Unsymmetrical nonplanar ‘push–pull’ β-octasubstituted porphyrins: facile synthesis, structural, photophysical and electrochemical redox properties

Pinki Rathi a, Ray Butcher b and Muniappan Sankar *a
aDepartment of Chemistry, Indian Institute of Technology Roorkee, Roorkee - 247667, India. E-mail: sankafcy@iitr.ac.in; Tel: +91-1332-286753
bDepartment of Chemistry, Howard University, Washington, DC 20059, USA

Received 4th July 2019 , Accepted 3rd September 2019

First published on 3rd September 2019


Mixed substitution at the β-position of porphyrins influences their photophysical and electrochemical redox properties. Two new series of asymmetrically mixed β-octasubstituted porphyrins viz. MTPP(Ph)2Br5X (X = NO2 or Br and M = 2H, Co(II), Ni(II), Cu(II), and Zn(II)) have been synthesized and characterized by various spectroscopic techniques. The single crystal X-ray structure of H2TPP(NO2)(Ph)2Br5 showed a nonplanar saddle shape conformation of the macrocyclic core. Furthermore, the fully optimized geometries confirmed the saddle shape conformation of H2TPP(Ph)2Br5X (X = NO2 or Br). Electronic spectra revealed a significant bathochromic shift by appending both electron donor and acceptor substituents at the β-position of the meso-tetraphenylporphyrin skeleton, which reflects the following order H2TPP < H2TPP(NO2) < H2TPP(NO2)(Ph)2 < H2TPP(Ph)2Br6 < H2TPP(NO2)(Ph)2Br5. H2TPP(Ph)2Br5X (X = NO2 or Br) exhibited a significant bathochromic shift (Δλmax = 53–61 nm) in the Soret band and (Δλmax = 90–95 nm) in the longest wavelength Qx(0,0) band as compared to H2TPP. Nonplanar conformations and electron withdrawing β-substituents induce higher protonation and deprotonation constants for H2TPP(NO2)(Ph)2Br5 and H2TPP(Ph)2Br6 as compared to precursor porphyrins viz. H2TPP, H2TPP(NO2) and H2TPP(NO2)(Ph)2. The electronic spectral properties and redox potentials of MTPP(Ph)2Br5X (X = NO2 or Br and M = 2H, Co, Ni, Cu and Zn) are affected by β-substituents at the periphery of the porphyrin core. Redox tunability was achieved by appending push–pull substituents at the β-position of the MTPP (M = 2H, CoII, NiII, CuII, and ZnII) skeleton of the macrocycle. CuTPP(Ph)2Br6 and CuTPP(NO2)(Ph)2Br5 exhibited a dramatically reduced HOMO–LUMO gap with a difference of 0.55 V and 0.62 V, respectively as compared to CuTPP due to the push–pull effect of β-substituents and nonplanarity of the porphyrin core.


Porphyrin is a colored, tetrapyrrolic pigment ubiquitous in nature. Porphyrin and its analogues are highly aromatic due to its 18-π electron system and are involved in many biological processes such as photosynthesis, electron transfer, detoxification, oxygen transport and storage.1,2 Porphyrins are used as models for heme proteins due to their conformational flexibility. They have many peripheral positions which are available for functionalization.3 The modulation of physicochemical properties of synthetic porphyrins can be studied by the introduction of β-substituents either as electron donors or as electron acceptors at the periphery of the macrocycle since these substituents are in direct conjugation with the porphyrin π-system.4 β-Nitroporphyrins are useful precursors to synthesize new derivatives with improved features which can be used for various material and medicinal applications. Curiosity in the synthesis of a new family of porphyrins arises due to the application of these compounds in the fields of biology, supramolecular chemistry and materials science.5 Mixed substituted porphyrins and their metal complexes have potential applications in molecular sensors,6 catalysis,7 dye-sensitized solar cells (DSSC),8 photodynamic therapy (PDT)9 and nonlinear optics (NLO).10 Increasing the number of β-substituents induces steric repulsion between the substituents and enhances the nonplanar conformation of the macrocyclic core and they serve as model compounds for heme.11,12 Perhalogenated metalloporphyrins exhibit enhanced robustness towards oxidative degradation so these porphyrins serve as model compounds for heme enzymes such as cytochrome P450.13 The substitution at the peripheral β-position of the macrocycle leads to the modulation of physicochemical and electrochemical redox properties of porphyrins.14Meso-substituted porphyrins exert less steric and electronic effects on the porphyrin π-system as compared to β-positions. β-Substitution at the periphery of the porphyrin core exerts steric repulsion between the substituents and as a result the pyrrole nitrogens incline from the plane of the porphyrin core to reduce the repulsion between peripheral substituents which results in nonplanarity of the macrocyclic core.15 β-Haloporphyrins are utilized as efficient catalysts for epoxidation and hydroxylation reactions.16 Highly substituted macrocycles exhibit exclusive physicochemical properties.17 Kadish and other research groups have reported the electrochemical studies of various mixed substituted porphyrins.18,19 β-Nitroporphyrins are good precursors for the synthesis of quinoxaline-fused porphyrins which have applications in dye-sensitized solar cells (DSSC).20 Our research group has synthesized β-nitroporphyrins with various functional groups such as thienyl, phenyl, phenylethynyl, methyl and cyano.18b The introduction of electron donor group(s) at the β-pyrrole position of nitroporphyrins leads to a ‘push–pull’ character which plays a vital role in NLO, anion sensing, DSSC, etc.4a,10c,21 In general, unsymmetrical mixed β-substituted push–pull porphyrins exhibit tunable photophysical and electrochemical properties with a high ground state dipole moment which are suitable for NLO application.4a,10c,18b,21,22 Several β-bromoporphyrins are known for catalytic application. Our research group has synthesized oxidovanadium perhaloporphyrins and utilized them for selective epoxidation and oxidative bromination reactions in aqueous media.7b,e Hence mixed β-substituted porphyrins have potential applications in NLO, anion sensing, DSSC and catalysis. Herein, we report the influence of mixed β-substitution with three different substituents on the electrochemical redox and photophysical properties of the porphyrin π-system. Hence we synthesized two new series of mixed β-substituted push–pull porphyrins viz. MTPP(Ph)2Br5X (X = NO2 or Br and M = 2H, Co(II), Ni(II), Cu(II) and Zn(II)) and explored their structural, photophysical and electrochemical redox properties. These porphyrins were synthesized for the first time and show a highly bathochromic shift in the absorption spectra and enhanced nonplanarity of the macrocyclic core as compared to MTPP (M = 2H, Co(II), Ni(II), Cu(II) and Zn(II)). Reports on β-substituted porphyrins are limited as compared to meso-functionalized porphyrins due to the lack of synthetic methodologies whereas the latter ones have been widely explored due to their facile synthesis and functionalization.23

