Study Of Near-Infrared BODIPY Fluorescent Probes For PH

4,4-Difluoro-4-bora-3a,4a-diaza-s-indacene （here after abbreviated to BODIPY） dyes have excellentphotophysics and photochemistry properties such as high molar extinction coefficient, high stability againstlight and chemical reactions, and high fluorescence quantum yields, etc．Near-infrared （NIR） dye with thespectral fluorescent emission range between650-900nm （so-called optical window） has great advantages:significant reduction of the background absorption, fluorescence, low light scattering, the detectablesensitivity and selectivity, and deep penetration of the NIR light. Near-infrared （NIR） dye based onBODIPY have been attracting increasing interest.A series of BODIPYs1-12with dialkylaminophenylbuta-1,3-dienyl group were successfullysynthesized by a Knoevenagel-type condensationin36-42%yields. The absorption and emission maxima(λ_abs=614-739nm; λ_em=655-776nm in CHCl₃) of such species1-12covered the visiblen ear-infraredregion. These dyes exhibited higher extinction coefficients (60000-150000M^-1.cm^-1) and possessed abroader full width at half maximum （fwhm=57-90nm）. In the investigation of the pH-responsiveproperties, probes1,2and5were almost non-fluorescent （Φ_f=0.0013-0.01） in DMSO/H2O （4:1, v/v）.Upon the protonation of the tertiary amine function, the strong fluorescence was released （Φ_f=0.97for1; Φ_f=0.43for2; Φ_f=1for5） and the florescence intensity was dramatically increased by one thousandfold. The sharp isosbestic point is at590nm during the formation of the protonated compound1-H+. Due tothe difference of pKa of the substituent, the respond of1to the pH （pH=3） was more sensitive than that（pH=2） of5; upon the addition of HCl the enhancement of absorption and fluorescence intensity reachingthe maximum was2M HCl for1but4M for5. In the di-substituent system, the protonated compounds 2-H+and2-2H+were c distinguished in fluorescence spetrum (λ_em=663nm for2-H+; λ_em=597nm for2-2H+). The probe2was a turn-on NIR pH fluorescent probe.As a reference material of1,2and5, BODIPYs14-17with styryl group had been designed andsynthesized The absorption and emission maxima was slightly blueshifted compared with those of thecorresponding dyes with dialkylaminophenylbuta-1,3-dienyl group. In the investigation of thepH-responsive properties, probes16and17were lower fluorescent （Φ_f=0.03⁰.02） in DMSO/H2O （4:1,v/v）. Upon the protonation of the tertiary amine function, the strong fluorescence was released （Φ_f=0.67for1; Φ_f=0.45for2） and the florescence intensity was increased by ten and forty fold, respectively.