Dimerization extends π‐conjugation of electron donor‐acceptor structures leading to phototheranostic properties beyond the sum of two monomers

Abstract Near‐infrared (NIR)‐II fluorescence imaging‐guided photothermal therapy (PTT) has attracted great research interest, and constructing donor‐acceptor (D‐A) electronic configurations has become an established approach to lower bandgap and realize NIR‐II emission. However, very few π‐conjugated phototheranostic agents can realize efficient NIR‐II guided PTT using a clinically safe laser power density, implying that sufficient photothermal performance is still desired. In addition to the continuously refreshed photothermal conversion efficiency levels, the strategies that focus on enhancing light absorptivity have been rarely discussed and endow a new direction for enhancing PTT. Herein, a dimerization π‐extension strategy is raised to synthesize π‐conjugated dimers with A‐D‐A monomers. We observe that the light absorptivity (ε) of the dimers is strengthened three times owing to the enhanced electronic coupling effect as a result of the π‐conjugation extension, thereby surpassing the 2‐fold increase in chromophore numbers from the monomer to dimers. Thanks to the enhancement in light absorption, the dimers could generate much more photothermal heat than the monomer in in vivo PTT treatments. Therefore, an efficient anti‐tumor outcome has been fulfilled by using dimers under a low laser power (0.3 W/cm 2 ). Moreover, the dimers with extended π‐conjugation structures become more favorable to the radiative excited state decay, thus exhibiting a distinguishing improvement in NIR‐II imaging compared with monomer. Collectively, due to the improved light absorptivity, the dimers can gain superior NIR‐II fluorescence brightness and photothermal performance over the recently reported material, which goes beyond the monomer in double doses for in vivo applications. All these results prove that dimerization is an effective strategy for designing high‐performance phototheranostic materials.