Aggregation States Manipulation Of Near-Infrared Squaraine Dyes And Its Application In Phototheranostics

Phototheranostic refers to the diagnosis and treatment of diseases with the joint participation of laser and photosensitizer,in which the photosensitizer is the core element of the phototheranostic process,and the series of photophysical processes（such as radiation/non-radiation transitions,inter-system scattering,etc.）that the photosensitizer undergoes after being irradiated to laser light correspond to the expression of different therapeutic properties.Currently,the relationship between the aggregated state structure of photosensitizers and the final therapeutic effect in organic small molecule-based optical therapeutic systems is not well understood,which affects the design and development of new photosensitizers.Squaraine dyes are a class of organic functional dyes with excellent optical properties,and their aggregation states can be controlled by simple structural modifications.Therefore,the aggregation state of the squaraine photosensitizers can be regulated by the design of molecular structure,and the relationship between the aggregation state structure and the phototheranostic effect can be elucidated.Finally,we can optimize the phototheranostic effect of the squaraine dyes and apply them to related biomedical fields.The details of the study are as follows:Firstly,by introducing hydrophilic polyethylene glycol（PEG）chains into the central tetrameric ring of the classical 1,3-substituted squaraine dyes（SQ1）,the originally hydrophobic SQ1 molecules are transformed into amphiphilic 1,2,3-tri-substituted squaraine dyes（PSQ）.nanospheres（PSQ-NSs）with H-dimers as substructures.This self-assembly process can be investigated and verified by molecular dynamics simulations.Furthermore,theoretical calculations of the excited-state reorganization energy of PSQ H-dimers show that the low-frequency out-of-plane vibrations of excited-state molecules significantly accelerate the rate of nonradiative transitions due to the formation of H-dimers.This facilitates the rapid conversion of the energy of the excited state molecules into thermal energy,which leads to efficient photothermal energy conversion.Thus,PSQ-NSs exhibit ultra-high photothermal conversion efficiency（η,81.2%）under the irradiation of near-infrared（NIR）laser(808nm,0.3 W·cm^-2).Subsequent in vitro and in vivo experiments demonstrated their effective tumor suppression in photothermal therapy.It can be expected that the self-assembled H-dimer nanospheres will become a unique platform for small-molecule photothermal reagents,which is expected to be applied in future clinical photothermal therapies.Secondly,considering the excellent photothermal properties of H-dimers in the previous work,a molecular platform of H-dimers was constructed for near-infrared region II（NIR-II）photothermal therapy by a"bottom-up"strategy in this study.Specifically,a malononitrile-functionalized 1,2,3-trisubstituted squaraine dye（NSQ1）with C_2v molecular symmetry was synthesized,thus forming an H-dimer molecular platform by dipole-dipole offset using the intrinsic ground state dipole moment（μ_g）.Subsequently,triphenylethylene or diphenylamine groups are introduced at the periphery of this H-dimer molecular platform to extend theπ-conjugation of the molecule,red-shifting the maximum absorption wavelength to the NIR-II region,and accelerating the intramolecular motion of the excited state for heat generation.Therefore,the synergistic engineering of"H-aggregation"and"intramolecular motion"greatly accelerates the non-radiative transition rate of the excited state and thus improves the photothermal conversion efficiency.Based on this synthesis,NSQ2 or NSQ3 co-assembled with Pluronic F127 to form nanoparticles,which showed ultra-high photothermal conversion efficiencies of 86.3%and 80.2%in water under NIR-II laser(1064 nm,1 W·cm^-2)irradiation,respectively.In addition,the H-dimer nanoparticles of NSQ2 exhibited excellent photothermal therapeutic properties in the application for in vivo tumor ablation in mice.This bottom-up molecular structure design provides a new design idea for the construction of efficient photosensitizers.Finally,in the previous two research works,the non-radiative transition rate of NIR squaraine was maximized by regulating the formation of H-aggregates.In the present work,we explored the optimization of the radiative transition channel by modulating the squaraine aggregation state to emit fluorescence and successfully applied to in vivo fluorescence imaging in NIR-II.Subsequently,a dye-protein nanocomplex composed of NSQ1270 and bovine serum albumin（BSA）protein was constructed by co-assembly.effect induced a significant increase in fluorescence brightness relative to that of the dye monomer.This allows the nanocomplex to be used for high signal-to-noise and longer time NIR-II dynamic angiography in mice due to the excellent biocompatibility of the protein molecule and the long blood circulation time.This work provides a novel vision for the application of NIR-II luminescent fluorophores of squaraine for in vivo imaging.In summary,the series of NIR-II-absorbing squaraine dyes prepared in this thesis for phototheranostic have the following innovative features:1)nanospheres with H-aggregate dimers as substructures were constructed by amphiphilic squaraine dyes,which have ultra-high photothermal conversion efficiency and were successfully applied to photoacoustic imaging-guided photothermal therapy;2)a class of receptor-substituted squaraine dyes was synthesized,which can be constructed by dipole-dipole counteraction strategy for H-dimer.On this platform,an"intramolecular motion"strategy was introduced to increase the non-radiative transition rate of H-dimers and optimize their photothermal conversion efficiency,which was applied to phototheranostic in the NIR-II region;3)A receptor with a maximum fluorescence emission wavelength of over 1200 nm was synthesized.The strategy of co-assembling this fluorophore with BSA achieved the enhancement of fluorescence intensity and was successfully applied to dynamic angiography in mice.