Abstract
<jats:p>While there is a significant amount of near-infrared (NIR) fluorophores emerging in the literature, organic molecules absorbing in the NIR-II window (1000 nm to 1700 nm) are relatively under-represented. To satisfy the need to shift the optical properties further into the NIR for utilisation in bioimaging, clever synthetic strategies and structural design are required. Following the adapted reported synthetic procedure, [1], we synthesised two heptamethine dyes absorbing and emitting deep into the NIR-II window with high purity using simple purification techniques. We introduce carboxylic acid as a functional group that enables the dye’s covalent attachment to a surface. By conducting the synthesis under a strict nitrogen atmosphere, we circumvented oxidation of the dyes, yielding final products with improved optical properties and stability. We show that the newly synthesised functionalised dyes are sensitive to oxidation and lose their optical properties with time. Density Functional Theory (DFT) and Time-Dependent DFT calculations elucidated the formation of different oxidised by-products. While the oxidation of one end group breaks the symmetry of the frontier orbitals, leading to an expected hypsochromic shift, the second by-product’s optical properties differ due to the interaction between the different cyclic bridges and the polymethine chain, significantly influencing the molecule’s HOMO-LUMO gap. This study establishes a reliable synthesis protocol for high-performance, easily functionalised NIR-II heptamethine dyes.</jats:p>