Solvent Effects on Spectral Property and Dipole Moment of the Lowest Excited State of Coumarin 343 Dye

Steady-state absorption and fluorescence spectra, and time-resolved fluorescence spectra of coumarin 343 (C343) were measured in different solvents. The effect of the solvent on the spectral properties and dipole moment of the lowest excited state of C343 were investigated. It was found that the absorption and fluorescence spectra red-shifted slightly and strongly with increasing solvent polarity, respectively, because the charge distribution of the excited state leaded to the increasing difference between the absorption and fluorescence spectra with increasing solvent polarity. The dipole moment of the lowest excited state of C343was determined from solvatochromic measurements and the quantum chemical calculation, and the results obtained from these two methods were fully consistent. Investigations of the time-resolved fluorescence of C343 in different solvents indicated that the fluorescence lifetimes increased nearly linearly with increasing solvent polarity from 3.09 ns in toluene to 4.45 ns in water. This can be ascribed to the intermolecular hydrogen bonding interactions between C343 and hydrogen donating solvents.     

Studies on fluorescent species of spiro-indolinonaphthooxazines and their fluorescence spectra

Photophysical behavior of spiro[1,3,3-trimethylindolino-2,3′-naphtho[2,1-b]-1,4-oxazine] (SP1) and spiro[1,3,3,2′-tetramethylindolino-2,3′-naphtho[2,1-b]-1,4-oxazine] (SP2) were studied. The fluorescent species and their spectra of SP1 and SP2 in polar solvents (acetonitrile and methanol) and non-polar solvent (cyclohexane) were investigated. The fluorescence decay in polar solvents was studied by picosecond time-correlated single photon counting. In most cases, fluorescence decay obeyed triexponential decay kinetics. The major fluorescent species is an excited intermediate which has similar conformation as its precursor (¹ SP ^*) formed after the bond cleavage between spirocarbon and oxygen in oxazine ring. The effects of molecular structure and solvent polarity on fluorescence spectra and fluorescence decay lifetime were studied.     

Synthesis of 1,4-dihydropyridines under solvent free conditions and investigation of their photo physical properties

Synthesis of five 4-aryl substituted 1,4-dihydropyridines was developed following condensation of multi component reaction strategy using yttrium triflate as a catalyst. The absorption and fluorescence properties of structurally related 4-aryl 1,4-dihydropyridines in different solvents of varied polarities was investigated. The absorption maxima of these compounds follow no order of solvent polarity and nature of substitution. The spectral characteristics are solvent and compound specific. Fluorophores with electron withdrawing group have larger fluorescence quantum yields and greater solvatochromism than the compounds with electron donating groups. Protic solvents yielded higher fluorescence quantum efficiency. The chemical shift of the proton attached to C-4 and the carbonyl stretching frequency of bis acetyl groups at 3 and 5-positions exhibited a linear relationship with Hammett's para substituent constants while no such relationship exists between the latter and electronic absorption maxima, fluorescence emission maxima, fluorescence quantum efficiency and Stokes shift.     

Photophysical Properties of 4′-(p-aminophenyl)-2,2′:6′,2′′-terpyridine

Spectral and photophysical investigations of 4′-(p-aminophenyl)-2,2′:6′,2′′-terpyridine (APT) have been performed in various solvents with different polarity and hydrogen-bonding ability.The emission spectra of APT are found to exhibit dual fluorescence in polar solvents, which attributes to the local excited and intramolecular charge transfer states, respectively. The two-state model is proven out for APT in polar solvent by the time-correlated single photon counting emission decay measurement. Interestingly, the linear relationships of different emission maxima and solvent polarity parameter are found for APT in protic and aprotic solvents, because of the hydrogen bond formation between APT and alcohols at the amino nitrogen N25. Furthermore, the effects of the complexation of the metal ion with tpy group of APT and the hydrogen bond formation between APT with methanol at the terpyridinenitrogen N4—N8—N14 are also presented. The appearance of new long-wave absorption and fluorescence bands indicates that a new ground state of the complexes is formed.     

