Photochemical & Photobiological Sciences PAPER Cite this: Photochem. Photobiol. Sci., 2019, 18, 2199 Received 20th December 2018, Accepted 25th February 2019 DOI: 10.1039/c8pp00586a rsc.li/pps Photo-induced spin switching in a modified anthraquinone modulated by DNA binding† Marco Bortolus, * a Giovanni Ribaudo, b Antonio Toffoletti, a Donatella Carbonera a and Giuseppe Zagotto b An anthraquinone modified with a nitroxide radical and able to intercalate into DNA has been synthesized to obtain a molecule the spin state of which can be manipulated by visible light and DNA binding. The doublet ground state of the molecule can be photo-switched to either a strongly coupled spin state (quartet + doublet), when isolated, or to an uncoupled spin state (triplet and doublet), when bound to DNA. The different spin state that is obtained upon photoexcitation depends on the intercalation of the quinonic core into double-stranded DNA which changes the conformation of the molecule, thereby alter- ing the exchange interaction between the excited state localized on the quinonic core and the nitroxide radical. The spin state of the system has been investigated using both continuous-wave and time-resolved EPR spectroscopy. Introduction The possibility of switching between spin states of different multiplicity is of great interest in the field of spintronics. 1–3 Temperature, chemical conditions and light have all been suc- cessfully used to switch the total spin of a molecule. 4–12 Among these the use of light as a trigger is the most attractive, given how easy it is to use in a device. Normally, spintronic applications are based on solid state materials, but the idea of extending the field to bio-compatible molecular systems is fascinating. One of the most flexible bio- logically compatible scaffolds is DNA which has been used both as a covalent and non-covalent scaffold upon which to build new molecules or assemble novel structures. 13–16 Additionally, DNA has the added benefit of being able to trans- fer electrons along its aromatic core, a feature that could be further used in spintronics. Moreover, photoactive compounds can be easily arranged on DNA. 17–20 1,4-Disubstituted 9,10-anthraquinone is well known to form a supramolecular complex intercalating into DNA 21 and for this reason display interesting biological activity. 22,23 By coupling an aromatic 9,10-anthraquinone to a single nitroxide stable radical, via the amide linkages in position 1 or, indistin- guishably, 4 (molecule 1 in Scheme 1), we obtained a system that can interact with DNA, has a paramagnetic spin state in the ground state, and absorbs light in the visible range where DNA does not absorb. As a reference in the analysis of the photophy- sics of the anthraquinonic core of 1, we also synthesized 2 (see Scheme 1), an analogue without the nitroxide moiety. To monitor the spin state of the system in the presence or in the absence of DNA, we used electron paramagnetic reso- nance (EPR) spectroscopy, both continuous-wave EPR (CW-EPR) for ground state experiments, and time-resolved EPR (TR-EPR) to monitor the evolution of the spin state as a function of time after the photoexcitation with a pulsed laser. TR-EPR is the technique of choice to monitor spin-active states and their evolution, and has been applied to triplet, 24,25 quartet, 26–29 and quintet states, 29–32 spin correlated radical pairs. 33,34 and other systems. In this work, we show that, following photoexcitation, 1 changes its total spin state going from a doublet ground state (the nitroxide) to either a strongly coupled excited state (doublet + quartet, D 1 +Q 1 ) or to an uncoupled excited state (doublet and triplet, D 0 and T 1 ) in the presence of DNA. Scheme 1 The modified anthraquinones studied in this work. † Electronic supplementary information (ESI) available: Syntheses, calculation details, additional EPR data, and full experimental procedures. See DOI: 10.1039/c8pp00586a a Department of Chemical Sciences, University of Padova, via Marzolo 1, 35131 Padova, Italy. E-mail: marco.bortolus@unipd.it b Department of Pharmaceutical Sciences, University of Padova, via Marzolo 5, 35131 Padova, Italy This journal is © The Royal Society of Chemistry and Owner Societies 2019 Photochem. Photobiol. Sci., 2019, 18, 2199–2207 | 2199