Expanded Porphyrins

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-- 08:35, 28 August 2017 (CEST)


Jcontreras (talk) 08:57, 28 August 2017 (CEST)[edit]


Judy Wu[edit]



In recent years, expanded porphyrins have emerged as a new class of functional molecules in light of their large conformational flexibility, rich metal coordination behaviour, unprecedented chemical reactivities and exceptional nonlinear optical properties [1,2,3].

Expanded porphyrins are macrocycles consisting of more than four pyrrole rings linked together either directly or through one or more spacer atoms in such a manner that the internal pathway contains a minimum of 17 atoms [4]. Owing to their tunable photophysical and chemical properties with external stimuli, expanded porphyrins represent a very promising platform to develop molecular switches for molecular electronic devices [5-7]. Molecular switches are regarded as the most basic component in molecular electronic devices that can reverse from an active/on state to a passive/off state. The switch between two or more states with distinct properties is triggered by external stimuli such as light, pH or voltage [8]. As opposed to normal switches, molecular switches are extremely tiny and their application in nanotechnology, biomedicine and computer chip design opens up whole new horizons.

Probably the most fascinating property of expanded porphyrins resides in their capacity to switch between several distinct π-conjugation topologies encoding different properties [11]. Besides the Hückel topology, expanded porphyrins can adopt a variety of conformations with Möbius and twisted-Hückel topologies (Scheme 1) that can be interconverted under certain conditions [12,13]. Recent experimental studies [14,15] proved that the photophysical and nonlinear optical (NLO) properties of expanded porphyrins strongly depend on their molecular topology and macrocyclic aromaticity.


--Henry (talk) 21:44, 2 September 2017 (CEST)[edit]

Mobius' own diagrams found in his papers after his death
The terms Möbius and twisted-Hückel might be considered as misleading. Hückel never really considered conjugated twisted systems (that was first done for chemistry by Heilbronner, in 1964 for one twist. Double twisted conjugated systems were first considered in 2005[1]). The Möbius band on the other hand is actually a family of bands, containing 1,2,3,4,5 twists. The famous recycling logo for example has three twists. I prefer to give Möbius the credit not merely for the single-twist band for which is is famous, but all the higher level twists which he clearly also described.

The degree of twisting is indeed described by a linking number Lk, which is a topological invariant with integer values (in units of π) for conjugated systems and is also associated with the linking number theorem; Lk = Tw + Wr (where W is writhe and T is local twist). Thus the classical Möbius system is Lk =±1π, the twisted-Hückel is probably Lk =±2 (but it could be ±4, ±6, etc), the trefoil is ±3 etc (where the sign of Lk indicates the handedness of the chirality, much like M and N). So rather than simply state Möbius or twisted-Hückel, we should really describe these higher order twisted systems using linking numbers Lk and if attribution is needed, to call the ones with Lk >1 after Möbius. Benzene of course is Lk = 0, which is a special case reserved for Hückel.[2][3][4].[5].

In discussing extended or expanded porphyrins, we should also take into consideration the theorem; Lk = Tw + Wr, and to consider what conformational effect the writhe is making to the system.

  1. DOI:10.1021/ol0518333
  2. See H. S. Rzepa in Six questions on topology in theoretical chemistry, DOI:10.1016/j.comptc.2014.09.028
  3. See S. M. Rappaport† and H. S. Rzepa, Intrinsically Chiral Aromaticity. Rules Incorporating Linking Number, Twist, and Writhe for Higher-Twist Möbius Annulenes, J. Am. Chem. Soc., 2008, 130, 7613–7619. DOI:10.1021/ja710438j for further details of this and the associated aromatic selection rules
  4. See See C S. M. Allan and H. S. Rzepa, Chiral Aromaticities. AIM and ELF Critical Point and NICS Magnetic Analyses of Möbius-Type Aromaticity and Homoaromaticity in Lemniscular Annulenes and Hexaphyrins, J. Org. Chem., 2008, 73, 6615–6622.DOI:10.1021/jo801022b for a pertinent discussion of extended porphyrins
  5. H. S. Rzepa, The Chiro-optical Properties of a Lemniscular Octaphyrin, Org. Lett., 2009, DOI:10.1021/ol901172g

--Jcontreras (talk) 23:02, 2 September 2017 (CEST)Fair enough! I have added it to the slides!

Jcontreras (talk) 09:12, 28 August 2017 (CEST)[edit]

Hence, it is fundamental to understand how the interesting properties at the macromolecular level might depend on the conformation, ultimately related to the bonding pattern. Based on the [36]octaphyrin I will show that the bond metallicity and delocalization index provide a qualitative description of the electron delocalization in octaphyrins and these changes are related to conductivity. The measures of bond metallicity indicate a local variation of the electron delocalization when the topological change is restricted to one part of the molecule and semi-local for global topological changes (e.g. folding). In the local cases, our approach enables to identify which conformational switch would be more efficient from an electronic device perspective. Since our indices were able to detect the difference between those two interconversions, they could also be used to perform a qualitative analysis (Scheme 2).



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