Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy

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1 Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 104 (2013) Contents lists available at SciVerse ScienceDirect Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: DFT/TD-DFT study of solvent effect as well the substituents influence on the different features of TPP derivatives for PDT application Mateusz Dulski a,, Marta Kempa a, Patrycja Kozub a, Justyna Wójcik a, Marcin Rojkiewicz b, Piotr Kuś b, Agnieszka Szurko a, Alicja Ratuszna a, Roman Wrzalik a a A. Chełkowski Institute of Physics, University of Silesia, Uniwersytecka 4, Katowice, Poland b Institute of Chemistry, University of Silesia, Szkolna 9, Katowice, Poland highlights " DFT and TD-DFT calculations of the physicochemical properties of photosensitizers. " Determination of geometrical parameters of test compounds. " Use LR and EI methods to determine the location of absorption and emission bands. " Solvent effect on the spectroscopic properties of compounds. " Comparison of theoretical and experimental data. graphical abstract article info abstract Article history: Received 5 June 2012 Received in revised form 6 November 2012 Accepted 23 November 2012 Available online 5 December December 2012 Keywords: DFT calculations TD-DFT calculations Electronic states Photodynamic therapy Photosensitizer Spectral characteristics study of meso-tetraphenylporphyrin derivatives (TPP1 and TPP2) used as photosensitizers for utilization in photodynamic therapy (PDT) has been performed by density functional theory (DFT) and time dependent DFT (TD-DFT) calculations at B3LYP/6-31G(d) level of theory using PCM solvation model. The geometrical parameters of porphyrins have been studied for ground and excited-state geometry to deduce the influence of various substituents as well as solvent effect on the deformation of porphyrin ring. Two theoretical approaches linear response (LR) and external iteration (EI) have been performed to replicate absorption and fluorescence emission spectra. Experimental and theoretical investigations have shown that EI method reproduces the absorption energies very well for both singlet singlet and triplet triplet transitions, whereas the LR approach is more coherent with experimental fluorescence emission spectra. Spectral features and HOMO LUMO band gap analysis have shown that TPP1 can be more useful in PDT. Calculations have revealed that two the highest occupied and two the lowest unoccupied molecular orbitals are responsible for the Q-band absorption and are located mainly on the porphyrin ring. In order to verify the substituent effect on the activity of tested compounds in their ground and excited states, the molecular electrostatic potential surfaces have been analyzed. Ó 2012 Elsevier B.V. All rights reserved. Introduction Corresponding author. Tel.: ; fax: addresses: mdulski@us.edu.pl (M. Dulski), mmalkiewicz@us.edu.pl (M. Kempa), pkozub@us.edu.pl (P. Kozub), dziewki130@interia.pl (J. Wójcik), marcin.rojkiewicz@us.edu.pl (M. Rojkiewicz), pkus@ich.us.edu.pl (P. Kuś), agnieszka. szurko@us.edu.pl (A. Szurko), alicja.ratuszna@us.edu.pl (A. Ratuszna), roman. wrzalik@us.edu.pl (R. Wrzalik). Photodynamic therapy (PDT) is a relatively new method applied mainly in the treatment of cancerous changes. It requires the presence of three components: oxygen, drug (chemical compound called the photosensitizer) and a light source emitting waves coherent with the absorption maximum of used photosensitizer. All of these components separately are nontoxic. Only if they are combined together they cause the destruction of cancer cells /$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.

2 316 M. Dulski et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 104 (2013) The main advantage of PDT is, on the contrary to the contemporary treatment techniques (chemotherapy and radiotherapy), the possibility of selective destruction of cancer tissues, what bears the testimony to its safety [1 3]. The photodynamic therapy is a technique which is constantly under development and for which new applications are found. Its possibilities are extending beyond the area of oncology. With success the PDT is used in dermatology, urology, ophthalmology and gynecology. Occasionally it is also used in cardiology, orthopedics and neurosurgery. This method is successfully used in treatment of eye diseases, for example macular age degeneration, and atherosclerosis [4 7]. A lot of photoactive compounds exist, but only some can be used in photodynamic therapy. Photosensitizers should have a series of relevant physical, biological and chemical properties, because this is the key to attain success in PDT [8 11]. Drug should exhibit the maximum of absorption in the tissue transmission optical window absorption band of photosensitizer could not overlap with the absorption bands of endogenous dyes and water. The shift of the longest wavelength band towards the red region of the spectrum (k P 650 nm) is also very important, because this allows deeper penetration of the light into the tissue [11]. An important role in the effectiveness of photodynamic therapy is played by the triplet state. Molecules of the photosensitizer in triplet state, excited with a light of appropriate wavelength, are undergoing a series of photochemical reactions with neighboring molecules especially oxygen molecules, what results in producing cytotoxic products (singlet oxygen and reactive oxygen species). They damage vital macromolecules, numerous intracellular structures leading to destruction of diseased tissue [8]. The requirements for the photosensitizers used in PDT are high and it is difficult to find compounds that satisfy all of them simultaneously. Furthermore, due to the different morphometric structures in variety of neoplasm (the connective tissue, tumor vasculature, etc.), it is difficult to imagine that one dye could be effective for different types of cancer. Therefore it is important to search for new photosensitizers which will have the desired properties for PDT, especially with higher absorption of light in the range of longer wavelengths and with higher generation of singlet oxygen. The basic criterion in investigation of photosensitizers is determining the wavelength for which the absorption is taking place. It allows for the verification whether the