Photochemistry

Materials science

Absorption (acoustics)

Side chain

Molecular engineering

Polymer

Physics

Nanotechnology

Organic chemistry

Cyanine

Planarity testing

Optics

Photothermal therapy

Fluorescence

Bathochromic shift

Composite material

Rational design

Crystallography

Chemistry

Angewandte Chemie International Edition

Developing conjugated small molecules (CSM) with intense NIR‐II (1000–1700 nm) absorption for phototheranostic is highly desirable but remains a tremendous challenge due to a lack of reliable design guidelines. This study reports a high‐performance NIR‐II CSM for phototheranostic by tailoring molecular planarity. A series of CSM show bathochromic absorption extended to the NIR‐II region upon the increasing thiophene number, but an excessive number of thiophene results in decreased NIR‐IIa (1300–1400 nm) brightness and photothermal effects. Further introduction of terminal nonconjugated alkyl chain can enhance NIR‐II absorption coefficient, NIR‐IIa brightness, and photothermal effects. Mechanism studies ascribe this overall enhancement to molecular planarity stemming from the collective contribution of donor/side‐chain engineering. This finding directs the design of NIR‐II CSM by rational manipulating molecular planarity to perform 1064 nm mediated phototheranostic at high efficiency.

Increasing Molecular Planarity through Donor/Side‐Chain Engineering for Improved NIR‐IIa Fluorescence Imaging and NIR‐II Photothermal Therapy under 1064 nm