Versatile Synthetic Route for β-Functionalized Chlorins and

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Versatile Synthetic Route for β‑Functionalized Chlorins and Porphyrins by Varying the Size of Michael Donors: Syntheses, Photophysical, and Electrochemical Redox Properties Nivedita Chaudhri, Nitika Grover, and Muniappan Sankar* Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee 247667, India

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S Supporting Information *

ABSTRACT: One-pot facile synthesis and characterization of novel βsubstituted meso-tetraphenylporphyrins and/or chlorins were described. The high regioselective reactivity of active methylene compounds in Michael addition reaction was reported to access β-substituted trans-chlorins. Sizedependent approach was applied for the fine-tuning of product formation from porphyrins to chlorins. Notably, we were able to isolate mono/ trisubstituted porphyrin and/or di/tetra-substituted chlorin from one-pot synthesis for the first time in porphyrin chemistry. Single-crystal X-ray diffraction analysis revealed the quasiplanar to moderate nonplanar conformation of chlorins due to trans orientation of β-substituents, whereas porphyrins exhibited higher mean plane deviation from 24-atom core (Δ24) as compared to chlorins. β-Functionalized chlorins exhibited lower protonation constants and much higher deprotonation constants as compared to porphyrins revealing the combined effect of the conformation of macrocyclic core and the electronic nature of β-substituents. Facile synthesis of porphyrins and/or chlorins based on the size of Michael donor employed and in turn resulted in tunable photophysical and electrochemical redox properties are the significant features of the present work.

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light harvesting,12 photodynamic therapy (PDT), and singlet oxygen generation.13 To better understand the effects of distinct substituents at specific positions, we have been working for some time to develop rational methods for the synthesis of stable tailorable chlorins and porhyrins, wherein each chlorin bears vicinal groups at the reduced pyrrole ring, whereas each porphyrin bears monosubstituent at the β-position. Recently, Brückner et al. reported the synthesis of various porphyrinoids by formal replacement of a pyrrole with several types of fused rings, for example, carbaporphyrinoids, benziporphyrins, oxypyriporphyrins, indaphyrins, indachlorins, etc.9b,c14 However, still there is a space to introduce cyclic moiety through C−C bond formation at reduced β-pyrrole (i.e., chlorin) or conjugated β-pyrrole (i.e., porphyrin) without breaking or mending the pyrrole ring. This inspired us to perform functionalization at the β-pyrrole positions of tetraphenylporphyrin (TPP) derivatives, which led to the synthesis of β-functionalized porphyrins and/or chlorins. Three distinct methods have been developed for the synthesis of chlorins and porphyrins bearing cyclic-1,3-dione derivatives. (a) Reaction of relatively small size cyclic-1,3-dione (cylcohexane-1,3-dione (CHD) and 1,3-dimethylbarbituric acid (DMBA)) with MTPP(NO2) and MTPP(NO2)Br2, where M = 2H and Ni(II), yielded mono- and tri-β-substituted porphyins,

INTRODUCTION meso-Tetraarylporphyrins are widely explored due to their facile synthesis, characteristic deep colors, intense fluorescence, high photochemical stability, and also their ability to chelate a wide variety of metal ions in their inner core.1 Therefore, they have wide range of utilities in biology and material chemistry2 such as artificial photosynthesis,3 catalysis,4 dye-sensitized solar cell (DSSC),5 nonlinear optics (NLO),6 and chemosensing.7 In most of these applications, it is enviable to tune the optical properties of porphyrin π-system by the modification of macrocyclic core through extended π-conjugation8 or by converting into chlorins,9,10 or by making them into nonplanar porphyrin core.11 Several methods have been reported in literature for the conversion of porphyrins into chlorins.9,10 The electronic properties of the porphyrin macrocycle can be modulated through functionalization at the β and/or meso positions. The insertion of single nitro group at the β-pyrrolic position of porphyrin is one of the successful examples that have been widely used to obtain chlorins.10 2-Nitro-5,10,15,20tetraphenylporphyrin (MTPP(NO2)) undergoes a variety of SNAr reaction with a wide range of nucleophiles and reveals the Michael acceptor character of this macrocycle. Generally, SNAr reactions are preferred with electron-deficient aromatic compounds. For that purpose, synthetically accessible βnitroporphyrins would be the better applicant over halogenated porphyrins. The unique electronic properties of chlorins allows them to capture more light of longer wavelength region for © 2017 American Chemical Society

Received: May 6, 2017 Published: September 18, 2017 11532

DOI: 10.1021/acs.inorgchem.7b01158 Inorg. Chem. 2017, 56, 11532−11545

Article

Inorganic Chemistry Chart 1. Molecular Structures of Synthesized Porphyrins

Chart 2. Molecular Structures of Synthesized Chlorins

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respectively. (b) Reaction of relatively moderate size nucleophile such as benzoylacetonitrile (BENAC) with NiTPP(NO2) and NiTPP(NO2)Br2 afforded mono- and tri-βsubstituted porphyins and di/tetra-substituted chlorins. (c) Reaction of relatively large size active methylene compound such as indane-1,3-dione (IND) with MTPP(NO2) and MTPP(NO2)Br2, where M = 2H, Ni(II), yielded only di/ tetra-substituted chlorins. However, no porphyrin formation was observed in the last case. Cyclic 1,3-diones are widely used in heterocyclic synthesis.15 Therefore, the introduction of these substituents at β-position of porphyrins makes them good substrates for further functionalization. Eight new families of βsubstituted porphyrins and chlorins were synthesized and characterized as represented in Charts 1 and 2.

