Controlling Of The Singlet/Triplet Excited States Of Aromatic Compounds And Its Application In Switchable Triplet-Triplet Annihilation Upconversion

Developing organic compounds with desired and tunable photophysical properties by regulation of their excited states has always been at the forefront of photochemistry.Recently,photon upconversion via triplet-triplet annihilation(TTA)has experienced a blowout type growth,due to its potential applications in the field of photocatalysis,photodynamic therapy,bioimaging and photovoltaics.Many breakthroughs in this field mainly focus on achieving high upconversion quantum yield and long excitation wavelength,by modifying the structures of triplet photosensitizers.Much attention is also paid to perform this upconversion in viscous matrix such as solid material and gel to make it practicable for applications.On the other hand,few studies has been reported to develop external stimuli-activatable TTA upconversion system to bring more functionality in this field.The present work highlighted the uniqueness and utility of strategies for controlling excited state properties into the TTA upconevrsion system to achieve desired goal.(1)Switchable TTA upconversion based on regulation of the singlet excited state.An efficient thiol-activated TTA upconversion system was devised with maleimide-caged perylene(Py-M)as the thiol-activatable triplet acceptor/emitter and with diiodobodipy as the triplet photosensitizer.The method is based on switching the singlet state(fluorescence)of the triplet acceptor in TTA upconversion.The triplet acceptor/emitter Py-M alone shows week fluorescence(ΦF = 0.8%)and no upconversion was observed.The quenching of fluorescence of Py-M is due to photoinduced electron transfer(PET)process from perylene to maleimide-caged part,which quenches the singlet-excited state of perylene.The fluorescence of Py-M was enhanced by 200-fold(ΦF = 97%)upon addition of thiols such as 2-mercaptoethanol and the upconversion quantum yield ΦUC was increased to 5.9%.The unique feature of this thiol-activated TTA upconversion is that the activation is based on addition reaction of the thiols with the caged acceptor/emitter,and no side products were formed that can be detrimental for TTA upconversion.With nanosecond transient absorption spectroscopy,we found that the triplet-excited state of Py-M was not quenched by any PET process,which is different from singlet-excited state(fluorescence)of the same compound Py-M.The results are useful for study of the triplet-excited states of organic chromophores and for designing activatable TTA upconversion.(2)Switchable TTA upconversion based on regulation of the triplet excited state.Photoirradiation generated radical from hexaphenyl-biimidazole(HPBI)was used for reversible switching of TTA upconversion.This method is based on quenching of the photosensitizer triplet state by radical-triplet pair mechanism.Upon 365 nm irradiation,the TTA upconversion was completely switched off due to quenching of triplet state of photosensitizer by photogenerated radical from HPBI,and the upconversion was recovered in darkness due to regeneration of HPBI dimer by the recombination of the radicals.The photophysical process involved in the photochromism and photoswitching of TTA upconversion were studied with steady-state UV-vis absorption spectroscopy,nanosecond transient absorption spectroscopy and time-resolved EPR spectroscopy.Electron spin polarization of the radical was observed.(3)Demonstration of the different quenching effect of PET on the singlet and triplet excited state.Fluorescent molecular probes are well known for which the fluorescence is able to be efficiently tuned OFF/ON by PET process.Now we demonstrate that the triplet-excited state of the probes is unable to be quenched by PET,which is in stark contrast to the singlet-excited state(fluorescence).This conclusion was proved by study of a fluorescent molecular probe BDP-DPA,which contains bodipy fluorophore and a dipicolylamine(DPA)ligand as the binding unit for Zn2+ cations.This probe shows enhanced fluorescence upon addition of Zn(II)salt.With fluorescence spectroscopy,nanosecond transient absorption spectroscopy and electrochemical studies,we found that the triplet-excited state of BDP-DPA was not quenched by any PET process,although the singlet excited state(fluorescence)of BDP-DPA was quenched.The results are useful for study of the triplet-excited states of organic chromophores and for designing of external stimuli-activatable triplet photosensitizers for TTA upconversion and photodynamic therapy.(4)Regulation of the singlet/triplet excited states by competing Forster-resonance energy transfer(FRET)and intersystem crossing(ISC).2,6-diiodobodipy-perylenebisimide(PBI)dyad and triad were prepared to control the triplet excited state properties,with the iodobodipy moiety as the singlet/triplet energy donor and PBI moiety as the singlet/triplet energy acceptor.Iodobodipy undergoes ISC but PBI is devoid of ISC,competition of intramolecular FRET with ISC of iodobodipy is established.Production of triplet state is high for the iodinated dyad and the triad(singlet oxygen quantum yield Φ△ = 80%).Therefore,our conclusion is that the ISC of the iodobodipy unit is not outcompeted by the FRET process.With nanosecond transient absorption spectroscopy,we confirmed that the triplet state is localized on PBI moiety in the iodinated dyad and the triad.Exceptionally long-lived triplet excited state was observed(τT = 150 μs)for PBI.With uniodinated reference dyad and triad,we demonstrated that the triplet state localized on the PBI moiety is not produced by charge recombination.These informations will be useful for design and study of the fundamental photochemistry of multi-chromophore organic triplet photosensitizers.