3.1. IR and Proton NMR Spectroscopic Data
The IR spectra of the H
2TClPP (
1) free-base porphyrin and the [Co
II(TClPP)] (
2) and [Co
III(TClPP)Cl] (
3) starting materials are depicted in
Figures S1, S6 and S7, respectively.
Complex
4 exhibits an IR spectrum characteristic of a
meso-arylporphyrin coordination compound and confirms the presence of the nicotinoyl chloride (NTC) axial ligand (
Figure S8). Indeed, the ν(CH) stretching frequency values of the TClPP porphyrinate and the NTC axial ligand of
4 are in the 3050–2854 cm
−1 range while the ν(C=C) and ν(C=N) stretching frequency values are 1496 cm
−1. The δ(CCH) bending frequency value of
4 is shown at 1007 cm
−1. The coordination of the NTC to the cobalt is confirmed by the absorption bands at 1708 cm
−1 corresponding to the ν(C=O) stretching frequency. This value indicates that this axial ligand is coordinated to the central Co(III) metal through the pyridyl group and not the carbonyl group of the chloride acid.
1H NMR spectroscopy is a successful method to check whether a cobalt metalloporphyrin is a paramagnetic cobalt(II) complex or a diamagnetic cobalt(III) coordination complex with 3d
7 and 3d
6 fundamental state electronic configurations of the Co(II) and Co(III) cations, respectively. Cobalt(II)
meso-arylporphyrin derivatives exhibit downfield-shifted
1H NMR spectra with β-pyrrole proton chemical shift values ~16 ppm and phenyl proton values in the range 13–9.8 ppm. On the other hand, for cobalt(III)
meso-arylporphyrin complexes the β-pyrrole protons and of the phenyl ring are slightly downfield-shifted with respect to those of the related free-base porphyrins, with chemical shift values ~8.9 ppm and in the range 7.65–8.80 ppm, respectively. The NMR spectrum of complex
4 is depicted in
Figure S9 while the chemical shift values of the phenyl and the protons β-pyrrole of
1–
4 and several related cobalt(II) and cobalt(III) metalloporphyrins are listed in
Table 1. The β-pyrrole protons of
4 resonate at 9.08 ppm while the chemical shift values of the aryl protons of this complex are in the range 8.09–7.75 ppm, which confirms that our Co-TClPP-NTC derivative is a paramagnetic cobalt(III) metalloporphyrin. The protons of the NTC axial ligand resonate between 7.5 and 6.5 ppm.
3.2. Photophysical Properties
The UV-vis spectra of our four synthetic porphyrinic species (
1–
4) are depicted in
Figure 1 while in
Table 2 is reported the UV-vis data of these compounds and several similar porphyrin species.
The λ
max value of the Soret band of the free-base porphyrin H
2TClPP is 421 nm while those of the Q bands are 522, 557, 599 and 651 nm which are characteristic for free-base
meso-arylporphyrins. The insertion of cobalt leads to a reduction in the number bands Q from four to two and to a shift of the Soret band towards the blue at 414 nm. The absorption spectrum of [Co
III(TClPP)Cl] (
3) shows a red-shift Soret band at 442 nm, which is an indication of the oxidation of cobalt(II) to cobalt(III) (
Table 2). In addition, the coordination of the nicotinoyl chloride (NTC) to the Co(III) center metal of (
3) leads to an important red shift of the Soret band (455 nm) and the Q bands (560 and 696 nm). For this Co(III) pentacoordinated metalloporphyrin (
4), the significant redshift of the Soret and Q bands explain the green color of this Co(III) metalloporphyrin, which is due to the important deformations of the porphyrin core (see the crystallographic section) reported by Weiss et al. [
36].
