tetrads are formed by the self-association of four guanine residues in a square-planar arrangement, stabilized by Hoogsteen hydrogen bonding and coordination to a central metal cation. The folding of the oligonucleotide strand causes the stacking of these tetrads to form the G4 architecture (Figure ). The presence of G4 forming sequences in the human genome is well-documented and their transient formation in vivo has been linked to essential genomic functions, such as transcription, replication, repair, and telomere maintenance. For instance, G4s are overrepresented in the promoter regions of oncogenes (e.g. c-myc, BCL2 and c-kit ) and it has been proposed that targeting of the folded G4 with small molecules can inhibit the binding of transcription factors leading to downstream silencing of oncogene expression. More recently, putative G4-forming sequences in the genomes of the protazoan parasites Trypanosoma brucei and Leishmania major have been reported, and bacteria (Escherichia coli, Pseudomonas aeruginosa and Mycobacterium bovis). G4s are also found in viral genomes such as the human papilloma virus (HPV), herpes simplex virus-1 (HSV-1), Epstein-Barr virus (EBV) human and the immunodeficiency virus 1 (HIV-1). The selective targeting of G4s with small molecule ligands has revealed a variety of promising therapeutic effects, both in cellular models and in whole organisms. During the course of our studies of G-quadruplex binding ligands, we became interested in the potential of G-quadruplex DNA as a target for photoresponsive molecules, the structure of which may switched, either irreversibly or reversibly, by irradiation with light, thus providing a non-invasive means to modulate the DNA binding properties and downstream function of the molecule with an external trigger in a biological or materials chemistry context. Indeed, the reversible photoregulation of oligonucleotide structure and function offers exciting opportunities for the control of biological function and for the development of novel tools for chemical biology, nanotechnology, material sciences, pharmacology and medicine applications. The use of light as an external stimuli offers significant advantages over alternative chemical methods, since it can be delivered with high spatiotemporal precision allowing localized control of the therapeutic activity, minimizing potential off-target effects. Importantly, control of the DNA architecture through the use of a photoresponsive supramolecular guest species obviates the need to engineer the photoresponsive functionality into the biomolecule itself, which presents several synthetic and practical challenges towards the potential applications. Whilst several studies have already investigated the potential of photoresponsive molecules to control DNA structure, these generally focus on classical duplex DNA helices. Very significant progress has been made in this research area, and photoresponsive DNA binders based on a variety of scaffolds including azobenzene, stilbene and diarylethene derivatives have been reported. However, despite the unique opportunities afforded by targeting G4 structures specifically, given their unique polymorphic nature, and their relevance in biological systems and materials chemistry, the development of photoresponsive G4 binding molecules has received little attention until recently. Figure . Regulation of G4 structures using photoresponsive ligands Towards the development of such photoresponsive G4 ligands, Wang and co-workers disclosed an azobenzene derivative that was able to reversible regulate G4 formation and dissociation in buffered aqueous conditions in the absence of metal cations (Figure ). However, the system proved ineffective in physiological conditions of high ionic strength.