THE PHOTOPHYSICS OF p-AMINOBENZOIC ACID

Abstract— The lowest-lying allowed UV transition in p -aminobenzoic acid (PABA) is assigned Γ→¹L_a based on quantitative absorption and fluorescence studies, as well as semiempirical PM3 multielec-tron configuration interaction calculations. The oscillator strengths, fluorescence quantum efficiencies and lifetimes are reported for PABA in several polar, nonpolar, protic and aprotic solvents (aerated) at 296 K. Reasonable agreement is found between the observed radiative rate constant and that calculated from the absorption and fluorescence spectra. Shifts in the absorption and fluorescence spectra in aprotic solvents are analyzed in terms of the Onsager reaction field model; results are consistent with an increase in dipole moment of ca 4 D between the relaxed S₀ and S₁, states. No evidence is found for the emission from the amino-twisted form of PABA in all solvents studied although calculations show that the amino-twisted S, state is highly polar, but higher in energy by ca 35 kJ/mol ( in vacuo ). The fluorescence efficiency is excitation wavelength independent in both methylcyclohexane and water. The temperature dependence of the nonradiative rate constant (from S₁) was studied in several solvents. Nonradiative decay may be due to intersystem crossing, which would be fast enough to compete with thermally activated intramolecular NH₂ twisting. The phosphorescence spectrum and lifetime obtained in an EPA glass at 77 K are reported, and the triplet energy of PABA is estimated.     

STUDIES ON THE CHEMILUMINESCENCE OF LUMINOL IN DIMETHYLSULFOXIDE AND DIMETHYLSULFOXIDE-WATER MIXTURES*

Abstract— The absorption, fluorescence, and chemiluminescence (CL) characteristics of luminol have been studied in basic dimethylsulfoxide (DMSO) and various basic DMSO — water mixed solvents. It has been shown that the luminol dianion can be produced quantitatively in carefully deoxygenated ‘dry’ DMSO using potassium t-butyl alcoholate (BTO) as the base. A direct correlation has been found between the intensity of CL and the concentration of luminol dianion, Indicating that the dianion is the reactive species in the chemiluminescent reaction in DMSO. Increasing concentrations of water in the mixed solvents greatly reduced the CL intensity because of the decrease in luminol dianion concentration. Solvent effects on the fluorescence of 3-aminophthalic acid samples was noted and identified. Chemiluminescent spectra of luminol were run as a function of solvent composition, and compared with fluorescence spectra in the same media. The need for correcting these spectra for comparison was noted. Stopped flow kinetic studies were run in DMSO and DMSO-water mixtures. These data can be interpreted on the basis of two second-order steps and one first-order step in the reaction. The first-order step is probably last and probably arises from decomposition of a ‘peroxy’ intermediate. The rate constant for this step is k= 1·2±0·3×10^-1 sec^-1.     

FLUORESCENCE AND THE STRUCTURE OF PROTEIN—XX. FLUORESCENCE OF 3-AMINOTYROSINE AND OTHER AMINOPHENOLS

Abstract— –The fluorescence of 3-NH₂ tyrosine and of simpler isomeric aminophenols was observed in aqueous solution at room temperature. Marked changes in both emission spectrum and fluorescence efficiency were observed upon change of pH and solvent composition. An explanation for the observed behavior is offered on the basis of two effects. First, peculiarities in the emission spectra were accountable in terms of proton dissociation from the cationic amino group in the excited singlet state so that emission characteristic of the free base was observed even in acidic solutions. Second, the unexpectedly low fluorescence efficiency from the free base was shown to result from quenching by the aqueous solvent. The relevance of these studies for the fluorescence of 3-NH₂ tyrosyl groups in proteins is discussed briefly.     

Investigation of solvatochromism in the low-lying singlet states of dithienyl polyenes

To understand the low-lying singlet states of dithienyl polyenes, we investigated the solvatochromism of a series of α,ω-di(2-dithienyl 3,4-butyl) polyenes having n=1–5 double bonds. Absorption and emission spectra were collected in a series of aprotic solvents. The absorption energy dispersion effect sensitivity increased smoothly with n, reaching asymptotic behavior as n approached 5. The emission energy had less solvent sensitivity. The trends gave evidence for the existence of a ¹B^*_u absorbing state and a ¹A^*_g emitting state. We observed sensitivity of the absorbing and emitting states to solute–solvent electrostatic interactions, suggesting the dithienyl polyenes had a polar ground state conformation.     

