The “phantom” photochemical singlet state of stilbene and its diphenylpolyene relative

Recently the conception on the cis-trans photoisomerization of stilbene has emerged that this process is governed by a higher excited ¹A_g state which exhibits a minimum at the perpendicular conformation crossing the lowest excited ¹B_u state upon bond rotation. An evaluation of the potential surfaces governing the photoisomerization process by a PPP SCF CI method revealed that there exists indeed such a photochemically active ¹A_g state which in the planar molecule lies about 1 eV above the lowest absorption band and involves the excitations of two electrons from the SCF ground state. Extensions of the calculations to diphenylpolyenes demonstrate that this ¹A_g state is related to the forbidden low-lying doubly excited ¹A_g state observed earlier in these molecules.     

Fully resonant CARS of cresyl violet in polyacrylic acid polymer films

Multi-resonant CARS data for ground and excited electronic state resonances (593.585 cm^?1) of cresyl violet perchlorate in polyacrylic acid are reported. The intensity of the excited state resonance (585 cm^?1) depends on the location of the ω₁-field (ω_as = 2ω₁ ? ω₂) within the severely inhomogeneously broadened absorption profile of the dye. Non-photochemical hole burning is used to determine the vibronic transitions which contribute to the absorption profile. It is argued that the linear electron-phonon interaction is an important mechanism for producing an egalitarian distribution of excited dye sites with a population sufficiently high to permit observation of the excited state resonance. A marked non-photochemical hole burning effect on the intensities of the CARS resonances is used for the assignment of 585 cm^?1 as an excited state resonance. The absence of line narrowing in the CARS and CSRS spectra is reported and discussed. Finally, a novel narrowing of the 593 cm^?1 ground state resonance with increasing temperature is reported and shown to occur only for restricted values (frequency) of ω₁.     

Polyene spectroscopy: Vibronic evidence for a forbidden transition in DECA-2,4,6,8-tetraene

The dimethylpolyene deca-2,4,6,8-tetraene was studied by absorption, fluorescence excitation and fluorescence spectroscopy in glasses at 77 K and in n-alkane crystals at 4.2 K. A strong transition to a ¹B_u excited state is observed with an origin at 32400 cm^?1 in isopentane at 77 K and at 31280 cm^?1 in n-undecane at 4.2 K. A weak transition to a ¹A_g excited state is observed with an origin at 28738 cm^?1 in the n-undecane matrix. The radiative fluorescence lifetime is 500 ns. In undecane the transition from the ground state to the ¹A_g excited state exhibits a classic Herzberg—Teller vibronic pattern indicating a symmetry forbidden transition.     

Theoretical study of the π→π* excited states of linear polyenes: The energy gap between 11Bu+ and 21Ag− states and their character

Multireference perturbation theory with complete active space self-consistent field (CASSCF) reference functions is applied to the study of the valence π→π* excited states of 1,3-butadiene, 1,3,5-hexatriene, 1,3,5,7-octatetraene, and 1,3,5,7,9-decapentaene. Our focus was put on determining the nature of the two lowest-lying singlet excited states, 1¹B_u⁺ and 2¹A_g⁻, and their ordering. The 1¹B_u⁺ state is a singly excited state with an ionic nature originating from the HOMO→LUMO one-electron transition while the covalent 2¹A_g⁻ state is the doubly excited state which comes mainly from the (HOMO)²→(LUMO)² transition. The active-space and basis-set effects are taken into account to estimate the excitation energies of larger polyenes. For butadiene, the 1¹B_u⁺ state is calculated to be slightly lower by 0.1 eV than the doubly excited 2¹A_g⁻ state at the ground-state equilibrium geometry. For hexatriene, our calculations predict the two states to be virtually degenerate. Octatetraene is the first polyene for which we predict that the 2¹A_g⁻ state is the lowest excited singlet state at the ground-state geometry. The present theory also indicates that the 2¹A_g⁻ state lies clearly below the 1¹B_u⁺ state in decapentaene with the energy gap of 0.4 eV. The 0–0 transition and the emission energies are also calculated using the planar C_2h relaxed excited-state geometries. The covalent 2¹A_g⁻ state is much more sensitive to the geometry variation than is the ionic 1¹B_u⁺ state, which places the 2¹A_g⁻ state significantly below the 1¹B_u⁺ state at the relaxed geometry. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 66 : 157–175, 1998     

