LUMINESCENCE OF TAUTOMERIC FORMS OF THYMINE MONOANIONS

Abstract— The fluorescence and phosphorescence emission and excitation spectra of monoan-ions of thymine. I-methyl-thymine and 3-methylthymine (in alkaline solutions, 10^-2N NaOH) at 300° and 77°K were measured. It was shown, that at 300°K only 3-HT- tautomeric thymine monoanion fluoresces; at 77°K emissions of both monoanionic species contribute to the observed luminescence spectrum.     

SPECTROSCOPY OF POLYENES–III. ABSORPTION AND EMISSION SPECTRAL INVESTIGATION OF POLYENE SCHIFF BASES AND PROTONATED SCHIFF BASES RELATED TO VISUAL PIGMENTS

Abstract— A number of n -butylamine Schiff bases of polyenals related to retinals as homologues and analogues, and their protonated forms, have been studied for absorption and emission spectral properties. The polyene Schiff bases exhibit the same general features in their absorption spectra as those of the parallel polyenals except that the ^lB_u←¹A_g and π*← n singlet transitions are at substantially higher energy in the Schiff bases (the shift being larger for the π *← n transition). The Schiff bases with short polyene chainlength ( n = 2, 3 where n is the number of double bonds including C=N) do not fluoresce or phosphoresce in 3-methylpentane in the temperature range 298–77 K. The Schiff bases with intermediate chainlength ( n = 4, 5) show fluorescence at 77 K with intensity strongly dependent on the nature of solvent. The Schiff bases with relatively long chainlength ( n = 5–7) show strong or moderately strong fluorescence at 77 K and very weak fluorescence at 298 K ( n = 7) with intrinsic radiative lifetimes much longer than those estimated from the oscillator strength of the low-energy, strong absorption band (¹B_u←¹ A_g ). A discussion on the possible state order and nature of the fluorescing state of the various polyene Schiff base systems is presented.     

THE PHOTOPHYSICS OF p-AMINOBENZOIC ACID

Abstract— The lowest-lying allowed UV transition in p -aminobenzoic acid (PABA) is assigned Γ→¹L_a based on quantitative absorption and fluorescence studies, as well as semiempirical PM3 multielec-tron configuration interaction calculations. The oscillator strengths, fluorescence quantum efficiencies and lifetimes are reported for PABA in several polar, nonpolar, protic and aprotic solvents (aerated) at 296 K. Reasonable agreement is found between the observed radiative rate constant and that calculated from the absorption and fluorescence spectra. Shifts in the absorption and fluorescence spectra in aprotic solvents are analyzed in terms of the Onsager reaction field model; results are consistent with an increase in dipole moment of ca 4 D between the relaxed S₀ and S₁, states. No evidence is found for the emission from the amino-twisted form of PABA in all solvents studied although calculations show that the amino-twisted S, state is highly polar, but higher in energy by ca 35 kJ/mol ( in vacuo ). The fluorescence efficiency is excitation wavelength independent in both methylcyclohexane and water. The temperature dependence of the nonradiative rate constant (from S₁) was studied in several solvents. Nonradiative decay may be due to intersystem crossing, which would be fast enough to compete with thermally activated intramolecular NH₂ twisting. The phosphorescence spectrum and lifetime obtained in an EPA glass at 77 K are reported, and the triplet energy of PABA is estimated.     

Temperature effects on femtosecond transient absorption kinetics of semiconducting single-walled carbon nanotubes

We report femtosecond transient absorption kinetics measured for selected semiconducting single-walled carbon nanotubes at different temperatures between 77 and 290 K. The nanotubes are embedded in a thin polymethylmethacrylate film, and the dominance of individual species enabled to probe selectively the kinetics associated with two desired tube types, the (6,5) and (7,5) tubes. A strikingly similar temperature dependence is found between the maximum change in the amplitude of transient absorption kinetics, the overall decay time and steady-state fluorescence emission intensity. The simplest explanation for our data is that the temperature dependence of the fluorescence intensity and the exciton decay kinetics are dominated by nonradiative decay and that the radiative decay rate is weakly temperature dependent.     

Fluorescence lifetime and quantum yield of diphenylketyl radical at low temperature

The fluorescence lifetimes of the diphenylketyl radical trapped in ethanol and EPA matrices are 16.8 ± 0.5 and 21.1 ± 0.6 ns, respectively in the temperature region of 77–130 K. The fluorescence quantum yield in ethanol matrix is 0.16 ± 0.6. These values are consistent with the radiative lifetime calculated from the absorption spectrum.     

