Spectral-luminescence properties of a norbornene-substituted tetraazachlorin and its metal complexes

The spectral-luminescence properties of a tetraazachlorin derivative with a norbornene fragment annelated to a reduced pyrrole ring and its complexes with zinc and palladium have been studied at 293 and 77 K. For the norbornene-substituted free base, differences in fluorescence from unsubstituted tetraazachlroin and its dibenzobarrelene-substituted analog are found. The fluorescence lifetime is observed to rise by ∼7 times for the free base and by ∼1.6 ties for the Zn complex on going from 293 to 77 K. An essential dependence of the photophysical parameters on the nature of the solvent is noted. The fluorescence polarization spectrum of the norbornene-substituted tetraazachlorin reveals in the Soret band region at least four electronic transitions. For the Pd complex, weak phosphorescence in the near IR region has been detected; the 0–0 band maximum is at 990 nm and the singlet–triplet interval amounts to 5800 cm^–1, which is larger by 400 cm^–1 than for Pd tetraazaporphine. The quantum yields of the photosensitized formation of singlet oxygen have been determined using a relative luminescence method.     

Effect of the Electron-Vibrational Interaction on the Second Electron Transition of Tetraazaporphin and Tetraazachlorin Molecules

We have traced the change in the complex structure of the intense 0–0 band of the electronic S ₂ S ₀ transition in the quasi-line absorption spectra (n-octane, 77 K) for individual types of impurity centers in the series of compounds tetraazaporphin–tetraazachlorin–di(tert-butylbenzo)barrelene-substituted tetraazachlorin. It has been concluded that the reason for the appearance of this structure is the interaction of the pure electronic S ₂ state with the vibronic S ₁ states rather than the Franck–Condon manifestation of low-frequency vibrations. Attempts to detect the short-wave S ₂ S ₀ fluorescence for all three compounds were unsuccessful. The reasons for the difference from the case of bacteriochlorin, in which such a fluorescence was observed earlier, are discussed.     

Fluorescence spectroscopy of naphthalene at high temperatures and pressures: implications for fuel-concentration measurements

Fluorescence of the S ₁→S ₀ transition of naphthalene vapour after laser excitation at 266 nm was studied in a heated cell. Experiments were carried out for temperature in the range 350–900 K, at pressure between 0.1 and 3.0 MPa and for oxygen molar fraction from 0 to 21%. The absorption cross section of naphthalene showed a non-monotonic dependence upon temperature, which may be attributed to the spectral structures present in the absorption spectrum of naphthalene. Under nitrogen atmosphere, naphthalene fluorescence bi-exponentially decreased by an order of magnitude as temperature increased, whereas it increased by about 10% with pressure. Strong influence of quenching by O₂ on naphthalene fluorescence was observed and Stern–Volmer plots were found to be linear for temperatures between 450 and 750 K. The dependence of naphthalene fluorescence on oxygen concentration suggests one to use this molecule for fuel-concentration measurements in turbulent flows.     

Thermally stimulated recombination processes and luminescence in Li<Subscript>6</Subscript>(Y,Gd,Eu)(BO<Subscript>3</Subscript>)<Subscript>3</Subscript> crystals

The thermally stimulated recombination processes and luminescence in crystals of the lithium borate family Li₆(Y,Gd,Eu)(BO₃)₃ have been investigated. The steady-state luminescence spectra under X-ray excitation (X-ray luminescence spectra), the temperature dependences of the X-ray luminescence intensity, and the glow curves for the Li₆Gd(BO₃)₃, Li₆Eu(BO₃)₃, Li₆Y_0.5Gd_0.5(BO₃)₃: Eu, and Li₆Gd(BO₃)₃: Eu compounds have been measured in the temperature range 90–500 K. In the X-ray luminescence spectra, the band at 312 nm corresponding to the ⁶ P _J → ⁸ S _7/2 transitions in the Gd³⁺ ion and the group of lines at 580–700 nm due to the ⁵ D ₀ → ⁷ F _J transitions (J = 0–4) in the Eu³⁺ ion are dominant. For undoped crystals, the X-ray luminescence intensity of these bands increases by a factor of 15 with a change in the temperature from 100 to 400 K. The possible mechanisms providing the observed temperature dependence of the intensity and their relation to the specific features of energy transfer of electronic excitations in these crystals have been discussed. It has been revealed that the glow curves for all the crystals under investigation exhibit the main complex peak with the maximum at a temperature of 110–160 K and a number of weaker peaks with the composition and structure dependent on the crystal type. The nature of shallow trapping centers responsible for the thermally stimulated luminescence in the range below room temperature and their relation to defects in the lithium cation sublattice have been analyzed.     

