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Photoactivity and Optical Applications of Organic Materials Containing Selenium and Tellurium Cite This: Chem Chemical Science View Article Online MINIREVIEW View Journal | View Issue Photoactivity and optical applications of organic materials containing selenium and tellurium Cite this: Chem. Sci.,2019,10,9182 a ab All publication charges for this article Gabrielle C. Hoover and Dwight S. Seferos * have been paid for by the Royal Society of Chemistry Sulfur-containing compounds, particularly derivatives of thiophene, are well studied for organic optoelectronic applications. Incorporating selenium or tellurium in place of sulfur imparts different physical properties due to the fundamental differences of these atoms relative to their lighter analogues. This has a profound influence on the properties of molecules and materials that incorporate chalcogens Received 23rd August 2019 that may ultimately lead to new opportunities and applications. This mini-review will focus on the Accepted 18th September 2019 quantitative and qualitative photophysical characteristics of organic materials containing selenium and DOI: 10.1039/c9sc04279b tellurium as well as their emerging applications as molecular photoactive species, including light- rsc.li/chemical-science emitting sensors, triplet sensitizers, and beyond. Creative Commons Attribution 3.0 Unported Licence. 1. Introduction has focused on semiconductor lms or nanocrystals such as CdSe or PbSe quantum dots or CdTe.6–12 In between lie classes of The chalcogens (O, S, Se, and Te) have attracted considerable organochalcogens, where selenium and tellurium have been attention for materials applications in the last few decades. In judiciously placed into otherwise organic materials. Such 13 14 the polymer electronics communities substantial attention has materials have been used as photovoltaics, transistors, 15 16 focused on the synthesis and properties of polythiophenes and thermoelectric generators, light emitting devices, and 17 their derivatives.1–5 In the inorganic materials space, much work sensors. The heavier chalcogens impart different physical properties than analogous lighter compounds due to funda- mental differences such as atom size and bond length, elec- This article is licensed under a aDepartment of Chemistry, University of Toronto, 80 St. George Street, ON M5S 3H6, tronegativity, and polarizability.18 These differences have Canada. E-mail: [email protected] a profound inuence on the morphology, crystallinity, and bDepartment of Chemical Engineering and Applied Chemistry, University of Toronto, charge transport of polymers.19–21 Many studies focus on 200 College Street, Ontario M5S 3E5, Canada Open Access Article. Published on 07 October 2019. Downloaded 1/29/2020 2:59:09 PM. Gabrielle Hoover is a PhD Dwight Seferos is a Professor of candidate in chemistry at the Chemistry, Chemical Engi- University of Toronto. She was neering, and Canada Research born in Syracuse, NY and grad- Chair in Polymer Nanotech- uated from the University at nology at the University of Tor- Albany, SUNY in 2015 where she onto. Seferos began his worked with fullerene fragments independent career at the under the supervision of Prof. University of Toronto in 2009, Marina Petrukhina. Her grad- and was promoted to Associate uate work has included the Professor in 2014 and Professor synthesis and surface conjuga- in 2017. Research in the Seferos tion of tellurophenes. She now group concerns the design, focuses on the synthesis of synthesis, characterization, and organic compounds containing selenium and tellurium toward device engineering of organometallic and organic materials for light-emitting applications. electronic and optical applications. Seferos has authored or coauthored over 100 publications, holds numerous patents, and has been recognized by many national and international awards including the DuPont Young Professor Award, Alfred P. Sloan Research Fellowship, CSC Strem Chemical Award, ACS Harry Gray Award, and E. W. R. Steacie Memorial Fellowship. 9182 | Chem. Sci.,2019,10,9182–9188 This journal is © The Royal Society of Chemistry 2019 View Article Online Minireview Chemical Science substituting heavier chalcogens in both semiconducting poly- direct effects of substituting sulfur with selenium or tellurium mers and small molecules, including organic photovoltaics13 on the rates of photophysical processes in a quantitative and as n-type materials.22 fashion. These studies are becoming increasingly necessary for The photophysical properties and optical applications the rational design of materials with heavy atoms, and to better heavier organochalcogens is also an emerging eld, yet it has understand the performance of these materials in sensors, received somewhat less attention. This is in part because triplet sensitizers, and other