Novel nonperipheral octa-3-hydroxypropylthio substituted metallo-phthalocyanines: synthesis, characterization, and investigation of their electrochemical, photochemical and computational properties

The current study describes the synthesis, electrochemical, computational, and photochemical properties of octa (3-hydroxypropylthio) substituted cobalt (II) ( 4 ), copper (II) ( 5 ), nickel (II) ( 6 ) and zinc(II) ( 7 ) phthalocyanine derivatives. These novel compounds were characterized by elemental analysis,1H,13C NMR, FT-IR, UV-Vis, and MS. The redox behaviors of these metallo-phthalocyanines were investigated by the cyclic voltammetric method. The optimized molecular structure and gauge-including atomic orbital (GIAO)1H and13C NMR chemical shift values of these phthalocyanines in the ground state had been calculated by using B3LYP/6–31G(d,p) basis set. The outcomes of the optimized molecular structure were given and compared with the experimental NMR values. The photochemical properties including photodegradation and singlet oxygen generation of zinc(II) phthalocyanine were studied in DMSO solution for the determination of its photosensitizer behaviors.

In this study, the novel metallo-phthalocyanines wherein the 3-hydroxypropylthio groups connected to nonperipheral positions of the Pc macrocycle were synthesized. Electron donating sulfur groups are known to shift the Q-band to the long wavelength in nonperipheral positions that is desirable for potential PDT applications. The newly synthesized compounds have been characterized by 1 H, 13 C NMR, UV-Vis, FT-IR, micrOTOF mass, electrochemical and computational studies as well as elemental analysis. The photochemical properties such as singlet oxygen generation and photodegradation of zinc(II) phthalocyanine was also investigated to determine possible usage of this compound as a photosensitizer for cancer treatment by photodynamic therapy technique. The theoretical 1 H and 13 C NMR data of the optimized geometry were also compared with the experimental chemical shift values.

Experimental
All information about the used materials, equipment, synthesis, electrochemical measurements singlet oxygen and photodegradation quantum yields as photophysical properties and theoretical calculations were showed in the "Supplementary materials".
In the 1 H NMR spectra of nickel(II) (6) and zinc(II) (7) phthalocyanines in DMSO-d 6 , the characteristic resonances of aromatic protons were observed at δ = 7.74-7.58 or 7.98 ppm, respectively. The other signals of compound 6 and 7 due to hydroxypropyl groups as multiplets at δ = 1.95 (6) or singlet at 2.11 ppm (7) for -CH 2 -protons, broad chemical shift at δ = 3.66 (6) and dublet at 3.77 ppm (7) for OCH 2 protons, broad singlet at δ = 3.33 (6) ppm as superimposed H 2 O proton, and singlet δ = 3.45 (7) ppm for SCH 2 protons and broad peaks at δ = 4.70 (6), 4.75 (7) ppm concerning OH ( Figures S4  and S5). 13 C NMR spectra concerning C≡N signals at δ = 116.9 ppm belonging to precursor compound (3) disappeared in the case of NiPc and ZnPc formations. In addition to that, the appearance of novel signals at δ = 145.5 ppm and δ = 152.7 related to the inner core of phthalocyanines also indicated the formulation of metallo-phthalocyanine structures ( Figures S6 and S7). The other 13 C NMR data of these molecules were almost identical to those of the precursor molecule (3) as anticipated. In the MS spectra of NiPc and ZnPc measured by the micrOTOF technique proved proposed structure due to the molecular ion peaks which observed at m/z = 1291.9 [M] + and 1298.9 [M] + , respectively ( Figures S8 and S9). In the FT-IR spectra of the compounds (4-7) (Figures S10-S13), the stretching vibrations concerning C≡N groups at 2221 cm -1 belong to phthalonitrile (3) disappear after the cyclotetramerization reaction. The deformation of these vibrations confirmed the formation of phthalocyanines. The rest of the FT-IR spectra showed very close similarity to the starting compound. The mass spectra of compounds 4 and 5 recorded by micrOTOF technique also confirmed the molecular ion peaks at m/z = 1291.0 [M+H] + and 1297.2 [M] + , respectively (Figures S14 and S15).
Theoretical 1 H and 13 C NMR chemical shifts of the compound 3 and ZnPc were calculated from B3LYP/6-31G (d,p) (see in the optimized molecular structure of the ZnPc in the ground state, Figure 1). The calculated NMR resonances concerning phthalonitrile compound 3 and ZnPc were given in Tables 1 and 2, respectively. The optimized geometric parameters of the ZnPc compound (bond lengths, bond angles, and dihedral angles) by B3LYP methods with 6-31G(d,p) as the basis set were presented in Tables S1 and S2. The correlations ( Figure 2) between the experimental and calculation of

