THE FLUORESCENCE PROPERTIES OF ORTHO AMINOBENZOATE ANESTHETICS IN DEFINED SOLVENTS AND PHOSPHOLIPID VESICLES

Abstract— The fluorescence properties of three ortho aminobenzoate local anesthetics have been determined in a variety of solvents. Results from these studies have been used to deduce how these drugs interact with phosphatidylcholine bilayers. The emission energy, fluorescence quantum yield and lifetime exhibited a biphasic dependence on solvent polarity. In aprotic solvents, alcohols and in ethanol-water mixtures containing less than 40% water, quantum yields and lifetimes were high (approximately 0.55 and 8.5 ns respectively). In ethanol-water mixtures containing >40% water, the strong fluorescence quenching was primarily due to an increase in the rate of non-radiative deactivation of the excited state. Both the radiative ( k_r ) and non-radiative ( k_nr ) rate constants show a biphasic dependence on solvent polarity. These studies suggest the presence of two singlet excited states for these molecules, a polar singlet excited state, S_1-p and a charge transfer excited state, S_1-ct with the latter predominating in ethanol-water mixture containing >40% water. In egg phosphatidylcholine bilayers, the fluorescence, lifetime and quantum yield are consistent with the view that these drugs are localized within the lipid head group region where the charge-transfer excited state can be stabilized by intermolecular hydrogen bonding.     

2-phenoxathiinyl-5-phenyloxazole and 5-phenoxathiinyl-2-phenyloxazole derivatives: experimental and theoretical study of emission properties

The absorption and emission spectra in cyclohexane and methanol of the title phenoxathiinyl-phenyloxazole derivatives are presented and discussed. Comparing to the unsubstituted diphenyloxazole (PPO), the experimental results show a bathochromic shift of the emission band, a significant dependence of the maximum on the solvent polarity and a drastic decrease of the fluorescence quantum yield. Semiempirical MO calculations (AM1) in both the ground and excited states support the experimental findings. A charge transfer from the phenoxathiin fragment to the oxazole ring is predicted in the excited state explaining the solvatochromism of the compounds. The values for the singlet-triplet gap, 3500-5000 cm-1 point to an enhanced probability of intersystem crossing (ISC) non-radiative deactivation processes, in agreement with the low fluorescence quantum yields.     

Spectroscopic studies of purine. II. Properties of purine substituted at the second and sixth carbons

Abstract— Substitution of an amino group at the C2 carbon of purine causes the two main components of the X absorption band to be separated by 200–500 Å: primarily this reflects the Occurrence of the lowest-lying transition at much longer wavelengths than in purine since, except in deoxyguanosine, there is an appreciable absorption component at ? 2420 Å in these compounds. Unlike purine, the wavelength shifts produced by different solvents are not simply related to the dielectric constant or hydrogen bonding capacity of the solvents. In agreement with previous predictions thatπ-π*states should be the lowest-lying excited singlets, no phosphorescence is observed from 2–aminopurine—the fluorescence quantum yield is close to unity. Besides strong fluorescence very weak phosphoresence can be detected when another amino group is added at C6 and when an oxygen is attached to the sixth carbon (guanine) fluorescence and phosphorescence of comparable intensities are observed: the sum of the quantum yields for both types of emission is of the order of unity. Thus, the lowest-lying singlet transition must be a π-π*state. At room temperature themal quenching is the most important parameter in determining the fluorescence intensity of these molecules, however, at lower temperatures the intensity—similar to the wavelength—f this emission apparently also depends upon the ability of a given solvent to reorient and/or perturb Franck-Condon surfaces. Discrepancies observed between absorption and excitation spectra in deoxyguanosine and azaguanine appear to reflect emission caused respectively by aggregation of molecules and by keto-enol shifts.     

