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The title compound (I) shows a 21-fold two-photon excited fluorescence enhancement in response to H2S.

Two-photon microscopy (TPM) has become an indispensible tool in biology and medicine owing to the capability of imaging the intact tissue for a long period of time. To make it a versatile tool in biology, a variety of two-photon probes for specific applications are needed. In this context, many research groups are developing two-photon probes for various applications. In this Focus Review, we summarize recent results on model studies and selected examples of two-photon probes that can detect intracellular free metal ions in live cells and tissues to provide a guideline for the design of useful two-photon probes for various in vivo imaging applications.

Elimination reactions of N-(arylsulfonoxy)-N-alkylbenzylamines 1-5 with MeONa−MeOH have been studied kinetically. The elimination reactions are regiospecific, producing only corresponding benzylidenalkylamines. The rate equation for the reactions is k obs =k 0 +k 2 [MeONa], indicating that the reactions proceed by competing solvolytic- and base-promoted pathways. The relative rates of elimination for the k 2 and k 0 pathways are 1, 0.67, 0.53, 0.35, and 0.27 for R=Me, Et, i-Pr, s-Bu, and t-Bu and 1, 4.1, 5.1, and 8.7 for R=Et, i-Pr, s-Bu and t-Bu, respectively. For MeONa-promoted elimination from 1-5, Hammett p and k H /k D decrease but ρ 1g and |β 1g | increase with a bulkier alkyl substituent

Reactions of (E)-O-arylbenzaldoximes 1-3 with MeONa-MeOH have been studied kinetically. The reactions proceed via competing E2 and S N Ar reactions, in which the first step is rate-determining. Although the reactions were strongly influenced by the electronic effect of the β- and O-aryl substituents, they were insensitive to the steric effect of the O-aryl group, except that the S N Ar reaction was retarded by the CF 3 group of 2. For eliminations from 1-3 promoted by MeONa-MeOH, the k H /k D value increased and the Hammett p value decreased with better leaving groups

Reactions of PhCH2CH(OTs)R [R = Me (1), Et (2), Pri(3), Bus(4), But(5)] with MeONa in MeOH have been studied. The reactions produce both conjugated and unconjugated alkenes. The yields of the conjugated alkenes are nearly the same for 1–4, while the E/Z ratios depend strongly upon the α-alkyl group. The rates of eliminations forming the conjugated E alkenes are decreased by a bulkier alkyl group as indicated by the relative rate of 1, 0.8, 0.7, 0.6, 0.2 for 1, 2, 3, 4, 5, respectively. On the other hand, the relative rates for the formation of the unconjugated alkenes are 1, 1.7, 2.8, 1.9 for 1, 2, 3, 4 respectively, indicating that the rate increases with the double bond stabilizing ability of the alkyl groups and decreases with their steric effect. From these results, the relative steric effect of the α- and β-alkyl groups in alkene-forming elimination is assessed.

The first evaluation of the β for 2-[2-(p-diethylaminophenyl)vinyl]furan derivatives is reported in comparison with the corresponding thiophene derivative.

Elimination reactions of (E)-2,4-dinitrobenzaldehyde O-aryloximes 1−3 promoted by EtO-, PhC(CH3)NO-/PhC(CH3)NOH, and CF3CH2O-/CF3CH2OH buffers in ethanol have been studied kinetically. The reactions produced 2,4-dinitrobenzonitrile and aryloxides as the only products. The observed second-order kinetics, Bronsted β = 0.55−0.75, |βlg| = 0.39−0.48 are consistent with the E2 mechanism. The Bronsted β decreased as the leaving group was made more nucleofugic and the |βlg| increased with a weaker base. The result can be described by a positive interaction coefficient, pxy = ∂β/∂pK1g = ∂β1g/∂pKBH > 0, which provides additional support for the E2 mechanism.

Elimination reactions of (Z)-thiophene- and (Z)-furan-2-carbaldehyde O-benzoyloximes 1 and 2 with DBU in MeCN have been investigated kinetically. The reactions are second order and exhibit substantial values of Hammett ρ and kH/kD values, and an E2 mechanism is evident. The relative rates of elimination from (Z)-ArCHNOC(O)C6H4Y are 1/1.1/0.6 for Ar = phenyl/thienyl/furyl, respectively. For reactions of 1 with DBU in MeCN, kH/kD = 8.2 ± 0.1, Hammett ρ = 1.22 ± 0.19, β1g = −0.43 ± 0.01, ΔH⧧ = 5.9 ± 0.1 kcal/mol, and ΔS⧧ = −28.5 ± 0.3 eu have been determined. The corresponding values for 2 are kH/kD = 8.8 ± 0.2, ρ = 1.87 ± 0.05, β1g = −0.55 ± 0.10, ΔH⧧ = 6.5 ± 0.1 kcal/mol, and ΔS⧧ = −29.0 ± 1.5 eu. The kH/kD, Hammett ρ, and |β1g| values increase as the β-aryl group is changed in the order phenyl < thienyl < furyl. The results indicate that the transition state structure for the nitrile-forming elimination changes slightly toward product-like with the change of the β-aryl group.