given drug will be subject to further research (suitable position of the last absorption band). It is also important to determine the fluorescence spectra and establishing the percentage of the absorbed energy lost throughout the process (lower value increases the probability of transferring the energy for production of singlet oxygen). Additionally, the fluorescence allows for monitoring the kinematics of changes of the accumulation and removal of the drug from tissue. Determination of above spectra by means of the theoretical calculations would enable the verification which from the interesting compounds has the greatest potential in terms of using them in the photodynamic therapy. Obtained information allows for drawing a conclusion, which compounds are suitable for synthesis, in order to carry out further analyses. Steady-state spectroscopy is one of the most fundamental tools for investigating equilibrium structures and potential energy surfaces for different electronic states. However, interpreting such experimental data is often not straightforward. Density functional theory (DFT) [12 14] and time-dependent DFT (TD-DFT) [15 20] calculations are the most useful methods for studying large molecular systems, like porphyrins [21]. Most DFT computations use Becke s three-parameter hybrid functional [22] with the Lee Yang Parr correlation functional (B3LYP) [22 24] due to high correlation between theoretical and experimental data [25]. Moreover researchers usually add only d -polarization function on heavy atoms to the standard 6-31G basis set [25], since it is the best alternative to get satisfactory results with relatively low computational cost. TD-DFT calculations including solvent effects could be performed using two different methods, that is, linear response (LR) and external iteration (EI) or state specific (SS) in older nomenclature [26 29]. In the LR-PCM the excitation energies are directly determined without computing the exact excited electron density. In turn, in the EI-PCM approaches a different effective Shrödinger equation is solved for each state of interest, up to the fully variational formulation of solvent effect on the excited-state properties. The dynamical solvent effect should also be considered during studying excited states in solution. Generally the total solvent polarization is always partitioned into two degrees of freedom, slow and fast [28,29]. The slow part can be regarded as the reorganization of the solvent molecules as a response to a change in the electronic density of the solute. The fast part can be regarded as the response of the electrons in the solvent to a change in the electronic density of the solute. In an equilibrium (EQ) solvation calculation, both components are in equilibrium with the electron density of the excited-state density. Additionally, the solvent reaction field depends on the static dielectric constant of the medium. In turn, in an non-equilibrium (NEQ) only solvent electronic polarization connected with fast degree of freedom is in equilibrium with the excited-state electron solute s density. Moreover, the slow solvent components remain equilibrated with the ground-state electron density. In LR-PCM solvent degrees of freedom are always equilibrated with the ground-state density. Therefore, a single-point LR-PCM/ TD-DFT calculation for absorption and emission energies are performed defaults to non-equilibrium solvation. EI-PCM/TD-DFT excitation energy calculations are performed by solving the fast components ( slow component comes from the origin state) of the solvent polarization self-consistently with the selected excited-state density. This description shows EI-PCM approach provides a more rigorous treatment of dynamical solvent effect than LR-PCM. Moreover, LR-PCM calculations have been shown to overestimate solvent effects on the intensities of particular bands [28,29]. The two other limitations of this method are connected to the treatment of the emission processes and the study of the electronic transitions involving a substantial electron density shift [28,29]. In turn, EI-PCM methods provide a balanced description of strong and weak electronic transitions and give accurate estimates of dynamical solvent effect on the absorption and fluorescence emission processes. Therefore, vertical excitation energy from the result of LR/TD-DFT geometry optimizations should be supplement by single-point EI/TD-DFT calculations. We have investigated the theoretical aspects of the geometrical and electronic structures of two meso-tetraphenylporphyrin (TPP): 5,10,15,20-tetrakis (3-methoxy-4-hydroxyphenyl)-porphyrin (TPP1) and 5,10,15,20-tetrakis (3,5-dimethoxy-4-hydroxyphenyl)-porphyrin (TPP2) presented in Fig. 1. Acetone solution has been used as a solvent in both calculations and experimental measurements. The effects of solvent, substrate, and electric field have been considered using a polarized continuum model (PCM) [30 36]. In order to characterize singlet singlet and triplet triplet absorption features and fluorescence spectra, the LR and EI approaches have been used. The main aim of the analysis is understanding the role of substituents on the ordering, energies of excited singlet and triplet electronic states as well as on the change of activity. Evaluation of influence of the solvent on the

3 M. Dulski et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 104 (2013) Absorption and fluorescence spectra were made with a Double Beam Spectrophotometer Hitachi U-2900 and Fluorescence Spectrophotometer Hitachi F-7000 devices respectively. Absorption spectra were collected in the range nm. Emission spectra were measured in the range nm using 421 nm excitation wavelength for TPP1 and 423 nm for TPP2. All data were collected with scanning speed of 800 nm/min and the sampling interval was equal to 1.0 nm. Transient triplet triplet spectra were observed with a Applied Photophysics LKS.60 Laser Flash Photolysis Spectrometer, using the third harmonic (355 nm) of a Brilliant Nd- YAG Q-switched laser (Quantel). The 150 W xenon lamp was used as the analyzing light source. Infiniium Agilent Technologies DSO9064A, 600 MHz oscilloscope and photomultiplier R-928 were used in the measurements. Spectra measurements were performed in the range from 290 to 650 nm with 1 nm interval. The quartz cell with optical path length of 10 mm was used in all