SPECTROSCOPIC STUDIES OF PURINES—I. FACTORS AFFECTING THE FIRST EXCITED LEVELS OF PURINE*

Abstract— Studies of purine absorption and emission in seven solvents differing greatly in dielectric constant and hydrogen bonding potential, reveal a variety of solvent effects. For example, the resolution of structure in the absorption spectrum, the position and/or intensity of the X₂ absorption band, the intensity of fluorescence, the magnitude of the long wave-lenth tail, and the position of the X₁ absorption band are differentially affected—in the order listed—by the solvents tested. Even though it is possible to correlate the extent of decrease in the n-π* tail with increasing solvent dielectric constant, probably alterations in all of these spectroscopic parameters depend most critically upon the ability of the various solvents to form hydrogen bonds with the hydrogen on N9 and/for with the non-bonding electrons on the purine nitrogens: it is tentatively concluded that the probability of hydrogen bonding is directly correlated with the electronegativity of the aza nitrogens (N7 > N3 > N1). In solvents like isopropanol not all of the non-bonding electrons must be solvated maximally in most purine molecules since there is appreciable fluorescence under conditions where a long wavelength tail is readily observed in the absorption spectrum (alternatively some noa-bonding electrons may not te relevant to fluorescence quenching.) Decreases in fluorescence yield are associated with red shifts in the fluorescence maximum, and in the solvents of highest polarity the fluorescence yield is again small indicating that glycerol and water can enhance radiationless tunneling—presumably by altering Franck-Condon configurations and/or improving electronic-vibrational coupling between solute and solvent. The quantum yield is uniform throughout the atsorption band for a given solvent, but studies in aqueous buffers varying from pH 1 to 11 show that the fluorescence yield is greater for charged than for neutral molecules. Further, the fluorescence excitation peak is red shifted in powders. Since phosphorescence is the predominant emission at 777deg;K and increases in fluorescence can be correlated with the presumed solvation of non-bonding electrons, the singlet excited state of lowest energy in ‘unperturbed’ purine must be n-π* in nature. The shape of the phosphorescence band and the decay lifetime of ? 1 sec at 77°K lead to the conclusion that the emitting triplet is a π-π* state. The eight vibrational structures in phosphorescence emission can be readily grouped into two progressions: there is an average separation of about 1300 cm^-1 between peaks within a given progression, and the two sets are mutually displaced by about 500 cm-^l. Individual vibrational peaks are favoured in different solvents and the whole band may be shifted up to 500 cm^-l. Even larger shifts are observed in charged purine molecules and in powders (up to 3000 cm^-l) and the presumed 0–0 band is not observed.     

CHARACTERIZATION OF THE SPECTROSCOPIC PROPERTIES OF A TETRAHYDROCHRYSENE SYSTEM CONTAINING A RIGIDIFIED HYDROXYNITROSTILBENE CHROMOPHORE: AN INHERENTLY FLUORESCENT LIGAND DESIGNED FOR THE ESTROGEN RECEPTOR

Abstract— A tetrahydrochrysene system that embodies a hydroxy- and nitro-substituted stilbene chromophore held rigidly near planarity by the tetracyclic nature of the compound was prepared as a fluorescent ligand for the estrogen receptor. It shows strong solvent-dependent fluorescence at long wavelengths. The solvent polarity dependence suggests that the fluorescence arises from an excited state with much nπ* character in cyclohexane; stronger emission comes from an intramolecular charge transfer state that has lower energy in more polar solvents, and finally progressive quenching of the charge transfer state occurs in solvents of higher polarity. The quenching effect is particularly evident in protic solvents. In water, however, the compound shows fluorescence of unusually high energy for an intramolecular charge transfer state, which suggests that photochemistry may be occurring. In solutions of gamma-cyclodextrin, emission from the nitrotetrahydrochrysene is red shifted and intensified relative to water. Photobleaching occurs in H₂O but not in ethanol or gamma-cyclodextrin solution. The change in dipole moment between the ground and excited states for the nitrochrysene is 12.9 D, similar to our previous measurements for related nitrostilbenes. The compound displays red-shifted emission in triethylamine, perhaps due to an excited state hydrogen-bonded complex, The absorption and emission properties of the corresponding nitrophenolate were also studied. The nitrophenolate exhibits reverse solvatochromism in its absorption spectra. In conclusion, the high sensitivity of the emission energy and quantum yield of the title compound make it of potential utility as a fluorescent probe.     

