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Graphical abstract Highlights • Five probes (1a-e) have excellent photo stability. • Four probes (1a-d) exhibit "OFF–ON" fluorescent responses towards nucleic acids. • Probe 1a can image mitochondrial nucleic acid. • Probe 1a was a low-toxic probe for live HeLa cells. Abstract Five cyanine dyes (1a-e) with azonia unit have been prepared by the reaction of pyrido[1,2−a]pyrimidinium ions and suitable aldehydes. Before further research, their photo and thermal stabilities were determined in the first place. At least 88% absorption remained after 5 h irradiating illustrated all of these dyes had excellent photo stability. Meanwhile, the decomposition temperature of dyes was around 200 °C which demonstrated they had good thermal stability. Therefore, the good photo and thermal stability made them possible to be probes. Subsequently, the optical properties of probes toward nucleic acids had been measured. TheirOFF–ON fluorescent responses (1a - d) towards nucleic acids were investigated. In addition, confocal microscopy imaging of probe 1a in live HeLa cells indicated that probe 1a could image mitochondrial nucleic acid. [ABSTRACT FROM AUTHOR]

Three 5 H -benzo[a]phenoxazin-5-one-based (benzoresorufin and nile-red) Cysteine (Cys) detection probes have been comparatively designed and synthesized in this paper. The optical experiments exhibit probe 1b with a crotonoyl group has no response toward Cys; while probes 1a and 1c have the same reaction site (acryloyl group), their optical responses to Cys are quite different. The benzoresorufin-based-probe 1a shows a turn-on fluorescence response (118-fold) to Cys at 631 nm and affords a very low detection limit (DL = 19.8 nM). Compared with probe 1a , the nile-red-based probe 1c displays gradually diminishing fluorescence intensity with increased Cys concentration at 665 nm. And the notable different fluorescence response mechanisms of probes 1a and 1c toward Cys can be interpreted by HRMS and time-dependent density functional theorety (TDDFT) calculations. Furthermore, both of the two probes indicate high sensitivity and selectivity toward Cys over other similar structured amino acids including homocysteine (Hcy) and glutathione (GSH). Further cellular applications of the two probes have been successfully performed in HeLa cells. Image 1 • Three Cys probes 1a − c based on benzoxazine (benzoresorufin and nile-red) were comparatively designed, synthesized and investigated, and the mechanism was proposed. • Their different responses to Cys could indicate that the same fluorophore with different auxochrome groups and detection groups have different properties. • Probes 1a and 1c exhibited high selectivity for Cys, and 1a had a low detection limit of 19.8 nM. • Off-on and attenuation response; long emission wavelength; cell imaging. [ABSTRACT FROM AUTHOR]

• pH related optical responses of 9-amino-9H-xanthene connected with electron-withdrawing groups were investigated. • The deprotonated-protonated equilibriums of 9-amino-9H-xanthene derivatives were evaluated. • The change of structures leads to target different organelles. Two xanthene compounds attached with 4-pyridinylimino (2a) and 2-quinolinylimino (2b) were prepared. Their deprotonated-protonated equilibrium between pyridine/quinoline/ xanthene units and the corresponding pyridinum/quinolinium/xanthylium units were evaluated in different pH by absorption and emission spectra. Probes 2a-b gave the colorimetric changes and emission responses at 550–750 nm, and their p K a s (2a : p K a 1 = 4.53, p K a 2 = 5.95; 2b : p K a 1 = 2.33, p K a 2 = 6.18) were calculated by emission spectra. To verify the protonation process of probes 2a-b , the other two xanthylene derivatives attached with pyridinium (3a) and quinolinium (3b) were designed and synthesized; they exhibited only the equilibrium between xanthene and xanthylium as expected. Probe 3a showed a ratiometric change at 525–750 nm, and probe 3b displayed an OFF–ON emission at 500–700 nm with the decrease of the solution acidity. The calculated p K a s were 6.18 and 1.49, respectively. Moreover, the fluorescent imaging experiments indicated that probes 2a – b were lysosome biomarkers while probes 3a – b were mitochondrial biomarkers for HeLa cells. [ABSTRACT FROM AUTHOR]

The involvement of biologically active nitrogen heterocycles in the construction of conjugated structures was important to improve the biocompatibility and optical properties of fluorescent probes. Herein, six pyrazolo [1, 5-a] pyrimidine derivatives (1a – f) were prepared by aldol or Suzuki reaction with the C-5 or C-7 sites of bioactive pyrazolo [1, 5-a] pyrimidine and different fluorophores. The optical properties in eight organic solvents were investigated; they showed excellent solvatochromism and the maximum emission wavelength range was from 443 nm to 678 nm. Besides, all of them exhibited high fluorescence quantum yields (0.44–0.98) in small polar solvents and they also exhibited excellent photostability. Subsequently, the fluorescent imaging experiments indicated that probes 1a – f could be used as lipid droplets (LDs) biomarkers for HeLa cells (cancer cells) and L929 cells (normal cells); moreover, the number of LDs in HeLa cells was found to be obviously more than in L929 cells, so probes 1a – f possessed great potential in distinguishing between normal cells and cancer cells, even in imaging and diagnosis of LDs-related diseases. Image 1 • The application of biological active heterocycles participate in the construction of novel fluorescent probes was reported in this paper. • Six pyrazolo[15-a]pyrimidine analogues and their optical properties in different solvents are investigated in detail. • Cell imaging experiments indicated they not only could label LDs but also possessed the potential to distinguish cancer cells and normal cells. [ABSTRACT FROM AUTHOR]