title: Twisting in the excited state of an N-methylpyridinium fluorescent dye modulated by nano-heterogeneous micellar systems.
abstract: A push-pull N-methylpyridinium fluorescent dye with a pyrenyl group as the electron-donor portion was investigated within the nano-heterogeneous media provided by some micellar systems. The molecule was studied by stationary and time-resolved spectroscopic techniques in spherical micellar solutions and viscoelastic hydrogels, in order to throw light on the role played by twisting in its excited state deactivation. As proven by femtosecond fluorescence up-conversion and transient absorption experiments, the excited state dynamics of the molecule is ruled by charge transfer and twisting processes, which, from the locally excited (LE) state initially populated upon excitation, progressively lead to twisted (TICT) and planar (PICT) intramolecular charge transfer states. The inclusion within micellar aggregates was found to slow down and/or limit the rotation of the molecule with respect to what had previously been observed in water, while its confinement within the hydrophobic domains of the gel matrixes prevents any molecular torsion. The increasing viscosity of the medium, when passing from water to micellar systems, implies that the detected steady-state fluorescence comes from an excited state which is not fully relaxed, as is the case with the TICT state in micelles or the LE state in hydrogels, where the detected emission changes its usual orange colour to yellow.
authors.affiliation: Department of Chemistry, Biology and Biotechnology and Centro di Eccellenza sui Materiali Innovativi Nanostrutturati (CEMIN), University of Perugia, via Elce di Sotto 8, 06123 Perugia, Italy. alex.cesaretti14@gmail.com.
authors.forename: A
authors.initials: A
authors.lastname: Cesaretti
authors.forename: B
authors.initials: B
authors.lastname: Carlotti
authors.forename: P L
authors.initials: PL
authors.lastname: Gentili
authors.forename: R
authors.initials: R
authors.lastname: Germani
authors.forename: A
authors.initials: A
authors.lastname: Spalletti
authors.forename: F
authors.initials: F
authors.lastname: Elisei
mesh_terms: D005456:Fluorescent Dyes / Q000737:chemistry*; D057927:Hydrophobic and Hydrophilic Interactions; D007700:Kinetics; D008823:Micelles; D049329:Nanostructures; D011726:Pyridinium Compounds / Q000737:chemistry*; D013050:Spectrometry, Fluorescence
chemical_list: D005456:Fluorescent Dyes; D008823:Micelles; D011726:Pyridinium Compounds; C014677:1-methylpyridinium