Understanding violations of Kasha’s rule in the heptamethine indolizine-cyanine dye SO3C7

The fluorescence properties of NIR-emitting molecules are normally governed by Kasha’s Rule and the Energy Gap Law; however, exceptions to these rules are rare and remain poorly understood. The heptamethine indolizine-cyanine dye (SO3C7) was previously reported by our group as an alternative to luminol in latent blood stain detection due to its optical emission wavelength and selective emission properties. Here, we employ steady-state and transient absorption spectroscopic studies analyzed using quantum chemistry calculations to investigate anti-Kasha fluorescence and possible violations of the Energy Gap Law manifested by excitation-dependent emission and inverted excited-state lifetimes in SO3C7.