The Intersystem Crossing (ISC) Property Of Triplet Photosensitizers Consisted With Electron Donor And Acceptor

One charge separation/charge recombination(CS/CR)-induced ISC mechanism,known as spin-orbit charge transfer intersystem crossing(SOCT-ISC),can be a novel strategy to eliminate the dark toxicity by attaching heavy atoms,such as Br,I,Pt,Ir,Ru,etc.Moreover,it shows great potential advantages compared with other design strategies for heavy-atom-free triplet Photosensitizers(PSs).First,synthetically demanding molecular structure motif and poor solubility will be avoided compared with chromophores attached with spin converters,such as C60.Second,PSs showing strong absorption in longer wavelength and guaranteeing the intersystem crossing(ISC)ability by further derivatization can be realized compared with n?*(?)??*mechanism.Finally,both the poor ISC ability attributed to the slow rate of ISC and synthetically demanding will be avoided compared with radical-pair intersystem crossing(RP-ISC),known as another CS/CR-induced ISC mechanism.Hence,it is becoming a promising strategy to attain efficient ISC in heavy atom-free organic molecules.However,triplet PSs based on SOCT-ISC mechanism have been limited to a few chromophores showing absorption in the ultraviolet region,moreover,the ISC property was not well studied.In this thesis,a series of compact Donor/Acceptor(D/A)organic chromophores based on SOCT-ISC mechanism have been prepared.Factors such as electron coupling(the magnitude of the matrix element,VDA),molecular orientation,the center distance between the donor and acceptor,solvent polarity and so on,controlling on the properties of charge transfer(CT)state were studied.Furthermore,more factors and regularities affecting ISC efficiency were explored,which favors the design of novel heavy-atom free triplet PSs.The research results are as follows:Naphthalenediimide(NDI)chromophore as an electron acceptor,attached with different electron donors(anthryl and alkylamino moieties,An and NH),to prepare compact electron D/A dyads/triads.UV-vis absorption spectroscopies indicated VDN of different electron donors did not show the add-up(or synergetic)effects.e.g.the VDA of Br-NDI-NH(alkylamino moieties as electron donor)was 0.55 eV and the VDA of 9-An-NDI-NH(both anthryl and alkylamino moieties as electron donors)was 0.36 eV.Femtosecond time-resolved absorption spectroscopies indicated the charge separation(CS,0.83 ps)and charge recombination(CR,50 ps)process of 9-An-NDI-NH,and the SOCT-ISC mechanism.The singlet oxygen quantum yield(??,13%)was not high for 9-An-NDI-NH with an orthogonal geometry(the dihedral angle of D/A is 88°).This result indicated that orthogonal geometry was not a sufficient factor to efficient ISC and other factors,i.e.the large energy gap between the CT state and triplet state or CT sate can not match the localized triplet excited(3LE)state,thus decreasing the ISC.In order to verify the above factors on ISC efficiency,a compact D/A system connected anthracene as an electron donor with naphthalimide(NI)an electron acceptor was designed.Efficient ??(88%in DCM)was observed for 9-An-NI by singlet oxygen photosensitizing experiment.This efficient ISC was not only attributed to the orthogonal geometry(the dihedral angle of D/A is 81°),other factors,ie.small energy gap(0.24 eV)between the the CT state and triplet state and dominate CR to triplet state(5.4 ps)compared with CR to ground state(63 ps)also favored ISC.Hence,ISC efficiency related to multiple factors.A compact D/A system,PTZ-BDP-2 and PTZ-BDP-1,in which boron-dipyrromethene(Bodipy)showing strong absorption in the visible region as electron acceptor and phenothiazine(PTZ)as electron donor,was designed by directly connecting the donor and acceptor and intervening an extra phenyl ring between the donor and accopter,respectively.We found that the energy gap(0.01 eV)between the CT state and triplet state for PTZ-BDP-1 was smaller compared with that(0.21 eV)for PTZ-BDP-2.However,the Gibbs free energy change of CS for PTZ-BDP-2 was larger and PTZ-BDP-2 had a more orthogonal geometry(the dihedral angle of D/A is 8 7°),which resulted that the ??(67%)of PTZ-BDP-2 was larger compared with ??(25%)for PTZ-BDP-1.Hence,all above factors contributed to ISC efficiency.To the best of our knowledge,triplet PSs based on SOCT-ISC mechanism showing strong absorption in the visible region applicated in triplet-triplet-annihilation upconversion(TTA UC)was not previously reported and blue upconverted fluorescence is visible even to unaided eyes by a 510 nm continuous laser.The upconversion quantum yield was determined as 3.2%.As the electron donating ability of electron donor can tune the energy level of CT state in a compact D/A system,compact D/A dyads/triads constructed-with Bodipy and an electron donor(5,10-dihydro-5,10-dimethylphenazine,PZ)with strong electron donating ability,was designed.UV-vis absorption spectroscopies indicated that a broad near-IR charge transfer(CT)absorption band(centred at 670 nm and FWHM=3610 cm-1)was observed for the dyad/triad showing more coplanar geometry.This illustrated that the strong electron coupling between the electron D/A can be obtained by controlling the molecular geometry,which further induced a new red-shifted absorption band.yet the intermolecular triplet photosensitizing experiment showed that the Bodipy localized triplet state energy(3LE,1.65?1.69 eV)was not compromised.Therefore,introduction of electron donor with strong electron donating ability is a potential method to design triplet PSs showing absorption in near-infrared region.The low-lying 3CT state(1.23 eV)of the dyad/triad in polar solvent vs.the triplet excited state located on Bodipy moiety(3LE state,1.65?1.69 eV)was confirmed by intermolecular triplet photosensitizing experiment and the time-resolved density function theory(TDDFT).