Versatile Synthetic Route for β-Functionalized Chlorins and Porphyrins by Varying the Size of Michael Donors: Syntheses, Photophysical, and Electrochemical Redox Properties

Indane-1,3-Dione: From Synthetic Strategies to Applications

Indane-1,3-dione is a versatile building block used in numerous applications ranging from biosensing, bioactivity, bioimaging to electronics or photopolymerization. In this review, an overview of the different chemical reactions enabling access to this scaffold but also to the most common derivatives of indane-1,3-dione are presented. Parallel to this, the different applications in which indane-1,3-dione-based structures have been used are also presented, evidencing the versatility of this structure.

Unsymmetrically β-Functionalized π-Extended Porphyrins: Synthesis, Spectral, Electrochemical Redox Properties, and Their Utilization as Efficient Two-Photon Absorbers

Two new series of unsymmetrically β-functionalized porphyrins, MTPP(NO₂)MA (1M), (MA = methyl acrylate) and MTPP(NO₂)MB (2M) (MB = mono-benzo) (where M = 2H, Co(II), Ni(II), Cu(II) and Zn(II)), were synthesized and characterized by various spectroscopic techniques. The saddle shape conformation of ZnTPP(NO₂)MAPy and ZnTPP(NO₂)MB was confirmed by single-crystal X-ray analysis. Density functional theory (DFT) calculation revealed that NiTPP(NO₂)MB has a severe nonplanar geometry possessing a high magnitude of ΔC_β = ±0.727 Å and Δ24 = ±0.422 Å values among all other porphyrins. Synthesized β-substituted porphyrins exhibited red-shifted B- and Q-bands corresponding to their parent molecule due to the electron-withdrawing peripheral substituents. Notable redshift (Δλ_max = 50-60 nm) in electronic spectral features and with weak-intensity emission spectral features were observed for the free-base porphyrins and Zn(II) complexes compared to H₂TPP and ZnTPP, respectively. The first-ring reduction potential of MTPP(NO₂)MA (1M) exhibited 0.21-0.5 V anodic shift, whereas 0.18-0.23 V anodic shift was observed in the first-ring oxidation potential compared to the corresponding MTPPs due to the presence of electron-withdrawing β-substituents at the periphery of the macrocycle. Interestingly, NiTPP(NO₂)MA (1Ni) has shown an additional Ni^II/Ni^III oxidation potential observed at 2.05 V along with two ring-centered oxidations. The first-ring reduction and oxidation potentials of MTPP(NO₂)MB (2M) have shown 0.39-0.46 and 0.19-0.27 V anodic shifts with respect to their corresponding MTPPs. The nonlinear optical (NLO) properties of all of the porphyrins were investigated, and the extracted nonlinear optical parameters revealed intense reverse-saturable absorption (RSA) behavior and the self-focusing behavior with positive nonlinear refractive index in the range of (0.19-1.75) × 10^-17 m²/W. Zn(II) complexes exhibited the highest two-photon absorption coefficient (β) and cross section (σ_TPA) of ∼95 × 10^-12 m/W and 19.66 × 10⁴ GM, respectively, among all of the metal complexes.

Nickel(II) monobenzoporphyrins and chlorins: synthesis, electrochemistry and anion sensing properties

A series of nickel(II) monobenzochlorins (MBCs) and monobenzoporphyrins (MBPs) containing β-appended or meso,β-fused indanedione (IND) or malononitrile (MN) groups were synthesized and characterized for their physicochemical, electrochemical and anion sensing properties. Each investigated compound contained four meso-phenyl rings and a single β,β'-fused 4,5-di(methoxycarbonyl)benzene ring, with the chlorins represented as NiMBC(Y)₂(R)₄ and the porphyrins as NiMBP(Y^F)₂, where Y is an indanedione (IND) or malononitrile (MN) group, R = H or Br and Y^F is a meso,β-fused IND or MN substituent. One of the investigated compounds, NiMBP(IND)₂, was structurally characterized and shown to possess a ruffled macrocyclic conformation. The monobenzochlorins, NiMBC(IND)₂, NiMBC(IND)₂Br₄ and NiMBC(MN)₂, reversibly respond to basic anions such as CN^-, F^-, OAc^- and H₂PO₄^- through a visible color change assigned to the deprotonation of the vicinal proton on the appended IND or MN substituents. The malononitrile-fused Ni^II monobenzoporphyrin, NiMBP(MN)₂, exhibited a selective but irreversible visual detection of cyanide ions (LOD = 2.23 ppm). This reaction afforded a tri-fused π-extended monobenzoporphyrin product represented as NiMBP(VCN)₂ (where VCN = meso,β-fused vinyl cyanide) in non-aqueous media and proceeded via anion induced electron transfer (AIET). The in situ generated π-extended porphyrin was also isolated and characterized as to its physicochemical and electrochemical properties and found to possess a narrow electrochemical HOMO-LUMO gap of 1.46 V along with a near-IR (NIR) absorption band located at 861 nm.