The values of the optical gap (E
g) of
1–
4 were calculated by applying the Tauc relationship
], where
A is a constant parameter depending on the transition probability,
hν is the incident photon energy and
α is the optical absorption coefficient extracted from absorbance data [
35]. The intercept from the solid lines and the x-axis allows the determination of E
g values (
Figure S10) of
1–
4 which are 1.88, 2.01, 1.96 and 1.73 eV, respectively (
Figure S10,
Table 2). The free-base porphyrin H
2TClPP (
1) exhibits a E
g value which is lower than those of [Co
II(Telp)] (
2) and [Co
III(TClPP)Cl] (
3) which could be justify by the fact that the non-metalated porphyrin presents a higher flexibility of the porphyrin core leading to the destabilization of the HOMO-LUMO orbitals and therefore lowering the energy of the HOMO-LUMO orbitals. In the case of [Co
III(TClPP)Cl(NTC)]·CH
2Cl
2 (
4), the E
g energy (1.73 eV) is lower than that of the H
2TClPP free-base porphyrin (1.88 eV), which is related to the very important distortion of the porphyrin macrocycle of this complex (see crystallographic section).
Porphyrins compounds usually present two emission transition types: (i) the S1 → S0 transition from the S1 first excited state to the S0 ground state corresponding to the Q bands [O(0,0 and Q(0,1)] and (ii) the S2→S0 transition from the second excited state S2 to the ground state S0 corresponding to the Soret band. Given that the S2→ S0 emission transition is very weak compared to the S1→S0 transition for porphyrin species, the former fluorescence is usually not considered for porphyrins and metalloporphyrins.
The fluorescence spectra of our four porphyrinic species (
1–
4) are presented in
Figure 2 and the fluorescence parameters of these compounds and some free-base
meso-arylporphyrins and Co(II) and Co(III) related metalloporphyrins are reported in
Table 3.
As shown in
Figure 2, the H
2TClPP porphyrin (
1) presents Q(0,0) and Q(0,1) emission band values at 650 and 714 nm, respectively. For the cobaltous [Co
II(TClPP)] (
2) complex, the O(0,0) and Q(0,1) bands are blue-shifted with λ
max values of 641 and 709 nm, respectively. Our two cobalt(III) complexes (
3–
4) show blue-shifted Q(0,0) and Q(0,1) bands at 637 and 699 nm, respectively. Generally, the fluorescence quantum yields (φ
f) of free-base porphyrins are greater than those of the corresponding metalated, which is explained by the electron transfer from the donor part of the metalloporphyrin to its acceptor part. The donor part of a porphyrin complex is the porphyrin core, and the acceptor part is the central metal. Indeed, the φ
f value of
1, which is 0.089, is higher than that of
2 with a value of 0.035. The fluorescence lifetime (τ
f) values have usually the same trend than the fluorescence quantum yields. Thus, the τ
f of
1–
2 are 7.40 and 6.40 ns, respectively. For the two Co(III) complexes [Co
III(TClPP)Cl] (
3) and [Co
III(TClPP)Cl(NTC)]·CH
2Cl
2 (
4), we noticed: (i) compounds
3–
4 present very close fluorescence lifetime values (~2.40 ns), (ii) for
3, the φ
f value is as expected smaller than that of the H
2TClPP with a value of 0.051 and (iii) compound
4 exhibits high φ
f value (0.065). An explanation of this behavior could be the very important deformation of the porphyrin core, which has an important effect on the acceptor–donor characteristics of the [Co
III(TClPP)Cl(NTC)] molecule.
3.3. X-ray Molecular Structure of Complex 4
The molecular structure of our Co(III)-TClPP-Cl-NTC derivative (
4) was determined by X-ray diffraction. Single crystals were obtained by slow diffusion of
n-hexane through a dichloromethane solution containing complex
4. The crystallographic data and the refinement of the structure of this species are presented in
Table S1 while a selection of distances and angles of the same species are given in
Table S2. Complex
4 crystallizes in the monoclinic crystal system with
P21/
c space group. One [Co
III(TMPP)Cl(NTC)] molecule and one disordered dichloromethane solvent molecules are the only constituents of the asymmetric unit of
4. The ORTEP drawing of this Co(III) metalloporphyrin is illustrated in
Figure 3. For this Co(III)-NTC porphyrin derivative, the cobalt(III) central ion is coordinated by the four pyrrole N atoms of the porphyrin. The chloride and the nicotinoyl chloride (NTC) axial ligands occupy the two apical sites of the distorted square-bipyramidal coordination polyhedra.
