1,3(n-π*) excited states of benzaldehyde

Geometries of the first triplet and first singletn-π* excited states of benzaldehyde have been optimized using thesindo 1 molecular orbital wave-function (with CI) and the Newton-Raphson method. The triplet excited state geometry of the molecule is found to be appreciably non-planar whereas that of the singlet excited state is planar. A crossing of molecular orbitals occurs in going from the ground state equilibrium geometry to the triplet and singletn-π* excited state equilibrium geometries. Existence of the para-directing effect of the singletn-π* transition for electronic charges found in an earlier work is confirmed by the present work. The tripletn-π* excitation rearranges electronic charges mainly on the CHO group. It is found that the dipole moment of the molecule would appreciably increase following the singletn-π* excitation whereas the same would appreciably decrease following the tripletn-π* excitation.     

Ab initio Calculation of Normal Vibrations of Chlorobenzene in the First Singlet Electron-Excitebd State 1 B 2

An analysis of experimental assignments of the frequencies of normal vibrations of chlorobenzene in the first singlet electron-excited state ¹ B ₂ has been performed with the use of two quantum-mechanical prediction methods — the method of frequency shifts and the method of transfer of scaling factors. Based on the data of this analysis, a new assignment of a number of vibration frequencies has been made. Normal vibrations of the chlorobenzene molecule in the excited state have been calculated by the CIS method with a 6-311 + G ^** basic set. An algorithm of autoscaling in dependent natural coordinates has been proposed. The force field of chlorobenzene has been scaled in dependent natural coordinates. It is shown that the scaling factors of benzene in the electron state ¹ B _2u can be used for calculating the frequencies of normal vibrations of chlorobenzene in the first singlet excited state ¹ B ₂.     

Rapid internal conversion in a symmetric molecule LD 700 studied by means of femtosecond fluorescence depletion

The rapid internal conversion dynamics at room temperature is determined by using the femtosecond time-resolved fluorescence depletion measurements of a complex solvated molecule of LD 700 (rhodamine 700) combined with steady-state absorption and fluorescence spectroscopy, as well as quantum chemical calculation. The molecule is excited by a 50fs laser pulse at 400nm which directly populated the highly excited singlet state, the rapid internal conversions (ICs) are observed, which leads to the directional changes of the emission transition moment following photoexcitation to the highly excited singlet state S₅ of LD 700.     

Molecular vibrations and lattice dynamics of ortho-terphenyl

Infrared and Raman spectra of crystalline, melted and solvated ortho-terphenyl and its perdeuterated isotopomer, D₁₄-ortho-tephenyl, have been recorded. Optimized geometries and vibrational frequencies were calculated by the semiempirical RHF/AM1 method and by DFT using the B3LYP functional and 6–31G(d) basis set. In both cases the lowest energy conformation is of C₂ symmetry. With the scaled AM1 and B3LYP/6-31G(d) force fields the average error in reproducing the experimental molecular vibrational frequencies is 13cm^?1 and 5cm^?1, respectively. The AM1 potential energy surface for phenyl torsions was mapped on a 15° grid. The barrier to concerted internal rotation is estimated to lie between 3 kJ mol^?1 and 6kJ mol^?1. The calculations of the lattice dynamics at k = 0 in the low temperature fully ordered crystal phase of parent and deuterated ortho-terphenyl were performed with inclusion of six low lying intramolecular vibrations. The conformational change of the ortho-terphenyl molecule induced by crystal packing forces was taken into account by re-defining the unperturbed molecular vibrational state. Although an accurate assignment of lattice vibrations was not possible, the calculated spectra give quite a reasonable picture of the low frequency dynamics in crystalline ortho-terphenyl. The relevance of the results obtained to the glass forming property of ortho-terphenyl is discussed.     

High Resolution B-Type Delayed Fluorescence of Dibenzanthracene in PMMA

B-Type delayed fluorescence of 1,2,3,4-dibenzanthracene in PMMA was experimentally observed for the first time. Dibenzanthracene molecules were exerted in a two-step process. In the first step, an excited singlet S₁ is created, which undergoes intersystem crossing to T₁ then T-T absorption creates an excited triplet dibenzanthracene molecule, which returns to the first exerted singlet level by intersystem crossing. The recreated first excited singlet of dibenzanthracene decays back to the ground state by emitting this new type of delayed fluorescence.     

Identification of excited singlet states of chlorophenol isomers

The UV spectra of optical absorption of para-, meta-, and ortho-chlorophenol are recorded in the gas phase. The bands of UV spectra are assigned to the electronic transitions of molecules to definite excited singlet states on the basis of calculations by the TDDFT B3LYP/6-311++G(d, p) method. In each case the electron configuration making the predominant contribution to the particular singlet state is determined. The energies of singlet electronic transitions are shown to depend on the energy spacing between the molecular orbitals involved in these transitions.     

