Fluorene Fluorescence

Brocklehurst et al. employed squalane as S and fluorene as M. They measured the time profile of fluorene fluorescence during and after pulse radiolysis and found that the fluorescence intensity was increased by a 0.3 T magnetic field as shown in Fig. 6-3(a). They also measured the time dependence of the magnetic field enhancement of the fluorescence intensity as shown in Fig. 6-3(b). This figure shows that the MFE is very small or zero during the pulse, but that it rapidly reaches an apparent plateau (40 % increase) after about 100 ns. This is due to the fact that the MFE grows in several tens ns, which is the order of the S-T conversion due to the HFCM as shown in Chapter 3. 

The use of near-IR-laser excited FT-SERS eliminates the disturbing fluorescence of impurities found with visible excitation, and provides SERS enhancement factors that are about 20 times larger than those found for excitation at 514.5nm [792]. For a strong Raman scatterer (fluorene), a typical detection limit of 500 ng is found for a 3-mm diameter spot. For weak scatterers, the detection limits may be in the high- xg region, which means that some compromise between chromatographic... 

Corredor CC, Huang Z, Belfield KD (2006) Two-photon 3D optical data storage via fluorescence modulation of an efficient fluorene dye by a photochromic diarylethene. Adv Mater 18 2910-2914... 

By the end of the nineteenth century around 600 fluorescent compounds had been identified [3], including fluorescein (A. von Baeyer, 1871), eosine (H. Garo, 1874), and polycyclic aromatic hydrocarbons (C. Liebermann, 1880) [5], Although it is generally accepted that fluorescence markers are relatively new analytical benefits, it is surprising to note that their chemical synthesis is rather old, such as the fluorescein reported by Baeyer, the 2,5-diphenyloxazole by Fisher in 1896, and the fluorene by Berthelot in 1867 [18],... 

Fluorene (FI) is a polycyclic aromatic compound, which received its name due to strong violet fluorescence arising from its highly conjugated planar n-electron system (Chart 2.44). 

Pei and coworkers [362] synthesized fluorene copolymer functionalized with imidazole ligands in the side chains (270). The PL emission of 270 was sensitive to the presence of metal cations in solution (particularly efficient quenching was due to Cu2+), which makes it a promising material for fluorescent chemosensing. 

When an electron-deficient BT unit was incorporated into the backbone of these polymers, an efficient energy transfer resulted in complete fluorescence quenching from the fluorene sites already at BT concentrations as low as 1% (for both neutral and quaternized copolymers, 377 and 378) [440] (Chart 2.93). These macromolecules emit green (544-550 nm, 377) to yellow (555-580 nm, 378) light and can be processed from environment-friendly solvents such as alcohols. The PLED fabricated with these polymers showed high 4>(]over 3 and 1% for 377 and 378, respectively (A1 cathode). 

Whereas all above water-soluble PFs are tetra-alkylammonium-based salts, Burrows et al. [443] reported on anionic fluorene-based copolymer 381 that showed a blue shift in PL (from 424 to 411 nm) as well as a dramatic increase in the fluorescence quantum yield (from 10-15 to 60%) when incorporated into w-dodecylpentaoxyethylene glycol ether micelles [443],... 

While retaining much of the substituted PT character (e.g., good hole-transport properties and stability), these materials exhibit significantly improved fluorescence efficiency in the solid state (cl>Pi up to 29%) that leads to (hllof UP to 0.1% for ITO/453/Ca PLED (Table 2.6). Other widely studied thiophene copolymers with aromatic 9,9-disubstituted fluorene units were already described above in Section 2.3. 

H.D. Burrows, Y.M.M. Lobo, J. Pina, M.L. Ramos, J.S. de Melo, A.J.M. Yelente, M.J. Tapia, S. Pradhan, and U. Scherf, Fluorescence enhancement of the water-soluble poly l,4-phenylene-[9,9-bis(4-phenoxybutylsulfonate) ]fluorene-2,7-diyl copolymer in M-dodecylpentaoxyethylene glycol ether micelles, Macromolecules, 37 7425-7427, 2004. 

Intensive effort has been devoted to the optimization of CCP structures for improved fluorescence output of CCP-based FRET assays. The inherent optoelectronic properties of CCPs make PET one of the most detrimental processes for FRET. Before considering the parameters in the Forster equation, it is of primary concern to reduce the probability of PET. As the competition between FRET and PET is mainly determined by the energy level alignment between donor and acceptor, it can be minimized by careful choice of CCP and C. A series of cationic poly(fluorene-co-phenylene) (PFP) derivatives (IBr, 9, 10 and 11, chemical structures in Scheme 8) was synthesized to fine-tune the donor/acceptor energy levels for improved FRET [70]. FI or Tex Red (TR) labeled ssDNAg (5 -ATC TTG ACT ATG TGG GTG CT-3 ) were chosen as the energy acceptor. The emission spectra of IBr, 9, 10 and 11 are similar in shape with emission maxima at 415, 410, 414 and 410 nm, respectively. The overlap between the emission of these polymers and the absorption of FI or TR is thus similar. Their electrochemical properties were determined by cyclic voltammetry experiments. The calculated HOMO and LUMO... 

The fluorescence lifetime was determined to be 1124ps for 35a, 785 ps for 35b, and 831 ps for 43 in dichloromethane, whereas in the corresponding amorphous films a nonexponential decay with shorter time constants was observed [118, 119]. These lifetimes are similar to the parent oligophenyls but different from fluorene (10 ns) [120, 121]. When applying oligophenyls as luminescent films, however, we must consider that photooxidation may occur if molecular oxygen is present [122, 123], The proposed pathway for the decomposition is... 

Steadily in the order 359, 385, 395, and 402 nm. The emission spectra exhibit a clearer vibrational fine structure than the absorption spectra. For spiro-sexiphe-nyl, 35b, a detailed analysis shows that the vibrational splitting of 0.20 eV corresponds to a phenyl breathing mode in the Raman spectrum [108]. If for spiro-sexiphenyl the outer biphenyl moieties are fixed parallel as in 4-Spiro (43), the absorption maximum is shifted from 346 to 353 nm (amorphous films) and the fluorescence maximum from 420 to 429 nm, maintaining the Stokes shift. The corresponding spectra are shown in Figure 3.17. The absorption signal at 310 nm in the spectrum of 43 can be attributed to the terminal fluorene moieties. The quantum yields for the fluorescence in the amorphous film are 38% for 35b and as high as 70 10% for 43 [89]. 

Fluorescence Anisotropy of Fluorenes under Two-photon Excitation. 124... 

The time-resolved emission spectra (TRES) and fluorescence lifetimes, ti, of the fluorene derivatives were measured in liquid solutions at room temperature with a PTI QuantaMaster spectrofluorimeter with 0.1 ns temporal resolution [20]. At this resolution, all investigated fluorenes exhibited TRES which were coincident with the corresponding steady-state fluorescence spectra. As an example, TRES for compounds 3 and 11 in hexane, THE, and ACN are presented in Eig. 8 for different nanosecond delays 0 ns (curves 2,4,6) and 5 ns, which modeled the steady-state condition (curves 3,5,7). No differences in the fluorescence spectra for these two delays were observed, indicating that all relaxation processes in the first excited state Si are sufficiently fast for fluorene molecifles and did not exceed the time resolution of the PTI system ( 0.1 ns). 

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