Phosphorescent Triplet State Research Articles

Optical detection of proton endor of short-lived phosphorescent triplet states of aromatic molecules

Electron nuclear double resonance of protons (proton ENDOR) of the short-lived triplet states of benzil in a benzophenone crystal and 2,3-dichloroquinoxaline (DCQ) in a 1,2,4,5-tetrabromobenzene crystal which are not detectable by the normal ENDOR was observed by monitoring phosphorescence intensity at ENDOR transitions are interpreted by a simple model. The spin densities on the carbons bonded to hydrogens in the triplet state of DCQ were found to be ϱ5 = ϱ8 = 0.239 and ϱ6 = ϱ7 = 0.072.

Spin—lattice relaxation in the phosphorescent triplet state of acridine and phenanthrene in zero-magnetic field

The three spin—lattice relaxation rates for the triplet state of acridine in zero-field have been determined in a biphenyl host from 1.7–3.2 K. The mean relaxation rate for the triplet state of phenanthrene was determined in a biphenyl host from 1.3–13 K. Two conclusions of a more general nature emerge: (1) the relaxation amongst the spin components of a given triplet state in zero-field may be markedly anisotropic; (2) the time scale on which relaxation occurs may vary by several orders of magnitude for different molecules.

Application of the stretched polymer-film method to the electron spin resonance study of phosphorescent triplet states

Using a stretched polyvinyl alcohol film as a host, electron spin resonance of phosphorescent triplet states of several aromatic compounds has been studied. Since the guest molecules have a tendency to orient biaxially, the determination of the fine structure tensor is rather straightforward. The advantage of the present method is discussed in relation with various conventional methods.

Zero-field magnetic resonance study on the phosphorescent triplet state of benzil

The properties of the lowest excited triplet state of benzil in neat crystals have been studied with zero-field experiments combining microwave spectroscopy and optical detection. Two emitting species, originating from X traps of benzil crystals, were detected. The two X traps were found to have quite different trap depths and zero-field splittings, yet their triplet kinetics proved to be the same. One of the three spin sublevels almost totally dominates the populating and depopulating activities of the phosphorescent triplet state. A new and more accurate method is described for the determination of the steady-state population ratios. A discussion of the geometry of the benzil triplet state is given, and it is proposed that benzil assumes a trans- (near) planar configuration in the lowest triplet state. Finally, a change in the zero-field resonance spectrum is reported near a second order phase transition of benzil crystals near 80 °K.

Coherence in multilevel systems. I. Coherence in excited states and its application to optically detected magnetic resonance in phosphorescent triplet states

A method for monitoring the time development of the coherent components in a multilevel system using an observable which can measure only the relative populations in the levels is presented. Using standard density matrix formulation, expressions are obtained for the conventional and this new ’’probe’’ pulse method. For several of the more common coherence experiments, optically detected magnetic resonance of molecular excited triplet states is used to illustrate the principles. The effect of coherent averaging of relaxation terms in the spin Hamiltonian is discussed and specifically applied to excited triplet states in zero field. Finally, experimental results of coherence measurements are presented along with a detailed description of the apparatus and methods used.

Level anticrossing and mixing effects in excited molecular triplet states. Direct observation by optical detection of magnetic resonance

This paper reports the direct observation of level anticrossing and mixing effects between a pair of Zeeman sublevels of a phosphorescent triplet state using optical detection of magnetic resonance (ODMR) methods. By carrying out the ODMR experiment at resonance fields below, at, and above the avoided crossing point, direct measurements of the interaction ( V) which couples the two states have also been made. For 13C-benzophenone in a single crystal of 4,4′-dibromodiphenylether at 1.6 K, we find | V| = 30 MHz with H| z , but in other orientations and systems different values of V are obtained.

Hyperfine and quadrupole coupling constants of deuterium nuclei in the phosphorescent triplet states of quinoline and quinoxaline

Results are presented of an ENDOR study on deuterium hyperfine and quadrupole interactions in the phosphorescent triplet state of two aza-aromatic molecules. Although the hamiltonian for a nucleus with spin I = 1 is more complicated than for a proton with I = 1 2 it is shown that in the case of deuterons one can obtain the hf and quadrupole interaction parameters and their orientational dependence with considerable precision.