Ab initio study of the solvent H-bonding effect on ESIPT reaction and electronic transitions of 3-hydroxychromone derivatives

The electronic transitions occurring in 4-(N,N-dimethylamino)-3-hydroxyflavone (DMAF) and 2-furanyl-3-hydroxychromone (FHC) were investigated using the TDDFT method in aprotic and protic solvents. The solvent effect was incorporated into the calculations via the PCM formalism. The H-bonding between solute and protic solvent was taken into account by considering a molecular complex between these molecules. To examine the effect of the H-bond on the ESIPT reaction, the absorption and emission wavelengths as well as the energies of the different states that intervene during these electronic transitions were calculated in acetonitrile, ethanol and methanol. The calculated positions of the absorption and emission wavelengths in various solvents were in excellent agreement with the experimental spectra, validating our approach. We found that in DMAF, the hydrogen bonding with protic solvents makes the ESIPT reaction energetically unfavourable, which explains the absence of the ESIPT tautomer emission in protic solvents. In contrast, the excited tautomer state of FHC remains energetically favourable in both aprotic and protic solvents. Comparing our calculations with the previously reported time-resolved fluorescence data, the ESIPT reaction of DMAF in aprotic solvents is reversible because the emitting states are energetically close, whereas in FHC, ESIPT is irreversible because the tautomer state is below the corresponding normal state. Therefore, the ESIPT reaction in DMAF is controlled by the relative energies of the excited states (thermodynamic control), while in FHC the ESIPT is controlled probably by the energetic barrier (kinetic control).     

Solvent-derived metal oxides in electrospray mass spectrometry of metal salt solutions

The electrospray mass spectra of MX₂and MX₃salts (where X is typically halide or nitrate) in protic and aprotic solvents, and solvent mixtures were examined. Comparisons of species responses with equilibrium aqueous solution concentrations were made. For MX₂salts, a good correlation between MOH⁺ response and MOH⁺ solution concentration was observed under conditions where the collision energy was nominally zero. Cu(II) is easily reduced in acetonitrile to Cu(I); Cu(I) was the principal species observed in the electrospray mass spectrum of Cu(II) in acetonitrile whereas Cu(II) was the principal species observed in that of Cu(II) in dimethyl sulphoxide. Gas-phase reactions between solvated clusters produced by electrospray and a second solvent vapour were examined. M³⁺clusters were the principal ions observed when an aqueous solution of MX₃was sprayed in the presence of aprotic solvent vapours.     

Ground state isomerism and dual emission of the β-carboline anhydrobase (N2-methyl-9H-pyrido[3,4-b]indole) in aprotic solvents

The spectral and photophysical properties of the betacarboline anhydrobase, N₂-methyl-9H-pyrido[3,4-b]indole, BCA, have been studied in aprotic solvents. The influence of solvent polarity and hydrogen-bonding interactions on the absorption, steady-state and time-resolved fluorescence spectra provides founded proofs on the existence of two ground state BCA isomers whose equilibrium concentrations change with the medium polarity. We propose that the isomers possess quinonoid, Q, and dipolar zwitterionic, Z, structures, respectively. Upon excitation, each isomer gives rise to a double emission. To account for this phenomenon, photophysical diagrams based on the simultaneous emission from locally excited states, LE, and charge transfer excited states, CT, are proposed.     

