Development of next generation of photosensitizer for fluorescent imaging and photodynamic therapy of cancer

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We have designed and synthesized a new class of rhodamine dyes with an extended ?-conjugated system, named 3’,3’’-bis(oxospiroisobenzofuran)-3,7-bis(diethylamino)benzopyrano-xanthene (ABPX01) dye. In this study, the investigations were carried out to reveal the relationships among the chemical species of ABPX01, the color, and the fluorescence emission in various solutions. ABPX01 had various forms: the spirolactone form (ABPX01^0), which is colorless; and the monocationic form (ABPX01H^+) and the dicationic form (ABPX01H_2^<2+>), which are colored. ABPX01H_2^<2+> was identified to be a red fluorescent species. Detailed spectroscopic and electron microscopic investigations led to the assumption that the ABPX01H_2^<2+> formed ion associates with Cl^- as counter anions in HCl aqueous solution, and the nanoand submicrometer-sized colloidal aggregates of ABPX01 hydrochloride exhibit fluorescence emission. Good performance in fluorescent imaging demands large fluorescence quantum yields, and solubility in hydrosoluble media of the markers. We design and synthesize new ABPX derivatives in which the amino groups are incorporated into rigid six-membered rings. The relative fluorescence quantum yields of ABPX101H_2^<2+>, ABPX102H_2^<2+>, and ABPX103H_2^<2+>were drastically increased relative to that of ABPX01H_2^<2+>. All ABPX derivatives commonly exhibited the well-developed bimodal fluorescence emission spectra over a wide range including red and NIR region. The intrinsic solubility of the ABPX dyes is favorable for the photosensitizers of fluorescent imaging and photodynamic therapy.