Ground state electronic absorption spectra
Phthalocyanine compounds show two strong absorption bands in their electronic absorption spectroscopy that correlate to π → π* transitions. One of them is the so-called Q-band and seen at around 600-800 nm, and the other is called as B band and  arise approximately 300-450 nm [24]. The ground-state electronic absorption spectra of Ni(II), Co(II), Cu(II) and Zn(II) phthalocyanines were measured in DMSO ( Figure 3). The most of phthalocyanines show characteristic absorption band in the visible region approximately at around 600-750 nm named the Q-band and in the UV region at around 300-400 nm named B or Soret band [25]. The Q-band absorption in DMSO of the four metallated phthalocyanine can be aligned in the sequence Ni(II) > Cu(II) > Zn(II) > Co(II). The Q-bands were seen at 811, 797, 792, and 767 nm for compounds 6, 5, 7, and 4, respectively. These single absorptions in lower energy regions should be related π→π* transitions of the phthalocyanine cores. The Q-bands of compounds are significantly red-shifted among the metallo-phthalocyanines. It is well known that the electron-releasing groups such as alkylthio are bound to eight α-benzo positions of the phthalocyanine skeleton,  the Q-band absorptions shift to longer wavelength. The transition metal ions have been settled in the phthalocyanine core may be expected they greatly affect absorption properties. Ni(II), Cu(II) and Co(II) have a similar electronegativity [26], so that the effect of electronegativity on the red-shifted is inferred to be similar [27]. The absorption maxima of 4 and 7 shifts to the shorter wavelength in the order of Co(II) and Zn(II) as central metals in the phthalocyanine core [28]. Ni(II) phthalocyanines have maximum red as an unusual shift among the metallatedphthalocyanines can be attributed the perfect planarity of the d 8 electronic configuration of this metal [14]. Aggregation is usually portrayed as a coplanar association of rings proceeding from monomer to dimer and higher order complexes. There are lots of parameters for aggregation in phthalocyanines; concentration, the nature of the solvent, nature of the substituents, complexed metal ions, and temperature [29]. The Q-band absorption maximum was independent of concentration and followed the Beer-Lambert law with a constant extinction coefficient in the studied concentration range [ Figure 4 as an example for ZnPc(7)] for all studied metallo-phthalocyanines and these phthalocyanines did not exhibit any aggregation in the studied concentration range.

Electrochemical studies
Voltammetric analyses of metallo-phthalocyanines have been performed with cyclic voltammetry (CV) as mentioned above. Figure 5 demonstrates the CV responses of the synthesized metallo-phthalocyanines recorded in the cathodic and anodic potential side in DCM:DMF (0.8:0.2)/TBP 6 electrolyte system on an ITO working electrode.
CoPc gives nonquasi-reversible metal-based reduction at 0.25 V (R1). Also, it is thought that nonquasi-reversible Pc based oxidation and reduction reactions were observed at 0.1 V(O1) and -0.25 V(R2). Most of the studies in the literature on reduction properties of the MPc complexes including that such complexes have two reduction processes as one metaland one ring-based [30,31].
Cyclic voltammetry graph of NiPc showed two oxidation peaks at 1.14 and 0.62 V and consecutive reduction peaks at 1.00 V and 0.38 V. When the electrochemical behavior of CuPc is examined, oxidation and reduction peak values have found to be lower than those of NiPc. In terms of CuPc, these values are 1.0 V and 0.56 V for oxidation and 0.18 V and 0.88 V for reduction. ZnPc had the lowest oxidation and reduction peak potential values among those of the other studied phthalocyanine derivatives. Compared to NiPc which has the oxidation peak values at 1.14 V and 0.62 V, ZnPc had lower redox peak values at 0.98 V and 0.49V. This result may be due to the smaller atomic radius of Zn metal comparing to the other metals' atomic radii.
The speed of the applied potential can be changed by changing the scan rate in cyclic voltammetry experiments. Higher peak current values were obtained at high scan rates due to a reduction in the size of the diffusion layer [32]. Figure 6 shows a series of cyclic voltammograms recorded at different scan rates for an electrolyte solution containing metallo-phthalocyanine. The linearity of the peak current values with the square root of the scan rates proved that the electrochemical reaction on the electrode surfaces was diffusion controlled as expected [32].

