title: Spatiotemporal control over the co-conformational switching in pH-responsive flavylium-based multistate pseudorotaxanes.
abstract: A multistate molecular dyad containing flavylium and viologen units was synthesized and the pH dependent thermodynamics of the network completely characterized by a variety of spectroscopic techniques such as NMR, UV-vis and stopped-flow. The flavylium cation is only stable at acidic pH values. Above pH ≈ 5 the hydration of the flavylium leads to the formation of the hemiketal followed by ring-opening tautomerization to give the cis-chalcone. Finally, this last species isomerizes to give the trans-chalcone. For the present system only the flavylium cation and the trans-chalcone species could be detected as being thermodynamically stable. The hemiketal and the cis-chalcone are kinetic intermediates with negligible concentrations at the equilibrium. All stable species of the network were found to form 1 : 1 and 2 : 1 host : guest complexes with cucurbit[7]uril (CB7) with association constants in the ranges 10(5)-10(8) M(-1) and 10(3)-10(4) M(-1), respectively. The 1 : 1 complexes were particularly interesting to devise pH responsive bistable pseudorotaxanes: at basic pH values (≈12) the flavylium cation interconverts into the deprotonated trans-chalcone in a few minutes and under these conditions the CB7 wheel was found to be located around the viologen unit. A decrease in pH to values around 1 regenerates the flavylium cation in seconds and the macrocycle is translocated to the middle of the axle. On the other hand, if the pH is decreased to 6, the deprotonated trans-chalcone is neutralized to give a metastable species that evolves to the thermodynamically stable flavylium cation in ca. 20 hours. By taking advantage of the pH-dependent kinetics of the trans-chalcone/flavylium interconversion, spatiotemporal control of the molecular organization in pseudorotaxane systems can be achieved.
authors.affiliation: Laboratório Associado para a Química Verde (LAQV), REQUIMTE, Departamento de Química, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, 2829-516 Monte de Caparica, Portugal. nuno.basilio@fct.unl.pt ajp@fct.unl.pt and Departamento de Química, Escola Superior de Tecnologia da Saúde de Lisboa - Instituto Politécnico de Lisboa, Lisboa, Portugal.
authors.forename: Ana Marta
authors.initials: AM
authors.lastname: Diniz
authors.affiliation: Laboratório Associado para a Química Verde (LAQV), REQUIMTE, Departamento de Química, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, 2829-516 Monte de Caparica, Portugal. nuno.basilio@fct.unl.pt ajp@fct.unl.pt.
authors.forename: Nuno
authors.initials: N
authors.lastname: Basílio
authors.affiliation: Laboratório Associado para a Química Verde (LAQV), REQUIMTE, Departamento de Química, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, 2829-516 Monte de Caparica, Portugal. nuno.basilio@fct.unl.pt ajp@fct.unl.pt.
authors.forename: Hugo
authors.initials: H
authors.lastname: Cruz
authors.affiliation: Laboratório Associado para a Química Verde (LAQV), REQUIMTE, Departamento de Química, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, 2829-516 Monte de Caparica, Portugal. nuno.basilio@fct.unl.pt ajp@fct.unl.pt.
authors.forename: Fernando
authors.initials: F
authors.lastname: Pina
authors.affiliation: Laboratório Associado para a Química Verde (LAQV), REQUIMTE, Departamento de Química, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, 2829-516 Monte de Caparica, Portugal. nuno.basilio@fct.unl.pt ajp@fct.unl.pt.
authors.forename: A Jorge
authors.initials: AJ
authors.lastname: Parola