Photostability of Cyanine Dyes and Its Correlation to Their Photothermal and Photodynamic Effects

Near-infrared (NIR)-absorbing small-molecule dyes are highly attractive for biological applications, including imaging, photothermal (PT), and photodynamic (PD) therapy. Among these, heptamethine cyanines are particularly promising, but their inherently low photostability (PS) remains a major limitation. While PS is recognized as critical for imaging, its importance for PT and PD performance is less well established. Moreover, reported PS values depend strongly on experimental conditions, making comparisons across studies challenging. Here, we systematically compare the PS of established and newly synthesized heptamethine cyanines using a carefully evaluated protocol designed to ensure reliable and reproducible measurements. We examine how commonly applied structural modifications affect PS and investigate the relationship between photosensitization of oxygen and photodegradation. We find no clear correlation between singlet oxygen generation, molecular structure, and PS, indicating that singlet oxygen is not the major pathway responsible for cyanine photodegradation. For selected dyes, we further investigate photoinduced heating and demonstrate that photostability is the limiting factor for all investigated compounds. These results highlight the critical importance of PS beyond imaging and suggest that photostability should be considered a key, yet often overlooked, parameter governing the performance of NIR dyes in photothermal and photodynamic applications.