Change of the luminescence properties of charge-transfer complexes in liquid solutions

The deactivation mechanism of the excited charge-transfer complex in non-polar and polar solvents has been discussed on the basis of measured quantum yields and decay times of fluorescence of charge-transfer complexes of pyromellitic dianhydride with some methylbenzenes. It has been pointed out that great changes of electronic structure in the excited state, caused by strong excited complex—solvent interaction, are responsible for the very low fluorescence quantum yield observed in polar solvents.     

PHOTOPHYSICAL PROPERTIES OF 3,3'-DIALKYLTHIACARBOCYANINE DYES IN HOMOGENEOUS SOLUTION

The photophysical properties of 3,3′-dialkylthiacarbocyanine iodides and chlorides were measured in various solvents. It was found that photoisomerization and fluorescence are the major contributors to the deactivation of the excited singlet state; intersystem crossing occurs with only a very low efficiency. In ethanol, a triplet yield of 0.004 and a singlet oxygen quantum yield of 0.002 were determined. The photophysical parameters of these dyes are not substantially influenced by the length of the alkyl chain or the size of the halide counterion. The substitution of an ethyl with an octadecyl-chain only slightly hinders photoisomerization, and the replacement of the chloride with an iodide reduces only marginally the fluorescence lifetimes and fluorescence quantum yields in chloroform. A significant external heavy-atom effect is observed using dibromoethane as a solvent: triplet and singlet oxygen yields increase7–10-fold, and the triplet lifetime decreases from 55 μs to 15 mUs.     

Photochromism, anomalous multi-banded fluorescence and laser properties of some amino- and tosyl-amino derivatives of oxadiazole

The multi-banded fluorescence and laser properties of 11 new amino- and tosylamino derivatives of 2,5-di(phenyl)-1,3,4-oxadiazole and oxadiazole in various solvents at 293 K are reported. All the compounds investigated possess intra-molecular hydrogen quasi-bond (IHB) of 4.6-5.2 kcal mol(-1) in the ground state. In the excited state they can undergo protolytic dissociation or intra-molecular photon-initiated transfer of proton and reveal anomalous fluorescence which cannot be explained within the framework of the Kasha and Kasha-Vavilov rules. Depending upon the excitation wavelength, solvent, concentration and pH of the medium, the compounds studied show a single, double, triple or even a four-banded fluorescence, which has not been reported earlier. The nature of multi-banded fluorescence is explained in terms of the possible photochromic processes in excited states. Quantum yields and decay times of the different fluorescence bands are reported. Anomalous dependence of quantum yield upon concentration of the solution is observed. Laser properties of the compounds studied are carefully tested. Laser action based on the fluorescence of the so-called bi-radical molecules is reported. Various possible arrangements of singlet and triplet levels of compounds investigated are discussed.     

ROOM-TEMPERATURE FLUORESCENCE PROPERTIES OF THE POLYNUCLEOTIDE POLYdA POLYdT

The room-temperature fluorescence spectrum of the non-alternating polynucleotide polydA.polydT is found to have its maximum at about 325 nm and, when exciting in the spectral region where both adenine (A) and thymine (T) absorb, to coincide with that obtained for excitation at 293 nm where thymine is selectively excited. The fluorescence anisotropy is found to be equal to 0.18 and independent of the excitation and emission wavelengths. These observations are consistent with: (i) emission stemming from T; and (ii) transfer of electronic energy from A to T being not efficient. These inferences are also supported by the observed dependence of the fluorescence quantum yield on the excitation wavelength.     

STUDIES ON FLAVINS IN ORGANIC SOLVENTS — III. SPECTRAL BEHAVIOUR OF LUMIFLAVIN

Abstract— The absorption and fluorescence emission spectra of lumiflavin have been studied in nonaqueous solvents of different polarity. The corrected emission spectra show apparent mirror symmetry to the longest wavelength absorption maximum. In view of high molar extinction coefficient of the longest wavelength maximum in absorption spectra, constant polarization of fluorescence across it in dioxane and high values of quantum yields of Buorescence, τ→τ* character of corresponding electronic transition was proposed. Roughly linear correlation was found between transition energies and quantum yields of fluorescence on the one hand, and polarity of solvents expressed in Kosower's 'Z' values (excepting acetic acid) on the other.
In preliminary experiments it was found that lumiflavin in dioxane and in acetic acid is strongly photodecomposed by the monochromatic(436 nm) light.     

