Photo-Fries rearrangement of phenyl salicylate studied by two-dimensional infrared spectroscopy $ Hideyuki Shinzawa*, Yuki Matsumoto, Seiji Tsuzuki, Yukihiro Shimoi, Hiroyuki Suda National Institute of Advanced Industrial Science and Technology (AIST), Japan A R T I C L E I N F O Article history: Received 30 September 2015 Received in revised form 5 November 2015 Accepted 5 November 2015 Available online 10 November 2015 Keywords: Photo-Fries rearrangement Phenyl salicylate Infrared (IR) spectroscopy Two-dimensional (2D) correlation spectroscopy Density functional theory (DFT) A B S T R A C T Photo-Fries rearrangement of phenyl salicylate was examined by real-time infrared (IR) monitoring in conjunction with density functional theory (DFT) calculation. Changes in the spectral features were readily captured during the photo-induced chemical reaction of the phenyl salicylate by means of two- dimensional (2D) correlation spectroscopy. The obvious variations of spectral intensities due to the production of 2,2 0 -dihydroxybenzophenone and 2,4 0 -dihydroxybenzophenone are clearly identified to provide in-depth understanding to the photo-Fries rearrangement. Namely, when exposed to UV- irradiation, the majority of the phenyl salicylate undergoes photothermal degradation. Further irradiation to the rest of the phenyl salicylate then results in the production of 2,2 0 -dihydroxybenzo- phenone followed by the development 2,4 0 -dihydroxybenzo-phenome products. Such delay in the production of 2,4 0 -dihydroxybenzo-phenome implies the presence of some more pathways, which makes its reaction efficiency lower than that of 2,2 0 -dihydroxybenzophenone. ã 2015 Elsevier B.V. All rights reserved. 1. Introduction Spectroscopic process monitoring of a chemical reaction is an analytical tool of practical interest. In particular, vibrational spectroscopy has been used for monitoring several chemical reaction processes by taking advantage of characteristic spectral changes due to consumption and production of chemical components in systems [1–6]. For example, each chemical functional group in a molecule vibrates at a unique frequency, providing a specific peak in spectrum. Steady variation of peak intensity observed in spectra thus can be used as useful indicator for the consumption or generation of chemical component during the reaction. Infrared (IR) spectroscopy is well suited for the in situ monitoring of chemical reaction process, particularly those carried out under the irradiation of UV light [7–12]. The photo-Fries rearrangement involves the transformation of molecules such as phenolic esters to hydroxy aryl ketones. This chemical reaction is important since the molecules generated by the rearrangement can be used as source for photoresist materials. Fig. 1 shows a schematic illustration of the reaction which occurs when an aromatic polyester is subjected to UV irradiation.The aromatic polyester first undergoes main-chain cleavage by free radical reaction and then subsequent multiple rearrangements to develop ortho-hydroxy ketone [13–15]. Initial interest in the photo-Fries rearrangement of aromatic polyesters was mainly due to the fact the reaction is accompanied by the formation of ortho- hydroxy aromatic compounds with great photostability, while the total amount of the reaction products are relatively small [14]. A considerable amount of attention has also been paid to the photo- Fries rearrangement of polymer pendant groups. It is also known that the rearrangements of some aromatic polyester are often incomplete and result in the formation of both the ortho and the para-hydroxyphenone rearranged products. For example, the photo-Fries rearrangement of phenyl salicylate generates major product (2,2 0 -dihydroxybenzophenone) and minor product (2,4 0 - dihydroxybenzophenone) as shown in Fig. 2 [13]. Unfortunately, the details of the reaction mechanism has not been fully understood yet. In this work, the first two-dimensional (2D) IR correlation spectra of the photo-Fries rearrangement are reported to demon- strate how the technique can elucidate the details of the reaction mechanism [16,17]. Namely, the photo-Fries rearrangement of phenyl salicylate was examined by IR spectroscopy to capture the evolutionary change induced by the reactions. The time-resolved IR spectra collected during the early stage of the photo-Fries rearrangement process are then subjected to 2D correlation spectroscopy. Density functional theory (DFT) calculation was also carried out to support the assignments of IR bands. The 2D $ Selected paper from 8th International Conference on Advanced Vibrational Spectroscopy, 12–17 July 2015, Vienna, Austria. * Corresponding author. E-mail address: h-shinzawa@aist.go.jp (H. Shinzawa). http://dx.doi.org/10.1016/j.vibspec.2015.11.001 0924-2031/ ã 2015 Elsevier B.V. All rights reserved. Vibrational Spectroscopy 81 (2015) 131–135 Contents lists available at ScienceDirect Vibrational Spectroscopy journa l homepage: www.e lsevier.com/locate/vibspec