Isomer-specific spectroscopy and conformational isomerization energetics of o-, m-, and p-ethynylstyrenes

The infrared and ultraviolet spectroscopy of o-, m-, and p-ethynylstyrene isomers (oES, mES, and pES) were studied by a combination of methods, including resonance-enhanced two-photon ionization (R2PI), UV−UV hole-burning spectroscopy (UVHB), resonant ion−dip infrared spectroscopy (RIDIRS), and rotationally resolved fluorescence excitation spectroscopy. In addition, the newly developed method of stimulated emission pumping−population transfer spectroscopy (SEP-PTS) was used to determine the energy threshold to conformational isomerization in m-ethynylstyrene. The S1 ← S0 origin transitions of oES and pES occur at 32 369 and 33 407 cm-1, respectively. In mES, the cis and trans conformations are calculated to be close in energy. In the R2PI spectrum of mES, the two most prominent peaks (32672 and 32926 cm-1) were confirmed by UVHB spectroscopy to be S1 ← S0 origins of these two conformers. The red-shifted conformer was identified as the cis structure by least-squares fitting of the rotationally resolved flu...