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The Dielectric Response of Phenothiazine-Based Glass-Formers

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The Dielectric Response of Phenothiazine-Based Glass-Formers www.nature.com/scientificreports OPEN The dielectric response of phenothiazine‑based glass‑formers with diferent molecular complexity M. Rams‑Baron1,2*, A. Jędrzejowska3, K. Jurkiewicz1,2, M. Matussek4, M. Musiał1,2 & M. Paluch1,2 We examined a series of structurally related glass‑forming liquids in which a phenothiazine‑based tricyclic core (PTZ) was modifed by attaching n‑alkyl chains of diferent lengths (n = 4, 8, 10). We systematically disentangled the impact of chemical structure modifcation on the intermolecular organization and molecular dynamics probed by broadband dielectric spectroscopy (BDS). X‑ray difraction (XRD) patterns evidenced that all PTZ‑derivatives are not ‘ordinary’ liquids and form nanoscale clusters. The chain length has a decisive impact on properties, exerting a plasticizing efect on the dynamics. Its elongation decreases glass transition temperature with slight impact on fragility. The increase in the medium‑range order was manifested as a broadening of the dielectric loss peak refected in the lower value of stretching parameter βKWW. A disagreement with the behavior observed for non‑associating liquids was found as a deviation from the anti‑correlation between the value of βKWW and the relaxation strength of the α‑process. Besides, to explain the broadening of loss peak in PTZ with the longest (decyl) chain a slow Debye process was postulated. In contrast, the sample with the shortest alkyl chain and a less complex structure with predominant supramolecular assembly through π–π stacking exhibits no clear Debye‑mode fngerprints. The possible reasons are also discussed. Dielectric spectroscopy is a powerful tool for studying the dynamics of self-assembling glass-forming systems over a wide range of temperatures and frequencies and also under high pressure. Much efort is now being made to fnd quantitative relationships between various parameters describing the dielectric response and identify those universal for a wide group of materials 1–3. Te strongly interacting liquids with the intermolecular organization going beyond the short-range order are usually excluded from such a discussion according to the idea of ‘under- standing the glass transition in its simplest form, before moving to the more general and probably more complex scenario’4. Indeed, in systems where we deal with any intermolecular forces (hydrogen bonds, ionic bonds, π–π stacking), the self-aggregation and nanoscale organization may result in a profound variability of their dynamics, depending on the size, architecture, and stability of the structures formed. Consequently, new phenomena and unusual dynamic behaviors compared to ordinary liquids may emerge. Tus, systematic studies allowing a deep understanding of the structure-dynamics correlations in this group of glass-forming materials are necessary. Particularly useful are investigations on structural analogs that allow in a systematic way to disentangle the efect of changes in the chemical structure on the dielectric response. To address this problem, we have studied the relaxation dynamics of a series of phenothiazine (PTZ)-based compounds using broadband dielectric spectroscopy (BDS). PTZs are a versatile group of materials attractive for various real-life applications. Tey are widely used in medicine and pharmacy as leading fragments of drugs with a broad spectrum of biological activities, including antipsychotic, antihistamine, and antiseptic5,6. Due to remarkable electronic properties, PTZ derivatives were found to be notably useful in optoelectronics and material science7. Te availability of the PTZ scafold for various chemical modifcations makes its derivatives a powerful platform for creating new compounds with desired properties. 1August Chełkowski Institute of Physics, University of Silesia in Katowice, 75 Pułku Piechoty 1, 41-500 Chorzów, Poland. 2Silesian Center for Education and Interdisciplinary Research, 75 Pułku Piechoty 1a, 41-500 Chorzów, Poland. 3PET Diagnostics Department, Maria Sklodowska-Curie Memorial Cancer Centre and Institute of Oncology, Gliwice Branch, Gliwice, Poland. 