π‑Bridge\nRegulates Donor–Acceptor\nInteractions for Long-Wavelength Photoacoustic Imaging and Enhanced\nPhotothermal Therapy

Acceptor–donor–acceptor (A-D-A) structured\nπ-conjugated\nmolecules emerge as a new class of phototheranostic materials. The\nπ-bridge has rarely been investigated for structure engineering\ncompared with the D and A units. This work reveals that changing ethylene\ninto a butadiene π-bridge can efficiently extend absorption\nwavelength and promote the nonradiative transition. Specifically,\nthe butadiene π-bridge not only extends the π-conjugation\nlength but also disrupts the S···O interlocking, which\nfacilitates\nintramolecular torsions and rotations and enhances the intramolecular\ncharge transfer for the molecule with butadiene π-bridge. As\na result, very broad absorption with a 70 nm red-shift is observed,\nand the power conversion efficiency (PCE) of BTVIC nanoparticles reaches\n82.6%. Ultimately, photoacoustic imaging (PAI)-guided photothermal\ntherapy (PTT) applications in vivo demonstrate a deep penetration\ndepth of over 7 mm and an efficient anti-tumor effect. This work illustrates\nthat modifying the π-bridge is a practical strategy for regulating\nA-D-A interactions and developing more efficient phototheranostic\nagents.