Probing Colossal Carbon Rings

Abstract

Carbon aggregates containing between 10 and 30 atoms preferentially arrange themselves as planar rings. To learn more about this exotic allotrope of carbon, electronic spectra are measured for even cyclo[<i>n</i>]carbon radical cations (<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msup><mml:mrow><mml:msub><mml:mrow><mml:mi>C</mml:mi></mml:mrow><mml:mrow><mml:mn>14</mml:mn></mml:mrow></mml:msub></mml:mrow><mml:mrow><mml:mo>+</mml:mo></mml:mrow></mml:msup></mml:math>-<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msup><mml:mrow><mml:msub><mml:mrow><mml:mi>C</mml:mi></mml:mrow><mml:mrow><mml:mn>36</mml:mn></mml:mrow></mml:msub></mml:mrow><mml:mrow><mml:mo>+</mml:mo></mml:mrow></mml:msup></mml:math>) using two-color photodissociation action spectroscopy. To eliminate spectral contributions from other isomers, the target cyclo[<i>n</i>]carbon radical cations are isomer-selected using a drift tube ion mobility spectrometer prior to spectroscopic interrogation. The electronic spectra exhibit sharp transitions spanning the visible and near-infrared spectral regions with the main absorption band shifting progressively to longer wavelength by ≈100 nm for every additional two carbon atoms. This behavior is rationalized with a Hückel theory model describing the energies of the in-plane and out-of-plane π orbitals. Photoexcitation of smaller carbon rings leads preferentially to neutral C₃ and C₅ loss, whereas rings larger than <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msup><mml:mrow><mml:msub><mml:mrow><mml:mi>C</mml:mi></mml:mrow><mml:mrow><mml:mn>24</mml:mn></mml:mrow></mml:msub></mml:mrow><mml:mrow><mml:mo>+</mml:mo></mml:mrow></mml:msup></mml:math> tend to also decompose into two smaller rings, which, when possible, have aromatic stability. Generally, the observed charged photofragments correspond to low energy fragment pairs, as predicted by density functional theory calculations (CAM-B3LYP-D3(BJ)/cc-pVDZ). Using action spectroscopy it is confirmed that <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msup><mml:mrow><mml:msub><mml:mrow><mml:mi>C</mml:mi></mml:mrow><mml:mrow><mml:mn>14</mml:mn></mml:mrow></mml:msub></mml:mrow><mml:mrow><mml:mo>+</mml:mo></mml:mrow></mml:msup></mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msup><mml:mrow><mml:msub><mml:mrow><mml:mi>C</mml:mi></mml:mrow><mml:mrow><mml:mn>18</mml:mn></mml:mrow></mml:msub></mml:mrow><mml:mrow><mml:mo>+</mml:mo></mml:mrow></mml:msup></mml:math> photofragments from <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msup><mml:mrow><mml:msub><mml:mrow><mml:mi>C</mml:mi></mml:mrow><mml:mrow><mml:mn>28</mml:mn></mml:mrow></mml:msub></mml:mrow><mml:mrow><mml:mo>+</mml:mo></mml:mrow></mml:msup></mml:math> rings have cyclic structures.