Study On The Luminescence Behaviors Upon Fe(?) Coordination

Iron is one of the most abundant essential transition metal in human body,and the transition between Fe3+ and Fe2+ plays vita roles in oxygen transportation,enzyme catalising reactions and transmission of electrons.In addition,the accessive iron may enhance the level of reactive oxygen species,which can lead to organ dysfunction and cell damage.Iron disorder has been associated with many diseases such as hepatitis,cancer and neurodegenerative diseases.Therefore exploring the transient of different oxidation state of iron should be helpful to understand the homeostasis of iron and the related functions.With the high sensitivity/selectivity,fluorescent probes have been frequently adopted as an invasive imaging technique to offer the temporal-spatial information of specific species.Therefore,the development of fluorescent probes for Fe3+ or Fe2+ to realize in situ tracking is of great significance.Most reported iron probes can be assigned into two families:the chelating probes and the chemodosimeters based on Fe-induced reaction.Chemodosimeters for Fe3+/Fe2+normally display the irreversible response and not suitable for the in situ monitoring of iron fluctuation.Since the chelating probes for Fe2+ possess the advantages of quick and reversible response,they are more attractive and appealing for the real time monitoring of Fe2+ fluctdation,yet the design of chelating probes is still challenging due to emission quenching effect of the paramagnetic nature of Fe2+.In the first part of this study,a NIR fluorescent probe for Fe2+,BTPY,was constructed via integrating a BODIPY fluorophore with terpyridine,a Fe2+-preferential chelator,at ?-position with a vinyl group.This probe displays an emission maximum at 678 nm upon exication at the maximum at 582 nm.The NIR emission and the visible exciation suggests that this probe is suitable for in vivo imaging in animal model.Spectroscopic study discosed that BTPY is able to bind Fe2+/Fe3+ selectively and trigger an bathochromic absorption shift from 573 to 607 nm.This indicates the colorimetric Fe3+/Fe2+sensing behavior of this probe,showing as the solution color change from red to pale blue?In addtion,this probe shows the selective fluorescence quenching effect on Fe2+ and Cu2+.However,the interference from Cu2+ and paramagnetic natur-induced quenching effect of Fe2+ make this probe not suitable for practical imaging.It is clear that understaning the luminescence behavior upon Fe2+ coordination and clarifying the paramagnetism effect of Fe2+ on the luminescence are essential for thedesign of fluorescent Fe2+ probe of the turn-on reposne.Altering the spin state of Fe2+to explore the related luminescence behaviors is a suitable means to realize this purpose.Therefore,a simple spin crossover(SCO)complex,[Fe(bpp)2][BF4]2(bpp=2,6-bis(pyrazol-l-yl)pyridine)and its analogue[Fe(Coubpp)2][BF4]2 formed by modifying the bpp ligand with a coumarin fluorophore were adopted to explore the relationship between spin crossover and luminescence.The results indicate that complex[Fe(Coubpp)2][BF4]2 shows no spin crossover phenomenon,and the percentage of high spin state complex at very low temperature(50 K)is still higher than 60%.Therefore,this complex is not suitable for the current study,and the thermal emission quenching effect can not be correlated to spin state of Fe2+.As to complex[Fe(bpp)2][BF4]2,it was found to luminesce with a broad band from 425 to 565 nm in solid state at ambient temperature although it is non-luminescent at temperature<250 K,and the abrupt luminescence enhancement was found at-260 K,which is exactly the temperature for spin transition from the low spin(LS)state to high spin(HS)state of the complex.This implies the luminescence is exactly triggered by the thermal spin transition from LS to HS state,which is different from the emission quenching effect of normal paramagnetic metal center.Theoretic study via DFT calculation suggested the excitation efficiency of the complex from the ground state to the lowest excited state for HS state is higher than that for LS state,which is proposed as the origin for the HS-triggered luminescence.The hole/electron excitation analysis and orbital component analysis all demonstrated that metal to ligand charge transfer(MLCT)upon excitation is responsible for the luminescence of this complex.This is the first report about the MLCT luminescence of SCO Fe(?)complex tuned by the thermal spin transition.All the study showed that regulating the spin state of Fe2+ is essential for the design of luminescent Fe2+ probes of turn-on response.