Ultraviolet spectroscopy of fundamental lignin subunits: Guaiacol, 4-methylguaiacol, syringol, and 4-methylsyringol

Abstract

Ultraviolet spectroscopy of the G- and S-type lignin subunits, guaiacol (G) and syringol (S), along with their para-methylated derivatives 4-methylguaiacol (4-MG) and 4-methylsyringol (4-MS), has been carried out in the cold, isolated environment of a supersonic jet. The excitation spectra and dispersed fluorescence (DFL) spectra of G and 4-MG show strong S0-S1 origins and Franck-Condon activity involving both the ring modes typical of aromatic derivatives, and the four lowest frequency out-of-plane modes (a") and lowest in-plane mode (a') involving the OH and OCH3 groups. The four low-frequency out-of-plane modes undergo extensive Duschinsky mixing between the ground and excited state. In 4-MG, combination bands involving methyl rotor levels with out-of-plane modes appeared with surprisingly high intensity, indicating a high degree of hindered rotor-vibration coupling in both S0 and S1. These mixing effects accompany the change in geometry upon π-π∗ electronic excitation going from a planar ground state to a non-planar excited state. Time-dependent density functional theory (TDDFT M05-2X∕6-311++G(d,p)) calculations predict a geometric distortion along the out-of-plane oxygen flapping coordinate, yielding a double minimum potential in S1 with a barrier to planarity of 195 cm(-1) in G. The excitation spectrum of S and 4-MS showed a much higher degree of spectral congestion and a larger geometry change evident by a shifted intensity distribution peaking ∼300 cm(-1) above the electronic origin. TDDFT calculations predict a larger geometry change in S compared with G, with the OH and H-bonded methoxy groups displaced in opposite directions above∕below the ring plane. Dispersed fluorescence from all S1 excited state levels in S∕4-MS yield only broad emission peaking far to the red of the excitation wavelength (-4500 cm(-1)). Several hypotheses regarding the source of this broad, redshifted emission were tested, but the cause remains unclear. p-Methylation was found to significantly redshift the UV absorption in both 4-MG and 4-MS, and methyl rotor transitions were assigned in both allowing for the determination of the shape and barrier heights of their respective potentials. These results provide a foundation for the discrimination of G- and S-chromophores in lignin oligomers, and demonstrate the potential for site-selective absorption.

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

“…The first is that in syringol, the initial composition of the wavepacket may only include a limited set of vibrational levels, composed primarily of τ OMe and φ modes. However, REMPI spectra indicate that the density of vibrational states in S 1 for syringol is far greater than that of guaiacol 14 , likely ruling out this conjecture. The second is that the enhanced ionization (detection) window is more localized in nuclear configuration space for syringol than in guaiacol.…”

“…5.0 ps). Given that the measured REMPI spectrum of syringol is significantly more congested than that for guaiacol, 14 it is not surprising that the dephasing time determined for syringol is faster; the greater density of states in the initial superposition drives faster dephasing of the vibrational wavepacket.…”

“…Unlike phenol, the presence of the methoxy-group(s) in guaiacol and syringol, ortho to the hydroxygroup, leads to an intramolecular H-bond between these two functional moieties, which distorts the geometry from planarity in the first electronically excited ππ* state (S 1 ), relative to the planar ground state (S 0 ) structure (Figure 1d-f). 11,14 In this contribution, we demonstrate how the different molecular structures of these three UV chromophores in their S 1 statesspecifically, the degree of H-bonding -can dramatically influence their excited state dynamics, and in turn the relative propensities for forming PhO sites (i.e. their relative 'photostabilities'), thereby taking a first step towards a more com- plete structure-dynamics-function picture of photodegradation in lignin.…”

Torsional Motion of the Chromophore Catechol following the Absorption of Ultraviolet Light

“…This means that any IVR, at first sight, is not possible around the S 1 origin. Calculations, however, suggest a small number of heavily mixed vibrational modes (reminiscent of those seen in guaiacol, which also has a nonplanar S 1 state minimum and undergoes Duschinsky mixing [39]), including τ 2 , that may fall within our excitation window [15,38]. This implies that IVR, while highly restricted, cannot be entirely ruled out.…”

Conformational Flexibility of Mephenesin

“…11,12 Other issues worth studying are the effect of the phenoxy group on the floppy structure of propane-1,2-diol found in previous studies, 13 −15 or the effect of substituents on the (nearly) free rotation of the methyl group, 16−20 adding a V 3 term to the V 6 barrier in toluene. Similar topics have been explored recently in the lignin monomers (p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol) 21 and their fundamental subunits (such as guaiacol and syringol), 22 finding that the presence of multiple flexible substituents locks in specific orientations of the molecular substituents.…”