An OFF-ON red-emitting phosphorescent thiol probe is designed by using the (3)MLCT photophysics of Ru(II) complexes, i.e., with Ru(II) as the electron donor. The probe is non-luminescent because the MLCT is corrupted by electron transfer from Ru(II) to an intramolecular electron sink (2,4-dinitrobenzenesulfonyl). Thiols cleave the electron sink, and the MLCT is re-established. Phosphorescence at 598 nm was enhanced by 90-fold, with a 143 nm (5256 cm(-1)) Stokes shift and a 1.1 mus luminescent lifetime.

Two highly selective OFF-ON green emitting fluorescent thiol probes (1 and 2) with intense absorption in the visible spectrum (molar extinction coefficient ε is up to 73 800 M(-1) cm(-1) at 509 nm) based on dyads of BODIPY (as electron donor of the photo-induced electron transfer, i.e.PET) and 2,4-dinitrobenzenesulfonyl (DNBS) (as electron acceptor of the PET process) were devised. The single crystal structures of the two probes were determined. The distance between the electron donor (BODIPY fluorophore) and the electron acceptor (DNBS) of probe 2 is larger than that of probe 1, as a result the contrast ratio (or the PET efficiency) of probe 2 is smaller than that of probe 1. However, fluorescence OFF-ON switching effects were observed for both probe 1 and probe 2 in the presence of cysteine (the emission enhancement is 300-fold for probe 1 and 54-fold for probe 2). The fluorescence OFF-ON sensing mechanism is rationalized by DFT/TDDFT calculations. We demonstrated with DFT calculations that DNBS is ca. 0.76 eV more potent to accept electrons than the maleimide moiety. The probes were used for fluorescent imaging of cellular thiols.

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 (l abs ¼ 498 nm, l em ¼ 511 nm) is a parent BODIPY moiety and the energy acceptor is based on 4-hydroxylstyryl BODIPY moiety (l abs ¼ 568 nm, l em ¼ 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 S 1 (oscillator strength f ¼ 0.0007 for S 0 / S 1 , based on the optimized S 1 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 S 1 state of the fluorophore (f ¼ 1.4317 for S 0 / S 1 ), 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 S 4 excited state (localized on the energy donor unit), the S 1 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).

Heteroleptic C^N cyclometalated iridium(iii) complexes showing strong NIR absorption were prepared.