optical devices. heavier organochalcogens are not traditional luminescent One study from Pensack et al. directly compared the photo- materials: the uorescence that occurs in lighter chalcogens is physics of chalcogen-containing polymers with substitutions oen quenched upon substituting heavier elements. Improved from sulfur to selenium to tellurium.27 The decrease in polymer spin orbit coupling resulting from the heavy atom effect photoluminescent quantum yield (PLQY) when moving down promotes intersystem crossing (ISC) to rapidly populate the Group 16 is indicative of more rapid ISC with heavier chalco- triplet state.23 The triplets will decay according one of three gens. The study compares three polymers (1) with identical pathways (Fig. 1). First, phosphoresce may occur, which is solubilizing side chains: poly(3-hexylthiophene) (P3HT), poly(3- uncommon at room temperature due to the spin-forbidden hexylselenophene) (P3HS), and poly(3-hexyltellurophene) nature of this relaxation. Alternatively, the triplet may decay (P3HTe). The PLQY of P3HT is 0.30, while those of P3HS and À À non-radiatively, which is oen faster. For example, uorescence P3HTe are drastically reduced to 4.2 Â 10 3 and 1.4 Â 10 4, quenching is observed when sulfur is substituted for oxygen in respectively. Furthermore, using transient absorption spec- 1 p* –3 p p* – perylene diimides through a (n, ) ( , )(S1 T1) transi- troscopy, this study reported simulated emission decay time tion.24 Third, the excited molecule may interact with another constants of 600 ps for P3HT, 26 ps for P3HS, and 1.8 ps for species nearby. These competing pathways may be modulated P3HTe, suggesting rapid population of the triplet state. This by chemical design and used for excited state chemistry, espe- study demonstrates that incorporating a heavier chalcogen cially if the photoexcited molecule is close in proximity to results in faster ISC in organochalcogen polymers.27 The Creative Commons Attribution 3.0 Unported Licence. another species. This minireview will summarize the main authors indicate that this would be advantageous for exploiting photophysical effects of heavy chalcogen substitution, recent triplet excitons in future work on organic photovolatics. advances in the understanding of how to exploit the unique Another study by Rodriguez-Serrano et al. directly compared photophysical properties of ‘heavy’ organochalcogens as the rate of ISC in oxonine, thionine, and selenine small mole- sensors and triplet sensitizers, and will nally discuss future cule dyes (2) using both experimental observations and strategies for the further advancement of optical materials quantum mechanics.28 The ISC rate of oxonine had previously À using this chemistry. been experimentally determined as 7.2 Â 105 s 1.29 In this study, its uorescence rate was estimated to be considerably À higher at 2.10 Â 108 s 1,28 rendering it highly uorescent in This article is licensed under a 2. Quantitative studies on heavy 30 ff water with an experimental quantum yield of 1.0. By chalcogen substitution e ects comparison, the ISC and uorescence rates of thionine were 1 À À Â 109 s 1 and 1.66 Â 108 s 1, indicating that triplet state pop- While the number of studies utilizing heavy chalcogen- ulation was an order of magnitude faster than uorescence. The Open Access Article. Published on 07 October 2019. Downloaded 1/29/2020 2:59:09 PM. containing organic materials for specic applications is 25,26 population of triplet states is also supported by a singlet oxygen growing (Fig. 2), only a small number have reported the Fig. 1 (a) Representative photophysical pathways of an organochalcogen chromophore/fluorophore include absorption, fluorescence, inter- system crossing (ISC), reverse ISC, phosphorescence, and non-radiative decay. (b) Representative photophysical pathways of an organo- chalcogen fluorescent probe that is off when non-radiative decay occurs through triplet quenching, and on when Forster¨ Resonance Energy Transfer (FRET) occurs to a fluorescent acceptor. This journal is © The Royal Society of Chemistry 2019 Chem. Sci.,2019,10,9182–9188 | 9183 View Article Online Chemical Science Minireview Creative Commons Attribution 3.0 Unported Licence. Fig. 2 Discussed photoactive compounds containing selenium and tellurium. quantum yield of 0.53 in water. Using density functional theory, intersystem crossing and instead emit from the singlet state. the rate of ISC in selenine was estimated to be around 1 Â 1010 This emission may be tailored to turn on or off upon proximity À s 1 while the rate of uorescence was estimated to be 1.6 Â 108 to another analyte which can quench uorescence. This is oen À s 1. This report demonstrates that in changing from sulfur to controlled through a photoinduced electron transfer (PET) selenium, the rate of triplet formation may
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