Abstract
Herein we describe the synthesis, computationally assisted spectroscopy, and lasing properties of a new library of symmetric bridged bis-BODIPYs that differ in the nature of the spacer. Access to a series of BODIPY dimers is straightforward through synthetic modifications of the pending ortho-hydroxymethyl group of readily available C-8 (meso) ortho-hydroxymethyl phenyl BODIPYs. In this way, we have carried out the first systematic study of the photonic behavior of symmetric bridged bis-BODIPYs, which is effectively modulated by the length and/or stereoelectronic properties of the spacer unit. The designed bis-BODIPYs display bright fluorescence and laser emission in non-polar media. The fluorescence response is governed by the induction of a non-emissive intramolecular charge transfer (ICT) process, which is significantly enhanced in polar media. The effectiveness of the fluorescence quenching and also the prevailing charge transfer mechanism (from the spacer itself or between the BODIPY units) rely directly on the electron-releasing ability of the spacer. Moreover, the linker moiety can also promote intramolecular excitonic interactions, leading to excimer-like emission characterized by new spectral bands and the lengthening of lifetimes. The substantial influence of the bridging moiety on the emission behavior of these BODIPY dyads and their solvent-sensitivity highlight the intricate molecular dynamics upon excitation in multichromophoric systems. In this regard, the present work represents a breakthrough in the complex relationship between the molecular structure of the chromophores and their photophysical signatures, thus providing key guidelines for rationalizing the design of tailored bis-BODIPYs with potential advanced applications.
Highlights
Modern avenues in dye chemistry are oriented to the development of single fluorophores with tailor-made molecular structures (De Moliner et al, 2017) but are focused on the rational design of multichromophoric architectures where the fluorophores are linked through covalent bonds (Ahrens et al, 2013; Fan et al, 2013)
Hereafter, we thoroughly describe the interplay between the molecular structure and the photophysical signatures, with special attention to the fluorescence response and the ongoing non-radiative channels related to intramolecular charge transfer and excitonic couplings
The computationally assisted spectroscopic characterization carried out unambiguously revealed the complex and intriguing excited state dynamics induced in these new multichromophoric systems
Summary
Modern avenues in dye chemistry are oriented to the development of single fluorophores with tailor-made molecular structures (De Moliner et al, 2017) but are focused on the rational design of multichromophoric architectures where the fluorophores are linked through covalent bonds (Ahrens et al, 2013; Fan et al, 2013). The boron-dipyrrin backbone is ready amenable to a wide range of post-functionalization routes (Boens et al, 2015), which might allow its ulterior covalent linkage to additional chromophoric units (Dumas-Verdes et al, 2010; Misra et al, 2014; Gartzia-Rivero et al, 2015; Kesavan et al, 2015; Arroyo-Córdoba et al, 2018; Xu et al, 2018; Zhang et al, 2018) Such tailoring of the molecular structure being available enables the modulation of the spectral bands of the BODIPY, leading to stable and bright dyes along the whole visible spectrum and even reaching the near-infrared region (Lu et al, 2014; Bañuelos, 2016). The computationally aided photophysical and laser study of this new set of bis-BODIPYs have contributed to the understanding of the structural controls behind the fluorescence response of these multichromophoric laser dyes
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