Rigid ground state structure of 1,2-dihydro-3H-benz[e]inden-3-one deformed in the lowest electronic triplet state

The X-ray diffraction of crystalline 1,2-dihydro-3H-benz[e]inden-3-one (DHBI) reveals that the molecular geometry is fully planar in the electronic ground state. Glassy solutions of naphthaldehyde, 2-acetonaphthone and methyl 2-naphthoate in the mixtures methylcyclohexane/iso-pentane (MIP) and methanol/ethanol (ME) are phosphorescent. DHBI in ME shows phosphorescence, but in MIP it is non-phosphorescent. The phosphorescence spectra of these compounds and of naphthalene have a strong resemblance. This is in accordance with a molecular distortion in the lowest triplet state, which decouples the π electron systems of the carbonyl group and the naphthyl group. The absence of phosphorescence of DHBI in MIP, indicates a geometry of the triplet state, having a non-planar naphthalene ring, when the molecule is in the non-hydrogen bonded form.     

A theoretical study on the quenching mechanisms of triplet state riboflavin by tryptophan and tyrosine

The reactions of tryptophan (Trp) and tyrosine (Tyr) with endogenous photosensitizer riboflavin (RF) have gained much interest for their crucial roles in various photobiological processes. In this paper, the quenching mechanisms of triplet state RF by Trp and Tyr have been explored employing density functional theory calculations. It is revealed that the H-atom transfer reaction from Trp and Tyr to triplet state RF is more favorable on thermodynamic grounds compared with direct energy transfer or direct electron transfer pathways. During the photosensitization, RF can photogenerate various reactive oxygen species (ROS) as intermediates, while the present study provides some deeper insights into the photosensitizing behaviors of triplet state RF by reacting directly with Trp and Tyr.     

First principles study of the structure,electronic state,and stability of CmN2 clusters

Geometric and electronic properties of C_mN₂ (m = 1–14) clusters have been investigated by density functional theory using the hybrid B3LYP functional and the 6‐311G(d) basis set. Harmonic frequencies for these clusters are given to aid in the characterization of the ground states. These results show that C_mN₂ (m = 1–14) clusters form linear structures with D_∞h symmetry. Two N atoms favor to bond at ends in linear isomers. The chains with odd m have triplet ground states whereas the ones with even m have singlet ground states. The calculated HOMO–LUMO gaps and ionization potentials all show that the C_mN₂ (m = 1–14) clusters with even m are more stable than those with odd m, which is consistent with the observed even–odd alternation of the time‐of‐flight signal intensities. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2004     

A theoretical study of the lowest-energy singlet and triplet electronic states p-iminophosphaalkyne and nitrilimine

The equilibrium geometries and fundamental vibrational frequencies of the two energetically lowest-lying electronic states of p-iminophosphaalkyne (HCPNH) and nitrilimine (HCNNH) were determined using a split-valence basis set with polarization functions on the heavy atoms. The most extensively correlated functions used in the geometry optimizations were of the Complete Active Space Self-Consistent Field (CASSCF) variety and included eight electrons distributed among seven active orbitals for HCPNH and six active orbitals for HCNNH. The effects on predicted geometry of the size of the CASSCF active space were investigated for HCPNH and are reported. Multi-Reference Configuration Interaction with Single and Double excitations calculations have been performed at the equilibrium geometries using larger basis sets to determine more accurately the relative energetics of the electronic states. It was found that nitrilimine has the expected singlet ground state, with the triplet lying 38.5 kcal mol⁻¹ higher in energy; however, p-iminophosphaalkyne has a triplet ground state, lying 9.7 kcal mol⁻¹ below the singlet.     

Synthesis and photochemistry of novel bromo substituted stilbene derivatives as triplet state sensitizers for the generation of singlet oxygen

We have presented the synthesis and characterization of three new bromo substituted stilbene derivatives,p-3,4,5-trimethoxy-p'- 2,3,4,5,6-pentabromostilbene(C1),p-N,N-dimethylamino-p'-2,3,4,5,6-pentabromostilbene(C2) and p-N,N-diphenylamino-p'- 2,3,4,5,6-pentabromostilbene(C3) in this letter.The UV/vis absorption and photoluminescence were investigated in various solvents.The maximal absorption wavelength of C1 exhibited blue-shift to those of C2 and C3 in different solvents.No florescence emission could...     

