Organic NIR-II Photoacoustic Agent Utilizing Combined Two-Photon and Excited State Absorption at 1064 nm

Photoacoustic (PA) imaging performed at a wavelength within the second optical window of biological transparency (NIR-II, 1000-1700 nm), where light scattering is greatly reduced, is of great significance for preclinical and clinical practices. However, the rarity of PA contrast agents strongly absorbing in the NIR-II region has greatly limited its practical applications. The study presented here reveals a new class of NIR-II PA agents using a near-infrared absorbing organic BODIPY molecule (2B-BDP) for NIR-II PA imaging via two-photon absorption (TPA) followed by excited state absorption (ESA). Femtosecond transient absorption (fs-TA) spectroscopy and nonlinear transmittance experiments unambiguously reveal an efficient TPA-induced ESA photophysical process for 2B-BDP. By virtue of sufficient absorption around 1064 nm through the combined contributions of TPA and ESA followed by a highly efficient nonradiative decay, 2B-BDP exhibits a strong NIR-II PA signal. This study can lead to the development of NIR-II organic PA agents with an easy, scalable, time-, and cost-effective approach.