The isomers of Si2H4: disilene and silylsilylene

Geometries and relative energies of the lowest singlet and triplet states of disilene and silylsilylene have been investigated using ab initio SCF and CI methods. At the most reliable level of theory employed, singlet disilene is predicted to be 8–10 kcal/mol more stable than singlet silylsilylene.     

The structure of protonated disilene

The open and cyclic forms of Si₂ H₅⁺ have been calculated at the SCF level and with inclusion of electron correlation energy. Our final results indicate that both structures are about equally stable. The proton affinity of disilene is calculated as 207 kcal/mol.     

Potential-energy surfaces in singlet and triplet silylene

MC SCF calculations with the 6-31G^** basis set, augmented by second-order configuration interaction, are used to analyze the potential-energy curves in the lowest singlet and triplet states of SiH₂. The adiabatic singlet-triplet splitting is predicted to be 17.7 kcal/mol, and the curves are predicted to cross at ≈ 129°. Thus, bulky substituents may open the angle in the singlet sufficiently to invert the ordering of the two states.     

Calculated photoelectron spectra of singlet and triplet methylene

Vertical ionization potentials for singlet and triplet methylene are calculated by a CI perturbation method based on ab initio SCF molecular orbitals (6–31 G^** basis). The shape and vibrational fine structure of the first photoelectron band are investigated using the MINDO/3 method. The computed singlet-triplet splitting for methylene is 16.4 kcal/mol, in reasonable agreement with the experimental value of 19.5 kcal/mol.     

Low-lying singlet and triplet electronic states of RhB

The low-lying XSigma+, a3Delta, A1Delta, b3Sigma+, B1Pi, c3Pi, C1Phi, D1Sigma+, E1Pi, d3Phi, and e3Pi electronic states of RhB have been investigated at the ab initio level, using the multistate multiconfigurational second-order perturbation (MS-CASPT2) theory, with extended atomic basis sets and inclusion of scalar relativistic effects. Among the eleven electronic states included in this work, only three (the X1Sigma+, D1Sigma+, and E1Pi states) have been investigated experimentally. Potential energy curves, spectroscopic constants, dipole moments, binding energies, and chemical bonding aspects are presented for all electronic states.     

Studies on the singlet and triplet states of unsaturated carbenes

Quantum chemical calculations and studies have been made on the different spin states of unsaturated carbenes X₂C?C: and X₂C?C?C: (X?H, Li, F). The geometries and relative stabilities of these carbenes with various groups X have been outlined. The ground states of unsaturated carbenes are all singlet. The energy splittings between the ground states and first excited states ΔE (¹A₁-³B₁) are generally within the value of 60 kcal/mol and change greatly with the electronegativities of groups X, but little with the sizes of the cumulidenes. The equilibrium conformations of ¹A₁ and ³B₁ are different.     

不饱和卡宾的单重态与三重态

对X2C=C:和X2C=C=C:(X=H,Li,F)进行了量子化学计算和研究,得到了不饱和卡宾单重态(^1A1)和三重态(^3B1)的几何构型及其相对稳定性随取代基X电负性变化的一般规律,结果表明,不饱和卡宾的基态都是单重态,第一激发态与基态的能量差ΔE(^1A1 - ^3B1)随X的电负性大小而变化,其值一般在60kcal.mol^-^1以内,累积烯基的大小对ΔE(^IA1-^3B1)的影响相对较小,^IA1和^3B1的平衡几何构型并不相同。     

Charge densities for singlet and triplet electron pairs

Analytic properties of charge densities associated with singlet and triplet electron pairs, ρ₀( r ) and ρ₁( r ), are presented. In an N‐electron system with total spin S, distributions ρ₀( r ) and ρ₁( r ) are independent of the spin projection quantum number M (spin rotation invariance), as opposed to the usual spin‐up and spin‐down electron densities, ρ_α( r ) and ρ_β( r ). We derive equations showing that in the case of a wave function which is a spin‐eigenfunction, ρ₀( r ) and ρ₁( r ) are linear combinations of the total charge density ρ( r ) and the uncompensated spin density ρ_s( r )=[ρ_α( r )−ρ_β( r )]/2M. For a wave function which is not an eigenfunction of $\mathcal{S}^{2}$, no such relationship exists. In a related discussion, a definition of the high‐spin solution corresponding to a given spin‐unrestricted Hartree–Fock wave function is proposed, and a notion of effectively unpaired electrons is introduced. The distributions ρ₀( r ) and ρ₁( r ) are shown not to be invariant under spin coupling between isolated systems. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 77: 651–660, 2000     