Arginine‐Substituted Phthalocyanine with Concentration‐Driven Self‐Disaggregation Performance: Synthesis,Properties and Mechanistic Study

An arginine‐substituted zinc phthalocyanine (ArgZnPc) capable of disaggregating at high concentrations in polar non‐aqueous solvents through concentration‐driven hydrogen bond type transformation has been prepared. The ArgZnPc was prepared through a guanidine‐meditated self‐catalytic ester hydrolysis reaction. The concentration‐driven disaggregation of ArgZnPc was confirmed by UV‐Vis absorption spectra, fluorescence emission spectra and lifetimes, and singlet oxygen quantum yield data.     

Photophysics of (1-butyl-4-(1H-inden-1-ylidene)-1,4-dihydropyridine (BIDP): an experimental test for conical intersections

Fluorescence experiments on (1-butyl-4-(1H-inden-1-ylidene)-1,4-dihydropyridine (BIDP) are reported in liquid and glassy solutions. The data indicate a fast decay in the fluid nonpolar, nonprotic solutions (decay times approximately 10(-12) s) and rapid but considerably slower decay in polar ones. In frozen solutions (polar and nonpolar), the fluorescence quantum yield is much higher (near 0.5 and around 0.1 in polar and nonpolar glasses, respectively). The rapid nonradiative transitions in fluid solutions are assigned to internal conversion in both solvent classes, as intersystem crossing is much slower and no net reaction is observed. These results are in agreement with predictions made for the closely related (in terms of electronic structure) but simpler molecule cyclopentadienyl-1,4-dihydropyridine (CPDHP) for which an S1/S0 conical intersection was recently proposed [Int. J. Quant. Chem. 2005, 102, 961]. The crossing of the two lowest singlet states is calculated to vanish in polar solvents such as methyl cyanide, leading to longer lifetime of S1 of CPDHP. As BIDP has a very similar electronic structure, the model predicts a corresponding change in this larger molecule. The strong fluorescence observed in the glassy environments is rationalized by the hindering of the internal torsion required to reach the geometry of the conical intersection.     

Interaction of 2-hydroxy-substituted Nile red fluorescent probe with organic nitrogen compounds

The fluorescent properties of 2-hydroxy Nile red dye (HONR) proved to be highly sensitive to the basicity of hydrogen bond acceptors. Fluorescence quantum yields and fluorescence decay profiles were measured as the function of the concentration of organic nitrogen compounds in solvents of various polarities. The detailed mechanism and the kinetics of the fluorescence quenching were revealed with the combined analysis of the steady-state and time-resolved spectroscopic data. The relative contribution of the competing reaction steps was found to be very sensitive to the basicity of the additive and to solvent polarity. The most profound change appeared in the unimolecular deactivation pathways of the excited hydrogen-bonded HONR, whereas the formation rate of this species varied to a lesser extent. The dissociation into excited HONR and ground-state base was able to compete with the energy dissipation only when 2,4,6-trimethylpyridine was used as hydrogen bond acceptor in toluene. The bimolecular quenching of the excited hydrogen-bonded complex played significant role in apolar solvents. Proton displacement along the hydrogen bond in the excited complex led to excited ion pairs in polar media.     

Fluorescence lifetimes and spectral properties of protochlorophyllide in organic solvents in relation to the respective parameters in vivo

In this work, absorption and fluorescence spectra of protochlorophyllide (Pchlide), as well as its fluorescence lifetime, were investigated in organic solvents having different physical properties. The obtained Pchlide spectral features are discussed in relation to the parameters describing solvent properties (refractive index and dielectric constant) and taking into account the specific solvent-Pchlide interaction. The correlation of Pchlide Qy and Soret absorption bands with solvent polarizability function ((n2 - 1)/(n2 + 2)) has been found; however, the dispersion of the observed points was rather high. A small Stokes shift of a magnitude between 50 and 300 cm(-1) was found, which indicates low sensitivity of Pchlide to nonspecific solvation. The fluorescence decay of Pchlide was single exponential in all the investigated solvents, with the lifetime value ranging from 5.2 ns for dioxane to 3.5 ns for methanol. Dependence of the obtained fluorescence lifetimes on the solvent orientation polarizability, a parameter being the function of both refractive index and dielectric constant, was discussed. In water-methanol mixtures, a further decrease of the fluorescence lifetime was observed, giving values of 2.9 ns for 25% methanol. Double-exponential decay of Pchlide fluorescence was found for Pchlide in a solution of 15% methanol with the lifetimes of 4.5 +/- 0.5 ns and 1.2 +/- 0.3 ns and in pure water with the lifetimes of 2.5 +/- 0.5 ns and 0.4 +/- 0.1 ns. The obtained results are discussed in relation to spectroscopic properties of Pchlide in vivo.     

