Generation of singlet oxygen by indotricarbocyanine dyes in low-polarity media

We present the results of a study of the spectral luminescence properties of three groups of indotricarbocyanine dyes, each of which is formed from compounds with the same cation and different anions. In high-polarity solvents, in the absorption and emission spectra of the dyes we see one type of center; in low-polarity solvents, due to the presence of different ionic forms of the dyes (free ions, contact ion pairs), we observe either one type or two types of centers. By analysis of the luminescence of molecular oxygen in the 1.27 μm spectral region, we determined the efficiency of photosensitization of ¹O₂ formation by dyes in deuterated solvents. We have shown that in low-polarity solvents, the yield for singlet oxygen generation is higher for indotricarbocyanine dyes which are found in the contact ion pair state and which also contain a heavy atom (I) in the anion. We have observed that an increase in the fraction of contact ion pairs in solution as the dye concentration increases or when an additional salt is introduced leads to an increase in the quantum yield for generation of singlet oxygen. In polar deuterated acetonitrile, the counterion has no effect on the efficiency of photosensitization of oxygen by the dyes. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 5, pp. 684–693, September–October, 2008.     

Spectral fluorescence and polarization characteristics of <Emphasis Type="Italic">Z,Z</Emphasis>-bilirubin IXα

We have studied the spectral fluorescence and polarization characteristics of Z,Z-bilirubin IXα, at room temperature in chloroform and in aqueous buffer medium, within an equilibrium complex with human serum albumin (HSA), and also under low temperature conditions (T = −100°C) in isobutyl alcohol. We have observed a bathochromic shift of the fluorescence spectra, which is most pronounced for the bilirubin-albumin complex. The following are considered as possible reasons for the observed dependence of the position of the fluorescence (fluorescence excitation) spectra on the excitation (detection) wavelength: structural and spectral differences between the chromophores making up the bilirubin molecule; conformational heterogeneity of the pigment in solution; a contribution to the fluorescence from molecules which have not completed the vibrational relaxation process; inhomogeneous orientational broadening of the levels; heterogeneity of the microenvironment of the chromophores in the protein matrix. We show that polarized fluorescence of bilirubin occurs at room temperature, due to the anomalously short fluorescence lifetime τ (picosecond or subpicosecond ranges). Despite such a short τ, the absorption and emission polarization spectra suggest the presence of intramolecular nonradiative singlet-singlet energy transfer when bilirubin is excited to high vibrational sublevels of the S₁ state (degree of polarization p = 0.11–0.12). When fluorescence is excited on the long-wavelength slope of the absorption band, no transfer occurs: the degree of polarization (p = 0.46−0.47) is close to the limiting value (p = 0.50). We discuss the question of the role played by exciton interactions between chromophores in the bilirubin molecule when it is excited. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 1, pp. 108–119, January–February, 2007.     

Dual fluorescence and laser emissions from fluorescein-Na and eosin-B

Dual laser emissions were observed from fluorescein-Na and eosin-B in ethanolic solutions individually in the concentration range from 10⁻² to 10⁻³ mol dm⁻³ under N₂ laser excitation. The first compound was found to lase at two distinct regions with wavelength maxima around 540, 550 nm, while the second one around 558, 574 nm. Steady-state absorption, fluorescence excitation, fluorescence polarization, fluorescence emission and decays of the dyes in various solvents under varying conditions of excitation and detection systems were carried out to identify the nature of the emitting species responsible for laser emissions in two distinct regions. Both the dyes exhibited concentration and excitation wavelength dependence of fluorescence and the effects were found to be more pronounced in binary solution. The fluorescence decays of dyes were monoexponential in ethanol, while in some other solvents used, the decays showed biexponential behavior. The absorption and excitation studies using thin layers of solutions revealed the formation of dimers with the dye concentration around 1×10⁻³ mol dm⁻³. Fluorescence polarization and decay studies confirmed the presence of dimers. The two laser bands observed in the shorter and longer wavelengths were respectively ascribed to monomeric and dimeric species.     

Absorption,excitation and fluorescence spectra of thallium activated cesium bromide

Absorption, excitation and fluorescence spectra of T1⁺ doped cesium bromide have been investigated at various thallium concentrations. At very low thallium concentration two absorption bands are obtained at 225 nm and 264 nm. With rise of thallium concentration additional absorption bands are obtained at 230, 244, 258, 270 and 285 nm. A single bell-shaped fluorescence band at 357 nm in the ultraviolet region is obtained at low thallium concentration. Two additional visible fluorescence bands appear at 440 and 540 nm with rise in thallium content. The excitation spectra for ultraviolet emission band and visible emission bands are found to be different. Accordingly the ultraviolet emission band is attributed to the characteristic A emission in T1⁺ ion and the visible bands are attributed to dimer centers havingD _4h site symmetry.     

