Raman spectroscopy of phthalocyanines and their sulfonated derivatives

Transcription

1 Journal of Molecular Structure (2005) Raman spectroscopy of phthalocyanines and their sulfonated derivatives B. Brożek-Płuska*, I. Szymczyk, H. Abramczyk Technical University of Lodz, Institute of Applied Radiation Chemistry Laboratory of Molecular Laser Spectroscopy, Wroblewskiego 15 street, Lodz Received 18 October 2004; revised 21 December 2004; accepted 21 December 2004 Available online 3 March 2005 Abstract The aggregation and photochemistry of the copper (II) 3, 4 00,4 00, tetrasulfonated phthalocyanine, free base phthalocyanine and copper (II) phthalocyanine have been studied by UV VIS absorption spectroscopy and resonance Raman spectroscopy (RRS). The vibrational mode n 3 of (Cu(tsPc) 4K has been used as a probe in RRS measurements. The photochemistry of monomers and dimers of (Cu(tsPc) 4K has been studied in liquid solutions of H 2 O and DMSO as well as in frozen matrices. Low temperature Raman measurements in a broad temperature range have been carried out for free base phthalocyanine and copper (II) phthalocyanine in DMSO to identify the nature of emissive bands observed in the Raman spectra. It has been shown that the dimerization equilibrium constant K for tetrasulfonated phthalocyanine Cu(tsPc) 4K is strongly shifted towards monomeric form in DMSO solutions and in human blood compared to aqueous systems. The emission band at around 682 nm in DMSO and aqueous solutions observed at 77 K for tetrasulfonated salt of copper(ii) phthalocyanine in concentrated solutions has been assigned to the radical transient species generated during the photoinduced dissociation with the electron transfer between the molecules of phthalocyanines. The emission at 527 nm in DMSO and at 556 nm in water has been preliminarily assigned to the fluorescence from the higher excited triplet state T n /T 1. q 2005 Elsevier B.V. All rights reserved. Keywords: Phthalocyanine derivatives; Resonance Raman spectroscopy; Absorption spectroscopy; Charge transfer 1. Introduction The methods of Raman spectroscopy are widely used to study photochemical and photophysical properties of phthalocyanines and their sulfonated derivatives, which are well known as green/blue dyes, catalysts, chemosensors and potential third generation photosensitizers in photodynamic anti-tumor therapy (PDT). Raman spectroscopy makes possible to investigate phthalocyanines in pigments, in solutions, in biological matrices as well as in frozen matrices and becomes suitable technique to study them [1]. Resonance Raman scattering and surface enhanced resonance Raman scattering have special importance making possible to obtain information about electronic structure, nature of excited states, the oxidation state of the central metal ion as well as about charge transfer processes. Structures, properties, and applications of phthalocyanines can be modified by introducing metals in * Corresponding author. address: brozek@mitr.p.lodz.pl (B. Brożek-Płuska). the macrocycle or adding axial and peripherical substituents. Understanding photochemical mechanisms of phthalocyanines plays a crucial role in evaluation of their photodynamic activity because photochemistry and mechanisms of vibrational relaxation of these drugs affect their photobiology. The main goal of our study was to investigate the influence of (1) a solvent, (2) ligand substituents, and (3) a central atom on photochemistry, chemical activity and particularly the aggregation of phthalocyanines and their sulfonated derivatives The tendency for aggregation for this class of compounds is especially important for photodynamic applications. We were interested in the aggregation of phthalocyanines in aqueous and DMSO solutions. We have also studied the photochemistry of phthalocyanines in human blood because the photosensitizers are distributed to the tissue through blood. In this paper we wish to concentrate on the aggregation of, copper (II) phthalocyanine-3, 4 00,4 00, tetrasulfonic anions (Cu(tsPc) 4K in aqueous solutions, in DMSO and in human blood by UV VIS absorption spectroscopy and /$ - see front matter q 2005 Elsevier B.V. All rights reserved. doi: /j.molstruc