Study of spectral and fluorescent properties and isomerization of merocyanines containing nitrile groups as acceptors

The spectral, fluorescent, and photochemical properties of new merocyanines containing nitrile groups as acceptors have been studied. Positive solvatochromism is observed for most of these compounds; however, for the dye containing three nitrile groups as acceptors—polyenic aminotrinitrile—negative solvatochromism is observed: the absorption spectrum is shifted to shorter wavelengths with increasing the solvent polarity. In all solvents used, the fluorescence and fluorescence excitation spectra of the merocyanines almost do not depend on the excitation and fluorescence registration wavelengths, respectively. Using the flash photolysis method, the formation of photoisomers of the merocyanines has been detected and the kinetics of their decay in solvents of different polarity has been studied. Backward (dark) isomerization of the photoisomers has been shown to take place more slowly in nonpolar solvents.     

Impact of Hydrogen Bonding on the Fluorescence of N‐Amidinated Fluoroquinolones

The fluorescence properties of AIE‐active N‐amidinated fluoroquinolones, efficiently obtained by a perfluoroaryl azide–aldehyde–amine reaction, have been studied. The fluorophores were discovered to elicit a highly sensitive fluorescence quenching response towards guest molecules with hydrogen‐bond‐donating ability. This effect was evaluated in a range of protic/aprotic solvents with different H‐bonding capabilities, and also in aqueous media. The influence of acid/base was furthermore addressed. The hydrogen‐bonding interactions were studied by IR, NMR, UV/Vis and time‐resolved fluorescence decay, revealing their roles in quenching of the fluorescence emission. Due to the pronounced quenching property of water, the N‐amidinated fluoroquinolones could be utilized as fluorescent probes for quantifying trace amount of water in organic solvents.     

4—甲基—7—羟基香豆素的发光特性研究

对比考察了多种溶剂介质中不同浓度４－甲基－７－羟基香豆素溶液的荧光光谱特性，发现其谱形状与峰位不仅随溶剂性质而异，且随浓度而变，在某些氢键溶剂中，激发光谱随浓度有很大变化，且发射光谱间存在某种内在联系。基于溶剂本身的性质及４－甲基－７－羟基香豆素在氢键溶剂中的二聚作用讨论了这种现象，本文还考察了这种香豆素衍生物在滤纸及聚酰胺膜基质上，以Ｐｂ（Ａｃ）２或Ｐｂ（ＮＯ３）２作重原子微扰剂时的室温磷光特性     

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.     

Electronic spectra and photophysics of the α-carboline (1-azacarbazole) monomer

The UV-vis electronic absorption and emission spectra of α-carboline or 1-azacarbazole, 9H-pyrido[2,3-b]indole, AC, have been investigated in aprotic solvents. Radiative, k(r), non-radiative, k(nr), rate constants and natural lifetimes, τ(N), of the AC monomer in hexane and acetonitrile, obtained from the experimentally determined fluorescence quantum yields and fluorescence lifetimes, have been compared with those theoretically estimated. The closeness between these experimental and theoretical data, the small Stokes shifts, the mirror image relationship between the absorption and fluorescence spectra and the close correspondence between the absorption and fluorescence excitation spectra, provide good evidences that the emission of AC monomer occurs directly from its lowest singlet excited state. The mono- and multi-parametric analyses of the AC solvatochromism indicate that the polarity-polarizability, the hydrogen bond donor and the hydrogen bond acceptor properties of the solvent preferentially stabilize the singlet excited over the ground state. These analyses also reveal that photoexcitation reinforces the hydrogen bond donor and acceptor properties of the AC, becoming the pyridinic nitrogen atom more basic and the pyrrolic group more acid.     