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.     

On the vibronic level structure in the NO3 radical. I. The ground electronic state

The model Hamiltonian approach of Koppel et al. [Adv. Chem. Phys. 57, 59 (1984)] is used to analyze the electronic spectroscopy of the nitrate radical (NO3). Simulations of negative ion photodetachment of NO3-, the X 2A2'<--B 2E' dispersed fluorescence spectrum of NO3, and the B 2E'<--X 2A2' absorption spectrum are all in qualitative agreement with experiment, indicating that the model Hamiltonian contains most or all of the essential physics that govern the strongly coupled X 2A2' and B 2E' electronic states of the radical. All 14 bands seen in the dispersed fluorescence spectrum below 2600 cm-1 are assigned based on the simulations, filling in a few gaps left by previous work, and 7 additional bands below 4000 cm-1 are tentatively assigned. The assignment is predicated on the assumption that the nu3 level of NO3 is near 1000 cm-1 rather than 1492 cm-1 as is presently believed. Support for this reassignment (which associates the 1492 cm-1 band with the nu1+nu4 level) comes from both the model Hamiltonian spectrum and a Fourier-transform infrared feature at 2585 cm-1 that is consistent with the large and positive cross anharmonicity between nu1 and nu4 needed for the alternative 1492 cm-1 assignment. An apparent systematic deficiency exists in the treatment of the model Hamiltonian for levels involving nu4. A discussion of the correlation between energy levels in the rigid D3h and C2v limits is illustrative, and provides insight into just how hard it is to treat the degenerate bending coordinate (q4) of NO3 accurately.     

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.     

A monomeric bacteriochlorophyll triplet state (B) in reaction centres of Rhodobacter sphaeroides R26, studied by absorption-detected magnetic resonance

Using absorption-detected magnetic resonance, a new triplet state was observed in reaction centres of Rhodobacter sphaeroides R26 at 7 K. Its zero-field splitting parameters &|;D&|; = 220.5 × 10⁻⁴ cm⁻¹ and &;|;E&|; = 65.4 × 10⁻⁴ cm⁻¹, its spin polarization and its microwave-induced absorption spectrum indicate that it belongs to one of the accessory monomeric bacteriochlorophyll molecules and that it is populated through the same excitation path as the primary donor triplet ³P₈₇₀, via the radical pair ³[P₈₇₀⁺H⁻].     

Effects of spin transitions degeneracy in pulsed EPR of the fullerene C70 triplet state produced by continuous light illumination

X-band echo-detected electron paramagnetic resonance (ED EPR) spectra of triplet state of fullerene C(70) generated by continuous light illumination were found to correspond below 30K to a non-equilibrium electron spin polarization. Above 30K spectra are characteristic of Boltzmann equilibrium. Spectra were simulated fairly well with zero-field splitting parameters D=153 MHz and E and distributed within the range of 6-42 MHz. The origin of E distribution is attributed to the Jahn-Teller effect, which in glassy matrix is expected to depend on the local surrounding of a fullerene molecule (a so-called E-strain). In the center of ED EPR spectra a narrow hole was observed. With increase of the microwave pulse turning angle this hole transforms into a single narrow absorptive line. Numerical simulations by density matrix formalism confirm that central hole originates from a simultaneous excitation of both allowed electron spin transitions of the triplet (T(0)?T(+) and T(0)?T(-)), because of their degeneracy at this spectral position. Also explanations are given why this hole has not been observed in the previously reported experiments on continuous wave EPR and on ED EPR under laser pulse excitation.     

A pulse radiolysis and pulsed laser study of the pyrromethene 567 triplet state

The triplet state of pyrromethene 567, a molecule with potential as a solid state laser dye, has been characterized in benzene by pulse radiolysis in terms of its absorption spectrum, lifetime, self-quenching, electronic excitation energy, triplet–triplet extinction coefficient and oxygen quenching rate constant. The use of laser flash photolysis has then allowed determination of the triplet quantum yield, efficiency of formation of singlet oxygen (¹Δ_g), and the rate constant for reaction of the latter species with the ground state. The affects of oxygen on the fluorescence and triplet yields demonstrate that oxygen-induced intersystem crossing is important, the sum of these parameters being unity within experimental error. The mechanism of reaction of P-567 with ¹Δ_g in benzene is predominantly physical in character with only a small (6%) contribution from chemical reaction.     

