Electronic structure of the lowest excited triplet state of 5,12-naphthacenequinone

Continuous-wave time-resolved EPR (cw-TREPR) and pulsed electron nuclear double resonance (ENDOR) studies have been carried out to clarify the electronic structure of the lowest excited triplet (Tl) state of 5,12-naphthacenequinone (5,12-NpQ) as well as 1,4-anthraquinone (1,4-AQ) and 6,13-pentacenequinone (6,13-PeQ). The Tl energy level and the D value of the zero-field splitting (ZFS) parameters only slightly decreased with the increasing pi-conjugated system. The Tl states of these linear para-acenequinones were assigned to the pi pi* character. In triplet 5,12-naphthacenequinone, more than 80% of the unpaired electron spins are localized on the naphthalene aromatic sub-system.     

ESR of the lowest triplet state of trans-stilbene

ESR of the lowest triplet state of trans-stilbene has been studied in glassy media at 77 K. This is the first report of ESR of a triplet polyene.     

Spin delocalization in the lowest triplet state of benzophenone

Ring atom hyperfine structure has been observed in the optically-detected magnetic resonance (ODMR) spectra of the lowest triplet states of 4,4′-difluoro- and 4,4′-dibromobenzophenone in a single crystal host at liquid helium temperatures. An analysis of this structure and its angular dependence yields a lower limit of 0.10 for the normalized π-orbital spin density in each of the para ring positions. These results, together with the demonstrated sensitivity of the zero-field splitting parameters to halogen substituents in the 4 and 4′ positions, suggest that a significant amount of spin density resides on the phenyl rings in the ³(n,π^*) state of benzophenone.     

Spin polarization in the lowest triplet state of chlorophyll

Chlorophyll-b in glassy solution has a spin-polarized lowest triplet state at and above 77 K. The magnitude of the effect is different for MTHF and ethanol as solvents, in contrast to what is found for the porphin free base. Chlorophyll-a does not exhibit spin-polarization under identical conditions as for chlorophyll-b. Zero-field parameters are found to be:chlorophyll-a (MTHF) D = (281 ± 6) × 10^?4 cm^?1; E = (39 ± 3) × 10^?4 cm^?1;chlorophyll-b (MTHF) D = (289 ± 4) × 10^?4 cm^?1; E = (49 ± 3) × 10^?4 cm^?1,From ESR signal kinetics it follows that for chlorophyll-b, population and depopulation mainly involve the spin level y?, describing a spin moving in a plane perpendicular to the molecular plane:P_y ? P_x ? P_z; k_x = 240 ± 40 s^?1; k_y = 600 ± 120 s^?1; k_z ? 75 s^?1,where P_i and k_i denote populating and decay rates. Thus, the kinetic scheme for the chlorophyll triplet is different from that of porphyrins with heavier metal ions, but very similar to that of the porphin free base. The spin-lattice relaxation time is found to be anisotropic and shorter than the decay rates of individual spin levels. Nevertheless, spin polarization can be observed, essentially because the ESR signal amplitude depends on population differences.     

Pseudo-jahn-teller distortion of pyridine in its lowest triplet state

We have performed an electron spin echo detected electron paramagnetic resonance study of the nitrogen hyperfine interaction in the lowest triplet state of pyridine. It is concluded that the molecule is non-planar in this state owing to pseudo-Jahn-Teller coupling between the ³B₁(nπ) and the close-lying ³A₁(ππ) states.     

Geometry of ammonia molecule in the lowest triplet state estimated theoretically

The geometrical parameters of the ammonia molecule in the lowest-lying triplet state have been estimated by means of the SCF method and by making an extrapolation to take into account electron correlation effects. The planar configuration has been obtained as the most probable geometrical arrangement (³ A₂″).     

Absorption and emission spectra of the lowest triplet state in hexachloroacetone

T₁ ← S₀ absorption and T₁ → S₀ phosphorescence spectra of neat cystalline hexachloroacetone have been analyzed at 4.2°K. From the lifetime and energy the upper state is assigned as ³nπ^*. The spectra are sharp compared to other aliphatic ketones, with the 0-0 band at 26 248 ± 2 cm ^?1. The phosphorescence shows two strong progressions; one involving the CO stretching mode at 1784 cm^?1 (x), the other a long progression of at least 8 bands involving a mode at 143 cm^t-1 (a). The 143 cm^?1 progression forming mode can best be asigned to the CO out-of-plane wagging vibration. The absorption shows the same two strong progressions, reduced in frequency to 1270 cm^t-1 and 123 cm^?1, respectively, but with the progression in mode a broadened with increasing n. The broadening is interpreted as arising from inversion doublets; the close harmonicity up to n = 5 allowing the potential barrier to inversion to be estimated as > 700 cm^?1. A feature of the spectra is the absence of low frequency torsional modes suggesting lack of pseudo Jahn-Teller distortion of the triplet state potential surface. For comparison, the phosphorescence of crystalline hexafluoroacetone was also studied at 4.2°K. The spectrum exhibits broad bandedness with a 00 band tentatively assigned at 26 870 ± 20 cm^?1.     