2 482 B. Brożek-Płuska et al. / Journal of Molecular Structure (2005) Raman spectroscopy as well as on the low temperature Raman spectroscopy of the free base phthalocyanine and the cooper(ii) phthalocyanine in DMSO and the tetrasulfonated copper (II) phthalocyanine in DMSO and in water. The vibrational mode n 3 of (Cu(tsPc) 4K has been used as a probe in the RRS measurements. 2. Experiment Spectrograde DMSO, free base phthalocyanine, copper (II) phthalocyanine-3,4 00,4 00, tetrasulfonic acid tetrasodium salt were purchased from Aldrich. The Raman spectra were recorded in the cryostat CF 1204 (Oxford Instruments Ltd). Raman spectra were measured in the region cm K1 in a broad temperature range from 293 to 77 K with Ramanor U1000 (Jobin Yvon) and spectra physics S argon ion laser operating at the line 514 nm, at power of 200 mw. The spectra slit width was 6 cm K1 in the full temperature range, which corresponds to the 500 mm mechanical slit of the spectrometer. The interference filter was used to purify the laser line. Absorption spectra at room temperature were recorded using Cary 5E (Varian) spectrometer. 3. Results and discussion To find the differences between the mechanisms of photochemical processes for monomers and dimers of Cu(tsPc) 4K we have studied the absorption spectra of Cu(tsPc) 4K in water and in DMSO. We have found that the Q band of Cu(tsPc) 4K in aqueous solutions consists of two bands at 624 and 661 nm. We assigned these components to the dimer (624 nm) and the monomer (661 nm) respectively. The tendency for aggregation of Cu(tsPc) 4K in aqueous solutions have been found even at low concentration 10 K5 mol/dm 3. The dimerization equilibrium constant K for the process 2Cu(tsPc) 4K /[Cu(tsPc) 4K ] 2 has been found to be 1.15!10 5 at 294 K (the extinction coefficient equals 1.29!10 4 [mol/dm 3 ] K1 cm K1 ). For comparison we have found that Cu(tsPc) 4K in DMSO systems exists predominantly as monomer, which is evident from the comparison between the absorption spectra in water and in DMSO (Fig. 1). Moreover, in pure DMSO the band at around 624 nm disappears which indicates that Cu(tsPc) 4K exists predominantly in monomeric form. The dimerization equilibrium constant K in DMSO is equal to 1.39!10 3. Additionally, in DMSO solutions the band at 608 nm can be seen. This band has been interpreted as a vibronic band of the Q electronic transition. We have estimated the frequency of the vibrational mode coupled to this electronic transition (1522 cm K1 ) which corresponds quite well to the frequency of the n 3 vibrational Fig. 1. Absorption spectra of Cu(tsPc) 4K at 294 K cz1!10 K4 mol/dm 3, optical path 0.2 mm in H 2 O (a) and in DMSO solutions (b) [2]. mode of Cu(tsPc) 4K at 1527 cm K1. The Raman spectra of the VV and VH components of the n 3 band have been shown in Fig. 2. One can see that the n 3 band consists clearly of two bands with the maximum peak positions at 1527 and 1538 cm 1. The depolarizations ratios calculated for both components (0.1 for the band at 1527 cm K1 and 0.33 for the band 1538 cm K1 ) confirm once again the coexistence of the monomeric at 1527 cm K1 (dashed line) and the dimeric at 1538 cm K1 (dotted line) forms in aqueous solutions. This conclusion has been supported by the concentration measurements [2]. Moreover, we have proved that the equilibrium of dimerization process is significantly shifted towards the monomeric form in comparison with aqueous solutions not Fig. 2. Resonance Raman spectra of the n 3 vibrational mode of Cu(tsPc) 4K in aqueous solution cz0.01 mol/dm 3 [2].