Synthesis and characterization

Two new series of mixed β-octasubstituted porphyrins have been synthesized and characterized by various spectroscopic techniques. Firstly, H2TPP was synthesized according to the reported literature followed by Cu(II) insertion in the core.24 Then, mono-nitration of CuTPP was carried out with 80% yield followed by acid demetalation yielding H2TPP(NO2). Furthermore, the regioselective dibromination of H2TPP(NO2) was carried out in order to synthesize H2TPP(NO2)Br2 which was subjected to the Pd catalyzed Suzuki cross coupling reaction with phenylboronic acid and resulted in the formation of H2TPP(NO2)Ph2.18b Then Ni(II) insertion of H2TPP(NO2)(Ph)2 was carried out in DMF. The bromination of NiTPP(NO2)(Ph)2 with 40 equivalents of liquid bromine resulted in the formation of NiTPP(NO2)(Ph)2Br5 and NiTPP(Ph)2Br6. NiTPP(Ph)2Br6 was obtained as the first fraction with 40% yield and the second fraction was NiTPP(NO2)(Ph)2Br5 with 35% yield. Subsequently, free base porphyrins were obtained by acid demetallation using con. H2SO4 in good yields (86–88%) as shown in Scheme 1. Metal (CoII, CuII and ZnII) complexes were prepared with excellent yields (90–94%) by conventional methods as described in the literature.22,23 The synthesized porphyrins (Scheme 1) were characterized by various spectroscopic techniques such as UV-vis, fluorescence, NMR (Fig. S1–S8 in the ESI), MALDI mass spectrometry (Fig. S9–S18 in the ESI), single crystal X-ray and elemental analyses.
image file: c9dt02792k-s1.tif
Scheme 1 Schematic scheme for the mixed β-octasubstituted porphyrins.

Mixed substituted porphyrins exhibited proton signals for meso-phenyl, β-substituents, and core imino protons. H2TPP(NO2)(Ph)2 shows proton resonance for β-pyrrole protons in the range of 8.98–8.58 ppm, for meso-phenyl 8.30–7.23 ppm and for β-phenyl 6.90–6.80 ppm.18b The meso-phenyl protons of H2TPP(Ph)2Br5X (X = NO2 or Br) were slightly upfield shifted (Δδ = 0.1–0.4 ppm) as compared to H2TPP(NO2)(Ph)2 whereas a marginal shift was observed in the case of β-phenyl protons of H2TPP(Ph)2Br5X (X = NO2 or Br). The core imino protons of H2TPP(Ph)2Br6 exhibited a characteristic proton signal at −1.64 ppm which is 0.67 ppm downfield shifted as compared to H2TPP(NO2)(Ph)2, due to the electron withdrawing effect of β-bromo groups and nonplanarity of the ring which reduces the ring current of the porphyrin π-system. In the 1H NMR spectra of MTPP(Ph)2Br5X (X = Br or NO2) (M = Zn(II) and Ni(II)) complexes, no –NH signal was found due to the presence of metal ions in the porphyrin core. The resonances of meso-phenyl protons for NiTPP(Ph)2Br5X (X = Br or NO2) are slightly upfield shifted (Δδ = 0.14–0.28 ppm) as compared to H2TPP(Ph)2Br5X (X = Br or NO2). β-Phenyl protons of NiTPP(Ph)2Br5X (X = Br or NO2) exhibit a marginal (Δδ = 0.04–0.13 ppm) upfield shift. In the case of ZnTPP(Ph)2Br5X (X = Br or NO2), meso-phenyl proton resonances are marginally upfield shifted (Δδ = 0.11–0.17 ppm) and β-phenyl protons exhibit a marginal (Δδ = 0.06–0.15 ppm) upfield shift as compared to H2TPP(Ph)2Br5X (X = Br or NO2). The NMR spectra of these porphyrins are shown in Fig. S1–S8 and the MALDI-TOF mass spectra of all synthesized porphyrins are shown in Fig. S9–S18 in the ESI. The observed molecular ion peaks are matching with the proposed structures.

Single crystal X-ray structure

The X-ray quality single crystals of H2TPP(NO2)(Ph)2Br5 were obtained by slow diffusion of CH3OH into the CH2Cl2 solution of H2TPP(NO2)(Ph)2Br5 at 298 K. The ORTEP diagram of H2TPP(NO2)(Ph)2Br5 is shown in Fig. 1. Crystallographic data of H2TPP(NO2)(Ph)2Br5 are given in Table S1 in the ESI. This porphyrin was crystallized in a monoclinic system with the P21/c space group. The crystal packing diagram of H2TPP(NO2)(Ph)2Br5 is shown in Fig. 2. Selected average bond angles and bond distances are listed in Table S2 in the ESI.
image file: c9dt02792k-f1.tif
Fig. 1 ORTEP diagram of H2TPP(NO2)(Ph)2Br5.

image file: c9dt02792k-f2.tif
Fig. 2 Crystal Packing Diagram of H2TPP(NO2)(Ph)2Br5 in a unit cell.

