Dynamics Of Ultrafast Intersystem Crossing In Organic Molecules

Triplet excited state in molecules is critical in many photophysical and photochemical processes.However,the rate of triplet excited state generation is usually2–3 orders of magnitude smaller than the deactivation of the initially populated singlet excited state,limiting triplet generation quantum yield in molecules.Thus,increasing the rate of intersystem crossing(ISC)while inhibiting other excited state relaxation pathways will be an effective method to enhance triplet generation quantum yield.In this dissertation,we studied ultrafast excited state dynamics of a series of well-designed compact organic molecules using time-resolved spectroscopy techniques combined with quantum chemical calculations.This study reveals the key role of the charge transfer state in ultrafast ISC and the structure-property relationships of ultrafast ISC in compact organic molecules.The results could provide guidance for further development of triplet photosensitizers.The dissertation contains the following four contents:(1)The excited state relaxation mechanisms of acridone and its derivatives(10-methylacridone and 3,6-bis(diethylamino)-10,10-diphenylanthrone)were studied.It is shown that there are always ultrafast ISC(6)6)_ISC?10¹⁰ s^-1)process and a singlet-triplet equilibrium state in acridone in all solvents studied.The equilibrium state could emit fluorescence or further internal conversion to the lowest triplet ~3??*state depending on the different solvent environment.10-methylacridone and 3,6-bis(diethylamino)-10,10-diphenylanthrone were designed by using the ketone carbonyl in acridone as electron acceptor and introducing methyl and diethylamino as electron donors,respectively.The ISC rate can vary by orders of magnitude in this molecule through changing the solvent polarity.(2)The excited state relaxation mechanism of spiro-conjugated molecules SF1-3,which were designed on the basis of the 3,6-bis(diethylamino)-10,10-diphenylanthrone,were studied.The electron donor and accepter are connected by a spiro carbon atom to ensure the two parts are locked orthogonally to each other,fulfilling the requirement of spin-orbit charge transfer ISC(SOCT-ISC).Energy level of the charge transfer state was regulated by varying the side groups and solvent polarity,resulting the ISC rate could vary by three orders of magnitude.The highest value of SOCT-ISC(up to?10¹¹s^-1)is registered in SF1-3.The ISC rates obtained in experiments were in line with the prediction of Fermi's golden rule,which proves that the charge transfer state plays a significant role in the ultrafast ISC process.(3)On the basis of SF1-3,the enhanced charge transfer character was achieved in spiro-conjugated molecules SCT-R,whose absorption are located at the visible region.The energy level of the charge transfer state and triplet state can be degenerated through side group engineering and the triplet excited state generation is achieved with assistance from multiple factors:n?*???*transition,SOCT-ISC and spin-vibronic coupling.The unique molecular structure also effectively suppresses other excited state relaxation pathways and near-unity triplet generation quantum yield is achieved.Potential applications of SCT-R as triplet photosensitizers and photodynamic therapy medicine were also demonstrated.(4)Finally,the excited state dynamics of Esi5b and Esi5d,which absorb in the near-infrared region,and their reference samples REF1 and REF2 were investigated.The excited state relaxation pathways of Esi5b and Esi5d are mainly internal conversion and fluorescence emission.With the help of REF1 and REF2,the critical role of molecular rigidity in the development of near-infrared triplet photosensitizers is revealed.In summary,this dissertation investigated the ultrafast ISC dynamics of a series of well-designed compact organic molecules.The results revealed the critical role of charge transfer state in ultrafast ISC and the relationship between molecular structure and its photophysical property,providing guidance for further development of novel triplet photosensitizers.