Senge et al. [
39] reported that the macrocycle of the porphyrin presents four ideal deformations: (i) the doming deformation (
dom) due to the displacement of the metal atom out of the 24-atom mean plane of the porphyrin core and the displacement of the nitrogen atoms to the axial ligand, (ii) the ruffling distortion (
ruff) is characterized by the high values of the displacement of the
meso-carbon atoms over and below the porphyrin mean plane (
Figure 4a,b), (iii) the saddle deformation (
sad) originates from the displacement of the pyrrole rings alternatively above and below the mean porphyrin core so that the pyrrole nitrogen atoms are out of the mean plane, and (iv) the waving distortions (
wav), for which the four fragments “β-carbon-α-carbon-
meso-carbon-α-carbon-β-carbon” are alternatively below and above the mean plane of the porphyrin macrocycle.
The most structural important feature of our new cobalt(III) chloride-nicotinoyl chloride derivative (
4) is the very important ruffling and waving deformations of the porphyrin core (
Figure 4c). Thus, as shown in
Figure 5, the displacements of the
meso-carbons from the mean plane of the 24-atom porphyrin core are −67, 63, −63 and 64.10
−2 Å, indicating a very important ruffling deformation. The four fragments “β-carbon-α-carbon-
meso-carbon-α-carbon-β-carbon” also present very high displacements vis-à-vis the mean plane of the 24-atom porphyrin core with values “−26 −35 −67 −39 −30”, “+22 +37 +63 +36 +27”, “−23 −35 −63 −36 −32” and “+16 +36 +64 +42 +27” 10
−2 Å, which confirms the significant waving deformation of the porphyrin core. It is noteworthy that the deformation of the porphyrin core and especially the ruffling distortion and the variation in the equatorial mean distance between the center metal and the nitrogen atoms of the porphyrin core (M
__Np, M = center ion) are related. Thus, the greater the ruffling deformation is, the smaller the Co–Np distance is and vice versa [
39].
Inspection of
Table 4 shows that (i) cobalt(III) metalloporphyrins with significant ruffling and waving deformations present very short Co–Np distances, while those with small deformations of the porphyrin core show longer Co–Np distance values as just indicated above, and (ii) our Co(III)-Cl-NTC derivative (
4) presents a very short Co–Np bond distance of 1.947(3) Å, which is in accordance with the fact that this species exhibits very important ruffling and waving deformations. Consequently, the Soret and Q bands of the spectrum of
4 and the Q bands of the fluorescence spectrum are very red-shifted.
The important ruffling and waving deformations of the porphyrin core of
4 are most probably due to coordination of the sterically hindered nicotinoyl chloride axial ligand to the Co(III) center ion. Thus, in order to minimize the interaction of this ligand with the phenyls or the TClPP porphyrinate, the porphyrin core is twisted severally so that the hydrogen atoms of the pyridyl group of the nicotinoyl chloride axial are as far as possible from the hydrogens of the phenyl closest to the porphyrin, leading to the formation of a “ligand binding pocket” in which the nicotinoyl chloride axial is located (
Figure 4c).
