ELECTRONIC SPECTRA OF AZA-AROMATICS IN POLYMER MATRICES†

Abstract— The absorption and fluorescence spectra of acridine, phenazine, their cations and phenazine-di-N-oxide are studied in several polymer matrices. There is a one-to-one correspondence between previously measured spectra in fluid media and those reported here for plastic media. The matrix-induced spectral shifts are dependent upon the polarity and hydrogen-bonding ability of the polymer substrate. Environmental perturbations are also produced by high pressures (0–27 kbar) applied to the polymer matrix. Specific interactions between polymer substrate and the electronically excited solutes are evident from pressure-induced red shifts of the fluorescence and changes in quantum yields under pressure. These interactions between solute and its microscopic environment are subtle and varied for this group of nitrogen hetero-cyclics, although their effects are smaller than those encountered in the adsorption of related dyes.     

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.     

Absorption and emission spectroscopic characterization of Ir(ppy)3

The absorption and emission spectroscopic behaviour of cyclometalated fac-tris(2-phenylpyridine) iridium(III) [Ir(ppy)₃] is studied at room temperature. Liquid solutions, doped films, and neat films are investigated. The absorption cross-section spectra including singlet–triplet absorption, the triplet–singlet stimulated emission cross-section spectra, the phosphorescence quantum distributions, the phosphorescence quantum yields and the phosphorescence signal decays are determined. In neat films fluorescence self-quenching occurs, in diluted solid solution (polystyrene and dicarbazole-biphenyl films) as well as deaerated liquid solution (toluene) high phosphorescence quantum yields are obtained, and in air-saturated liquid solutions (chloroform, toluene, tetrahydrofuran) the phosphorescence efficiency is reduced by triplet oxygen quenching. At intense short-pulse laser excitation the phosphorescence lifetime is shortened by triplet–triplet annihilation. No amplification of spontaneous emission in the phosphorescence spectral region was observed indicating higher excited-state absorption than stimulated emission.     

Solvent and Media Effects on the Photophysics of Naphthoxazole Derivatives

The photophysical properties of 2‐phenyl‐naphtho[1,2‐d][1,3]oxazole, 2(4‐N,N‐dimethylaminophenyl)naphtho[1,2‐d][1,3]oxazole and 2(4‐N,N‐diphenylaminophenyl) naphtho[1,2‐d][1,3]oxazole were studied in a series of solvents. UV–Vis absorption spectra are insensitive to solvent polarity whereas the fluorescence spectra in the same solvent set show an important solvatochromic effect leading to large Stokes shifts. Linear solvation energy relationships were employed to correlate the position of fluorescence spectra maxima with microscopic empirical solvent parameters. This study indicates that important intramolecular charge transfer takes place during the excitation process. In addition, an analysis of the solvatochromic behavior of the UV–Vis absorption and fluorescence spectra in terms of the Lippert–Mataga equation shows a large increase in the excited‐state dipole moment, which is also compatible with the formation of an intramolecular charge‐transfer excited state. We propose both naphthoxazole derivatives as suitable fluorescent probes to determine physicochemical microproperties in several systems and as dyes in dye lasers; consequence of their high fluorescence quantum yields in most solvents, their large molar absorption coefficients, with fluorescence lifetimes in the range 1–3 ns as well as their high photostability.     

Photodissociation dynamics of hydroxybenzoic acids

Aromatic amino acids have large UV absorption cross-sections and low fluorescence quantum yields. Ultrafast internal conversion, which transforms electronic excitation energy to vibrational energy, was assumed to account for the photostability of amino acids. Recent theoretical and experimental investigations suggested that low fluorescence quantum yields of phenol (chromophore of tyrosine) are due to the dissociation from a repulsive excited state. Radicals generated from dissociation may undergo undesired reactions. It contradicts the observed photostability of amino acids. In this work, we explored the photodissociation dynamics of the tyrosine chromophores, 2-, 3- and 4-hydroxybenzoic acid in a molecular beam at 193 nm using multimass ion imaging techniques. We demonstrated that dissociation from the excited state is effectively quenched for the conformers of hydroxybenzoic acids with intramolecular hydrogen bonding. Ab initio calculations show that the excited state and the ground state potential energy surfaces change significantly for the conformers with intramolecular hydrogen bonding. It shows the importance of intramolecular hydrogen bond in the excited state dynamics and provides an alternative molecular mechanism for the photostability of aromatic amino acids upon irradiation of ultraviolet photons.     

