EXTERNAL HEAVY ATOM EFFECTS ON THE S1 AND T1 ELECTRONIC STATES OF INDOLE

Abstract— The absorption, fluorescence and phosphorescence spectra along with the phosphorescence decay function of indole perturbed by various amounts of halocarbons have been studied in a 1:1 ethanol-ether glass at 77 K. For carbon tetrachloride. chloroform, halothane and methylene chloride used as quenchers. the biexponential nature of the phosphorescence decay prompted us to assume the existence of a triplet state complex (exciplex) between indole and the halocarbons. On the other hand. propyl bromide gives rise to a non-exponential decay and to an increase in the phosphorescence intensity suggesting the occurrence of a normal external spin-orbital coupling interaction between indole and the propyl bromide molecule.     

Spin alignment in the charge transfer phosphorescent state of the tetracyanobenzene-biphenyl complex in crystalline state

Intermolecular charge-transfer phosphorescence and fluorescence were observed for a biphenyl single crystal containing 1,2,4,5-tetracyanobenzene as a guest at various temperatures between 29 and 1.47°K. The non-exponential phosphorescence decay due to the spin alignment in the charge-transfer, triplet state was first observed and the phosphorescence lifetimes of the three sublevels were determined to be 0.9, 2.5, and 7.9 see at 1.47°K.     

Lowest excited triplet states of hypericin and isohypericin

The fluorescence and phosphorescence of hypericin and isohypericin were studied in an ethanol matrix at 1.2 K. The prompt fluorescence spectra are mirror images of the absorption around the 0-0 transition, as expected. The 0-0 vibronic lines of the phosphorescence are found at 13190 and 13622 cm⁻¹, and the phosphorescence decay times are 2.79 and 6.6 ms at 1.2 K for hypericin and isohypericin respectively. The fluorescence excitation spectrum of isohypericin reveals a small trace of a different pigment, possibly a tautomeric form of isohypericin. The decay of the phosphorescence of isohypericin is biexponential. The slow component is attributed to an unknown contaminant with a high quantum yield of phosphorescence.     

Multiphoton tea CO2 laser excitation of the triplet state of propynal

Multiple infrared photon excitation of propynal triplet molecules gives rise to a strongly perturbed phosphorescence. Following absorption of a few IR photons per molecule the phosphorescence spectrum extends to higher energy, the intensity increases, the decay — deviating from the original exponential decay — accelerates and the emission quantum yield drops dramatically. These findings are explained in terms of temperature sensitive radiative (T₁ → S₀) and radiationless (T₁ → S₀) processes with the vibrational temperature as the determining factor. During the perturbed triplet decay, the IR excitation initially confined to the vibrational degrees of freedom becomes distributed among all degrees of freedom which results in a decrease in the vibrational temperature and thus a complex phosphorescence decay.     

Characterization of room temperature phosphorescence of propranolol and the chiral discrimination between R- and S-isomers

Upon addition of a small amount of bromocyclohexane, propranolol displays room temperature phosphorescence in γ-cyclodextrin solution without deoxygenation. Several factors including the pH, and the concentration of γ-cyclodextrin and bromocyclohexane, which affect the room temperature phosphorescence (RTP) intensity and room temperature phosphorescence lifetime of propranolol are studied in detail. Under optimal conditions, the room temperature phosphorescence lifetimes of propranolol enantiomers are measured. The experimental results show that the associated phosphorescence decay curves can be best fitted to mono-exponential patterns and room temperature phosphorescence lifetimes of R- and S-propranolol are 4.60 ms and 5.74 ms, respectively. The difference of the room temperature phosphorescence lifetimes of R- and S-propranolol is 22.05%. Based on that, chiral discrimination of propranolol enantiomers is carried out successfully by time-resolved phosphorescence.     

