Deuteration of Heptamethine Cyanine Dyes Leads to Enhanced Emission Efficacy

Short-wave infrared region (SWIR; 900-2000 nm) enables in vivo bioimaging with unrivaled spatial and temporal resolution, but its full potential is locked behind the lack of highly emissive organic fluorophores. Their design remains a grand challenge and understanding the structure-property relationship is the key to their rational design. Here we investigate the effects of deuteration on the photophysical properties of a series of heptamethine (Cy7) dyes bearing different terminal heterocycles, the absorption of which spans the near-infrared (NIR) and SWIR regions. Using cheap deuterium sources, we demonstrate that deuteration is a strategy applicable across the Cy7 family that leads to enhanced quantum yields of fluorescence, longer-lived singlet excited states and suppressed rates of non-radiative deactivation processes. Jointly with exclusion of the central cyclohexenyl ring, the approach furnishes the brightest SWIR-emitting Cy7 fluorophore disclosed to this date.