Spectral and photophysical studies on cruciform oligothiophenes in solution and the solid state

The photophysical and spectroscopic properties of a new class of oligothiophene derivatives, designated as cruciform oligomers, have been investigated in solution (room and low temperature) and in the solid state (as thin films in Zeonex matrixes). The study comprises absorption, emission, and triplet-triplet absorption spectra, together with quantitative measurements of quantum yields (fluorescence, intersystem crossing, internal conversion, and singlet oxygen formation) and lifetimes. The overall data allow the determination of the rate constants for all decay processes. From these, several conclusions are drawn. First, in solution, the main deactivation channels for the compounds are the radiationless processes: S(1) --> S(0) internal conversion and S(1) --> T(1) intersystem crossing. Second, in general, in the solid state, the fluorescence quantum yields decrease relative to solution. A comparison is made with the analogous linear alpha-oligothiophenes, revealing a lower fluorescence quantum efficiency and, in contrast to the normal oligothiophenes, that internal conversion is an important channel for the deactivation of the singlet excited state. Replacement of thiophene by 1,4-phenylene units in the longer-sized cruciform oligomer increases the fluorescence efficiency. The highly efficient generation of singlet oxygen through energy transfer from the triplet state (S(Delta) approximately 1) provides support for the measured intersystem crossing quantum yields and suggests that reaction with this may be an important pathway to consider for degradation of devices produced with these compounds.     

THE PHOTOPHYSICS OF MEROCYANINE 540. A COMPARATIVE STUDY IN ETHANOL AND IN LIPOSOMES

Abstract— Absorption and fluorescence emission spectra, fluorescence lifetimes, fluorescence quantum yields, photoisomerization quantum yields and triplet quantum yields were measured for Merocyanine 540 (MC540) in ethanol and in large unilamellar dimyristoyl phosphatidylcholine vesicles. The major differences in the photophysics between the two media are the increase of the fluorescence quantum yield from 0.15 in ethanol to 0.6 in vesicles at 25° C, and the appearance of a second fluorescence decay with a lifetime of 1.87 ns in the latter medium. Upper and lower limits for the photoisomerization quantum yields were determined by combining the data from laser flash photolysis and optoacoustic spectroscopy. The decrease in photoisomerization quantum yield upon incorporation of the dye into the lipid bilayer by a factor 2 suggests that this process competes directly with fluorescence. The temperature dependence of the fluorescence and photoisomerization quantum yields in solution supports this model. In both media MC540 has a very low triplet quantum yield with values 0.002 > (> ø_T > 0.02 in ethanol and 0.01 > ø_T- > 0.09 in liposomes Our data are consistent with the model whereby the dye is incorporated into the lipid bilayer as a monomer with two different orientations and this model is adopted on the basis of the biexponential behaviour of the fluorescence and photoisomer decay.     

9-乙烯基蒽和对-N,N-二甲氨基苯乙烯激基络合物的生成和光二聚反应

测定了9-乙烯基蒽和对-N,N-二甲氨基苯乙烯体系在不同极性的溶剂中的荧光光谱和荧光量子产率。发现随着溶剂极性的增加,激基络合物的荧光发生红移,荧光量子产率降低。同时9-乙烯基蒽和对-N,N-二甲氨基苯乙烯之间的光二聚产物的相对产率也降低。并讨论了经过激基络合物中间体的光二聚反应机理。     

EXCITATION WAVELENGTH DEPENDENCE OF PROTOTROPIC DISSOCIATION AND TAUTOMERISM OF SALICYLAMIDE IN THE LOWEST EXCITED SINGLET STATE

Abstract—The fluorescence spectra of salicylamide in cyclohexane, ethanol and at different pH in water were studied. The short and long wavelength fluorescences observed in the organic solvents originate from emissions of a weakly or non-intramolecularly hydrogen bonded conformer and from phototautomerization of a strongly intramolecularly hydrogen bonded conformer, respectively. Evidence for at least 2 conformers in the ground state exists in the excitation wavelength dependence of the ratio of short wavelength to long wavelength emission. In water, prototropic dissociation of the phenolic group of salicylamide in the lowest excited singlet state also shows an excitation wavelength dependence, indicating that the weakly or non-intramolecularly hydrogen bonded conformer in water is predominately responsible for photodissociation.     

