Optical Features of BODIPY Dyes Controlled by Local Environmental Interactions

To control the optical features of BODIPY dyes through local environmental interactions, three derivatives with distinct meso-substituents are investigated in different solvation systems. The large Stokes shift and broad bandwidth in optical transitions of BODIPY with a meso-substituent of an acetyl group (Ac-BDP) are attributed to significant structural changes in the excited state for the enhancement of electronic conjugation, which further influence dynamics and quantum yields. In contrast, the small Stokes shifts and narrow bandwidths are found in BODIPY with meso-substituents of hydroxyalkyl and methyl groups, due to minor structural changes in the excited state. At the freezing point of solvents, the sudden variation of peak position and quantum yield is observed in Ac-BDP, suggesting the effects of constrained structural changes in a glassy matrix. In a polymer matrix, structural changes are also influenced by the less-flexible environment, although the gradual evolution with temperature in the polymer matrix differs from the sudden variation in the glassy matrix. Notably, a narrow bandwidth of fluorescence is achievable in a wire-shaped solid of Ac-BDP, which is ascribed to waveguiding effects. Our findings propose novel strategies for modifying characteristics of BODIPY using the substituents and local environments, including temperature and flexibility of solvation systems, to enhance the tunability and efficiency of BODIPY for various applications.