Environmental effects on the absorption and luminescence properties of aromatic amines: Aniline

Emission and absorption spectra of aniline have been studied in different solvents at 300 K and 77 K. This molecule exhibits broad and structureless fluorescence and poorly resolved phosphorescence in glassy polar ethanol and non-polar methylcyclohexane and n-hexane matrices at 77 K. Unlike the behaviour in a polar medium, fluorescence is found to be more intense than phosphorescence in non-polar media. In contrast to the case of glassy matrices, aniline shows highly stuctured emission and excitation spectra in crystalline cyclohexane and benzene matrices at 77 K. Excited state dipole moment calculation shows that both the singlet states contain charge transfer character, which is responsible for the strong phosphorescence in polar glassy medium. Spectral analysis indicates that the aniline molecule is slightly distorted in both the singlet and triplet states. The marked reduction in phosphorescence intensity of this molecule observed in non-polar matrices, particularly in a benzene crystalline matrix, relative to that in an ethanol matrix, is attributable to the decreased intersystem crossing rate and increased non-radiative rate from the emitting triplet state.     

Polyene spectroscopy: Vibronic evidence for a forbidden transition in DECA-2,4,6,8-tetraene

The dimethylpolyene deca-2,4,6,8-tetraene was studied by absorption, fluorescence excitation and fluorescence spectroscopy in glasses at 77 K and in n-alkane crystals at 4.2 K. A strong transition to a ¹B_u excited state is observed with an origin at 32400 cm^?1 in isopentane at 77 K and at 31280 cm^?1 in n-undecane at 4.2 K. A weak transition to a ¹A_g excited state is observed with an origin at 28738 cm^?1 in the n-undecane matrix. The radiative fluorescence lifetime is 500 ns. In undecane the transition from the ground state to the ¹A_g excited state exhibits a classic Herzberg—Teller vibronic pattern indicating a symmetry forbidden transition.     

The peculiarities of the spectral luminescence properties of N-anthryl-substituted pyridinium cations

A structureless fluorescence band λ^max_f ≈ 530 nm) in liquid solution and a fluorescence band with vibrational structure (λ = 376–480 nm) in rigid media (ethanol, 77 K and poly(methyl methacrylate) (PMMA), 296 K) were observed for N-anthryl-substituted pyridinium cations. In liquid solution the long-wave fluorescence is attributed to the biradical structure resulting from the photoinduced electron transfer (PET) from anthracene to the pyridinium ring, and the long-wave, low intensity absorption (and fluorescence excitation) band (λ=465 nm) is attributed to this interfragment PET which is weakly allowed by the torsional vibrations. In rigid media the PET is strongly forbidden (in the strict orthogonal pyridinium-anthracene structure) and radiative deactivation of the anthracene fragment is observed.     

Fluorescence properties of fluoranthene as a function of temperature and environment

The fluoranthene fluorescence properties were investigated in a variety of environments, i.e. in different solvents and temperatures, in the solid state, and in the vapor phase. The emission maximum was found to be independent of environment. The absorption spectrum in different solvents exhibits only minor changes. In solution, the fluorescence lifetime shows a slight inverse relationship to the solvent dielectric constant. With water/methanol mixtures of varying composition as the solvent, the lifetime decreases linearly with increasing mole fraction of water. At 77K, the fluoranthene fluorescence lifetime in frozen polar and nonpolar solvents are the same within experimental error. In hexane the fluorescence lifetime is independent of temperature (77±3 and 82±7 ns, at room temperature and 77 K, respectively). In methanol the lifetime is 64±3 ns at room temperature and increases linearly to 80±4 ns at 77 K. In the vapor phase the lifetime is 32±1 ns. No fluorescence quantum yield change was observed for either S₁ or S₂ manifold excitation.     

CHEMILUMINESCENCE FROM CARBENE OXIDATION

Abstract— The chemiluminescent reaction between carbenes and O₂ in Freon 113 at and above 77 K was investigated. The reaction was found to be general for phenylated carbenes and independent of precursor. Spectra obtained by optical multichannel analyzer (OMA) are consistent with the emission being from the excited ketone formed. Kinetics studies at 77 K indicate that the decay of the chemi-luminescence is significantly faster than the decay of the carbene as monitored by fluorescence and electron paramagnetic resonance.     

