Electronic states of model hydrocarbon chromophores investigated by Synchrotron Radiation Linear Dichroism (SRLD) spectroscopy on aligned samples

Conventional UV-VIS absorption spectroscopy provides information on transition energies and intensities. Linear dichroism (LD) spectroscopy on aligned molecular samples yields additional information on transition moment directions, thereby frequently leading to resolution of otherwise overlapping, differently polarized absorption bands, and to symmetry assignments of the observed molecular states. This information is highly useful in the study of molecular symmetry and reactivity. Partially aligned molecular samples may be produced in a simple fashion by the use of stretched polymers as anisotropic solvents, for example stretched polyethylene. With a traditional UV light source (deuterium lamp, Glan prism polarizer), LD spectroscopy on thin polyethylene samples can be performed in the spectral range up to 47 000 cm^-1 (210 nm). But with synchrotron radiation as a source of linearly polarized UV light, the region can be extended considerably. Previous SRLD experiments performed with the UV1 beamline at ISA (Aarhus University) obtained excellent LD spectra up to 57000 cm^-1 (175 nm), thereby increasing the investigated range by no less than 10000 cm^-1. In this communication, we present corresponding results for four hydrocarbons which are of interest as model compounds for molecular wires and switches:
1)  Diphenylethyne (tolane, DPA)
2)  1,4-Bis(phenylethynyl)benzene (BPEB)
3)  (E)-1,2-Diphenylethene (stilbene, DPE)
4)  (E,E’)-1,4-Diphenyl-1,3-butadiene (DPB)