The role of water in the dual luminescence of 4-N,N-dimethylaminobenzonitrile

The observation of the dual luminescence of the title compound in very concentrated solutions in specially dried acetonitrile rules out water impurities as being at the origin of the anomalous emission. Water produces a quenching of the anomalous emission as well as a red-shift of the emission spectrum.     

Solute-solvent exciplexes in the photoinduced formation of radical ions of 4-N,N-dimethylaminobenzonitrile and related compounds

Measurements of laser-pulse-induced transient optical absorptions and transient electrical and conductance reveal that excitation of 4-N,N-dimethylaminobenzonitrile and related compounds in acetonitrile or water at 308 nm leads to formation of radical cations of the solutes. In acetonitrile the cations are formed in a single-photon process via a singlet state solute-solvent exciplex, whereas in water they are formed mainly by a biphotonic process. It follows from the transient absorptions of the compounds in cyclohexane, 1,4-dioxane and acetonitrile that the solutes in their lowest electronic triplet state do not form exciplexes with these solvents.     

Laser-induced fluorescence, dispersed fluorescence and lifetime measurements of jet-cooled chloro-substituted benzyl radicals

We measured the laser-induced fluorescence (LIF) and dispersed fluorescence (DF) spectra of jet-cooled -, o- and m-chlorobenzyl radicals after they were generated by the 193 nm photolysis of the corresponding parent molecules. The vibronically resolved spectra were obtained to analyze their D₁–D₀ transitions. The fluorescence lifetimes of -, o-, m- and p-chlorobenzyls in the zeroth vibrational levels of the D₁ states were measured to estimate the oscillator strengths of a series of benzyl derivatives. It was found that the -substitution is inefficient to break the ‘accidental forbiddenness' of the D₁–D₀ transition of benzyl, while the ring-substitution enhances the oscillator strength by 50%.     

Spectroscopy of the A(1B2)-X(1A1) transition of jet-cooled fluorobenzene: laser-induced fluorescence, dispersed fluorescence, and pathological Fermi resonances

A detailed study of the S1((1)B2)-S0((1)A1) electronic transition of jet-cooled fluorobenzene has been carried out using laser-induced fluorescence and dispersed fluorescence (DF) spectroscopies. Analysis of over 40 single vibronic level DF spectra resulted in the assignment of 16 fundamental frequencies in the excited electronic state. Progressions in totally symmetric modes, particularly in the ring-breathing mode nu9, feature in both types of fluorescence spectrum. There is also significant activity in non-totally-symmetric modes, with activity in Franck-Condon (FC)-allowed overtones, FC-forbidden combinations induced by Duschinsky mixing, and symmetry-forbidden transitions induced by the same Herzberg-Teller vibronic coupling mechanism that induces the benzene S1-S0 transition. Fermi resonances (FRs) are extensive throughout the spectrum, especially in the important FC-active a1 modes. A consequence of these extensive FRs is that several important previous assignments are shown to be incorrect and have been reassigned here. Ab initio and density functional theory calculations have also been performed to support the experimental assignments.     

Dispersed fluorescence spectroscopy of jet-cooled HCF and DCF: Vibrational structure of the X 1A' state

We recorded dispersed fluorescence (DF) spectra following excitation of the pure bending levels 2(0) (n) and the combination states 1(0) (1)2(0) (n) and 2(0) (n)3(0) (1) in the A 1A"<--X 1A' system of HCF and DCF. Spectra were measured with a 0.3 m spectrograph equipped with a gated intensified charge coupled device (CCD) detector and obtained under jet-cooled conditions using a pulsed discharge source. The DF spectra reveal rich detail concerning the vibrational structure of the X state up to 10 000 cm(-1). For HCF, resonances among the nearly degenerate levels 1(1)2n, 2n+13(1), and 2n+2 produce a polyadlike structure in the spectrum, and the usual effective spectroscopic Hamiltonian (Dunham expansion) poorly reproduces the experimental term energies. In contrast, this Hamiltonian works well for the term energies of DCF. Density functional calculations of the ground state vibrational frequencies were performed; the results are in excellent agreement with the experimentally derived vibrational parameters. The search for perturbations involving the low-lying a 3A" state is described.     

