Quantum-chemical analysis of vibrational intensity distributions in the S0-S1 absorption spectra and raman excitation profiles of azulene

The vibronic, Franck—Condon and Dushinsky activity of the totally modes of azulene in S₀-S₁ transition is investigated by quantum-chemical methods. It is found that the 900 and 1400 cm⁻¹ mode mixing proposed earlier to explain the relative intensities of Raman fundamentals in the region of the S₁ state is not supported by the calculations. The alternative mechanism arising from vibronic activity of the a₁ modes produces signs of the non-Condon parameters in agreement with those found from fitting the Raman excitation profiles; however, the absolute value of this parameter for the 900 cm⁻¹ mode is almost one order of magnitude too small. Thus, although quantum-chemical calculations favour non-Condon effects over Dushinsky effects in shaping the relative intensities of Raman fundamentals, they do not give a conclusive answer. It is argued that such an answer can be obtained from measurements of the 900 + 1400 cm⁻¹ combination band Raman excitation profiles.     

Laser excited S2 → S1 and S1 → S0 emission spectra and the S2 → Sn absorption spectrum of azulene in solution

We present the S₁ → S₀ fluorescence spectrum, between 740 and 940 nm, of azulene solutions (10^?3 M in methanol) excited with a Q-switched ruby laser. The nitrogen-laser excited S₂ → S₁ fluorescence spectrum, between 700 and 930 nm, is also reported. The transient S₁ → S_n spectrum between 500 and 650 nm was studied, using synchronous nitrogen laser and dye laser excitation. The S₅ (¹B₁(3)) state of azulene was found to be located at 45500 cm^?1 and the cross section σ₂₅ of the transient absorption S₂ → S₅ is estimated to be 3 × 10^?18 cm²/molecule.     

S2 → S0 fluorescence in an aromatic thioketone, xanthione

In addition to the red phosphorescence (T₁(³ A₂n, π^*) → S₀) xanthione exhibits in solution an emission with a maximum at ≈ 23 000 cm⁻¹ and φ_f(298°) = 5 × 10⁻³. It is shown that this emission is fluorescence from the second excited singlet state (S₂ (¹A₁ π, π^*) → S₀).     

Distinction of the delayed fluorescences S1 → S0 And S2 → S0 of pyrene by selective quenching Of S1 → S0

In the spectrum of the delayed fluorescence (DF) of pyrene, caused by triplet-triplet annihilation T₁ + T₁ → S_n + S_o (n = 1,2), a strong DF S₁ → S_o and a very weak DF S₂ → s₀ are observed. The DF S₁→ S_o is quenched selectively by compounds like N-diethylanine or triethylamine which do not quench T₁ of pyrene.     

Relative quantum yield of the S2 → S1 fluorescence from azulene

The quantum yield ratio r = φ_{2 → 0}/φ_{2 → 1} of the S₂ → S₀ and S₂ → S₁ fluorescences from azulene has been redetermined. With azulene in isopentane at 190 K, r = 455 ± 100. This value agrees with the lower limit, given by Huppert, Jortner and Rentzepis, but is an order of magnitude lower than that given by Gillispie and Lim.     

Charge transfer and van der Waals states of jet-cooled dialkylaniline complexes with anthracene

We report here a study of the influence of a physical parameter (i.e. the ionization energy of different donor aromatic molecules) on the spectroscopic and dynamic properties of a series of molecular complexes A-D (A acceptor, D donor) where A is the anthracene molecule and D is one of the following dialkylanilines: dimethyl, diethyl, dipropyl or dibutyl. All complexes exhibit the same spectroscopic behavior already observed for dimethylaniline and diethylaniline cases and tentatively explained by the existence of two isomeric forms for each complex. Decay times and the broad band maximum frequency shifts for the exciplex-like emission type are experiencing a continuous variation in agreement with the change of the ionization energy of the donors. This is confirmed by calculations done using a very simple model based on the interactions between the diabatic A*D and A⁻D⁺ states of the complexes. This agreement is in favor of the initial assumption, that most physical parameters (complexation geometry, coupling between the diabatic states) are only weakly perturbed when changing the donor molecule and this despite an expected increased steric hindrance.     

Comparative sensibility of S1 ←S0 and S2 ←S0 indole electronic transitions to environment perturbations. The positions of the 0—0 bands in polar media

The modifications under environmental perturbations of the well separated two first electronic systems of carbazole are used to demonstrate the possibility of including solute-ether complexes in polyethylene films. The application of this technique to indole allows to differentiate the overlapping ¹L_a and ¹L_b transitions in the spectrum of hydrogen bonded indole. The ground state interactions of indole, 5-methoxyindole and 3-methylindole in several well defined environment conditions (hydrogen bond in a polar cage, hydrogen bond in a nonpolar cage) are investigated through UV absorption spectroscopy between 293 and 88 K. A strong red shift of the ¹L_a bands under the hydrogen bonding occurs with all the indoles investigated. The comparative analysis of the spectra in three types of environment, allows to give the accurate location of the first bands for the two overlapping vibronic systems of these molecules in polar media. Particularly, it is unambiguously shown that the first ¹L_a band and the first ¹L_b band of indole in alcohols are superposed at 288 nm. These results offer reliable new bases for the study of the emission properties of indole and its derivatives.     

