title: Suppression of Kasha's rule as a mechanism for fluorescent molecular rotors and aggregation-induced emission.
abstract: Although there are some proposed explanations for aggregation-induced emission, a phenomenon with applications that range from biosensors to organic light-emitting diodes, current understanding of the quantum-mechanical origin of this photophysical behaviour is limited. To address this issue, we assessed the emission properties of a series of BF2-hydrazone-based dyes as a function of solvent viscosity. These molecules turned out to be highly efficient fluorescent molecular rotors. This property, in addition to them being aggregation-induced emission luminogens, enabled us to probe deeper into their emission mechanism. Time-dependent density functional theory calculations and experimental results showed that the emission is not from the S1state, as predicted from Kasha's rule, but from a higher energy (>S1) state. Furthermore, we found that suppression of internal conversion to the dark S1state by restricting the rotor rotation enhances fluorescence, which leads to the proposal that suppression of Kasha's rule is the photophysical mechanism responsible for emission in both viscous solution and the solid state.
authors.affiliation: Department of Chemistry, Dartmouth College, Hanover, New Hampshire 03755, USA.
authors.forename: Hai
authors.initials: H
authors.lastname: Qian
authors.affiliation: Department of Chemistry, University of Vermont, Burlington, Vermont 05405, USA.
authors.forename: Morgan E
authors.initials: ME
authors.lastname: Cousins
authors.affiliation: Department of Chemistry, University of Vermont, Burlington, Vermont 05405, USA.
authors.forename: Erik H
authors.initials: EH
authors.lastname: Horak
authors.affiliation: Department of Chemistry, Dartmouth College, Hanover, New Hampshire 03755, USA.
authors.forename: Audrey
authors.initials: A
authors.lastname: Wakefield
authors.affiliation: Department of Chemistry, University of Vermont, Burlington, Vermont 05405, USA.
authors.forename: Matthew D
authors.initials: MD
authors.lastname: Liptak
authors.affiliation: Department of Chemistry, Dartmouth College, Hanover, New Hampshire 03755, USA.
authors.forename: Ivan
authors.initials: I
authors.lastname: Aprahamian