Porphyrins have delocalized π electrons. The EPR signals of these delocalized π electron systems are generated by the interaction of the porphyrin, or in the case of metal-porphyrins, by the unpaired electrons of a metal ion with the magnetic field (Fig. 2).

Fig. (2) Spectral behavior of the porphyrin 1.

The copper porphyrin complex exhibits an anisotropic EPR spectra typical of copper (II) centers with well determined hyperfine lines in the g_ǁ. All signals show the dependence g_ǁǁ> g_┴ >ge which is diagnostic of a d_x²-_y² ground state for copper (II), the hyperfine structure is obtained from the contact between delocalized π electrons and the nuclear spin I_Cu=3/2. The values obtained by simulated spectra are shown in Table 1. Additionally, in the spectra, a super hyperfine structure can be observed due to the coupling of with the nuclei of pyrrole nitrogens of Cu-porphyrin I_N =1 (shfcc). The simulated spectra show the values in Table 1.

The tetrahedral distortion that frequently occurs for square planar copper complexes is observed by the ratio f = g_ǁǁ/A_ǁǁ and it is considered for the determination of the distortion and the deviation from perfect geometry depending on the coordinated ligands [26Labanova, M.; Bidzinska, E.; Para, A. EPR investigation of Cu(II)-complexes with nitrogen derivatives of dialdehyde starch. Carbohydr. Polym., 2012, 87, 2605-2613.
[http://dx.doi.org/10.1016/j.carbpol.2011.11.034] , 27Pogni, R.; Baratto, M.C.; Diaz, A.; Basosi, R.J. Inorg. Biochemistry, 2000, 79, 333-339.
[http://dx.doi.org/10.1016/S0162-0134(99)00166-X] ]. The f value in the range 110-120 is typical for signals of the copper complex with planar geometry, however, its increase to 150 indicates a small or modest distortion of the planar symmetry, and the higher values, 180- 250, suggest a strong deviation from square planar geometry. In general, as a complex becomes more tetrahedral, the g_ǁ value and the g_iso value increase and the A_iso and A_ǁǁ values decrease [28Cline, S.J.; Wasson, J.R.; Hatfield, W.E.; Hodgson, D.J. Structure and spectroscopic properties of bis(N-cyclohexyl-3-methoxysalicylideneiminato)-copper(II). J. Chem. Soc., Dalton Trans., 1978, 1051-1057.
[http://dx.doi.org/10.1039/dt9780001051] ]. Additionally, the EPR spectra can help with an estimation of the geometry that the compounds have adopted, by using the isotropic parameters A_iso= (A_|| +2A_┴ )/3 and g_iso= (g_|| + 2g_┴ )/3. The values for planar symmetry are between A_iso= 75 -90 cm^-1 and g_iso= 2.06 - 2.11 [29Suresh, E.; Bhadbhade, M.M.; Srinivas, D. Molecular association, chelate conformation and reactivity correlations in substituted o-phenylenebis (salicylidenato) copper(II) complexes: UV-visible, EPR and X-ray structural investigations. Polyhedron, 1996, 15, 4133-4144.
[http://dx.doi.org/10.1016/0277-5387(96)00178-7] , 30Joseph, M.; Kuriakose, M.; Kurup, M.R.; Suresh, E.; Kishore, A.; Bath, S.G. Structural, antimicrobial and spectral studies of copper(II) complexes of 2-benzoylpyridine N(4)-phenyl thiosemicarbazone. Polyhedron, 2006, 25, 61-70.
[http://dx.doi.org/10.1016/j.poly.2005.07.006] ].

Table 1

EPR data of porphyrins 1, 2, 3, 6 and 7 in methanol at 295 K and at 77 K.

The results show no variation in the tetrahedral distortion (Table 1) among all compounds, since all the values are close to f = 106 cm^-1 indicating a planar geometry. No increase in the tetrahedral distortion of the porphyrin ring was observed because of ring stiffness. The values ​​found for g_iso (2.09 to 2.10) and A_iso 75 to 85 cm^-1 are consistent with a square planar geometry.

The absorption spectra of the metalloporphyrins 1, 2, 3, 5, 6 and 7 showed similar Soret or B and Q bands (Fig. 3). A small ipsochromic change of the B band by a some nm can be perceived in the pentamer porphyrin array when compared to the monomeric porphyrin Cu(II), which signifies a faint excitonic coupling amid the subunits in the porphyrin, independent of the concentration (1 x 10⁻⁶ M⁻¹ and 3 x 10⁻⁸ M⁻¹). The porphyrins are quite undisturbed and the porphyrins are alone mild electronically coupled [31Holten, D.; Bocian, D.F.; Lindsey, J.S. Probing electronic communication in covalently linked multiporphyrin arrays. A guide to the rational design of molecular photonic devices. Acc. Chem. Res., 2002, 35(1), 57-69.
[http://dx.doi.org/10.1021/ar970264z] [PMID: 11790089] ].

