Effective conjugation lengths

Conjugation of the 7t-electrons of the carbon-carbon double bond with the LUMO sulfur 3d-orbitals would be expected to stabilize the Hiickel 4n -I- 2 (n = 0) array of n-electrons in the thiirene dioxide system. No wonder, therefore, that the successful synthesis of the first member in this series (e.g. 19b) has initiated and stimulated several studies , the main objective of which was to determine whether or not thiirene dioxides should be considered to be aromatic (or pseudo-aromatic ) and/or to what extent conjugation effects, which require some sort of n-d bonding in the conjugatively unsaturated sulfones, are operative within these systems. The fact that the sulfur-oxygen bond lengths in thiirene dioxides were found to be similar to those of other 802-containing compounds, does not corroborate a Hiickel-type jr-delocalization... 

Commonly, the increase in the conjugation chain length leads to the bathochromic shift of the absorption bands. The optical and photoelectrical properties analysis needs to take into account not only formal polyconjugation following from the molecule chemical structure but effective conjugation defined by sterical factors too. 

Allinger NL, Chen K, Katzenellenbogen JA, Wilson SR, Anstead GM. Hyper-conjugative effects on carbon-carbon bond lengths in molecular mechanics (MM4). J Comput Chem 1996 17 747-755. 

The noticeable distortion of the C3 ring and measured C-C, C-O and U-O bond lengths reflect the conjugation effect, the chemical environment of individual atoms, steric congestion around the respective uranium centers, and variation caused by experimental errors. 

The molecular hyperpolarizability of charged ionic compounds was not directly accessible for measurement until the HRS method was reestablished [36]. Therefore only recently research has been focused on the effect of donor or acceptor substitution and elongation of the conjugation path length to demonstrate the engineering guidelines for enhancing molecular optical nonlinearities. 

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