Photophysical properties of the lowest excited singlet and triplet states of thio- and seleno-psoralens

The decay processes of the lowest excited singlet and triplet states of five heteropsoralens (HPS) were investigated by steady-state and shift-phase fluorometry and by laser-flash photolysis in different solvents. The emission spectra of HPS are detectable only in trifluoroethanol (TFE), where fluorescence lifetimes (τ_F) and quantum yields (φ_F) were measured. The triplet lifetimes (τ_T), triplet (φ_T) and singlet-oxygen production (φ_Δ) quantum yields were determined in benzene, ethanol and TFE by laser-flash photolysis. Semiempirical (INDO/1-CI) calculations allowed the nature of the lowest excited singlet and triplet states and transition probabilities to be obtained. Theoretical and experimental results indicate that the two lowest excited singlet states S₁ and S₂ of HPS are close-lying and different in nature (π,π* and n,π*). The "proximity effect" between these two states controls the photophysical properties of HPS as it does for the other furocoumarins. However, HPS have a peculiar behavior with respect to the related compounds because they are fluorescent and have, in three cases, detectable intersystem crossing only in TFE. This behavior can be tentatively explained by a different energy gap and/or order between the S₁ and S₂ states.     

Quantum-chemical study of the structure of the acetyl fluoride molecule in the ground and lowest excited singlet and triplet electronic states

The structure of the conformationally flexible acetyl fluoride molecule (CH3CFO and CD3CFO) in the ground (S0) and lowest excited triplet (T1) and singlet (S1) electronic states was calculated by different quantum-chemical methods (RHF, UHF, MP2, CASSCF). The equilibrium geometric parameters and harmonic vibrational frequencies of the molecules in these electronic states were estimated. The calculations demonstrated that the electronic excitation causes considerable conformational changes involving the rotation of the CH3(CD3) top and a substantial deviation of the CCFO carbonyl fragment from planarity. For large-amplitude vibrations, namely, for the torsional vibration in the S0 state and the torsional and inversion (nonplanar carbonyl fragment) vibrations in the T1 and S1 states, the quantum-mechanical problems were solved in one-dimensional (1D) and two-dimensional (2D) approximations. The results of calculations are in good agreement with experimental data.     

Characterization of the singlet and triplet excited states of 3-chloro-4-methylumbelliferone

An extensive photophysical characterization of 3-chloro-4-methylumbelliferone (3Cl4MU) in the ground-state, S(0), first excited singlet state, S(1), and lowest triplet state, T(1), was undertaken in water, neutral ethanol, acidified ethanol, and basified ethanol. Quantitative measurements of quantum yields (fluorescence, phosphorescence, intersystem crossing, internal conversion, and singlet oxygen formation) together with lifetimes were obtained at room and low temperature in water, dioxane/water mixtures, and alcohols. The different transient species were assigned and a general kinetic scheme is presented, summarizing the excited-state multiequilibria of 3Cl4MU. In water, the equilibrium is restricted to neutral (N*) and anionic (A*) species, both in the ground (pK(a) = 7.2) and first excited singlet states (pK(a)* = 0.5). In dioxane/water mixtures (pH ca. 6), substantial changes of the kinetics of the S(1) state were observed with the appearance of an additional tautomeric T* species. In low water content mixtures (mixture 9:1 v:v), only the neutral (N*) and tautomeric (T*) forms of 3Cl4MU are observed, whereas at higher water content mixtures (water mole fraction superior to 0.45), all three species N*, T*, and A* coexist in the excited state. In the triplet state, in the nonprotic and nonpolar solvent dioxane, the observed transient signals were assigned as the triplet-triplet transition of the neutral form, N*(T(1)) → N*(T(n)). In water, two transient species were observed and are assigned as the triplets of the neutral N*(T(1)) and the anionic form, A*(T(1)) (also obtained in basified ethanol). The phosphorescence spectra and decays of 3Cl4MU, in neutral, acidified, and basified solutions, demonstrate that only these two species N*(T(1)) and A*(T(1)) exist in the lowest lying triplet state, T(1). The radiative channel was found dominant for the deactivation of the anionic species, whereas with the neutral the S(1) ? S(0) internal conversion competes with fluorescence. For both N* and A* the intersystem crossing yield represents a minor deactivation channel for S(1).     

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.     

A diffusion quantum Monte Carlo study on the lowest singlet and triplet electronic states of BN molecule

Ab initio calculation of both the lowest singlet and triplet electronic states of BN has been performed by the fixed-node Ornstein-Uhlenbeck diffusion quantum Monte Carlo method with the floating spherical Gaussian orbitals and spherical Gaussian geminals. The Monte Carlo calculation gives equilibrium bond lengths and equilibrium harmonic frequencies of 1.3317(7) A and 1529(7) cm(-1), respectively, for the lowest triplet state and 1.2751(7) A and 1709(8) cm(-1), respectively, for the lowest singlet state. Also, the Monte Carlo calculation reports an energy separation of 178(83) cm(-1) between the two electronic states and recommends the ground state is the lowest triplet state.     