Photochemistry of 1,2-dihydronaphthalene oxide: concurrent triplet and singlet processes via singlet excitation

The photochemistry of 1,2-dihydronaphthalene oxide (254 nm) was reexamined and indan was found to be a primary photoproduct, as well as the traditionally assumed secondary photoproduct. Quenching studies demonstrated that indan, as a primary photoproduct, is derived from a triplet pathway, competing with a singlet route, back to the ground state surface. CASSCF calculations strongly suggest that the triplet pathway consists of a dissociation of the oxirane moiety to give a triplet carbene and aldehyde, which via hydrogen abstraction-decarbonylation-ISC recloses to give indan. Conical intersections corresponding to the presumed 1,2-hydrogen shift and 1,2-alkyl shift to give 2-tetralone and 1-indancarbaldehyde, respectively, were located computationally.     

硅甲基硼烯异构化反应机理研究所

用量子化学理论方法研究了硅甲基硼烯(SiH~3B)单重态和三重态异构化反应,计算了有关化合物的电子结构, 优化了过渡态构型, 讨论了两个反应的控制类型。     

Theoretical study of the excited singlet and triplet states of alloxan

The possibility of excited‐state protomeric shifts in the biologically important molecule, alloxan, is investigated. We have focused on the S₁ and T₁ excited states of alloxan and its hydroxy tautomers. Modifications brought in by excitation on the relative stabilities, activation barriers, and optimized geometries, computed at the MNDO, AM1, and PM3 levels of approximation, have been discussed for both excited electronic states. The absorption and fluorescence spectra for the three tautomers are also discussed. Results show significant changes in the geometries on excitation, although the changes are similar for the singlet and triplet excited states. Though the relative stability orders do not change, the 2‐hydroxy tautomer is stabilized, while the 4‐hydroxy tautomer gets destabilized on excitation. The excited states are (n,π*) states, involving the promotion of a nonbonding oxygen lone pair from the CO? CO? CO moiety, which explains why the oxygens of this group become less basic and the 4‐hydroxy tautomer gets destabilized on excitation. However, the activation barriers do not reduce significantly on excitation, and this precludes the possibility of ground‐ or excited‐state proton transfer in the gas phase. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001     

Saturable absorption dynamics in the triplet system and triplet excitation induced singlet fluorescence of some organic molecules

The triplet saturable absorption behaviour of the xanthene dyes eosin Y, erythrosin B, and rose bengal and of the fullerene molecule C₇₀ is studied. The molecules are excited to the S₁-state by intense picosecond pulses (wavelength λ_P=527 nm). They relax dominantly to the triplet system by intersystem crossing. The triplet–triplet saturable absorption is investigated with time-delayed intense picosecond pulses (wavelength λ_L=1054 nm) in the transparency region of the molecules in the singlet ground state. Higher excited-state triplet absorption cross-sections and higher excited-state triplet relaxation times are determined by numerical simulation of the experimental results. Time-resolved fluorescence measurements reveal higher excited-state triplet to singlet back-intersystem-crossing and multi-step triplet photoionization. Additionally the two-photon absorption cross-sections at λ_L=1054 nm are determined by measurement of the fundamental pulse two-photon induced fluorescence relative to the second-harmonic pulse single-photon induced fluorescence.     

Solvent effects on the singlet - triplet equilibrium and reactivity of a ground triplet state arylalkyl carbene

Results from intramolecular singlet and triplet specific reactivity in solvents of different Polarity suggest that the spin state equilibrium of 1,2-diphenyl-1-butylidene, a triplet ground state carbene. is largely susceptible to solvent polarity. The results are consistent with stabilization of the zwitterionic singlet state in solvents of high polarity.     