HOMO-LUMO studies
The electrostatic potential map of a molecule supply knowledge about the electron acceptor and electron donor regions. This knowledge may help us to see the relationships between the atoms regarding intramolecular and intermolecular hydrogen bonds. The distinctive values of the electrostatic potential at the area of the map are referred to by varied colours: blue refers to the most positive electrostatic potential, red refers to the most electronegative electrostatic potential site and green refers to the zero potential sites.
The electronic properties of a molecule can be calculated depending on HOMO and LUMO energies. In the calculations, the electron affinity (A=-LUMO) and the ionization potential (I=-HOMO) are the basic parameters. The other parameters such as absolute electronegativity (c=(I+A)/2), softness (S =(I-A)/2), and absolute hardness (h=(I-A)/2) can be calculated accordingly.  The dispersions of the HOMO and LUMO orbitals calculated for the B3LYP/6-31G(d, p) level for the compounds 3 and ZnPc were shown in Figures 7 and 8, respectively. In our calculations, ZnPc had a total of 1588 orbitals out of which 339 were filled and the rest were 1249 empty orbitals. The orbital numbered as 339 accounted for HOMO and 340 accounted for LUMO orbitals. The corresponding energy values were calculated as -4.29 eV for the HOMO and -2.42 eV for the LUMO energies with B3LYP/6-31G(d, p) level. The parameters for the 3,6-bis-(3-hydroxypropylthio)phthalonitrile were calculated at the same levels and the results were presented in Table 3.
As shown in Figure 9, the red region was localized on the nitrogen atoms and vicinity of the sulphur atoms in both the phthalonitrile and the ZnPc, whereas the blue region was delocalized on the OH groups. Hence, it was found that the  ZnPc was useful to both bond metallically, and it has intermolecularly interacted. This result also supports the evidence of the charge analyses part. NBO analysis is a tool for the determination of intramolecular interactions. The NBO analysis is used to specify the interactions between filled and empty orbitals of a molecule with the help of DFT method [33][34][35]. The NBO analysis, especially charge transfer, indicates the role of intermolecular orbital interaction in the compound. In tandem with this, the stabilization energy E (2) linked with electron delocalization between donor and acceptor is predicted for each donor NBO (i) and acceptor NBO (j) as follows: where q i is the orbital occupancy of the i th donor, E j and E i are the diagonal elements (orbital energies) and F (i,j) is the off-diagonal NBO Fock matrix element. The hyper conjugative σ→σ* interactions play an extremely significant role in the molecule represent the weak departures from a strictly localized natural Lewis structure that constitutes the primary "noncovalent" effects [36]. The results of the NBO analysis of the ZnPc collected with B3LYP/6-31G(d,p) basis set presented in Table S3.
The calculated visible absorption maxima at TD-B3LYP/6-31G(d,p) of λ which are a function of the electron availability were displayed in Table 3. The most likely transition for the molecule is the HOMO-LUMO transition at 339→340 because the maximum f = 0.4356 (oscillator strength) value is in the excited state-1 at 735.51 nm. HOMO-LUMO+1 transition was calculated at 339→341 molecular orbital, excited state-2: 735.21 nm. Typically, the energy bandgap in inorganic materials is ~ 1.5 eV, in organic materials is in the range of 1.5-3.5 eV. In accordance with this, the compound is capable of being a potential molecule for inorganic semiconductor materials [37]. Additionally, according to ligand, this value reveals that the compound becomes more conductive electrically.

Singlet oxygen generation properties
PDT is a treatment for cancer where light, molecular oxygen, and photosensitizer are used in combination to produce cytotoxic forms of oxygen such as singlet oxygen. The generation of singlet oxygen is the key to show PDT potential of the compounds [38]. The singlet oxygen production of studied zinc(II) phthalocyanine (7) was determined with the chemical method in DMSO. 1,3-diphenylisobenzofuran (DPBF) was used as a singlet oxygen scavenger which causes the formation of endoperoxide species. A time-dependent decrease of DPBF absorbance at 417 nm was observed for phthalocyanine photosensitizer 7. There was no change in the Q-band intensity during the Φ Δ determinations and it supports that studied phthalocyanine did not show any degradation by used light irradiation ( Figure 10). The singlet oxygen production of studied zinc(II) phthalocyanine (7) was found higher compared to unsubstituted zinc(II) phthalocyanine in DMSO (Figure 10, inset). The singlet oxygen generation properties of other metallo-phthalocyanines (4, 5, and 6) studied in this work did not investigate due to paramagnetic behavior of the used Co, Ni, and Cu metal ions the cavities of these phthalocyanines because paramagnetic metal ions reduce the photoactivity of the molecules.
The synthesis of the phthalocyanine compounds bearing different alkylthio groups on the phthalocyanine ring were given in the literature but the photochemical properties of these derivatives were studied limitedly. On the other hand, the phthalocyanine derivatives substituted at the nonperipheral octa positions of the phthalocyanine macrocycle are very rare in the literature. The studied zinc(II) phthalocyanine (7) showed higher singlet oxygen production in comparison with other octa nonperipheral substituted photosensitizers containing different alkylthio groups such as 2-propoxy, benzyloxy or 3,5 bis(benzyloxy)benzyloxy groups [39]. Similarly, the studied zinc(II) phthalocyanine (7) showed higher singlet oxygen production in comparison with alkylthio substituted pyridoporphyrazines [40]. Additionally, the phthalocyanine 7 exhibited similar singlet oxygen generation when compared to the nonperipherally octa-sulfanyl substituted zinc phthalocyanine [41].

