Ultrafast intramolecular charge transfer and internal conversion with tetrafluoro-aminobenzonitriles.

The five 2,3,5,6-tetrafluoro-4-aminobenzonitriles XABN4F with a dimethyl-amino (DMABN4F), diethyl-amino (DEABN4F), azetidinyl (AZABN4F), methyl-amino (MABN4F) or amino (ABN4F) group undergo ultrafast intramolecular charge transfer (ICT) at room temperature, in the polar solvent acetonitrile (MeCN) as well as in the nonpolar n-hexane. ICT also takes place with the corresponding non-fluorinated aminobenzonitriles DMABN, DEABN and AZABN in MeCN, whereas for these molecules in n-hexane only minor (DMABN, DEABN) or no (AZABN) ICT fluorescence is detected. For the secondary (MABN) and primary (ABN) amines, an ICT reaction does not occur, which makes ABN4F the first electron donor/acceptor molecule with an NH(2) group for which ICT is observed. The ICT state of the XABN4Fs has a dipole moment of around 14 D, clearly smaller than that of DMABN (17 D). This difference is attributed to the electron withdrawing from the CN group to the phenyl ring, exerted by the four F-substituents. The reaction from the initially prepared locally excited (LE) to the ICT state in n-hexane proceeds in the sub-picosecond time range: 0.35 ps (DMABN4F), 0.29 ps (DEABN4F) and 0.13 ps (AZABN4F), as determined from femtosecond transient absorption measurements. In the highly polar solvent MeCN, an ICT reaction time of around 90 fs is observed for all five XABN4Fs, irrespective of the nature of their amino group. This shows that with these molecules in MeCN the ICT reaction rate is limited by the solvent dielectric relaxation time of MeCN, for which a value of around 90 fs has been reported. It is therefore concluded that, during this ultrashort ICT reaction, a large-amplitude motion such as a full 90 degrees twist of the amino group is unlikely to occur in the XABN4Fs. The ICT state of the XABN4Fs is strongly quenched via internal conversion (IC), with a lifetime tau'(0) (ICT) down to 3 ps, possibly by a reaction passing through a conical intersection made accessible due to a deformation of the phenyl group by out-of-plane motions induced by vibronic coupling between low-lying pisigma* and pipi* states in the XABN4Fs.     

Oxidative addition reactions of dicyclopentadienyltin(II) and of tin(II) bis(acetylacetonate) with organic halides. The preparation of compounds of the type RSn(acac)2X

Oxidative addition reactions of tin(II) bis(acetylacetonate) with organic halides provide a route to new monoorganotin(IV) bis(acetylacetonate) halides. Dicyclopentadienyltin(II) undergoes oxidative addition with methyl iodide, diiodomethane and ethyl bromoacetate, but with allyl bromide, benzyl bromide and triphenylmethyl bromide carbon—carbon coupling reactions and the formation of the corresponding cyclopentadienyltin(II) halide take place. Some of the reactions are accelerated by light. NMR spectroscopic data show that the RSn(acac)₂X compounds have a cis-configuration. The compounds YCH₂Sn(acac)₂X (e.g. Y = I, CHCH₂, COOEt) provide the first examples of diastereo-topic non-equivalence of methylene protons in organotin compounds containing a hexacoordinate chiral tin centre.     

Spectroscopic observation of the resonance-stabilized 1-phenylpropargyl radical

The gas-phase laser-induced fluorescence (LIF) spectrum of a 1-phenylpropargyl radical has been identified in the region 20,800-22,000 cm(-1) in a free jet. The radical was produced from discharges of hydrocarbons including benzene. Disregarding C2, C3, and CH, this radical appears as the most strongly fluorescing product in a visible wavelength two-dimensional fluorescence excitation-emission spectrum of a jet-cooled benzene discharge. The structure of the carrier was elucidated by measurement of a matching resonant two-color two-photon ionization spectrum at m/z = 115 and density functional theory. The assignment was proven conclusively by observation of the same excitation spectrum from a low-current discharge of 3-phenyl-1-propyne. The apparent great abundance of the 1-phenylpropargyl radical in discharges of benzene and, more importantly, 1-hexyne may further underpin the proposed importance of the propargyl radical in the formation of complex hydrocarbons in combustion and circumstellar environments.     