Time-Resolved Vacuum Ultraviolet Spectroscopy of Er<Superscript>3+</Superscript> Ions in the SrF<Subscript>2</Subscript> Crystal

The photoluminescence and photoexcitation spectra as well as the luminescence decay kinetics of Er³⁺ ions in the visible ultraviolet and vacuum ultraviolet (VUV) regions have been studied by the method of low-temperature, time-resolved VUV-spectroscopy on excitation by synchrotron radiation. In the VUV spectral region of the luminescence of SrF₂:1% Er³⁺, the 146.5-nm band with a time of decay of less than 0.6 nsec was revealed together with the well-known emission band at 164.3 nm (decay constant in the microsecond range). Its possible nature is discussed. The specific features of the formation of photoexcitation spectra of the f-f and f-d transitions in the Er³⁺ ion are considered. Competition between the processes of excitation of f-f and d-f luminescence has been revealed. It manifests itself in the inverse relationship of their photoexcitation spectra in a range of energies of incident photons that are close to the position of the 4fⁿ⁻¹5d configuration levels. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 4, pp. 519–523, July–August, 2005.     

ESIPT reaction starting from S2 and S3 singlet states in 3-hydroxyflavone

Spectra fluorescence at excitation of emission within spectral intervals of S₃, S₂ and S₁ bands of absorption, and excitation at registration of fluorescence at maxima of blue and green bands of emission have been studied in 3-hydroxyflavone solution. A possibility of ESIPT reaction starting from S₃ and S₂ states of parent 3HF molecule was shown for the first time follows from observations of differences in excitation spectra of normal and tautomer forms of 3HF, and from different ratios of these forms intensities upon excitation in different ranges of UV spectra of absorption.     

Particular features of fluorescence of jet-cooled vapor of 2,3-diazabicyclooctene

The fluorescence and fluorescence excitation spectra of azocompound, 2,3-diazabicyclo[2.2.2]oct-2-ene, have been studied using jet cooling. The violation of the mirror symmetry between the fluorescence and fluorescence excitation spectra due to an intense long-wavelength emission that manifests itself in solutions and in the gas phase cannot be eliminated even by jet cooling. It has been revealed that the bands of pure electronic and vibronic transitions are split by 0.55 cm^?1, which is caused by tunneling with accompanying emission from an intermediate short-lived state and which may be the reason for the violation of the mirror symmetry of the spectra upon tunnel inversion.     

Interpretation of the soret band of porphyrins based on the polarization spectrum of N-methyltetraphenylporphin fluorescence

Low-temperature (77 K) absorption, fluorescence, and fluorescence-excitation spectra and the fluorescence polarization spectrum for N-methyl-meso-tetraphenylporphin (N-methyl-TPP) are measured. Based on the polarization spectrum the absorption spectrum in the visible region (a “generic” porphyrin spectrum) is interpreted. In particular, it is shown that the fifth absorption band (the “Longo band”) that manifests itself in some porphyrins is a Q_y(0–2) band and does not belong to the individual electron transition. Emphasis is placed on the region of the Soret band. It is inferred that at least two allowed electron transitions (G →B_x and G→B_y) polarized mutually perpendicularly manifect themselves in this region. The interpretation of the Soret band of porphyrins [2] that attributes this band to one electron transition G→B_x is thereby rejected. This interpretation is confirmed by computer modeling of the polarization spectrum. Special features of the experimental polarization spectrum are explained by a more developed vibrational structure and, possibly, a larger half-width of the B_x band than the half-width of the B_y band. The contribution of the states of intramolecular charge transfer to the formation of the Soret band is discussed. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 1, pp. 65–72, January–February, 1999.     

Excitonic nature of the dual fluorescence of 2,3-diazabicyclo[2.2.2]oct-2-ene and 1,4-dimethyl-(2,3-diazabicyclo[2.2.2]oct-2-ene)

Absorption, fluorescence excitation, and fluorescence spectra and the dependence of the degree of fluorescence polarization on the emission wavelength are measured for glass-like ethanol solutions of 2,3-diazabicyclo[2.2.2]oct-2-ene and 1,4-dimethyl-(2,3-diazabicyclo[2.2.2]oct-2-ene) at a temperature of 77 K. The analysis of the spectral polarization data shows that two excited electronic states S₁ and S₂ that contribute to the emission of the compounds are related to the exciton splitting and correspond to the symmetric (S₂) and antisymmetric (S₁) wave functions.     

