π-Bridge Regulates Donor–Acceptor Interactions for Long-Wavelength Photoacoustic Imaging and Enhanced Photothermal Therapy

Acceptor–donor–acceptor (A-D-A) structured π-conjugated molecules emerge as a new class of phototheranostic materials. The π-bridge has rarely been investigated for structure engineering compared with the D and A units. This work reveals that changing ethylene into a butadiene π-bridge can efficiently extend absorption wavelength and promote the nonradiative transition. Specifically, the butadiene π-bridge not only extends the π-conjugation length but also disrupts the S···O interlocking, which facilitates intramolecular torsions and rotations and enhances the intramolecular charge transfer for the molecule with butadiene π-bridge. As a result, very broad absorption with a 70 nm red-shift is observed, and the power conversion efficiency (PCE) of BTVIC nanoparticles reaches 82.6%. Ultimately, photoacoustic imaging (PAI)-guided photothermal therapy (PTT) applications in vivo demonstrate a deep penetration depth of over 7 mm and an efficient anti-tumor effect. This work illustrates that modifying the π-bridge is a practical strategy for regulating A-D-A interactions and developing more efficient phototheranostic agents.