The peripheral β-substituents of free base porphyrins induce nonplanarity in the macrocyclic core. Furthermore, it was enhanced by two CH3OH solvent molecules which undergo hydrogen bonding with core imino nitrogens. The Cβ–Cβ bond length (1.366 Å) of H2TPP(NO2)(Ph)2Br5 (bearing two Ph, one NO2 and one Br group) is longer than Cβ′–Cβ′ (1.273 Å) (bearing four Br groups) due to the presence of mixed β-substituents having different shapes and sizes at the periphery of the macrocycle. The pyrrole units of H2TPP(NO2)(Ph)2Br5 tilted up and down alternatively from the mean plane of the porphyrin core. The Cβ–Cβ substituents tilted to one face of the porphyrin and the Cβ′–Cβ′ substituents tilted to the other face of the porphyrin macrocycle. The increment in the Cβ–Cα–Cm bond angle with the decrement in the N–Cα–Cm revealed the nonplanar conformation of the porphyrin core. Hence H2TPP(NO2)(Ph)2Br5 exhibited a saddle shape nonplanar conformation with an average deviation of 24 core atoms (Δ24) by ±0.558 Å and a displacement of β-carbon (ΔCβ) by ±1.23 Å from the mean plane of the porphyrin macrocycle. Overall, H2TPP(NO2)Ph2Br5 exhibited an enhanced nonplanar conformation as compared to the precursor (H2TPP(NO2)(Ph)2) (ΔCβ = ±0.671 Å and Δ24 = ±0.320 Å) due to the steric effect of five β-bromo substituents at the periphery of the porphyrin macrocycle.18b

DFT studies

The DFT studies of mixed β-octasubstituted free base porphyrins (H2TPP(Ph)2Br5X (X = Br or NO2) were carried out in the gas phase using the B3LYP and LANL2DZ functional and basis set, respectively.25 Average bond angles and bond lengths are given in Table S3 in the ESI. Fig. S19 shows the optimized geometries (top and side views) of the synthesized free base porphyrins and the deviation of core atoms from the mean plane of the porphyrin.

To reduce the repulsion between the β-substituents observed at the periphery, the pyrrole rings are tilted up and down (saddle shape conformation) with respect to the mean plane of the porphyrin core. The bending of the pyrrole ring increases the Cβ–Cα–Cm bond angles as well as the Cβ–Cβ bond length with a decrease in in N–Cα–Cm bond angles. H2TPP(Ph)2Br5X (X = NO2 or Br) exhibits a 3–4° change in the Cβ–Cα–Cm bond angles as compared to H2TPP which evidences the enhanced nonplanarity in the mixed β-substituted porphyrins. While there is no more change in the Cβ–Cβ bond length of H2TPP(Ph)2Br5X (X = NO2 or Br), the N–Cα–Cm bond angle of H2TPP(Ph)2Br5X (X = NO2 or Br) decreased by 1.4–1.8° as compared to the precursor H2TPP(NO2)(Ph)2 and 3–4° compared to H2TPP. The increase in the Cβ–Cβ bond length and Cβ–Cα–Cm bond angles and the decrement in the bond angles of N–Cα–Cm revealed the nonplanar conformation of the porphyrin core. Furthermore, the bromo substitution at the remaining β-positions exhibited high nonplanarity as compared to H2TPP. The bond length of Cβ′–Cβ′ (1.391–1.392 Å) of bromo containing β-pyrrole in H2TPP(Ph)2Br5X (X = NO2 or Br) is longer relative to the Cβ–Cβ bond distance (1.383–1.389 Å) bearing four β-substituents. H2TPP(Ph)2Br6 exhibited comparable ΔCβ with respect to H2TPP(NO2)(Ph)2Br5 which indicates that H2TPP(Ph)2Br6 and H2TPP(NO2)(Ph)2Br5 have comparable nonplanarity. Both H2TPP(NO2)(Ph)2Br5 and H2TPP(Ph)2Br6 exhibited a higher nonplanarity (ΔCβ = ±1.238 and ±1.251 Å respectively) as compared to H2TPP(NO2)(Ph)2 (ΔCβ = ±0.671 Å). Hence, the optimized geometries of H2TPP(Ph)2Br5X (X = NO2 or Br) revealed an enhanced nonplanar saddle shape conformation of the macrocyclic ring.

Electronic spectral studies

Optical absorption spectra of MTPP(Ph)2Br5X (M = 2H, Co(II), Ni(II), Cu(II), Zn(II) and X = NO2 or Br) are influenced by the shape, size and electronic nature of β-substituents. The electronic spectra of all synthesized porphyrins were measured in CH2Cl2 at 298 K. The optical absorption spectra of H2TPP(Ph)2Br6, and H2TPP(NO2)(Ph)2Br5 are shown in Fig. 3a. The increment in the number of substituents at the β-pyrrole positions of porphyrins induced nonplanarity in the macrocyclic system which results in a bathochromic shift in the absorption spectra due to the enhanced steric repulsion between the β-substituents which results in the destabilization of HOMOs.26 The Soret band of H2TPP(Ph)2Br5X (X = NO2 or Br) is broader than that of H2TPP due to the distortion of the macrocyclic ring (nonplanar conformation) and intramolecular charge transfer. The Soret and longest wavelength Qx(0,0) bands of H2TPP(Ph)2Br5X (X = NO2 or Br) exhibit a Δλmax = 53–61 nm and Δλmax = 90–95 nm dramatic bathochromic shift, respectively as compared to H2TPP. H2TPP(NO2)(Ph)2Br5 exhibited an 8 nm red shift in the Soret band and 3 nm blue shift in the Qx(0,0) band as compared to β-octabromotetraphenylporphyrin, H2TPPBr8 due to the electron withdrawing effect of the –NO2 group. H2TPP(Ph)2Br6 exhibited a 1 nm (marginal shift) and 8 nm blue shift in the Soret band and Qx(0,0) band, respectively as compared to H2TPPBr8 due to the presence of two phenyl groups in H2TPP(Ph)2Br6. As a result the HOMO–LUMO gap decreased as compared to H2TPPBr8. The optical absorption spectra of MTPP(Ph)2Br5X (M = Co(II), Ni(II), Cu(II), Zn(II) and X = NO2 or Br) are shown in Fig. 3c and d. MTPP(NO2)(Ph)2Br5 (M = CoII, NiII, CuII and ZnII) exhibited a red shift in the Soret band but a blue shift in the Qx(0,0) band as compared to MTPPBr8 (M = CoII, NiII, CuII and ZnII). MTPP(Ph)2Br6 (M = CoII, NiII, CuII and ZnII) exhibited an ∼4 nm blue shift in the Soret band and 4–8 nm blue shift in the Qx(0,0) band as compared to MTPPBr8, due to the presence of a higher number of Br groups in MTPPBr8. The charge transfer from the HOMO of the porphyrin core to the LUMO of the nitro group is attributed to a greater FWHM (full width half maximum) of H2TPP(NO2)(Ph)2Br5 as compared to H2TPP and H2TPP(Ph)2Br6. The bathochromic shift in the Soret band and longest wavelength Qx(0,0) band was observed due to the electron withdrawing nature of β-substituents and the nonplanar conformation of the macrocyclic core which follows the order: H2TPP < H2TPP(Ph)2Br6 < H2TPP(NO2)(Ph)2Br5.
image file: c9dt02792k-f3.tif
Fig. 3 (a) Optical absorption spectra of H2TPP(NO2)(Ph2)Br5 and H2TPP(Ph)2Br6, (b) emission spectra of H2TPP, H2TPP(NO2)(Ph)2Br5 and H2TPP(Ph)2Br6. (c) Optical absorption spectra MTPP(NO2)(Ph)2Br5 and (d) MTPP(Ph)2Br6, M = Co(II), Ni(II), Cu(II), Zn(II) in CH2Cl2 at 298 K.