Fluorescent probe based on proton transfer in 4′-(diethylamino)-3-hydroxyflavone: Spectral and luminescent characteristics upon selective excitation

The spectral characteristics of acetonitrile solutions of 4′-(diethylamino)-3-hydroxyflavone dye with dual fluorescence are studied under selective excitation. This dye is a structural analog of 3-hydroxyflavone and exhibits excited-state proton transfer, which, in contrast to 3-hydroxyflavone, has a thermodynamic rather than a kinetic character. The fluorescence spectra at different excitation photon energies and the excitation spectra of different fluorescence bands are studied. It is found that the intensity ratio of the normal and tautomeric fluorescence bands lying near 507 and 570 nm, respectively, depends on the excitation wavelength, namely, this ratio is 1.45 and almost does not change in the region of the main absorption band (370–420 nm), while, in the region of the second singlet band (near 280 nm), it decreases to 1.15. This can be explained by an increase in the probability of proton transfer with formation of a tautomeric form in the case of excitation into the second band. Another interesting feature is the existence of a latent third emission band peaked at 535 nm, which was found and reliably recorded upon excitation at wavelengths of 470–500 nm. Addition of water quenches this emission, which indicates that it belongs to the anionic form of the dye.     

Fluorescence studies of retinol and retinyl acetate

The polarized emission study of all-trans retinol and retinyl acetate in solid films at room temperature has been carried out. The shape of the absorption and fluorescence is typical of Franck-Condon forbidden bands. The high positive polarization of the fluorescence excitation and the emission (when excited into main absorption band due to 1_Bu ← 1_Ag transition) suggests that the lowest excited singlet state of retinols has a large amount of 1_Bu character. The explanation for the high positive polarization involving strong vibronic interactions suggesting 1_Ag - (a very weakly allowed state) being the lowest excited singlet has also been considered.     

Fluorescence Study on the Structure of Ionic Liquid Aggregates in Aqueous Solutions

Although ionic liquids are a relatively novel class of materials, it is well documented that they form micelles through aggregation of cation aliphatic tails. However, anion self-assembly has not yet been reported. In this study, we analyzed the intrinsic fluorescence of p-toluenesulfonate groups (tosylate) as part of the ionic liquid 1-ethyl-3-methylimidazolium tosylate ([emim][TOS]) and p-toluenesulfonic acid (pTSA), in aqueous solution. pTSA was found to have overlapping monomer and excimer emissions for chromophore concentrations from 10⁻³ to 1 M, whereas [emim][TOS], in the same conditions, showed monomer emission slightly broadened by much weaker excimer emission. These different photophysical behaviors of the same chromophore in the two compounds are explained by the formation of ion pairs by [emim][TOS], which can also be inferred from the loss of vibrational structure of the absorption spectra with respect to pTSA. Despite this different behavior regarding ion pairing, anion aggregation was observed in the excitation spectra of both pTSA and [emim][TOS]. While the absorption spectra corresponded to single chromophores, the excitation spectra changed from those characteristic of a single chromophore (below 10⁻³ M) to red-shifted narrow bands (above 0.1 M) typical of J aggregates. Between those concentrations, the excitation spectra split into blue- and red-shifted bands with relative intensities that changed with concentration as the chromophores rearranged in their clusters from head-to-head to head-to-tail aggregates. Differences between the absorption and excitation spectra were ascribed to aggregation-induced fluorescence enhancement.     

Excited state intramolecular proton transfer in amino 2-(2′-hydroxyphenyl)benzazole derivatives: Effects of the solvent and the amino group position

The excited-state intramolecular proton transfer (ESIPT) mechanism in six amino 2-(2′-hydroxyphenyl)benzazole derivatives were investigated in different solvents by means of UV-vis absorption and steady-state fluorescence. The amino benzazoles are fluorescent in the blue-orange region under UV radiation. Changes in the absorption, emission and excitation spectra were analyzed and correlated to the position of the amino group and the solvent polarity. The equilibrium between the conformers in solution in the ground state, confirmed by the solvatochromic effect, reflects the dual fluorescence emission presented by these dyes.     