Vibronic interactions in the phosphorescent triplet state of p-chloroaniline as studied by the method of microwave induced delayed phosphorescence

The vibronic interactions involved in the phosphorescent triplet state of p-chloroaniline in p-xylene have been studied by the method of microwave induced delayed phosphorescence. It is shown that vibrations of b 2 symmetry appear strongly in the emission from the T Z sublevel. This unusual appearance of a b 2 mode is discussed in some detail and the pseudo-Jahn-Teller distortion is suggested as a possible explanation.

Phosphorescence spectrum and triplet state of Sn(IV) tetraphenylporphin

Phosphorescence spectrum and triplet state of Sn(IV) tetraphenylporphin

Spin-forbidden radiationless processes in isoelectronic molecules: Anthracene, acridine and phenazine

Microwave induced delayed phosphorescence (MIDP) experiments have been performed to study the populating and decay of the phosphorescent triplet state T 0 of seven aromatic three-ring molecules : anthracene (-h 10, -h 2d8, and -d10), acridine (-h9 and -d9) and phenazine (-h8 and -d8), all diluted in biphenyl crystals. We chose these molecules because the deactivation of T 0 goes dominantly via radiationless processes and the total decay rates of the individual spin components of T 0 determined in the experiment thus equal the radiationless decay rates. The phosphorescence emission, used as a monitor for the triplet state population, here gives an insignificant contribution to the decay. MIDP experiments only yield the true decay rates if thermal isolation between the levels is maintained throughout, and the present investigations, in which the intensity of excitation has been varied over a wide range, uncover a possible source of systematic error that has been overlooked thus far. The principal results of...

The determination of the structure of low temperature triplet energy traps using PMDR methods. I. The magnetic axes of hexachlorobenzene excimers

Using linearly polarized microwave radiation, the optically detected microwave signal of the zero-field transitions of the hexachlorobenzene broad excimer-type emission is studied as a function of the angle between the microwave H field and the crystal axes. The dependence of the PMDR signal on the angle between the H field of the linearly polarized microwave and crystal axes is examined. From these studies, the exact orientation of the three orthogonal magnetic axes of a phosphorescent triplet state in a single crystal host can be elucidated if three microwave polarization ratios are determined. The method is applied to determine the principal magnetic axes of the triplet state giving rise to the excimer type emission of the hexachlorobenzene crystal at 1.6°K.

Optically detected zero field magnetic resonance in the lowest triplet nπ* state of p-benzoquinone

Optically detected magnetic resonance (ODMR) techniques are used to determine the orbital assignment of the phosphorescent triplet state of p-bezonquinone in the condensed phase. The principal spin—orbit coupling routes are discussed in terms of the observed spin—vibronic activity.

OPTICALLY DETECTED MAGNETIC RESONANCE OF THE TRYPTOPHAN PHOSPHORESCENT STATE IN NATIVE PROTEINS*

Abstract— The phosphorescent triplet state of tryptophan has been studied by the method of optically detected magnetic resonance (ODMR) at pumped helium temperatures in zero magnetic field. Only one of the triplet sublevels is found to be significantly radiative; the other two decay radiationlessly. Although the phosphorescence and ODMR decay lifetimes are influenced by spin–lattice relaxation processes at T= 1.3°K, the lifetime of the radiative level can be estimated as approximately 2 s, whereas the lifetimes of the non–radiative levels are in excess of 10 s. Comparison of the ODMR signals and the phosphorescence spectra has been made for tryptophans in native proteins with the following results: the ODMR signals of the two types of tryptophan sites in horse liver alcohol dehydrogenase can be resolved due to a shift in the D and E values of the respective triplet states; binding of the substrate tri‐N‐acetylglucosamine to hen lysozyme leads to a considerable narrowing of the phosphorescence peaks and ODMR signals as well as to a shift in the E value of the triplet state.The following tentative conclusions can be reached: the tryptophan triplet D and E values are measurably affected by the environment of the chromophore in the protein, as are the linewidths of the magnetic resonance transitions. The |E| value is reduced and the magnetic resonance linewidth is increased with increasing exposure of the tryptophan to hydroxylic solvent. Although a considerable part of the width of the magnetic resonance transition can be ascribed to a heterogeneity of environments in the sample, there appears to exist an intrinsic line–broadening process which at present is not understood.

