The lowest triplet state of pyrimidine in a crystalline host at 1.2 K. Total phosphorescence and Tx sublevel excitation spectra

Total phosphorescence and T_x sublevel excitation spectra of pyrimidine in benzene and cyclohexane are reported. With increasing excitation energy absorption into the¹B₁ nπ* vibronic ladder leads to an increase of the populating rate of T_y, T_z relative to T_x. For pyrimidine in benzene (p-xylene) a second electronic origin is observed 2416 (2260) cm^?1 above the S₁ S₀ 00 band and assigned to¹A₂ nπ*. The spin-orbit coupling mechanism that accounts for the intersystem crossing into T_x is discussed.     

Kinetics of the phosphorescent triplet states of tetracyanobenzene complexes as studied by microwave induced delayed phosphorescence

Using microwave induced delayed phosphorescence techniques we determined the populating and depopulating rate constants for the individual spin-sublevels of the phosphorescent states of tetracyanobenzene (TCNB) and its charge-transfer complexes with benzene and hexamethylbenzene (HMB) in n-hexane at 1.25 K. It was shown that the non-radiative decay process from the shortest lifetime sublevel is most responsible for the previously observed shortening in the phosphorescence lifetime of the TCNBHMB complex.     

The lowest triplet state of pyrimidine in a crystalline host at 1.2 K. The spin-sublevel emission spectra and their vibrational assignment

Spin-sublevel phosphorescence spectra of pyrimidine in benzene are reported. The spectra related to the in-plane spin axes consist of progressions of totally symmetric bands, whereas that of the third sublevel in addition shows strong non-totally symmetric bands. Assignments are given, and some out-of-plane modes previously in dispute are identified (ν_10b = 808, ν_17b = 960, ν_s = 992 cm^?1, A₂). As a check similar experiments were carried out of pyrimidine incorporated in p-xylene and cyclohexane.     

Sublevel decay kinetics of the triplet state of bacteriochlorophyll a and b in methyltetrahydrofuran at 1.2 K

The decay rates of the triplet sublevels of monomeric BChl a and b have been reexamined with pulsed microwave excitation. Our values of k_x and k_y are significantly higher than published values. The new results are discussed in relation to the exciton model of the special pair making up the primary electron donor in photosynthetic purple bacteria.     

X-traps in isotopically mixed benzene crystals: phosphorescence spectrum and zero-field splitting of the lowest triplet state

We have observed temperature-dependent X-trap phosphorescence in C₆H₆ crystals doped with C₆D₆. The observed X-trap is identified as C₆H₆ which is perturbed by the presence of C₆D₆ in a nearest-neighbor crystal site. Zero-field optical-microwave double resonance measurements on the X-traps give for the zero-field anisotropy parameter |E| a value of 20 ± 5 MHz. This is an order of magnitude smaller than that observed for C₆H₆ in C₆D₆ and indicates for the X-traps a nearly hexagonal distribution of spin density in the ³B_1u state. This difference shows that the electronic structure of the ³B_1u state is sensitive to subtle changes in its environment.     

Investigation of the lowest triplet state of free base porphin by microwave induced changes in its fluorescence

The zero-field splitting of the non-phosphorescent lowest triplet state of free base porphin is measured by observing microwave induced changes in the fluorescence of this molecule as a guest in a Shpolskii-matrix of _n-octane at 4.2°K.The doublet character of the fluorescence spectrum of the system also manifests itself in the microwave spectrum. It is proved that this doublet structure is caused by two physically distinct free base porphin molecules in the host lattice.     

The lowest triplet state of benzene: Differences in electronic structure in two crystalline modifications of a cyclohexane host

Microwave induced delayed phosphorescence (MIDP) measurements have been performed in the 00 bands of the phosphorescence spectrum of benzene dissolved in two phases of cyclohexane. From the relative radiative rates for decay of the three zero-field levels it is concluded that on the average the symmetry of the electronic structure is lower than D_2h. In the monoclinic low temperature stable phase of cyclohexane (phase II) the structure is approximately antiquinoidal, and in the metastable phase III it is approximately quinoidal.     

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.     

Products of the triplet excited state produced in the radiolysis of liquid benzene

The radiation chemical yields of the products derived from the triplet excited state produced in the radiolysis of liquid benzene with gamma-rays, 10 MeV 4He ions, and 10 MeV 12C ions have been determined. Iodine scavenging techniques have been used to examine the formation and role of radicals, especially the H atom and phenyl radical. For all irradiation types examined here, the increase in hydrogen iodide yields with increasing iodine concentration matches the increase in iodobenzene yields. This agreement suggests that the benzene triplet excited state is the common precursor for the H atom and the phenyl radical. Pulse radiolysis studies in liquid benzene have determined the rate coefficients for the reactions of phenyl radicals with iodine and with the solvent benzene to be 9.3 x 10(9) M(-1) s(-1) and 3.1 x 10(5) M(-1) s(-1), respectively. Direct measurements of polymer formation, which refers to trimers (C18) and higher order compounds (>C18), in liquid benzene radiolysis using gamma-rays, 4He ions, and 12C ions at relatively high doses have been performed using gel permeation chromatography. The yields of trimers increase from gamma-rays to 12C ions due to the increased importance of intratrack radical-radical reactions that can be scavenged by the radical scavenging reactions of iodine. On the other hand, the >C18 product yields decrease from gamma-rays to 12C ions. The structure of the polymer consists of a partly saturated ring as determined by infrared and gas chromatography/mass spectrometry studies. A schematic representation for the radiolytic decomposition of the benzene triplet excited state is presented.     