Solvent effect on the non-radiative S2(ππ*) decay of xanthione

The fluorescence quantum yield of the S₂(ππ*) state of xanthione has been measured in various solvents at room temperature. With these data, the strong solvent effect on the subnanosecond, non-radiative S₂ decay has been assessed.     

Rotational spectra of the van der Waals complexes of molecular hydrogen and OCS

The a- and b-type rotational transitions of the weakly bound complexes formed by molecular hydrogen and OCS, para-H2-OCS, ortho-H2-OCS, HD-OCS, para-D2-OCS, and ortho-D2-OCS, have been measured by Fourier transform microwave spectroscopy. All five species have ground rotational states with total rotational angular momentum J=0, regardless of whether the hydrogen rotational angular momentum is j=0 as in para-H2, ortho-D2, and HD or j=1 as in ortho-H2 and para-D2. This indicates quenching of the hydrogen angular momentum for the ortho-H2 and para-D2 species by the anisotropy of the intermolecular potential. The ground states of these complexes are slightly asymmetric prolate tops, with the hydrogen center of mass located on the side of the OCS, giving a planar T-shaped molecular geometry. The hydrogen spatial distribution is spherical in the three j=0 species, while it is bilobal and oriented nearly parallel to the OCS in the ground state of the two j=1 species. The j=1 species show strong Coriolis coupling with unobserved low-lying excited states. The abundance of para-H2-OCS relative to ortho-H2-OCS increases exponentially with decreasing normal H2 component in H2He gas mixtures, making the observation of para-H2-OCS in the presence of the more strongly bound ortho-H2-OCS dependent on using lower concentrations of H2. The determined rotational constants are A=22 401.889(4) MHz, B=5993.774(2) MHz, and C=4602.038(2) MHz for para-H2-OCS; A=22 942.218(6) MHz, B=5675.156(7) MHz, and C=4542.960(7) MHz for ortho-H2-OCS; A=15 970.010(3) MHz, B=5847.595(1) MHz, and C=4177.699(1) MHz for HD-OCS; A=12 829.2875(9) MHz, B=5671.3573(7) MHz, and C=3846.7041(6) MHz for ortho-D2-OCS; and A=13 046.800(3) MHz, B=5454.612(2) MHz, and C=3834.590(2) MHz for para-D2-OCS.     

Fluorescence of DPH derivatives. Evidence for emission from S2 and S1 excited states

Absorption, fluorescence excitation and emission spectra, quantum yields, and fluorescence decay times have been measured for several derivatives of all-trans 1,6-diphenyl-1,3,5-hexatriene (DPH) in a number of solvents. Emission occurs from the low-lying ¹Ag^*_g state and the ¹B^*_u which is at slightly higher energy. Accepted models for the photophysics of DPH are shown to be inadequate.     

Structures of 1,2,3-trihydroxybenzene in the S0 and S1 states

In this paper we report on the structure and vibrations of gaseous pyrogallol (1,2,3-trihydroxybenzene) in the electronic ground state (S₀) and its first electronically excited state (S₁). Both ab initio CASSCF/CASMP2 calculations as well as R2PI spectroscopy have been performed. From the ab initio calculations three minimum energy structures are obtained and the vibrations of two structures are observed in the R2PI spectra. The minimum energy structures differ by their OH torsional angles. The full three-dimensional potential energy surface of the coupled torsional motions is investigated and the three-dimensional eigenvalues are calculated. The most stable structure of pyrogallol contains two intramolecular hydrogen bonds and turns out to be planar in the S₀ state. In the S₁ state the free OH group is rotated out of the plane of the aromatic ring by about 40°. The strong change in geometry of this structure is predicted by the CASSCF calculations and confirmed by the R2PI spectra of pyrogallol and its deuterated species. The low frequency region of the R2PI spectra can be explained by a torsional motion and the out of plane vibration 17b.     

High resolution infrared spectra of H2-Kr and D2-Kr van der Waals complexes

Infrared spectra of weakly bound hydrogen-krypton complexes have been studied at high spectral resolution (0.04 cm(-1)) using a long-path (154 m) low temperature (100 K) absorption cell and a Fourier transform spectrometer. In addition to spectra from the regions of the H(2) and D(2) fundamental vibrational bands in the midinfrared, the results also include the region of the pure rotational S(0)(0) transition of H(2) in the far infrared. A total of 219 measured line positions from these spectra have been fully assigned to specific quantum transitions and form the basis for determining a greatly improved semiempirical three-dimensional intermolecular potential energy surface for hydrogen-krypton in an accompanying paper.     

