Photodissociation of thioglycolic acid studied by femtosecond time-resolved transient absorption spectroscopy

Steady-state and time-resolved spectroscopies were employed to study the photodissociation of both the neutral (HS-CH2-COOH) and doubly deprotonated (-S-CH2-COO-) forms of thioglycolic acid (TGA), a common surface-passivating ligand used in the aqueous synthesis and organization of semi-conducting nanostructures. Room temperature UV-Vis absorption spectroscopy indicated strong absorption by the S-1 and S-2 excited states at 250 nm and 185 nm, respectively. The spectrum also contained a weaker absorption band that extended to approximately 550 nm, which was assigned to the pi(CO)* <- n(O) transition. Femtosecond time-resolved transient absorption spectroscopy was performed on TGA using 400 nm excitation and a white-light continuum probe to provide the temporally and spectrally resolved data. Both forms of TGA underwent a photoinduced dissociation from the excited state to form an alpha-thiol-substituted acyl radical (alpha-TAR, S-CH2-CO center dot). For the acidic form of TGA, radical formation occurred with an apparent time constant of 60 +/- 5 fs; subsequent unimolecular decay took 400 +/- 60 fs. Similar kinetics were observed for the deprotonated form of TGA (70 +/- 10 fs radical formation; 420 +/- 40 fs decay). The production of the a-TAR was corroborated by the observation of its characteristic optical absorption. Time-resolved data indicated that the photoinduced dissociation of TGA via cleavage of the C-OH bond occurred rapidly (<= 100 fs). The prevalence of TGA in aqueous semiconducting nanoparticles makes its absorption in the visible spectral region and subsequent dissociation key to understanding the behavior of nanoscale systems. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3526746]