There are obviously no classical hydrogen bonds in the structure of
4 for which the crystal stability is described as follows (
Figures S1–S4): (i) the chlorine Cl
2 in
para position of a phenyl group of one [Co
III(TClPP)Cl(NTC)] molecule and the C8 atom of one pyrrole ring of a neighboring TClPP porphyrinate of a [Co
III(TClPP)Cl(NTC)] complex are hydrogen bonded with a C8
__H8
…Cl
2 distance of 3.398 (4) Å, (ii) the carbon atoms C31 and C32 of one phenyl ring of one [Co
III(TClPP)Cl(NTC)] complex are H bonded to the centroid Cg4 of the N4/C16-C19 pyrrole ring and the Cl5 chloride axial ligand of the same [Co
III(TClPP)Cl(NTC)] closest neighbor, respectively (
Table S3). The C31
__H31
…Cg4 and C32
__H32
…Cl5 distance values are 3.503(3) and 3.374 (3) Å, respectively, (iii) the carbon C47 of the phenyl ring of the nicotinoyl chloride (NTC) axial ligand is weakly linked to the centroid Cg3 of the N3/C11-C14 of a nearby [Co
III(TClPP)Cl(NTC)] molecule with a C47
__H47
…Cg3 distance of 3.458(5) Å and (iv) the centroid Cg1 of the N1/C2-C4 of the later molecule is weakly H bonded to the carbons C51A and C51B of the disordered dichloromethane solvent molecule with a C50A
__H51B
…Cg1 and C50B
__H51C
…Cg1 distance values of 3.675(6) and 3.61(5) Å.
We noticed the presence of small voids 1.2 Å and a grid of 0.7 Å perpendicular to the [010] direction (
Figure S4). These voids correspond to 2% of the cell volume with a total volume of 95.12 Å
3 per cell.
3.4. Hirshfeld Surface Analysis
In order to further understand the intermolecular interactions in the crystal of compound
4, Hirshfeld surface (HS) analysis was carried out by using Crystal Explorer 17.5 [
47].
The white surfaces in the HS plotted over d
norm, indicates contacts with distances equal to the sum of van der Waals radii while the red and blue colors indicate distances shorter or longer than the van der Waals radii, respectively [
48]. As shown in
Figure 6a, the red spots in the Hirshfeld surface presented by the dnorm surface correspond to the C8
__H8
…Cl2, C32
__H32
…Cl5, C31
__H31
…Cg4, C47
__H47
…Cg3, C51A
__H51B
…Cg1 and C51B
__H51C
…Cg1. As already indicated by the PLATON calculations [
49], the most abundant contributor of the total Hirshfeld surface around complex
4 is the H…Cl/Cl…H interaction which contributed by 33.2%. The other noticeable contributors are: H…H (29.7%), C…Cl/Cl…C (7.4%), Cl-Cl (5.1%) H…O/O…H (3.6%) (
Figure 7).
Both the curvature and the shape indices can also be utilized to make identifications of typical stacking modes and the manners in which closely spaced molecules engage with each other. The shape index for
4 shows a red concave area on the surface around the acceptor atom and a blue area around the donor H-atom (
Figure 6b) [
50,
51]. The curvedness is a function of the root mean square curvature of the surface and the maps of curvedness on the HS for complex
4 indicate no plain surface patches, indicating that there is no stacking interaction between the molecules (
Figure 6c).
3.5. Cyclic Voltammetry Investigation on [CoIII(TClPP)Cl(NTC)]·CH2Cl2 (4)
The electrochemical properties of
4 were assessed by cyclic voltammetry with tetra-n-butylammonium hexafluorophosphate (TBAPF
6) used as supporting electrolyte (0.1 M) in dichloromethane under an argon atmosphere. The voltammogram of this cobalt(III) metalloporphyrin is illustrated by
Figure 8.