Two Step Chemical Vapor Deposition of In2Se3/MoSe2 van der Waals Heterostructures

Two-dimensional transition metal dichalcogenides heterostructures have stimulated wide interest not only for the fundamental research,but also for the application of next generation electronic and optoelectronic devices.Herein,we report a successful two-step chemical vapor deposition strategy to construct vertically stacked van der Waals epitaxial In₂Se₃/MoSe₂ heterostructures.Transmission electron microscopy characterization reveals clearly that the In₂Se₃ has well-aligned lattice orientation with the substrate of monolayer MoSe₂.Due to the interaction between the In₂Se₃ and MoSe₂ layers,the heterostructure shows the quenching and red-shift of photoluminescence.Moreover,the current rectification behavior and photovoltaic effect can be observed from the heterostructure,which is attributed to the unique band structure alignment of the heterostructure,and is further confirmed by Kevin probe force microscopy measurement.The synthesis approach via van der Waals epitaxy in this work can expand the way to fabricate a variety of two-dimensional heterostructures for potential applications in electronic and optoelectronic devices.     

Photochemistry in van der Waals complexes: Observation of the intermediate state of the Hg*,Cl2 reaction

A novel method designed to observe the collision complex of a photochemical reaction is reported here. The reactants Hg, Cl₂ are frozen in a van der Waals complex (HgCl₂), and then promoted by an optical excitation (250 nm) to the reactive state. The broad complex action spectrum, presumably due to the Hg⁺-Cl₂^? intermediate, is monitored through the HgCl(B ²Σ⁺) fluorescence.     

Bi2SeO5: A single-crystalline van der Waals dielectric beyond h-BN

<正>1School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore;2CINTRA CNRS/NTU/THALES, UMI 3288, Research Techno Plaza, Singapore 637553, Singapore     

Interaction potential for He-H2 in the region of the van der Waals minimum

Interaction energies for the He-H₂ system have been computed by a VB method for the intermolecular distances 5.2 ? R ? 20.0 bohr and two different orientations of the H₂ molecule (θ = 0°, 90°). The results, especially for the anisotropy, are in good agreement with experiment. The dispersion constants C₆ and C₈ and the corresponding anisotropy constants α₆ and α₈ are presented.     

Lithium ions in the van der Waals gap of Bi2Se3 single crystals

Insertion/extraction of lithium ions into/from Bi₂Se₃ crystals was investigated by means of cyclic voltammetry. The process of insertion is reflected in the appearance of two bands on voltammograms at ∼1.7 and ∼1.5 V, corresponding to the insertion of Li⁺ ions into octahedral and tetrahedral sites of the van der Waals gap of these layered crystals. The process of extraction of Li⁺ ions from the gap results in the appearance of four bands on the voltammograms. The bands 1 and 2 at ∼2.1 and ∼2.3 V correspond to the extraction of a part of Li⁺ guest ions from the octahedral and tetrahedrals sites and this extraction has a character of a reversible intercalation/deintercalation process. A part of Li⁺ ions is bound firmly in the crystal due to the formation of negatively charged clusters of the (LiBiSe₂.Bi₃Se₄⁻) type. A further extraction of Li⁺ ions from the van der Waals gap is associated with the presence of bands 3 and 4 placed at ∼2.5 and ∼2.7 V on the voltammograms as their extraction needs higher voltage due to the influence of negative charges localized on these clusters.     

Delayed fluorescence S2 → S0 of azulene in isopentane,due to hetero-triplet—triplet annihilation of 3azulene* and 3fluoranthene*

The lowest triplet state of azulene, T₁(Az), can be populated efficiently by triplet energy transfer from the lowest triplet state of fluoranthene, T₁(F1). In isopentane at temperatures 120 K ? T ? 193 K a delayed fluorescence S₂(Az) → S₀(Az) is found, caused by hetero-triplet—triplet annihilation T₁(Az) + T₁(Fl) → S₂(Az) + S₀(F1).     

Structure and bonding studies of ClX-C2H4 (X = H or Br) van der Waals complexes

High-quality ab initio calculations at the MPn (n = 2, 3 or 4) levels for the π-donor complex formed by HCl and ethene, and the recently characterized ClBr-ethene complex are presented. Interaction energies were calculated with the inclusion of both basis-set superposition error and zero-point energy corrections, resulting in values of about 1 kcal mol⁻¹ for both complexes. The total charge densities for both complexes yielded molecular graphs indicative of the weak binding in these molecules, and correspondingly, calculated charges showed that the XCl moiety has an overall slightly negative charge, while the ethene moiety is slightly positive. Analysis of the Laplacian of the charge density showed that the geometry of the ClBr- C₂H₄ complex may be understood in terms of the VSEPR model as an example of an AXYE₃ molecule.     

Effect of spin-orbit coupling in para-dihalogenobenzenes on the environment-induced frequency changes in their S1 ← S0 spectra

The consequences of spin-orbit coupling in p-dihalogenobenzenes for solvent-induced shifts or S₁ ← S₀ spectral transitions, and for changes of S₁-state vibrational frequencies compared with the S₀ state, are assigned to mixing low-energy triplet states into the singlet-state system.