Theoretical study of solvent influence on the electronic absorption and emission spectra of kynurenine

Neutral/zwitterionic form equilibrium, excited state wave functions, absorption and emission spectra of kynurenine (KN) in various solvents (water, methanol, ethanol, and dimethylsulfoxide) have been studied theoretically. The ground electronic state geometries have been optimized by density functional theory methods; the geometries of the first two singlets excited electronic states have been optimized using the CASSCF technique. The influence of the solvent was taken into account by the calculation of the solvation free energies using the Polarizable Continuum Model (PCM). The spectra of electronic absorption and fluorescence emission have been calculated by the CS‐INDO S‐CI and SDT‐CI methods [Momicchioli, Baraldi, and Bruni, Chem Phys, 1983, 82, 229]. The calculated data reproduce the experimental positions of maxima and the solvent‐induced shifts of the absorption and emission bands well. The energy gap between the two lowest excited states of KN increases from aprotic to protic solvents. This fact suggests that the “proximity effect” cannot be responsible for the ultrafast decay of KN fluorescence in protic solvents. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Isotope effects and medium effects on sulfuryl transfer reactions

Kinetic isotope effects and medium effects have been measured for sulfuryl-transfer reactions of the sulfate ester p-nitrophenyl sulfate (pNPS). The results are compared to those from previous studies of phosphoryl transfer, a reaction with mechanistic similarities. The N-15 and the bridge O-18 isotope effects for the reaction of the pNPS anion are very similar to those of the p-nitrophenyl phosphate (pNPP) dianion. This indicates that in the transition states for both reactions the leaving group bears nearly a full negative charge resulting from a large degree of bond cleavage to the leaving group. The nonbridge O-18 isotope effects support the notion that the sulfuryl group resembles SO(3) in the transition state. The reaction of the neutral pNPS species in acid solution is mechanistically similar to the reaction of the pNPP monoanion. In both cases proton transfer from a nonbridge oxygen atom to the leaving group is largely complete in the transition state. Despite their mechanistic similarities, the phosphoryl- and sulfuryl-transfer reactions differ markedly in their response to medium effects. Increasing proportions of the aprotic solvent DMSO to aqueous solutions of pNPP cause dramatic rate accelerations of up to 6 orders of magnitude, but only a 50-fold rate increase is observed for pNPS. Similarly, phosphoryl transfer from the pNPP dianion to tert-amyl alcohol is 9000-fold faster than the aqueous reaction, while the sulfuryl transfer from the pNPS anion is some 40-fold slower. The enthalpic and entropic contributions to these differing medium effects have been measured and compared.     

A novel fluorophore with dual fluorescence: local excited state and photoinduced electron-transfer-promoted charge-transfer state.

Absorption and emission spectra of 9-N,N-dimethylaniline decahydroacridinedione (DMAADD) have been studied in different solvents. The fluorescence spectra of DMAADD are found to exhibit dual emission in aprotic solvents and single emission in protic solvents. The effect of solvent polarity and viscosity on the absorption and emission spectra has also been studied. The fluorescence excitation spectra of DMAADD monitored at both the emission bands are different. The presence of two different conformation of the same molecule in the ground state has lead to two close lying excited states, local excited (LE) and charge transfer (CT), and thereby results in the dual fluorescence of the dye. A CTstate involving the N,N-dimethylaniline group and the decahy droacridinedione chromophore as donor and acceptor, respectively, has been identified as the source of the long wavelength anomalous fluorescence. The experimental studies were supported by ab initio time dependent-density functional theory (TDDFT) calculations performed at the B3LYP/6-31G* level. The molecule possesses photoinduced electron transfer (PET) quenching in the LE state, which is confirmed by the fluorescence lifetime and fluorescent intensity enhancement in the presence of transition metal ions.     

Theoretical Study of Fluorescence Spectra Utilizing the pKa Values of Acids in Their Excited States

Assignment of the fluorescence spectrum of firefly luciferin in aqueous solutions was achieved by utilizing not only emission energies but also theoretical absorption spectra and relative concentrations as estimated by pK_a values. Calculated Gibbs free energies were utilized to estimate pK_a values. These pK_a values were then corrected by employing the experimental results. It was previously thought that the main peak near 550 nm observed in the experimental fluorescence spectra at all pH values corresponds to emission from the first excited state of the luciferin dianion [Ando et al. (2010) Jpn. J. Appl. Phys. 49, 117002–117008]. However, we found that the peak near 550 nm at low pH corresponds to emission from the first excited state of the phenolate monoanion of luciferin. Furthermore, we found that the causes of the red fluorescence at pH 1–2 are not only the emission from phenol monoanion but also the emission from the protonated species at nitrogen atom in the thiazoline ring of dianion.     