measurements. Fig. 1. Labeling scheme for the meso-tetraphenylporphyrin (TPP) and studied compounds [created in ChemCraft based on Gaussian09 calculations]. geometry and analyzed absorption and fluorescence emission spectra will also be performed. Calculations The starting point for geometry optimization of two studied analogs of meso-tetraphenylporphyrin (TPP1 and TPP2) with various substituents was taken from X-ray structure of TPP [37]. DFT and TD-DFT calculations for two studied compounds were carried out with aid of the Gaussian 09 software package [38] using the B3LYP exchange correlation functional [22 24] and 6-31G(d) basis set. Additionally excitation energies from the lowest triplet energy state to higher triplet states (T 1? T n ) were calculated using UB3LYP/6-31G(d) theory level. Solvent effects were evaluated by using the PCM model [35,36] in which the cavity is created via a series of overlapping spheres [35] with standard dielectric constants (e) of for acetone solution. In order to simulate absorption spectra, the lowest most-probable 7 spin-allowed singlet singlet (S 0? S n ) and spin-forbidden singlet triplet (S 0? T 1 ) have been considered in the 2nd (the LR approach) and 3rd (the EI approach) step. Whereas for reproduction of fluorescence emission spectra, only the lowest 2 spin-allowed S n? S 0 transitions have been investigated in the 4th (the LR approach) and 5th (the EI approach) step. Additionally 28 triplet triplet (T 1? T n ) transitions along with their intensities were determined for optimized geometry of the lowest triplet state T 1. The vacuum calculations have involved only the first, second and fourth step. The molecular electron densities and the molecular electrostatic potential surfaces of TPP1 and TPP2 were determined from the wave functions using CUBE option implemented in Gaussian 09 and visualized using GaussView 5.0. Additional visualization of structures and molecular energy levels was possible in ChemCraft and Chemissian software, respectively. Experiment All investigated compounds were synthesized according to well-known Adler Longo method [39] in Institute Chemistry, University of Silesia. Results Geometry The computed structural parameters of two examined compounds obtained in vacuum and acetone solution for both ground (S 0 ) and excited-state (S 1 ) geometries are presented in Table 1. Both molecules were assumed to belong to the C 2 point group symmetry during these computations. The bond lengths and plane angles of TPP core are almost the same for both compounds and slightly changed in solution as well as in excited state S 1 (see Table 1). The primary modifications are related to dihedral angles appeared to be the most sensitive parameters (their percentage differences vary from 0.06% to 116%). It is worthy of note, that the C a 0 C m C a N a and C a C m C a 0 N a 0 dihedral angles describe the distortion of the pyrrole rings out of the plane of the porphyrin core the higher deviation from 0 the more deformation. In turn, the N a C a C b C c and N a 0 C a 0 C b 0 C c 0 dihedral angles determine the distortion from planarity of the pyrrole rings (like previously, the higher deviation from 0 is, the more deformed rings are). Whereas, the C a C m C1 C2 and C a 0 C m 0 C1 0 C2 0 dihedral angles describe the spatial orientation of the phenyl rings relative to the porphyrin core (the smaller deviation from 0, the more planar molecule). In the case of acetone solution as a solvent, the most significant changes occur in C a C m C a 0 N a 0 (the percentage difference is about 7%) and N a C a C b C c (the percentage difference is about 6%) dihedral angles for the ground state as well as N a 0 C a 0 C b 0 C c 0 (the percentage difference is about 14%) and C a 0 C m C a N a (the percentage difference is about 4%) for the excited state. The higher values observed for the TPP1 make this structure more distorted from planarity than the TPP2. This can be caused by the influence of an asymmetric charge distribution derived from the single methoxy substituent at the meta-position of the TPP1. On the other hand, the values of C a C m C1 C2 and C d 0 C m 0 C1 0 C2 0 dihedral angles are smaller for the TPP1; this suggest the more planar conformation of the phenyl rings relative to the porphyrin core. The above dependency can be formulated as follows: the more planar porphyrin ring becomes, the more perpendicular the orientation of the phenyl ring is. The situation is slightly different in the case of vacuum. Although C a C m C1 C2 and C d 0 C m 0 C1 0 C2 0 dihedral angles are still smaller for the TPP1, the other ones, describing distortion of porphyrin ring, have not the same types of changes. This can be easy observed for N a 0 C a 0 C b 0 C c 0 (higher for the TPP2 at about 15% and 11% in the ground and excited state, respectively) and C a C m C a 0 N a 0 (higher at about 7% and 4% for the TPP1 in the ground and excited state, respectively). That is why, in the

4 318 M. Dulski et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 104 (2013) Table 1 The structural parameters of TPP1 and TPP2 obtained for vacuum and acetone solution for both ground and excited states by using TD-DFT/B3LYP/6-31G(d)//DFT/B3LYP/6-31G(d) calculations. Vacuum Acetone solution Ground state Excited state Ground state Excited state TPP1 TPP2 TPP1 TPP2 TPP1 TPP2 TPP1 TPP2 Bond lengths (Å) C b C c C b 0 C c C a C b C a 0 C b C a N a C a 0 N a C a C m C a 0 C m N a 0 H C b H/C c H C b 0 H/C c 0 H Bond angles ( ) C m C a N a C m C a 0 N a C a C m C a C d N a C a C d 0 N a 0 C a Dihedral angles ( ) N a C a C b C c N a 0 C a 0 C b 0 C c C a 0 C m C a N a C a C m C a 0 N a C a C m C1 C C d 0 C m 0 C1 0 C Intermolecular distances (Å) N a N a case of vacuum, it is not possible to clearly determine which ring (TPP1 or TPP2) is more distorted from planarity. The greatest solvent effect for the TPP1 is observed in N a 0 C a 0 C b 0 C c 0 angle, that increases at about 41% and 45% for the ground and excited state, respectively. Whereas for the TPP2, the most significant increase is observed for C a C m C a 0 N a 0 angle at about 23% and 29% for the ground and excited state, respectively. This analysis suggests that the TPP1 is much more susceptible to the solvent effect. The excitation causes the most significant geometrical changes, especially in the case of dihedral angles. The ones connected