Absorption,fluorescence, and fluorescence excitation spectra of free molecules of indole and its derivatives

We have obtained and analyzed the absorption, fluorescence, and fluorescence excitation spectra of indole vapor, N-acetyl-L-tryptophan vapor, and 3-indole aldehyde vapor. From analysis of the dependence of the fluorescence spectrum of the free indole molecules on the wavelength of the exciting radiation λ_ex, it follows that emission of fluorescence occurs when the molecules undergo a transition from the one electronically excited state ¹L_b. The fluorescence spectra of the studied compounds are insignificantly different, suggesting a major role for the indole chromophore in formation of the compounds. The absorption spectrum of N-acetyl-L-tryptophan, in which the group of atoms is added to the indole ring through a-C-C bond, is similar to the spectrum of indole, while the spectrum of 3-indole aldehyde is significantly different from the indole spectrum due to the effect of the C=O group conjugated with the indole ring. The fluorescence excitation spectra are considerably different from the absorption spectra. This is associated with the strong dependence of the quantum yield for the free molecules on λ_ex. Qualitatively, they are mirror-symmetric to the fluorescence spectra of the stodied compounds. Analysis of the data obtained provides a basis for assuming that in the case of free molecules of indole and its derivatives, the ¹L_a absorption in the extreme long-wavelength region of the spectrum does not overlap ¹L_b absorption. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 2, pp. 218–222, March–April, 2007.     

2-Aminopurine optical spectra: Solvent,pentose ring,and DNA helix melting dependence

2-Aminopurine (2AP) absorption and fluorescence excitation and emission spectra in a series of solvents have been measured to assess effects of solvent polarity. Emission spectra of the free base shift to the red in solvents of a higher dielectric constant, including water but excepting dioxane. Excitation spectra also red-shift, except in water. A change in dipole moment of 2.8 D upon excitation is obtained from a Bilot-Kawski plot which includes data from potentially anomolous solvents such as alcohols but which excludes dioxane and aqueous solvents. Attachment of ribose or 2-deoxyribose causes 1 to 2-nm shifts in the position of fluorescence excitation and emission spectra of 2AP in water and little change in fluorescence yield. Melting of the DNA duplex d[CTGA(2AP)TTCAG]₂ yields a blue shift of the excitation and no shift of the emission spectrum-results consistent with increased exposure to water and formation of 2AP-water H bonds in the ground state. The spectral shift occurs 5°C or more below the helix melting temperature, implying a premelting structural change in the decamer.     

Spectroscopic and nonlinear characteristics of anthracene-containing silicon-organic polymers in solution

Spectroscopic characteristics, i.e. absorption, fluorescence, fluorescence excitation spectra, fluorescence decay time, fluorescence polarization degree of novel silicon-containing organic polymers including main chain anthracene groups were investigated. Three kinds of emission spectra were revealed and assigned to polaron–exciton, anthracene and anthracene dimer. The measured fluorescence polarization spectra gave evidence of directed excitation energy migration along the disordered polymeric chain. Strong quenching of anthracene fluorescence during the polaron–exciton lifetime was interpreted as a result of the interaction between two excitations that causes anthracene anion-radical formation. The third-order nonlinear susceptibility of the polymers in solution measured by the Z-scan technique at 1054 nm is 190×10⁻¹⁴ cm² W⁻¹.     

Modulation of the proton transfer rate by excitation photons

The effect of the exciting photon energy on the excited state proton transfer in a dye with dual fluorescence—FET (4′-diethylamino)-3-hydroxyflavone)—is studied. The steady-state fluorescence spectra are studied upon selective excitation by photons with different energies in the region of the main absorption band, as well as at its long-wavelength wing, in the temperature range of 2–30°C. It is found that, at all temperatures, the ratio of the integral emission of the normal and tautomeric forms, which are observed at 480 and 570 nm, respectively, depends on the excitation wavelength; namely, this ratio noticeably decreases with increasing excitation wavelength in the region of the main absorption band and its long-wavelength wing at 390–440 nm, and the rate of this decrease depends on temperature. In the same region, the long-wavelength excitation effect, which is atypical for inviscid solvents at room temperature, is observed; i.e., a short-wavelength emission band is bathochromically shifted by 6–15 nm depending on temperature. This spectral shift is directly related to the inhomogeneous broadening of the electronic spectra of the normal FET form, which is very large due to a considerable (>10 D) difference in the dye dipole moments. Most probably, the excitation creates the possibility of emission from nonrelaxed nonequilibrium orientational sublevels because their lifetime becomes shorter due to the proton transfer reaction, the rate of which in acetonitrile is comparable with the rate of intermolecular orientational relaxation. It is proposed to explain these dependences using energy diagram taking into account the dependence of the free energy on the orientational polarization of the solvent.     

