Synthesis and circularly polarised luminescence of helically chiral 4,4-difluoro-4-bora-3a,4a-diaza-s-indacenes (BODIPYs)

Abstract

II The differential emission of left- or right-handed circularly polarised light is termed circularly polarised luminescence (CPL).1,2 CPL emission has promising applications in a wide range of fields, such as the development of CPL microscopes.3,4 Commonly chiral lanthanide complexes are employed in CPL-related studies, due to their high luminescence dissymmetry factors (glum).5–7 However these systems are limited by their typically low fluorescence quantum yields (φF), leading to low overall CPL efficiencies (|glum|·φF). Therefore there is considerable interest in the development of small organic molecules capable of CPL emission (CPL-SOMs). The 4,4′-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) dyes are a class of organic fluorophores which have desirable photophysical properties, including typically high fluorescence quantum yields. Furthermore BODIPYs have highly tuneable absorption and emission bands which can be modulated through varied and robust synthetic transformations. In this work, we have focused on the development of helically chiral BODIPY dyes for use as CPL-SOMs. Herein we describe the synthesis and chiroptical characterisation of a novel, helically chiral N,N,O,C-BODIPY, which we have shown is capable of CPL emission upon irradiation in solution (glum = 3.7×10-3, figure A.1).8 Subsequent mechanistic investigation has allowed us to identify an improved synthetic route to the N,N,O,C-BODIPYs, and thus the expansion of the N,N,O,C-BODIPY series. Study of the chiroptical properties of the N,N,O,C-BODIPYs has revealed that this architecture is amenable to the addition of functional groups, without detriment to CPL-emission. This opens up the possibility of introducing more complex functionality to this architecture, whilst retaining the desired chiroptical activity.Furthermore our investigations into the fundamental molecular properties of the N,N,O,C-BODIPY system has given us insight into design strategies towards development of BODIPY CPL-SOMs with increased glum. We describe our attempts towards the synthesis of helically chiral BODIPYs containing sulfur atoms, and the synthesis of a π-extended helically chiral N,N,O,O-BODIPY system. Through these syntheses, we have made a preliminary assessment of the validity of these design strategies