Photodegradation studies
Photodegradation quantum yield can be used to study the stability of photosensitizer during the photocatalytic reaction in PDT [42]. The current study shows that photodegradation properties of studied zinc(II) phthalocyanine (7) were determined in DMSO by monitoring the collapse of their absorption bands underused light irradiation with increasing time (Figure 11). The Φ d value of zinc(II) phthalocyanine was found the order of 3.79 × 10 -5 (between 10 -3 and 10 -6 for ideal photosensitizer) in DMSO [21]. The Φ d value of the investigated zinc(II) phthalocyanine (7) was found slightly higher than unsubstituted zinc(II) phthalocyanine (Φ d = 2.61 × 10 -5 ) [18]. On the other hand, the phthalocyanine 7 exhibited lower photodegredation quantum yield value when compared to the non-peripherally octa-sulfanyl substituted zinc phthalocyanine [41] which means that the studied zinc(II) phthalocyanine (7) exhibited higher stability to light irradiation.

Conclusion
In this study, the phthalonitrile derivative substituted with 3-hydroxypropylthio groups at 3 and 6 positions as ligand and its non-peripheral octa substituted metallo-phthalociyanines [M =Zn(II), Ni(II), Cu(II) and Co(II)] were synthesized and characterized. Electrochemical properties of the proposed compounds also investigated because of these kinds of compounds show the possible potential usage in electro-catalysis, electrosensing, and electrochromic devices. When the ΔE values in Table 3 are compared, the ZnPc compound is electrically more conductive than phthalonitrile. In this electrical conductivity, zinc(II) plays an important role. Besides, the molecular geometry and GIAO 1 H and 13 C NMR chemical shift values of the molecule in the ground state had been estimated by applying B3LYP with 6-31G(d,p) basis set. Also, the photochemical properties such as singlet oxygen generation and photodegradation under light irradiations were studied for the determination of possible photosensitizer ability of zinc(II) phthalocyanine derivative 7. These properties of the other studied metallo-phthalocyanines did not investigate because of the paramagnetic behavior of metal ions (Co, Ni, and Cu) in the cavities of these phthalocyanines. The absorbance of the new zinc(II) phthalocyanine (7) was studied in DMSO solutions at different concentrations for determination of the most suitable concentration for further photochemical properties. The singlet oxygen production of this phthalocyanine (7) was determined in DMSO using a chemical method. The singlet oxygen production of studied zinc(II) phthalocyanine (7) was found higher compared to unsubstituted zinc(II) phthalocyanine in DMSO. In this study, the photodegradation behavior of the zinc(II) phthalocyanine (7) was determined in DMSO. The Φ d value of zinc(II) phthalocyanine was found in the order of 3.79 × 10 -5 in DMSO. This value is slightly higher than unsubstituted zinc(II) phthalocyanine [

Experimental
All reagents were supplied from commercial suppliers. All reactions were carried out under the argon atmosphere. All solvents were dried and purified according to standard methods [S1]. 1 H NMR and 13 C NMR spectra were measured by a Varian Mercury 300 NMR spectrometer. FT-IR and mass spectra were recorded on a Perkin-Elmer Spectrum One FT-IR spectrometer and on a micrOTOF mass spectrometer. UV-Vis spectral measurements were performed by Shimadzu UV-1601 spectrometer at room temperature. Melting points were measured by an electrothermal apparatus and are uncorrected. 3,6-dibromo phthalonitrile was synthesized according to the literature procedures [S2].