Spectroscopic correlations between supermolecules and molecules. Anatomy of the ion-modulated electronic properties of the nitrogen donor in monoazacrown-derived intrinsic fluoroionophores

The synthesis, absorption and emission spectra, fluorescence quantum yields, and fluorescence lifetimes of three compound series of trans-4,4'-disubstituted aminostilbenes (1-3) are reported. The chromo-/fluoroionophoric behavior of the monoaza-15-crown-5- (A15C5) and monoaza-18-crown-6 (A18C6)-derived species (1A(5)()-3A(5)() and 1A(6)()-3A(6)()) in acetonitrile and dichloromethane are also investigated. Great similarities in electronic spectroscopic properties (chemical shifts, wavelength, intensity, and lifetime) between the metal ion-complexed supermolecules and the corresponding chloro-substituted molecules have been observed: namely, 1A(5)()/Ca(2+)-3A(5)()/Ca(2+) approximately 1A(6)()/Ba(2+)-3A(6)()/Ba(2+) approximately 1C-3C in acetonitrile and 1A(5)()/Na(+)-2A(5)()/Na(+) approximately 1A(6)()/K(+)-2A(6)()/K(+) approximately 1C-2C in dichloromethane. Such spectroscopic correlations allow us to define the metal ion-modulated electronic character of the azacrown nitrogen atom in the ground and excited states and, in turn, to gain insights into the observed fluoroionophoric behavior of these probes in terms of the size and direction of fluorescence shifts and intensity variations.     

Solvatochromism of 3-[2-(4-diphenylaminophenyl)benzoxazol-5-yl]alanine methyl ester. A new fluorescence probe

The photophysical properties of 3-[2-(4-diphenylaminophenyl)benzoxazol-5-yl]alanine methyl ester (1b) and its Boc derivative (1a) were studied in a series of solvents. Its UV-Vis absorption spectra are less sensitive to the solvent polarity than the corresponding fluorescence spectra which show pronounced solvatochromic effect leading to large Stokes shifts. Using an efficient solvatochromic method, based on the molecular-microscopic empirical solvent polarity parameter E(T)(N), a large change of the dipole moment on excitation has been found. From an analysis of the solvatochromic behaviour of the UV-Vis absorption and fluorescence spectra in terms of bulk solvent polarity functions, f(epsilon(r),n) and g(n), a large excited-state dipole moment (mu(e) = 11D), almost perpendicular to the smaller ground-state dipole moment, was observed. This demonstrates the formation of an intramolecular charge-transfer excited state. Large changes of the fluorescence quantum yields as well as the fluorescence lifetimes with an increase of a solvent polarity cause that the new non-proteinogenic amino acid, 3-[2-(4-diphenylaminophenyl)benzoxazol-5-yl]-alanine methyl ester, is a new useful fluorescence probe for biophysical studies of peptides and proteins.     

A theoretical, spectroscopic, and photophysical study of 2,7-carbazolenevinylene-based conjugated derivatives

A combined theoretical and experimental study of the structure, optical, and photophysical properties of four 2,7-carbazolenevinylene-based derivatives in solution is presented. Geometry optimizations of the ground states of PCP, PCP-CN, TCT, and TCT-CN were carried out using the density functional theory (DFT/B3LYP/6-31G*). It is found that PCP and TCT are nearly planar in their ground electronic states (S0), whereas the cyano derivatives are more twisted. The nature and the energy of the first singlet-singlet electronic transitions have been obtained from time-dependent density functional theory (TDDFT) calculations performed on the optimized geometries. For all the compounds, excitation to the S1 state corresponds mainly to the promotion of one electron from the highest-occupied molecular orbital to the lowest-unoccupied molecular orbital, and the S1 <-- S0 electronic transition is strongly allowed and polarized along the long axis of the molecular frame. The optimization (relaxation) of the first singlet excited electronic state (S1) has been done using the restricted configuration interaction (singles) (RCIS/6-31G*) approach. It is observed that all four investigated compounds become more planar in their S1 relaxed excited state. Electronic transition energies from the relaxed excited states have been obtained from TDDFT calculations performed on the S1-optimized geometries. The absorption and fluorescence spectra of the carbazolenevinylenes have been recorded in chloroform. A good agreement is obtained between TDDFT vertical transitions energies and the (0,0) absorption and fluorescence bands. The change from phenylene to thiophene rings as well as the incorporation of cyano substituents induce bathochromic shifts in the absorption and fluorescence spectra. From the analysis of the energy of the frontier molecular orbitals, it is believed that thiophene rings and CN substituents induce some charge-transfer character to the first electronic transition, which is responsible for the red shifts observed. Finally, the fluorescence quantum yield and the lifetime of the compounds in chloroform have been obtained. In sharp contrast with many oligothiophenes, it is observed that TCT possesses a high fluorescence quantum yield. On the other hand, the CN-containing derivatives exhibit much lower fluorescence quantum yields, probably due to the combined influence of steric effects and charge-transfer interactions caused by the cyano groups.     