Ratiometric, fluorescent BODIPY dye with aza crown ether functionality: synthesis, solvatochromism, and metal ion complex formation

A new pH and metal ion-responsive BODIPY-based fluorescent probe with an aza crown ether subunit has been synthesized via condensation of 4-(1,4,7,10-tetraoxa-13-aza-cyclopentadec-13-yl)-benzaldehyde with the appropriate 1,3,5,7-tetramethyl substituted boron dipyrromethene moiety. Steady-state and time-resolved fluorometries have been used to study the spectroscopic and photophysical characteristics of this probe in various solvents. The fluorescence properties of the dye are strongly solvent dependent: increasing the solvent polarity leads to lower fluorescence quantum yields and lifetimes, and the wavelength of maximum fluorescence emission shifts to the red. The Catalan solvent scales are found to be the most suitable for describing the solvatochromic shifts of the fluorescence emission. Fluorescence decay profiles of the dye can be described by a single-exponential fit in nonprotic solvents, whereas two decay times are found in alcohols. Protonation as well as complex formation with several metal ions are investigated in acetonitrile as solvent via fluorometric titrations. The aza crown ether dye undergoes a reversible (de)protonation reaction (pKa = 0.09) and shows a approximately 50 nm blue shift in the excitation spectra and a 10-fold fluorescence increase upon protonation. The compound also forms 1:1 complexes with several metal ions (Li(+), Na(+), Mg(2+), Ca(2+), Ba(2+), Zn(2+)), producing large blue shifts in the excitation spectra and significant cation-induced fluorescence amplifications.     

Effects of organic and aqueous solvents on the electronic absorption and fluorescence of chloroaluminum (III) tetrasulphonated naphthalocyanine

The effects of various solvents on the ground and excited states of chloroaluminum (III) tetrasulphonated naphthalocyanine (AlNcS(4)) were studied. Both the absorbance and fluorescence spectra were found to be influenced by the hydrogen bond donating ability of various solvents. As the hydrogen bond donating ability of the solvent increased, hypsochromic and bathochromic shifts in the absorbance and fluorescence spectra were observed in protic and aprotic solvents respectively. Plots of the absorbance and fluorescence maxima versus the E(T)(30) solvent parameter showed linear relationships in binary mixtures of protic-protic (methanol-H(2)O) and aprotic-protic (DMSO-H(2)O) solvents. Aggregation was indicated by a broad band in the ground state absorption spectra and a low quantum efficiency 0.04 relative to the efficiency observed in organic solvents. A face-to-face conformation of the monomeric subunits of the dimer is suggested due to the red-shifted absorbance band. The acid-base properties of the dye were studied and were indicative of a multi-step process. In acidic conditions (pH 1), protonation of the bridging nitrogen atoms was identified by a broad band appearing red-shifted to those obtained at higher pH values. Under slightly acidic conditions a pKa value of 6.7 was determined for one of the meso-nitrogen. In alkaline conditions a pKa of 11.5 was determined for another meso-nitrogen and a second fluorescence band emerged at 804 nm, red-shifted to the emission maxima.     

Structure and medium effects on the photochemical behavior of nonfluorinated quinolone antibiotics

The photophysical behavior of the quinolone antibiotics, oxolinic (OX), cinoxacin (CNX) and pipemidic (PM) acids was studied as a function of pH and solvent properties. The ground state of these compounds exhibits different protonated forms, which also exist in the first excited states. Theoretical calculations of the Fukui indexes allowed to assigning the different protonation equilibria. The pK values indicate that the acidity of the 3-carboxylic and 4-carbonyl groups increases with the N-atom at position 2 in CNX. It has been found that fluorescence properties are strongly affected by pH, the more fluorescent species is that with protonated carboxylic acid, protonated species at the carbonyl group and the totally deprotonated form present very low fluorescence. The fluorescence behavior also depends on the chemical structure of the quinolone and on the solvent properties. The analysis of the solvent effect on the maximum and the width of the fluorescence band of OX, using the linear solvent-energy relation solvatochromic equation, indicates that the polarizability and hydrogen bond donor ability are the parameters that condition the spectral changes. The hydrogen bond acceptor ability of the solvents also contributes to the spectral shifts of CNX. The compound bearing the piperazinyl group at the position 7, PM only is fluorescent in high protic solvents. These results are discussed in terms of the competition between the intra- and intermolecular hydrogen bonds. The irradiation of OX, CNX and PM using 300 nm UV light led to a very low photodecomposition rate. Under the same conditions the nalidixic acid (NA), a structurally related quinolone, photodecomposes two orders of magnitude faster.     

