On infinitenes - Reliable calculation of λ∞ and molecular modeling of lemniscate structured carotenoids

The absorption limit for an infinite number of conjugated double bonds has been a topic of considerable interest for over seventy years. Establishing the limit (hereafter lambda(infinity)) consists in collecting the spectral data of homologous polyenes (in most previous investigation with <= 17 double bonds), fitting the absorption values to a formula with one or more adjustable parameters, and identifying lambda(infinity) by extrapolation. Here we report establishing the unsettled value of lambda(infinity) by taking as basis three recently synthesized long carotenoid families culminating in 19, 23 and 27 conjugated double bonds. Excellent fits of experimental data have been obtained by using new formulas allowing a confident lambda(infinity)-extrapolation to carotenoids with infinite length. UV visible lambda(max) values of long carotenoids were also calculated by using Time Dependent Density Functional Theory (TDDFT). The theoretical results agree well with the experimental trend. Molecular modeling confirmed that accumulating infinite conjugated double bonds in carotenoids imparts to the molecule the shape of a lemniscate, the symbol for infinity. The name infinitene is proposed for a hypothetical lemniscate carotenoid. (C) 2017 Elsevier B.V. All rights reserved.