Photoinduced electron transfer in solutions of A Pd-porphyrin-quinone complex

Photoinduced electron transfer (PET) in a Pd-porphyrin-quinone complex (Pd-P-Q) was investigated using the flash photolysis method in microsecond and picosecond range and by luminescence. The investigations were performed for toluene solutions. Intramolecular PETs in the lower excited singlet state (k_CT=1.7·10¹⁰ sec⁻¹) and in the triplet state (k_CT=2.5·10⁸sec⁻¹) were observed. For a quantum yield of the triplet state that is close to 1 in Pd-porphyrin (Pd-P) this yield is equal to 0.4 in Pd-P-Q. This decrease is the result of PET in the excited state and, possibly, in an unrelaxed triplet state. Triplet Pd-P-Q molecules were efficiently quenched by unexcited molecules, and this process is related to intermolecular charge transfer. This electron transfer is likely to occur from the Pd-P protion of an excited molecule to the quinone protion of an unexcited molecule in the collisional complex (Pd-P-Q)₂. Charged radicals, formed once the collisional complex is transferred and separated, have a wide absorption band with a maximum of about 960 nm. The average lifetime of the radicals was about 2 msec. Institute of Molecular and Atomic Physics, Academy of Sciences of Belarus, 70, F. Skorina Ave., Minsk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No 1 pp. 61–70, January–February, 1997.     

Theoretical calculation of an external heavy-atom effect on the spin-orbit coupling of the benzene molecule

The mechanism of direct spin-orbit coupling arising from electrostatic interaction of the electrons on the perturbed molecule with the nuclei of the perturber is investigated theoretically for the case of benzene in the presence of external heavy atoms. Matrix elements are evaluated using Slater-type orbitals and the von Neumann expansion, and by using gaussian-type orbitals. Results of the present calculation confirm the experimental evidence (from polarization studies by Giachino and Kearns and phosphorescence measurements by Hofeldt, Sahai and Lin) that mixing by the perturber of the triplet state of the molecule with its singlet states is not the mechanism for introducing intensity into the radiative T ₁ - S ₀ transition.     

Intramolecular excitation transfer. The lowest n→π* transitions in pyrazine

In the pyrazine molecule, which has filled non-bonding orbitals para to each other, two orbitally degenerate n→π* transitions are expected, one symmetry forbidden and one symmetry allowed. Interelectronic interactions remove this degeneracy. The lowest pair of (n, π*) triplet states and the corresponding pair of singlet states are studied with a view towards determining the magnitude of such interactions and the ordering of the forbidden and allowed components. Absorption spectra are obtained of pyrazine in crystalline hydrogen and rare gases at 4·2°k, of pure crystalline pyrazine at 4·2°k, and of 42 metres of pyrazine vapour at various pressures. The phosphorescence spectrum of pyrazine in crystalline rare gases at 4·2°k also is studied. The splitting between the two singlet components is found to be approximately 435 cm^?1 with the forbidden component lying lowest. The forbidden singlet-singlet transition gains some of its intensity through vibronic mixing with a (π, π*) state, but vibronic coupling between the two (n, π*) states also may be present. The strongest part of the singlet-triplet absorption spectrum is found to involve the same upper state as the phosphorescence spectrum, and the transition is shown to be symmetry allowed. It is strongly suggested that the ordering of the allowed and forbidden components of the triplet state is inverted from that of the singlet, or the two states may lie very close together. Using an ‘independent systems’ model, a calculation of the splitting, with the inclusion of exchange is made. The theory indicates that the singlet states are indeed split by a coulomb term having roughly the expected magnitude with the forbidden component lying lowest; in addition, there is a very small exchange term with the opposite sign. The splitting between the triplets is shown to involve only the small exchange term, and the order of the allowed and forbidden components becomes reversed. There is no indication of pyrazine fluorescence at 4·2°k in any of the solids used. This fact illustrates the high efficiency of the singlet-triplet radiationless process even under these conditions where the rate might be expected to be a minimum for condensed phases. An interesting alternation of spacing has been discovered in the first few quantum levels of the 600 cm^?1 a _1g ring-bending vibration of the ground electronic state of the molecule.     