Theoretical study of singlet and triplet excitation energies in oligothiophenes

We have analyzed singlet and triplet excitation energies in oligothiophenes (up to five rings) using time-dependent density-functional theory (TD-DFT) with different exchange-correlation functionals and compared them with results from the approximate coupled-cluster singles and doubles model (CC2) and experimental data. The excitation energies have been calculated in geometries obtained by TD-DFT optimization of the lowest excited singlet state and in the ground-state geometries of the neutral and anionic systems. TD-DFT methods underestimate photoluminescence energies but the energy difference between singlet and triplet states shows trends with the chain-length similar to CC2. We find that the second triplet excited state is below the first singlet excited state for long oligomers in contrast with the previous assignment of Rentsch et al. (Phys.Chem. Chem. Phys. 1999, 1, 1707). Their photodetachment photoelectron spectroscopy measurements are better described by considering higher triplet excited states.     

Study of singlet—triplet coupling in glyoxal by level anticrossing spectroscopy. V. Nature of singlet—triplet interaction

We observe in glyoxal cooled in a supersonic free jet the fluorescence of individual rotational levels of the S₁ state excited by a cw laser. We use the technique of singlet—triplet magnetic resonance near an anticrossing to measure matrix elements V₃₁ as a function of rotational quantum numbers N_s, N_t, K_s, K_t. The experimental results are compared with theoretical models of singlet—triplet couplings and we show that the spin-vibronic interaction is the dominant singlet—triplet interaction in glyoxal.     

Quantitative aspects of all-trans-retinol singlet and triplet quenching by oxygen

In benzene and methanol, the oxygen quenching of all-trans-retinol singlet results in intersystem crossing (enhanced) and energy transfer (to form ¹O₂^*, ¹Δ_g), both with quantum efficiencies considerably less than unity (0.3–0.5). ¹O₂^* formation from retinol triplet quenching by ³O₂ is closer to quantitative (≈ 0.8).     

Hole-induced quenching of triplet and singlet excitons in conjugated polymers

Quantitative information on the mechanisms and rates of hole (radical cation)-induced quenching of triplet and singlet excitons in the conjugated polymer poly[2-methoxy-5-(2'-ethylhexyloxy)-p-phenylene vinylene] has been acquired by a new technique, fluorescence-voltage time-resolved single molecule spectroscopy (FV-TR-SMS). FV-TR-SMS measures the fluorescence intensity of a single conjugated polymer molecule that is embedded in a capacitor-like device while simultaneously modulating the bias on the device and the irradiation intensity. The results demonstrate that triplet excitons are efficiently quenched by holes in conjugated polymers for hole densities >10(16) charges/cm(3), while singlet excitons are quenched with a much lower efficiency. Detailed kinetic analysis shows that the greater efficiency for quenching of triplets by holes (compared to that for singlets) is due to a >10(6) times longer exciton lifetime for triplets. In fact, the results suggest that while singlet quenching is less efficient due to a much shorter singlet lifetime, the rate constant for the quenching of singlets by holes actually exceeds that for triplets by several orders of magnitude.     

Dual emission of singlet and triplet states boost the sensitivity of pressure-sensing

Pressure-related sensing materials in mechanochromic luminescent materials have received wide attention. However, at present, most piezochromic luminescence (PCL) materials have problems such as aggregation-caused quenching (ACQ) effect due to the presence of powder form, complicated preparation methods and fluorescence quenching effect under high pressure. To solve these problems, we employ three components containing carbon dots (CDs), layered double hydroxides (LDHs) and polyvinyl alcohol (PVA) to construct the CDs-LDHs/PVA film. The LDHs can provide a rigid environment for CDs and improve the luminescent efficiency of CDs. The film shows high sensitivity, stability and reversibility. Moreover, the compressed film can recover to its original state by heating. Therefore, the PCL film with dual emission (fluorescence and phosphorescence) characteristic is constructed, which boosts the sensitivity of pressure-sensing.