with a deformation in the porphyrin ring are % higher in the excited states; the greater increase (from about 6 to 11 in the case of vacuum and from 7 to 14 in the case of acetone solution) applies to the C a 0 C m C a N a angle. Whereas, the angles associated with the orientation of the phenyl rings are 14 19% higher in the ground states. To conclude, according to expectations, the structures of the ground state are much more planar than the excitedstate ones. Moreover the decrease of C m C a N a bond angle and increase of N a N a and C a C m bond distances indicate that porphyrin rings of TPP1 and TPP2 get longer along the N a N a direction and simultaneously close up in the opposite direction under the influence of excitation. Electrostatic surface potential Molecular electrostatic potential maps of TPP1 and TPP2 calculated in acetone solution for both ground and excited state are presented in Fig. 2. Color maps for potential surfaces were chosen to yield maximum contrast of a given net charge as well the relative locations of partial positive and negative charge were found. Positively and negatively charged groups may serve as donors and acceptors for different atoms, respectively. The ground-state potential surfaces differ only because of additional methoxy group causes that negative charge is concentrated on the lone pairs of oxygen atoms in this group. For the TPP1 the substantial negative charge (red) is accumulated inside the porphyrin ring on the dissubstituted nitrogen atoms, making them very reactive. Slightly less negative charge is formed by the lone pairs of oxygen in the hydroxyl groups. All carbon atoms in the porphyrin ring have partial negative charge (yellow) while hydrogen atoms have intermediate electron density (green). Substantial positive charge (dark blue) is only concentrated on the hydrogen atoms in hydroxyl groups and partial positive charge (blue) is located on the methyl groups. The molecular electrostatic potential surfaces of TPP2 can be interpreted in exactly the same way as described above. Since excited-state geometry is slightly distorted, the significant increase in absolute values of positive and negative charges is observed, in relation to the ground state. However, the general charge distribution of both compounds in their excited state remains unchanged. Absorption analysis The experimental absorption spectra of porphyrins are divided into two characteristic spectral regions intense B-band also called Soret band located near 400 nm and weak Q-band consisting of four bands in the region nm [40]. The long-wavelength Q bands are of interest to photodynamic therapy since light from this region possesses maximum penetration power into most human tissues. Unfortunately just two out of four Q bands, i.e. Q y (0 0) and Q x (0 0), can be obtained during vertical excited calculation [41 43]. Thus the S 0 S 1 transition should be assigned to Q y band (0 0). Remaining two bands, i.e. Q y (1 0) and Q x (1 0) have vibronic character [41 43] and this effect is not taken into account

5 M. Dulski et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 104 (2013) Fig. 2. The molecular electrostatic potential surfaces of TPP1 (left) and TPP2 (right) calculated for both ground (upper) and excited (lower) states by using PCM/TD-DFT/ B3LYP/6-31G(d)//PCM/DFT/B3LYP/6-31G(d) calculations [created in GaussView 5.0 based on Gaussian 09 calculations]. here. The x, y labels refer to the direction of polarization (see Figs. 1 and 4) as shown by Improta et al. [44]. The absorption energies for spin-allowed singlet singlet (S 0? S n ) transitions obtained for vacuum and acetone solution by using linear response (LR) and external iteration (EI) [26 29] TD-DFT/B3LYP/6-31G(d)//DFT-B3LYP/6-31G(d) calculations compared with available experimental data are presented in Tables 2 and 3 for TPP1 and TPP2, respectively. The corresponding UV VIS absorption spectra of S 0? S n transitions are illustrated in Fig. 3. The theoretical absorption energies for S 0? S n transitions obtained with two considered methods are close to each other for both TPP1 and TPP2 where the percentage differences are less than 0.4% for the Q-bands and 3.0% for the B ones. In the case of TPP1 the Q y (0 0) and Q x (0 0) bands as well as the most intense peaks of the Soret bands are observed experimentally at 651, 553, 421, 403 nm. The corresponding theoretical data are located at 599, 563, 425, 407 nm for the LR approach and 597, 561, 424, 401 nm for the EI one. For TPP2 the Q y,q x, B band positions at 598, 563, 425, 401 nm for the LR method and 597, 561, 419, 389 nm for the EI one correspond with experimental data found near 650, 554, 423, 402 nm. It should be noted that the description of the Soret band is much more complicated than for TPP [44]. As can be seen from Tables 2 and 3 the four-orbital Gouterman s model [45] cannot be directly used to describe the observed transitions. Our calculation shows that four intense transitions built Soret band range. These transitions are not only due to HOMO-1, HOMO, LUMO and LUMO+1 excitations because the excitation from lower energy occupied orbitals play important role. Although small shift towards the red region of the spectrum is observed for the LR-PCM approach in relation to the EI-PCM one, both ways of calculation give results in agreement with experimental data. For both TPP derivatives, the percentage differences of the LR-PCM approach are less than 9%, 2% for the Q y and Q x bands and 1.1% for the most intense B ones, while for the EI-PCM method the corresponding values are 9%, 1.5% and 1.4%. This indicates that the EI approach is the better way to estimate the Q bands position and simultaneously gives better fit to the B-bands. The computed values with assigned experimental data of TPP2 show the blue-shift trend in comparison to the corresponding values of TPP1. This fact indicates that the lack of an additional OCH 3 group on the phenyl ring makes the TPP1 more useful in PDT application from the viewpoint of absorption spectroscopy. The difference between the LR and EI approaches is more visible in the case of oscillatory strength parameter (f) (i.e. Fig. 3 for TPP1). It should be noticed that the LR-PCM calculations give higher oscillator strengths for both the B and Q bands. EI-PCM method reflects better than the LR-PCM reality of experimental spectra and, therefore, was