Towards Understanding Photodegradation Pathways in Lignins: The Role of Intramolecular Hydrogen Bonding in Excited States

The photoinduced dynamics of the lignin building blocks syringol, guaiacol, and phenol were studied using time-resolved ion yield spectroscopy and velocity map ion imaging. Following irradiation of syringol and guaiacol with a broad-band femtosecond ultraviolet laser pulse, a coherent superposition of out-of-plane OH torsion and/or OMe torsion/flapping motions is created in the first excited (1)pi pi* (S-1) state, resulting in a vibrational wavepacket, which is probed by virtue of a dramatic nonplanar -> planar geometry change upon photoionization from S-1 to the ground state of the cation (D-0). Any similar quantum beat pattern is absent in phenol. In syringol, the nonplanar geometry in S-1 is pronounced enough to reduce the degree of intramolecular H bonding (between OH and OMe groups), enabling H atom elimination from the OH group. For guaiacol, H bonding is preserved after excitation, despite the nonplanar geometry in S-1, and prevents O-H bond fission. This behavior affects the propensities for forming undesired phenoxyl radical sites in these three lignin chromophores and provides important insight into their relative "photostabilities" within the larger biopolymer.