A study of interaction in the lowest singlet and triplet states of H2

The potential energy curves of the ground state and the first excited state of H₂ are examined in terms of the electronic force acting on each nucleus. The results reveal the detailed course of events that occur when two hydrogen atoms with parallel and antiparallel electron spins approach one another from a large internuclear separation.     

QM/MM dynamics of a Peridinin model in triplet state in three prototypical solvents

Peridinin (Per) is a carbonyl-containing carotenoid playing a key role in light harvesting and photoprotection in dinoflagellates. This carotenoid plays its photoprotective role by quenching the potentially dangerous ³Chl-a triplet state through the formation of the non-reactive ³Per triplet state through Dexter energy transfer mechanism. We have investigated by means of Quantum Mechanics/Molecular Mechanics dynamics simulations at room temperature the structural and dynamical properties of a Peridinin model system (PMS) in triplet state in three different solvents: cyclohexane, apolar/aprotic; acetonitrile, polar/aprotic; and methanol (MeOH), polar/protic. Our results of ³PMS in MeOH show that the lactonic carbonyl has a stronger tendency to accept hydrogen bonds compared to the corresponding singlet ground state (¹PMS). This effect may play some so far overlooked role in Per-containing proteins (notably the water soluble Peridinin-Chlorophyll-Proteins – PCPs).The vibrational properties of the ³PMS dynamics in the three solvents have been analyzed by means of decomposition of the vibrational density of states in effective normal modes. The results show that the solute-solvent interactions can influence some vibrational bands of ³PMS; in particular, they are able to modulate the position of the lactonic CO stretching band. The situation is particularly evident in the case of MeOH, where the dynamics of the MeOH⋯OC hydrogen bond interactions can strongly influence the band position and shape. As vibrational spectroscopy (notably step-scan FTIR difference spectroscopy) has been largely used to investigate ³Per in PCPs, especially using the lactonic carbonyl stretching as a marker band to investigate the different photophysical role of each Per in the protein complex, this study represents an important step to understand the experimental spectra and to identify the Per(s) molecule(s) bearing the triplet in PCPs.     

On the stability of the autodissociative ground electronic state of BeH2+

It is shown that the mechanism of spontaneous infrared emission enhances substantially the stability of the long-lived ground electronic state of the exotic BeH²⁺, whose autodissociation becomes possible only via tunneling. The system, initially situated in any vibrational level (except the high-lying ones for which dissociation is predominate) and statistically distributed over all rotational states, reaches stability through a cascade of dipole emissions toward lower levels. The rapid spatial variation of the dipole moment results in fast radiative processes and might suggest experimental observations easier to perform. The methodology is based on a recently presented analytic discrete variable representation (DVR ) [D.T. Colbert and W.H. Miller, J. Chem. Phys. 96 , 1982 (1992)] that facilitates calculations. This might prove particularly useful to spectroscopists, allowing a check of the quality of potentials produced by inversion methods or the reliable calculation of spectrochemical quantities. © 1995 John Wiley & Sons, Inc.     

Studies on the structural transitions and self-organization behavior of polyacrylic acids with complementary polymers in aqueous solution by laser flash photolysis method using the triplet state of covalently bound phenosafranine