Merging of a Perylene Moiety Enables a RuII Photosensitizer with Long-Lived Excited States and the Efficient Production of Singlet Oxygen

Multichromophoric systems based on a Ru^II polypyridine moiety containing an additional organic chromophore are of increasing interest with respect to different light-driven applications. Here, we present the synthesis and detailed characterization of a novel Ru^II photosensitizer, namely [(tbbpy)₂Ru((2-(perylen-3-yl)-1H-imidazo[4,5-f][1,10]-phenanthrolline))](PF₆)₂ RuipPer , that includes a merged perylene dye in the back of the ip ligand. This complex features two emissive excited states as well as a long-lived (8 μs) dark state in acetonitrile solution. Compared to prototype [(bpy)₃Ru]²⁺-like complexes, a strongly altered absorption (ϵ=50.3×10³ M⁻¹ cm⁻¹ at 467 nm) and emission behavior caused by the introduction of the perylene unit is found. A combination of spectro-electrochemistry and time-resolved spectroscopy was used to elucidate the nature of the excited states. Finally, this photosensitizer was successfully used for the efficient formation of reactive singlet oxygen.     

Construction and applications of symmetrized valence bond wave functions

A method constructing symmetry-adapted bonded Young tableau bases is proposed, based on the symmetry properties of bonded tableaus and the projection operator associated with a point group. Several examples including the ground states and π excited states of O₃⁻, O₃, O₃⁺, and C₃⁻ are shown for instruction to construct the symmetrized valence bond (VB) wave function. Excitation energies of transitions from the ground states to π excited states of O₃⁻, C₃H₅, and C₃⁻ are calculated with an optimized symmetrized valence bond wave function in the σ–π separation approximation. Good agreement between the VB and experimental excitation energies is observed. The bonding features of the ground state and the first π excited singlet and triplet states for S₃ are discussed according to bonding populations from VB calculations. Both the singlet-biradical and the dipole structures have significant contributions to the ground state X ¹A₁ of S₃, while the excited state 1 ¹B₂ is essentially composed of the dipole structures, and the 1 ³B₂ excited state is comprised from a triplet-biradical structure. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 66 : 1–7, 1998     

MRD CI calculations of excited states of the SiH3 radical,including potential curves for the inversion mode and the dissociation SiH3 → SiH2 + H

Using the MRD CI method and large basis sets the vertical spectrum of silyl radical (SiH₃) has been calculated. The lowest excited state is the 4s Rydberg state, 41000 cm⁻¹ (5.2 eV) above the ground state. Only one excited valence state (2²E) was encountered, all other states are of Rydberg type. From potential curves for the inversion mode (symmetric bending motion) it was inferred that all Rydberg states are planar, whereas the valence excited state is highly pyramidalized. The investigation of the dissociation reaction SiH₃ → SiH₂ + H leads to the conclusion that the first excited state is dissociative.     

Reversible radiationless decay in methylglyoxal

We report an examination of the fluorescence decay of methylglyoxal vapor excited from the ground ¹A′(S₀) electronic state to the excited ¹A″(S₁) electronic state as a function of additive pressure and excitation wavelength. To explain the low-pressure behavior we postulate a kinetic model which includes a “reversible” collison-free intersystem crossing (S₁-T₁) rate. Quantum mechanical considerations suggest that the optically excited single vibronic level is not coupled with pure T₁ levels.     

Azomethine ylide-formation from N-phthaloylglycine by photoinduced decarboxylation: A theoretical study

In this work, we report the first CASPT2//CASSCF study of the mechanism of the photodecarboxylation of N-phthaloylglycine. The charge transfer excited state S CT ( 1∏∏*) is initially populated upon irradiation at 266 nm. As a result of a fast internal conversion to the lowest excited singlet state S CT-N ( 1∏∏*), this state becomes a favorable precursor state for proton transfer, which triggers decarboxylation. Actually, the excited state intramolecular proton transfer (ESIPT) and decarboxylation processes proceed in an asynchronous concerted way. The ESIPT process is accomplished in the S CT-N ( 1∏∏*) state, but the CO 2 molecule is finally formed in the ground state via the S CT /S 0 conical intersection. Azomethine ylide is formed in the ground state as a complex with CO 2 . A barrier of ~15 kcal/mol indicates that azomethine ylide is stable in the ground state, which is consistent with the experimental findings. This work provides mechanistic details about the formation of azomethine ylide by photoreaction of N-phthaloylglycine.     

