Characterization of Triphenylamine and Ferrocenyl Donor-π-Donor Vinyl BODIPY Derivatives as Photoacoustic Contrast Agents^†

The photophysical and electrochemical properties for a series of BODIPY dyes with incremental 3- and 3,5-vinyl conjugation, as well as incremental electron-donating groups (anisole < triphenylamine < ferrocenyl), are presented. Insight into the influence of each vinyl-conjugated electron-donating group on both vis-NIR absorption and fluorescence emission properties is provided. These trends are further corroborated by density functional theory computational analysis. Two of this series containing the 3,5-bis(vinyltriphenylamine) and 3,5-bis(vinylferrocenyl) substituents exhibit significant absorption cross sections in the biological transparency window justifying further investigation of their photoacoustic emission properties via both optical photoacoustic z-scan and photoacoustic tomography experiments. Both the 3,5-bis(vinyltriphenylamine) and 3,5-bis(vinylferrocenyl) substituted BODIPY dyes exhibit quantitative photoacoustic quantum yields. Relative to the commercially available methylene blue and indocyanine green molecular photoacoustic contrast agents, the 3,5-bis(vinyltriphenylamine)-derived BODIPY exhibits the greatest photoacoustic emission and contrast upon excited-state absorption at 685 nm excitation at a low power laser fluence (<20 mJ cm^-2).