The synthesized MTPP(Ph)2Br5X (M = 2H, Zn, X = NO2 or Br) porphyrins were characterized by fluorescence spectroscopy to determine the effect of mixed substitution at β-positions of the porphyrin macrocycle. The photophysical data of MTPP(Ph)2Br5X (X = NO2 or Br and M = 2H, Zn) are listed in Table 1. H2TPP(Ph)2Br5X (X = NO2 or Br) exhibits extremely weak emission due to the nonplanar conformation of the porphyrin core and a heavy atom effect of bromo substituents. The fluorescence spectra of H2TPP, H2TPP(Ph)2Br6, and H2TPP(NO2)(Ph)2Br5 are compared as shown in Fig. 3b. H2TPP(NO2)(Ph)2Br5 exhibited an extremely low quantum yield as compared to H2TPP due to the heavy atom effect of bromo substituents and the nonplanar conformation of the porphyrin core. We were unable to calculate the quantum yield of H2TPP(Ph)2Br6 due to the feeble intensity of the porphyrin. The fluorescence spectra of ZnTPP(Ph)2Br5X (X = NO2 or Br) are shown in Fig. S20 in the ESI.

Table 1 Photophysical data of synthesized free base and Zn(II) Porphyrins in CH2Cl2 at 298 K
Porphyrin λ ex, nm FWHM (nm) λ em, nm ϕ f τ (ns)
λ ex = Excitation wavelength (nm), λem = Emission wavelength (nm), FWHM – full width half maximum, ϕf – quantum yield relative to H2TPP and ZnTPP in CH2Cl2, τ represents excited state lifetime.
H2TPP(NO2)(Ph)2Br5 477 57 816 0.003 0.0985
H2TPP(Ph)2Br6 469 39 808 0.0059
ZnTPP(NO2)(Ph)2Br5 472 39 769, 807 0.001 0.0503
ZnTPP(Ph)2Br6 463 29 720 0.0016 0.1760


Protonation and deprotonation studies

The acid–base properties can be tuned by appending the mixed substituents at β-pyrrole positions of the macrocycle. Many research groups have reported the acid–base properties of nonplanar porphyrins.27 The protonation and deprotonation studies of H2TPP(Ph)2Br5X (X = NO2 or Br) have been carried out to examine the influence of mixed substitution on the porphyrin π-system. These studies were performed in toluene using TFA and TBAOH, respectively. Fig. 4 and S21 in the ESI show the absorption spectral variations of H2TPP(NO2)(Ph)2Br5 and H2TPP(Ph)2Br6, respectively in toluene. H2TPP(NO2)(Ph)2Br5 has shown UV-vis spectral changes upon increasing the conc. of TFA (0.42–9.31 × 10−6 M) and tetrabutylammonium hydroxide (0.39–2.34 × 10−6 M), respectively. In the case of H2TPP(NO2)(Ph)2Br5, the absorption band at 477 nm with a concomitant rise in the new band at 501 nm shows a 24 nm red shift in the Soret band upon increasing the conc. of TFA. As protonation proceeds, H2TPP(NO2)(Ph)2Br5 exhibited a new Q band at 757 nm in place of multiple Q bands which was broader and 17 nm red shifted as compared to previous bands. Fig. S21a (inset) shows the electronic spectral changes of H2TPP(Ph)2Br6 in toluene upon increasing the conc. of TFA (0.12–1.14 × 10−5 M) and TBAOH (0.19–1.69 × 10−5 M), respectively. In the case of H2TPP(Ph)2Br6, a decrement in the absorption intensity at 469 nm and a concomitant rise at 496 nm were observed upon increasing the conc. of TFA. The red-shift was found to be a 27 and 13 nm bathochromic shift in Soret and Qx(0,0) bands, respectively. In both cases, the Hill plot showed a slope value of 2 (Fig. 4a inset and S21a inset for H2TPP(NO2)(Ph)2Br5 and H2TPP(Ph)2Br6, respectively) which clearly indicates the formation of diprotonated porphyrin. As shown in Table 2, H2TPP(NO2)(Ph)2Br5 exhibited an 87 fold lower protonation constant (β2) as compared to H2TPP(NO2)Br6 due to the strong electron withdrawing nitro substituent. Fig. 4b shows the decrement in the absorbance at 477 nm of H2TPP(NO2)(Ph)2Br5 and the appearance of a new band at 521 nm upon increasing the conc. of TBAOH. With the disappearance of all Q bands, a new band arises at 779 nm for H2TPP(NO2)(Ph)2Br5. Nonplanarity and the electronic nature of substituents at the β-position of the porphyrin core influence the deprotonation. H2TPP(Ph)2Br6 shows an 89 fold higher protonation constant (β2) than H2TPP(NO2)(Ph)2Br5 which shows the electron withdrawing effect of the nitro group on core basicity. The inset of Fig. S21b shows the decrement in the absorption band at 469 nm of H2TPP(Ph)2Br6 while another band appears at 505 nm. A new band appears at 740 nm while all other Q bands diminish. Fig. 4b (inset) and Fig. S21b (inset) show the Hill plots which exhibit a slope value of 2 revealing the formation of dianionic species in both cases. H2TPP(Ph)2Br6 shows an 83 fold higher deprotonation constant than H2TPP(NO2)(Ph)2Br5. The logβ2 value of free base porphyrins follows the order: H2TPP(Ph)2Br6 > H2TPP(NO2)(Ph)2Br5 which suggests that the effect of increased nonplanarity of the porphyrin core and the electronic nature of β-substituents influence the acid–base behavior of the porphyrin core.
image file: c9dt02792k-f4.tif
Fig. 4 UV-vis spectral titration of H2TPP(NO2)(Ph)2Br5 with (a) TFA and (b) TBAOH in toluene at 298 K, respectively. Insets show the consistent Hill plots.
Table 2 Protonation and deprotonation constants of H2TPP(NO2)(Ph)2Br5 and H2TPP(Ph)2Br6 in comparison with H2TPP(NO2)Br5 in toluene at 298 K
Porphyrin TFA TBAOH
log β2 Slope r 2 log β2 Slope r 2
H2TPP(Ph)2Br6 12.15 2.20 0.85 12.23 2.30 0.93
H2TPP(NO2)Br6 11.29 2.32 0.93 11.54 2.05 0.94
H2TPP(NO2)(Ph)2Br5 10.20 1.90 0.89 10.31 2.10 0.95