used to analyze the excitations of Q and B transitions. Calculations indicate that the introduction of an additional OCH 3 group to the phenyl ring does not imply any changes in the composition of Q-bands. According to Tables 2 and 3, the Q y -band represents mainly the configuration HOMO LUMO (about 35% for TPP1 and TPP2) with a smaller contribution from HOMO-1 LUMO+1 (about 13% for TPP1 and TPP2), while the Q x one represents mostly the HOMO LUMO+1 transition (about 35% for TPP1 and TPP2) with smaller amount of HOMO-1 LUMO (about 13% for TPP1 and TPP2). However, the Soret band represents different configuration for both compounds. Taking into account the most intense Soret band for TPP1 is composed mainly of HOMO- 3 LUMO (about 30%) configuration with considerably smaller contribution from HOMO-2 LUMO+1 (about 8%), while the second intense one is composed of three approximately equal contributions from HOMO-4 LUMO+1 (about 12%), HOMO-1 LUMO+1 (about 12%) and HOMO-3 LUMO (about 10%). Whereas, in the

6 Table 2 The absorption energies of TPP1 for spin-allowed singlet singlet and triplet triplet obtained for vacuum and acetone solution by using linear response (LR) and external iteration (EI) TD-DFT/B3LYP/6-31G(d)//DFT/B3LYP/6-31G(d) calculations compared with experimental data. Spin-allowed singlet singlet (S 0? S n ) transitions Calculations Experiment Vacuum Acetone solution Acetone solution MO LR (%) k LR (nm) (E (ev)) f MO LR (%) EI (%) k LR (nm) (E (ev)) f k EI (nm) (E (ev)) f k exp (nm) Type Assign. H-1? L (2.10) H-1? L (2.07) (2.08) p? p Q y (0 0) H-0? L+1 32 H-0? L Q y (1 0) H-1? L (2.23) H-1? L (2.20) (2.21) p? p Q x (0 0) H-0? L+0 32 H-0? L Q x (1 0) 484 H-3? L (2.30) H-2? L (2.85) (2.86) p? p B H-1? L+0 13 H-2? L H-1? L H-3? L (3.04) H-2? L (2.87) (2.87) p? p B H-2? L H-5? L (3.06) H-3? L (2.92) (2.93) p? p B H-3? L H-1? L+1 19 H-2? L H-1? L H-5? L (3.10) H-3? L (2.93) (2.94) p? p B H-3? L+0 26 H-3? L H-1? L+0 9 H-2? L H-7? L (3.10) H-4? L (3.04) (3.09) p? p B H-3? L H-5? L+1 21 H-1? L Triplet triplet (T 1? T n ) transitions H-0? L (0.45) H-0? L (0.39) (0.36) ? T 2 H-5? H (1.64) H-7? L (1.26) (1.29) ? T 5 H-3? H-1 85 H-5? L H-8? H (1.96) H-0? L (1.67) (1.72) ? T 9 H-2? L (2.11) H-2? L (1.97) (1.98) ? T 10 H-2? H-1 11 H-9? L H-1? L (2.30) H-3? L (2.15) (2.16) ? T 13 H-9? H-1 27 H-9? H H-14? H (2.47) H-1? L (2.27) (2.29) ? T 15 H-0? L (2.80) H-15? H (2.56) (2.59) ? T 21 H-0? L+4 48 H-2? L H-0? L (2.80) H-2? L (2.74) (2.76) ? T 22 H-0? L (2.83) H-0? L (2.80) (2.78) ? T 23 H-17? H-1 28 H-18? H H-0? L (2.87) H-0? L (2.82) (2.80) ? T 25 H-2? L+0 22 H-0? L M. Dulski et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 104 (2013)

7 Table 3 The absorption energies of TPP2 for spin-allowed singlet singlet and triplet triplet obtained for vacuum and acetone solution by using linear response (LR) and external iteration (EI) TD-DFT/B3LYP/6-31G(d)//DFT/B3LYP/6-31G(d) calculations compared with experimental data. Spin-allowed singlet singlet (S 0? S n ) transitions Calculations Experiment Vacuum Acetone solution Acetone solution MO LR (%) k LR (nm) (E (ev)) f MO LR (%) EI (%) k LR (nm) (E (ev)) f k EI (nm) (E (ev)) f k exp (nm) Type Assign. H-1? L (2.10) H-1? L (2.07) (2.08) p? p Q y (0 0) H-0? L+1 34 H-0? L Q y (1 0) H-1? L (2.24) H-1? L (2.20) (2.21) p? p Q x (0 0) H-0? L+0 34 H-0? L Q x (1 0) 483 H-3? L (3.00) H-3? L (2.85) (2.87) p? p B H-1? L+0 17 H-1? L H-5? L (3.06) H-3? L (2.89) (2.89) p? p B H-1? L+1 19 H-3? L (3.08) H-5? L (2.91) (2.96) p? p B H-1? L H-3? L (3.10) H-5? L (2.96) (2.97) p? p B H-3? L H-5? L (3.12) H-5? L (3.10) (3.19) p? p B H-1? L Triplet triplet (T 1? T n ) transitions H-2? H (0.65) H-0? L (0.37) (0.35) ? T 2 H-4? H (1.60) H-5? H (1.27) (1.29) ? T 5 H-5? H (1.58) H-0? L (1.66) (1.71) ? T 9 H-1? L (2.13) H-2? L (2.06) (2.05) ? T 14 H-2? H-0 36 H-3? L H-2? L (2.23) H-2? L (2.25) (2.26) ? T 15 H-13? H-1 81 H-13? H H-14? H (2.29) H-1? L (2.27) (2.28) ? T 16 H-13? H-1 1 H-15? H H-17? H (2.48) H-13? H (2.45) (2.47) ? T 19 H-1? L (2.84) H-2? L (2.66) (2.70) ? T 22 H-0? L+2 89 H-1? L H-2? H-0 2 H-3? L H-0? L (2.85) H-1? L (2.76) (2.79) ? T 23 H-0? L+8 2 H-17? H H-0? L (2.88) H-3? L (2.82) (2.81) ? T 24 H-0? L+8 3 H-0? L M. Dulski et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 104 (2013)

8 322 M. Dulski et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 104 (2013) Fig. 3. Experimental UV VIS absorption spectrum of TPP1 with theoretical absorption spectra obtained in acetone using the LR and EI PCM/TD-DFT/B3LYP/6-31G(d)//PCM/DFT/B3LYP/6-31G(d) calculations for singlet singlet transitions. case of TPP2 suitable line represents the configuration HOMO-5 LUMO+1 (about 22%) with a comparable contribution from HOMO-1 LUMO (about 17%) and HOMO-5 LUMO (about 21%) transition with very small amount from HOMO-1 LUMO+1 (about 9%). The graphical presentation of the highest occupied and lowest unoccupied molecular orbitals of TPP1 and TPP2 is presented in Fig. 4. Generally, the calculations of singlet singlet absorption spectra for vacuum, for both studied porphyrins, exhibit a blue shift of the absorption bands in comparison with the experimental data in solvent the percentage differences are less than 0.5% for the Q bands and 5.0% for the B ones. These results indicate, that the influence of solvent is significant and only calculations using PCM model [30 36] allow for the replication of real experimental conditions. The calculations also enabled the determination of the energy gap between the ground singlet and low-lying triplet excited state (E S T ) has been computed. To excite oxygen from its triplet ground state to its singlet state the energy gap needs to be above 1 ev. Moreover, the triplet state should be generated with the high quantum yield (u > 0.4) and lifetimes (>1 ls) [40]. Thus the LR- PCM and EI-PCM methods can be also helpful in predicting energy gap between the ground singlet and low-lying triplet excited. For TPP1 according to our theoretical data E S T is located at 1.33 ev for the LR-PCM approach while at 1.32 ev for the EI-PCM ones. Fig. 4. The graphical presentation of the highest occupied and lowest unoccupied molecular orbitals of TPP1 (left side of the column) and TPP2 (right side of the column) obtained by using PCM/TD-DFT/B3LYP/6-31G(d)//PCM/DFT/B3LYP/6-31G(d) calculations [created in GaussView 5.0 based on Gaussian 09 calculations].