3,6-bis-(3-hydroxypropylthio)phthalonitrile (3)
A mixture of 3,6-dibromo phthalonitrile (1) (1.43 g, 5 mmol), excess amount of anhydrous Na 2 CO 3 (2.65 g, 25 mmol) and 3-mercapto-1-propanol (2) (1.15 g, 12.5 mmol) in dry DMF (25 mL) were placed in a round-bottom two flask under argon atmosphere. This suspension was stirred at 50 °C for 10 h. The reaction mixture was monitored by TLC [silica gel (chloroform:methanol)(95: 5)]. At the end of this period, the reaction mixture was cooled to room temperature and filtered. The filtrate was evaporated under reduced pressure to dryness and then the solid product purified by column chromatography on silica gel using the mixture of chloroform:methanol (95:5) as eluent to give a pale yellow solid. Yield

Electrochemical measurements
All electrochemical measurements were actualized with Ivium potentiostat using the three-electrode system at room temperature. The Pt and Ag wires were used as counter and reference electrodes, respectively. Optically transparent indium tin oxide (ITO) coated glass slides from Delta Technologies (7 × 50 × 0.5 mm thickness and 8 -12 ohm.sq −1 ) were used as a working electrode. Electrochemical characterizations of the materials were carried out in DCM/DMF (0.8 / 0.2) solution containing 0.1 M tetrabutylammonium hexafluorophosphate (TBP 6 ). The cyclic voltammetry technique was applied for the electrochemical characterization of materials. The ITO-coated glass electrode, Ag wire, and Pt wire have been plunged into the electrochemical cell. Different voltage has been applied on the working electrode through the potentiodynamic method and has been controlled using the Ivium Compact stat. The Ag wire electrode was calibrated versus Ag/AgCl (3M KCl) electrode.

Singlet oxygen quantum yields
Singlet oxygen production determinations were carried out using the experimental set-up described in the literature [S4]. Typically, a 3 mL portion of the respectively substituted zinc(II) phthalocyanine (7) solution (concentration = 1 × 10 −5 M) containing the singlet oxygen quencher was irradiated in the Q band region with the photo-irradiation set-up described in the reference [S4]. Singlet oxygen production was determined in the air using the relative method using unsubstituted ZnPc as a standard. 1,3-diphenylisobenzofuran (DPBF) was used as a chemical quencher for singlet oxygen in DMSO. To avoid chain reactions induced by DPBFin the presence of singlet oxygen [S5],the concentration of quenchers (DPBF) was lowered to ~ 3 × 10 −5 M. Solutions of sensitizer (1 × 10 -5 M) containing DPBF was prepared in the dark and irradiated in the Q-band region using the setup described and degradation of DPBF at 417 nm was monitored.

Photodegradation quantum yields
Photodegradation quantum yield (Φ d ) determinations were carried out using the experimental set-up described in the literature [S4]. Photodegradation quantum yields were determined using Equation (1), where C 0 and C t are the sample (7) concentration before and after irradiation respectively, V is the reaction volume, N A is the Avogadro's constant, S is the irradiated cell area, it is the irradiation time and I abs is the overlap integral of the radiation source light intensity and the absorption of the sample (7). A light intensity of 2.17 × 10 16 photons s -1 cm -2 was employed for Φ d determinations.

Theoretical calculations
Density functional theory (DFT) is one of the most useful quantum chemistry tools in calculating the ground state properties of compounds. In the modeling, the initial guess of the compound was provided from the X-ray coordinates. The molecular structures were optimized to get the global minima by using DFT/B3LYP/6-31G (d,p) level in the gas phase. The electronic properties were also calculated using 6-31G(d,p) and 6-31G+(d, p) levels. All the calculations were carried out with the Gaussian 16 B.01 [S6] package program and GaussView 6.0. 16 [S7] was used for the visualization of the structure. The 1 H and 13 C NMR chemical shielding constants were calculated using GIAO-B3LYP in the gas phase and DMSO. For the NBO analysis, the same calculation procedure in the gas phase was also used. The 1 H and 13 C-NMR chemical shifts were converted to the TMS scale by subtracting the calculated absolute chemical shielding of TMS (d = ∑ 0 -∑), where d is the chemical shift, ∑ is the absolute shielding and ∑ 0 is the absolute shielding of TMS, whose values (reference shielding for 1 H and 13 C) are at 31.883 ppm and 191.80 ppm, respectively, for B3LYP/6-31G(d,p). Besides, molecular electrostatic potential (MEP) of the title molecules were investigated by B3LYP/6-31G(d,p). The energy difference between HOMO and LUMO levels was described as the optical bandgap for the HOMO to LUMO excitation energy (TDDFT) and the electronic band gap for excitation energy difference (ΔE = LUMO-HOMO). The visible absorption maxima of the molecule were corresponded to the electron transition from HOMO to LUMO by using calculations of molecular orbital geometry. Figure S1. 1