RESULTS AND DISCUSSION

Synthesis and Characterization. A wide variety of mixed β-substituted chlorins and porphyrins, specifically, β-di/tetrasubstituted tetraphenylchlorins (MTPC(R)X2, X = H, Br and R = BENAC and IND) and β-mono/tri-substituted tetraphenylporphyrins (MTPP(R)X2, X = H, Br and R = CHD and DMBA) were synthesized (Schemes 1 and 2) and characterized by various spectroscopic techniques. The precursor porphyrins NiTPP(NO2) and NiTPP(NO2)Br2 were synthesized according to literature methods.16 Nucleophilic substitution of NiTPP(NO2)X2, where X = H and Br with different active methylene compounds including 1,3-diones, for example, CHD, IND, DMBA, and BENAC, was performed to obtain various β-substituted chlorins and porphyrins as shown in Schemes 1 and 2. 11533

DOI: 10.1021/acs.inorgchem.7b01158 Inorg. Chem. 2017, 56, 11532−11545

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formation of 2,3-disubstituted trans-chlorins and mono/trisubstituted porphyrins in one pot for the first time in porphyrinoid chemistry. To extend this work, we examined the reactivity of both Ni(II) and free base β-nitroporphyrin toward a wide variety of active methylene compounds. Interestingly, the reaction of free base nitroporphyrin with cyclic 1,3-diones and BENAC afforded only single product either porphyrin or chlorin. In principle, the addition of unsymmetrical active methylene compounds (benzoylacetonitrile) to MTPP(NO2)X2 (where X = H, Br) should yield diastereomers due to presence of chiral centers. However, BENAC reacts in a regioselective manner and produces single diastereomer in good yield. Evidence for regioselectivity is revealed by 1H NMR spectrum that shows two doublets between 3.5 and 5.0 ppm. In contrast, subjecting NiTPP(NO2)/NiTPP(NO2)Br2 to Michael addition with BENAC carbanion afforded both porphyrins and chlorins (Scheme 1). In case of DMBA, we were able to isolate nitrochlorins as a side product. However, we were unable to obtain nitrochlorin while subjecting H2TPP(NO2)Br2 with DMBA due to in situ complete conversion of nitrochlorin into porphyrin by eliminating HNO2, while subjecting the NiTPP(NO2)X2 (where X = H and Br) to DMBA afforded only porphyrin as a single product due to complete in situ conversion of nitrochlorin into β-functionalized porphyrin (Scheme 2). The reaction of CHD with MTPP(NO2) (M = 2H and Ni(II)) led to the selective formation of porphyrin, whereas treatment of IND with MTPP(NO2) (M = 2H and Ni) results in the selective formation of 2,3-disubstituted trans chlorins (Scheme 2). Scheme 3 shows the plausible mechanism for product formation. In route (i) the nitronate adduct was formed as an intermediate (a), which resulted into the formation of cyclopropylchlorin due to intramolecular nucleophilic substitution (c). The chlorins (c) underwent further nucleophilic substitution to afford 2,3-disubstituted trans chlorins (d). In route (ii) the nitronate adduct (e) was converted into (f) instead of (b), which led to the formation of β-substituted porphyrin (g) via HNO2 elimination as shown in Scheme 3. Crystal Structure Discussion. The single crystals of 2a (C50H32N4O2Br2Ni), 8a (C62H40N6O2Ni), 9 (C62H42N6O2Br2· 2CHCl3), and 11b (C67H41N5O4Br2Zn) were grown at room temperature from toluene−methanol for 2a, CHCl3−hexane for 8a and 9, and CHCl3−hexane with a few drops of pyridine for 11b. Nonhydrogen atoms were refined in anisotropic approximation except those for the disordered fragments.

Scheme 1. Synthetic Routes for the Preparation of Di/Tetraβ-Substituted Tetraphenylchlorinsa

a

(a) DMSO, K2CO3, BENAC, 3 h; (b) DMSO, K2CO3, IND, 3 h.

Scheme 2. Synthetic Route for the Preparation of Mono/triβ-Substituted Tetraphenylporphyrinsa

a

(a) DMSO, K2CO3, CHD/DMBA, 1.5 h; (b) − HNO2.

A methodology based on 1,4-addition of cyclic active methylene derivatives with MTPP(NO2) or MTPP(NO2)Br2 as a suitable route for the selective preparation of di/tetra-βsubstituted tetraphenylchlorin and mono/tri-β-substituted tetraphenylporphyrin was developed. The product distribution can be controlled by varying the size of cyclic carbanion, reaction time, and/or temperature, as well as the absence or presence of metal ion in the TPP(NO2) core. On the one hand, higher temperature (∼110−120 °C) combined with large size cyclic carbanion, favored the formation of the trans-chlorins, whereas lower temperature and small cyclic carbanion afforded porphyrins. On the other hand, moderate carbanion with higher temperature and longer reaction time resulted in the

Scheme 3. Plausible Mechanism for the Synthesis of Porphyrins and Chlorins

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Figure 1. ORTEP diagrams showing top and side views of NiTPPBr2(CHD) (2a) (a, b), NiTPC(BENAC)2 (8a) (d, e), and H2TPCBr2(BENAC)2 (9) (g, h), respectively. Solvates are not shown for clarity; the β-substituents and β-/meso-phenyls are omitted for clarity in side view. (c, f, i) Numbering of carbon atoms in the skeleton.

atom (0.076 Å). The observed C2−C3 bond length (1.52 Å) was larger than other Cβ−Cβ bond length (1.35 Å), which clearly indicates that the proposed structure was 2,3disubstituted chlorin. ΔCβ and Δ24 were found to be ±0.262 and ±0.132 Å, which revealed the quasiplanar confirmation of the macrocyle. ZnTPCBr2(IND)2(py) crystallized in the triclinic crystal system with P1̅ space group in which two molecules were present in the asymmetric unit. In both chlorins, each Zn atom was coordinated to one pyridine unit. Zn1 was 0.342 Å deviated from the mean plane, whereas Zn2 was 0.352 Å deviated from mean plane. Similar to H2TPCBr2(BENAC)2 (9), both IND groups were trans to each other, but in contrast to previous chlorin C2′−C54′ and C3′−C45′ were equal (1.56 Å). C3′ atom was 0.251 Å deviated above the mean plane, while C2′ was 0.233 Å deviated below the mean plane. In the case of second molecule in the same asymmetric unit, C3 was 0.397 Å deviated above the mean plane, and C2 was 0.080 Å deviated below the mean plane. In both molecules, the reduced Cβ−Cβ (C2′−C3′ and C2−C3) bond lengths were equal to 1.53 Å, but a remarkable difference was observed in the values of Δ24 and ΔCβ. Both the chlorin rings were moderately distorted having Δ24 values ±0.174 and ±0.099 Å, while ΔCβ values were found to be ±0.327 and ±0.146 Å, respectively. The displacement of β-pyrrole carbons (ΔCβ) and 24 core atoms (Δ24) from the porphyrin mean plane follows the order