Theoretical study of the structure of the CClF2NO and CCl2FNO molecules in the ground and lowest excited singlet and triplet electronic states

The potential energy surfaces of the nitroso compounds CClF₂NO and CCl₂FNO in the ground and lowest excited singlet and triplet electronic states were studied by various ab initio methods (including multiconfigurational methods). The equilibrium geometric parameters, vibrational frequencies, internal rotation potential functions, and rotational contours of bands in the S₁ S₀ vibronic spectrum of the CClF₂NO molecule were calculated. For the molecules under consideration, the quantum-mechanical problem on torsional motion was solved. The results of calculations are, on the whole, in good agreement with experiment.     

Geometrical relaxation in the excited singlet states of propylene

The consequences of the twist around the double bond in propylene for the properties of its low lying excited singlet states have been investigated by the ab initio large-scale multireference configuration interaction method (MRD-CI). A substantial increase in the dipole moments of the S₁ and S₂ excited states was found for a large interval of the twist angel θ = 50–130°. The variation of the VB covalent VB ionic contributions to the correlated wavefunctions of these two states a function of twisting has been analyzed. The connection with the occurence of an avoided crossing of the two excited singlets near the twist angel θ = 75°, which results in no change in dipole moment directions, is pointed out. The existence of destructive or constructive interference between acceptor and donor substitution has been investigated on the example of the pyramidalization at one of the vinylic C atoms. A competition of opposing effects matrix can invert the dipole moment direction in the excited states. Preliminary investigation of the nonadiabatic coupling elements indicates that the “sudden polarization” effect willnot disappear through vibronic coupling, and that the return of excited molecules to the ground electronic state will not be immediate.     

Influence of excited state aromaticity in the lowest excited singlet states of fulvene derivatives

The absorption spectra and excited state dipole moments of four differently substituted fulvenes have been investigated both experimentally and computationally. The results reveal that the excited state dipole moment of fulvenes reverses in the first excited singlet state when compared to the ground state. The oppositely polarized electron density distributions, which dominate the ground state and the first excited singlet state of fulvenes, respectively, reflect the reversed π-electron counting rules for aromaticity in the two states (4n + 2 vs. 4n, respectively). The results show that substituents indeed influence the polarity of fulvenes in the two states, however, cooperative interactions between the substituents and the fulvene moiety are most pronounced in the ground state.     

Conformational relaxation dynamics in the excited electronic states of benzil in solution

Relaxation dynamics in the excited singlet (S₁) state of benzil have been studied in solution using pico and subpicosecond transient absorption spectroscopic techniques. The triple exponential decay dynamics of the S₁ state indicates that the process of conformational change from the cis-skewed to the trans-planar form takes place via the formation of a meta-stable intermediate conformer resulting the involvement of two consequent barrier crossing processes. The barrier crossing dynamics is governed by both the polarity of the solvent, which alters the barrier heights by ‘static' interactions, as well as the viscosity of the solvent via ‘dynamical' interactions.     

Ab initio study of the structure of 2,2-difluoroethanal in the ground and lowest excited triplet electronic states

The molecular structure of 2,2-difluoroethanal (DFE) in the ground (S₀) and lowest excited triplet (T_i) electronic states was investigated byab initio quantum-chemical methods. In the S₀ state, the DFE molecule exists as the only stablecis conformer. The T_i↓S₀ electronic excitation is accompanied by the rotation of the top and the deviation of the carbonyl fragment from planarity. For the DFE molecule in the T_i state, six minima corresponding to three pairs of enantiomers were found on the potential energy surface. Based on this potential energy surface, the problems on torsion and inversion nuclear motions were solved in the one- and two-dimensional approximations, and the interaction between these motions was revealed. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 989–995, June, 2000.     

Studies of electronic structures of the lowest excited triplet states of substituted benzenes and pyridines by EPR

Single crystal EPR studies of a series of substituted benzenes and pyridines have been made. It was observed that the electronic structures of these systems are primarily determined by the locations of the substituents. The observed results were qualitatively interpreted on the basis of the excited configurations of benzene.     

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     

Resonance CARS spectra of the lowest excited singlet states of rhodamines

The resonance CARS spectra of the S₁ states of rhodamine 6G, rhodamine B and sulforhodamine were obtained by choosing ω₁ resonant with the S₁ ← S₀ and S₃ ← S₁ transitions simultaneously and by varying the laser beam power density of ω₁ or ω₂. The vibrational frequencies for the S₀ and S₁ states are similar, implying that the structure of the S₁ state is not distorted significantly.     

Spectrometric evidence ford-orbital participation in the lowest excited singlet states of naphthalenethiols

Tellurium can be determined polarographically in the range 10^?5–10^?8M by means of the Te⁰_ads→Te^2- reduction in 1M perchloric acid as supporting electrolyte. Pulse polarography, a.c. polarography and linear sweep cyclic voltammetry can be used to determine tellurium in the p.p.b. range. Copper(II), arsenic(III) and selenium(IV) interfere, but the interferences can be overcome by a standard addition method.     