LUMINESCENCE OF L-TRYPTOPHAN IN THE DRY STATE AND IN AQUEOUS SOLUTION INDUCED BY X-RAYS AND U.V.-LIGHT AT 77°K

Abstract— The luminescence from tryptophan in the dry state and in aqueous glass. induced by u.v.-light and X-rays at 77°K, has been studied. The spectra of the luminescence from the dry material, which were identical for the two types of irradiation, were attributed to the same excited singlet level as that giving rise to the fluorescence from the aqueous sample. The strong phosphorescence exhibited by the latter. could not be detected for the dry material with either type of irradiation. This was attributed to a strong quenching of the radiative triplet level occurring upon crystallization of the substance. The yield of X-ray induced fluorescence of tryptophan in aqueous glass was found to be similar to that observed in the dry state. It was concluded that there is no extensive transfer of energy between the solvent and the solute molecules contributing to the fluorescence yield of tryptophan in aqueous glass.     

LUMINESCENCE AND PHOTOCHEMISTRY OF 4-THIOURIDINE IN AQUEOUS SOLUTION

Abstract— In bidistilled water, 4-thiouridine (4TU) exhibits a weak unusual luminescence, the quantum yield of which is 3 × 10^-4 at 25°C. The excitation spectrum corresponds well to the 4TU absorption spectrum. The emission lies at longer wavelengths (Λ_max 550 nm) than the 4TU phosphorescence observed at 77 K (Λ_max, 470–480 nm). From the emission signal obtained after an excitation flash of 3 ns half-width, an "apparent" rate constant for the radiative deactivation process, shorter than 5 × 10⁶ s, can be inferred. The 300 K emission is efficiently quenched by halides and by oxygen: quenching involves a long-lived intermediate (⋍ 200 ns).
Clearly the emissive state X is populated through the S₀-S₁ electronic transition π→π* of 4TU. The nature of X cannot be unambiguously determined: it cannot be an excimer but can be either the 4TU triplet state or another chemical state distinct from the 4TU excited singlet or triplet states.
An interesting finding is that the 300 K emission and the ability of 4TU to photoreact are related: they are quenched with the same efficiency by halide anions. This indicates that quenching occurs at the same long-lived intermediate species , which is either a precursor of the emitter or the emitter itself.     

Luminescence studies in polymers—IV. Effect of orientation,tacticity and crystallinity on polystyrene and polyvinylcarbazole fluorescence

The fluorescence spectra of amorphous atactic, amorphous isotactic and crystallized isotactic polystyrene films have been compared. The effect of chain orientation has also been analysed on amorphous atactic samples. The results show that the fluorescence yield increases with crystallinity at room temperature and 77°K. The contribution of excimer fluorescence at 77°K increases according to the sequence: atactic < atactic oriented < isotactic amorphous < isotactic crystallized. An increase of the fluorescence yield with crystallinity was also observed for polyvinylcarbazole samples although the contribution of excimer fluorescence at 77°K is independent of crystallinity for this polymer. The results are interpreted in terms of energy migration.     

Fluorescence reabsorption in anthracene single crystals: Lifetime variations with emission wavelength and temperature

Fluorescence spectra and lifetimes of anthracene melt-grown single crystals and sublimation flakes have been examined at 298 and 77°K, using a mono-photon counting technique for the lifetime measurements. The observed emission decay times were nearly independent of the excitation wavelength, though a small dependence of the fluorescence spectrum on the excitation wavelength was noted. By contrast, large variations of fluorescence lifetimes in thick crystals were found as a function of emission wavelength. For thick melt-grown single crystals at 298°K the lifetime was found to increase from 9.8 nsec at 405 nm to 20.4 nsec at 445 nm. For sublimation flakes at 77°K and at 298°K and for thick melt-grown crystals at 77°K, the lifetimes were less than 10 nsec and were nearly independent of emission wavelength. Despite these relatively large variations in lifetimes, the decay rates at each separate wavelength remained exponential, within experimental error. Theoretical calculations were made of emission lifetimes based on a model with one reabsorbing state. The calculations are in substantial agreement with the experimental results.     

Luminescence studies in polymers—III. Copolymers styrene-methylmethacrylate and styrene-methylacrylate

The emission spectra of copolymers styrene-methylmethacrylate and styrene-methylacrylate with various compositions have been analyzed. Films and solutions have been studied at 77 and 300 K. With the exception of glassy solutions at 77 K, normal and excimer fluorescence are emitted. Their relative and absolute intensities vary with temperature and copolymer composition. An interpretation of the results, which takes into account the statistical composition of the copolymers, is proposed. It shows that energy migration can occur from isolated to non-isolated styrene units.     

