Dual sensitivity of spiropyran-functionalized carbon dots for full color conversions

Multicolor phosphors that respond rapidly to external stimuli are highly desirable for many applications, including lighting, optical displays and sensors. Herein, spiropyran (SP)-functionalized carbon dots (CDs) were synthesized with a broad-spectrum output that were responsive to both ultraviolet (UV)/visible light and pH. The SP-CDs possessed strong ester linkages between the UV-absorbing/blue-emitting CDs and blue-absorbing/red-emitting surface SP groups, allowing efficient Forster resonance energy transfer (FRET) between the donor and acceptor. UV irradiation or acid addition cause the SP ring opening to merocyanine (MC) or protonated MCH+ forms, respectively, together with the formation of sheet-like aggregates. These processes enhanced the red emissions by the SP groups and attenuated blue emissions from the CDs. These changes were fully reversible under visible light or basic conditions, respectively, allowing dynamic regulation of the fluorescence properties (emission colors from blue to red, photoluminescence (PL) intensity) under photoirradiation on the timescale of minutes. As proof-of concept, we demonstrate that the emission properties of the SP-CDs can be used to construct UV and pH sensing materials as well as light emitting diode (LED) with different colors.