Photoinduced omega-bond dissociation in the higher excited singlet (S2) and lowest triplet (T1) states of a benzophenone derivative in solution

Photochemical properties of photoinduced omega-bond dissociation in p-benzoylbenzyl phenyl sulfide (BBPS) in solution were investigated by time-resolved EPR and laser flash photolysis techniques. BBPS was shown to undergo photoinduced omega-bond cleavage to yield the p-benzoylbenzyl radical (BBR) and phenyl thiyl radical (PTR) at room temperature. The quantum yield (phi(rad)) for the radical formation was found to depend on the excitation wavelength, i.e., on the excitation to the excited singlet states, S2 and S1 of BBPS; phi(rad)(S2) = 0.65 and phi(rad)(S1) = 1.0. Based on the CIDEP data, these radicals were found to be produced via the triplet state independent of excitation wavelength. By using triplet sensitization of xanthone, the efficiency (alpha(rad)) of the C-S bond fission in the lowest triplet state (T1) of BBPS was determined to be unity. The agreement between phi(rad)(S1) and alpha(rad) values indicates that the C-S bond dissociation occurs in the T1 state via the S1 state due to a fast intersystem crossing from the S1 to the T1 state. In contrast, the wavelength dependence of the radical yields was interpreted in terms of the C-S bond cleavage in the S2 state competing with internal conversion from the S2 to the S1 state. The smaller value of phi(rad)(S2) than that of phi(rad)(S1) was proposed to originate from the geminate recombination of singlet radical pairs produced by the bond dissociation via the S2 state. Considering the electronic character of the excited and dissociative states in BBPS showed a schematic energy diagram for the omega-bond dissociation of BBPS.     

Electron-impact investigation of excited singlet states in 1,3-butadiene

Electron-impact energy-loss spectra of 1,3-butadiene have been obtained at impact energies from 35 eV to 90 eV, at a scattering angle of 0°, and at a     

Evolution of lowest singlet and triplet excited states with transition metals in group 10-12 metallaynes containing biphenyl spacer

A new series of thermally stable group 10 platinum(II) and group 12 mercury(II) poly-yne polymers containing biphenyl spacer trans-[-Pt(PBu₃)₂CC(p-C₆H₄)₂CC-]_n and [HgCC(p-C₆H₄)₂CC-]_n were prepared in good yields by Hagihara’s dehydrohalogenation reaction of the corresponding metal chloride precursors with 4,4′-diethynylbiphenyl HCC(p-C₆H₄)₂CCH at room temperature. We report the optical spectroscopy of these polymetallaynes and compare the results with their bimetallic model complexes trans-[Pt(Ph)(PEt₃)₂CC(p-C₆H₄)₂CCPt(Ph)(PEt₃)₂] and [MeHgCC(p-C₆H₄)₂CCHgMe] as well as the group 11 gold(I) counterpart [(PPh₃)AuCC(p-C₆H₄)₂CCAu(PPh₃)]. The structural properties of all model complexes have been studied by X-ray crystallography. The influence of the heavy metal atom in these metal alkynyl systems on the intersystem crossing rate and the spatial extent of lowest singlet and triplet excitons is systematically characterized. Our investigations indicate that the organic triplet emissions can be harvested by the heavy-atom effect of group 10-12 transition metals (viz., Pt, Au, and Hg) which enables efficient intersystem crossing from the S₁ singlet excited state to the T₁ triplet excited state.     

Theoretical analysis of singlet and triplet excited states of nickel porphyrins

Local density and generalized gradient approximation time-dependent density functional methods have been used for calculation of the singlet and triplet excited states of nickel-porphine, Ni-tetraphenyloporphine, and Ni-octaethyloporphyrine. Special attention is paid to metal-ligand transitions and d-d transitions. It is shown that the lowest exited singlet states of the three compounds can be described as a transfer of an electron from the porphine ring to the d(x2-y2) orbital of the nickel atom. On the other hand, the lowest excited triplet state arises from promotion of an electron between two nickel d orbitals, an occupied d(z2) and an empty d(x2-y2). It is proposed that a rapid quenching of the excited singlet states is due to an ultrafast intersystem crossing between 1Eg)and 3Eg or 3B1g states.     

Theoretical study of photoinduced singlet and triplet excited states of 4-dimethylaminobenzonitrile and its derivatives

Singlet and triplet low-lying states of the 4-dimethylaminobenzonitrile and its derivatives have been studied by the density functional theory and ab initio methodologies. Calculations reveal that the existence of the methyl groups in the phenyl ring and the amino twisting significantly modify properties of their excited states. A twisted singlet intramolecular charge-transfer state can be accessed through decay of the second planar singlet excited state with charge-transfer character along the amino twisting coordinate or by an intramolecular charge-transfer reaction involved with a locally first excited singlet state. Plausible charge-transfer triplet states and intersystem crossing processes among singlet and triplet states have been explored by spin-orbit coupling calculations. The intersystem crossing process was predicted to be the dominant deactivation channel of the photoexcited 4-dimethylaminobenzonitrile.