Photophysical characters of rationally designed hetero-ring-expanded guanine analogues and effect of cytosine pairing

We present the results of the CIS and TDB3LYP calculations of the optical absorption and emission spectra of some newly designed guanine (G) analogues and their Watson-Crick base pairs. Compared with natural G, the onset absorption peaks of these newly designed analogues are red-shifted, while the fluorescence peaks are blue-shifted. In general, the first excited singlet states (pipi*) of these analogues are nonplanar for all bases considered here. But, the Stokes shifts for the designed G-analogues are much smaller than that of natural G, suggesting that they have stronger molecular rigidity and higher fluorescence quantum yields than those of natural G. The first excited states of these Watson-Crick base pairs essentially originate from those of their isolated purine moieties, as demonstrated from the S1 geometries of their Watson-Crick base pairs. For G and its analogues, A1 and A2 (they are ring-expanded with one-bond intercalation at the C5 site), the pairing with cytosine reduces the oscillator strengths of both the first absorption peak (by 27%-60%) and the fluorescent emission (by 19%-23%), while for the analogues A3, A4, and xG in which G is ring-expanded with a two-bond intercalation at the C5 site, the pairing, in contrast, increases the oscillator strengths of both the first absorption peak (by 11%-15%) and the fluorescent emission (by 3%-20%). These observations indicate that the pairing with cytosine can quench the fluorescence for G, A1, and A2 but enhance the fluorescence quantum yields for A3, A4, and xG. The significant shifts induced by ring-expansion in the ring-expanded G with a two-bond intercalation at the C5 site reveal a possibility for their fluorescent detections.     

Synthesis and fluorescence characterization of MEHPPV oligomers

Trimer, tetramer, and pentamer oligomers based on the polymer backbone structure of poly[2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylenevinylene] (MEHPPV) have been synthesized by Horner-Wadsworth-Emmons reactions. The fluorescence spectra, emission quantum yields, and lifetimes of the oligomers have been characterized in dilute chloroform solutions. The oligomers exhibit a sequential increase in absorption and emission wavelength maxima and a decrease in fluorescence lifetime as the π conjugation length is increased. The shortening in excited state lifetime is shown to be due to an increase in the rates of both radiative and nonradiative processes. The absence of a mirror-image relationship for the absorption and fluorescence spectra of the oligomers is attributed to the photoexcitation of a range of torsional configurations followed by relaxation to a more planar arrangement that then emits.     

新型蓝色上转换荧光分子的合成、表征与双光子吸收性能

设计并合成了3 个新的受体-给体-受体(A-D-A)构型上转换荧光分子,用傅里叶变换红外光谱、核磁共振氢谱、质谱和元素分析进行了表征. 测定了它们在不同溶剂中的线性吸收光谱、单光子荧光光谱和荧光量子产率. 以飞秒激光作为光源,研究了它们的双光子吸收和上转换荧光特性. 结果表明：该类化合物的荧光量子产率为0.20-0.68,双光子吸收截面为16×10^-50-101×10^-50 cm⁴·s·photon^-1,具有较强的蓝色上转换荧光发射.     

Dual Solvatochromism of Neutral Red

Abstract— The effect of solvent polarity on the electronic absorption and fluorescence properties of neutral red (NR), a phenazine-based dye of biological importance has been investigated in several neat and mixed solvents. An unusual dual solvatochromic behavior has been observed that reveals the existence of two closely spaced electronic excited states in NR. In low-polarity solvents the fluorescence of the NR is mainly emitted from the localized excited state, whereas in high-polarity solvents the emission from the charge transfer state dominates. The dipole moments of the localized and charge transfer states of NR have been estimated from the solvatochromic shifts. The dipole moment of the localized excited state (4.8 D) is only slightly higher than that of the ground state (2.0 D), while that of the charge transfer state is drastically higher (17.5 D). Fluorescence quantum yields and the life-times of NR have been determined in different solvents and correlated with the solvatochromic shifts.     