Twisted internal charge transfer fluorescence of para-N,N-dimethylaminobenzonitrile in rigid matrix at room temperature

The dual fluorescence of para-N,N-dimethylaminobenzonitrile is observed for the first time in a rigid matrix at room temperature (polyvinylalcohol). A relationship between the twisted internal charge transfer (TICT) fluorescence and the formation of a ground-state hydrogen bonded complex is shown.     

Interaction between intramolecular charge transfer state of p-N,N-dimethylaminobenzonitrile and amines. Stereo-controlled fluorescence quenching

The first excited singlet state, an intramolecular charge transfer state, of p-N,N-dimethylaminobenzonitrile (DMABN) is quenched by tertiary amines. The quenching rate cannot be correlated with the oxidation potential of the amines, but is controlled by the size of the amine alkyl group. This unusual steric effect indicates a short-range interaction, which we attribute to three-electron bonding. Excited-state three-electron bonding interaction should be a general phenomenon and can lead to the formation of sigma-bonded exciplexes. The implication of this result on the origin of the anomalous dual fluorescence of DMABN is discussed.     

Laser induced fluorescence and resonant two-photon ionization spectroscopy of jet-cooled 1-hydroxy-9,10-anthraquinone

We carried out laser induced fluorescence and resonance enhanced two-color two-photon ionization spectroscopy of jet-cooled 1-hydroxy-9,10-anthraquinone (1-HAQ). The 0-0 band transition to the lowest electronically excited state was found to be at 461.98 nm (21,646 cm(-1)). A well-resolved vibronic structure was observed up to 1100 cm(-1) above the 0-0 band, followed by a rather broad absorption band in the higher frequency region. Dispersed fluorescence spectra were also obtained. Single vibronic level emissions from the 0-0 band showed Stokes-shifted emission spectra. The peak at 2940 cm(-1) to the red of the origin in the emission spectra was assigned as the OH stretching vibration in the ground state, whose combination bands with the C=O bending and stretching vibrations were also seen in the emission spectra. In contrast to the excitation spectrum, no significant vibronic activity was found for low frequency fundamental vibrations of the ground state in the emission spectrum. The spectral features of the fluorescence excitation and emission spectra indicate that a significant change takes place in the intramolecular hydrogen bonding structure upon transition to the excited state, such as often seen in the excited state proton (or hydrogen) transfer. We suggest that the electronically excited state of interest has a double minimum potential of the 9,10-quinone and the 1,10-quinone forms, the latter of which, the proton-transferred form of 1-HAQ, is lower in energy. On the other hand, ab initio calculations at the B3LYP/6-31G(d,p) level predicted that the electronic ground state has a single minimum potential distorted along the reaction coordinate of tautomerization. The 9,10-quinone form of 1-HAQ is the lowest energy structure in the ground state, with the 1,10-quinone form lying approximately 5000 cm(-1) above it. The intramolecular hydrogen bond of the 9,10-quinone was found to be unusually strong, with an estimated bond energy of approximately 13 kcal/mol (approximately 4500 cm(-1)), probably due to the resonance-assisted nature of the hydrogen bonding involved.     

Electronic spectra of the jet-cooled 1-methylvinylthio radical

Electronic spectra of the B?-X? transition of the 1-methylvinylthio radical were observed in a discharged jet of propylene sulfide by laser-induced fluorescence spectroscopy. Identification of the spectral carrier was made by comparing the observed spectra with results of molecular orbital calculations, in particular, for vibrational frequencies, rotational contour simulations, and the Franck-Condon simulations. Vibrational structures observed in the electronic spectra indicate that the 1-methylvinylthio radical can be regarded as a molecule with C(s) symmetry at the zero-point levels of both the excited and ground states.     