ON THE ROOM TEMPERATURE PHOSPHORESCENCE OF WOOL KERATIN

Abstract—A study has been made of the room-temperature phosphorescence behaviour of wool keratin and its major constituent proteins. The 290 nm induced phosphorescence of wool, previously thought to decay by second-order kinetics, is shown to consist of two separate components which decay exponentially with lifetimes of 0.18 × 0.03 and 1.45 × 0.25 s. Selective oxidation of the tryptophyl residues eliminates the long-lived phosphorescence, but has no effect on the short-lived component. The latter is variable in its intensity, and depends upon the history of the fibre. The origins of the different emissions are discussed, and the possible involvement of the phosphorescent species in the photochemical reactions leading to discoloration of the fibre are considered.     

Temporal characteristics of the S1 and T1 thiophosgene Cl2CS in the gas phase: Comparison of the T1 decay with theoretical predictions

The picosecond optical-optical double resonance experiment in a supersonic free jet as well as the vapor-phase phosphorescence indicates that the decay of T(1) Cl(2)CS belongs to the intermediate case of the classification scheme for electronic relaxation. The A(fast)/A(slow) pre-exponential ratio in the biexponential T(1) decay is much greater under picosecond excitation than under nanosecond excitation. In vapor phase at low pressure, the phosphorescence exhibits a decay time that varies with the coherence width of the laser used for excitation. Both the T(1) and the S(1) decay times of Cl(2)CS depend strongly on temperature, indicating that Coriolis coupling plays an important role in mode mixing (intramolecular vibrational redistribution).     

Resolution of uranium isotopes with kinetic phosphorescence analysis

This study was conducted to test the ability of the Chemchek? Kinetic Phosphorescence Analyzer Model KPA-11 with an auto-sampler to resolve the difference in phosphorescent decay rates of several different uranium isotopes, and therefore identify the uranium isotope ratios present in a sample. Kinetic phosphorescence analysis (KPA) is a technique that provides rapid, accurate, and precise determination of uranium concentration in aqueous solutions. Utilizing a pulsed-laser source to excite an aqueous solution of uranium, this technique measures the phosphorescent emission intensity over time to determine the phosphorescence decay profile. The phosphorescence intensity at the onset of decay is proportional to the uranium concentration in the sample. Calibration with uranium standards results in the accurate determination of actual concentration of the sample. Different isotopes of uranium, however, have unique properties which should result in different phosphorescence decay rates seen via KPA. Results show that a KPA is capable of resolving uranium isotopes.     

Phosphorescence and its characteristics in pyridine vapor

The phosphorescence emission of pyridine-d₀ and -d₅ has been observed in the vapor phase by means of time-resolved spectroscopy. The results of the experiments, which are described in full detail in this paper, on the phosphorescence spectrum, the excitation spectrum, the phosphorescence decay and sensitization of biacetyl phosphorescence indicate that the emission concerned is the genuine phosphorescence of the pyridines. For pyridine-d₀ (-d₅), the wavelength of the phosphorescence maximum is 450 nm (440 nm), the phosphorescence quantum yield 1.5 × 10^?6) (1.7 × 10^?6) and the phosphorescence lifetime is 1.2 μs (2.1 μs), the values for pyridine-d₅ being given in parentheses. The phosphorescence characteristics of pyridine are compared with those of other related molecules. The nature of the phosphorescent triplet state of pyridine is discussed with particular regard to the exceptionally fast non-radiative decay from that state.     

Dynamics of the tyrosine triplet state from magnetic resonance-saturated phosphorescence decay measurements

Analysis of the phosphorescence decays measured during magnetic resonance saturation of sublevel populations has been carried out on tyrosine and tyrosinate triplet states at 1.17°K in zero field. The individual sublevel decay constants and spin-lattice relaxation rate constants are derived. Relative intersystem crossing rates to the sublevels are obtained from flash excitation microwave-induced delayed phosphorescence measurements. Intersystem crossing is not spin-selective in tyrosine, but becomes so upon ionization near pH = 12. The spin-selective intersystem crossing mechanism in tyrosinate is discussed in terms of a model proposed by Bersohn.     