INFLUENCE OF THE ENVIRONMENT ON THE EXCITATION WAVELENGTH DEPENDENCE OF THE FLUORESCENCE QUANTUM YIELD OF INDOLE

Abstract— The influence of excitation wavelength, pH, oxygen and solvents, upon fluorescence quantum yields, were measured for insole Indole in neutral aqueous solution exhibits the same wavelength dependence as tryptophan, which indicates that COO- absorption is not responsible for the effect. Parameters such as pH and oxygen influence only the absolute fluorescence quantum yields but not their relative variation with wavelength, indicating competition with fluorescence emission for S1 deactivation. without any influence upon deactivation of higher excited states. In contrast, solvents exhibit a specific influence upon the wavelength dependence; for indole, the decrease of fluorescence yield excited around 215 nm, compared with the yield in the first absorption band is about 40% in water, 10% in acetonitrile, 70% in n-hexane and cyclohexane, whereas no appreciable decrease occurs in methanol, ethanol or n-butanol. These observations, together with the Stokes shifts of the emission spectra, may be well correlated with Kosower's Z-values, expressing microscopic solvent-solute interactions.     

Fluorescence characteristics and photostability of benzoxazole derived donor-acceptor dyes in constrained media

The spectroscopic properties and photoreactivity of 2-(4-dimethylaminostryl)benzoxazole and its butadienyl analogue (abbreviated as DMASBO and BODB, respectively) were investigated in thin films of PS, PMMA and PVA polymer matrices as well as immobilized on silica surface. The experimental results showed that the investigated benzoxazole derivatives are sensitive to the polarity of its microenvironment even in the glassy matrices and in the solid state. The micropolarity of PVA and silica surface is greater than that of PMMA and PS. The combined results of fluorescence quantum yield and photoisomerization indicated that the size of the free volume within the polymer matrix influence strongly the photochemical reactivity of the used dyes. In PVA, which offers the smaller free volume, the photoreaction is totally blocked while the fluorescence yield is extremely enhanced relative to those in solutions and other polymer matrices. This is associated with an emission of green laser from DMASBO-PVA doped film when pumped by nitrogen laser (337 nm). The results showed that the present dyes are excellent probes for non-fluorescent systems presenting constrained geometry. The spectra of amorphous solid thin films of both dyes were also measured and show large red shifts relative to those in solutions indicating symmetric aggregation of molecules in the solid crystals.     

Fluorescence enhancement of trans-4-aminostilbene by N-phenyl substitutions: the "amino conjugation effect"

The synthesis, structure, and photochemical behavior of the trans isomers of 4-(N-phenylamino)stilbene (1c), 4-(N-methyl-N-phenylamino)stilbene (1d), 4-(N,N-diphenylamino)stilbene (1e), and 4-(N-(2,6-dimethylphenyl)amino)stilbene (1f) are reported and compared to that of 4-aminostilbene (1a) and 4-N,N-dimethylaminostilbene (1b). Results for the corresponding 3-styrylpyridine (2) and 2-styrylnaphthalene analogues (3) are also included. The introduction of N-phenyl substituents to 4-aminostilbenes leads to a more planar ground-state geometry about the nitrogen atom, a red shift of the absorption and fluorescence spectra, and a less distorted structure with a larger charge-transfer character for the fluorescent excited state. Consequently, the N-phenyl derivatives 1c-e have low photoisomerization quantum yields and high fluorescence quantum yields at room temperature, in contrast to the behavior of 1a, 1b, and most unconstrained monosubstituted trans-stilbenes. The isomerization of 1c and 1d is a singlet-state process, whereas it is a triplet-state process for 1e, presumably due to a relatively higher singlet-state torsional barrier. The excited-state behavior of 1f resembles 1a and 1b instead of 1c-e as a consequence of the less planar amine geometry and weaker orbital interactions between the N-phenyl and the aminostilbene groups. Such an N-phenyl substituent effect is also found for 2 and 3 and thus appears to be general for stilbenoid systems. The nature of this effect can be described as an "amino conjugation effect".     