Spectral properties of molecular charge-transfer probe QMOM

The spectral characteristics of solutions of a dye with dual fluorescence, 1-methyl-2-(4-methoxy)phenyl-3-hydroxy-4(1H)-quinolone, in acetonitrile are studied upon selective excitation. This dye is a structural analogue of 3-hydroxyflavone and also exhibits excited-state proton transfer, which, as well as in the case of 3-hydroxyflavone, has a kinetic nature. The fluorescence spectra are studied upon excitation by photons of various energies, and the excitation spectra are recorded at wavelengths of different fluorescence bands. It is found that the intensity ratio of the emission of the normal and tautomeric forms (at wavelength of 415 and 518 nm, respectively) is almost the same (0.23–0.25) for excitation in the regions of the main and the second absorption bands. At the same time, in the case of excitation between these bands, this ratio decreases to 0.19. The second interesting feature is the existence of a third latent emission band peaked at about 480 nm, which is reliably detected upon excitation at wavelengths in the region of 400–450 nm. This study shows that this emission belongs to the anionic form of the dye. This form is also responsible for a decrease in the intensity ratio of the emission of the two main forms in the case of excitation between the first and second absorption bands.     

Native Fluorescence and Time Resolved Fluorescence Spectroscopic Characterization of Normal and Malignant Oral Tissues Under UV Excitation—an In Vitro Study

The present work aims to investigates the native fluorescence and time resolved fluorescence spectroscopic characterization of oral tissues under UV excitation. The fluorescence emission spectra of oral tissues at 280 nm excitation were obtained. From the spectra, it was observed that the alteration in the biochemical and morphological changes present in tissues. Subsequently, the Full width at Half Maximum (FWHM) of every individual spectra of 20 normal and 40 malignant subjects were calculated. The student’s t-test analysis reveals that the data were statistically significant (p?=?0.001). The fluorescence excitation spectra at 350 nm emission of malignant tissues confirms the alteration in protein fluorescence with respect to normal counterpart. To quantify the observed spectral differences, the two ratio variables R₁?=?I₂₇₅/I₃₁₀ and R₂?=?I₃₁₀/I₃₂₈ were introduced in the excitation spectra. Among them, the Linear Discriminant Analysis (LDA) of R₁ reveals better classification with 86.4 % specificity and 82.5 % sensitivity. The fluorescence decay kinetics of oral tissues was obtained at 350 nm emission and it was found that the decay kinetics was triple exponential. Then the ROC analysis of fractional amplitudes and component lifetime reveals that the average lifetime shows 77 % sensitivity and 70 % specificity with the cut off value 4.85 ns. Briefly, the average lifetime exhibits better statistical significance when compared to fractional amplitudes and component lifetimes.     

Activation Energy of Light Induced Isomerization of Resveratrol

Isomerization of trans-stilbenes is known to be induced by light. The two isomers have distinct absorption, fluorescence excitation and emission spectra. Resveratrol, 3,4',5-trihydroxystilbene, is a member of the stilbene family. The interest of the scientific community in resveratrol has increased over the last years due to its biomedical properties. Whereas there is a growing confidence that trans-resveratrol is non-toxic, very little is known about the pharmacology of cis-resveratrol. Of this very reason there is considerable interest in knowing the energetics of the trans-cis conversion. Cis-resveratrol is characterized by a large fluorescence quantum yield when compared to trans-resveratrol. In the present paper we report a detailed analysis of the spectral changes induced in trans-resveratrol upon 260?nm excitation for different time periods. Spectral changes have been monitored with UV-visible absorption and steady-state fluorescence spectroscopy at pH?4 at 20, 25, 30, 35, 40, 45 and 50?°C. Continuous 260?nm excitation induces a blue shift in the absorption and fluorescence excitation spectra of resveratrol and a 14?nm blue shift in its fluorescence emission. The photoisomerization yield is reported as a function of 260?nm excitation time. 330?min continuous excitation led to ~60% isomerization yield. The kinetics of trans-cis isomerization has been monitored following the increase in fluorescence quantum yield upon continuous 260?nm excitation of trans-resveratrol. The study was carried out at the above mentioned temperatures in order to obtain the Arrhenius activation energy of photoisomerization. Activation energy and pre-exponential factor were 3.7?±?0.3?kcal.mol(-1) and 10.6?±?1.6 s(-1), respectively. The activation energy is comparable with previously reported values for the photoisomerization of other stilbenes.