Non-exponential phosphorescence decay of phenanthrene in biphenyl

Non-exponential phosphorescence decays of phenanthrene in biphenyl polycrystals have been observed. It is found that excitation with short duration quickens the decay while the decay is slower after excitation with weaker intensity. The origin of the non-exponentiality is ascribed to the distance-dependent interactions between guest molecules in the lowest excited triplet state.     

Sensitized phosphorescence of benzil-doped ladder-type methyl-poly(para-phenylene)

The delayed luminescence and phosphorescence of ladder-type methyl-poly(para-phenylene) (MeLPPP) doped with benzil at a concentration of 20% by weight has been measured. The introduction of benzil leads to a dramatic reduction of the polymer singlet emission. At the same time, a new band with maximum at 611 nm appears, corresponding to the phosphorescence of MeLPPP. The phosphorescence decay on the short time scale is close to an exponential law with a time decay of 15 ms. This indicates that benzil can efficiently sensitize the phosphorescence of the polymer. In addition, a broad and featureless emission is observed in the delayed luminescence spectra of benzil-doped MeLPPP, which is attributed to an exciplex formed between the polymer host and the dopant. We further observe that the delayed fluorescence is enhanced by the addition of benzil. It is concluded that the delayed fluorescence of benzil-doped MeLPPP is mainly due to the annihilation of triplet excitons on the polymer. Finally, efficient triplet-triplet energy transfer from the benzil-doped polymer to the red-emitting phosphorescent dye Pt(II)octaethylporphyrin is established.     

Comparison of different alcohols in a caged lumophore for measuring supersaturated dissolved oxygen

Intense phosphorescence is observed when an alcohol is introduced to an aqueous solution containing 1-bromonaphthalene and a cyclodextrin complex. The enhancement of the phosphorescence intensity of 1-bromonaphalene is related closely with the alcohol’s effectiveness in shielding the photo-excited lumophore from dissolved oxygen. The phosphorescence intensity decreases and the phosphorescence decay rate increases as the dissolved oxygen concentration increases. In this paper, we measure the phosphorescence intensity and decay rate of the complex using different alcohols and explain their respective advantages. By relating the concentration of oxygen to the intensity and the lifetime of the phosphorescence, the dissolved oxygen concentration can be measured. The sensitivities and measurement ranges with the complexes containing different alcohols are compared. Currently, this technique can measure dissolved oxygen concentrations from 0 to 40.2 mM (31,400% saturation). The range of the measurement is 40 times larger than previous studies.     

Distinctive characteristics of the decay function for phosphorescence in the presence of reabsorption

The phosphorescence decay under the effect of significant excited-state absorption has a distinctive signature: it is initially concave, switching to convex (pure exponential decay) after a certain time. A simple one-parameter decay function satisfactorily reproducing the experimental decays is discussed, and some of its peculiar mathematical properties analyzed.     

Manipulating the Ultralong Organic Phosphorescence of Small Molecular Crystals

Ultralong organic phosphorescence (UOP) of metal-free organic materials has received considerable attention recently owing to their long-lived emission lifetimes, and the fact that they present an attractive alternative to persistent luminescence in inorganic phosphors. Enormous research effort has been devoted on improving UOP performance in metal-free organic phosphors by promoting the intersystem crossing (ISC) process and suppressing the non-radiative decay of triplet state excitons. This minireview summarizes the recent advances in the rational approaches for manipulating the UOP properties of small molecular crystals, such as phosphorescence lifetime, efficiency, and emission colors. Finally, the present challenges and future development of this field are proposed. This review will provide a guideline to rationally design more advanced metal-free organic phosphorescence materials for potential applications.     

