temperature controlled cell holder. Temperature was controlled within ±0.1 K by circulating water from a constant temperature bath (Heto Holten, Denmark). 1 H NMR spectra of the meso-tetraarylporphyrin in CD3 CN medium have been recorded in a Bruker Ac-200 (200 MHz) spectrometer. 2.3. Analysis of spectral data Equilibrium constant corresponding to TPP–Zn2+ interaction in ground state was determined by using Benesi–Hildebrand equation [21] of the form. −

1 1 1 = + ε[TPP]0 d K1 ε[TPP]0 [Zn2+ ]

(1)

where [TPP]0 is the initial concentration of the porphyrin, and d is the absorbance of the complex at 512 nm against the sol0 ] where d 0 vent as reference i.e., d = [dmix − dTPP mix and dTPP are the absorbances of the donor–acceptor mixture and TPP solutions at the same molar concentration, present in the mixture, at the same wavelength against solvent as reference. K1 is the formation constant of the complex. Eq. (1) is valid [21] under the condition [Zn2+ ]  [TPP]0 for 1:1 EDA complexes. The linearity of the BH plot (Fig. 1) in all cases ensures 1:1 molecular complex formation between TPP and the zinc ion. The equilibrium constant values calculated using the BH model, in both the solvent media are given in Table 1. Equilibrium constant for the interaction between TPP in excited state and zinc ion was determined by a previously described

-1

25.0k

[Zn ] mol .dm

30.0k

35.0k

3

Fig. 1. Benesi–Hildebrand plot of TPP/Zn2+ system in ground state in acetonitrile medium.

method [22]. A measured volume of TPP solution in a given solvent was taken in a stoppered quartz cuvette and 0.2 ml solution of Zn(ClO4 )2 in that solvent was added to it. The fluorescence spectra were then determined. Addition of Zn(ClO4 )2 solution was repeated several times and the spectra were measured after each addition. The observed maximum fluorescence energy, E(F), can be considered as mole fraction average of ES and EM , the maximum fluorescence energy of the two species, viz, the solvated and complexed forms of TPP [23–26]. Thus, where C represents molar concentrations of the species, one gets: E(F) =

(CS ES + CM EM ) (CS + CM )

(2)

Eq. (2) can be rearranged to give: E(F) = ES + EM K2 CZn+2 − K2 E(F)CZn+2

(3)

where, K2 is the equilibrium constant for the following process: S . . . TP (S1 state) + Zn2+ = Zn . . . TP (S1 state) + S

(4)

values of ES , EM and K2 that fit Eq. (4) can thus be obtained by a linear regression analysis. 3. Results and discussions 3.1. Absorption studies The Soret absorption band of the porphyrin in both acetone and acetonitrile shifts to the red as Zn(ClO4 )2 is added to the system. Only the cation of the electrolyte has been found to be effective in

Table 1 Ground state association constant values of the complex in both the solvent media at 298 K. Solvent

ACN

AC

[Zn2+ ] (×105 mol dm−3 ) 2.887 5.609 8.212 10.727 13.111 15.391 0.878 1.61 2.23 2.76 3.22 3.62

[TPP] (×105 mol dm−3 )

4.90

9.86

Absorbance at CT (d ) 0.2348 0.5009 0.7379 1.3984 1.8105 2.3723 0.0519 0.1255 0.2340 0.3845 0.5013 0.5801

K1 (×10−3 dm3 mol−1 )

35.80 ± 8.39

19.23 ± 3.31

T. Chaudhuri et al. / Spectrochimica Acta Part A 79 (2011) 131–136

B

2.0

2.0

1.5

Absorbance

Absorbance

A

1.0

0.5

TPP TPP-Zn

133

+2

1.5

1.0

0.5

0.0 400

450

500

550

600

650

700

Wavelength (nm)

0.0 300

400

500

600

700

800

Wavelength (nm)

Fig. 2. Ground state isobestic formation in (A) acetone and in (B) acetonitrile. Q-band region shown in the (B) inset.

