The electronic spectroscopy of resonance-stabilised hydrocarbon radicals

Resonance-stabilised hydrocarbon radicals serve as reaction intermediates in flames, plasmas, atmospheres and interstellar space. Their stability is conferred by delocalisation of the radical electron through a conjugated -system. As such, they tend to have low-lying electronic states which engender a rich optical spectroscopy. Over the last sixty years, and intensively over the last decade, the spectra of a great many such radicals have been measured, ranging from three to 17 carbon atoms. The excitation wavelengths of transitions to the first excited states of these radicals range from the ultraviolet to the near infrared, spanning the visible region. In this review, the electronic spectroscopy of the various chromophores presented by resonance-stabilised radicals is reviewed. The theoretical methods used to predict the excitation spectra of resonance-stabilised radicals is critically evaluated, and two emergent rules-of-thumb are demonstrated which allow one to empirically predict the approximate excitation energy of certain radicals.