“Dark” Excited States of Diphenylacetylene Studied by Nonresonant Two-Photon Excitation Optical-Probing Photoacoustic Spectroscopy

Optical probing photoacoustic spectroscopy (OPPAS), a photothermal calorimetric technique, was applied to investigate one-photon forbidden “dark” electronically excited singlet states of diphenylacetylene (DPA), belonging to the D2h point group. When 502 nm light from a pulsed OPO laser was focused into a DPA hexane solution, an acoustic wave was detected as a transient angular deflection of the probe beam (He–Ne laser). The laser power dependence of the OPPAS signal was estimated to be 1.9 ± 0.2, suggesting two-photon absorption for acoustic generation. With the OPPAS technique, we successfully obtained a nonresonant two-photon absorption spectrum of DPA in solution for the first time. The two-photon allowed 11B3g and 21Ag states were found to be above the one-photon allowed 11B1u state. These results show that OPPAS is a highly sensitive technique applicable to two-photon absorption spectral measurements of nonfluorescent species.