Triplet Photosensitizers Based On Spiro Rhodamine:Synthesis And Study Of Photophysical Properties

The heavy-atom free triplet photosensitizers have the advantages of low synthesis cost,low toxicity and long-lived triplet state,which are the future trend of triplet photosensitizers.The commonly used heavy-atom free triplet photosensitizers,such as based on spin converter of C₆₀ to induce intersystem crossing are usually synthetically demanding.The electron donor/acceptor systems without heavy atoms which can induce spin-orbital charge transfer intersystem crossing（SOCT-ISC）are easy to synthesize,and the structure of the dyads are simple.Therefore,the construction and research of the heavy-atom free triplet photosensitizers which possess SOCT-ISC have more prospects.The charge transfer（CT）states of the dyads involved in SOCT-ISC process show strong solvent polarity dependence.Theses dyads provide the possibility to attain long-lived locally excited triplet state（³LE）or charge transfer triplet state（³CT）.This mechanism has never been used to obtain ³CT state with long-lived and high energy level.In this thesis,SOCT-ISC has been realized by constructing novel electron donor/acceptor compact dyads based on spiro rhodamine chromophore,and photophysical properties of molecules were studied by steady/transient spectroscopy.First of all,conceptually novel,fully rigid,spiro compact electron donor（Rhodamine B,lactam form,RB）/acceptor（naphthalimide;NI）orthogonal dyad（RB-NI）has been prepared.Femtosecond and nanosecond transient absorption spectra indicate that the first charge separation（CS）（¹NI*→¹CT）takes place within 2.5 ps,subsequent SOCT-ISC（¹CT→³NI*）within 8 ns,and ultimately,the ³NI*state leads to the slow formation（125 ns）of a long-lived³CT state（0.94?s in fluid solution at room temperature）with high energy of ca.2.12 eV.The proposed cascade photophysical processes of the dyad RB-NI are ¹NI*→¹CT→³NI*→³CT.With time-resolved electron paramagnetic resonance（TREPR）,an EEEAAA electron-spin polarization pattern was detected,which excludes radical-pair intersystem crossing（RP ISC）.Our approach is different as compared to previous electron spin-control methods based on transition-metal coordination or chromophores exhibiting intrinsic ISC ability.Second,two spiro rhodamine（RB）-coumarin（Cou）compact dyads（RB-Cou-CF₃ and RB-Cou-CN）have been prepared,with the aim to induce SOCT-ISC.Twisted intramolecular charge transfer（TICT）emission was detected in these two dyads when measured temperature-dependent luminescence spectra.The potential energy curves of the dyads indicate that the flexible molecular geometry can lead to insufficient SOCT-ISC and low singlet oxygen quantum yields.Based on the analysis of DFT calculations,electrochemical study and transient absorption data,we ultimately access ³Cou*state only in n-hexane via SOCT-ISC mechanism in the dyads.Thus,the proposed photophysical path for RB-Cou compact dyads upon photoexcitation is as follow:¹Cou*→¹CT→³Cou*,the triplet lifetimes are 15.6?s（RB-Cou-CF₃）and 11.0?s（RB-Cou-CN）.This work indicates that PECs can play an important part in molecule design since it can give a great insight about the flexibility of the molecular geometry.To the best of our knowledge,this thesis has first reported the case to obtain long-lived³CT state with high energy via the electron-spin control method based on SOCT-ISC.These results are significant for artificial photosynthesis,photovoltaics,photocatalysis as well as fundamental photochemistry studies.