Manifestation of vibronic intensity borrowing in fluorescence spectra of metalloporphyrins

In selective laser excitation and at T=4.2 K, fine-structure spectra of the fluorescence of the complexes of Mg, Zn, In, and TiO with octaethylporphyrin are obtained. It is shown that the fine-structure fluorescence spectra of these compounds are formed to a large aegree by ribronic interaction between quasi-degenerate S₁ and S₂ electron states. An increase in the relative intensities of a number of negatively polarized fluorescence lines in displacement of the excitation line from a longwave edge of the band of an S₀→S₁ transition in the shortwave direction is revealed. Reported at the VIIIth International Conference on Spectroscopy of Porphyrins and Their Analogs, Minsk, September 22–26, 1998. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 4, pp. 518–522, July–August, 1999.     

The fluorescence and electronic structure of phenyl-substituted tetraazachlorin molecules

The effect of the addition of phenyl groups to pyrrole rings of tetraazachlorins, a new class of tetrapyrroles, on the photophysical properties and electronic structure of the molecules has been investigated by a complex of experimental and theoretical methods. Characteristics of fluorescence at 293 and 77 K have been determined for phenyl-substituted tetraazachlorins. The objects of this study include unsubstituted tetraazaporphine. The introduction of phenyl groups affords a marked increase in the fluorescence quantum yield. For tetraazaporphine and phenyl-substituted tetraazachlorins, fluorescence buildup occurs as the temperature is decreased from 293 to 77 K, but to a lesser extent than for tetraazachlorins having no phenyl groups, which were earlier studied by the authors. The fluorescence buildup mechanism is discussed. The singlet oxygen generation quantum yield has been determined for the tetrapyrroles examined. This characteristic increases upon tetrapyrrole is phenylation. The electronic structure and absorption spectra of unsubstituted porphine and chlorin, tetraazaporphine, tetraazachlorin, octaphenyltetraazaporphine, and tetramethylhexaphenyltetraazachlorin have been calculated by the INDO/Sm method (original modification of the INDO/S method) with molecular geometry optimization using DFT. The results of the quantum-chemical calculation of the absorption spectra are in good agreement with experimental data for transitions to the lowest excited electronic states Q _x (S ₁) and Q _y (S ₂).     

Luminescence and thermally stimulated recombination processes in Li<Subscript>6</Subscript>Gd(BO<Subscript>3</Subscript>)<Subscript>3</Subscript>:Ce<Superscript>3+</Superscript> crystals

The luminescence and thermally stimulated recombination processes in lithium borate crystals Li₆Gd(BO₃)₃ and Li₆Gd(BO₃)₃:Ce have been studied. The steady-state luminescence spectra under X-ray excitation (X-ray luminescence), temperature dependences of the intensity of steady-state X-ray luminescence (XL), and thermally stimulated luminescence (TSL) spectra of these compounds have been investigated in the temperature range of 90–500 K. The intrinsic-luminescence 312-nm band, which is due to the ⁶ P _J → ⁸ S _7/2 transitions in Gd³⁺ matrix ions, dominates in the X-ray luminescence spectra of these crystals; in addition, there is a wide complex band at 400–420 nm, which is due to the d → f transitions in Ce³⁺ impurity ions. It is found that the steady-state XL intensity in these bands increases several times upon heating from 100 to 400 K. The possible mechanisms of the observed temperature dependence of the steady-state XL intensity and their correlation with the features of electronic-excitation energy transfer in these crystals are discussed. The main complex TSL peak at 110–160 K and a number of minor peaks, whose composition and structure depend on the crystal type, have been found in all crystals studied. The nature of the shallow traps that are responsible for TSL at temperatures below room temperature and their relation with defects in the lithium cation sublattice are discussed.     