9 M. Dulski et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 104 (2013) The theoretical values for TPP2 are similar to values from TPP1 and there are located at 1.34 ev for LR-PCM approach while for EI-PCM methods are shifted towards lower energy and there are observed at 1.33 ev. The E S T energies for both studied samples are the same and our outcomes allow to predict that both compounds have enough energy to create reactive form of oxygen (to excite molecular oxygen to their singlet state). According to these results both TPP1 and TPP2 can be proposed as photosensitizes in PDT. This parameter has been estimated also for vacuum where the lack of solvent molecules does not change the E S T energy for the long-wavelength peaks giving similar results as for solvent but in the case the second band energy is higher than for acetone solution calculations. LR approach gives E S T 1.34 ev for both compounds. One of two processes that determine uses porphyrins in PDT treatment is the energy transfer from photosensitizer triplet state directly to molecular oxygen (oxygen is excited from ground triplet state to very reactive singlet state). For this reason it is essential to know the energy of triplet triplet transitions. Experimental absorption spectrum for triplet triplet transitions (T T) with predicted excitation energies obtained (taking the solvent effect into consideration) by using the LR and EI PCM/TD- DFT/B3LYP6-31G(d)//PCM/DFT-B3LYP/6-31G(d) calculations (see Fig. 5 for TPP1). The absorption energies for T 1? T n transitions obtained with two considered methods are very close to each other for both examined samples and their percentage differences are less than 0.8%. Energy of the lowest triplet state calculated is equal to 0.15 ev and 0.14 ev for TPP1 and TPP2, accordingly. In the case of TPP1 the theoretical data with the highest oscillator strength are found at 628, 545, 452, 441 nm for the LR-PCM method. In turn, the data from EI-PCM approach allow to predict the maxima positions and character of transitions on the triplet triplet spectrum at 626 nm (T 1? T 10 ), 540 nm (T 1? T 15 ), 449 nm (T 1? T 22 ) and 443 nm (T 1? T 25 ). Presence of additional methoxy group in TPP2 causes the results to be shifted by 2 10 nm in the direction of longer wavelengths, in comparison to TPP1. Thus for TPP2 bands are located at 546, 505, 465 nm for the LR-PCM method, while values obtained using the EI-PCM approach allow for indicating the locations and character of triplet triplet transitions at 544 nm (T 1? T 16 ), 502 nm (T 1? T 19 ), 460 nm (T 1? T 22 ). These results show higher values of oscillatory strength in the LR calculations, in the NEQ states [28,29], while calculations for the EQ state using EI-PCM method cause significant decrease of oscillation constant. They show also why the experimental spectra of T T transitions have such low intensities. Full excitation analysis for triplet triplet transitions is very complicated due to unrestricted density functional calculations. Limiting the discussion to the most intense bands, we can conclude as shown in the Tables 2 and 3 that the most intensity peak in the T T spectrum of TPP1 near 449 nm is the mixture of HOMO-2 LUMO (about 30% for the LR-PCM and 14% for the EI-PCM) and Fig. 5. Experimental absorption spectrum for triplet triplet transitions of TPP1 with predicted excitation energies of triplet triplet transitions obtained by using the LR and EI PCM/TDDFT-B3LYP/6-31G(d) calculations. Fig. 6. Experimental fluorescence emission spectrum of TPP1 at 421 nm excitation wavelength with theoretical fluorescence emission spectra obtained in acetone solution by using the LR and EI PCM/TD-DFT/B3LYP/6-31G(d)//PCM/DFT/B3LYP/6-31G(d) calculations for singlet singlet transitions. HOMO-15 HOMO-1 (about 12% for the LR-PCM and 16% for the EI-PCM) transitions. In turn, for TPP2 (see Tables 2 and 3) the peak with maximum intensity, located at 460 nm, is made up mainly of HOMO-2 LUMO (about 29% for the LR-PCM and 6% for the EI- PCM) configuration with much smaller amount of contribution from HOMO-1 LUMO (about 7% for LR and 2% for the EI methods) and from HOMO-3 LUMO (about 40% for LR-PCM and 73% for the EI-PCM methods). Graphical presentation of the highest occupied and lowest unoccupied molecular orbitals of TPP1 and TPP2 is presented in Fig. 4. Calculations of triplet triplet spectrum in case of vacuum have shown that for both analyzed porphyrins the band blue-shift in comparison with data obtained in the presence of solvent. The highest differences were observed in case of oscillation force, values of which are very low in comparison to transitions calculated in solution. Fluorescence emission analysis The fluorescence emission energies for spin-allowed singlet singlet (S n? S 0 ) transitions obtained in acetone solution by using linear response (LR) and external iteration (EI) PCM/TD-DFT/ B3LYP/6-31G(d)//PCM/DFT/B3LYP/6-31G(d) calculations are compared with experimental data (see Fig. 6 for TPP1). Computed oscillator strength values and schematic description of the most relevant excited states of TPP1 and TPP2 in vacuum and acetone solution are summarized in Tables 4 and 5. The experimental spectra show two singlet singlet fluorescence emission bands (Q(0,0), Q(0,1)) located at 723 and 659 nm for TPP1 as well as 722 and 659 nm for TPP2. The calculation of fluorescence emission using the LR-PCM method gives two peaks located at 663.9, nm for TPP1 and 663.6, nm for TPP2. Whereas the solvent effect correction implemented in EI- PCM approach gives peaks located at 626.4, nm for TPP1 as well as 628.0, nm for TPP2. The percentage differences between theoretical and experimental data for LR-PCM are less than 8% and 3% while for EI-PCM approach are about 13% and 4%. Thus, the calculations of fluorescence emission spectra indicate that the LR-PCM method gives better results than the EI-PCM approach what is in opposite to the previous calculation of absorption spectra. Additionally, the theoretical peaks have similar intensities and are located close to each other (see Tables 4 and 5). As shown in experimental data for studied porphyrins, both fluorescence emission bands have a large FWHM (25 nm). One can expect that the low intense high-wavelength band will have vibronic character therefore the vertical excitation energies calculation allows to reconstruct only one experimental fluorescence emission band. Thus, it seems that both calculated peaks are associated only with the intense Q (0,0) band in a maximum observed at 659 nm.