Solvent Polarity and pH Effects on the Spectroscopic Properties of Neutral Red: Application to Lysosomal Microenvironment Probing in Living Cells

Neutral red is a lysosomal probe and a biological pH indicator. In aqueous solutions, the protonated (NRW) and neutral (NR) forms of monomeric neutral red exhibit distinct absorption maxima (535 and 450 nm, respectively) but have the same fluorescence with a maximum at 637 nm and a quantum yield of 0.02. The similarity of the fluorescence spectra at acidic and basic pH suggests deprotonation of cationic species in the first singlet excited state. The NR fluorescence strongly depends on the solvent polarity as shown by addition of increasing amounts of water to pure dioxane, which gradually shifts the fluorescence maximum from 540 nm in pure dioxane to 637 nm in water. The fluorescence quantum yield increases from 0.17 in dioxane to 0.3 upon addition of 7% water and then decreases, reaching 0.02 in pure water. Immediately after incubation of human skin fibroblasts with neutral red, excitation with 435 nm light produces a fluorescence whose maximum is recorded at 575 nm. This fluorescence is located in the perinuclear region and originates from large fluorescent intracytoplasmic spots, suggesting staining of the endoplasmic reticulum-Golgi complex. At longer times, this fluorescence is shifted to 606 nm, suggesting slow diffusion of the lysosomotropic dye toward the more hydrated and acidic interior of ly-sosomes. Addition of a lysosomotropic detergent to cells previously incubated with neutral red shifts the fluorescence to the blue. Thus, in complex biological systems, this probe cannot be a good pH indicator but is a very sensitive probe of lysosomal rnicroenvironrnents.     

Excited state solvatochromic and prototropic behaviour of 4-aminodiphenylamine and 4,4'-diaminodiphenylamine--a comparative study by electronic spectra

Solvatochromic and prototropic behaviour of 4-aminodiphenylamine (4ADA) and 4,4'-diaminodiphenylamine (DADA) have been investigated in the solvents of different polarity and at various acid-base concentrations in the ground and excited states using absorption and fluorescence spectra. Solvatochromic shifts have been analysed and observed shifts are explained by the hydrogen bonding interactions. The prototropic study reveals that (i) absorption maximum of monocation of DADA is red shifted to its neutral form, and (ii) the fluorescence of 4ADA is red shifted on protonation. The abnormal fluorescence of 4ADA+ is found to be due to large solvent relaxation in polar medium.     

可溶性的含羧基苝酰亚胺类化合物的合成及其光谱性质研究

本文主要以酐(1)为起始物,通过在环母体结构的1、6、7、12位上引入苯氧基和对甲基苯氧基合成了另外两种酐化合物(2a和2b);所得到的酐化合物与6-氨基已酸进行反应,合成了3种可溶性的含有羧基的酰亚胺化合物(3a、3b和3c),使用红外、紫外、荧光、¹H-NMR,元素分析对所合成的化合物进行了表征.经研究发现,环母体结构的1、6、7、12位上苯氧基和对甲基苯氧基的引入,能使得所合成的含羧基酰亚胺类化合物的最大吸收波长分别红移了44.1 nm和50.1 nm;Stokes位移变大,分别为34.1 nm和30.0 nm;而荧光量子产率有所降低但下降趋势不明显.     

Spectral-luminescence and generation characteristics of 1-R-vinyl-3-aryl(heteryl)benzo[f]quinolines

The absorption and fluorescence spectra and the fluorescence quantum yields of 1-R-vinyl derivatives of benzo[f]quinoline containing aryl and 2-, 4-, and 6-quinolyl groups in the vinylene grouping and in the 3 position were studied. The relationship between the spectral-luminescence characteristics and the structures of the molecules was examined. It was established that solutions of compounds that contain a dimethylamino group attached to the phenyl ring give forced emission when they are excited with the second harmonic of a ruby laser. The effect of the solvent on the spectral-luminescence and generation characteristics of the indicated compounds was investigated.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 4, pp. 511–515, April, 1978.     

Synthesis and optical properties of new chalcones containing 4-[bis(2-hydroxyethyl)amino]phenyl fragment

New chalcones with 4-[bis(2-hydroxyethyl)amino] phenyl fragment were obtained from 4-[bis(2-acetoxyethyl) amino]- benzaldehyde by the Claisen–Schmidt reaction. From their UV-VIS absorption and emission spectra, optical band gap values were calculated based on the Stokes shifts as well as the molar absorption coefficients and fluorescence quantum yields were estimated. The dependence of the absorption and emission maxima on solvent polarity and pH was evaluated.