Design And Research Of Thionated Naphthalimide-based Molecules For Efficient Two-photon Imaging And Photodynamic Therapy

As a typical two-photon fluorescent dye,naphthalimide-based molecules have advantages such as large stokes shift and good photostability,which are widely used in fields of photodynamic therapy and biological visualization.Compared with other two-photon fluorescent dyes,the synthesis pathway of naphthalimide-based molecules is simple and the structure is easy to modify.At present,efficient thionated naphthalimide-based photosensitizers designed for two-photon photodynamic therapy(TP-PDT)process have attracted widespread attention from researchers due to their easily adjustable optical properties,low dark toxicity and cost.TP-PDT,as a treatment technology with less damage and deep tissue penetration,has been an effective method for treating cancer and skin diseases in recent years.However,the development of these photosensitizers is still limited by their small two-photon absorption(TPA)cross section value,insufficient triplet state lifetime,as well as the complicated relationship between molecular structure and intersystem crossing efficiency.On the other hand,thionated naphthalimide-based molecules can also be used for fluorescence detection of hypochlorite.Hypochlorite plays a crucial role in the natural defense system of organisms,and its excessive formation can cause various diseases.Therefore,monitoring and imaging the position of hypochlorite in living cells is crucial.However,the reported two-photon fluorescence probes for detecting hypochlorite require improvement in both the TPA intensity and the fluorescence intensity.Based on the above,this paper investigates in detail the effects of the structural changes on the two-photon fluorescence properties and TP-PDT related photophysical properties of the molecules by introducing electron donor substituents and π-connecting groups(forπ-extension of the molecule).The microscopic mechanisms of the studied molecules through TP-PDT process and fluorescence imaging process are fully simulated and analyzed.Finally,this paper elucidates the photosensitization mechanism of thionated naphthalimide molecules(the relationship between type Ⅰ/Ⅱ mechanism and the structure of photosensitizers),improves the performance of such photosensitizers/fluorescent probes(TPA strength,triplet state lifetime,PDT penetration depth,photosensitization/fluorescence imaging efficiency,etc.),and provides strong theoretical guidance for the design and synthesis of naphthalimide-based molecules for detecting hypochlorite and for TP-PDT.The research content of this work includes the following two parts:1.The effect of the 4-position electron donor substitution of naphthalimide derivatives on their photophysical properties.In different systems,the structure-activity relationship between molecular structure,TPA intensity,and triplet state lifetime,as well as the photodynamic processes experienced by molecular excited states,are not yet clear.In this part of the work,based on the experimentally thionated naphthalimide derivative,we proposed some novel modification strategies for optimizing molecular optical properties,thereby obtaining efficient fluorescent probes for detecting hypochlorite and excellent photosensitizers for TP-PDT.We simulated the TPA process,radiative decay and non-radiative decay processes of the designed molecules by using density functional theory(DFT)and time-dependent density functional theory(TD-DFT).Our results show that the introduction of different electron-donating groups at the position 4 of naphthalimide can effectively improve their TPA and emission properties.Specifically,3s with a N,N-dimethylamino group has large triplet state lifetime(τ=699 μs)and TPA cross section value(δTPA=314 GM),which can effectively achieve TP-PDT;4s(with electron-donating group 2-oxa-6-azaspiro[3.3]heptane in naphthalimide)effectively realizes the dual-function of TP-PDT(τ=25122 μs,δTPA=351 GM)and detection of ClO-(Φf=29%of the product 4o).Moreover,we elucidated from a microscopic perspective the reasons why the transition property of 3s and 4s(1π-π*)from S1 to S0 is different from that of 1s and 2s(1n-π*).Based on the above,this part of work improved the TPA cross section and triplet state lifetime of thionated naphthalimide photosensitizers by introducing 4-position electron donating groups,and designed 4s that can effectively achieve ClO-detection and TP-PDT dual-mode process,providing valuable theoretical clues for the design and synthesis of naphthalimide-based photosensitizers and fluorescent probes for detecting ClO-.2.The influence of the π-bridge of naphthalimide derivatives on their photophysical properties.At present,π extension is an effective means to regulate the TPA properties and the energy gap of the singlet-triplet state,which is widely used to optimize the molecular structure of photosensitizers.Here,we designed two sets of naphthalimide-based molecules with different π connections(ls-ph-4s-ph connected by phenyl and ls-thio-4s-thio connected by thiophene)and characterized their photophysical properties to explore the effects of different π-linkers(phenyl and thiophene)on the photophysical properties of naphthalimide-based photosensitizers.Our results reveal that using thiophene as a π-linker can greatly improve the optical properties of the molecule,such as increasing the planarity,δTPA value,intersystem crossing value,andτ,while reducing the energy gap of the singlet-triplet state.Among them,molecules 3s-ph,2s-thio～4s-thio have large δTPA value(1028～1384 GM,8.3～11.2 times of molecule 1s)and τ(1337～11609 μs,1.6～14.2 times of molecule 1s),making them effective for TP-PDT process;molecule ls-thio acts as a promising photosensitizer candidate that can effectively undergo type I process and circumvent type II process,thus avoiding aggravating hypoxic environment and having stronger phototoxicity.Moreover,we revealed the reason why ls-thio only undergoes type I mechanism by analyzing the overlap integral(SH-L)of HOMO and LUMO of the studied molecules.Based on the above,this part of the work further improves the TPA cross section and triplet state lifetime of naphthalimide photosensitizers by introducing π linkers,and optimizes their photosensitization mechanism(transform photosensitizers from that can undergo type Ⅰ/Ⅱ mechanism to that only undergo type Ⅰ mechanism),providing new ideas for the design and synthesis of effective naphthalimide-based photosensitizers.