Phosphorescence-detected triplet state microwave resonance of a protoporphyrin molecule in n-octane

Optically detected magnetic resonance and phosphorescence decay measurements for protoporphyrin IX in n-ocatane at 5 K have been carried out. The results are interpreted taking into account spin—orbit and vibronic coupling terms.     

The triplet state of magnesiumporphin · ethanol in an n-octane crystal studied by microwave induced changes in the fluorescence intensity

The zero-field splitting of the triplet state of magnesiumporphin solvated by ethanol is represented by D = 0.035 cm^?1 and |E| = 0.010 cm^?1. The decay rates of the upper two spin components both are found to be about 20 s^?1, while that of the bottom component (where the spin lies in the molecular plane) is about 2 s^?1.     

Experimental study of triplet exciton diffusivity in crystalline pyrene at 300 K

Preliminary measurements of the triplet diffusivity tensor in crystalline pyrene at 300 K are reported. Maximum diffusivity occurs along the b axis and D_ab = (1.25 ± 0.3) × 10^?4 cm² s^?1. Diffusion in the ac′ plane is nearly isotropic with D ≈ (0.3 ± 0.1) × 10^?4 cm² s^?1. These results are tentatively interpreted as diffusion dominated by nonlocal scattering.     

The lowest triplet state of K2Cr2O7. Electron spin resonance in an external magnetic field at 1.2 K

ESR experiments on the luminescent triplet state of triclinic K₂Cr₂O₇ are reported. For the A site D = 24.300, E = 2.324 GHz, for t     

Crystal field effects on the ground and lowest excited triplet states of benzene isotopes in a borazine host crystal

The phosphorescence spectra of the C₆H₆C₆H₅D₁p-C₆H₄D₂symp-C₆H₃D₃, C₆D₆ and ¹³CC₅D in a borazine host crystal are analyzed at high resolution. The spectral lines are sharp (～2 cm^?1 wide) indicating that the impurity molecules occupy a unique site in the borazine lattice which is probably substitutional. The phonon sidebands are weak,giving clean, well-resolved spectra much like those of isotopic mixed crystals. In contrast, however, the crystal field effects on the ground state vibrational levels are much smaller than those found for isotopic mixed crystals. The gas-to-crystal shifts are very small, the vibrational degeneracies are not removed and orientational splittings are only observable for a few select vibrational levels. For most vibrational levels and for the derivation of selection rules one can asume the effective crystal site symmetry to be D_3d. The data provide the first conclusive evidence that the splitting observed in the benzene phosphorescence spectrum results from a distortion of the molecule when excited to the zeroth vibrational level of the T₁ state. Furthermore, the data suggest that the distortion is intrinsic in nature (i.e.,is not caused by the crystal field).     

Photosensitization mechanisms of triplet excited state beta-lapachone. A density functional theory study

Beta-Lapachone is a natural product with multiple pharmacological activities and mechanistic studies indicated that reactive oxygen species (ROS) generated by beta-lapachone play significant roles in its pharmacological actions. As photosensitization is an important ROS-generating pathway, in the present work, the photosensitization mechanisms of beta-lapachone are explored on the basis of density functional theory estimated triplet excited state characters. Starting from triplet excited state beta-lapachone, the possible generating pathways of 1O2 and O2*- are elucidated and the solvent effects on the photosensitizing reactions are also discussed.     

The lowest triplet state of K2Cr2O7. Optical spectra and optically detected electron spin resonance at 1.2 K

High-resolution emission and excitation spectra are reported for a K₂Cr₂O₇ crystal at 1.2 K, and the effect of microwave saturation on thes     

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.     

A flash photolysis study of the triplet state quenching of anthracene by oxygen in the presence of polystyrene

The rate constant for the quenching of triplet anthracene by molecular oxygen in the presence or absence of polystyrene was measured. The quenching ability is not affected by the presence of polymer.     

Solid state vibrational spectroscopy of disordered crystals. A study of dimethylsulphoxide at 213 K

The vibrational spectrum of crystalline dimethylsulphoxide at 213 K has been studied by i.r. and Raman spectroscopy. Such an experiment is proposed as an example to show how much the dipole-dipole interaction forces can affect the molecular disorder often observed in molecular crystals of compounds containing sulphur atoms.