Electrochemical redox properties

To examine the influence of mixed electron-donor and acceptor substituents at the β-position of the porphyrin macrocycle, the electrochemical studies of mixed β-octasubstituted porphyrins were carried out using cyclic voltammetry (CV) in CH2Cl2 containing TBAPF6 at 298 K. The electrochemical redox data (vs. Ag/AgCl) are summarized in Table 3.
Table 3 Redox potential data of synthesized porphyrins and their precursors (V vs. Ag/AgCl) in CH2Cl2 containing 0.1 M TBAPF6 with scan rate 0.1 V s−1 at 298 K
Porphyrin Oxidation (V) Reduction (V) ΔE (V) MII/III MII/I
I II I II III
a Refers to irreversible potential. b Data obtained from DPV.
H2TPP 1.00 1.34 −1.23 −1.54 2.23
H2TPPNO2 1.10 1.28 −0.87 −1.08 1.97
H2TPP(NO2)(Ph)2 1.00 1.13 −0.85 −1.03 1.85
H2TPP(NO2)(Ph)2Br5 1.27 1.63a −1.06 −1.29 2.33
H2TPP(Ph)2Br6 0.84 1.16 −0.80 −1.17 1.64
CoTPP 1.06 1.31 −1.38 2.44 0.85 −0.86
CoTPPNO2 1.17 1.42 −1.30 2.47 0.91 −0.66
CoTPP(NO2)(Ph)2 1.13 1.34 −1.29 2.42 0.88 −0.64
CoTPP(NO2)(Ph)2Br5 1.33 1.47 −1.16 −1.64b 2.49 0.95 −0.34
CoTPP(Ph)2Br6 1.35 −1.38b 0.92 −0.42
NiTPP 1.02 1.32 −1.28 −1.72 2.30
NiTPPNO2 1.19 1.32 −0.95 −1.21 2.14
NiTPP(NO2)(Ph)2 1.12 1.24 −0.94 −1.20 2.06
NiTPP(NO2)(Ph)2Br5 1.27 1.95 −0.77 −1.03 2.04
NiTPP(Ph)2Br6 1.22 −0.92 −1.24 2.14
CuTPP 0.99 1.36 −1.36 −1.73 2.35
CuTPPNO2 1.08 1.44 −0.97 −1.22 2.05
CuTPP(NO2)(Ph)2 0.96 1.38 −0.96 −1.21 1.92
CuTPP(NO2)(Ph)2Br5 0.98 1.55 −0.74 −1.01 1.72
CuTPP(Ph)2Br6 0.88 1.46 −0.91 −1.19 1.79
ZnTPP 0.84 1.15 −1.36 −1.77 2.20
ZnTPPNO2 0.91 1.22 −1.05a 1.19 −1.50 1.96
ZnTPP(NO2)(Ph)2 0.85 1.07 −1.06a −1.20 −1.49 1.91
ZnTPP(NO2)(Ph)2Br5 0.89 1.17 −0.98 −1.22 1.87
ZnTPP(Ph)2Br6 0.84 1.07 −0.98 −1.17 1.82


