Contents lists available at ScienceDirect Journal of Photochemistry & Photobiology A: Chemistry journal homepage: www.elsevier.com/locate/jphotochem Quinoline Schiff-base ligands as long-wavelength photosensitizers for diphenyliodonium salt Binnur Aydogan Temel a,b, *, Belma Zengin Kurt a , Irem Akar b , Merve Keklik b a Department of Pharmaceutical Chemistry, Faculty of Pharmacy, Bezmialem Vakif University, Fatih, Istanbul 34093, Turkey b Department of Biotechnology, Institute of Health Sciences, Bezmialem Vakif University, Fatih, Istanbul 34093, Turkey ARTICLE INFO Keywords: Photosensitization Cationic polymerization Schiff-base ligands Diphenyliodonium salt ABSTRACT This study describes the use of quinoline Schiff-base ligands as photosensitizers for diphenyliodonium salt using long-wavelength UV light. 4-Bromo-2-((quinolin-8-ylimino)methyl)phenol (BQIP) and 2-methoxy-6-((quinolin- 8-ylimino)methyl)phenol (MQIP) were synthesized and characterized by 1 H NMR, 13 C NMR, FT-IR, UV–vis and fluorescence measurements. The cationic polymerization of cyclohexene oxide monomer was initiated at room temperature upon LED exposure (395-480 nm) in the presence of BQIP/MQIP and diphenyliodonium hexa- fluorophosphate (DPI + PF 6 - ). Free energy change (ΔG S ) calculations revealed that electron transfer from singlet excited states of the Schiff-bases to DPI + PF 6 - is thermodynamically favorable. 1. Introduction The long wavelength initiating systems for cationic polymerization received increased attention in recent years [1–3]. Dental filling ma- terials [4], printing inks [5], highly pigmented coatings [6], circuit board imaging [7], microelectronics and 3D-printing technologies [8,9] are some of the applications based on long wavelength acting systems. The photochemistry of conventional cationic photoinitiators such as iodonium [10], sulfonium [11] and alkoxypyridinium [12] salts has been studied in detail. These photoinitiators are efficient when irra- diation is carried out in the short- to mid-wavelength UV regions (220-300 nm). Strategies employed for improving the performance of the onium salt photoinitiators at long wavelengths include i) oxidation of free radicals by onium salts [13–16], ii) electron transfer between a photoexcited molecule and an onium salt [17–27], and iii) excitation of charge-transfer complexes of onium salts [28–30]. A major problem with the oxidation of free radicals is the limited number of photo- initiators capable of forming electron-donating radicals. On the other hand, the excitation of charge-transfer complexes is only possible with alkoxypyridinium salts. As a result, electron-transfer photosensitization appears to be the most efficient and applicable strategy. The general mechanism of electron transfer reaction between the photosensitizer and diphenyliodonium salt is presented in Scheme 1. It is a photo- induced redox process starting with the absorption of light by the photosensitizer to give the corresponding excited species [PS]* (eq. 1). In the second step, an excited state complex (exciplex) is formed between diphenyliodonium salt and the excited photosensitizer (eq. 2). Subsequently, a diphenyliodine radical and a photosensitizer radical cation are formed by one electron-transfer between the two reaction partners (eq. 3). The resulting unstable diphenyliodine radical decom- poses rapidly preventing back electron transfer and making the overall process irreversible (eq. 4). Cationic polymerization can be initiated by photosensitizer radical cation or by dication formed after radical di- merization of photosensitizer radical cation (eq. 5). Electron-rich polynuclear aromatic compounds such as anthracene, perylene, pyrene and phenothiazine are known to be the most efficient electron-transfer photosensitizers [31–34]. Despite various potential applications of these photosensitizers, there are also some serious drawbacks limiting their use such as expensiveness, toxicity and poor solubility in most reactive monomer systems. Therefore, there is a continuing need for long-wavelength-active photosensitizers in order to overcome these limitations. Several recent studies investigating the development of long-wavelength acting photosensitizers have been carried out [20–22,25–27]. Schiff-bases are easily synthesized from chemical reactions occur- ring between primary amines and aldehydes or ketones [35]. They have been receiving increasing attention due to their wide spectrum appli- cations particularly as metal ion complexing agents, catalyst carriers, corrosion inhibitors and thermo-stable materials [36]. Moreover, Schiff-base containing coordination compounds have a remarkable applicability in biological systems by acting as antibacterial, antifungal, antiviral, antitubercular, antitumor, anti-inflammatory, analgesic, and https://doi.org/10.1016/j.jphotochem.2020.112715 Received 3 May 2020; Received in revised form 15 June 2020; Accepted 16 June 2020 ⁎ Corresponding author at: Department of Pharmaceutical Chemistry, Faculty of Pharmacy, Bezmialem Vakif University, Fatih, Istanbul 34093, Turkey. E-mail address: baydogan@bezmialem.edu.tr (B.A. Temel). Journal of Photochemistry & Photobiology A: Chemistry 400 (2020) 112715 Available online 18 June 2020 1010-6030/ © 2020 Elsevier B.V. All rights reserved. T