Study Of Triplet Photosensitizers With Intersystem Crossing Enhanced By Restricted Configuration

Triplet photosensitizers?PSs?have attracted much attention due to wide applications,such as photocatalysis and triplet-triplet annihilation upconversion?TTA UC?.PSs with heavy atoms are expensive and difficult to synthesize and preserve.Moreover,dark toxicity of the heavy atoms is also a drawback in areas,such as PDT.Therefore,it is important to develop new heavy atom-free triplet PSs.The electron donor/acceptor dyads have been reported to produce triplet states,with Spin-Orbital Charge Transfer Intersystem Crossing?SOCT-ISC?mechanism.It is relatively easy to synthesize such molecules.Given the electron donor and acceptor adopt orthogonal geometry,charge recombination-insulted ISC is enhanced.Meanwhile,it can be affected by external environment,such as the solvent polarity.Therefore,SOCT-ISC mechanism has become a promising method to design new PSs.By introducing conjugated aromatic groups to the organic chromophores,the twisted non-planar configuration is adopted by the molecules due to increased steric hindrance,with that ISC is enhanced by Twisted-Induced Spin-Orbital Coupling mechanism.Therefore,it is promising to design new heavy atom-free PSs with this designing strategy.Based on the above two different design strategies,two PSs with special steric configuration are designed and prepared.The photophysical properties,especially triplet state properties,have been detailed studied in detail by steady state/transient spectroscopy,electrochemistry and theoretical calculations.1.Bodipy-phenylethynyl anthracene dyad.The orthogonal configuration of the compound BDP-An is achieved by attaching phenylethynyl group to the anthracene moiety of Bodipy-anthracene dyad.In polar solvents,the singlet oxygen quantum yield??37??35??was determined as 95%.SOCT-ISC mechanism was confirmed by femtosecond transient absorption?fs TA?spectra,and the CS time constant was 1.6 ps,the time constant of charge recombination?CR?,i.e.ISC,was determined as 3.8 ns.In nanosecond transient absorption?ns TA?spectra,long-lived triplet states??_T=108?s?were observed,which are delocalized on both parts of the dyad,i.e.there is a triplet excited-state equilibrium.To our best knowledge,this is the first report on the triplet excited-state equilibrium observed in an electron donor/acceptor dyad showing SOCT-ISC mechanism.With BDP-An as the triplet donor and perylene as the triplet acceptor,TTA UC was achieved,and the upconversion quantum yield was up to 18.9%.Meanwhile,the lifetime of TTA-based delayed fluorescence was determine as 70.8?s.2.Aza[7]helicene.Helicene molecules have twisted configurations due to the intramolecular strain.By introducing heteroatoms,the molecular structures of helicenes can be significantly changed,and the photophysical properties are adjusted.Based on carbazole,an aza[7]helicene compound HEL was synthesized.The fluorescence quantum yield was 18%,which indicates weak radiative transition.?37??35?was determined as 35%,meanwhile a long-lived??_T=40?s?triplet state was observed with ns TA spectra,which confirm the ISC mechanism.Through fs TA spectra,the ISC time constant was determined as 1.1 ns.The twisted configuration was confirmed by theoretical calculation,and the excited state properties were determined.The energy levels of S₁ state and T₄ state are close,thus it is reasonable that T₄state is firstly generated by ISC,and T₁ state is produced by internal conversion.