The difference in the absorption intensity of the porphyrin-core derivatives 1-6 in comparison with the pentaporphyrin array is caused by the porphyrin substituents on the porphyrin-connected phenyl rings. As the porphyrin derivative size intensities, no shifts in the fluorescence and absorption bands were observed, indicating that the substituents, which are targeted outward from the porphyrin center, do not engage with the dyes [32Falk, J.E. Porphyrins and Metalloporphyrins; Elsevier: New York, 1964, pp. 85-87.]. Therefore, the substituents are not assumed to change either the character of the photoexcitation or the consequent photophysical processes of the porphyrin moiety.

When excited at 425 nm, all Cu-porphyrins emit at 605 nm and at 659 nm (Fig. 4).

Table 2 shows the absorption spectral data of compounds 1-7 in CH₂Cl₂ at 24 °C, where the molar extinction coefficient (ε) of the absorbance of the compounds 1-7 showed an increase for the compounds 2-4, and decrease for the compounds 5-7, while the quantum yield showed slightly changes.

Table 2

Absorption, fluorescence profiles of porphyrins (1-7).

In Fig. (5), the surface morphology of the film from compound 7 is shown. From this image, a very regular distribution is observed. In addition, the surface coverage is complete which suggests a homogenous deposit on the ITO substrate.

The surface morphology was further analyzed using atomic force microscopy. In Fig. (6), an AFM image of compound 7, is observed, which shows a granular aspect. From this image, it is clear that the surface coverage is not homogeneous, since small defects or holes can be seen irregularly distributed on the film surface. The thickness of the film was 30nm and the mean roughness value was 1.33 nm, which nevertheless, suggests the presence of a very flat film.

In addition to the morphological characterization, the surface conductivity of the film of 7 was measured. In Fig. (7), a conductivity (V vs. I) plot of the bare ITO substrate and film in the presence and absence of light is shown. From these curves, the linear resistance of the bare ITO was calculated as 82.17 ohms, while the film shows a linear resistance of 43.05 ohms. This change suggests that the film exhibits a larger conductivity in comparison to the ITO substrate. In the presence of light, there is a slight decrease in linear resistance (42.02 ohms), which suggests a very small photoconductive response.

Fig. (3) Absorption spectra of the compounds 1-7 in CH2Cl2 at room temperature.

Solvents and reagents were purchased as reagent grade. ¹H and ¹³C NMR spectra were recorded on a Varian Unity-300 MHz in CDCl₃. The UV-Vis absorption and emission spectra were performed using a Shimadzu 2401 PC spectrophotometer and Perkin-Elmer LS-50 spectrofluorimeter. Quantum yields (ΦF) were determined using a quinine solution in CH₂Cl₂ as fluorescence standard (ΦF (std) = 0.546). All compounds were dissolved in dichloromethane (n = 1.445) and the fluorescence standard (n_std= 1.465). The EPR measurements were made in a Jeol JES-TE300 spectrometer. The g- factor values were calculated according to Weil [33Weil, J.A.; Bolton, J.R. Electron Paramagnetic Resonance; Wiley Interscience: New York, 2007. ]. The films for the photoelectric measurements were prepared on glass plates covered with a semitransparent layer of indium tin oxide (ITO) with a sheet resistance of 10 ohms/sq. The substrates were cleaned and nitrogen was dried before each evaporation. The Cu(II) porphyrin-PAMAM-Cu(II) porphyrin compound was deposited by using a NORM vacuum thermal evaporator VCM 600 working at a 10^-6 mbar pressure. The morphological properties of the film were evaluated with a JEOL FE7800F scanning electron microscope (SEM) working at 1kV in the gentle beam mode, in order to minimize the destruction and charging of the films. In addition, atomic force microscopy (AFM) images were obtained with a JEOL JSPM4210 instrument in the tapping mode to determine the morphology, thickness and roughness of the films.

Fig. (4) Emission spectra of the compounds 1-7 in CH2Cl2 at room temperature.

The photoconductive response of the film was evaluated with a Keithley 2400 digital SourceMeter instrument and an Oriel LCS-100 solar simulator lamp with an AM1.5G filter.

Fig. (5) SEM image of the metalloporphyrin film. Image magnification is 1,000X.