4Institute of Chemistry, University of Silesia in Katowice, Szkolna 9, 40-006 Katowice, Poland. *email: [email protected] Scientifc Reports | (2021) 11:15816 | https://doi.org/10.1038/s41598-021-95127-y 1 Vol.:(0123456789) www.nature.com/scientificreports/ Scheme 1. Schematic representation of the synthesis of investigated PTZs. Te goal of this study is to systematically elucidate how the structural and molecular properties, in particular diferences in the intermolecular confguration, govern the dielectric response of a series of structurally related glass-forming molecules based on the PTZ backbone. We examined three PTZ derivatives with increasing alkyl chain length, i.e., butyl (assigned as PTZ-C4), octyl (PTZ-C8), and decyl (PTZ-C10). Teir chemical structures are sketched in Scheme 1. Te choice of these materials was dictated by diferences in their afnity to supramo- lecular structuring revealed by X-ray difraction (XRD). In such π-conjugated systems, the intermolecular π–π stacking interactions are believed to be the driving force behind their self-organization8–10. We expected that the length of the alkyl chain might have a decisive impact on the confguration of aromatic PTZ units, diferentiat- ing the observed X-ray scattering response in the low-scattering vector range covering the nanoscale structure. Indeed, the XRD results confrmed two types of supramolecular organization in the PTZs, which vary with the length of the side chain attached to the identical tricyclic core. Te frst one is related to the parallel π–π stacking of aromatic units, while the second one results from an organization of alkyl chains along their long axis. Tus, these structurally related glass-forming molecules with diferent self-assembling abilities are perfect materials to study structure-dynamics interplays. Te question we ask is if the diferences in their structure found in the XRD patterns translate into the molecular dynamics in the supercooled and glassy states probed by dielectric spectroscopy. Do we observe any peculiarities resulting from the molecular association of these systems, or is the dielectric response of PTZs similar to that registered for glass-forming liquids lacking specifc interactions and a tendency to self-aggregation? Material and methods Synthetic procedures and characterization. All reagents were purchased from Sigma-Aldrich or TCI Chemicals and were used without further purifcation. Tetrahydrofuran was heated to refux with sodium and benzophenone and distilled before use. Column chromatography was carried out on Merck silica gel (230–400 mesh). Tin-layer chromatography (TLC) was performed on silica gel (Merck TLC Silica Gel 60, F254). NMR spectra were taken on Bruker Avance 400 (400 MHz) spectrometer. Chemical shifs (δ) are reported in parts per million (ppm) relative to traces of CHCl 3 (δH = 7.26 ppm, δC = 77.0 ppm). Te key 10-alkylated PTZs were synthesized according to the well-known literature procedure with minor modifcations 11. Phenothiazine (10 g, 50.2 mmol) was dissolved in 300 mL of anhydrous tetrahydrofuran and the solution was cooled to 0 °C using an ice bath. Te reaction system was kept under an argon atmosphere. Sodium hydride (2.17 g, 90.4 mmol, 1.8 eq) was added to a solution in one portion, and the resulting mixture was stirred at room temperature for 1 h. Afer this time, 1-bromobutane (8.94 g, 7.1 mL, 65.3 mmol), 1-bromooctane (12.60 g, 11.3 mL, 65.3 mmol) or 1-bromodecane (14.44 g, 13.5 mL, 65.3 mmol) was added, and the resulting mixture was vigorously stirred and heated under refux for 24 h. Te progress of the reaction was monitored by thin-layer chromatography (TLC). Ten, ice was added, and the mixture was extracted with ethyl acetate (3 × 150 mL). Te combined organic layers were dried over MgSO 4, fltered to remove solids, and the fltrate was evaporated under reduced pressure. Te residue was purifed by column chromatography. 10-butylphenotiazine (PTZ-C4). Te crude product was purifed by column chromatography (silica gel, hexane), to give PTZ-C4 as a yellowish 1 oil liquid (11.2 g, 87% yield). H NMR (400 MHz, CDCl 3) δ: 7.21–7.08 (m, 4H), 6.96–6.82 (m, 4H), 4.00–3.76 13 (m, 2H), 1.88–1.76 (m, 2H), 1.53–1.42 (m, 2H), 0.96 (t, J = 7.4 Hz, 3H). C NMR (101 MHz, CDCl 3) δ: 145.42, 127.51, 127.27, 125.06, 122.42, 115.54, 47.20, 29.13, 20.29, 13.94. 10-octylphenothiazine (PTZ-C8). Scientifc Reports | (2021) 11:15816 | https://doi.org/10.1038/s41598-021-95127-y 2 Vol:.(1234567890) www.nature.com/scientificreports/ Te crude product was purifed by column chromatography (silica gel, hexane), to give PTZ-C8 as a yellowish 1 oil liquid (13.8 g, 88% yield). H NMR (400 MHz, CDCl 3) δ: 7.22–7.11 (m, 4H), 7.02–6.81 (m, 4H), 4.00–3.62 (m, 2H), 1.90–1.76 (m, 2H), 1.49–1.40 (m, 2H), 1.37–1.21 (m, 8H), 0.89 (t, J = 6.8 Hz, 3H). 13C NMR (101 MHz, CDCl3) δ: 145.43, 127.50, 127.25, 125.05, 122.39, 115.50, 47.52, 31.88, 29.34, 29.23, 27.08, 27.03, 22.77, 14.25. 10-decylphenothiazine (PTZ-C10). Te crude product was purifed by column chromatography (silica gel, hexane), to give PTZ-C10 as a white 1 waxy solid (12.4 g, 73% yield). H NMR (400 MHz, CDCl3) δ: 7.22–7.12 (m, 4H), 7.03–6.88 (m, 4H), 4.01—3.67 (m, 2H), 1.88–1.75 (m, 2H), 1.50–1.38 (m, 2H), 1.36–1.19 (m, 12H), 0.90 (t, J = 6.8 Hz, 3H). 13C NMR (101 MHz, CDCl3) δ: 145.59, 127.66, 127.43, 125.31, 122.57, 115.70, 47.66, 32.35, 29.99, 29.94, 29.75, 29.66, 27.29, 27.22, 23.15, 14.63.
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