Solute-solvent exciplexes as the source of anomalous fluorescence from 4-N,N-dimethylamino-ethylbenzoate in 1,4-dioxane and in polar solvents

The behaviour of 4-N,N-dimethylamino-ethylbenzoate in its lowest electronically excited singlet and triplet states in solutions in apolar and polar solvents has been investigated by measuring the lifetime and quantum yield of the fluorescence, quantum yield of the triplet state and transient dielectric loss induced by laser pulse excitation. The major conclusion is that the anomalous fluorescence of the compound in 1,4-dioxane and in polar solvents is emitted by solute-solvent exciplexes. The previously reported anomalous fluorescence of the compound in alkanes has not been observed.     

Picosecond excitation of jet-cooled pyrazine: magnetic field effects on the fluorescence decay and quantum beats

Quantum beats and fluorescence decays of the vibrationless S₁ state of jet-cooled pyrazine have been studied as a function of magnetic field. Fourier analysis of the modulated portions of these decays has yielded results which are consistent with first- and second-order Zeeman effects occurring in the excited state. An increase in the number of triplets states effetively coupled to the singlet manifold with increasing magnetic field is discussed in terms of Zeeman interactions     

Ultrafast fluorescence risetime of a polyatomic molecule in solution and under collision-free conditions

The fluorescence rise in solution and in the vapor phase was investigated for the same molecule, dimethyl-POPOP. After optical excitation the structured fluorescence spectrum of the solution evolves in a time shorter than 2 ps. Likewise the fluorescence in the vapor phase rises within less than 2 ps. This finding suggests rapid intramolecular vibrational relaxation in S₁ For excitation with a large excess energy of 11000 cm^?1 the relaxation to fluorescing states in S₁ is still fast under collision-free conditions.     

An efficient one-pot, two-step synthesis of 4-substituted 1-heteroarylpiperazines under microwave irradiation conditions

A highly efficient one-pot, two-step microwave procedure has been developed for the synthesis of 4-substituted 1-heteroarylpiperazines. Microwave heating of heteroaryl chlorides with 1,4-diazabicyclo[2.2.2]octane (DABCO) at 160 °C for 15 min yielded 1-heteroaryl-4-(2-chloroethyl)piperazines, which could be further reacted with various nucleophiles, again under microwave irradiation conditions, to give an array of 4-substituted 1-heteroarylpiperazines in good to excellent yields.     

Rotationally resolved laser-induced fluorescence and zeeman quantum beat spectroscopy of the V 1B2 state of jet-cooled CS2

The rotationally resolved laser-induced fluorescence (LIF) excitation spectrum of V system bands (V¹B₂≈X¹Σ¹_g transition) of CS₂ cooled in a supersonic jet has been observed. In a supersonic jet of CS₂/Ar or He mixture, the rotational temperature of CS₂ is reduced to less than 10 K, and thus the LIF excitation spectrum is simplified significantly. Two types of rotational structure are found; one is composed of P and R branch transitions from even J″ levels and the other is of P, Q, and R branch transitions from even as well as J″. The bands with the former rotational structure are assigned to transitions to K′ = O levels of ¹B₂ state, the bands with the latter structure to transitions to K′ = 1 levels from the (O, 1¹, O) level of the electronic ground state, i.e. vibrationally hot bands. This assignment is supported by the further evidence that these hot bands disappear when the supersonic jet includes a third-body gas such as NH₃ which enhances the vibrational relaxation of CS₂. Calculation of transition moments for respective leads to the conclusion that the upper levels of the V system bands are located in the region close to or higher than the potential barrier of the bending vibration of excited CS₂. The radiative lifetime of CS₂ in single rovibronic levels of the ¹B₂ state is in the range of 2–8 μs which is of the same order of magnitude as that calculated from the absorption coefficient. It tends to be longer for higher J levels or for higher vibronic levels. Zeeman quantum beating is observed in the fluorescence decay of excited CS₂ for a number of rovibronic levels under a weak magnetic field, and thus a magnetic moment associated with each rovibronic level can be determined. The g values are around 0.02 and tend to be smaller in higher J levels for some vibronic states. Based on the the observed radiative lifetime and the g value, it is suggested that the ¹B₂ state is perturbed by a spin-rotation interaction with two spin components, A₁ and B₁ of the ³A₂ orbital state besides a strong spin-orbit coupling with the R ³B₂ state.     