band shifts. The band maximum in aprotic dipolar solvent (AC and ACN) shows a moderate red shift for lower concentration of zinc ion. For higher salt concentration a dramatic red shift is observed and a second band emerges which shows no appreciable shift on further addition of the salt. Such type of spectral variation agrees well with the observation made by Pocker and Ciula [27] and Rezende et al. [28] where a dye–cation complex formation has been proposed. For a fixed TPP concentration an isosbestic point appeared in the absorption spectrum in both the solvents containing varying amounts of the salt (Table 2). Fig. 2 shows representative absorption curves in solvents containing zinc perchlorate. This fact clearly indicates the existence of an equilibrium between two forms, viz, the solvated and the Zn2+ ion complexed TPP. At a very high concentration of Zn(ClO4 )2 the complexed form of the TPP exists almost exclusively in the solution and the absorption band is due to this species only. Thus the complexed TPP absorbs at 421.5 nm in ACN and at 422 nm in AC. This may be compared with the absorption maximum at 412.5 nm and 414 nm for TPP in pure ACN and AC respectively. The position of the absorption maximum, however, does not change when salts containing bulky organic ions e.g. tetramethyl ammonium (TMA) ions or tetrabutyl ammonium (TBA) ions are added to AC/ACN solution of the TPP. In aprotic dipolar solvents like AC or ACN a weak intermolecular interaction takes place between the solute and the surrounding solvent dipoles. With the addition of an electrolyte the positively charged cation competes with the solvent dipoles for the negative centre in the TPP molecule, namely the pyrrolic nitrogens. At a low concentration of the electrolyte the cation cannot replace the solvent molecule in the vicinity of the porphyrin core. As a result, at low salt concentration the solvated dye molecule experiences relatively weak perturbation due to metal ion and the shift is less pronounced. At higher salt concentration, however, the cation replaces a solvent molecule and weak bonds between the metal ion and the pyrrolic nitrogens of the TPP moiety are formed. Beyond this point there is no substantial perturbation on the TPP–zinc ion complex and consequently no significant change in spectral property is observed. Formation of TPP–cation complex is rationalisable in view of soft–soft interaction of nitrogen centre and Zn2+ ions. The equilibrium constant, K1 , representing TPP (S0 state)–Zn2+ ion Table 2 Isosbestic formed in ground and excited state in two solvent media. Solvent

ACN AC

Isosbestic points Ground state (nm)

Excited state (nm)

417.0 417.0

626.6 629.0

interaction, have been calculated from the absorbance (d ) values at 512 nm, Q band position of the complex formed, using Eq. (1). The calculated K1 values for AC and ACN have been listed in Table 1. Value of K1 depends on the nature of the solvent and in the present case it follows the order KAC < KACN . The relative order of K values in two solvents are KACN /KAC = 1.86 for the ground state interaction. 3.2. Steady state emission studies The wavelengths of emission maxima of TPP shift to the red as concentration of Zn2+ ion in the solution is increased, as seen from Fig. 3. However, the wavelengths of the twin peaks of fluorescence maxima approach to the limiting values of 605.0 nm and 656.8 nm for ACN media while to the values of 603.0 nm and 652.8 nm for AC media as concentration of Zn2+ ion is increased. Apart from the shift of band positions, there is also a change in the relative intensity of the twin peaks and they grow gradually almost too similar heights with increasing Zn2+ ion concentration. The experimental observation lead to the findings that the fluorescence maxima at 651.2 nm and 715.6 nm correspond to pure TPP in ACN media while the maxima at 605.0 nm and 656.8 nm correspond to ZnTPP [22], the set of emission curves passing through an isosbestic at 626.6 nm. This definitely leads to conclude that there are two emitting species in equilibrium – one the solvated TPP and the other, metal complexed TPP. Since the entire process of crossing over from one species to the other is accompanied through fluorescence emission, we may emphasize the process to have gone through the S1 excited state. Similar work in AC media has led to the solvated emission maxima at 655.2 nm and 716.6 nm while the metal complexed maxima at 603.5 nm and 652.8 nm, passing through an emission isosbestic at 629.0 nm. It may be noted from Table 1 that there is a little solvent dependence of the emission isosbestic in that the values are 629.0 nm in AC and 626.6 nm in ACN. But in absorption isosbestic, there is almost no solvent dependence on the observed wavelength at 417.0 nm. It may be noted that TMA and TBA ions do not affect the fluorescence band maximum in ACN or AC solvent. Fig. 4 shows plots of E(F) values, the energy of maximum fluorescence at the new peak (∼600 nm) corresponding to metal complexed species, calculated by using the relation: E(F)/kcal mol−1 = 28590/(/nm), 2+ as a function of zinc ion concentration, CZn . For a particular sol2+ vent the slope of E(F) versus CZn plot is large for small values of 2+ CZn and the slope decreases with salt concentration and finally E(F) tends towards a limiting value. The equilibrium constant, K2 , representing TPP (S1 state)–Zn2+ ion interaction (Eq. (4)), along with the values of ES and EM have been calculated from the observed E(F) values using Eq. (3). The calculated values for AC and ACN have been listed in Table 3. It may also be noted that value of ES is smaller

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A 8000

Fluorescence Intensity (a.u)

7000

6000

5000

626.6 nm 4000

3000

2000

1000

0 575

600

625

650

675

700

725

750

Wavelength (nm)

B

5000

Fluorescence Intensity (a.u.)