The reduction in [Co
III(TClPP)Cl(NTC)]·CH
2Cl
2 (
4) has uncoupled cathodic and anodic peaks that are tagged
1a (reduction) and
1b (oxidation) (
Scheme 2) and were spectro-electrochemically analyzed by Kadish et al. [
52] for the [Co
III(TPP)Cl] (TPP =
meso-tetraphenylporphyrinate) coordination compound as inlaying a Co(III)/Co(II) reduction. In the case of [Co
III(TPP)Cl], the reduction (
1a) occurs at E
cp = −0.20 V (E
cp = cathodic potential) and the reoxidation of the generated Co(II) complex (peak
1b) occurs at E
ap = 0.57 V (E
ap = anodic potential) (
Figure 8,
Table 5). For complex
4, the E
cp (reduction
1a) and E
ap (oxidation
1b) values are −0.37 V and −0.75 V, respectively (
Figure 9 Table 5). The large difference in the Ecp and E
ap for the Co(III)-Cl-TPP and the Co(III)-Cl-NTC-TClPP could be caused by the important deformation of the porphyrin core in the case of complex
4 leading to the narrowing the energy levels of the HOMO and LUMO orbitals of this species. The uncoupled Co(III)/Co(II) reactions of cobalt(III) porphyrin complexes were explained [
52] by the “box mechanism” shown in
Scheme 2. In this mechanism, the reduction and reoxidation of the [Co
III(TPP)Cl] occur via two distinct reversible electron-transfer steps, each a chemical reaction:
As reported by Kadish et al. [
52], the second reduction of [Co
III(TPP)Cl] occurs at E
cp = −0.90 V [process (3) in
Figure 8] involving Co(II)/Co(I) reduction and indicates the characteristics of a couple of chemical reactions (
Scheme 2). For our Co(III)-Cl-NTC-TClPP derivative, the E
cp value for the second reduction occurs at −1.29 V, which is shifted toward the negative potential, due probably to the significant deformation of porphyrin core of
4. The third reduction of [Co
III(TPPCl] reported by Kadish et al. involves the ring reduction of the electrogenerated [Co
I(TPP)(CH
2Cl] with E
cp value of −1.42 V. For complex
4, a similar third reduction occurs at E
cp = −1.65 V. Based on the scheme proposed by Kadish et al. for the second reduction of [Co
III(TPP)Cl] (
Scheme 3), is proposed to describe the second reduction of our Co(III) porphyrinic complex
4:
The anodic part of the cyclic voltammogram of 4 contains two reversible one-electron oxidations assigned to the oxidation of the porphyrin ring where the E1/2 values are 1.06 and 1.43 V, respectively. These values are shifted to more positive potential than those of [CoIII(TPP)Cl] (0.90 and 1.15 V, respectively) which also could be attributed to the significant deformation of the porphyrin macrocycle of 4.
The photodegradation reactions of the malachite green (MG) dye using
1–
4 as catalysts were monitored by UV spectroscopy. The λ
max of the absorption band of the MG at 618 nm was utilized to make an approximate assessment of the decolorization rate of the organic dye. The variation of λ
max of the absorption of MG dye upon radiation time, using
1–
4 is reported in
Figure S11. As shown in this figure, the degradation of MG dye did not occur without adding compounds
1–
4.
The effective degradation of MG dye (C
t/C
o versus time curve) using
1–
4 as photocatalysts indicates that [Co
III(TClPP)Cl(NTC)]·CH
2Cl
2 (
4) shows the higher degradation efficiency after 60 min of irradiation with a yield value of 95% (
Figure 9). For compounds
1–
3, the degradation yields are 90%, 80% and 84%, respectively.
The photocatalytic discoloration of a dye is believed to take place according to the following mechanism. When a catalyst is exposed to UV radiation, electrons are promoted from the valence band to the conduction band. As a result, an electron–hole pair is produced [
53], and
and
are the electrons in the conduction band and the electron vacancy (holes) in the valence band, respectively. Both these entities can migrate to the catalyst surface, where they can enter a redox reaction with other species present on the surface. In most cases,
can react easily with surface bound H
2O to produce
•OH radicals, whereas
can react with O
2 to produce superoxide radical anions of oxygen [
54] (
Scheme 4). This reaction prevents the combination of the electron and the hole that are produced in the first step. The
•OH and O
2• produced in the above manner can then react with the dye to form other species and is thus responsible for the discoloration of the dye.
The reusability of the photocatalysts was considered. Thus, our four porphyrinic derivatives (
1–
4) used as catalysts were separated by filtration, washed with distilled water after each run, then dried and further subjected to subsequent runs under the same conditions.
Figure S12 indicates that the regeneration process could be repeated for 4 cycles, without appreciable activity loss. The regenerated catalysts were also characterized by FTIR analyses after each cycle and no change was observed.
The excellent photocatalytic degradation yields, and the cyclic stable performance testified that 1–4 could be candidates for other photocatalysis reactions.