UV polarisation spectroscopy of 1,4-diethynylbenzene

1,4-diethynylbenzene is investigated by UV linear dichroism polarisation spectroscopy using synchrotron radiation on molecular samples partially aligned in stretched polyethylene. The investigation covers the range from the absorption onset around 32000 cm^–1 (310 nm) to 58000 cm^–1 (170 nm) in the far UV. The polarisation spectra lead to resolution of otherwise overlapping spectral features and to experimental symmetry assignments of the observed molecular states. The observed transition moment directions of the differently polarised components of the weak absorption band between 32000 and 35000 cm^–1 (310 and 285 nm) support previously suggested vibronic assignments. The observed bands are compared with electronic transitions computed with TD–CAM-B3LYP, CIS and CIS(D). Magnetic Circular Dichroism (MCD) B-terms computed with the semiempirical LCOAO method are also reported. The calculated ordering of electronic states is found to be sensitive to the theoretical model. The absorption in the far UV is predicted to involve the in-plane π and π* components of the C≡C triple bonds and orbitals of mixed valence-Rydberg character.