X-ray- and U.V.-induced excitation of adenine, thymine and the related nucleosides and nucleotides in solution at 77 degrees K

Abstract— The light emitted by adenine and thymine and the related nucleosides and nucleotides in ethylene glycol/water glass during irradiation at 77°K with X-rays and 270 nm U.V. light has been studied. It was found that the same radiative levels were excited by both types of radiation. However, the phosphorescence to fluorescence ratios observed with X-rays were considerably enhanced relative to those obtained with U.V. light. This observation was taken to mean that a major fraction of the excitation results from ion recombination, although direct excitation of the triplet manifold as well as intersystem crossing between upper excited levels may have contributed to the enhancement as well. The yields of the X-ray-induced excitation of radiative levels. which were calculated by combining the yields of X-ray-induced emission and the quantum yields observed with u.v.-excitation, corresponded to: G≥ 4.0 excitations per 100 eV of dose absorbed in the solute for adenine and G≥ 1.8 for thymine. The excitation yields obtained for the nucleosides and nucleotides were somewhat smaller than those observed for the corresponding bases. It was inferred from the yields that a substantial fraction of the dose absorbed in the ribose of the nucleosides was transferred to the radiative levels of the constituent bases.     

THERMAL REQUIREMENTS OF PHOTOSENSITIZED PYRIMIDINE DIMER SPLITTING

Abstract— A cis, syn -pyrimidine dimer (derived from thymine and orotate) covalently linked to 5-methoxyindole has been studied as a mechanistic model of photosensitized pyrimidine dimer splitting. In this dimer-indole, photoinitiated electron transfer to the dimer causes splitting in a manner that parallels the mechanism by which the DNA photolyases are thought to act. Dissolved in EPA (diethyl ether-isopentane-ethyl alcohol, 5: 5: 1, by vol) at room temperature, the dimer-indole exhibited indole fluorescence quenching and underwent splitting upon irradiation at 300 nm. In an EPA glass at 77 K, however, no splitting was detectable. To distinguish the effects of temperature and immobilization, photolysis experiments were performed on PMM [poly(methyl methacrylate)] films containing dimer-indole. In PMM at room temperature, dimer-indole underwent splitting when irradiated at 300 nm, which indicated that immobilization per se was not responsible for the failure of dimer-indole to split at low temperature. Furthermore, no splitting was observed when dimer-indole was irradiated in PMM at 77 K. These results imply that a step following photoinitiated, intramolecular electron transfer from indole to dimer has an insurmountable activation barrier at 77 K. The mechanistic implications for the photolyases are considered.     

Study of near resonant energy transfer from laser excited CO2 to SO2

Collisional energy transfer from CO₂ to SO₂ was studied subsequent to pumping of CO₂ (ν₃) by a Q-switched laser. The measurements were made in the temperature range 300–800 K and in the pressure range 1–30 Torr. The fluorescence from the ν₃ level of CO₂ was monitored with the help of a Ge:Au detector at 77 K with an estimated response time of ≈2 μs. The probability of the energy transfer was found to be increasing with increasing temperature. The probable kinetic models for the V---V relaxation pathways were discussed and the experimentally measured energy transfer rate is related to the cross-over transfer processes. Theoretical calculations using both a simple SSH-breathing sphere model and the Sharma-Brau theory were carried out to evaluate the probabilities of the involved cross-over energy transfer processes and the results were compared with the experimental rates.     

Delayed fluorescence in a pyrene-doped naphthalene crystal: homofusion and heterofusion of triplet excitons

Prompt and delayed emissions from a naphthalene crystal doped with pyrene have been investigated in the temperature range 77–300 K. It is shown that delayed fluorescence (DF) in this crystal originates from heterofusion and homofusion of two different types of trap triplets at low temperature (T< 130 K) and from homofusion of pyrene triplets at high temperature (T >200 K). Pyrene concentration effects on DF suggest that free triplet excitons are trapped more efficiently at monomeric pyrene sites than at nascent excimeric pyrene sites.     

Temperature dependence of radiationless processes. Isoquinoline in solution

The temperature dependence of the fluorescence quantum yield φ_f, the fluorescence lifetime τ_f, and the oscillator strength f(S₀→S₁) of isoquinoline in solution has been measured between room temperature and 77 K. Following an Arrhenius type expression, φ_f in ethanol increases from 0.012±0.002 at 295 K to 0.61±0.03 at 77 K paralleled by an increase of τ_f from 0.25±0.10 ns to 9.0±0.2 ns. Over the same temperature range f(S₀→S₁) and the radiative fluorescence lifetime remain constant. By analyzing the temperature dependent data, it is shown that a spin-allowed internal conversion process with an activation energy of ～1500 cm^?1 is responsible for the observed temperature effect. A mechanism is proposed based upon a thermally activated depopulation of the S₁(ππ^*) state of isoquinoline via a slightly higher state, presumably the S₂(ππ^*) singlet state. An extremenly fast process involving the dissociation of the hydrogen bond deactivates this latter state, by possing S₁.