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

Abstract

One-pot facile synthesis and characterization of novel β-substituted 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. Size-dependent 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.

β‐meso‐Annulated meso‐Tetraarylchlorins: A Study of the Effect of Ring Fusion on Chlorin Conformation and Optical Spectra

“…The Soret band of Ni(II)DFPs was found to be 38–53 nm red‐shifted (for 32 Ni ( IND ) H 2 : λ max =463 nm; for 32 Ni ( MN ) H 2 : λ max =465 nm) as compared to their corresponding non‐fused nickel(II) trans ‐chlorin precursors 31 Ni . Similarly, free base H 2 DFCs exhibited ∼16–22 nm red shifts in their Soret band (for 33 ( IND ) H 2 : λ max =434 nm; for 32 ( MN ) H 2 : λ max =423 nm) (Table , entries 7–12) compared to precursor free‐base trans ‐chlorins 31 H 2 . The UV‐visible spectrum of indanedione‐appended free‐base DFCs had more red‐shifted longest wavelength Q y bands (for 33 ( IND ) Br 2 : λ max =681 nm) compared to dicyanomethyl‐appended free‐base DFCs (for 33 ( MN ) Br 2 : λ max =667 nm) (Table , entries 7–12) and the spectral feature of both were indeed ‘chlorin‐like’.…”

“…All the synthesized double fused metalloporphyrinoids showed a red‐shifted Soret absorption band due to the extended π‐conjugation and twisted macrocyclic conformation, as confirmed by single crystal X‐ray diffractometry . The Soret band of Ni(II)DFPs was found to be 38–53 nm red‐shifted (for 32 Ni ( IND ) H 2 : λ max =463 nm; for 32 Ni ( MN ) H 2 : λ max =465 nm) as compared to their corresponding non‐fused nickel(II) trans ‐chlorin precursors 31 Ni . Similarly, free base H 2 DFCs exhibited ∼16–22 nm red shifts in their Soret band (for 33 ( IND ) H 2 : λ max =434 nm; for 32 ( MN ) H 2 : λ max =423 nm) (Table , entries 7–12) compared to precursor free‐base trans ‐chlorins 31 H 2 .…”

“…The higher electron density of the indanone ring rendered the moiety easier to oxidize as revealed by electrochemical studies, while the more electron‐withdrawing malononitrile group made electrochemical oxidation difficult; implying a reduced HOMO‐LUMO gap in the indandione series compared to the malononitrile series . Overall, greater degree of non‐planarity induced by multiple ring fusions, does induce moderate to strong Q y band red‐shifts in both indanone and malononitrile based DFCs compared to the Q y bands of their precursor trans‐ chlorins (Q y =∼648 nm, Δλ max =24–40 nm in favor of the DFCs). This was also influenced by the nature of the opposite β,β’‐substituents (H, Br, Ph) .…”

Electrochemistry of Tri‐substituted Porphyrins with β‐Appended Ethyl Acetoacetate and Acetylacetone in Neutral and Basic Nonaqueous Solvents

“…The easiest to oxidize porphyrin in Figure has two β ‐Ph groups ( E 1/2 =0.83 and 1.14 V) and the hardest to oxidize has two β ‐Br groups ( E 1/2 =0.94 and 1.24 V), thus following the expected trend in electron‐donating or ‐withdrawing properties of the β ‐pyrrole substituents. In line with this expectation, a more facile first reduction is observed for the dibromo‐substituted porphyrin ( E 1/2 =−1.12 V) but there is little difference in potential between E 1/2 values for the first reduction of the copper β ‐Ph or β ‐H substituted compounds (−1.29 and −1.27 V), as was previously reported in the literature for related tri‐substituted porphyrins with these two β ‐substituents …”

“…It is significant to note the quite large substituent effect of the two electron‐withdrawing β ‐Br groups on the electroreductions of the investigated EAA‐appended porphyrins. In general, adding two β ‐Br substituents to a tetraphenylporphyrin will result in a positive shift of each reduction potential, where the magnitude of the shifts amounts to 50–60 mV per Br group . However, in the case of CuTPP(EAA)Br 2 and CuTPP(EAA), the difference in half‐wave potentials between the first reduction amounts to 85 mV per Br group (170 mV total) while the Δ E 1/2 between half‐wave potentials for the second reversible reduction of the two copper complexes is 300 mV, a shift of 150 mV per Br group as seen in Figure ( E 1/2 =−1.69 and −1.39 V).…”