Meso-tetraarylporphyrins are widely explored due to their facile synthesis, characteristic deep colors, intense fluorescence, and high photochemical stability. Moreover, porphyrinoids have ability to chelate a wide variety of metal ions in their inner core.¹ The electronic properties of porphyrins can be easily modulated by appending different substituents at β-and/or meso-positions. The substituents at β-positions of the porphyrins exert much larger steric and electronic effects on the porphyrin π-system than substituents at the meso-aryl positions.² The optical properties of porphyrin π-system can be tuned by the modification of macrocyclic core through extended π-conjugation^3a or by converting into chlorins^3b or by tuning the conformation of the macrocycle.^3c

Chart 1. Molecular structures synthesized monofused porphyrins and difused chlorins/porphyrins.

Herein, we report the facile regioselective synthesis of monofused porphyrin and difufused chlorins/porphyrins (Chart 1) in good to excellent yields. Single crystal X-ray diffraction analysis revealed the quasi planar to moderate nonplanar conformation of difused chlorins due to trans-orientation of β-substituents (Figure 1), whereas difused porphyrin exhibited higher mean plane deviation from 24-atom core (Δ24) as compared to chlorins.⁴

Figure 1. ORTEP diagrams showing top and side views of NiDFP(IND)₂ (a and c) and trans-NiDFC(MN)₂ (b and d), respectively.

Difused porphyrins exhibited large red-shift (Δλ_max = 40-55 nm) in their Soret band as compared to their precursors, i.e. Ni(II) chlorins whereas difused chlorins showed marginal red-shift (Δλ_max = 16-22 nm) as compared to the corresponding precursors, i.e. free base trans-chlorins. The significant red-shift in the Soret band was attributed to the twisted macrocyclic core as shown in the ORTEP side views (Figures 1c and 1d).

Figure 2. Comparative electronic absorption spectra of trans-Ni(II)chlorin, NiTPC[CH(CN)₂]₂ and the corresponding difused porphyrin, NiDFP.

These fused porphyrinoids have shown interesting electrochemical redox properties. Notably, antipodal-NiDFP(IND)₂(Ph)₂ exhibited three oxidations, whereas H₂DFC(IND)₂X₂ and NiDFP(IND)₂X₂ (X = Br, Ph) have shown three reversible reductions. These multiple oxidations and reductions aroused due to combined ‘push-pull’ effect of β-substituents and fusion induced twisted macrocyclic core. The first oxidation and reduction potentials of H₂DFC(IND)₂ were cathodically shifted (130 mV and 240 mV) as compared to their precursors, i.e H₂TPC(IND)₂ due to extended π-conjugation. In this presentation, the synthesis, spectral, intriguing redox properties will be discussed in detail.

Figure 3. Comparative cyclic voltammograms of (a) NiDFPs (b) H₂DFCs.

References

1. The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: San Diego, 2000−2003; Vol. 1−20.
2. (a) Medforth, C. J.; Senge, M. O.; Smith, K. M.; Sparks, L. D.; Shelnut, J. A. Am. Chem. Soc. 1992, 114, 9859−9869. (b) Retsek, J. L.; Medforth, C. J.; Nurco, D. J.; Gentemann, S.; Chirvony, V. S.; Smith, K. M.; Holten, D. J. Phys. Chem. B 2001, 105, 6396−6411.
3. (a) Akhigbe, J.; Luciano, M.; Zeller, M.; Brückner, C. Org. Chem. 2015, 80, 499−511. (b) Brückner, C.; Samankumara, L.; Ogikubo, J. In Handbook of Porphyrin Science; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; World Scientific: River Edge, NY, 2012; Vol. 17, pp 1−112. (c) Kojima, T.; Nakanishi, T.; Harada, R.; Ohkubo, K.; Yamauchi, S.; Fukuzumi, S. Chem. Eur. J. 2007, 13, 8714−8725.
4. (a) Chaudhri, N.; Grover, N.; Sankar, M.; Chem. 2017, 56, 11532–11545. (b) Chaudhri, N.; Grover, N.; Sankar, M.; (Submitted for Publication).

MS sincerely thanks DST, India and Indo-US Science and Technology Forum (IUSSTF), New Delhi for financial support.