The conformational transition of polyacrylic acids and the formation of interpolymer complexes with synthetic polymers in aqueous solution are investigated using the triplet state of the cationic dye phenosafranine covalently attached to the polymer chain. Laser excitation of the phenosafranine dye covalently bound to polymethacrylic acid at 532 nm shows that the absorption spectrum of the triplet state shifts to red region by 40 nm as compared to that of the free dye in aqueous solution and the triplet state lifetime is enhanced by 20-fold. Laser flash excitation shows that the environment of the triplet state of the dye bound to the polyelectrolyte at pH ?5.5 in aqueous solution is more rigid and less polar resulting in a highly compact globular nature of the polymer. The decay of the triplet state of the dye bound to the polymer is attributed to the quenching of the excited state by the carboxylate groups of polyacrylic acids and to the decay process of the triplet in the tightly coiled polymer environment in the pH range 2.0–5.0. The spectra of the triplet dye molecules bound to the polymer at different degree of ionization of the polyelectrolyte suggest that the structural transition from compact globular structure to stretched rod like structure is cooperative involving a series of structural transitions. The observation of diprotonated triplet state of the PMAA bound dye at higher pH (i.e. pH ∼7.0) reveals the existence of an intermediate structure akin to a micellar segment in PMAA prior to the formation of elongated linear chain. The self-organization of PMAA adduct formation with complementary macromolecules, PVP, PEO and PVA primarily due to hydrogen bonding makes the environment of the dye in the adduct more compact and rigid; in particular poly(vinylpyrrolidone), PVP, has the tendency to form more compact interpolymer complex at pH 4.5 than poly(vinyl alcohol), PVA, and poly(ethylene oxide), PEO as revealed from the laser flash photolysis studies of the polymer bound dye using triplet state of the phenosafranine as the marker.     

Quantum chemical study of the electronic structure of NiCH2 + in its ground state and low-lying electronic excited states

The electronic structure of NiCH(2) (+), representative of transition metal carbene ions, is investigated by means of several methods of quantum chemistry. The relative stabilities of the four low-lying doublet electronic states ((2)A(1), (2)A(2), (2)B(1), and (2)B(2)) are determined at the coupled cluster singles and doubles level (CCSD) and triples level [CCSD(T) and CCSDT-3] with both a Hartree-Fock and density functional theory (Kohn-Sham) reference. The equation-of-motion coupled cluster for treatment of excited states in singles and doubles approximation (EOM-CCSD) is used to characterize the transition energies from the (2)A(1) electronic ground state to the low-lying doublet excited states. The (2)A(2) and (2)B(1) states are nearly degenerate, found to be separated by 940 cm(-1) at the EOM-CCSD level, in agreement with the CASSCF energy ordering. The (2)B(2) state is calculated to be higher in energy by more than 1.0 eV. The spin purity of the low-lying doublet and quadruplet states described by CCSD calculations based on the unrestricted open-shell Hartree-Fock reference is discussed.     

Characterization of the ground state pyrene complex in ethylene-propylene copolymer solutions

Pyrene-labeled functionalized ethylene-propylene (EP) copolymer was prepared by grafting 1-pyrenebutyrylhydrazine onto EP copolymer through maleic anhydride pendants. The EP copolymer contained 60 mol % ethylene; its weight-average molecular weight (M^w) was 148,000. The pyrene-labeled amide functionalized EP copolymer, PA-EP(60/40), was made to simulate the amine functionalized EP copolymers that are commonly used as dispersant additives in motor oils. UV absorption spectra, fluorescence emission and excitation spectra, and fluorescence decay profiles of the pyrene were studied to determine the copolymer conformation and dynamics in methylcyclohexane and tetrahydrofuran (THF). The pyrene fluorescence characteristics of PA-EP(60/40) were highly dependent on the solvent. The dependence of fluorescence emission intensity on the excitation wavelength was large in methylcyclohexane and moderate in THF. A frequency shift of about 2 nm was observed between the excitation spectrum obtained with the emission line at 377 nm and that at 550 nm in the methylcyclohexane solutions, but no shift was found in the corresponding tetrahydrofuran solutions. The ratios of the preexponential factors (a₂₁/a₂₂) of the excimer decays obtained in both methylcyclohexane and THF solutions were different from ?1.0. However, the deviation of the excimer formation process from the Birks scheme is small in THF but large in methylcyclohexane. In addition, the Huggins constants obtained from intrinsic viscosity measurements of the PA-EP(60/40) copolymer solutions suggest that copolymer aggregation occurs in methylcyclohexane but not in THF. H-bonding between two pyrene-containing pendants is apparently the main driving force for the formation of the ground state pyrene complex. THF is found to be effective in inhibiting the H-bonding formation. © 1995 John Wiley & Sons, Inc.