All mixed substituted porphyrins exhibited two successive reversible one electron oxidation and one electron reduction waves in CV studies. The comparative cyclic voltammograms of Cu(II) and Co(II) porphyrins are shown in Fig. 5 and 6, respectively. The first ring reduction potential of CuTPP was found at −1.36 V. After nitration of CuTPP, the first ring reduction potential was 0.39 V anodically shifted as compared to CuTPP (Fig. 5). Furthermore, diphenyl substitution at the antipodal position of –NO2 exhibited a marginal shift in reduction potential as compared to CuTPPNO2 whereas the first oxidation potential was cathodically shifted (0.13 V) due to the electron donating phenyl substituents. The first ring reduction potential of CuTPP(NO2)(Ph)2Br5 was found at −0.74 V which led to a drastic anodic shift (0.62 V) as compared to CuTPP because of the electron accepting substituents at the β-pyrrole position of the porphyrin core and 0.22–0.23 V anodic shift as compared to CuTPP(NO2) and CuTPP(NO2)(Ph)2. While in the case of CuTPP(Ph)2Br6, the first reduction potential exhibited a 0.45 V anodic shift as compared to CuTPP and a 0.17 V cathodic shift than CuTPP(NO2)(Ph)2Br5 (Fig. 5). CuTPP(NO2)(Ph)2Br5 exhibited the first oxidation and reduction potentials which were 0.07 V and 0.12 V anodically shifted, respectively as compared to CuTPPBr8 due to the presence of the strong electron withdrawing –NO2 group at the β-position of the porphyrin macrocycle. CuTPP(Ph)2Br6 exhibited the first oxidation and reduction potentials which were 0.03 V and 0.05 V cathodically shifted with respect to CuTPPBr8 due to a smaller number of bromo groups and two electron donating phenyl groups at the β-position of the porphyrin macrocycle which made this porphyrin easy to oxidize and difficult to reduce as compared to both CuTPPBr8 and CuTPP(NO2)(Ph)2Br5. These porphyrins follow the order of the first reduction potential,CuTPP < CuTPPNO2 < CuTPP(NO2)(Ph)2 < CuTPP(Ph)2Br6 < CuTPPBr8 < CuTPP(NO2)(Ph)2Br5.


image file: c9dt02792k-f5.tif
Fig. 5 Cyclic voltammograms of Cu(II) porphyrins using Ag/AgCl as reference electrode in CH2Cl2 containing 0.1 M TBAPF6 at 298 K.

image file: c9dt02792k-f6.tif
Fig. 6 Cyclic voltammograms of CoTPP(NO2)(Ph)2Br5 and CoTPP(Ph)2Br6 with precursor porphyrins using Ag/AgCl as reference electrode in CH2Cl2 containing 0.1 M TBAPF6 at 298 K.

ZnTPP(NO2)(Ph)2Br5 exhibited a marginal anodic shift in the first oxidation potential due to the strong electron withdrawing effect of –NO2 while a 0.05 V cathodic shift in the first reduction potential as compared to ZnTPPBr8. This is due to the presence of two phenyl groups which make it difficult to reduce. The first oxidation and reduction potentials of ZnTPP(Ph)2Br6 were found to be 0.04 and 0.05 V cathodically shifted as compared to ZnTPPBr8. A similar trend was observed for other metal derivatives. MTPP(Ph)2Br6 is easy to oxidize and difficult to reduce due to the ‘push–pull’ effect of β-substituents as compared to MPPBr8 and MTPP(NO2)(Ph)2Br5. Almost all porphyrins exhibited the same trend except Co(II) porphyrins (Fig. 6 and Table 3). In general, Co(II) porphyrins first undergo metal centered oxidation CoII/CoIII and reduction CoII/CoI followed by ring centered oxidation and reduction. The reactivity changes after metal centered oxidation and reduction and hence Co(II) porphyrins slightly deviate from the expected trend. Fig. S22 shows the cyclic voltammograms of MTPP(Ph)2Br6 and MTPP(NO2)(Ph)2Br5 (M = 2H, Ni(II), Zn(II) and X = NO2 or Br) in the ESI.

The HOMO–LUMO variation is represented in Fig. 7. As we increase the number of β-substituents (electron donor and acceptor) the HOMO–LUMO energy gap decreases gradually. The HOMO–LUMO energy gap of CuTPP(NO2)(Ph)2Br5 was found to be 1.72 V which is 0.62 V less as compared to CuTPP due to the stabilization of the LUMO and electron withdrawing nitro and bromo substituents and destabilization of the HOMO due to β-phenyl substituents and nonplanarity of the macrocycle. This huge reduction in the HOMO–LUMO gap has been ascribed to mixed-substitution (Ph, NO2 and Br) on the porphyrin macrocycle and higher nonplanarity of the core. The energy gap between these porphyrins follows the order CuTPP > CuTPPNO2 > CuTPP(NO2)(Ph)2 > CuTPP(Ph)2Br6 > CuTPP(NO2)(Ph)2Br5 due to the push–pull β-substituents a having difference in shape, size and electronic nature and nonplanarity of the macrocycle. These results suggest that the redox tunability can be achieved by means of mixed β-substitution at the peripheries with a reduced HOMO–LUMO gap.


image file: c9dt02792k-f7.tif
Fig. 7 HOMO–LUMO variation of Cu(II) porphyrins.

Conclusion

Two new series of mixed substituted porphyrins have been synthesized and characterized by various spectroscopic techniques. The crystal structure of H2TPP(NO2)Ph2Br5 revealed the saddle shape nonplanar conformation with an average deviation of 24 core atoms, Δ24 = ± 0.558 Å and a displacement of β-carbons, ΔCβ = ± 1.23 Å, from the mean plane of the porphyrin core. These unsymmetrical porphyrins exhibit a 53–61 nm bathochromic shift in the Soret band as compared to H2TPP. These porphyrins show higher protonation and deprotonation constants due to the enhanced nonplanarity and electronic effect of β-substituents. The 1H NMR spectra of H2TPP(Ph)2Br6 exhibited a 0.67 ppm downfield shift of –NH protons as compared to H2TPP(NO2)(Ph)2. The HOMO–LUMO gap of CuTPP(NO2)(Ph)2Br5 is decreased to 0.62 V as compared to CuTPP due to the push–pull β-substituents having a different shape, size and electronic nature and enhanced nonplanarity of the porphyrin core. Redox tunablity of these porphyrins was accomplished by appending the electron donating/accepting groups at the periphery of the porphyrin core which results in a push–pull effect of β-substituents on the porphyrin π-system. Mixed ‘push–pull’ β-substituents induce the higher order of nonplanarity which was confirmed by single crystal X-ray structural analysis, DFT calculations and photophysical data as well as red-shifted spectral features with redox tunability.