Effect of bridging aza-substitution on electronic structure and spectral-luminescent properties of isobacteriochlorin NH-isomers

We have investigated the spectral characteristics, quantum yield ϕ_F, lifetime τ_F, and polarization of fluorescence of NH-isomers of dibenzobarrelene disubstituted tetraazaisobacteriochlorin at 293 and 77 K. The following information has been obtained from low-temperature measurements: 1) the weak shoulder on the edge of the long-wavelength 0-0 absorption band disappears at 77 K and, therefore, can be assigned to the cis-NH-isomer [the principal component of the tautomeric mixture is the trans-isomer a (b)]; 2) the low value of ϕ_F at room temperature is due to temperature-viscosity quenching whereas ϕ_F at 77 K is close to 1 and τ_F = 5.3 nsec; 3) the energy of the weak electronic transition G → Q₂ and the interval have been determined. Quantum-chemical calculations found that replacing the methine bridges by nitrogen stabilizes the non-aromatic trans-isomer a (b) whereas the non-planarity of the macrocycle and the alternation of bond lengths increase for the cis-isomers. The calculated electronic absorption spectra of the a isomer reproduce rather well the experimental data. It is shown that the four-orbital model is applicable for the lower excited states Q₁ and Q₂ for all NH-isomers. __________ Translated from Zhurnal Prikladnoi Spektroskopii Vol. 74, No. 3, pp. 395–407, May–June, 2007.     

Polarized fluorescence of β-carotene and related polyenes

The spectral and polarization characteristics of trans-isomers ofβ-carotene, lycopene, retinal, and 1,6-diphenylhexatriene (DPH) in solutions have been investigated by the methods of polarized fluorescence and semiempirical calculations of AM1 and RM3. The high and constant values of the degree of polarization P ∼0.46, closely coinciding with the limiting value P₀=0.5, according to the S₂→S₀-fluorescence and S₂←S₀-fluorescence excitation spectra ofβ-carotene and lycopene in n-hexane at room temperature and also high and coinciding according to the S₁→S₀-fluorescence and S₂←S₀-fluorescence excitation spectra for retinal in EPA at 77 K and DPH in a polyvinylbutyral film, point to the coincidence of the directions of absorption and emission oscillators. These oscillators are aligned along the major axis of the polyene molecule, and the longwave S₂←S₀-absorption band is formed only by one electronic¹B _u ⁺ ←¹A _g ⁻ -transition. The oscillators of shortwave S_a←S₀-absorption bands are aligned at an angle to the oscillators of the S₂←S₀-transitions, which is related to the distortions in the structure of molecules (β-carotene, lycopene) or, as in the case of DPH, to the influence of transitions localized on the orbitals of phenyl rings and polarized perpendicularly to the major axis of the molecule. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 67, No. 1, pp. 69–74, January–February, 2000.     

Spectral luminescence properties of Eu(III) complexes with tetracycline antibiotics and hydrogen peroxide

We have studied the spectral luminescence properties of mixed-ligand complexes of Eu(III) ions with tetracyclines and hydrogen peroxide. We have established that incorporation of hydrogen peroxide into the inner sphere of these complexes leads to a significant increase (by a factor of 10–25) in the intensity of luminescence of the Eu(III) ions, redistribution of the intensities of the ⁵D₀ → ⁷F_j transitions, in particular to an anomalous increase in the intensity of the band for the forbidden transition ⁵D₀ → ⁷F₀. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 3, pp. 310–314, May–June, 2007.     

Calculation of Normal Vibrations and Interpretation of the Quasiline Spectra of Tetraazachlorin

Quasiline fluorescence and fluorescence-excitation spectra of tetraazachlorin and its N-deuterated derivative in n-octane at 77 K have been investigated and the frequencies of the normal vibrations for the electronic states S ₀ and S ₁ have been determined. Calculation of the normal vibrations of these molecules has been done and used to interpret the experimental data. Based on the results of analysis of the intensities in the spectra, the change in the structure of the molecule in the electronic state S ₁ is discussed.     