Photofragmentation of H2S in the first continuum

The dynamics of the photofragmentation of H₂S in the first continuum has been investigated at laser wavelengths 193, 222 and 248 nm. The HS fragments are produced mainly in υ = 0, as expected, but a non-negligible part of the HS radicals produced are vibrationally excited. The distribution of the vibrationally excited radicals peaks at υ = 3 or 4 at 193 nm and at υ = 2 at 222 nm. From the angular distribution of the H fragments it can be concluded that the anisotropy parameter β decreases with increasing wavelength. These results can be explained by a predissociation mechanism. Excitation in the first continuum takes place from the ¹A₁ ground state to the bound ¹B₁ state. This state is predissociated by a repulsive ¹A₂ state producing electronically ground-state H and HS fragments, of which the HS fragments can be vibrationally excited, resulting in a bimodal vibrational distribution. The increase of β with increasing wavelength is caused by a non-negligible lifetime in the ¹B₁ state.     

Photoluminescence of Acenaphthenone in Organic Solvents and Aqueoug Media

The configuration of the lowest excited state of acenaphthenone, S₁(π, π^*) or T₁(π, π^*), depending on the solvent, dominates photoluminescence. The T₁(n, π^*) state in aprotic organic solvents is responsible for the phosphorescence of acenaphthenone. The wavelengths of the phosphorescence measured in benzene are 576 nm and 635 nm (vibronic) with 3.3 × 10^?4 quantum efficiency. However, the S₁(π, π^*) state in protic solution which dominates the fluorescence emission depending upon acidity is the most distinctive feature of acenaphthenone. The wavelengths of the emissions are 446 nm under water solvation with 0.185 quantum efficiency and 538 nm with 0.097 quantum efficiency under high acidity. The emission at 446 nm is assigned from a H-bonded keto-form excited state, whereas the emission at 538 nm is probably due to the excited state of protonated keto-form. The pK_a value in aqueous solution measured by diminution of fluorescence in basic solutions is 12.5 ± 0.4.     

EXCITED STATE BEHAVIOR OF PHYTOCHROME Pr

The primary excited state processes of phytochrome, the plant chromoprotein responsible for most photomorphogenetic responses, are investigated by picosecond intensity-dependent transmission measurements. At sufficient high excitation intensity (starting at photon flux densities of about 1′10²⁵ cm^?2 s^?l) a moderate bleaching of the absorption is observed. For the quantitative analysis of the experimental data a rate equation system approach for the energy levels responsible for the ultrafast transformation P_r→ lumi-R and the photon transport through the sample was utilized. The following was found for the excited state parameters of the phytochrome P_r: (1) an excited state absorption of the same order of magnitude as the ground state absorption (cross section: S?_exc= 4′10^?16 cm^?2 at 650 nm), (2) a fast relaxation of the higher excited singlet state on the femtosecond time scale.     

Low temperature luminescence spectra of the d10s2 complexes Cs2MX6 (M = Se,Te and X = Cl,Br). The Jahn—Teller effect in the Γ−4(3T1u) excited state

The emission spectra of the title compounds in microcrystalline form have been measured at 10 K. The extensive vibrational progression in the e_g mode is indicative of a tetragonal Jahn—Teller distortion in the Γ^?₄(³T_1u) excited state. The vibronic coupling of a threefold electronic state with a doubly degenerate e_g mode (T—e coupling), linear in the nuclear coordinates, has been reinvestigated considering spin—orbit coupling up to second order perturbation on energy levels which result from an a¹_1gt¹_1u electron configuration. For an estimation of Jahn—Teller coupling constants, the intensity distributions in the progressions were compared with the theoretical line shape functions which were obtained from a model which also permits the determination of potential energy minima and vibrational fundamentals of the excited state. The unusually large increase in the e_g vibrational frequency compared to the ground state is due to Jahn-Teller forces which distort the potential surface, yielding steeper excited state energy curves.     

Singlet Sigma: The "Other" Singlet Oxygen in Solution

The lowest excited electronic state of molecular oxygen, O₂(a^1-DL_g), is often called simply singlet oxygen. This singlet delta state is an acknowledged and well-studied intermediate in many solution-phase photosystems. However, the second excited electronic state of oxygen, O₂(b¹δ_g+), is also a singlet. It has recently become possible to monitor this singlet sigma state in solution, which, in combination with studies of the singlet delta state, contributes to a better understanding of a variety of general problems in chemistry.     

