In this perspective we introduce the basic photophysics of the excited-state intramolecular proton transfer (ESIPT) chromophores, then the state-of-the-art development of the ESIPT chromophores and their applications in chemosensors, biological imaging and white-light emitting materials are summarized. Most of the applications of the ESIPT chromophores are based on the photophysics properties, such as design of fluorescent chemosensors by perturbation of the ESIPT process upon interaction with the analytes, their use as biological fluorescent tags to study DNA–protein interaction by probing the variation of the hydration, or design of white-light emitting materials by employing the large Stokes shift of the ESIPT chromophores (to inhibit the Föster energy transfer of the components). The photophysical mechanism of these applications is discussed. Furthermore, a new research topic concerning the ESIPT chromophores is proposed based on our group's results, that is, to develop organic triplet sensitizers with ESIPT chromophores.

Triplet–triplet annihilation (TTA) is a promising upconversion approach due to its low excitation power density (solar light is sufficient), high upconversion quantum yield, readily tunable excitation/emission wavelength and strong absorption of excitation light. This review focuses on the reported TTA based upconversion examples, the challenges that are facing the developments of TTA upconversion and the design rationales for the triplet sensitizers and triplet acceptors.

A red-emitting BODIPY-based fluorescent-resonance-energy-transfer (FRET) molecular probe 1 for selective detection of cysteine and homocysteine was designed. The fluorescence OFF–ON switch is triggered by cleavage of the 2,4-dinitrobenzensulfonyl (DNBS) unit from the fluorophore by thiols. The FRET energy donor (λ_abs = 498 nm, λ_em = 511 nm) is a parent BODIPY moiety and the energy acceptor is based on 4-hydroxylstyryl BODIPY moiety (λ_abs = 568 nm, λ_em = 586 nm). The unique C–C linker between the energy donor and acceptor was established using a Suzuki cross-coupling reaction. A polyether chain was also introduced into the probe to improve solubility in aqueous solution. While probe 1 itself is non-fluorescent, in the presence of cysteine or homocysteine a red emission at 590 nm is switched on (excitation at 505 nm), producing a pseudo-Stokes shift of up to 77 nm, which is in stark contrast to the small Stokes shift (ca. 10 nm) observed for typical BODIPY dyes. Excitation of the energy donor leads to the red emission from the acceptor of the probe, and demonstrates a high energy transfer efficiency. The probe was used for in vivo fluorescent imaging of cellular thiols. The fluorescence sensing mechanism of the probe and the photophysical properties of the fluorescent intermediates were fully rationalized by DFT calculations. The lack of fluorescence of probe 1 is attributed to the dark excited state S₁ (oscillator strength f = 0.0007 for S₀ → S₁, based on the optimized S₁ state geometry), which is due to the electron sink effect of the DNBS moiety. Cleavage of the DNBS moiety from the fluorophore by thiols re-establishes the emissive S₁ state of the fluorophore (f = 1.4317 for S₀ → S₁), thus the red emission can be observed in the presence of thiols (fluorescence is turned on). The FRET effect of the probe was rationalized by DFT calculations which indicated that upon excitation into the S₄ excited state (localized on the energy donor unit), the S₁ state (localized on the energy acceptor, i.e. styryl-BODIPY) is populated via internal conversion (IC), thus red emission from the styryl-BODIPY energy acceptor is observed (Kasha's rule).