Effect of fused indanedione (IND) groups and antipodal β-substituents on electrochemical properties of unsymmetrical metalloporphyrins

The interacting effect of fused indanedione (IND) groups and antipodal Br [Formula: see text]-substituents on the electroreduction of free-base and transition metal tetraphenylporphyrins (TPPs) are elucidated through the use of cyclic voltammetry. A systematic study of the reduction potentials for mono- and di-fused derivatives of MTPP(IND)(R)2 and MTPP(IND)2(R)2 where R = Br or H and M = Cu[Formula: see text], Ni[Formula: see text], Zn[Formula: see text] or H2 is carried out in pyridine containing 0.1 M tetrabutylammonium perchlorate (TBAP) as supporting electrolyte and demonstrates how the overall electrochemical properties of these compounds can be tuned by systematic changes in the number of meso-[Formula: see text]fused IND groups on the molecule and the presence or absence of electron-withdrawing Br [Formula: see text]-substituents. Under these solution conditions, two reversible one-electron reductions are seen for H2TPP, while three reversible reductions are obtained for H2TPP(IND) or H2TPP(IND)Br2 and four for H2TPP(IND)2Br2. Multiple reductions also occur for the other investigated mono- and di-fused metalloporphyrins. Plots of measured [Formula: see text] values for the two ring-centered reductions of compounds in a given series vs. the number of fused IND groups are linear and with slopes varying in magnitude between 15 and 230 mV/IND depending upon the product of the electron transfer (anion radical or dianion), the specific central metal ion and the presence or absence of Br substituents on the pyrrole ring opposite the fused IND group(s). It is anticipated that the information from this study can be used for predicting the reduction potentials for yet to be synthesized compounds where a specific redox reactivity is desired.

Synthesis, photoinduced amination and topological indices of novel porphyrin dyads

Transition-metal-catalyzed homo and hetero coupling is a rapidly growing area of research. This work refers to the nickel-catalyzed photoinduced amination study of [Formula: see text]-bromotetraarylporphyrin and its Ni(II), Zn(II) and Cu(II) complexes. The selective mono [Formula: see text]-bromination of 5,10,15,20-tetrakis(4[Formula: see text]-isopropylphenyl)porphyrin was achieved with [Formula: see text]-bromosuccinimide. Under similar conditions, [Formula: see text]-bromination of Ni(II), Zn(II) and Cu(II) complexes of 5,10,15,20-tetrakis(4[Formula: see text]-isopropylphenyl)porphyrin successfully afforded the corresponding 2-bromometalloporphyrins. The [Formula: see text]-bromoporphyrin/metalloporphyrins were coupled with three different amines through the creation of the C–N bond by using an economical and air-tolerant photoactive catalyst (NiBr[Formula: see text] · 3H2O) at room temperature under 365 nm radiations. Nickel-catalyzed amination yields are compared with the traditional Buchwald–Hartwig amination yields. Due to the low operational cost, photoinduced nickel-catalyzed C–N couplings were found to be more economical than the Buchwald–Hartwig amination procedure, although the latter afforded higher yields. The nickel-catalyzed photoamination reaction was also extended for the one-pot synthesis of pyridine-3,5-diamine bridged porphyrin dyad. The intramolecular cyclization of the pyridine-3,5-diamine-bridged porphyrin dyad afforded a novel quinolino-fused porphyrin dyad. Degree- and distance-based topological indices of the newly synthesized porphyrins were calculated and correlated with their molar refractivity. All newly synthesized porphyrins are characterized by UV-vis, FTIR, 1H NMR, elemental analysis and mass spectrometry.