Saturable absorption dynamics of mode-locking dye DODCI

0 –S₁–S_m–S_n multilevel system. An S_m→S₁ absorption recovery time of τ^S1 _A=(600±100) fs is determined. The picosecond and femtosecond pulse excitation leads to S₀→S₁ ground-state absorption bleaching and S₁→S_m first excited-state absorption bleaching. The excited-state absorption cross-sections σ^S1 _ex(S₁→S_m) and σ^Sm _ex(S_m→S_n) are determined. Received: 3 June 1996     

Optical properties of new indotricarbocyanine dye as a limiter of laser radiation power

We present results of experimental and theoretical studies of the optical characteristics of a new indotricarbocyanine dye that is capable of effectively limiting the power of laser radiation in the visible spectral range. The spectral-luminescent and energy characteristics of the dye molecules and their absorption spectra from the excited state with nanosecond resolution are investigated experimentally. Quantum-chemical methods are used to calculate electronic absorption spectra from the ground (S₀ → S_n) and excited (S₁ → S_n) states and to determine the nature of electronic states of the molecule and the rate constants of intramolecular photophysical processes. The results of the theoretical research agree with experimental data. It is shown that the investigated dye has singlet-singlet absorption at 400–600 nm. Nonlinear absorption of the dye upon excitation by radiation of the second harmonic of a Nd:YAG laser is studied by z-scanning with an open diaphragm. The ratio of dye absorption cross sections from the excited and ground states at 532 nm is determined in the framework of a three-level model. The results are compared with those for previously studied compounds. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 4, pp. 473–480, July–August, 2007.     

Franck-Condon transitions and thermodynamics of photoinduced processes of orientational relaxation in polar solutions

Polar luminescence probe+polar medium subsystems are considered. True minimization parameters of the free energy F are obtained. It is shown that Franck-Condon transitions between F-terms of different electronic states are generally not “vertical”. A complete thermodynamic classification of photoinduced processes of orientational relaxation in the excited S₁ and ground S₀ states is presented. Depending on the excitation conditions they can be either exo- or endothermic, spontaneous or induced (not spontaneous), and accompanied by an increase or decrease in entropy. Indifferent processes also take place. Regularities of shifts of fluorescence spectral bands and burned holes in time-resolved experiments are discussed. Institute of Molecular and Atomic Physics, Academy of Sciences of Belarus, 70, F. Skorina Ave., Minsk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 6, pp. 753–765, November–December, 1997.     

Spectral and kinetic properties of fluorescence of immunoactive steroids in the gas phase

We have used steady-state fluorescence spectroscopy and pulsed kinetic fluorimetry with high time resolution to experimentally study the spectral and temporal characteristics of luminescence of immunoactive 8-aza-D-homogona-1,3,5(10), 13-tetraene-12,17a-dione (8-aza-D-homogonane) molecules in the gas phase. We have established the existence of several centers emitting long-wavelength and short-wavelength fluorescence. The fluorescence of high-temperature 8-aza-D-homogonane vapor (T = 550 K) when excited by radiation with λ_ex = 266 nm is represented by two spectrally separated emission bands with maxima λ _max ^fl = 465 nm and λ _max ^fl = 365 nm. Analysis of the distribution of fluorescence decay times for 8-aza-D-homogonane showed that in the spectral range of the emission wavelengths 360–590 nm, the fluorescence decay kinetics contains three components with average lifetimes <200 psec, 2.8 nsec and 13.5 nsec. The multicenter nature of luminescence of 8-aza-D-homogonane was confirmed by direct measurements of instantaneous fluorescence spectra in different stages of luminescence decay. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 3, pp. 309–314, May–June, 2006.     