Rational Design of Emissive NIR‐Absorbing Chromophores: RhIII Porphyrin‐Aza‐BODIPY Conjugates with Orthogonal Metal–Carbon Bonds

The facile synthesis of Group 9 Rh^III porphyrin‐aza‐BODIPY conjugates that are linked through an orthogonal Rh?C(aryl) bond is reported. The conjugates combine the advantages of the near‐IR (NIR) absorption and intense fluorescence of aza‐BODIPY dyes with the long‐lived triplet states of transition metal rhodium porphyrins. Only one emission peak centered at about 720 nm is observed, irrespective of the excitation wavelength, demonstrating that the conjugates act as unique molecules rather than as dyads. The generation of a locally excited (LE) state with intramolecular charge‐transfer (ICT) character has been demonstrated by solvatochromic effects in the photophysical properties, singlet oxygen quantum yields in polar solvents, and by the results of density functional theory (DFT) calculations. In nonpolar solvents, the Rh^III conjugates exhibit strong aza‐BODIPY‐centered fluorescence at around 720 nm (Φ_F=17–34 %), and negligible singlet oxygen generation. In polar solvents, enhancements of the singlet‐oxygen quantum yield (Φ_Δ=19–27 %, λ_ex=690 nm) have been observed. Nanosecond pulsed time‐resolved absorption spectroscopy confirms that relatively long‐lived triplet excited states are formed. The synthetic methodology outlined herein provides a useful strategy for the assembly of functional materials that are highly desirable for a wide range of applications in material science and biomedical fields.     

Photophysics of Glycosylated Derivatives of a Chlorin,Isobacteriochlorin and Bacteriochlorin for Photodynamic Theragnostics: Discovery of a Two‐photon‐absorbing Photosensitizer

The photophysical properties of a chlorin, isobacteriochlorin and bacteriochlorin built on a core tetrapentafluorophenylporphyrin (TPPF₂₀) and the nonhydrolyzable para thioglycosylated conjugates of these chromophores are presented. The photophysical characterization of these compounds was done in three different solvents to correlate with different environments in cells and tissues. Compared with TPPF₂₀ other dyes have greater absorption in the red region of the visible spectrum and greater fluorescence quantum yields. The excited state lifetimes are from 3 to 11 ns. The radiative and nonradiative rate constants for deactivation of the excited state were estimated from the fluorescence quantum yield and excited state lifetime. The data indicate that the bacteriochlorin has strong absorption bands near 730 nm and efficiently enters the triplet manifold. The isobacteriochlorin has a 40–70% fluorescence quantum yield depending on solvent, so it may be a good fluorescent tag. The isobacteriochlorins also display enhanced two‐photon absorption, thereby allowing the use of 860 nm light to excite the compound. While the two‐photon cross section of 25 GM units is not large, excitation of low chromophore concentrations can induce apoptosis. The glycosylated compounds accumulate in cancer cells and a head and neck squamous carcinoma xenograft tumor model in mice. These compounds are robust to photobleaching.     

THE EFFECT OF pH ON THE ABSORPTION AND FLUORESCENCE SPECTRA OF SANGUINARINE

Abstract— The influence of pH in the range 1.0 to 13.0 on the structure of sanguinarine has been investigated by spectrophotometric and spectrofluorimetric measurements. The data on absorption maxima, molar extinction coefficient, fluorescence emission maxima, relative fluorescence intensity and fluorescence quantum yield of the sanguinarine under various pH are presented. It is suggested that the pH dependent absorbance and fluorescence property of sanguinarine is due to the formation of the carbinolamine by hydroxylation at C-6 at alkaline pH.     

