In the comparison of IR spectra of 2-PAP and its photocatalyzed products (Figure ), there is a clear broadening in the 3600-3400 cm -1 region corresponding to -OH group of phenol. The second most obvious difference in IR spectrum can be observed in the carbonyl region (1600-1800 cm -1 ) due to the formation of acetophenone (1), benzaldehyde (5) and benzoic acid (8) having C=O stretching peaks at 1682 cm -1 , 1700 cm -1 and 1735 cm -1 , respectively. In a nutshell, photocatalyzed reaction of 2-PAP using PAB consumed 2-PAP yielding acetophenone as the major product (coming from the photocleavage of both 2-PAP and PAB) and phenol, benzaldehyde, benzoic acid, coupling species as side products. Encouraged by these results, we next attempted to photocleave SL using PAB as photocatalyst under similar conditions. As previously described in Scheme 3, two major reactions are expected from this photocatalytic system; first one is a redox neutral process that cleaves β-O-4 bonds in Norrish type I manner and the second one is an oxidation process which produces different carbonyl compounds through interaction with molecular oxygen (O2). In a typical experiment, 64 mg of SL (2 x 10 -5 mol according to SEC result) was mixed with 20 mg of PAB (1 x 10 -4 mol) in 5 mL of aqueous ACN inside a 25 mL round-bottom flask with open neck, and the resulting solution was irradiated for 8 h under continuous stirring, inside the same photoreactor used for the 2-PAP, under ambient conditions. After the photoreaction, ACN solvent was evaporated from the dark brown colored mixture resulting in an oily (highly viscous) liquid (Figure ). Different spectral, chromatographic, diffractometric and thermal investigations were employed in order to speculate on the products of PAB-photocatalyzed reaction of SL (PSL). Firstly, SEC investigation was performed to compare the chromatograms of SL and PSL (Figure ). While SEC traces of SL contained high molecular weight fractions (Mn ~3.24 kDa) with high dispersity (Đ = 1.78), SEC traces of PSL mainly exhibited a Mn shift from ∼3.24 kDa to ∼0.13 kDa and with a dispersity of Đ = 1.08. Furthermore, area corresponding to low molecular weight fractions increased significantly in the chromagotram of PSL compared to those of SL indicating the formation of monomeric species, thus, successful depolymerization. Following SEC investigations, direct injection LC-HRMS analysis was employed to get insight into the possible depolymerization products (Figure ). Direct injection doesn't require any sample pre-treatment and the required sample volume for direct injection (100 μL) is much smaller than a sample requiring pre-treatment (10 mL), thus improving the depth and time resolution that can be attained from SL. It is possible to observe looking at Figure , that SL is not a random polymer since there is constant regularity along the whole mass spectrum (217.10 to 833.47 m/z) with a difference of 44.03 m/z which corresponds to C2H4O units. This finding evidences that soluble part of SL is formed by oligomeric species. On the other hand, mass spectrum of PSL shows a strong peak at m/z = 187.10 corresponding to lithium adduct of guaiacyl alcohol, according to literature, and other peaks at higher m/z with much lower intensity. These higher m/z values are not related to the peaks observed in SL's mass spectrum indicating structural changes in SL and the possible presence of trace amount of brominated species that aren't totally depolymerized, which were also present in SEC traces (Figure , inset photo). Simliarly, comparison of 1 H and C NMR spectra of SL and PSL (Figure ) revealed the formation of several aromatic aldehydes and carboxylic acids having proton peaks between 7.3-8.0 ppm and carbon peaks in 185-200 ppm region, respectively. The strong broad peak between 3.5-4 ppm can be associated with the methoxyl group (-OCH3) of lignin units, and other signals around 3.30 and 2.50 ppm correspond to water and DMSO solvent, respectively. The relative decrease in the area of methoxyl units from SL to PSL is most probably due to the demethylation followed by phenolation reaction in low concentration of Lewis acids, as it was previously demonstrated that dilute HBr treatment was found to be effective in both increasing lignin hydroxyl group content and cleaving of -O-4 bonds. The chemical shifts in PSL spectrum between 4.0 -6.0 ppm indicates the presence of aliphatic protons. Spectra observed at 7.7-7.3 ppm can be associated with guaiacyl unit. Control experiment performed in the absence of PAB under identical conditions resulted in no change in the 1 H NMR spectrum (Figure ). In the comparison of IR spectra of SL, PSL and PAB (Figure ), there is a significant change near 3000 cm 1 in PSL spectrum corresponding to the aromatic C-H stretching peaks of newly formed aromatic species. In addition, broad C=O stretching peak near 1700 cm 1 is due to different unconjugated carbonyl compounds (aldehydes and quinones) and indicates the formation of new aromatic aldehyde and carboxylic acid species as this was also evidenced in the C NMR spectrum of PSL (Figure ). The relative values of the peaks at 1458 cm 1 in SL, corresponding to C-O deformation of methoxyl group decreased significantly in PSL due to demethylation/phenolation by low concentration of HBr. Aromatic C-H stretching peaks (~ 3000 cm 1 ) which are absent in SL's spectrum, are present in PSL's spectrum due to the formation of new aromatic species by uncrosslinking of SL. Comparison of UV-vis spectra of SL and PSL revealed the significantly higher UV absorption of PSL compared to that of SL's (Figure ). The absorption peak around 290 nm corresponds to the characteristics of phenolic groups. Higher absorption of PSL in 300 nm and visible region is due to the formation of new products with lower molecular weights, that are rich in carbonyl and phenolic groups. Crystallinity of SL and PSL were investigated using powder x-ray diffractometry (Figure ). While SL exhibited a totally amorphous structure with no assignable peaks, PSL revealed a lot of sharp peaks corresponding to crystalline monomeric species and a broad peak ~ 26 o . This broad peak is due to the breaking of the cross-linked bonds of SL forming monomeric and oligomeric molecules which stack through  interactions. 78 Thermal stabilities of SL and PSL were probed by employing TGA (Figure ) and DSC measurements (Figure ). Examination of TGA thermograms disclosed the occurrence of first weight loss (~4%) in both compounds between 50-100 o C due to the evaporation of absorbed water. Remarkable amount of absorbed water in both SL and PSL can be related to the hygroscopic functional groups such as phenolics forming hydrogen bonds with ambient water molecules. A significant change can be observed starting from 100 o C when thermograms of SL and PSL are compared. While SL possessing a cross-linked structure showed thermal stability up to 300 o C (only ~%10 weight loss), PSL lost most of its weight (~73%) at that temperature. This huge weight loss is a consequence of the boiling of different monomeric species produced by the successive photo-depolymerization of SL. After 300 o C, while SL underwent a rapid pyrolytic degradation, PSL's weight loss is much smaller. Efficient depolymerization process of SL was also supported by DSC results (Figure ). Endothermic peaks in both compounds, starting from 75 o C stand for the gradual evaporation of the ambient water absorbed by phenolic monomers. However, after 100 o C, there is a notable change between the DSC curves of SL and PSL. While PSL exhibited a broad exothermic peak between 75-150 o C corresponding to the melting points of monomeric species, SL exhibited no obvious exothermic peak after 100 o C due to its cross-linked nature. Based on all these spectral, chromatographic, diffractometric and thermal data, we propose the following reaction mechanism which is depicted in Scheme 4. Scheme 4. Plausible mechanism for the PAB-photocatalyzed depolymerization of soda lignin, yielding guaiacyl alcohol as main lignin-derived product.