Hydrogen atoms of these chlorins and porphyrin structures were geometrically relocated at the chemically meaningful positions. Figure 1a clearly shows that the product is monosubstituted porphyrin, whereas Figures 1g,d and S1 show 2,3-disubstituted trans-chlorins, respectively. The crystallographic data of above-mentioned chlorins and porphyrins are listed in Table S1 in the Supporting Information. The observed bond distances and bond angles of these porphyrins and chlorins are given in Table S2 in the Supporting Information and are in close agreement with those reported in the literature.17 ORTEP top and side views are shown in Figures 1 and S1 in the Supporting Information. NiTPPBr2(CHD) (2a) crystallized in the triclinic crystal system with P1̅ space group. The CHD moiety is present in the half-chair conformation. The macrocycle exhibited saddle shape conformation. Interestingly, tri-β-substituted porphyrins have shown higher magnitude of the displacement of the βpyrrole carbon (ΔCβ = ±0.66 Å) and 24 core atoms (Δ24 = ±0.430 Å) as compared to β-tetrasubstituted chlorins. NiTPC(BENAC)2 (8a) crystallized in the triclinic crystal system with P1̅ space group in which one full molecule was present in the asymmetric unit. Both BENAC groups were trans to each other. Δ24 value was found to be 0.281 Å. H2TPCBr2(BENAC)2 (9) crystallized in the monoclinic crystal system with a C2/c space group. H2TPCBr2(BENAC)2 (9) showed two CHCl3 as lattice solvate one of them was disordered. The X-ray structure revealed that both BENAC groups were trans to each other. The observed C3−C45 bond length (1.57 Å) was close to C2− C54 bond length (1.59 Å). Both were slightly higher than C−C bond length present in sp3 hybridized carbons. C3 atom was more deviated (0.364 Å) from mean plane as compared to C2

H 2TPCBr2(BENAC)2 (9) < ZnTPCBr2(IND)2 (py) (11b) < NiTPC(BENAC)2 (8a) < NiTPPBr2(CHD) (2a) 11535

DOI: 10.1021/acs.inorgchem.7b01158 Inorg. Chem. 2017, 56, 11532−11545

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distance (2.6 Å) was shorter as compared to those reported in literature.18e DFT Studies. To gain insight into the effect of βsusbtituents on spectral and photophysical properties as a function of molecular characteristics, DFT calculations were performed. The ground-state geometries of synthesized free base porphyrins/chlorins were optimized in the gas phase using B3LYP functional and 6-31G basis set (Figures S3 and S4 in the Supporting Information).19 The structures obtained by DFT methods were in close agreement with those obtained by X-ray crystallography. The frontier molecular orbitals (FMOs) of synthesized free base compounds are shown in Figures 3 and S5−S11 in the Supporting Information. According to Gouterman’s four orbital model, the highest occupied molecular orbital−lowest unoccupied molecular orbital (HOMO− LUMO) and HOMO−1 to LUMO+1 configuration make roughly equal contribution to the S1 wave function in case of porphyrin, whereas in chlorins the HOMO−LUMO gap is greater. The HOMO of H2TPC(IND)2 (10) has contributions mainly from the macrocycle, whereas the LUMOs of H2TPC(IND)2 (10) contribute to the IND group, which indicates charge transfer from the conjugated ring to the βsubstituents. However, this contribution is small as compared to the π−π* transition. DFT calculations of free base chlorins have shown the contribution from the tetrapyrrole macrocycle and the β-substituents (IND and BENAC) to give linearcombination of orbitals, the HOMO−1, HOMO, LUMO, and LUMO+1 of which have mixed chlorins/β-substituents character. Absorption Spectral Studies. The optical absorption spectra of each synthesized compound were recorded in CH2Cl2 at 298 K. The electronics of the porphyrinic π-systems depend on their degree of saturation. The fully conjugated porphyrins and the chlorins in which one of the Cβ−Cβ double bond is reduced, all possess characteristic UV−vis spectra. The presence of core metal, type and position of substituents, degree of macrocyclic conjugation, and the conformation of the macrocyclic core are also important factors that determine the

Notably, an oxygen−oxygen (O···O) interaction between the nearest CHD moiety of NiTPPBr2(CHD) (2a) was found at an O···O distance of 2.6 Å (Figure 2), which was comparable to

Figure 2. Packing diagram showing O···O interaction between the nearest CHDs in NiTPPBr2(CHD) (2a).

O···O interaction found in ice.18 These kinds of noncovalent interactions can be attractive candidates for the formation of supramolecular assemblies. There are several reports in which intramolecular noncovalent interactions between S and S or O or N atoms has also been observed in a large number of organosulfur complexes controlling the conformation of small and large molecules.18 These oxygen−oxygen (O···O) types of interactions may be the result of local polarization effects, due to the electron-rich and electron-deficient regions in the O atoms, which results in the complementary electrostatic interactions.18d These forces are responsible for the selfassembly of large molecules, crystal packing, and biological pattern recognition. We analyzed that the observed O···O

Figure 3. Pictorial representation of FMOs of H2TPP(CHD) (1). 11536

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Figure 4. Electronic absorption spectra of (a) H2TPP(CHD) (1), H2TPP(DMBA) (3), H2TPC(IND)2 (10), H2TPC(BENAC)2 (8) and (b) H2TPPBr2(CHD) (2), H2TPPBr2(DMBA) (4), H2TPCBr2(IND)2 (11), H2TPCBr2(BENAC)2 (9) in CH2Cl2. Intensity of Q-bands was enhanced by 5 times for clarity. Circled area shows the absorption due to BENAC and IND.