Photophysical properties of borondipyrromethene analogues in solution

The photophysical properties of seven new 8-(p-substituted)phenyl analogues of 4,4-difluoro-3,5-dimethyl-8-(aryl)-4-bora-3a,4a-diaza-s-indacene (derivatives of the well-known fluorophore BODIPY) in several solvents have been studied by means of absorption and steady-state and time-resolved fluorimetry. For each compound, the fluorescence quantum yield and lifetime are lower in solvents with higher polarity owing to an increase in the rate of nonradiative deactivation. Increasing the electron withdrawing strength of the p-substituent on the phenyl group in position 8 also leads to lower fluorescence quantum yields and lifetimes. When the p-substituent on the phenyl group in position 8 is a tertiary amine [8-(4-piperidinophenyl), 8-(4-N,N-dimethylaminophenyl), and 8-(4-morpholinophenyl)], the low quantum yields of these compounds in more polar solvents can be rationalized by the inversion of the energy levels of an apolar, highly fluorescent and a polar, nonfluorescent excited state, where charge transfer from the tertiary amine to the BODIPY unit occurs. These amine analogues can be protonated at low pH in aqueous solution. Fluorescence titrations yielded pK(a) values of their conjugate ammonium salts which are in agreement with the electron donating tendency of the amine group: piperidino (4.15) > dimethylamino (2.37) > morpholino (1.47), with the pK(a) values in parentheses. The rate constant of radiative deactivation (k(f)) is the same for all compounds in all solvents studied (k(f) = 1.4 x 10(8) s(-1)).     

PHOTOCHEMICAL PROPERTIES OF PORPHYRIN c: AN AGENT FOR USE IN TUMOR PHOTOTHERAPY

Abstract— The absorption and fluorescence properties of porphyrin c (P c ), the porphyrin chromophore present in cytochrome c , have been determined in several solvents and micellar environments. In aqueous buffer solutions at pH 7.5 Pc may exist in both a fluorescent monomeric form with quantum yield of fluorescence, (Φ_f,) ∼ 0.03, and fluorescence lifetime, (τ_f) ∼ 8 ns, and as a non-fluorescent aggregate. The proportion of monomeric form is higher in organic solvents and micelles but is reduced with increasing porphyrin concentrations in aqueous solutions. Porphyrin c readily complexes with Zn²⁺ to produce a fluorescent chelate (Zn-P c ) with Φ_f, ∼ 0.02 and τ_f, ∼ 2 ns at pH 7.5. The yields of singlet excited oxygen formation from Pc and the Zn-P c complex are higher than observed for hematoporphyrin derivative (HpD). Both P c and Zn-P c are effective agents in tumor phototherapy and do not induce the prolonged cutaneous photosensitivity observed with the use of HpD.     

THE EFFECT OF pH ON THE AEROBIC and ANAEROBIC PHOTOLYSIS OF TRYPTOPHAN AND SOME TRYPTOPHAN-CONTAINING DIPEPTIDES

Abstract— Rates of photolysis, quantum yields of fluorescence, and fluorescence emission maxima for the dipeptides glycyltryptophan (Gly-Trp) and tryptophylglycine (Trp-Gly) and for free tryptophan (Trp) were determined under both degassed and aerated conditions in the pH range 4.5-10.0. The photolyses were performed at 25°C using 290 nm radiation from a 1000 W xenon lamp. Photolysis rates were determined by monitoring tryptophan fluorescence loss with time. It was found that Trp-Gly and free Trp showed similar behavior in that their fluorescence quantum yields and photolysis rates increased significantly above neutral pH. In contrast, the Gly-Trp fluorescence yield was smaller than that of Trp or Trp-Gly, showing no significant increase at high pH and the photolysis rate for Gly-Trp decreased with increasing pH. In comparing aerated to degassed samples, it was found that degassing had a far greater effect on the photolysis rates of Trp and Trp-Gly than on the photolysis rate of Gly-Trp especially at higher pH. But, degassing did not change the relative fluorescence quantum yields or fluorescence emission maxima of any of the three compounds. Possible mechanisms for photolysis under various experimental conditions were examined in light of the data.