Comparison of a series of donor-acceptor-donor type croconium dyes on pH response, near-infrared fluorescence/photoacoustic imaging and photothermal properties

Eight donor-acceptor-donor type croconium dyes with different donor moieties, such as indole, N-methylindole, benzo[e]indole, N-methylbenzo[c,d]indole, benzothiazole and quinoline groups, have been designed and discussed their pH response, near-infrared fluorescence/photoacoustic (NIR/PA) imaging and photothermal properties in solutions and tumor cells. Croconium dye with bisbenzothiazole CBT exhibited the best NIR-imaging. Croconium dye with bisindole CBI-1 showed the best PA-imaging. Dye CBI-3 with bisbenzo[e]indole showed the best photothermal. Four dyes with either indole or benzo[e]indole group CBI-1, CBI-3, CIT and CIQ showed pH response on fluorescence and photoacoustic spectra and photothermal. This study proves that preserving the indole group in croconium dyes was beneficial for their pH response and NIR/PA imaging, and the introduction of quinoline and thiazole groups was detrimental to the photostability. Gathered with pH response, NIR/PA imaging and photothermal property, dye CBI-1 is suitable for structural modification in the future for developing better cancer theranostic reagents.