Solvatochromic behavior of donor-acceptor-polyenes: dimethylamino-cyano-diphenylbutadiene

4-(Dimethylamino)-4'-cyano-1,4-diphenylbutadiene (DCB) and 4-(dimethylamino)-2,6-dimethyl-4'-cyano-1,4-diphenylbutadiene (DMDCB) have been characterized spectroscopically. Quantum chemical calculations were performed for comparison. Solvatochromic shifts of the fluorescence were strong and showed a linear dependence on the solvent polarity parameters, whereas shifts in the absorption spectra are very weak only correlate better with the polarizability of the solvents. Excited state dipole moments derived from fluorescence using the Onsager model are very large and similar for both compounds. It is concluded that a strongly allowed and highly dipolar pi, pi* state is the lowest excited state in polar solvents. The strong difference in absorption and fluorescence solvatochromic slopes suggests that the simple Onsager model with a point dipole approximation is not sufficient here.     

TIME-RESOLVED FLUORESCENCE OF NITROBENZOXADIAZOLE-AMINOHEXANOIC ACID: EFFECT OF INTERMOLECULAR HYDROGEN-BONDING ON NON-RADIATIVE DECAY

The fluorescence kinetics of the nitrobenzoxadiazole (NBD) chromophore were studied at low concentrations in solvents with varying polarity and hydrogen-bonding donor strength. The emission decay was essentially single exponential in all solvents studied. While the absorption and fluorescence solvatochromism is determined largely by the solvent polarity, the S1 state decay kinetics are strongly modulated by the solvent H-bonding capacity. The NBD emission lifetime, generally approximately 7-10 ns in the aprotic solvents, is reduced to 0.933 ns in water. The solvent deuterium isotope effect on the fluorescence decay is substantial in D2O and in methanol-d4, but is insignificant in DMSO-d6. These results are consistent with acceleration of S1----S0 internal conversion through an accepting vibrational mode created by intermolecular hydrogen-bonding of the NBD chromophore to an H atom-donating solvent. This work bears on the practically of using NBD as a fluorophore in assays for estrogen and progesterone receptors.     

Solvent effects on the electronic absorption spectra and acid strength of some substituted pyridinols.

The electronic absorption spectra of some substituted pyridinols in organic solvents of different polarities are studied. Also, the solvent effects on the intramolecular charge transfer bands are discussed using various solvent parameters. The acid-base equilibria of the compounds used are studied spectrophotometrically in various mixed aqueous solvents at 25 degrees C and 0.1 M ionic strength (NaClO4). Furthermore, the influence of the solvents on the dissociation constants and tautomeric equilibria of a pyridinol derivatives are discussed. The effect of molecular structure of the pyridinols on the pK's is also examined.     

Photophysical Properties of the Cationic Form of Neutral Red

Abstract— Photophysical properties of the cationic form of neutral red (NRH⁺), a phenazine-based dye of biological importance, have been investigated in several protic and aprotic solvents using optical absorption, steady-state and time-resolved fluorescence and picosecond laser flash photolysis techniques. Absorption and fluorescence characteristics of the dye in protic solvents indicate the existence of intermolecular hydrogen bonding between the NRH⁺ and solvent molecules in the ground state as well as in the excited state. Measurements of the fluorescence lifetime in normal and heavy water also support the formation of intermolecular hydrogen bonding. Time-resolved transient absorption spectra obtained in the picosecond laser flash photolysis experiments show only the absorption band due to the S_n← S₁ absorption. The picosecond transient absorption results do not indicate any spectral shifts attributable to the hydrogen bond formation dynamics between the excited NRH⁺ and the protic solvent molecules. It is inferred that the hydrogen bonding dynamics are much faster than the time resolution of our picosecond setup (∼35 ps).