Design strategy for tautomerization-based small panchromatic molecules

We report a design strategy for developing small panchromatic molecules through structural tautomerism. Bridged rhodamine (1) is soluble in water and exhibits excellent light absorption, its molar absorption coefficient exceeding 5000 across the entire UV-visible region from 300 to 750 nm in DMSO. DFT calculations suggest that the barrierless interconversion of the energetically equivalent C10-hydroxy (1a) and C10-carbonyl (1b) forms of 1 contributes to the broadening of the absorption spectrum.