Control of population flow in coherently driven quantum ladders

A technique for adiabatic control of the population flow through a preselected decaying excited level in a three-level quantum ladder is presented. The population flow through the intermediate or upper level is controlled efficiently and robustly by varying the pulse delay between a pair of partly overlapping coherent laser pulses. The technique is analyzed theoretically and demonstrated in an experiment with Na2 molecules.     

A new ultrafast technique for measuring the terahertz dynamics of chiral molecules: the theory of optical heterodyne-detected Raman-induced Kerr optical activity

Optical heterodyne-detected Raman-induced Kerr optical activity (OHD-RIKOA) is a nonresonant ultrafast chiroptical technique for measuring the terahertz-frequency Raman spectrum of chirally active modes in liquids, solutions, and glasses of chiral molecules. OHD-RIKOA has the potential to provide much more information on the structure of molecules and the symmetries of librational and vibrational modes than the well-known nonchirally sensitive technique optical heterodyne-detected Raman-induced Kerr-effect spectroscopy (OHD-RIKES). The theory of OHD-RIKOA is presented and possible practical ways of performing the experiments are analyzed.     

8-Hydroxyquinoline benzoates as highly sensitive fluorescent chemosensors for transition metal ions

[graph: see text] 8-Hydroxyquinoline benzoates were developed as a new set of 8-HQ derivatives for highly sensitive fluorescent chemosensors for transition metal ions. A prominent fluorescence enhancement was found in the presence of transition metal ions such as Hg2+ and Cu2+, and this was suggested to result from the suppression of radiationless transitions from the npi state in the chemosensors.     

Intramolecular switching between dipolar resonance structures in singlet and triplet excited states detected by TRMC

Transient changes in the real and imaginary component of the complex permittivity of a flash-photolyzed solution can be monitored with nanosecond time resolution using the time-resolved microwave conductivity (TRMC) technique. Effects caused by rapid (picosecond), flip—flop switching between dipolar resonance structures in the S₁ and T₁ states of molecules with mirror symmetry have been observed.     

Formation and reactivity of metal carbonyl anions in the gas phase

The reactions of metal carbonyl anions (M(CO)_n^?; M = Cr, Mn and Fe; n = 1–3) with n-heptane, water and methanol were studied with use of a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer equipped with an external ion source. The M(CO)_n^? ions were formed in the FT-ICR cell by collision-induced dissociation of the most abundant primary ion generated by electron impact of the appropriate metal carbonyl compound present in the external ion source. The M(CO)_n^? ions were allowed subsequently to undergo non-reactive collisions with argon in order to remove possible excess internal/translational energy prior to the ion/molecule reaction. Only the Cr(CO)₃^?, Mn(CO)₃^? and Fe(CO)₂^? ions react with n-heptane. This reaction proceeds by loss of H₂ from the collision complex and the Cr(CO)₃^? and Fe(CO)₂^? ions react about three times more efficiently than the Mn(CO)₃^? ion. With water, Mn(CO)^? and Fe(CO)₃^? are unreactive, whereas the other ions react by loss of one or two CO molecules from the collision complex. The rate of the reaction with water decreases in the order Cr(CO)₃^?, Fe(CO)₂^?, Cr(CO)₂^?, Fe(CO)^?, Mn(CO)₃^? and Mn(CO)₂^?. With methanol, the Cr(CO)₂^? ion reacts by loss of two CO molecules from the collision complex, whereas loss of one CO molecule and elimination of CO + H₂ occur in the reaction with Cr(CO)₃^?. Competing loss of CO and one or two H₂ molecules occurs in the reactions of Mn(CO)₃^? and Fe(CO)₂^? with methanol. The rate of the reaction with methanol decreases in the order Cr(CO)₃^?, Fe(CO)₂^?, Cr(CO)₂^? and Mn(CO)₃^?.