Theoretical study of electronic structure of rhodium mononitride and interpretation of experimental spectra

Potential energy curves of 22 electronic states of RhN have been calculated by the complete active space second‐order perturbation theory method. The X¹Σ₀⁺ is assigned as the ground state, and the first excited state a³Π₀+ is 978 cm^?1 higher. The ¹Δ(I) and B¹Σ⁺ states are located at 9521 and 13,046 cm^?1 above the ground state, respectively. The B¹Σ⁺ state should be the excited state located 12,300 cm^?1 above the ground state in the experimental study. Moreover, two excited states, C¹Π and b³Σ⁺, are found 14,963 and 15,082 cm^?1 above the X¹Σ⁺ state, respectively. The transition C¹Π₁–X¹Σ₀⁺ may contribute to the experimentally observed bands headed at 15,071 cm^?1. There are two excited states, D¹Δ and E¹Σ⁺, situate at 20,715 and 23,145 cm^?1 above the X¹Σ⁺ state. The visible bands near 20,000 cm^?1 could be generated by the electronic transitions D¹Δ₂–a³Π₁ and E¹Σ⁺₀–X¹Σ⁺₀ because of the spin–orbit coupling effect. © 2013 Wiley Periodicals, Inc.     

Anomalous vibrational relaxation of a hot S*1 state of 3,4-benzpyrene in 2-methylpentane

Two anomalous emission bands in the fluorescence spectrum of 3,4-benzpyrene, dissolved in 2-methylpentane, have been studied as a function of temperature. These emissions originate from the second excited singlet state S₂, and from a vibrationally excited S₁ (S^*₁) respectively. From the temperature dependence of the relative yield and the decay time of the S^*₁ emission it can be concluded that the vibrational relaxation of this state is hampered. The rate constant for this relaxation process is smaller that 4 > 62;x 10⁷ sec^?1.     

Vacuum ultraviolet mcd and absorption spectra of P4 isolated in an ar matrix using synchrotron radiation

The VUV MCD and absorption spectra of P₄ isolated in an argon matrix were obtained using synchrotron radiation. Four well resolved ¹A₁ → ¹T₂^J transitions are observed (J = A, B, C, D from low to high energy). A Jahn-Teller effect is clearly evident in the ¹T₂^C excited state. The ¹T₂^D excited state contains substantial 3d character.     

Kinetic Evidence for Methanol Trimer Assisted Proton Transfer: Transient Absorption Study on Excited Triplet State of 4-Hydroxy-4′-nitrobiphenyl

Hydroxyaromatic compounds have a wide range of applications in catalytic synthesis and biological processes due to their enhanced acidity upon photo-excitation. Most hydroxyaromatic compounds with a medium excited state acidity are unable to deprotonate in non-aqueous solvents such as alcohol due to their short-lived excited singlet states. The nitro group in 4-hydroxy-4'-nitrobiphenyl (NO₂-Bp-OH) increases the spin-orbit coupling between excited singlet states and the triplet state, resulting in ultrafast intersystem crossing and the formation of the long-lived lowest excited triplet state (T₁) with a high yield. Using transient absorption spectroscopy and kinetic analysis, we discover that, despite its moderate acidity, the T₁ state of NO₂-Bp-OH (³NO₂-Bp-OH) is able to transfer proton to methanol. Following the formation of the hydrogen-boned complex between ³NO₂-Bp-OH and three methanol molecules in a consecutive process, proton transfer occurs very fast. This finding suggests that the long lifetime of the photoacid excited state allows for the formation of alcohol oligomer with sufficient basicity to induce photoacid deprotonation.     

High-resolution absorption spectra of diphenylnmethylenes

High-resolution absorption spectra of the following diphenylmethylenes (DPM_s) dispersed in benzophenone crystals at liquid-helium temperatures are presented: DPM-h₁₀, DPM-d₁₀, 4-chloro-DPM, and 4-bromo-DPM. The substituent effects concerning the electronic structure, transition energy and intensity are discussed. From polarization measurements, the electronic configurations of the ground and the first excited triplet states of these DPM_s are assigned as (pπ)¹(pσ)¹ and (pσ)¹(π*)¹, respectively. Further studies reveal a second excited triplet state, designated as (pπ)¹(π*)¹, which lies less than 1000 cm^-1 above the first excited triplet state of DPM. Diffuse broad bands appear as common features in all the spectra. Such diffuseness is discussed in terms of electron-phonon coupling of the low-lying excited states.