Table 1. Photophysical Properties of Synthetic Chlorins and Porphyrinsa λabs

porphyrin H2TPP(CHD) (1) H2TPPBr2(CHD) (2) H2TPP(DMBA) (3) H2TPP Br2(DMBA) (4) H2TPC(NO2)(DMBA) (7) H2TPC(BENAC)2 (8) H2TPCBr2(BENAC)2 (9) H2TPC(IND)2 (10) H2TPCBr2(IND)2 (11) a

419(5.49), 422(5.41), 419(5.50), 428(5.37), 435(5.46), 255(4.56), 256(4.52), 229(4.80), 230(4.91),

518(4.18), 519(4.13), 515(4.18), 525(4.12), 537(4.03), 416(5.34), 423(5.27), 419(5.18), 427(5.27),

552(3.76), 550(3.43), 550(3.71), 600(3.53), 580(4.29), 516(4.21), 525(4.13), 518(4.08), 527(4.16),

λem

591(3.68), 598(3.51), 595(3.65), 670(3.79) 618(3.64), 544(4.18), 595(3.97), 545(3.97), 596(3.90),

648(3.66) 655(3.61) 648(3.54) 675(4.00) 593(3.88), 646(4.47) 645(4.09) 595(3.71), 649(4.31) 648(4.12)

657, 663, 666, 716 683, 652, 653, 656, 655,

721 720 722 753 718 717 721 718

Q/B

∑Q/∑B

f

0.015 0.016 0.011 0.026 0.054 0.133 0.066 0.136 0.044

0.02 0.02 0.01 0.04 0.03 0.06 0.03 0.12 0.02

0.019 0.023 0.022 0.025 0.026 0.031 0.028 0.023 0.032

The values in parentheses refer to log ε, integrated (∑Q/∑B) intensity ratios of the Q and Soret (B) absorption, f = oscillator strength.

electronic properties of macrocycle.20 Figures 4, S12, and S13 represent the comparative UV−vis spectra of synthesized porphyrins and chlorins in CH2Cl2 at 298 K. Tables 1 and S3 list absorption spectral data of all synthesized chlorins and porphyrins. The red region band is a characteristic property that distinguishes chlorins from porphyrins. On the one hand, the presence of reduced pyrrole causes a relative increase in intensity of the last Q-band. On the other hand, the porphyrin exhibited a significant decrement in last Q-band intensity as compared to chlorins; for example, Q/B ratio of H2TPC(IND)2 (10) is ∼9 times higher as compared to H2TPP(CHD) (1). Lindsey and Coworkers reported photophysical properties of meso-arylsubstituted chlorins.20f In contrast to mesosubstitution, we observed that Q/B ratio altered upon substitution at β-position, whereas position of last Q-band remains same. Q/B ratio of 8 and 10 reduced to nearly half as compared to H2TPC. Q/B ratio of H2TPC was nearly 4 and 6 times higher as compared to 9 and 11, respectively. The last Qband of synthesized free base chlorins were nearly 8−10 nm blue-shifted as compared to H2TPC. Blue shift was observed possibly due to stabilization of HOMO. All dibromosubstituted compounds exhibited red shift in their B and/or Q bands as compared to corresponding unbrominated ones. The Soret band of NiTPPBr2(CHD) (2a) was ∼8 nm redshifted as compared to NiTPP(CHD) (1a), whereas the Soret band of NiTPCBr2(IND)2 (11a) was ∼11 nm red-shifted as compared to NiTPC(IND)2 (10a). On the one hand, the Soret (B) band is observed as a result of S0 to S2 transition; hence, the red shift in the Soret band was possibly due to stabilization of LUMO+1. On the other hand, the position of the Qy(0,0)

band in dibromo-substituted chlorins remain unchanged, yet the relative intensity was decreased as compared to unbrominated ones, which may be due to reduced energy gap between LUMO and LUMO+1. Position of Qy(0,0) band remained same, and therefore the HOMO−LUMO gap remains unaltered. The similar shifts were observed for other synthesized chlorins and pophyrins as listed in Table 2. Oscillator strength of above-synthesized compounds ranges Table 2. Photophysical Properties of Synthetic Chlorins and Porphyrinsa

a

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porphyrin

ϕf

τs (ns)

knr−1 (ns)

kr−1 (ns)

H2TPP(CHD) (1) H2TPP(CHD)Br2 (2) H2TPP(DMBA) (3) H2TPC(NO2)(DMBA) (5) H2TPC(BENAC)2 (8) H2TPC(BENAC)2Br2 (9) H2TPC(IND)2 (10) H2TPC(IND)2Br2 (11) ZnTPP(CHD) (1b) ZnTPP(CHD)Br2 (2b) ZnTPP(DMBA) (3b) ZnTPP(DMBA)Br2 (4b) ZnTPC(BENAC)2 (8b) ZnTPC(BENAC)2Br2 (9b) ZnTPC(IND)2 (10b) ZnTPC(IND)2Br2 (11b)

0.097 0.008 0.095 0.028 0.402 0.004 0.317 0.005 0.038 0.004 0.041 0.002 0.154 0.010 0.047 0.003

5.23 0.42 0.056 7.72 7.63 1.00 6.71 1.15 1.56 0.44 5.71 0.02 1.51 0.506 0.522 0.004