Bromine Hyperfine and quadrupole fine structure in the MODR spectrum of sym-tetrabromobenzene in its lowest triplet state

Bromine hyperfine and quadrupole fine structure has been observed in the 3 cm MODR spectrum of the photoexcited ³B_1u state of symmetric tetrabromobenzene (s?TBB) in a durene host crystal at liquid helium temperature. A preliminary analysis of the observed structure yields the two in-plane values of the bromine hyperfine tensor |A_y/gβl = 10.3 and |A_z/gβ| = 16.5 (for ⁸¹Br), and a comparison of these values with those recently observed for the α-bromomaleic acid radical suggests that the correct zero-field scheme in the lowest triplet state of s-TBB is τ_N > τ_M > τ_L.     

On the intrinsic population of the lowest triplet state of thymine

The population of the lowest triplet state of thymine after near-UV irradiation has been established, on the basis of CASPT2//CASSCF quantum chemical calculations, to take place via three distinct intersystem crossing mechanisms from the initially populated singlet bright 1pipi* state. Two singlet-triplet crossings have been found along the minimum-energy path for ultrafast decay of the singlet state at 4.8 and 4.0 eV, involving the lowest 3npi* and 3pipi* states, respectively. Large spin-orbit coupling elements predict efficient intersystem crossing processes in both cases. Another mechanism involving energy transfer from the lowest 1npi* state with much larger spin-orbit coupling terms can also be proposed. The wavelength dependence measured for the triplet quantum yield of pyrimidine nucleobases is explained by the location and accessibility of the singlet-triplet crossing regions.     

Polarization of zero-field transitions of the lowest triplet state of benzophenone

The polarization characteristics of the zero-field (zf) transitions of benzophenone in bis(4-bromophenyl) ether are examined at 1.6 K using optical detection. The results indicate that the transitions are mostly magnetic dipole in nature and that the size of the electric dipole transition moment resulting from spin-orbit perturbation is less than 10^?4 debye. This is being blamed on the small moment of the n, π^? π, π^* triplet-triplet transition that could provide electric allowance in these magnetic transitions. Anomalous magnetic dipole results are obtained on the ab face that can be explained by mixing with host crystal states.     

Assignment of the lowest triplet state of 9,10-anthraquinone single crystals

The lowest triplet state of 9,10-anthraquinone single crystals is assigned as ³B_1g located at 22153 cm^?1. The pure electronic transition to this state is forbidden in the centrosymmetric crystal, but is induced in an electric field. Combined Stark—Zeeman measurements provide an unambiguous symmetric assignment of this transition and clear up the difficulties of previous assignments.     

Time-resolved infrared spectroscopy of the lowest triplet state of thymine and thymidine

Vibrational spectra of the lowest energy triplet states of thymine and its 2′-deoxyribonucleoside, thymidine, are reported for the first time. Time-resolved infrared (TRIR) difference spectra were recorded over seven decades of time from 300 fs to 3 μs using femtosecond and nanosecond pump-probe techniques. The carbonyl stretch bands in the triplet state are seen at 1603 and 1700 cm⁻¹ in room-temperature acetonitrile-d₃ solution. These bands and additional ones observed between 1300 and 1450 cm⁻¹ are quenched by dissolved oxygen on a nanosecond time scale. Density-functional calculations accurately predict the difference spectrum between triplet and singlet IR absorption cross sections, confirming the peak assignments and elucidating the nature of the vibrational modes. In the triplet state, the C4O carbonyl exhibits substantial single-bond character, explaining the large (70 cm⁻¹) red shift in this vibration, relative to the singlet ground state. Femtosecond TRIR measurements unambiguously demonstrate that the triplet state is fully formed within the first 10 ps after excitation, ruling out a relaxed ¹nπ^* state as the triplet precursor.     

