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ABSTRACT: The hydrogen-bonded clusters Coumarin 151/(H2O)1 and Coumarin 152A/(H2O)1 have been studied via IR–UV double-resonance spectroscopy. Each cluster exists in two structural forms having distinct electronic properties. Evidence is presented to show that the structures involve water hydrogen bonded to groups at opposite ends of the molecular dipole. These results were supported by semiempirical structure calculations and, for C151, by infrared measurements in the S1 state.
Chemical Physics Letters 318:440-447. · 2.34 Impact Factor
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ABSTRACT: Two-photon pump—probe experiments involving absorption—depletion (hole-burning) spectroscopy have measured the vibronic spectra of CO2 complexes with perylene. Four types of Franck—Condon active mode are indicated by the spectrum of the 1:1 aggregate: (a) Many of the totally symmetric (in-plane) modes of perylene are only weakly perturbed by CO2 complexation. (b) Certain out-of-plane modes are significantly perturbed, including changes in both Franck—Condon factor and frequency. (c) Low-frequency progressional structure due to intermolecular vibrational modes indicates an unstable conformation for the CO2 complexes, which can be attributed to multipolar interactions. (d) The spectrum of perylene/(CO2)1 at higher internal energies exhibits doubling of some progressions.
Chemical Physics Letters.
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ABSTRACT: Vibronically induced conformational relaxation is reported between two distinct structures of the jet-cooled cluster coumarin 151/(H2O)2. The process involves transfer of the water dimer in the electronically excited state over a distance of ≈8 Å between sites on the coumarin molecule, governed by an activation energy of .
Chemical Physics Letters.
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ABSTRACT: We have used infrared spectroscopy in both the ground and excited electronic states to determine the structural geometries of hydrogen-bonded clusters involving aminophthalimide molecules. This provides a higher level of structural information than available through an inertial measurement such as rotational coherence spectroscopy. Clusters of interest include the two conformers each of the water n=1 cluster with 4-aminophthalimide (4AP) and 4-amino-N-methylphthalimide (4ANMP), which are shown to be attached by proton donation to different >CO groups. The corresponding n=1 water cluster of 3-aminophthalimide (3AP) is present as only a single conformer, consistently with the presence of an intramolecular hydrogen bond. Both 3AP/(H2O)1 and 4AP/(H2O)1 are found to form bridging structures between the imide N–H bond and an adjacent >CO group. On the other hand, the water dimer clusters of 4AP and 4ANMP bridge the NH2 and the adjacent >CO group.Pure dimers of the aminophthalimides have simple infrared spectra in the electronic ground states, which indicate symmetrical structures. The 4AP and 4ANMP dimers form a double hydrogen-bonded bridge across the 2,4 positions and their complex infrared spectra in the excited state show evidence of strong interactions, which may involve excited-state intermolecular double proton transfer. On the other hand, the 3AP dimer is doubly hydrogen bonded across the 1,7 positions, and appears to show evidence of excited-state single intramolecular proton transfer. Possible reasons for these differences are discussed.
Chemical Physics.
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ABSTRACT: The jet-cooled dimer of 4-amino-N-methylphthalimide (4ANMP) has four NH bonds, but the ground-state infrared spectrum shows just two strong N–H-stretch resonances in the range 3100–3800 cm−1. This confirms a symmetric structure (C2). In the excited state, there are six strong resonances in the same region, falling into two groups of three. A similar pattern is observed for the dimer of 4-aminophthalimide (4AP). These observations suggest that double proton transfer may be occurring across the two N–H⋯O hydrogen bonds.
Chemical Physics Letters.
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ABSTRACT: Ultraviolet fluorescence spectra are presented for 3,4-benzpyrene, 1,12-benzeperylene and 3,4,9,10-dibenzpyrene, which were obtained by single-laser, consecutive two-proton excitation. Experiments using two-laser excitation have permitted the study of fluorescence line-narrowing in the spectrum of dibenzpyrene at 300 K in hexane solution. These unique observations are interpreted in terms of optical site-selection.
Chemical Physics Letters.
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ABSTRACT: Resonance fluorescence effects are absent in the two-photon-excited upper state fluorescence spectra of a centrosymmetric aromatic hydrocarbon. Spectra of states populated by radiationless relaxation are structured and sensitive to the excitation conditions. The results are interpreted in terms of vibration mode-selectivity of the internal conversion process between two highly excited singlet states.
