Switching Of Triplet-triplet Annihilation Upconversion

Triplet-triplet annihilation(TTA)assisted upconversion is an important photon-upconversion method.It has already been applied in various research fields,e.g.,luminescence bioimaging,photocatalysis and photovoltaics.Many investigations into TTA upconversion have focused on how to synthesis high efficiency photosensitizers,obtain oxygen non-sensitivity upconversion system,prepare solid state upconversion and develop new application.However,activatable,or switchable TTA upconversion was rarely studied.In order to build on the complexity and functionality of TTA upconversion,a highly desirable goal is the development of systems which are able to respond to external stimuli.The present thesis aims to develop some activatable upconversion system.The mechanism of the activatable upconversion was studied with steady state UV-vis absorption,fluorescence emission spectroscopy,nanosecond/femtosecond transient absorption spectroscopy,electrochemistry analysis and DFT/TDDFT calculations.Firstly,two photoswitchable fluorescent triads based on two 9,10-diphenylanthracene(DPA)/bodipy and one dithienylethylene(DTE)moiety are prepared for photoswitching of TTA upconversion.The DPA/Bodipy is the triplet acceptor,while the DTE is a photochromism unit.Upon the irradiation of UV/Visible light,the singlet/triplet exited state of the acceptors can be switched on/off,as a result the corresponding upconversion will be turn-on/off.Three different mechanisms are responsible for the fluorescence photoswitching effect,i.e.the photoactivated fluorescence resonance energy transfer(FRET),the intramolecular electron transfer,as well as the photoactivated intermolecular triplet energy transfer(TTET)between the photosensitizer and DTE-(c)moiety.The upconversion based on Bodipy as emitter shows green/near IR emission changes with a large anti-Stokes shift of 0.76 eV.Whereas the upconversion based on DPA as emitter shows yellow(?)blue emission changes.Both the two photoswitchable upconversion systems exhibit a favorable reversibility.Secondly,two activatable upconversion systems based on the respond to the endogenous bioactive molecules were devised.The nitric oxide(NO)and thiols were chosen as activatable units.In the NO-activatable TTA upconversion system,a substituted Ru(?)complex as a phosphorescent probe(Ru-1)which is activated in the presence of NO/O2,and DPA as a triplet acceptor/emitter.In the presence of NO/O2,the triplet state of Ru-1 can be switched on by the termination of a photo-induced electron transfer(PET)reaction involving the 1,2-diaminobenzene moiety as an electron donor,which quenches the emission.As a result,the apparent triplet state quantum yield is dramatically increased from 22.2%to 85.5%.Thus,TTA upconversion is switched on in the presence of NO/O2.In the thiols-activatable upconversion system,the photosensitizer(IBDP)and acceptor(Perylene)were linked together by a disulfide bond(dyad BP-1),which can be specifically cleaved by thiols.In the absence of thiols,the upconversion emission was quenched by the singlet energy transfer from perylene to IBDP.However,in the presence of thiols,the FRET can be ceased by the cleavage of disulfide bond,as a result the upconversion emission was switched on.Lastly,a series of triplet acceptors(aminomethyleneanthracene and Bodipy-Tempo derivatives)which singlet excited states can be switched on/off based on the mechanism of photo-induced electron transfer(PET)and intersystem crossing(ISC)were designed and synthesized.The structural modifications of anthracene tune the ISC,the fluorescence as well as the distance between the electron donor(amino group)and the fluorophore by using methylene(A-1 and A-2)or a benzyl moiety(A-3)as a linker.The TTA upconversion can be switched on by inhibition of the PET through acetylation and protonation.The singlet excited states of Bodipy dimer attached a free radical Tempo was tuned by the mechanism of radical enhanced intersystem crossing(EISC)and conformation changes.The formation of singlet excimer,as well as the triplet excimer was observed due to the existence of a strong intramolecular ground state interaction between the bodipy chromophores in the triad.Upon the addition of protonic acid or variation of temperature,the singlet state of bodipy was tunable.The TTA upconversion quantum yield was determined as 4.7%with the bodipy dimer as triplet photosensitizer.