Excited States of the Allene Chromophore: Photoelectron, Circular Dichroism, and Absorption Spectroscopy of Alkyl- and Halogenoallenes

Photoelectron (PE) as well as near- and vacuum-ultraviolet (300–150 nm) absorption and circular dichroism (CD) spectra of the tert-butylallenes (CH₃)₃CCH=C=CHX (X = C(CH₃)₃, Cl, Br, I) are reported. The emphasis of the work is on the excited states and electronic structures of the allenes. In the PE spectra interactions between the substituents across the allenic C=C=C system are observed. These can be rationalized in terms of Coulombic field effects and “hyperconjugation”. Effects of vibronic coupling observed in the PE spectra are discussed qualitatively. By the combined use of absorption and CD spectroscopy, a large number of excited states resulting from valence-shell and Rydberg transitions are observed. The assignments of Rydberg states rely essentially on the absorption spectra using ionization energies from the PE spectra. Referring to the hydrocarbon as a representative of the allene chromophore, the sequence of excited states resulting from intravalence e → e (HOMO →LUMO) transitions is found to be ¹A₂< ¹B₁ < ¹B₂ < ¹A₁. The intravalence excited states of the halogenoallenes originating with the two highest energy orbitals can be correlated with those of the allene chromophore. The two lowest energy excited states of chloro- and iodoallene are shown to have bent C=C=C geometries. In the CD spectra of the halogenoallenes low-energy excited states are detected with the corresponding electronic transitions terminating at the antibonding C-X σorbitals (σ*_c-_x) and σ*_c-_x comprising halogen d atomic orbitals.