PHOSPHORESCENCE MAXIMA AND TRIPLET STATE LIFETIMES OF NAD + AND ε-NAD+ IN TERNARY COMPLEXES WITH HORSE LIVER ALCOHOL DEHYDROGENASE

This paper describes the phosphorescence emission and decay times of NAD+ and its fluorescent etheno derivative, epsilon-NAD+, in the pyrazole ternary complex with horse liver alcohol dehydrogenase (ADH). We show that the epsilon-NAD+ triplet state, as well as the tryptophan triplet state, can be utilized to monitor the coenzyme-enzyme interaction. The decays of NAD+ and AMP are single exponential, and the lifetimes are the same within experimental error. The phosphorescence lifetimes, evaluated as single exponentials, are slightly shorter in epsilon-NAD+ than they are in epsilon-AMP. Whereas the decay of epsilon-AMP was adequately fit by a single exponential with a time constant of very close to 0.5 s, it was necessary to fit the decay of epsilon-NAD+ to a double exponential. Ternary complexes with NAD+ excited at 297 nm exhibit decay kinetics nearly identical to those of ADH by itself. On the other hand, when excitation of the epsilon-NAD+ ternary complex is provided at 313 nm, where there is very little absorption by either tryptophan residue, the decay law of the ternary complex is similar to that of epsilon-NAD+ in solution. Our results demonstrate that NAD+ and epsilon-NAD+ quench tryptophan phosphorescence in ADH. Normalizing the phosphorescence intensity to the 0-0 vibronic band assigned to Trp-15 (blue-edge), we calculate a 21% decrease in the phosphorescence associated with Trp-314 at stoichiometric saturation of the coenzyme binding sites with NAD+ in the ternary complex. When the active sites are saturated with epsilon-NAD+, the relative phosphorescence due to Trp-314 decreases by 63%.(ABSTRACT TRUNCATED AT 250 WORDS)     

Determination of the spin sublevel origin of radiative and non-radiative decay routes of phosphorescing benzene derived from precision lifetime measurements

The increase in the phosphorescence lifetime as temperature is lowered has been quantitatively determined in the 2–30 K range. Applying the Boltzmann population expression yields the information that virtually none of the radiative decay originates from the lowest spin sublevel (A_2g).     

REMARKABLE INHIBITION OF OXYGEN QUENCHING OF PHOSPHORESCENCE BY COMPLEXATION WITH CYCLODEXTRINS

The phosphorescence of the 4-bromo-l-naphthoyl group is readily quenched by molecular oxygen in homogeneous solvents. However, when this lumophore is complexed with γ-cyclodextrin in aqueous solution at room temperature, its phosphorescence is observed even under 1 atm of oxygen! Phosphorescence decay data indicated that two types of probe/cyclodcxtrin complexes are formed with lifetimes of 600 u,s and 3.5 ms. Oxygen completely quenches the fast decay, but only partially quenches the slow decay.     

Double exponential phase plane method for decay analysis in room temperature phosphorimetry

The phase plane (PP) method for evaluation of decay parameters is applied to the determination of room-temperature phosphorescence lifetimes on filter paper substrates. The method proposed here is an extension of the double exponential PP method, to include a correction for distortion in the decay data mainly due to stray light and instrument electronics. As a result, short lifetimes can be accurately determined. The ability to analyze decay curves with a high background phosphorescence and the detection of double exponentials where the semilogarithmic method provides a single decay component are other advantages of the method proposed here.     

Luminescence behavior of the pyridine—BCl3 complex

A phosphorescence and CNDO study of the 1:1 electron donor-acceptor complex formed between pyridine and boron trichloride has been shown to account satisfactorily for the electronic states and induced luminescence of pyridine resulting from complexation. The phosphorescence yield and formation constant of the complex in ethyl alcohol were determined to be 0.38 ± 0.04 and 1.3 × 10⁴ M⁻¹ respectively. CNDO calculations show that the lowest n,π* levels are shifted to significantly higher energies, thereby removing efficient radiationless decay routes available with these states.