Calculation of the exciton band structure of the lowest energy singlet state of fluorene

The exciton band structure of the 33000 cm⁻¹ transition of fluorene has been calculated in the dipole approximation. It is shown that the interac     

Position-dependent heavy atom effect in the subrates of the lowest triplet state of dichloronaphthalenes

By means of the microwave-induced delayed phosphorescence (MIDP) technique, the dynamics of the lowest triplet state T₀ of several dichloronaphthalenes in naphthalene and durene are investigated. It is shown that the position dependence is very selective in the nonradiative decay of the spin-sublevels: The out-of-plane spin state is nearly not affected, while the in-plane states are strongly affected. The radiative rates, on the other hand, are not selectively influenced by the position of substitution. Besides, the results show that the dynamics of the lowest triplet state is determined by the type of substitution, rather than by the symmetry of the molecule. The spin—orbit coupling of the in-plane states seems to profit from some distortion the guest molecule experiences in the naphthalene host lattice.     

Temperature effects on the intramolecular decay of the lowest triplet state of benzophenone

By monitoring the emission specturm, yield, and lifetime from 77 to 400°K for benzophenone in poly(chlorotrifluoroethylene), we have been able to establish the thermally activated delayed fluorescence process and have also observed a large temperature effect on the rate constant for nonradiative decay from the lowest triplet state to the ground singlet state.     

Quenching of the lowest benzene triplet state in the vapor phase by nitric oxide

the rate constant for quenching of the triplet state (³B_1u) of benzene vapor by nitric oxide has been determined from a study of the flash photosensitization of biacetyl phosphorescence. The rate constant has been found to be (1.08 ± 0.2) × 10^?11 cm³ molecule^?1 sec^?1. Although this value is much larger than expected, it is in agreement with rate constants reported for quenching of some liquid phase aromatics.     

Quantum chemistry-based interpretations on the lowest triplet state of luminescent lanthanides complexes. Part 1. Relation between the triplet state energy of hydroxamate complexes and their luminescence properties

In this paper, we evaluate the potential use of theoretical calculations to obtain an energy scale of the lowest ligand-centred triplet excited state in luminescent terbium(III) complexes. In these complexes, non-radiative deactivation of the terbium emitting state via a back-energy transfer process (T1<--Tb(5D4)) is a common quenching process. Consequently the prediction of the energy gap between these two excited states should be useful for programming highly luminescent Tb(III) systems. We report on a strategy based upon experimental and theoretical investigations of the excited state properties of a series of four simple aromatic hydroxamate ligands coordinated to Tb(III) and Gd(III) ions. By using previously reported crystallographic data, the structural and energies properties of these systems were investigated in the ground and first excited triplet states at the density functional theory (DFT) level of calculations. Our theoretical results are consistent with a triplet excited state T1 which is localised on one ligand only and whose the energy level is independent of the lanthanide ion nature (Tb(III), Gd(III)). A good agreement between the calculated adiabatic transition energies and experimental data derived from emission spectra is obtained when a corrective term is considered. These satisfactory results are an indication that this type of modelling can lead to discriminate in terms of the position of the lowest ligand triplet energy level the best antenna among a family of chromophoric compounds. In addition this theoretical approach has provided indications that the difference between the adiabatic transition energies of all the investigated complexes can be mainly explained by metal-ligand electrostatic interactions. The influence of the number of antennae on the quantum yield and the luminescence lifetime is discussed.     

The resonance Raman spectrum of tetramethylbutadiene in its lowest excited triplet state

The resonance Raman spectrum of tetramethylbutadiene in its lowest excited triplet state, with a reported lifetime of 80 ns, is presented. The triplet state was produced by laser flash photolysis using acetone as sensituer in acetonitrile. Transient Raman bands were seen at 1620, 1395, 1353, 1275, 1239, 1048, and 521 cm⁻¹.     

Time-resolved electron paramagnetic resonance of the lowest excited triplet state of phenazinium cation

Time-resolved and steady-state electron paramagnetic resonance (EPR) spectra have been observed for the lowest excited triplet (T(1)) states of phenazine (Phz) and its singly protonated cation (phenazinium) in sulfuric acid-ethanol mixtures at 77K. The single protonation appears to have little effect on the anisotropic sublevel populating rates of the T(1) state of phenazine. However, the zero-field splitting (ZFS) parameter D decreases on the protonation, reflecting the increase of delocalization of the two unpaired electrons. The sublevel preferentially populated by intersystem crossing (ISC) is T(y) in both phenazine and phenazinium (the y-axis is parallel to the in-plane long axis). From the analysis of the observed anisotropy in the ISC rates and the semi empirical molecular orbital calculations of the ZFS parameters, we concluded that the T(1) state of phenazinium is the (3)A(1)(pipi*) state.