5.85 0.42 0.06 7.95 12.76 1.00 9.82 1.16 1.63 0.45 5.96 0.03 1.79 0.51 0.55 1.22

54.49 52.50 0.60 276.01 18.96 250.00 21.17 230.00 41.16 110.83 139.34 12.52 9.83 50.62 11.11 0.004

Values were calculated in CH2Cl2 at 298 K. DOI: 10.1021/acs.inorgchem.7b01158 Inorg. Chem. 2017, 56, 11532−11545

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Inorganic Chemistry from 0.010 to 0.034. Experimental oscillator strengths were estimated by the following formula f = 4.61 × 10−9εδ

Here, ε = molar absorption coefficient, δ = full width at half maxima. Fluorescence Spectral Studies. Free base and Zn(II) complexes of the mono/trisubstituted porphyrins and di/tetra substituted chlorins were characterized by fluorescence spectroscopy to elucidate the role of substitution. Figures S14 and S15 in the Supporting Information show the emission profile of synthesized free base and Zn(II) porphyrinoids. The fluorescence maxima of all synthesized chlorins were generally within ∼10 nm of the corresponding Qy(0,0) absorption maximum, reflecting a relatively small Stokes shift. Porphyrins exhibited higher Stokes shift as compared to chlorins. Among all synthesized compounds, H2TPPBr2(DMBA) (4) showed highest Stokes shift. Bromo-substituted porphyrinoids exhibited lower quantum yields due to heavy atom effect of β-bromo substituents and nonplanar conformation of the macrocyclic core. We were unable to calculate quantum yield for H2TPPBr2(DMBA) (4) due to very feeble fluorescence intensity. In case of bromochlorins, quantum yield profoundly decreased as compared to unbrominated chlorins due to heavy atom effect and low Q/B ratio. Table 2 represents the fluorescence yields (ϕf) and singlet excited-state lifetimes (τs) for the newly synthesized compounds. Quantum yields (ϕf) were calculated by using following formula

Figure 5. Fluorescence lifetime decay profiles of (a) H2TPP(CHD) (1) and H2TPC(BENAC)2 (8) and (b) H2TPC(BENAC)2 (8) and H2TPCBr2(BENAC)2 (9).

ns−1). However, in case of bromo-substituted chlorins and porphyrins radiative rate constants were invariably lower than those reported in literature for related systems (Table 2).21b,c The distinct differences in photophysical properties of mono/ di- and tri/tetra-β-substituted MTPPs/MTPCs differ strikingly from their parent molecules due to the nature of β-substituents, effect of substituents on molecular-orbital energy gaps, and excited-state configurational mixing and conformation of the macrocyclic core.20 In summary, we can conclude that (a) the integrated intensity of S0−S1 manifold decreased upon βsubstitution, which results into low ϕf and kr of synthesized free base and Zn(II) chlorins as compared to meso-tetraphenylchlorins and that (b) reduced LUMO and LUMO+1 gap may be the another cause of low fluorescence intensity in synthesized β-substituted chlorins. (c) Finally, we conclude that, by means of mixed β-substitution photophysical properties of meso-tetraphenylchlorin can be tuned. NMR Studies. All the synthesized porphyrins and chlorins exhibited characteristic chemical shifts arising from β-pyrrole protons, β-substituents (CHD, DMBA, IND, BENAC), mesophenyl, and imino protons in case of free base porphyrins and chlorins. Figures S16−S41 in the Supporting Information represent the 1H NMR spectra of all newly synthesized porphyrinoids. The 1H NMR spectra of 2,3-disubstituted transchlorins show different features from that of β-substituted porphyrin. The presence of a sharp singlet with one proton intensity in the range from 3.5 to 5.0 ppm clearly indicates the formation of porphyrins (1−6, 1a−4a, and 1b−4b), whereas the presence of two doublets of two proton intensity between 3.5 and 5.0 ppm demonstrates the formation of 2,3disubstituted trans-chlorins (7−11, 8a−11a, and 8b−11b). The β-pyrrole resonances of these synthesized free base TPCs were downfield shifted (∼0.44 ppm) as compared to H2TPC.20e Inner core NH signals of 7, 8, and 9 were upfield shifted (0.03−0.19 ppm) as compared to H2TPC, whereas no significant shift was observed for 10 and 11. In contrast to H2TPC, reduced β-pyrrolic protons of synthesized chlorins resonated at higher frequencies. Significantly, these protons were 0.66−1.44 ppm downfield shifted as compared to H2TPC. Downfield shift of β-protons and upfield shift of inner core NH was observed due to electron-withdrawing nature of βsubstituents. Figure 6 shows the 1H NMR spectra of the imino proton region of synthesized free base porphyrins and chlorins in CDCl3 at 298 K. The β-pyrrole resonances of the synthesized free base TPPs were downfield-shifted as compared to synthesized free base TPCs. The core imino protons of synthesized porphyrins resonate at higher regions as compared to nitro-substituted porphyrins, whereas synthesized chlorins resonate in lower regions. The major differences for the position of core imino

(ϕf )sample = (ϕf ref × A sample × εref )/(A ref × εsample)

and follow the order H 2TPCBr2(BENAC)2 (9) < H 2TPCBr2(IND)2 (11) < H 2TPPBr2(CHD) (2) < H 2TPP(DMBA) (3) < H 2TPP(CHD) (1) < H 2TPC(NO2 )(DMBA) (7) < H 2TPC(BENAC)2 (8) < H 2TPC(IND)2 (10)

The studies performed in the present work on β-substituted porphyrins and chlorins provide an opportunity to compare the quantum yields, lifetimes, and rate constants for the two S1 excited-state decay pathways across two classes of macrocycles having similar skeletons. The span of τs values (0.004−7.72 ns) for chlorins were wider as compared to porphyrins (0.02−5.53 ns). Singlet-state lifetime values of 8 and 10 were low as compared to H2TPC (13.1 ns); however, lifetime values of 9 and 11 were lower than those of 8 and 10, respectively. In this way, β-substitution leads to dramatic shift in singlet-state lifetime. Figure 5a shows the comparative lifetime plot of H2TPP(CHD) (1) and H2TPC(BENAC)2 (8). The radiative (kr) and nonradiative (knr) rate constants were calculated according to the method reported by Holten and co-workers.21

k r = ϕf /τS k nr = (1 − ϕf )/τS k = 1/τS

k nr = k − k r

These rate constants (Table 2) were cast as the associated time constants (in nanoseconds). The calculated kr values were significantly lower than that of meso-tetraphenylporphyin (120 11538

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Figure 6. Representative 1H NMR Spectra of imino proton region of synthesized free base porphyrins and chlorins.

