Ultrafast Spectroscopy Study On The Excited State Dynamics Of Photosensitizer Under The Influence Of Solvent Environment

Photosensitizers are widely used in photocatalysis,photodynamic therapy,molecular probes,triplet-triplet up-conversion,photoelectric conversion,etc.Among them,the triplet state of photosensitizer has important research value because it is an important intermediate carrier for energy transfer and electron transfer in these applications.On the other hand,the application of photosensitizers mostly occurs in the solvent environment.It is of great significance to explore the influence of the solvent environment such as acidity,alkalinity,polarity,viscosity,etc.on the triplet state of the photosensitizer.Significant differences in the microenvironment between tumor tissue and normal tissue are becoming a new breakthrough for targeted cancer treatment.The development and utilization of tumor acidic microenvironment in photodynamic therapy has become a research hotspot.In order to research the influence of microenvironment-selective to photosensitizer,two pH-responsive photosensitizers named CS-2 and CS-3 have been designed to utilize tumor acidic microenvironment,which were introduced tertiary amine substituent and morpholine ring group to enhance the cellular uptake,lysosomal localization ability,prolong the wavelength of the absorption,and increase the phototoxicity.The differences between the spectra of two photosensitizer molecules and their ability to generate reactive oxygen species were explored,and the ultrafast spectroscopy has been used to study the dynamic of the effect of two different substituent groups and environmental pH on the performance of reactive oxygen species.Furthermore,the therapeutic effect of photosensitizer molecules on cancer cells was verified.The conclusions are of significance for the design of photosensitizers and photodynamic therapy of cancer.Polarity is an important physical and chemical property of solvent systems.Hence it is of a great significance to study the dynamics mechanism of excited photosensitizers influenced by the solvent polarity.In this paper,we have synthesized a push-pull molecule named BDP,which contains BDPIPY and carbazole groups.Measurements such as steady-state spectra,fluorescence lifetime and femtosecond/nanosecond transient absorption have been performed on BDP in different polar solvents in order to research spectroscopic characters of its excited states.The red-shifted fluorescence spectra with increasing solvent polarity show that it is possible to form an intramolecular charge transfer（ICT）state in excited BDP.Furthermore,fluorescence lifetimes measured by means of time-correlated single photon counting（TCSPC）also prove the existence of ICT state since the fluorescence lifetime of BDP in high-polar solvent is shorter than that in low-polar solvent.Femtosecond transient absorption experiments reveals that a solvation-stabilized relaxed ICT state（ICT’）is formed only in high-polar solvent because of its solvation effect.In addition,nanosecond transient absorption measurements show different lifetimes of the first excited triplet state（T₁ state）for BDP in high/low-polar solvent under air or N2 condition proves that the energy of T₁ state in high-polar solvent becomes lower accordingly,which will cause a lower singlet oxygen yield because of the disadvantage in energy transfer.The following DPBF experiment confirms the result above,BDP in low-polar solvent can generate more singlet oxygen than in high-polar solvent.This study provides an effective method for the design of molecular structures and the investigation of polarity effect of photosensitizers.