New Hindered BODIPY Derivatives: Solution and Amorphous State Fluorescence Properties

Four new hindered boron-dipyrromethene (BODIPY) dyes have been engineered and synthesized by modification of the pyrrole substituents. The chromophores were designed to shift their absorption and emission wavelengths to the red and to show quantitative emission in solid amorphous deposits. [2.2]Paracyclophane (PcP) was grafted on the BODIPY core as well as alkyl chains of various length to force the molecules away one from each other and prevent π-π stacking. The optical spectroscopic and electrochemical properties in solution are presented. Absorption, emission and FLIM were done on drop-casted films of the fluorophores. The addition of alkyl substituents (-Me, -Et, n-Pr) next to the PcP group clearly affects the emission properties in solid state since the dyes display sharper emission bands than in solution and their relative fluorescence quantum yield increases with the number of carbons in the alkyl chain. Fluorescence lifetime images analysis reveals complex decay profiles. Quite a fair description is given by a biexponential fit: the short component is attributed to aggregates and the long component to monomer-like molecules. This population shows a shorter lifetime (2-3.9 ns) compared to solution (6 ns) due to a quenching by aggregates. This quenching is decreasing as the number of carbons on the alkyl side chain is increasing. The combination of a PcP group next to a n-Pr group on a BODIPY is a good combination to get an excellent fluorophore in the solid state.