Figure 7. UV−vis spectral titration of (a) H2TPPBr2(DMBA) (4) and (b) H2TPCBr2(IND)2 (11) in toluene at 298 K. (insets) The corresponding Hill plots.

protons and β-pyrrole proton arise due to reduced ring current in chlorins. The number and positions of carbon signals were clearly in accordance with the proposed structures. However, we were unable to record 13C NMR spectrum of 2b, 4b, 10, and 10b due to very poor solubility in CDCl3 and deuterated dimethyl sulfoxide (DMSO-d6). Mass Spectrometric Studies. The MALDI-TOF mass spectra of all the synthesized compounds were recorded using 2-(4′-hydroxyphenylazo)benzoic acid (HABA) as matrix. The positive ion mode mass spectra of synthesized porphyrins are shown in Figures S42−S68 in the Supporting Information. BENAC and IND appended chlorins showed fragmentation pattern in mass spectra due to removal of labile one or two βsubstitutents as shown in Figure S55−S68 in the Supporting Information. Protonation and Deprotonation Studies. To determine the effect of mixed β-substitution on the nonplanarity and acid−base properties of central coordination entity H2N4 of macrocycles, we performed the protonation and deprotonation studies of synthesized free base porphyrins and chlorins using trifluoroacetic acid (TFA) and tetrabutylammonium hydroxide (TBAOH), respectively, in toluene at 298 K. The acid−base properties of electron-deficient nonplanar porphyrins are welldocumented in literature,16c,22 whereas there is no report on protonation and deprotonation studies of chlorins. Figure 7 shows the UV−vis spectral changes upon increasing the concentration of TFA (0.26 × 10−5 to 3.84 × 10−5 M) for porphyrin and H2TPPBr2(DMBA) (4) (Figure 7a) and (0.34 × 10−3 to 5.74 × 10−3 M) for chlorin and H2TPCBr2(IND)2 (11) (Figure 7b) in toluene at 298 K. Figure 7a represents the concomitant decrement in the absorbance of H2TPPBr2(DMBA) (4) at 430 nm and rising of a new band at 456 nm upon increasing [TFA]. As protonation proceeds, the multiple Q bands disappeared, and a new single broad band arose at 694 nm accompanied by the red shift of 20 nm in

Qx(0,0) band. A similar behavior was observed for other free base porphyrins as shown in Figure S69 in the Supporting Information. In protonation studies, chlorins also exhibited similar spectral changes in B band, whereas different spectral changes were observed in Q bands as compared to porphyrins. Figure 7b shows the UV−vis spectral titration of H2TPCBr2(IND)2 (11) while increasing [TFA]. The absorbance concomitantly decreased at 428 nm with the emergence of a new B band at 453 nm accompanied by a minimal (5 nm) blue shift in the last Qy(0,0) band. Similar spectral changes were observed for remaining synthesized free base chlorins (Figure S70 in the Supporting Information). The protonated porphyrins show enhanced Qx(0,0) band with 11−19 nm red shift, whereas in chlorins after protonation the intensity of last Qy(0,0) slightly decreased accompanied by 4−15 nm blue shift. The protonation constants were calculated using Hill equation. Table 3 lists the protonation constants for various Table 3. Protonation and Deprotonation Constants (log β2)a of Free Base Mixed Substituted Chlorins and Porphyrins in Toluene at 298 K protonation log β2

nb

r2

log β2

nb

r2

H2TPP(CHD) (1) H2TPPBr2(CHD) (2) H2TPP(DMBA) (3) H2TPPBr2(DMBA) (4) H2TPC(BENAC)2 (8) H2TPCBr2(BENAC)2 (9) H2TPC(IND)2 (10) H2TPCBr2(IND)2 (11)

10.97 11.36 10.29 10.50 5.94 6.04 5.41 6.77

2.0 2.1 2.1 2.1 2.2 2.3 1.9 2.3

0.93 0.96 0.97 0.97 0.99 0.97 0.98 0.98

10.03 9.67 14.12 15.59

1.9 1.9 2.5 2.8

0.97 0.95 0.95 0.96

a

11539

deprotonation

porphyrins/chlorins

Within the error ±0.05 for log β2. bn refers to stoichiometry. DOI: 10.1021/acs.inorgchem.7b01158 Inorg. Chem. 2017, 56, 11532−11545

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Figure 8. UV−vis spectral changes of (a) H2TPP(DMBA) (3) and (b) H2TPC(IND)2 (10) while increasing the concentration of TBAOH in toluene at 298 K. (inset) The corresponding Hill plots.

