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MO: Molekülphysik

MO 31: Ultrafast Photochemistry

MO 31.5: Talk

Monday, March 7, 2005, 11:30–11:45, HU 2091

Solvent Dependent Photoacidity State of Pyranine as Monitored with Transient Mid-Infrared Spectroscopy — •Omar F. Mohammed¹, Jens Dreyer¹, Ben-Zion Magnes², Ehud Pines², and Erik T. J. Nibbering¹ — ¹Max Born Institut, Max Born Strasse 2A, D-12489 Berlin, Germany — ²Ben Gurion Univeristy of the Negev, Beer-Sheva 84125, Israel

Photoacids are well known molecular systems used to optically trigger proton transfer reaction dynamics. Understanding the nature of photoacidity, i.e. the property of enhanced acidity in electronic excited states, is thus of fundamental importance. We investigate with femtosecond mid-infrared spectroscopy the vibrational mode characteristics of the electronic states involved in pyranine (HPTS), and its methoxy derivative (MPTS). We compare the observed vibrational band patterns of MPTS and of HPTS after electronic excitation in the solvents deuterated dimethylsulfoxide and H2O/D2O, from which we conclude that for MPTS and HPTS photoacid the first excited singlet state reached within our time resolution (150 fs) appears to have charge transfer (CT) properties in water. In contrast, in aprotic dimethylsulfoxide the photoacid appears to be in a less-polar electronic excited state. We compare our results with existing models for photoacidity. For the fingerprint vibrations we do not observe dynamics on a time scale of a few picoseconds, and with our results obtained on the O-H stretching vibration we argue that the dynamical behaviour observed in previous UV/vis pump-probe studies is likely to be related to solvation dynamics.