Study On The Photophysical Properties Of The Heavy Atom-Free BODIPY With Twisted Molecular Structure

Intersystem crossing（ISC）is one of fundamental research area of photochemistry,and it is also critical for triplet photosensitizers,as well as their applications in photodynamic therapy（PDT）,photocatalysis,photon upconversion,etc.Due to the advantages of low cost,low toxicity and long triplet lifetime,the heavy atom-free triplet photosensitizers（PSs）become alternatives of the conventional PSs which relies on the heavy atom effect.However,it’s still a challenge to design efficient heavy atom-free triplet PSs,because the ISC is difficult to predict for organic chromophores contains no heavy atoms.Accordingly,this thesis mainly focuses on the heavy atom-free triplet PSs of fluoropyrrole（BODIPY）based on the mechanism of ISC induced by distortedπ-conjugated structure.Steady state/transient spectroscopy and theoretical calculations were used to investigate the molecular structure-ISC relationship.First,we report a heavy atom-free,twisted BODIPY derivative（helical-BDP）,with efficient ISC and long triplet excited lifetime.Theoretical computation and time-resolved EPR spectroscopy show that the ISC process of helical-BDP was mainly caused by the reduced symmetry of BODIPY molecules with twisted structure.Due to the intense long wavelength absorption(ε=1.76×10⁵M^-1cm^-1 at 630 nm),efficient ISC（triplet quantum yield,Φ_T=52%）,and long triplet state lifetime（τ_T=492μs）of helical-BDP,nanoparticles constructed with this compound having twisted molecular structure achieved outstanding PDT-mediated antitumor immunity amplification at very low dose of reagent concentration（0.25μg/kg）and low light dose（6 J/cm²）.It is worth mentioning that due to its excellent photophysical properties,the dosage of helical-BDP nanoparticles is two orders of magnitude lower than the minimum dosage（0.1 mg/kg）of clinical PDT reagents such as ALA,Photofrin,m THPC,etc.helical-BDP can also be used in triplet annihilation upconversion system of neat organic compounds,and transform red light into bright yellow upconversion luminescence.Second,in order to explore whether ISC efficiency is proportional to the degree of molecular geometry distortion,two BODIPY derivatives with different distortion angles were synthesized（BDP-B and BDP-P）.The study shows that BDP-B has a planarπ-conjugation framework,which S₁ state is a dark state.Therefore,its steady-state spectra are different from classical BODIPY compounds,displaying weak absorption(ε=3.8×10⁴ M^-1cm^-1at 570 nm)and extremely weak fluorescence（fluorescence quantum yield,Φ_F<0.1%）,short-lived singlet-excited state（fluorescence lifetime,150 ps）.On the contrary,BDP-B has much better triplet properties,with higher ISC efficiency（Φ_T=23%）and longer triplet lifetime（τ_T=132μs）.In comparison,the more twisted BDP-P shows almost opposite photophysical properties:stronger absorption(ε=9.8×10⁴ M^-1cm^-1 at 640 nm),stronger fluorescence（Φ_F=70%）and longer S₁state lifetime（6.4 ns）,but negligible ISC（Φ_Δ=5.5%）and shorter triplet state lifetime（τ_T=18.9μs）.Herein we demonstrated it doesn’t work for twisted BODIPY compounds,i.e.twistingπ-conjugation framework doesn’t always induce efficient ISC.Time-resolved paramagnetic resonance spectra shows that the triplet state wave function of BDP-P is more confined,although it is with largerπ-conjugation framework.It’s the reason why BDP-P shows red-shifted absorption but high T₁ state energy（1.44-1.61 e V）as compared to the native BODIPY chromophore.For BDP-B,the triplet wave function is delocalized to the entire molecular framework.