10 324 M. Dulski et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 104 (2013) Table 4 The fluorescence emission energies of TPP1 for spin-allowed singlet singlet transitions obtained in vacuum and acetone solution by using linear response (LR) and external iteration (EI) TD-DFT/B3LYP/6-31G(d)//DFT/B3LYP/6-31G(d) calculations compared with experimental data. Spin-allowed singlet singlet (S n? S 0 ) transitions Calculations Experiment Vacuum Acetone solution Acetone solution MO LR (%) k LR (nm) (E (ev)) f MO LR (%) EI (%) k LR (nm) (E (ev)) f k EI (nm) (E (ev)) f k exp (nm) Type Assign Q (0 1) L+1? H (1.97) L+0? H (1.87) (1.98) p? p Q (0 0) L+0? H-0 39 L+0? H (2.14) L+0? H (1.93) (1.96) L+1? H-0 36 L+1? H Table 5 The fluorescence emission energies of TPP2 for spin-allowed singlet singlet transitions obtained in vacuum and acetone solution by using linear response (LR) and external iteration (EI) TD-DFT/B3LYP/6-31G(d)//DFT/B3LYP/6-31G(d) calculations compared with experimental data. Spin-allowed singlet singlet (S n? S 0 ) transitions Calculations Experiment Vacuum Acetone solution Acetone solution MO LR (%) k LR (nm) (E (ev)) F MO LR (%) EI (%) k LR (nm) (E (ev)) f k EI (nm) (E (ev)) f k exp (nm) Type Assign Q (0 1) L+1? H (1.97) L+0? H (1.87) (1.97) p? p Q (0 0) L+0? H-0 38 L+0? H (2.15) L+0? H (1.93) (1.96) L+1? H-0 36 L+1? H In the case of fluorescence emission spectra calculated for vacuum we observe higher energy values than for obtained for acetone solution (about 5 15%). This clearly reveals the need to incorporate of solvent effect on the fluorescence spectra. The highest occupied and lowest unoccupied molecular orbitals (MOs) of TPP1 and TPP2, obtained by using PCM/TD-DFT/B3LYP/6-31G(d)//PCM/DFT/B3LYP/6-31G(d) calculations, are illustrated in Fig. 10. Our outcomes indicate, that considered singlet singlet and triplet triplet absorption bands arise from p? p transitions. As can be seen from Tables 4 and 5 fluorescence emission spectra are dominated by emission from the two lowest unoccupied orbitals (LUMO, LUMO+1) to the two highest occupied orbitals (HOMO- 1, HOMO). The fluorescence emission process arises from p? p transitions for both compounds. It are worth noting that excitation does not result in a change in shapes and location of molecular orbital. Energy levels analysis The molecular orbital energy levels obtained in both vacuum and acetone solution is illustrated for TPP1 and TPP2 in Figs. 7 and 8. One should observe that LUMO and LUMO+1 orbitals exist in nearly double degenerate state for steps connected with ground-state geometry of both TPP1 and TPP2; these levels are separated by less than 0.03 ev. Whereas this degeneracy vanishes completely for the steps related to excited-state geometry; the distance between LUMO and LUMO+1 orbitals increases to about 0.20 ev for both porphyrins. Fig. 7. Molecular orbital energy levels of TPP1 calculated by PCM/TD-DFT/B3LYP/6-31G(d)//PCM/DFT/B3LYP/6-31G(d) for ground (a, c, and d) and excited-state geometries (b, e, and f) in vacuum (a and b) and acetone solution (c f) by using the LR (a c, and e) and EI (d and f) method [created in Chemissian based on Gaussian 09 calculations]. Fig. 8. Molecular orbital energy levels of TPP2 calculated by PCM/TD-DFT/B3LYP/6-31G(d)//PCM/DFT/B3LYP/6-31G(d) for ground (a, c, and d) and excited-state geometries (b, e, and f) in vacuum (a and b) and acetone solution (c f) by using the LR (a c, and e) and EI (d and f) method [created in Chemissian based on Gaussian 09 calculations].

11 M. Dulski et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 104 (2013) Table 6 The features of the excited and emission states (partial electric dipole moment) for TPP1 obtained for acetone solution by using linear response (LR) and external iteration (EI) PCM/TD-DFT/B3LYP/6-31G(d)//PCM/DFT/B3LYP/6-31G(d) calculations compared with experimental data. LR-PCM approach EI-PCM approach Spin-allowed singlet singlet (S 0? S n ) transitions Spin-allowed singlet singlet (S 0? S n ) transitions Electric Dipole Moment (EDM) Electric Dipole Moment (EDM) d 0? n Dipole strength f d 0? n Dipole strength f (S 0? S n ) x y z x y z ? S ? S ? S ? S ? S ? S ? S 7 Spin-allowed singlet singlet (S n? S 0 ) transitions LR-PCM approach Spin-allowed singlet singlet (S n? S 0 ) transitions EI-PCM approach d n? 0 Dipole strength f d n? 0 Dipole strength f S n? S 0 x y z x y z S 2? S 2? Table 7 The features of the excited and emission states (partial electric dipole moment) for TPP2 obtained for acetone solution by using linear response (LR) and external iteration (EI) PCM/TD-DFT/B3LYP/6-31G(d)//PCM/DFT/B3LYP/6-31G(d) calculations compared with experimental data. LR-PCM approach Spin-allowed singlet singlet (S 0? S n ) transitions Electric Dipole Moment (EDM) EI-PCM approach Spin-allowed singlet singlet (S 0? S n ) transitions Electric Dipole Moment (EDM) d 0? n Dipole strength f d 0? n Dipole strength f S 0? S n x y z x y z ? S ? S ? S ? S ? S ? S ? S 7 Spin-allowed singlet singlet (S n? S 0 ) transitions LR-PCM approach Spin-allowed singlet singlet (S n? S 0 ) transitions EI-PCM approach d n? 0 Dipole strength f d n? 0 Dipole strength f S n? S 0 x y z x y z S 2? S 2? The decrease in band gap (E gap ) at ground-state geometry causes the absorption spectrum to be red-shifted. In exactly the same way the band gap at excited-state geometry influences the fluorescence emission spectrum. This is especially true for bands that represent the HOMO LUMO configuration (in particular Q y and Q x ). Calculations indicate that the E gap for the TPP1 is smaller than the TPP2 one in all considered cases where the difference is about 0.02 ev. This is in agreement with previous results, since the wavelength shift to longer values is observed for TPP1 in absorption and fluorescence emission spectra, obtained by using both the LR-PCM and EI-PCM methods (see Figs. 3 and 6), as well as in experimental ones. It is confirmed, once again, that TPP1 can be more useful in PDT application. Moreover, there is a small difference between the band gaps calculated in two different approaches for considering solvent effects the correction implemented in EI-PCM method gives about ev lower E gap values than the LR-PCM method. This result suggests, that a shift towards the red region of the spectrum should be observed for the EI-PCM approach in relation to the LR-PCM one. However, this is in conflict with the calculated absorption and fluorescence emission spectra for spin-allowed transitions. Another contradictory relationship can also be seen if we compare the energies of the HOMO LUMO transitions presented in Tables 2 5 with the E gap values given in Figs. 7 and 8. The energies obtained from molecular orbital analysis (the steps describing absorption and fluorescence emission equaling, respectively, 2.55, 2.53, 2.23, 2.21 ev for the TPP1 and 2.56, 2.55, 2.25, 2.23 ev for the TPP2) are significantly higher than corresponding values of the absorbed and emitted photon (2.07, 2.08, 1.82, 1.88 ev for TPP1 and 2.07, 2.08, 1.84, 1.96 ev for the TPP2). Additional features analysis of dipole moments Additional information about studied porphyrins provides the analyzing of dipole moments. It is worth to notice that x and y axis are oriented in the plane of main porphyrin ring, while the z axis is

