Intramolecular Excimer Formation for Covalently Linked Boron Dipyrromethene Dyes

Abstract

Photophysical properties have been recorded for a small series of covalently linked, symmetrical dimers formed around boron dipyrromethene (Bodipy) dyes. Within the series, a control dimer is unable to adopt a cofacial arrangement because of steric factors, while a second dimer possesses sufficient internal flexibility to form the cofacial geometry but with little overlap of the Bodipy units. The other three members of the series take up a cofacial arrangement with varying bite angles between the planes of the two Bodipy units. Fluorescence quantum yields and excited-state lifetimes indicate differing extents of electronic interaction between the two Bodipy head-groups, but only the compound with the smallest bite angle exhibits excimer emission in solution under ambient conditions. Time-resolved fluorescence studies show dual-exponential decay kinetics in each case, while temperature-dependent emission studies reveal reversible coupling between monomer and lower-energy excimer states. The latter is weakly fluorescent, at best, and is seen clearly only for dimers having small bite angles. The application of high pressure to dilute solutions of these dimers promotes excimer formation in certain cases and leads to loss of monomer-like fluorescence. Under high pressure, excimer emission is more evident, and the overall results can be discussed in terms of subtle structural rearrangements that favor excimer formation.

Unexpected Nucleophilic Substitution Reaction of BODIPY: Preparation of the BODIPY–TEMPO Triad Showing Radical‐Enhanced Intersystem Crossing

“…With decreasing temperature, the monomer/excimer emission intensity ratio increased, which indicates that there is an activation energy for the formation of the excimer from the S 1 state of the monomer. This feature is opposite to that previously observed in BODIPY dimers with co‐facial geometries …”

“…This indicates that the excitation at the low‐energy side of the absorption band is more efficient for the formation of excimer emission than excitation at the major absorption band at 514 nm. For the reference compound BT, no such redshifted excitation band was observed (see Figure S16 in the Supporting Information), which is consistent with this postulate …”

“…Recently we prepared TEMPO‐labeled BODIPY, but the linker in the current DBT is much shorter (Scheme ) and two BODIPY chromophores exist in DBT, which may exert a substantial effect on the photophysical properties of the compound . For instance, because two BODIPY chromophores are present, excimer formation could be expected …”

“…[67] For instance, because two BODIPY chromophores are present, excimer formation could be expected. [68][69][70] According to previous reports, the preparation of the triad from meso-chloro-BODIPY should be a rather straightforward substitution reaction at the 8-chloromethyl site (see DBT′, expected structure, Scheme 1). [33][34][35][36][37][38][71][72][73] However, after the reaction, the 1 H NMR spectrum of the product indicated that the four methyl groups on the BODIPY chromophore are inequivalent (see Figures S3 and S4 in the Supporting Information for the spectrum).…”

Recent advances in artificial photosynthetic systems at Newcastle University

“…Alamiry et al have done an extensive study on how subtle changes in the molecular conformation affect the fluorescence properties and, in turn, charge transfer kinetics using high pressures [23].…”