飞秒时间分辨拉曼光谱用于研究β-胡萝卜素单重激发态内转换和振动弛豫过程

搭建了飞秒时间分辨受激拉曼光谱(FSRS)装置,并用于研究全反式β-胡萝卜素单重电子激发态超快内转换和振动弛豫过程.基于三脉冲“抽运-探测”方案搭建的时间分辨受激拉曼光谱装置同时实现了150fs的时间分辨率和23.7cm^-1的光谱分辨率,光谱检测范围为300—4000cm^-1.对全反式β-胡萝卜素电子激发态的飞秒时间分辨拉曼光谱研究表明,β-胡萝卜素被激发到S₂态后,经由寿命约为0.3ps的中间态S_X态实 关键词： 飞秒时间分辨拉曼光谱 β-胡萝卜素 激发态内转换 振动弛豫     

Microwave spectroscopy of the weakly bound CO-<Emphasis Type="Italic">ortho</Emphasis>-D<Subscript>2</Subscript> molecular complex

A van der Waals complex that consists of a deuterium molecule in the ortho state and a carbon monoxide molecule, CO-ortho-D₂, was studied in the frequency range 85–130 GHz with the help of an intracavity orotron-based spectrometer. Nine new lines, which correspond to rotational transitions and belong to the R and Q branches, were measured and identified. The positions of the rotational energy levels of CO-ortho-D₂ were refined by comparative analysis of the transition frequencies measured in this work and previously reported microwave and infrared data.     

The weakly bound complex NH3-CO: Observation and analysis of spectra in the infrared (C-O stretching) and millimetre wave (76–106 GHz) regions

The van der Waals complex NH₃-CO has been studied in the infrared and millimetre wave regions, and analysed on the basis of the distinction between the ortho- and para- NH₃-CO nuclear spin species. The spectrum and energy level pattern of ortho-NH₃-CO are relatively simple, closely resembling those of the Ne-CO and Ar-CO complexes. The situation for para-NH₃-CO is somewhat more complicated, with observed states having K values which differ by ±1 from that of the analogous orthoNH₃-CO state. This indicates that the lowest energy orientation of para-NH₃-CO in the complex lies in a II configuration. The present results correlate very well with the only previous study of NH₃-CO, in the microwave region, and clarify the nature of the rotational series observed there. The infrared band centre of NH₃-CO is only very slightly shifted (<0.2cm^?1) from that of the free CO molecule, indicating that the CO vibration has little effect on the intermolecular forces.     

Photoluminescence study of new lanthanide complexes with benzeneseleninic acids

New lanthanide complexes with benzeneseleninic (ABSe) and 4-chloro-benzeneseleninic (ABSeCl) acids have been synthesized and characterized by elemental analysis, infrared and UV–visible spectroscopies. The emission spectra of the trivalent europium complexes presented the typical electronic ⁵D₀→⁷F_J transitions of the ion (J=0–4). The ground-state geometries of the europium complexes have been calculated by using the Sparkle/AM1 model. From these results, the 4f–4f intensity parameters and energies of the ligand singlet and triplet excited states have been obtained. The lower emission quantum yield for the [Eu(ABSe)₃(H₂O)₂](H₂O)₂ compound, as compared to the [Eu(ABSeCl)₃(H₂O)₂] one, can be associated to the higher numbers of water molecules, in the first and second coordination spheres, that contribute to the luminescence quenching. The [Eu(ABSe)₃(H₂O)₂](H₂O)₂ complex presents an intermediate state whose energy difference with respect to the first excited singlet state is resonant with three phonons from the water molecules, favouring a multiphonon relaxation process from the singlet state followed by a fast internal conversion process; this effect is less pronounced in the complex with the ABSeCl ligand. The luminescence decay curves of the gadolinium complexes indicate that the level responsible for the intramolecular energy transfer process has a triplet character for both compounds. The nephelauxetic effect in these compounds was investigated under the light of a recently proposed covalency scale based on the concept of overlap polarizability of the chemical bond.     

Ab initio SCF CI study of the electronic spectrum of s-tetrazine

Configuration interaction (CI) studies of ground, n→ π* and π→ π* electronically excited states are reported for s-tetrazine. The first n→ π* singlet excited state (¹ B _3u ), which is responsible for the purple-red colour of the molecule, is calculated at 2·80 eV, compared to the experimental transition energy of 2·22–2·70 eV. The singlet-triplet split of the first n→ π* states (¹ B _3u and ³ B _3u states) is calculated to be 0·76 eV.

The interaction of nitrogen lone pair orbitals (n-orbitals) is studied in terms of the ordering of the n π* excited states and found that the SCF orbital ordering is qualitatively in accord with the ordering of the n π* excited states in the CI level.

The first π→ π* excited state (¹ B _2u ) is calculated at 5·99 eV, slightly above the observed range of absorption. Numerous other high-lying singlet states as well as the triplet states have been calculated and they are used to verify several proposals relating to the excited state dynamics in the photo-physical studies of s-tetrazine.     

On estimating probability of excited-state intramolecular proton transfer

Higher singlet states can play an important role in various intramolecular processes. Recent investigations of the time-resolved (with a picosecond resolution) spectra of the dual fluorescence of 3-hydroxyflavone molecules excited in the region of the S ₁ and S ₂ absorption bands by pulses with a duration of ∼44 ps have directly shown the occurrence of the proton transfer from the carboxyl to the carbonyl group of the molecule upon excitation into the second singlet absorption band. The reaction times estimated from the emission characteristics are comparable with the electronic level lifetime (several picoseconds), as a result of which the direct measurements are rather difficult. The proton transfer through the S ₂ state is also recorded in the steady-state fluorescence excitation spectra. In this study, it is shown how the reaction rate can be estimated from these data.     