Dimethylsilyl- and methylene-bridged aromatic groups : II. Phosphorescent triplet states

The zero-field splitting parameters D and E, or D, were measured by electron spin spectroscopy for the triplet states of bis(4-biphenylyl)dimethylsilane, 4,4′-bis-(4-biphenylyldimethylsilyl)biphenyl, 1,2-diphenyltetramethyldisilane, several monosilanes, bis(4-biphenylyl)methane, 4,4′-bis(4-phenylbenzyl)biphenyl, and dimethyldiphenylmethane. The zero-field splitting values of compounds having biphenyl subunits bridged with either SiMe 2- or CH 2-groups, compared with those of mono- and disubstituted biphenyls, show that the triplet electrons are localized mainly on one biphenyl group in each of the bridged compounds on the time scale of the ESR observations.

Intersystem Crossing Quantum Yields of Phthalazine and Pyridazine

Abstract The quantum yields of S1→T1 intersystem crossing have been determined for phthalazine and pyridazine in EPA at 77°K by using the technique of T–T energy transfer. Phthalazine and pyridazine were used as the energy donor, and naphthalene-h8 and naphthalene-d8 as energy acceptor. The intersystem crossing quantum yields were found to be 0.49 for phthalazine and 0.24 for pyridazine. These values lead to the conclusion that the weakness or lack of phosphorescence in the compounds is due to an anomalously high rate of the radiationless transition from their phosphorescent triplet state as compared with the rate of the corresponding radiative transition.

Optically Detected Adiabatic Inversion in Phosphorescent Triplet States and the Measurement of Intramolecular Energy Transfer Processes

The theory and observation of microwave induced population inversion within triplet magnetic sublevels in zero field are presented. It is shown that inversion via adiabatic fast passage can be measured quantitatively by changes in triplet phosphorescence and that population inversion can be accomplished easily and with microwave field strengths even lower than are required for cw saturation. Parameters affecting the ability to invert the sublevel populations, such as dipolar interactions, microwave power, and sweep rates, are studied. General equations relating the intensity of phosphorescence and the fraction of inversion to parameters associated with energy transfer processes into and from the triplet state such as intersystem crossing and radiative and radiationless relaxation are derived and measured for the π π*3 state of 2,3-dichloroquinoxaline.

Optical detection of the zero field splittings in the phosphorescent triplet state of neat SO 2 crystals

The zero field transition energies in the phosphorescent triplet state of neat SO 2 are directly measured by optical detection at 1.4°K, leading to | D| = 0.275 cm −1 and | E| = 0.0157 5 cm −1 for the usual spin hamiltonian parameters. Two spin states are found to be active in the phosphorescence with the dominant spin state having a lifetime of 2 msec.

Phosphorescence process in phthalazine

The phosphorescence and its polarization characteristics have been studied for phthalazine in EPA at 77°K. The phosphorescence spectrum consists of at least two progressions. One originating from the O-O band, which is relatively weak, is polarized out of the molecular plane; the other commences at 580 cm −1 from the O-O band and is polarized along the long axis of the molecule. The intensity of the phosphorescence is ascribed predominantly to the latter progression involving vibronic interaction. These observations, together with the measured phosphorescence lifetime and quantum yield, 0.40 and 0.04 sec respectively, indicate that the phosphorescent triplet state is the 3 B 2(π,π ∗)( 3 L a) state. It is concluded that the following spin-orbit coupling mechanism is important in the phosphorescence process of phthalazine: ▪

Effect of methyl substitution on phosphorescent triplet states of naphthalene via electron spin resonance

Zero-field splittings of mono- and dimethylnaphthalenes in EPA glass at 77°K were obtained from ΔM s = ±1 transitions in the X-band. The deviations of X and Y values for dimethylnaphthalenes were discussed in terms of substituent locations.

The dynamics of populating and depopulating the phosphorescent triplet state as studied by microwave induced delayed phosphorescence

It is known that at low temperatures relaxation between the spin components of a phosphorescent triplet state of an organic molecule may become very slow as compared to their individual rates of decay. Spectacular changes in the intensity emitted can then be induced by suddenly sweeping a microwave field through one of the zero-field resonances during phosphorescence decay. These microwave induced signals arise because molecules are transferred from a populated non-radiative level to an empty radiative one. In this paper a system of experiments based on this phenomenon is presented which enables one to solve the dynamics of populating and depopulating the individual levels of the phosphorescent state. As a demonstration results of experiments on the aza-naphthalenes quinoline and quinoxaline are given.