The Helicenes: Potential Carriers of Diffuse Interstellar Bands

In this work, the helicenes are postulated as potential carriers of the diffuse interstellar bands, DIBs. The helicenes are nonplanar cata-condensed molecules structurally related to the polyacenes, and polyacenes have been already proposed as conceivable carriers of the DIBs. In here, the possible identity of the carrier of the DIB at 4502 angstrom is studied and potentially identified as a cata-condensed helicene using spectroscopic, statistical, and stability methods. First, statistic and spectroscopic information allow the fitting of the regression equation that governs the variation of the location of the published absorbance p-bands as a function of the number of fused aromatic rings (nFAR) for planar and helical cata-condensed polycyclic aromatic hydrocarbons with 1-14 FAR, and the application of this equation to establish the identity of the possible carrier of the 4502 angstrom DIB, that is, [26]helicene (C106H56). The difference between the calculated location of the p-band from the fitted equation and the DIB location under study is equal to 0.21%. Subsequently, an aromaticity argument demonstrates the presence of pi-delocalizing benzene effect in helicenes with the general formula C2+4NH2N+4, for example, [26]helicene, that would theoretically provide supplementary stability to the molecule to justify its occurrence. Finally, a feasible mechanism of formation of helicenes in the interstellar medium is proposed that involves successive additions of n-butane units to a phenanthrene moiety. To the best of our knowledge, this is the first report about the possible presence of cata-condensed helical molecules in the DIBs and the first study that theorizes that [26]helicene is the possible carrier of the DIB at 4502 angstrom.

Automated summary

This work explores the potential of helicenes as carriers of the diffuse interstellar bands, with [26]helicene identified as a possible candidate for the 4502 angstrom DIB. Statistical and spectroscopic analysis support this identification, while aromaticity arguments suggest additional stability for helicenes. The study also proposes a feasible mechanism for the formation of helicenes in the interstellar medium.