Improving quantum state transfer in a general XY chain via the Dzyaloshinskyben Moriya interaction

We study the state transfer of Bell states in a general XY spin chain using the Dzyaloshinsky-Moriya interaction. Two symmetries of fidelity with the anisotropy parameter are found. The maximum fidelity is shown to be significantly enhanced in cases of an odd number of sites. Enhancement of fidelity on a singlet state is greater than that on the other Bell states in such cases.     

Steric Effect of an ortho-Alkyl Group on the Stretching Vibration Frequency of Phenol Hydroxyl

It is shown that the derivatives of mono-ortho-alkylsubstituted phenols, which in the ortho-position have primary and secondary alkyl groups, and also tertiary alkyl groups have different correlations between the frequencies of the stretching vibration of the OH bond and Pal'm's steric constants E _s ⁰.     

Singlet and Triplet State Properties and Singlet Oxygen Yield of an Amino-Acyl-Quinoxalinone Yellow Dye

Amino-acyl-quinoxalinone yellow dyes are cyclised analogues of the yellow azomethine dyes developed for, and still used in, silver halide colour photography. Unlike image azomethine dyes, which are rapidly deactivated in their excited states by torsion about the azomethine bond, amino-acyl-quinoxalinone dyes have an interesting photophysics because torsion is not possible due to their cyclised structure. We report results from studies on singlet and triplet state properties, and singlet oxygen yields, of the yellow dye, 7-diethylamino-3-(2,2-dimethyl-propionyl)-5-methyl-1-phenyl-1H-quinoxalin-2-one, in polar and nonpolar solvents. The dye photophysics is characterised by a weak fluorescence, with a solvent dependent emission yield (Φ_F?≈?0.002–0.004), and short singlet state lifetime (τ_expt?≈?20–50 ps), both increasing by a factor of ≈2 in going from polar acetonitrile to non-polar dioxane as solvent. DFT ZINDO calculations show a transition involving significant electron transfer from the diethyl-amino group into the carbonyl region of the molecule. In solution, in the presence of oxygen, the triplet state decays almost exclusively by oxygen quenching, and singlet oxygen is produced in high yield (Φ_??≈?0.5–0.55). The triplet state absorbs across the 450–750 nm region with maxima around 480 and 650 nm, and moderate molar absorption coefficients (ca. 6000–8000 M^?1 cm^?1). In a glass at 77 K, triplet decay gives a red phosphorescence, with λ_max?≈?640–650 nm, and a ?≈?0.25 s lifetime. If singlet oxygen yields are a good indication of triplet yields, then internal conversion and intersystem crossing occur with roughly equal efficiency.

     

Theoretical insight into the open‐shell singlet diradical character of single molecular solvated dielectron e2@CnFn (n = 20, 28, 36, 50, 60, and 80)

Can there exist an open‐shell (OS) singlet diradical in the electronic ground state of the solvated dielectron? Here, we presented a comparison study by different cage‐shaped e₂@C_nF_n (n = 20, 28, 36, 50, 60, and 80) at unrestricted broken spin‐symmetry density functional theory. It is found that both the stability and the singlet diradical character of the molecule increase with increasing excess electron encapsulation space (size of the cage). For the e₂@C_nF_n (n = 50, 60, and 80), the electronic ground states have obvious special OS singlet diradical characters. Among these OS e₂@C_nF_n (n = 50, 60, and 80), the e₂@C₅₀F₅₀ with intermediate diradical character (0.658) is the most stable. The two semispheres in each highest occupied molecular orbital (α and β) of these diradicals suggest that the two excess electrons are simultaneously encapsulated inside the different regions of the cage, respectively, to form special broken s‐type 3excess electron pair.     

Excitation of lowest electronic states of the uracil molecule by slow electrons

The excitation of lowest electronic states of the uracil molecule in the gas phase has been studied by electron energy loss spectroscopy. Along with excitation of lowest singlet states, excitation of two lowest triplet states at 3.75 and 4.76 eV (±0.05 eV) and vibrational excitation of the molecule in two resonant ranges (1?C2 and 3?C4 eV) have been observed for the first time. The peak of the excitation band related to the lowest singlet state (5.50 eV) is found to be blueshifted by 0.4 eV in comparison with the optical absorption spectroscopy data. The threshold excitation spectra have been measured for the first time, with detection of electrons inelastically scattered by an angle of 180°. These spectra exhibit clear separation of the 5.50-eV-wide band into two bands, which are due to the excitation of the triplet 1³ A?? and singlet 1¹ A?? states.