Observation of slow charge redistribution preceding excited-state proton transfer.

The photoacid 8-hydroxy-N,N,N',N',N',N'-hexamethylpyrene-1,3,6-trisulfonamide (HPTA) and related compounds are used to investigate the steps involved in excited-state deprotonation in polar solvents using pump-probe spectroscopy and time correlated single photon counting fluorescence spectroscopy. The dynamics show a clear two-step process leading to excited-state proton transfer. The first step after electronic excitation is charge redistribution occurring on a tens of picoseconds time scale followed by proton transfer on a nanosecond time scale. The three states observed in the experiments (initial excited state, charge redistributed state, and proton transfer state) are recognized by distinct features in the time dependence of the pump-probe spectrum and fluorescence spectra. In the charge redistributed state, charge density has transferred from the hydroxyl oxygen to the pyrene ring, but the OH sigma bond is still intact. The experiments indicate that the charge redistribution step is controlled by a specific hydrogen bond donation from HPTA to the accepting base molecule. The second step is the full deprotonation of the photoacid. The full deprotonation is clearly marked by the growth of stimulated emission spectral band in the pump-probe spectrum that is identical to the fluorescence spectrum of the anion.

[1]  A. Goun,et al.  Deprotonation dynamics and stokes shift of pyranine (HPTS). , 2007, The journal of physical chemistry. A.

[2]  A. Goun,et al.  Identification and properties of the La1 and Lb1 states of pyranine , 2006 .

[3]  D. Huppert,et al.  Excited-state proton transfer: indication of three steps in the dissociation and recombination process. , 2005, The journal of physical chemistry. A.

[4]  O. Mohammed,et al.  Solvent-dependent photoacidity state of pyranine monitored by transient mid-infrared spectroscopy. , 2005, Chemphyschem : a European journal of chemical physics and physical chemistry.

[5]  Noam Agmon.  Elementary steps in excited-state proton transfer. , 2005, The journal of physical chemistry. A.

[6]  G. Fleming,et al.  Femtosecond pump-probe measurements of solvation by hydrogen-bonding interactions. , 2004, Chemphyschem : a European journal of chemical physics and physical chemistry.

[7]  Z. Rappoport,et al.  The chemistry of phenols , 2003 .

[8]  G. Granucci,et al.  Intermolecular photochemical proton transfer in solution: new insights and perspectives , 2002 .

[9]  J. Hynes,et al.  Substituent and Solvent Effects on the Nature of the Transitions of Pyrenol and Pyranine. Identification of an Intermediate in the Excited-State Proton-Transfer Reaction† , 2002 .

[10]  T. Elsaesser.  Ultrafast Excited State Hydrogen Transfer in the Condensed Phase , 2002 .

[11]  T. Elsaesser,et al.  Ultrafast hydrogen bonding dynamics and proton transfer processes in the condensed phase , 2002 .

[12]  D. Huppert,et al.  Photochemistry of “Super”-Photoacids. Solvent Effects , 1999 .

[13]  D. Huppert,et al.  Solvatochromic Shifts of “Super” Photoacids , 1998 .

[14]  M. Maroncelli,et al.  Subpicosecond Measurements of Polar Solvation Dynamics: Coumarin 153 Revisited , 1995 .

[15]  Martin Klessinger,et al.  Excited states and photochemistry of organic molecules , 1995 .

[16]  Alan E. Johnson,et al.  Interplay of solvent motion and vibrational excitation in electron-transfer kinetics : experiment and theory , 1992 .

[17]  Corwin Hansch,et al.  A survey of Hammett substituent constants and resonance and field parameters , 1991 .

[18]  D. Huppert,et al.  Solvent motion controls the rate of intramolecular electron transfer in solution , 1983 .

[19]  L. Brand,et al.  Excited-State Proton Transfer , 1983 .

[20]  P. Barbara,et al.  Picosecond kinetics of p-dimethylaminobenzonitrile , 1981 .

[21]  P. Wyatt,et al.  Acid-Base Properties of Electronically Excited States of Organic Molecules , 1976 .

[22]  V. Gold,et al.  Proton-Transfer Reactions , 1975 .

[23]  R. P. Bell.  The proton in chemistry , 1973 .

[24]  J. Murrell.  The theory of the electronic spectra of organic molecules , 1963 .

[25]  A. Weller.  Protolytische Reaktionen angeregter Oxyverbindungen , 1958 .

[26]  John R. Platt,et al.  Classification of Spectra of Cata-Condensed Hydrocarbons , 1949 .