Experimental section

Materials

Pyrrole, N-bromosuccinamide, potassium carbonate, phenyl boronic acid, HCl, methanol, and hexane were purchased from Alfa Aesar Rankem, SRL and used without further purification. M(OAc)2·nH2O (M = CoII, CuII, NiII and ZnII), DMF, Na2SO4, TBAPF6, P2O5 and Br2 were purchased from HiMedia, and used as received. Pd(PPh3)4 was purchased from Sigma-Aldrich and used as received. Toluene, CHCl3 and CH2Cl2 were distilled over P2O5. NBS was used after being recrystallized from hot water and then dried under vacuum up to 8 h at 75 °C. TBAPF6 was used after recrystallizing twice from hot ethanol and then dried at 25 °C for 2 days.

UV-vis and fluorescence spectra were recorded using a Cary 100 spectrophotometer and a Hitachi F-4600 spectrofluorometer respectively. All 1H NMR spectra were recorded using a JEOL ECX 400 MHz spectrometer in CDCl3 at 298 K. An Elementar EL III Instrument was used to carry out elemental analysis. A Bruker UltrafleXtreme-TN MALDI-TOF/TOF mass spectrometer was used to record mass spectra using 2-(4′-hydroxybenzene-azo)benzoic acid as a matrix. Electrochemical studies were performed using a CHI-620 electrochemical workstation. An electrode system with Pt-wire as the counter electrode, Pt as the working electrode and Ag/AgCl as the reference electrode was used. All measurements in electrochemical studies were carried out in triple distilled dichloromethane containing 0.1 M TBAPF6 as the supporting electrolyte. Computational studies of the synthesized free base porphyrins were carried out in the gas phase using the LANL2DZ basis set and B3LYP functional.

Synthesis of 2,3,7,8,17,18-hexabromo-12,13-diphenyl-meso-tetraphenylporphyrin and 2-nitro-3,7,8,17,18-pentabromo-12,13-diphenyl-meso-tetraphenylporphyrin and their metal complexes

NiTPP(NO2)(Ph)2 was synthesized according to the method developed in our lab.18b NiTPP(NO2)(Ph)2 (0.120 g, 0.138 mmol) was added to 40 ml of distilled CHCl3 in 250 ml RB flask. To this, 40 eq. of liquid bromine in 20 ml of CHCl3 was added dropwise and then the reaction mixture was stirred for 4 h at room temperature. Then, 1.24 ml of pyridine in 30 ml of CHCl3 was added to the reaction mixture and again stirred for 6 h at room temperature. Then the reaction mixture was neutralized by sodium meta-bisulphite and washed with water. Finally the organic layer was dried over Na2SO4. The crude product was chromatographed on a silica gel column using the CHCl3/hexane mixture (1[thin space (1/6-em)]:[thin space (1/6-em)]1 v/v) as the eluent, two fractions were obtained. The first fraction NiTPP(Ph)2Br6 was obtained with (0.05 g) 40% yield and NiTPP(NO2)(Ph)2Br5 as the second fraction with (0.042 g) 35% yield.

Demetalation of NiTPP(Ph)2Br5X (X = NO2 or Br)

75 mg of NiTPP(Ph)2Br5X (X = NO2 or Br) was dissolved in a minimum amount of chloroform. A few drops of conc. sulphuric acid were added dropwise and stirred for 25 min at 0 °C. Then 40 ml of distilled H2O was added to this solution. The organic layer was washed with water (100 ml) and 10% of aq. ammonia. Finally, excess ammonia was removed by washing with distilled water (100 ml) and then the organic layer was passed through the anhydrous Na2SO4. The crude product was purified on a silica gel column using chloroform as the eluent. Yield: H2TPP(NO2)(Ph)2Br5 76% (57 mg, 0.047 mmol) and H2TPP(Ph)2Br6 78% (59 mg, 0.048 mmol).
H2TPP(NO2)(Ph)2Br5. UV-vis (CH2Cl2, λmax (nm) (log ε): 477 (4.88), 644 (3.69), 745 (3.64). 1H NMR in CDCl3 (400 MHz): δ (ppm) 8.24 (t, 6H, J = 8 Hz, meso-o-Ph), 7.85–7.70 (m, 10H, meso-o,m-Ph), 7.38–7.27 (m, 4H, meso-p-Ph), 6.85–6.79 (m, 10H β-Ph). 13C NMR in CDCl3 (100 MHz) δ: 137.38, 137.32, 137.03, 136.94, 136.67, 131.69, 130.14, 130.01, 128.83, 128.49, 128.21, 128.21, 127.45, 126.75, 126.66, 126.43. MALDI-TOF-MS (m/z) = [M]+ 1204.89, calcd 1204.84. Anal. calcd for C56H32Br5N5O2: C, 55.75; H, 2.67; N, 5.81%. Found: C, 55.05; H, 2.30; N, 5.32%.

H2TPP(NO2)Ph2Br5 was dissolved in 10 ml of CHCl3. To this solution, 10 eq. of M(OAc)2·nH2O (M = Co(II), Cu(II) and Zn(II)) in 2 ml of methanol was added and the mixture was refluxed for 35 minutes. Then the reaction mixture was cooled to room temperature and washed with water. The organic layer was passed through anhydrous Na2SO4 and the crude product was purified by column chromatography using CHCl3 as the eluent. Yield: 90–94%.

