Spectral and luminescent properties of an indotricarbocyanine dye are studied in solutions and after deposition on quartz or silicon substrates. It is found that the dye molecules self-assemble in aqueous EtOH solutions to form H^*-aggregates. The absorption band of the H^*-aggregates shows a hypsochromic shift of 192 nm (5291 cm^–1) relative to the absorption maximum of dye monomers (706 nm) and has a full width at half maximum of 21 nm (797 cm^–1). The morphology of the H^*-aggregates of the indotricarbocyanine dye is studied for the first time. It is found that the aggregates are rod-like species ~10 nm high, 100 nm wide, and several micrometers long. H-aggregates with a fluorescence maximum at 560 nm and Stokes shift of 325 cm^–1 in addition to non-fluorescent H^*-aggregates form in aqueous EtOH solutions and are nanoparticles with a height of 1–3 nm and lateral dimensions of ~100 nm.

The spread of multidrug-resistant mycobacterium strains requires the development of new approaches to combat diseases caused by these pathogens. For that, photodynamic inactivation (PDI) is a promising approach. In this study, a tricarbocyanine (TCC) is used for the first time as a near-infrared (740 nm) activatable PDI photosensitizer to kill mycobacteria with deep light penetration. For better targeting, a novel tricarbocyanine dye functionalized with two trehalose units (TCC2Tre) is developed. The photodynamic effect of the conjugates against mycobacteria, including Mycobacterium tuberculosis, is evaluated. Under irradiation, TCC2Tre causes more effective killing of mycobacteria compared to the photosensitizer without trehalose conjugation, with 99.99% dead vegetative cells of M. tuberculosis and M. smegmatis. In addition, effective photoinactivation of dormant forms of M. smegmatis is observed after incubation with TCC2Tre. Mycobacteria treated with TCC2Tre are more sensitive to 740 nm light than the Gram-positive Micrococcus luteus and the Gram-negative Escherichia coli. For the first time, this study demonstrates the proof of principle of in vitro PDI of mycobacteria including the fast-growing M. smegmatis and the slow-growing M. tuberculosis using near-infrared activatable photosensitizers conjugated with trehalose. These findings are useful for the development of new efficient alternatives to antibiotic therapy.