The core-modified porphyrins resulting from the replacement of one or two pyrrole rings have received less attention inspite of their novel properties such as stabilization of metals in unusual oxidation states. Our group at IIT-Bombay, has been interested in synthesis of novel core-modified porphyrins to explore their potential for various applications in place of normal porphyrins (N4 core). We are working on different aspects of core-modified porphyrin chemistry and developed some new method(s) to synthesize new heteroporphyrin systems with desirable functional groups as well as some complicated heteroporphyrin based systems as outlined below:

We synthesized a series of heteroporphyrin building blocks with N2S2, N3S and N3O porphyrin cores containing functional groups such as iodo-, ethyne-, aldehyde-, pyridyl etc groups. These porphyrin building blocks were used to construct a covalently linked unsymmetrical pentamer containing one N2S2 and four N4 porphyrin cores and also non-covalently linked trimer containing one N3S and two N4 porphyrin cores. We also synthesized donor appended N3O, N3S and N2S2 porphyrin systems and demonstrated energy transfer from donor to heteroporphyrin core.

1. Eur. J. Org. Chem. 2500-2517 (2005) 3. Inorg. Chim. Acta. 358, 2671-2679 (2005) 5. Chem. Phys. Lett. 395, 87-91 (2004) 7. Tetrahedron Lett. 42, 8547-8550 (2001) 9. JCS Perkin Trans. 1 1644-1648 (2001) 11. Tetrahedron Lett. 41, 3709-3712 (2000)

2. SynLett 2199-2203 (2005) 4. Tetrahedron, 60, 8437-8444 (2004) 6. Tetrahedron 58, 5347-5356 (2002) 8. SynLett 1635-1637 (2001) 10. Chem. Lett. 836-837 (2000)

On introduction of substituents at β-positions, the electronic properties are expected to alter significantly because the substituents are in direct conjugation with the porphyrin π-system. Interestingly, there are no reports available on β-substituted heteroporphyrins. We synthesized several N2S2 and N3S porphyrins containing long alkoxy groups such as butoxy, octyloxy and dodecyloxy groups at β-thiophene carbons. These compounds are expected to show good liquid crystalline properties. We also synthesized several β-pyrrole and β-thiophene substituted thiaporphyrins and studied their effects on electronic properties of the porphyrins.

We synthesized a series of meso-thienyl and meso-furyl substituted heteroporphyrins with N3S, N2S2 and N3O porphyrin cores. The five membered thienyl and furyl groups at meso carbons instead of six membered aryl groups altered the electronic properties significantly and these compounds may have potential applications in materials chemistry.

18. Eur. J. Org. Chem. 4392-4400 (2003) 20. Tetrahedron Lett. 43, 9453-9455 (2002)

19. Tetrahedron 59, 6131-6139 (2003)

Porphyrins with free meso carbons are highly desirable synthetic precursors to construct complex porphyrin based systems. Thiaporphyrins with one, two and four meso free carbons were synthesized by using easily available thiophene diols. The reactivity of meso free carbons was demonstrated by carrying out some reported reactions and some important functionalized thiaporphyrins were synthesized.

We also synthesized N3S, N3O, N2S2, N2O2 and N2SO porphyrins with one free meso carbons which were used to synthesize phenylethyne bridged unsymmetrical porphyrin dimers.

We recently developed for the first time a simple one step rapid synthetic route to prepare mono-functionalized N3S and N3O porphyrins using easily available thiophene and furan mono-ols respectively. We also developed an alternate unsymmetrical diol method to improve the yields of mono-functionalized N3S porphyrins and the method was then extended further to synthesize mono-functionalized N2S2 and N2SO porphyrins. These mono-functionalized heteroporphyrin building blocks were used to prepare several complex porphyrin systems such as unsymmetrical covalent and non-covalent dimers containing two different porphyrin cores. This kind of systems were not accessible earlier due to unavailability of the proper heteroporphyrin building blocks.

24. Eur. J. Org. Chem. 1693-1697 (2004)

25. J. Org. Chem. 69, 6796-6811 (2004)

Interestingly, the same mono-ol method can also be used to synthesize the tri-functionalized 21-thia and 21-oxaporphyrin systems which were not possible earlier. The tri-functionalized heteroporphyrin systems were used to synthesize non-covalent unsymmetrical porphyrin tetramers. Thus the mono-ol method developed by us has opened up a lot of avenues for the development of heteroporphyrin chemistry.