CoTPP(NO2)(Ph)2Br5. Yield: 90% (13.6 mg, 0.01 mmol) UV-vis (CH2Cl2, λmax (nm) (log ε): 455 (4.88), 573 (3.91). MALDI-TOF-MS (m/z) = found [M + K + H]+ 1303.40, calcd 1303.72. Elemental analysis calcd For C56H30Br5CoN5O2: C, 53.24; H, 2.39; N, 5.54%. Found: C, 53.15; H, 2.04; N, 5.14%.
NiTPP(NO2)(Ph)2Br5. UV-vis (CH2Cl2, λmax (nm) (log ε): 454 (4.86), 568 (3.87), 617 (3.68). 1H NMR in CDCl3 (400 MHz): δ (ppm) 7.96–7.92 (m, 4H, meso-o-Ph), 7.74–7.44 (m, 12H, meso-o,m-Ph), 7.22–7.13 (m, 4H, p-Ph), 6.88–6.66 (m, 10H β-Ph). MALDI-TOF-MS (m/z) = found [M + K]+ 1301.97, calcd 1301.76. Elemental analysis calcd For C56H32Br5N5NiO2: C, 53.25; H, 2.39; N, 5.54%. Found: C, 53.92; H, 2.64; N, 5.02%.
CuTPP(NO2)(Ph)2Br5. Yield: 94% (14.2 mg, 0.011 mmol) UV-vis (CH2Cl2, λmax (nm) (log ε): 465 (4.79), 585 (3.91), 635 (3.64). MALDI-TOF-MS (m/z) = [M + H]+ 1268.93, calcd 1268.77. Elemental analysis calcd For C56H30Br5CuN5O2: C, 53.05; H, 2.38; N, 5.52%. Found: C, 52.94; H, 2.30; N, 5.32%.
ZnTPP(NO2)(Ph)2Br5. Yield: 92% (13.8 mg, 0.01 mmol), UV-vis (CH2Cl2, λmax (nm) (log ε): 471 (4.98), 605 (3.71), 669 (3.77). 1H NMR in CDCl3 (400 MHz): δ (ppm) 8.12 (t, 4H, J = 8 Hz, meso-o-Ph), 7.82–7.60 (m, 12H, meso-o,m-Ph), 7.22–7.09 (m, 4H, p-Ph), 6.80–6.60 (m, 10H, β-Ph). MALDI-TOF-MS (m/z) = found [M + H]+ 1269.48, calcd 1269.75. Elemental analysis calcd For C56H30Br5N5O2Zn: C, 52.97; H, 2.38; N, 5.52%. Found: C, 52.67; H, 2.20; N, 5.46%.
H2TPP(Ph)2Br6. UV-vis (CH2Cl2, λmax (nm) (log ε): 468 (5.01), 571 (3.64), 629 (3.76), 738 (3.85). 1H NMR in CDCl3 (400 MHz): δ (ppm) 8.23 (d, 6H, J = 8 Hz, meso-o-Ph), 7.83–7.74 (m, 10H, meso-o,m-Ph), 7.34–7.28 (m, 4H, p-Ph), 6.83–6.76 (m, 10H β-Ph), −1.64 (bs, 1H, imino-H). 13C NMR in CDCl3 (100 MHz) δ: 139.41, 137.68, 134.21, 129.57, 128.63, 126.64, 126.35, 121.92, 114.18, 33.92, 32.02, 29.79, 29.61, 29.46, 29.25, 29.04, 22.79, 14.22. MALDI-TOF-MS (m/z) = found [M + H]+ 1240.51, calcd 1240.77. Elemental analysis calcd For C56H32Br5N5O2: C, 55.75; H, 2.67; N, 5.81%. Found: C, 55.95; H, 2.97; N, 5.92%.
CoTPP(Ph)2Br6. Yield: 89% (13.4 mg, 0.01 mmol), UV-vis (CHCl2) λmax (nm) (logε): 445 (5.02), 565 (3.94). MALDI-TOF-MS m/z = [M + H]+ 1297.67, calcd 1297.68. Elemental analysis calcd For C56H30Br6CoN4: C, 51.85; H, 2.33; N, 4.32%. Found: C, 51.99; H, 2.56; N, 4.67%.
NiTPP(Ph)2Br6. UV-vis (CHCl2, λmax (nm) (log ε): 446 (5.18), 561 (4.06), 603 (3.74). 1H NMR in CDCl3 (400 MHz): δ (ppm) 7.94 (d, 4H, J = 8 Hz, meso-o-Ph), 7.75–7.62 (m, 8H, meso-o,m-Ph), 7.53 (d, 4H, J = 8, p-Ph), 7.15 (t, 4H J = 8 Hz, p-Ph), 6.82–6.61 (m, 10H β-Ph). MALDI-TOF-MS m/z) = [M + H]+ 1298.34, calcd 1298.68. Anal. calcd for C56H30Br6N5Ni: C, 51.86; H, 2.33; N, 4.32%. Found: C, 51.92; H, 2.94; N, 4.67%.
CuTPP(Ph)2Br6. Yield: 93% (14 mg, 0.01 mmol), UV-vis (CH2Cl2, λmax (nm) (log ε): 444 (5.06), 461 sh (5.01), 579 (4.14), 626 (3.73). MALDI-TOF-MS m/z = found [M + H]+ 1301.97, calcd 1301.69. Elemental analysis calcd C56H30Br6CuN4: C, 51.67; H, 3.32; N, 4.30%. Found: C, 51.09; H, 3.76; N, 4.87%.
ZnTPP(Ph)2Br6. Yield: 91% (13.7 mg, 0.010 mmol), UV-vis (CH2Cl2, λmax (nm) (log ε): 463 (5.07), 597 (3.78), 653 (3.68). 1H NMR in CDCl3 (400 MHz): δ (ppm) 8.14 (d, 4H, J = 8 Hz, meso-o-Ph), 7.79–7.71 (m, 10H, meso-o,m-Ph), 7.32–7.27 (m, 3H, p-Ph), 7.23–7.18 (m, 3H, p-Ph), 7.79–7.64 (m, 10H β-Ph). MALDI-TOF-MS m/z = found [M]+ 1303.40, calcd 1303.68. Elemental analysis calcd for C56H30Br6N4Zn: C, 51.59; H, 2.32; N, 54.30%. Found: C, 51.87; H, 2.45; N, 54.46%.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

MS thanks the Science and Engineering Research Board (SERB/EMR/2016/004016), New Delhi, India for the financial support. PR thanks the Ministry of Human Resource Development for providing a Senior Research Fellowship.

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Footnote

Electronic supplementary information (ESI) available: Single crystal X-ray diffraction, geometry optimized structures, UV-vis, fluorescence, NMR and mass spectra and comparative CV diagrams. CCDC 1921730. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/C9DT02792K

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