Chemical Physics Letters.
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ABSTRACT: Consecutive two-photon laser excitation has been used to populate elevated singlet states of the xanthene dyes rhodamine 6G and rhodamine B. Fluorescence has been observed from several states in each molecule, terminating in the ground state, in the range 240-400 nm. It is demonstrated that the shape of the spectrum depends on both excitation energy and polarisation, and the hydrocarbon rubrene is used as an example of the excitation intensity dependence of the weak emission spectrum from a fluid solution.
Chemical Physics Letters.
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ABSTRACT: Infrared-optical double-resonance measurements have been made for electronically excited, hydrogen-bonded clusters involving Coumarin 151 molecules under jet-cooled conditions. Two main differences are reported between the ground and excited states:1.Infrared resonances attributed to “donor”-OH and -NH bonds, as well as the symmetric-stretch modes of non-hydrogen-bonded NH2 tend to downshift in energy and are usually intensified in the excited state. Also, “donor”-NH groups tend to develop an additional strong infrared resonance in the excited state, which is observed for clusters of C151 with methanol, ethanol, di-isopropyl ether, and for the “B” conformers of the water n=1, 2 and acetone n=1 clusters. The effect evidently requires no activation energy, since it is observed following electronic-origin excitation. A similar effect is also observed in the doubly-hydrogen-bonded dimers of some aminophthalimide molecules. This result is discussed in terms of possible excited-state proton transfer and Fermi resonance interactions.2.Several clusters have shown changes in the infrared spectra consistent with vibronically-induced conformational rearrangement. The driving force is the relative stabilization of the NH2 proton-donor site in the excited state and the formation of a strong NH⋯O hydrogen bond. The cases noted here are C151/(H2O)2, C120/(H2O)2 and C151/((CH3)2CO)1. One surprising observation here is that the activation energy for the conformational relaxation of C151/(H2O)2 is measured to be only 60 cm−1, and yet the displacement of the water dimer within the cluster exceeds 7 Å.
International Journal of Mass Spectrometry.
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ABSTRACT: Time-correlated single-photon counting experiments at ≈ 25 ps resolution have been used to measure the S2 fluorescence decay time of xanthione in different fluid solvents. Systematic differences in S2 lifetime in the range 20–30 ps are revealed for a homologous series of n-alkane solvents, and an increase in lifetime for extensive chain branching is also noted. Solutions of xanthione in aromatic solvents and in alcohols are found to exhibit fluorescence decay limes < 20 ps. Little evidence is seen which would support a nonradiative relaxation mechanism involving Franck-Condon selective energy transfer to high-energy vibrational modes of the solvent. Instead, the current results provide support for a mechanism in which a reversible solvent-solute chemical reaction, probably involving bond insertion, is responsible for the solvent dependence of the S2 fluorescence decay time.
Chemical Physics Letters.
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ABSTRACT: Fluorescence excitation spectra are reported for van der Waals complexes of 2,5-diphenylfuran (PPF), 2,5-diphenyloxazole (PPO) and 2,5-diphenyloxa-1,3,4-diazole (PPD), with a number of normal alkanes ranging from pentane to dodecane, measured under supersonic jet conditions. It is shown that referencing the spectral red shifts for alkane complexes against the corresponding pyrene S0 → S2 data provides a useful means of detecting structural variations. The PPF complexes with alkanes from heptane to dodecane show an odd—even effect, where n-alkane guests of different symmetries give rise alternately to single and doubled resonances. Thus, even-alkane complexes have two indistinguishable isomers of C1 symmetry, which contribute to a single spectrum. On the other hand, odd-alkane complexes also have a Cs isomer, which has a slightly different electronic origin resonance, so that a double spectrum results, with spacing on the order of 10 cm−1. These conclusions are consistent with the general predictions of atom—atom pair-potential calculations. We also show that further splitting may be induced in the spectra by complexation of the asymmetrical molecule 2,5-diphenyloxazole (PPO). Complexes with the shorter alkanes pentane and hexane show increased complexity in the spectrum, which is attributed to greater flexibility in the complexation geometry.
Chemical Physics.