3 B. Brożek-Płuska et al. / Journal of Molecular Structure (2005) Fig. 3. Resonance Raman spectra of the n 3 vibrational mode of Cu(tsPc) 4K in human blood [2]. only in DMSO but also in the human blood, which is very important especially in the context of PDT applications. The results are presented in Fig. 3. Photochemistry of Cu(tsPc) 4K has been also investigated by resonance Raman spectroscopy. Fig. 4 shows Raman spectra of Cu(tsPc) 4K water system as a function of temperature for the concentration cz10 K3 mol/dm 3. The similar pattern of behavior has been recorded for other concentrations ranging from cz10 K2 to 10 K5 mol/dm 3.The narrow peaks in the region cm K1 correspond to the internal vibrations of Cu(tsPc) 4K, the peaks at around 3000 cm K1 correspond to the stretching modes of water. The broad structureless band with the maximum at around 4800 cm K1 (682 nm) corresponds to the emission with the intensity increasesing with temperature decreasing. Many phthalocyanines were reported to have fluorescence of the Q band in this region [3 6]. However, as we can see from Fig. 4 the intensity of this emission increases not decreases with temperature decreasing, which indicates that it should be assigned rather to the emission of transient Fig. 4. Raman spectra of Cu(tsPc) 4K water system as a function of temperature for the concentration cz10 K3 mol/dm 3. Fig. 5. Raman spectra of Cu(tsPc) 4K in DMSO as a function of temperature, cz10 K3 mol/dm 3. species generated by the VIS-light irradiation that are stabilized at low temperatures. The similar pattern of behavior has been recorded for Cu(tsPc) 4K anions in DMSO. Fig. 5 shows Raman spectra of Cu(tsPc) 4K in DMSO as a function of temperature, cz10 K3 mol/dm 3. Once again the intensity of the emission at around 682 nm increases with temperature decreasing. In contrast to water systems, a very intensive emission with the maximum at around 527 nm (500 cm K1 ) is observed. The emission at around 527 nm is observed only for liquid solution and it decreases in direct correspondence with the increasing intensity at 682 nm. This evidently indicates that these bands represent processes that are linked together. Although the nature of the emissive states in this region is not clear yet, it is obvious that the emission at 527 nm cannot originate from the phosphorescence from the lowest lying excited ligand-centered triplet T 1 state that is shifted significantly to the near infrared region [6]. It has been suggested that the emission at 527 nm originates from the higher excited singlet S n or triplet T n states of phthalocyanines [3] for the Q transition or from the lowest lying excited states of the phthalocyanine radicals [7]. The assignment of the fluorescence at 556 nm in water and 527 nm in DMSO to the T n /T 1 raises the question about the mechanisms of population for the triplet states. The mechanism of intersystem crossing transition from the S 1 state to the lowest excited triplet state T 1 is supported by the clear vibrational structure of the Q electronic transition in DMSO (Fig. 2). To investigate photochemistry of Cu(tsPc) 4K in more details we have recorded Raman spectra of phthalocyanine without any substituents and without a central metal atom (H 2 Pc) as well as without any substituents and with a central atom (copper (II) phthalocyanine (CuPc)). Fig. 6 shows Raman spectra of Cu(tsPc) 4 (cz1!10 K5 mol/dm 3 ), H 2 Pc

4 484 B. Brożek-Płuska et al. / Journal of Molecular Structure (2005) Fig. 6. Raman spectra of Cu(tsPc) 4K (cz1!10 K5 mol/dm 3 ), H 2 Pc (cz 6!10 K5 mol/dm 3 ) and CuPc (cz1!10 K5 mol/dm 3 ) in DMSO at 293 K (a) and 77 K (b). (cz6!10 K5 mol/dm 3 ) and CuPc (cz1!10 K5 mol/dm 3 ) in DMSO at 293 K (a) and 77 K (b). The narrow peaks in the region cm K1 correspond to the internal vibrations of phthalocyanines or DMSO, whereas the broad bands are related to the emission from the excited electronic states. The results presented in Fig. 6 show that the photochemistry of Cu(tsPc) 4K and Fig. 7. Raman spectra of Cu(tsPc) 4K (cz0.01 mol/dm 3 ), Cu(tsPc) 4K (cz1!10 K5 mol/dm 3 ), H 2 Pc (cz6!10 K5 mol/dm 3 ) and CuPc (cz1! 10 K5 mol/dm 3 ) in DMSO at (a) 293 K and (b) 77 K. CuPc at 293 K in diluted solutions is similar whereas for more concentrated H 2 Pc solution the weak emission at 682 and 527 nm is observed. At 77 K the emission at 682 nm disappears for H 2 Pc but the emission at around 527 nm is still observed. The emission at around 628 nm observed at 293 K has been assigned to the fluorescence of the Q band. At 77 K no emission is observed at 682 nm, which indicates