Fluorescence spectroscopy of 1,2,4-trimethylbenzene at high temperatures and pressures: application to temperature measurements

Fluorescence of the S₁→S₀ transition of 1,2,4-trimethylbenzene vapour after laser excitation at 266 nm was investigated in a heated cell. Experiments were performed for temperature between 350 and 900 K, at pressure between 0.1 and 3.0 MPa and for oxygen molar fraction between 0 and 21%. The absorption cross section was found to reduce by about 20% between 350 and 900 K. 1,2,4-trimethylbenzene fluorescence exponentially decreased by three orders of magnitude as temperature increased. A similar behaviour was observed with pressure, although fluorescence reduced only by a factor of 2.5 between 0.1 and 3.0 MPa. Additionally, Stern–Volmer plots were found to be linear for temperatures between 450 and 750 K. Using the dependence of 1,2,4-trimethylbenzene fluorescence on temperature, potential use of this molecule for temperature measurements in turbulent flows is suggested.     

Fine fluorescence and absorption spectra of hypericin in a polymer matrix at a temperature of 4.2 K

Monochromatic photoexcitation and selective registration in bands of purely electronic S₁↔S₀ transitions resulted in fine fluorescence and absorption (fluorescence excitation) spectra of hypericin in polyvinylbutyral at a temperature of 4.2 K. Vibration analysis of the spectra is done and conclusions are made about the form of many normal vibrations. It is found that the formation of intramolecular hydrogen bonds in the hypericin structure has a substantial effect on the relative position of the electronic vibrational levels of the pigment molecule. Institute of Molecular and Atomic Physics of the National Academy of Sciences of Belarus, 70, F. Skorina ave., Minsk 200072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 4, pp. 539–545, July–August, 1998.     

Determination of intramolecular energy-transfer efficiency in ytterbium chelates

The quantum yield of²F_5/2 →²F_7/2 luminescence of the Yb³⁺ ion has been measured for ytterbium tris-thenoyltrifluoroacetonate upon excitation in the UV S₀ → S₁ absorption band of a ligand and in the IR²F_7/2 →²F_5/2 absorption band of the Yb³⁺ ion. It has been established that the quantum efficiency of intramolecular transfer of electron excitation energy from the lowest triplet state of the ligand to the excited²F_5/2 level of Yb³⁺ is equal to unity. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 67, No. 4, pp. 539–542, July–August, 2000.     

Transient absorption spectra of tetraazaporphine and its reduced derivatives upon picosecond excitation

Transient absorption spectra of tetraazaporphine and substituted derivatives of tetraazachlorin and tetraazabacteriochlorin were obtained upon picosecond excitation with various probe pulse delay times in order to resolve the open question about the reasons for fluorescence quenching in tetraazaporphine hydrogenated derivatives. The quantum yield of triplet state T ₁ formation has been estimated. It has been shown that in all investigated cases radiationless de-excitation of the fluorescent level S ₁ occurs by two channels: S ₁ ⤳ T ₁ and S ₁ ⤳ S ₀, the latter being predominant. As the S ₁ level becomes lower, the transition rate for this channel increases. For tetraazaporphine and its derivatives, a quasi-photochemical mechanism is proposed that accounts for the anomalous efficiency of the S ₁ ⤳ S ₀ channel in the dissipation of the electronic excitation energy.     

Absorption and fluorescence spectra of C<Subscript>60</Subscript> fullerene concentrated solutions in hexane and polystyrene at 77–300 K

The locations of the 0₀⁰ 0_0^0 -bands for S₁← S₀ and S₁ → S₀ transitions have been found for C₆₀ solutions in hexane. It is shown that the profile of the S₁ ← S₀ band is mainly shaped by h_u(4), t_1u(4)- and h_g(1), a_g(2)- modes that are active in absorption. Bands involving the h_u(4)- and t_1u(4)-modes in the emission process have also been identified in the fluorescence spectrum. The appearance of the 0₀⁰ 0_0^0 -band in the forbidden 1¹T_1g ← 1¹A_g transition is explained by symmetry reduction in the C₆₀+environment system due to the interaction of electrons with local phonons. The temperature coefficients of the red shift for the 256.3- and 328.3-nm bands of allowed ¹T_1u ← 1¹A_g transitions for C₆₀ in hexane are equal to –1.45 and –0.46 cm^–1·K^–1, respectively. The peak and half-width values of the 337.2-nm band for C₆₀ in polystyrene remain unchanged on cooling to 77 K. Absorption in the 700–800-nm region for concentrated hexane solutions of fullerene at 292 K results from the production of (C₆₀)_n-clusters.