Optical imaging with fluorescence microscopy is a vital tool in the study of living systems. The most common method for cell imaging, one-photon microscopy (OPM), uses a single photon of higher energy to excite the fluorophore. However, two-photon microscopy (TPM), which uses two photons of lower energy as the excitation source, is growing in popularity among biologists because of several distinct advantages. Using TPM, researchers can image intact tissue for a long period of time with minimum interference from tissue preparation artifacts, self-absorption, autofluorescence, photobleaching, and photodamage. However, to make TPM a more versatile tool in biology, researchers need a wider variety of two-photon probes for specific applications. In this Account, we describe a series of two-photon probes that we developed that can visualize the distribution of intracellular metal ions, acidic vesicles, and lipid rafts in living cells and tissues. The development of these probes requires a significant two-photon cross section for the bright image and receptors (sensing moieties) that triggers the emission of the two-photon excited fluorescence upon binding with the ions or membrane in the living system. These probes also must be sensitive to the polarity of the environment to allow selective detection of cytosolic and membrane-bound probes. In addition, they need to be cell-permeable, water-soluble for the staining of cells and tissues, and highly photostable for long-term imaging. The resulting probes-AMg1 (Mg(2+)), ACa1-ACa3 (Ca(2+)), AZn1 and AZn2 (Zn(2+)), AH1, AH2, and AL1 (acidic vesicles), and CL2 (membrane)-use 2-acetyl-6-aminonaphthalene as the fluorophore and receptors for the target ions or membrane. All of these two-photon turn-on probes can detect the intracellular free metal ions, acidic vesicles, and lipid rafts at 100-300 microm depth in live tissues. Moreover, with ACa1-AM, we could simultaneously visualize the spontaneous Ca(2+) waves in the somas of neurons and astrocytes at approximately 120 microm depth in fresh hypothalamic slices for more than 1000 s without appreciable decay. Furthermore, AL1 could visualize the transport of the acidic vesicles between cell body and axon terminal along the axon in fresh rat hippocampal slices at approximately 120 microm depth.

The development of two-photon materials has been growing very fast during the last decade due to the potential applications in optical limiting, 3D microfabrication, and multiphoton microscopy. In this article, the structure–two-photon absorption property relationships of dipolar, quadrupolar, and octupolar molecules as well as the derivatives of triphenylamine, [2.2]paracyclophanes, multi-annulenes, and porphyrins are briefly reviewed to provide a guideline for the design of efficient two-photon materials.

1,3-Alternate calix[4]arene-based fluorescent chemosensors bearing two-photon absorbing chromophores have been synthesized, and their sensing behaviors toward metal ions were investigated via absorption band shifts as well as one- and two-photon fluorescence changes. Free ligands absorb the light at 461 nm and weakly emit their fluorescence at 600 nm when excited by UV-vis radiation at 461 nm, but no two-photon excited fluorescence is emitted by excitation at 780 nm. Addition of an Al(3+) or Pb(2+) ion to a solution of the ligand causes a blue-shifted absorption and enhanced fluorescence due to a declined resonance energy transfer (RET) upon excitation by one- and two-photon processes. Addition of a Pb(2+) ion to a solution of 1.K(+) results in a higher fluorescence intensity than the original 1.Pb(2+) complex regardless of one- or two-photon excitation, due to the allosteric effect induced by the complexation of K(+) with a crown loop.

La «base complexe» utilisee est le melange NaNH 2 -NaOtBu dans le THF; obtention du produit de syn-difluorhydratation

Reactions of N-halogenobenzylmethylamines (1) and (2) with Et3N–MeCN have been investigated kinetically. Eliminations from (1) and (2) were quantitative and regiospecific, producing only benzylidenemethylamines. For the elimination reaction of (1) with Et3N, kH/kD 5.7, p 0.88, ΔH‡ 9.1 kcal mol–1, and ΔS‡–42.4 cal mol–1K–1 were determined. The transition-state structure is assessed as being highly symmetric with similar extents of Cβ–H and Nα–Cl bond cleavage, little carbanionic character, and significant π-bond formation. The structure of the transition state changes only slightly with the change in the leaving group from Cl to Br.

Reaction d'elimination des dinitro-2,4 phenyloxime et picryloxime du benzaldehyde. Les resultats observes sont en faveur d'un mecanisme E2