5 B. Brożek-Płuska et al. / Journal of Molecular Structure (2005) that for frozen matrices of diluted solutions the electron transfer between the macrocycles does not occur and the transient radicals are not generated. The emission at around 527 nm has been assigned like for Cu(tsPc) 4K in frozen matrices of concentrated solutions (Fig. 5) to the fluorescence from the higher excited triplet state (T n /T 1 ). To understand the photochemistry of phthalocyanines we have compared the concentration effect on the emission intensity for Cu(tsPc) 4K in DMSO. Results are shown in Fig. 7a for 293 K and in Fig. 7b for 77 K. One can see that for Cu(tsPc) 4K in concentrated mixture (cz10 K2 mol/dm 3 ) the photochemistry is different from that observed in diluted systems (cz10 K5 mol/dm 3 ). Indeed, the intense band at 527 nm observed for Cu(tsPc) 4K for cz10 K2 mol/dm 3 at 293 K (Fig. 7a) is not observed (or is much weaker) for Cu(tsPc) 4K for cz10 K5 mol/dm 3. Moreover, the band at 527 nm is significantly suppressed at 77 K (Fig. 7b) in contrast to the band at 682 nm which intensity increases with temperature decreasing. We suggest that this emission at 682 nm should be assigned to the emission of the transient radicals stabilized at low temperatures. The radicals are generated via electron transfer between the phthalocyanine macrocycles that form the stacked columns in concentrated solutions. The experiments performed with different rates of cooling and radical anion scavengers have confirmed that irradiation with 514 nm of concentrated systems of Cu(tsPc) 4K generates the Cu(tsPc) 4K phthalocyanine radicals that emit the fluorescence with the maximum at around 682 nm. This kind of emission for Cu(tsPc) 4K phthalocyanine confirms the utility of the tetrasulfonated phthalocyanines in PDT where the radicals of Cu(tsPc) 4K can participate the in I-type of photooxidation. In this type of photooxidation the sensitizer reacts directly with another chemical entity by hydrogen or electron transfer to yield transient radicals, which react further with oxygen. To provide additional arguments supporting the assignment of the emissive band at around 527 nm to the fluorescence from the higher excited triplet state we have performed also the measurements for degassed and nondegassed samples [8]. The intensity of the emission at 527 nm was significantly suppressed in nondegassed solutions which confirms that oxygen is involved in the process of deactivation of the triplet state. 4. Conclusions (1) Dimerization constant of tetrasulfonated phthalocyanine Cu(tsPc) 4K is predominantly shifted towards monomeric form in DMSO solutions and in human blood compared to aqueous solutions. (2) The emission at 682 nm was observed at low temperatures only for the frozen matrices of concentrated solutions of tetrasulfonated salt of copper (II) phthalocyanine and has been assigned to the radical transient species. (3) The emission at 527 nm in DMSO and at 556 nm in H 2 O for Cu(tsPc) 4K is tentatively assigned to the fluorescence from the higher excited triplet state T n /T 1. Acknowledgements We gratefully acknowledge the support from the B. WŁ. 844/04, Dz.S/04 and KBN grant 4 T09A References [1] C.C. Leznoff, A.B.P. Lever (Eds.), Phthalocyanines. Properties and Applications vol. 3, VCH, New York, [2] H. Abramczyk, I. Szymczyk, J. Mol. Liq. 110 (2004) [3] K. Tokumaru, J. Porphyrins Phthalocyanines 5 (2001) 77. [4] G.P. Johari, A. Hallbrucker, E. Mayer, J. Chem. Phys. 92 (1990) [5] A.M. Schaffer, M. Gouterman, E.R. Davidson, Theor. Chim. Acta 30 (1973) 9. [6] G. Ferraudi, in: C.C. Leznoff, A.B.P. Lever (Eds.), Phthalocyanines. Properties and Applications, VCH, New York, 1989, p [7] J. Simon, P. Bassoul, in: C.C. Leznoff, A.B.P. Lever (Eds.), Phthalocyanines. Properties and Applications, VCH, New York, 1989, p [8] H. Abramczyk, I. Szymczyk, G. Waliszewska, A. Lebioda, J. Phys. Chem. 108 (2004) 264.