Ultrafast Spectroscopic Study Of Photoexcited State Dynamics Of Compounds With Charge Transfer

Solvent polarity is an important factor influencing the properties of intramolecular charge transfer(ICT)molecular,and the study of the excited state dynamic mechanism of ICT molecular in different solvents is the focus of photochemical mechanism research.To conceive an effective molecular design strategies and a clearly photophysical process analyze is still a challenge for charge separation and charge recombination research.In this study,in order to study the relationship between polarity and kinetics of excited states of ICT compounds,we designed and prepared NI-PXZ-LT and NI-PTZ-LT,in which the naphthalimide(NI)unit is the electron acceptor,the phenoxazine/ phenothiazine unit is the electron donor,where the two units adopt an almost orthogonal geometry.The photo-physical properties of the dyads were studied with steady absorption spectra,fluorescence lifetime,fluorescence quantum yield,femtosecond transient absorption spectroscopies and DFT computations.From the rigorous photophysical investigation of configuration twist,we elucidate the role of CT states,and the role of the D to A orientation in the mechanism.Moreover,excited-state dynamics in a intermolecular charge-transfer system was studied by using of femtosecond transient absorption techniques and establish a series of suitable photophysical model.In methanol with higher polarity,the charge transfer,solvation process and torsion relaxation are faster,and the energy of charge transfer state is lower,which makes the molecule more prone to radiation-free transition back to the ground state,resulting in lower fluorescence quantum yield and unable to measure the triplet state lifetime.Triplet lifetime can be measured only in low polarity toluene.This study will contribute to the development of molecular design and application of functional ICT luminescent materials such as viscosity probes,temperature sensor by controlling the orthogonal configuration of molecules,obtaining dual fluorescence and precise-tuning charge transfer.Our study provides help in designing of ICT compound with improvement of photon utilization for efficient.Here a photosensitizer BDP-Cz,which was composed of diiodo-BODIPY as acceptor and carbazole as donor,was prepared with intramolecular electron push-and-pull structure.To investigate the influence of solvent polarity on the excited state properties of BDP-Cz,a series of measurements were performed including steady-state spectra,fluorescence lifetime and quantum yield,femtosecond/nanosecond transient absorption spectra,excited state lifetime,triplet state quantum yield as well as singlet oxygen generation experiment.It was shown that BDP-Cz has intramolecular charge transfer property.Faster charge transfer and solvation processes were observed in DMSO with higher polarity,which was because the lower CT state energy of BDP-Cz in DMSO made nonradiative decay more favorable and led to a lower fluorescence quantum yield.The introduction of heavy atom iodine can promote the intersystem crossing(ISC)from the CT state to the triplet state of BDP-Cz,but it also promotes the ISC process from the triplet state to the ground state,so its triplet state lifetime is relatively short.Polarity has little influence on triplet state lifetime and quantum yield.Triplet BDP-Cz can transfer energy to ground state oxygen and produce singlet oxygen which can be used in photodynamic therapy by damaging cell structure or affecting cell function.In toluene with lower polarity,BDP-Cz achieved higher fluorescence quantum yield(24.6%),satisfying triplet quantum yield(20.7%)and also stronger singlet oxygen generation ability,which makes BDPCz a promising candidate for diagnosis and targeted photodynamic therapy.Our results provide a guidance for molecular design of photosensitizer and an effective strategy for the study of polarity-regulated excited state dynamic mechanism of photosensitizer.