12 326 M. Dulski et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 104 (2013) perpendicular to it (see Fig. 1). The values of partial electric dipole moments for molecule in the ground state along the z axis are equal to 0.184, for TPP1 and 1.634, for TPP2, accordingly for the calculations for vacuum and taking into consideration the solvent effect. The values for the remaining axis are equal to zero. This result indicates deviation from the planarity of porphyrin ring, while a higher deformation takes place for TPP2. It should also be emphasized that according to the theory, TPP2 will be better solved in polar solvents (higher value of dipole moment than for TPP1). Partial electric dipole moments connected with S 0? S n transitions for both analyzed porphyrins are presented in Tables 6 and 7. Their values along x and y axis are different from zero. It means that the electron transitions are connected only with main porphyrin ring and are taking place between pyridine rings. Moreover bending of pyridine ring along the x axis increases the value of partial electric dipole moment along y axis. This is caused by electron pairs shifting accordingly to type of excitation. Additionally, Tables 6 and 7 show that LR-PCM and EI-PCM methods provide values similar to values in the case of Q bands, while for transitions connected with Soret band both methods give higher differences between partial electric dipole moments. Partial electric dipole moments connected with S n? S 0 transitions are changing insignificantly in relation to S 0 in case of vacuum. These che calculations show that the porphyrin ring deforms more in the presence of solvent, what results in doubling the value of partial electric dipole moment along the z axis in relation to the ground state. Moreover the LR-PCM method indicates high values of dipole force, while values for the EI-PCM approximation are significantly lower. This suggests that the effect connected with solvent influence on molecule in excited state has greater impact on calculations done using LR-PCM method opposing to the second method. Conclusions DFT and TD-DFT calculations at the TD-B3LYP/6-31G(d)//B3LYP/ 6-31G(d) level of theory were performed in order to analyze structural and optical properties of two meso-tetraphenylporphyrin derivatives which could be of interest to PDT. This was done using two different approximations (linear response LR and external iteration EI) in order to determine the influence of the solvent on both geometry and characteristics of absorption and fluorescence emission spectra. Calculations of ground-state geometry indicate planarity of porphyrin ring and the presence of solvent does not cause any important changes to bond lengths and bond angles. However, solvent solute interaction leads only to small changes of dihedral angles. Similar changes in the geometry are observed for the molecules in the first excited state in the vacuum. The solvent solute interaction has significant influence on the excited-state geometries the deformation of the porphyrin ring occurs and meaningful changes of dihedral angles between porphyrin ring and phenyl group are observed. These geometry changes might lead to strengthening of the intensity of the transition in solvent in relation to the vacuum. The results of these calculations are confirmed by the experimental data showing that the absorption spectra and the fluorescence emission spectra are red-shifted as a result of the influence of solvent on the molecule. Theoretical and experimental singlet singlet spectrum for TPP2 is insignificantly shifted in the direction of shorter wavelengths in comparison with TPP1. In case of triplet triplet spectrum an inverse trend was observed. Bands in the TPP2 spectrum are redshifted in comparison to TPP1. It is connected with introducing an additional methoxy group. Theoretical calculation allowed also to find the location of the maximum of triplet triplet transition which are difficult to designate with experiment and are important from the viewpoint of PDT. It was also shown that the E gap distance for TPP1 is smaller than in the case of TTP2 both for ground state geometry and excited state geometry, what indicates that TPP1 would be more suitable for use in PDT. Moreover, basing on calculations, the energy difference between ground singlet state and the lowest triplet state was determined. Values of energy E S T for both analyzed porphyrins are similar and sufficient for generation of 1 O 2. The conducted analysis showed differences in energy levels for next stages of calculations. First, interaction with the solvent leads to significant changes in the energy levels of TPP1 and TPP2 molecules in the ground state, and secondly the two lowest unoccupied molecular orbitals (LUMO and LUMO+1) in case of absorption spectra are degenerated why in the excited state these levels are getting apart causing elimination of degeneration. The theoretical absorption and fluorescence emission spectra shown that the Q-band can be successfully described by the Gouterman model [45] while the origin of the band B requires a more complex description the dominant contribution to the B-band coming from the lower energy levels below HOMO-1. In the case of triplet triplet spectra the molecular orbital analysis has shown noticeable impact on the structure of bands additional transitions only between occupied HOMO orbitals. The analysis of electrostatic surface potential has shown that the most important is the electronegativity located inside the porphyrin ring and on the oxygen in the hydroxyl group in TPP1. Introduction of the additional substituent in TPP2 implies only that the electronegativity is located on the oxygen in the methoxy group. To conclude, both studied porphyrins because of its spectral properties can be considered as potential dye in PDT. For both compounds the last Q band is located in the desired range of the spectrum for cancer treatment. Moreover the band gap between the singlet ground state and the lowest excited triplet state is appropriate to generate highly reactive singlet oxygen, which takes part in the destruction of cancer cells. By analyzing values of partial dipole moments it is possible to check the influence of particular substituents on solubility of compounds in polar and non-polar solutions. An important parameter for anticancer drugs is good solubility in water solutions. Additionally OCH 3 group in TPP2 causes higher value of partial dipole moment what may lead to better solubility in polar solvents. Acknowledgements The authors are deeply thankful for the possibility of calculations on the Zeus Cluster of AGH-UST CYFRONET in Cracow in the PL-Grid Project (MNiSW/IBM_BC_HS21/UŚląski/051/2011). The studies were partly financed by the project R supported by Polish National Center for Research and Development. The authors are grateful to Dr J. Peszke for his help valuable guidance. Justyna Wojcik acknowledges a scholarship from the TWING project co-financed by the European Social Fund. Marta Kempa received a grant for the project DoktoRIS Scholarship Program for Innovative Silesia co-financed by the European Union under the European Social Fund. References [1] P. Agostinis, K. Berg, K.A. Cengel, T.H. Foster, A.W. Girotti, S.O. Gollnick, S.M. Hahn, M.R. Hamblin, A. 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