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)ππ* (S1) state, resulting in a vibrational wavepacket, which is probed by virtue of a dramatic nonplanar → planar geometry change upon photoionization from S1 to the ground state of the cation (D0). Any similar quantum beat pattern is absent in phenol. In syringol, the nonplanar geometry in S1 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 S1, 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.
†Department of Chemistry, University of Warwick, Library Road, Coventry CV4 7AL, United Kingdom.
Jamie D
JD
Young
†Department of Chemistry, University of Warwick, Library Road, Coventry CV4 7AL, United Kingdom.
Michael
M
Staniforth
§Department of Chemistry, Purdue University, West Lafayette, Indiana 47907-2084, United States.
Jacob C
JC
Dean
†Department of Chemistry, University of Warwick, Library Road, Coventry CV4 7AL, United Kingdom.
Gareth M
GM
Roberts
‡Lens, Polo Scientifico e Tecnologico dell'Universitá di Firenze, Via Nello Carrara 1, 50019 Sesto Fiorentino, Florence, Italy.|⊥Dipartimento di Chemica, Polo Scientifico e Tecnologico dell'Universitá di Firenze, Via della Lastruccia 3, 50019 Sesto Fiorentino, Florence, Italy.
Federico
F
Mazzoni
#School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1TS, United Kingdom.
Tolga N V
TN
Karsili
#School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1TS, United Kingdom.
Michael N R
MN
Ashfold
§Department of Chemistry, Purdue University, West Lafayette, Indiana 47907-2084, United States.
Timothy S
TS
Zwier
†Department of Chemistry, University of Warwick, Library Road, Coventry CV4 7AL, United Kingdom.
Vasilios G
VG
Stavros
dynamics; femtosecond; quantum beating; spectroscopy; ultrafast; wavepacket