Photoinduced structural volume changes in aqueous solutions of blepharismin

Employing time-resolved photoacoustics we measured the structural volume ch anges (Delta V-ri) occurring after photoexcitation of blepharismin (BPR) aq ueous solutions; an expansion occurring in the subnanosecond timescale is f ollowed by a back contraction within some hundreds of nanoseconds. The magn itude of the Delta V-ri strongly depends on pH, allowing the determination of the pK(a) of BPR with this method. The values so measured are very close to those found by means of UV-visible absorption spectroscopy. The presenc e of water-soluble electron accepters or donors (hexacyanoferrate [III] hex acyanoferrate [II]) as well as the concentration of oxygen do not affect th e magnitude or the kinetics of the structural volume changes. On the contra ry, we detect a strong deuterium effect; this suggests that the observed De lta V-ri are related to an altered hydrogen bond pattern of the excited sta te of the pigment with respect to the ground state. Comparative measurement s with the parent compound hypericin are also reported, suggesting that the photoinduced expansion-contraction pattern is a general characteristic of polyhydroxylated quinones.