4000

629 nm

3000

2000

1000

0 600

625

650

675

700

725

750

775

Wavelength (nm) Fig. 3. Excited state isobestic formation (A) in acetonitrile and (B) in acetone for ex = 530 nm.

than that of EM for both the solvents. Moreover, these values agree well with the values of wavelength of maximum fluorescence for the corresponding species as obtained experimentally. Values of K2 depend on the nature of the solvent and in this case also it follows the order KAC < KACN . The relative order of K2 values in two solvents Table 3 The best fit values of ES , EM , and K2 as a function of solvent at 298 K. Solvent

ES (kcal mol−1 )

EM (kcal mol−1 )

ACN AC

43.98 ± 0.77 43.63 ± 0.07

47.33 ± 0.16 47.45 ± 0.06

K2 (×10−3 dm3 mol−1 ) 530.45 ± 12.80 275.34 ± 15.80

are similar to that obtained from absorption study. This indicates that TPP has undergone equivalently strong complexation in both the ground and excited states. The relative order of K values in two solvents is KACN /KAC = 1.92 for the excited state binding and this is comparable to that in the ground state. It is interesting to compare the effect of addition of Zn(ClO4 )2 to solutions of TPP in acetonitrile and acetone. On addition of Zn(ClO4 )2 a pronounced change is observed for acetonitrile than that for acetone. Note that the E(F) values change more sharply when Zn(ClO4 )2 is added to acetonitrile (Fig. 4). Thus, TPP–solvent interaction is found to be higher in acetonitrile than in acetone. Again, TPP–solvent interaction is found to be higher in excited state

T. Chaudhuri et al. / Spectrochimica Acta Part A 79 (2011) 131–136

A

B 47.6 47.5 -1

47.55

E(F) Kcal.mol

-3

47.60

E(F) Kcal.mol

135

47.50 47.45

47.4 47.3 47.2 47.1 47.0

47.40 46.9 47.35

46.8 5.0µ 10.0µ 15.0µ 20.0µ 25.0µ 30.0µ 35.0µ 40.0µ 2+

-3

[Zn ] mol.dm

9.0µ

18.0µ

27.0µ 2+

[Zn ] mol.dm

36.0µ

45.0µ

-3

Fig. 4. Plot of E(F) as a function of the concentration of Zn(ClO4 )2 in (A) acetonitrile and (B) acetone.

than in ground state. This is also reflected from the values of isosbestics formed in ground and in excited states. Thus it can be stated that, S1 state of TPP will be more polar and is expected to interact with the solvent to a greater extent. 3.3.

1H

NMR studies

Interaction in the ground state between Zn(ClO4 )2 and TPP has been demonstrated through 1 H NMR spectra. Due to simplicity of 1 H NMR spectra of TPP, the complexation between zinc ion and

Fig. 5.

1

TPP could be monitored NMR spectrometrically. The aromatic protons of uncomplexed TPP in CD3 CN medium appeared as a singlet at 8.841 ppm (3.288 × 10−4 mol dm−3 ). However, in the complex mixture at the same concentration of TPP, there was an upfield shift to 8.798 ppm in the presence of Zn2+ (2.68 × 10−4 mol dm−3 ) i.e., there was a shift of 8.6 Hz on complexation (shown in Fig. 5). Such values are much greater than that expected due to solvation (ca. 0.5 Hz). Again the pyrrolic ␤-protons appearing in the region 7.10–7.90 ppm shifted to appear in 5.90–6.50 ppm, nearly a 1.30 ppm (i.e., 260 Hz) shift was observed along with a change in

H NMR spectra of blank TPP (3.288 × 10−4 mol dm−3 ) (A) and in the presence of Zn(ClO4 )2 (2.68 × 10−4 mol.dm−3 ) (B) in CD3 CN medium.

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colour of the solution from purple to green indicating the formation of a new compound in ground state. 4. Conclusion (1) The purple colour of the TPP solution gradually changes to green with the addition of Zn(ClO4 )2 indicating the formation of a new compound (complex). (2) Energy of maximum absorption and the steady state fluorescence of TPP depend significantly on the presence of Zn2+ ion. (3) Absorption spectral studies point to the formation of a cation–TPP complex in the ground (S0 ) state. (4) Steady state emission studies indicate that TPP forms a complex with Zn2+ in the S1 state. (5) 1 H NMR shift of 8.6 Hz in the aromatic protons and 260 Hz in the pyrrolic ␤-protons of TPP take place due to interaction with zinc ion in solution. (6) Tetraphenylporphyrin (TPP), a new fluorescence ratiometric sensitizer for zinc ion has been proposed. References [1] A.P. Demchenko, Introduction to Fluorescence Sensing, Springer, Germany, 2009. [2] X. YuFang, L. Feng, X. ZhaoChao, C. TanYu, Q. XuHong, Sci. China Ser. B: Chem. 52 (2009) 771–779.

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