Structural, Photophysical, and Electrochemical Properties of Doubly Fused Porphyrins and Related Fused Chlorins

The electrochemical and physicochemical properties of tetraphenylporphyrins and tetraphenylchlorins with two fused indanedione (IND) or malononitrile (MN) groups and two antipodal Br, Ph, or H β-substituents are investigated in nonaqueous media. These compounds were synthesized by oxidative fusion of free-base trans-chlorins, followed by metalation. The corresponding free-base di-fused chlorins were also isolated as intermediates and characterized for comparisons. The examined di-fused porphyrins (DFP) and di-fused chlorins (DFC) are represented as MDFP(Y)₂(R)₂ and H₂DFC(Y)₂(R)₂, where M = 2H, Cu^II, Ni^II, Zn^II, and Co^II, Y is a fused indanedione (IND) or malononitrile group (MN), and R = H, Br, or Ph. The IND- and MN-appended compounds in both series exhibit the expected two one-electron oxidations but quite different redox behavior is observed upon reduction, where the free-base IND-appended chlorins show four reversible one-electron reductions, compared to only two for the related free-base MN-appended chlorins. Although porphyrin trianions and tetraanions have been recently described for derivatives with highly electron-withdrawing and/or π-extending substituents, this seems not to be the case for the doubly fused IND-chlorins, where the first two one-electron additions are proposed to be located at the conjugated macrocycle and the last two at the fused IND groups, each of which is reduced at a different potential, consistent with the behavior expected for two equivalent and interacting redox centers. Unlike the examined chlorins, which are all stable in their electroreduced forms, the electrogenerated anionic forms of the di-fused porphyrins are all highly reactive and characterized by cyclic voltammograms having reduction peaks not only for the synthesized compounds added to solution but also for one or more new redox active species formed at the electrode surface in homogeneous chemical reactions following electron transfer. Comparisons are made between electrochemical behavior of the structurally related porphyrins and chlorins and the sites of electron transfer assigned on the basis of known electrochemical diagnostic criteria. One of the compounds, ZnDFP(MN)₂, was also structurally characterized as having a ruffled and twisted macrocyclic conformation.

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

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.

Structure and conformation of photosynthetic pigments and related compounds. 15. Conformational analysis of chlorophyll derivatives – implications for hydroporphyrinsin vivo

Controlling the function of chlorophylls depends in part on their 3D conformation. The NSD program presents a powerful tool to identify the distortion modes in phytochlorins.

Nickel-Induced Skeletal Rearrangement of Free Base trans-Chlorins into Monofused NiII-Porphyrins: Synthesis, Structural, Spectral, and Electrochemical Redox Properties

Several ruffled β-to- ortho-phenyl monofused metalloporphyrins (M^IIMFPs) with 1,3-indanedione functionality have been synthesized by oxidative fusion of free base trans-chlorins via nickel insertion followed by the skeletal rearrangement of macrocycle. The synthesized monofused porphyrins exhibited red-shifted electronic spectral features as compared to precursor, trans-chlorins or structurally related unfused porphyrins (mono/tri-β-substituted porphyrins) due to extended π-conjugation and nonplanar conformation of the macrocyclic core. Four of the synthesized porphyrins were structurally characterized by X-ray diffraction analysis. Ring fusion resulted in twisted macrocyclic conformation, and the twist angles (the angle between 24-atom core and fused part) were found to be in the range of 20.97° to 27.97°. NiMFP(IND)Ph₂ (3b) exhibited modestly ruffled conformation of the macrocyclic core which was further confirmed by higher Δ24 (0.362 Å) and ΔC_β (0.279 Å) values. Free base monofused porphyrins have shown upfield shifted (Δδ = 0.27-0.29 ppm) inner core NHs as compared to precursors ( trans-chlorins), whereas much downfield shifted (Δδ = 0.9 ppm) as compared to H₂TPP. Electron-rich Ni(II) complexes, that is, NiMFP(IND)R₂ (where R = H and Ph) (1b and 3b), exhibited metal centered oxidation (Ni^II/Ni^III) due to extended π-conjugation of macrocyclic core and electron-donating β-substituents. Facile synthesis with good yields (60-90%), unexpected nickel-induced oxidative fusion, and selective conversion of trans-chlorins into monofused Ni^II-porphyrins are the significant features of the present work.