Synthesis of 4-(1-methyl-2-pyrrolidinyl)isoquinoline under physiological conditions : a model for the biosynthesis of 4-pyrrolidinylisoquinoline alkaloids

1,2,3,4-Tetrahydroisoquinoline-3-carboxylic acid was oxidatively decarboxylated with sodium hypochlorite, affording a dihydroisoquinoline which on condensation with N-methyl-Δ¹-pyrrolinium acetate, and subsequent aerial oxidation yielded 4-(1-methyl-2-pyrrolidinyl)isoquinoline.     

A laser-induced fluorescence study of the jet-cooled nitrous oxide cation (N2O+)

Laser-induced fluorescence and wavelength resolved emission spectra of the A? (2)Σ(+) - X? (2)Π(i) electronic transition of the jet-cooled nitrous oxide cation have been recorded. The ions were produced in a pulsed electric discharge at the exit of a supersonic expansion using a precursor mixture of N(2)O in high pressure argon. Both spin-orbit components of the 0(0) (0) band were studied at high resolution and rotationally analyzed to provide precise molecular constants for the combining states. Emission spectra were obtained by laser excitation of the 0(0) (0), 2(0) (1), 3(0) (1), and 2(0) (2) absorption bands, providing extensive data on the ground state bending, stretching, and combination vibrational levels. These data were fitted to a Renner-Teller model including spin-orbit, anharmonic, and Fermi resonance terms. The observed energy levels and fitted parameters were found to be comparable to those in the literature predicted from an ab initio potential energy surface.     

Theoretical characterization of stable eta1-N2O-, eta2-N2O-, eta1-N2-, and eta2-N2-bound species: intermediates in the addition reactions of nitrogen hydrides with the pentacyanonitrosylferrate(II) ion

The addition of nitrogen hydrides (hydrazine, hydroxylamine, ammonia, azide) to the pentacyanonitrosylferrate(II) ion has been analyzed by means of density functional calculations, focusing on the identification of stable intermediates along the reaction paths. Initial reversible adduct formation and further decomposition lead to the eta(1)- and eta(2)-linkage isomers of N(2)O and N(2), depending on the nucleophile. The intermediates (adducts and gas-releasing precursors) have been characterized at the B3LYP/6-31G level of theory through the calculation of their structural and spectroscopic properties, modeling the solvent by means of a continuous approach. The eta(2)-N(2)O isomer is formed at an initial stage of adduct decompositions with the hydrazine and azide adducts. Further conversion to the eta(1)-N(2)O isomer is followed by Fe-N(2)O dissociation. Only the eta(1)-N(2)O isomer is predicted for the reaction with hydroxylamine, revealing a kinetically controlled N(2)O formation. eta(1)-N(2) and eta(2)-N(2) isomers are also predicted as stable species.     

Time-resolved fluorescence polarization as a probe of the rotational dynamics of electronically excited molecules: Fluorescence from single rotational (J) level of jet-cooled 0001B1 pyrimidine

The polarization of fluorescence from R-branch excited 0₀⁰¹B₁ pyrimidine decays with a rate which increases with increasing J. This observation confirms the occurrence of rotation-induced K mixing, which takes place during the lifetime of the excited electronic state.     

Photoinduced intramolecular charge-transfer reactions in 4-amino-3-methyl benzoic acid methyl ester: A fluorescence study in condensed-phase and jet-cooled molecular beams

Photoinduced intramolecular charge-transfer reactions in 4-amino-3-methyl benzoic acid methyl ester (AMBME) have been investigated spectroscopically. AMBME, with its weak charge donor primary amino group, shows dual emission in polar solvents. Absorption and emission measurements in the condensed phase support the premise that the short wavelength emission band corresponds to local emission and the long wavelength emission band to the charge transfer emission. Laser-induced fluorescence excitation spectra show the presence of two low-energy conformers in jet-cooled molecular beams. Theoretical calculations using density functional theory help to determine structure, vibrational modes, potential energy surface, transition energy and oscillator strength for correlating experimental findings with theoretical results.