Figure 9. 1H NMR spectral changes of H2TPCBr2(IND)2 (11) upon addition of TBAOH in CDCl3 at 298 K. *Correspond to TBAOH proton signals.

nature of the porphyrin core, which prevents formation of dianionic species. Moreover, in excess of TBAOH, the bromocontaining porphyrins H2TPPBr2(R) (R = DMBA and CHD) showed a small decrement in the B-band and emergence of a very less intense shoulder; this means that the reactant was in equilibrium with the anionic species and did not move forward to completion, which indicated the slightly nonplanar conformation as compared to H2TPP(R) (R = DMBA and CHD). Interestingly, all the free base chlorins were easily deprotonated even at very low concentration of TBAOH. Table 3 lists the deprotonation constant data in toluene at 298 K. Figure 8b shows the concomitant decrement in absorbance of H2TPC(IND)2 (10) at 418 nm and emergence of a new band at 429 nm with multiple isosbestic points (425, 521, 539, 553, 601, and 655 nm) while increasing the concentration of TBAOH (0.20 × 10−5 to 1.00 × 10−5 M). Simultaneously, the last Qy(0,0) band of H2TPC(IND)2 (10) concomitantly decreased at 650 nm, and a new band rose at 658 nm. As we increased the concentration of TBAOH from 1.00 × 10−5 to 2.18 × 10−5 M, the newly generated B-band at 429 nm

free base mixed substituted chlorins and porphyrins. In all cases, we could obtain the diprotonated chlorin and porphyrin species, which was further confirmed by Hill plot having a slope of ∼2 as shown in insets of Figures 7 and S69 and S70 in the Supporting Information. The bromo porphyrins exhibited higher log β2 as compared to the porphyrins not having bromo groups due to nonplanar conformation of the macrocyclic core. The diprotonated bromochlorins have shown minimal blue shift of ∼4 nm in last Qy(0,0) band as compared to neutral bromochlorins, whereas a prominent blue shift of ∼15 nm was observed after the protonation of unbrominated chlorins. UV−vis titration was performed to determine the deprotonation constants of all the synthesized free base chlorins and porphyrins. All the synthesized free base porphyrins were unable to deprotonate even at a higher concentration of TBAOH, as expected. H2TPP(R) (where R = CHD and DMBA) did not show any spectral changes even after addition of excess of TBAOH (Figure 8a and S71 in the Supporting Information), which was possibly due to the electron-rich 11540

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Article

Inorganic Chemistry Table 4. Electrochemical Redox Dataa of Porphyrinoids in CH2Cl2 at 298 K oxidation(V) porphyrins/chlorins H2TPP(CHD) (1) H2TPPBr2(CHD) (2) H2TPP(DMBA) (3) H2TPPBr2(DMBA) (4) H2TPC(NO2)(DMBA) (7) H2TPC(BENAC)2 (8) H2TPCBr2(BENAC)2 (9) H2TPC(IND)2 (10) H2TPCBr2(IND)2 (11) NiTPP(CHD) (1a) NiTPPBr2(CHD) (2a) NiTPP(DMBA) (3a) NiTPPBr2(DMBA) (4a) NiTPP(BENAC) (5) NiTPPBr2(BENAC) (6) NiTPC(BENAC)2 (8a) NiTPCBr2(BENAC)2 (9a) NiTPC(IND)2 (10a) NiTPCBr2(IND)2 (11a) ZnTPP(CHD) (1b) ZnTPPBr2(CHD) (2b) ZnTPP (DMBA) (4b) ZnTPPBr2(DMBA) (4b) ZnTPC(BENAC)2 (8b) ZnTPCBr2(BENAC)2 (9b) ZnTPC(IND)2 (10b) ZnTPCBr2(IND)2 (11b) a

I

II b

0.93 0.98b 1.10b 1.10b 1.01 1.04 1.18b 0.41b 0.42b 1.08 1.09b 1.06 1.14 1.09 1.18 0.95 1.06 0.58b 0.93 0.85b 0.94b 0.88 0.92 0.81 0.90 0.44b 0.49b

1.46 1.40 1.28b 1.18b 1.39 1.42 1.40b 0.97b 1.04b 1.49 1.515 1.45 1.45 1.30 1.32 1.31 1.30 0.82 1.22 1.07 1.42b 1.13 1.13 1.16 1.17 0.70 0.80

reduction(V) III

1.62 1.61

1.30 1.27

1.24 1.34b

1.06 1.09

ΔE1/2

I

II

2.26 2.22 2.40 2.04 1.98 2.09 2.22 1.78 1.84 2.07 2.08 2.25 2.11 2.07 2.17 2.13 2.02 1.91 1.84 1.82 1.95 2.26 1.81 1.95 2.06 2.24 2.29

−1.33 −1.24 −1.30 −0.94 −0.97b −1.05 −1.04 −0.81b −0.80c −0.99b −0.99b −1.19b −0.97 −0.98c −0.99b −1.18 −0.96 −1.09b 0.91b −0.97 −1.01c −1.38 −0.89b −1.14b −1.16 −1.54 −1.49

−1.68 −1.40b −1.62 −1.20 −1.08 −1.22b −1.22 −1.51 −1.11c −1.31 −1.30 −1.45c −1.27 −1.35c −1.27 −1.29 −1.19 −1.38 −1.16 −1.08c −1.27c −1.66b −1.18c −1.41 −1.31 −1.76

III

−1.40

−1.34c

−1.29 −1.67c −1.59c −1.51b

Versus Ag/AgCl reference electrode. bIrreversible peaks. cData obtained from DPV.

addition. 1H NMR studies support the very high deprotonation constants log β2 (9.67−15.59) of free base chlorins (Table 3). Electrochemical Redox Properties. Redox potentials of macrocyclic core are known to be influenced by conformations of the macrocycle, the nature of the β- and meso-substituents, the solvent polarity, and the core metal ion.23 To determine the influence of macrocyclic conformation and the electronic nature of β-pyrrole substituents, cyclic voltammetric studies of synthesized porphyrinoids were performed in CH2Cl2 containing 0.1 M TBAPF6. Table 4 lists the redox potential data of the synthesized chlorins and porphyrins in CH2Cl2 at 298 K. Figure 10 represents the comparative cyclic voltammograms (CVs) of Ni(II) and Zn(II) chlorins. The CVs of free base chlorins and porphyrins are shown in Figures S73 and S74, respectively in the Supporting Information. The synthesized porphyrins exhibited two one-electron ring-centered oxidations and reductions. However, DMBA substituted free base porphyrins (3 and 4) have shown third oxidation at 1.62 V. The first ring-centered reduction potentials of the MTPPBr2(R) (R = CHD and DMBA; M = 2H, Ni and Zn) exhibited anodic shift in the reduction potentials (0.09−0.36 V) indicating extensive stabilization of the LUMO, whereas there were no observable shifts in oxidation potentials, which was due to destabilization of HOMO induced by nonplanar conformation of macrocyclic core. The free base mono/tri-βsubstituted porphyrins exhibited the following trend in anodic shifts of their first ring-centered reduction potentials:

sequentially decreased at the same wavelength accompanied by 5 nm blue shift (658−653 nm) in the last Qx(0,0) band as indicated by the red line spectrum in Figure 8b. These spectral changes may be attributed to the removal of indanedione protons attached to the β-pyrrole position of the chlorin. The deprotonation constants were calculated using Hill equation. The Hill plot (Figure 8b inset) showed a straight line between log[(TBAOH)] and log(Ai − A0/Af − Ai) having a slope value greater than 2, which further confirmed the deprotonation of the outer proton (active methylene protons). Similar spectral changes were observed for other three free base chlorins (Figure S72 in the Supporting Information). All the chlorins exhibited very high deprotonation constants log β2. The bromochlorins H2TPCBr2(R)2 (R = IND and BENAC) showed slightly higher log β2 values as compared to the H2TPC(R)2 (R = IND and BENAC), which may be due to the combined effect of electronic nature of β-substituents as well as outer proton abstraction. The deprotonation of inner core −NH and abstraction of outer CH proton was also confirmed by 1H NMR spectroscopy in CDCl3 at 298 K. Figure 9 shows the changes in proton signals after addition of TBAOH in CDCl3 at 298 K. The singlet corresponds to two protons of the inner core −NH at −1.56 ppm disappeared after addition of TBAOH as indicated by blue circle in Figure 9. Likewise the doublet of two protons at 3.23 ppm, which corresponds to −CH of IND moiety attached to β-pyrrole position of chlorin, also nearly disappeared as shown by red circle in Figure 9. The proton signals corresponding to other βprotons and phenyl rings were unaffected after TBAOH 11541

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the porphyrin core was more susceptible toward protonation, whereas deprotonation of chlorin macrocycle was more facile.

■

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01158. ORTEP diagrams, crystal structure data, selected average bond lengths and angles, optimized geometries, illustrated frontier molecular orbitals, absorption, fluorescence, and NMR spectra, MALDI-TOF mass spectra, UV−vis spectral titrations, cyclic voltammograms, Cartesian coordinates for synthesized free base (PDF) Accession Codes

CCDC 1532468, 1533453, 1534972, and 1547059 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/ data_request/cif, or by emailing [email protected]. uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

Figure 10. Comparative CVs of (a) Ni(II) chlorins and (b) Zn(II) chlorins.

MTPPBr2(DMBA) > MTPPBr2(CHD) > MTPP(DMBA)

■

> MTPP(CHD)

By introducing electron-withdrawing groups such as indane1,3-dione or benzoylacetonitrile, H2TPC(IND)2 (10) and H2TPCBr2(IND)2 (11) exhibited 60−130 mV anodic shift in oxidation potential with respect to H2TPC, whereas H2TPC(BENAC)2 (8) and H2TPCBr2(BENAC)2 (9) exhibited 130− 270 mV anodic shift in oxidation potentials due to the presence of electron-withdrawing substituents at 2,3-position of the macrocyclic core. Zn(II) and Ni(II) complexes of synthesized chlorins exhibited a similar trend in redox potentials. In contrast to porphyrins, MTPCBr2(R)2 exhibited higher oxidation potentials as compared to MTPC(R)2. Indane-1,3dione substituted chlorins exhibited an irreversible oxidation potential at 0.42 V, which is due to oxidation of β-substituted indane-1,3-dione moiety. Observation of this peak is further proved by recording the CV of indane-1,3-dione under similar conditions. In general, the HOMO−LUMO gap of chlorins was much lower as compared to synthesized porphyrins. H2TPC(IND)s exhibited lowest HOMO−LUMO gap among all synthesized free base porphyrins and chlorins.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +91-1332-284753. Fax: +91-1332-273560. ORCID

Muniappan Sankar: 0000-0001-6667-3759 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We are grateful to Mr. V. Ramkumar, Indian Institute of Technology Madras, for helping single-crystal X-ray analysis of compound 11b and to Ms. N. Singh and Prof. U. P. Singh, Indian Institute of Technology Roorkee, for collecting singlecrystal X-ray data. We are grateful for the financial support provided by Council of Scientific and Industrial Research (01(2694)/12/EMR-II), the Science and Engineering Research Board (SB/FT/CS-015/2012), and the Board of Research in Nuclear Science (2012/37C/61/BRNS). N.C. and N.G. thank Council of Scientific and Industrial Research (CSIR) and the Ministry of Human Resource development (MHRD) India, respectively, for their senior research fellowship.

■

CONCLUSIONS A new synthetic strategy has been provided to access various βsubstituted porphyrins and chlorins to expand the porphyrinoids chemistry. The regioselectivity displayed in these reactions highlights the importance of method that facilitates the direct and predictable introduction of cyclic Michael donors at β-pyrrolic position. The orientation of β-substituents and conformation of macrocyclic core was revealed by single-crystal X-ray analysis. A hypsochromic shift (∼8−10 nm) in the longest wavelength (Qy) absorption band was observed upon substitution of electron-withdrawing groups (IND and BENAC) at 2,3-postion of meso-tetraphenylchlorin. Notably, HOMO−LUMO gap was altered upon substitution at 2,3 position of meso-tetraphenylchlorin, whereas bromo-substitution at 12,13 position of 2,3-disubstituted trans-chlorins reduces the gap between LUMO and LUMO+1. Electronic nature of synthesized macrocyclic skeletons was authenticated by protonation and deprotonation studies, which revealed that

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