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.     

Dipolar aprotic–protic solvent influences in the hydrolysis of diol monoesters

The kinetics of the alkaline hydrolysis of five diol monoesters and related esters in aqueous DMSO, aqueous acetone, and aqueous ethanol are reported. A novel solvent dependency of the reactivity pattern of the various esters is noticed. The results are explained on the basis of an internal stabilization of the transition state and the influences of the various solvent systems on such transition states. It is suggested that such dipolar aprotic–protic solvent influences on reaction rates can be used as a criterion in the assessment of anchimeric assistance in such reactions.     

STUDIES ON TETRAPYRROLS PIGMENTS—I. ABSORPTION AND FLUORESCENCE OF BILIVERDIN DIMETHYL ESTERS OF THE IX SERIES

Abstract— Each of the four isomeric biliverdin dimethyl esters of the IX series give a fluorescent spectrum at ambient temperature, the intensity of which increases considerably when these biliverdins are complexed with zinc. A systematic study of the influence of protic and aprotic solvents failed to yield significant results. In methanol. variations in the pH greatly affect the emission absorption spectra of zinc complexes of biliverdins.     

Photoluminescence of Acenaphthenone in Organic Solvents and Aqueoug Media

The configuration of the lowest excited state of acenaphthenone, S₁(π, π^*) or T₁(π, π^*), depending on the solvent, dominates photoluminescence. The T₁(n, π^*) state in aprotic organic solvents is responsible for the phosphorescence of acenaphthenone. The wavelengths of the phosphorescence measured in benzene are 576 nm and 635 nm (vibronic) with 3.3 × 10^?4 quantum efficiency. However, the S₁(π, π^*) state in protic solution which dominates the fluorescence emission depending upon acidity is the most distinctive feature of acenaphthenone. The wavelengths of the emissions are 446 nm under water solvation with 0.185 quantum efficiency and 538 nm with 0.097 quantum efficiency under high acidity. The emission at 446 nm is assigned from a H-bonded keto-form excited state, whereas the emission at 538 nm is probably due to the excited state of protonated keto-form. The pK_a value in aqueous solution measured by diminution of fluorescence in basic solutions is 12.5 ± 0.4.     

Solvatochromic effects in the electronic absorption and nuclear magnetic resonance spectra of hypericin in organic solvents and in lipid bilayers

The natural product hypericin was tested in recent years as a biological photosensitizer with a potential for viral and cellular photodamage. We thus studied extensively its spectroscopy and membrane partitioning. Absorption, fluorescence excitation and emission spectra of the sodium salt (HyNa) were measured in 36 protic and aprotic, polar and apolar, solvents. Electronic transition bands as well as vibrational progressions were identified. Aggregation in some nonpolar solvents and protonation in organic acids were demonstrated. Modeling solvatochromism was done by Lippert equation, by the ET(30) parameter and by the Taft multiparameter approach. In all cases, separation into protic and aprotic solvents gave much better fits to the models. 13C chemical shift data could also be correlated with solvent polarity. They correlated best with Lippert's delta f polarity measure, but tended to fall into two distinct solvent groups--each along different lines--corresponding to protic and aprotic media, respectively. This interesting phenomenon suggests that in the case of the charged and slightly water soluble HyNa, two mechanisms of solvation are involved, each resulting in its own line equation. In aprotic media, dipole-dipole interaction is the predominant solvation mechanism. In protic solvents, the most effective means of solvation is likely to be hydrogen bonding. When intercalated into the liposomal phospholipid bilayer, HyNa is oriented at an angle to the interface, thus experiencing a gradient of solvent polarities: a highly polar environment (similar to methanol) for C-2/5, suggesting that they lie not far from the interface; a moderately polar environment (similar to that of n-propanol) for C-6a/14a, which are somewhat deeper within the bilayer; and a more lipophilic environment (akin to n-hexanol) for C-10/11. The fluorescence excitation peak in liposomes also correlates with an aprotic medium of relatively high polarity, as might be excepted from a molecule in a shallow position in the bilayer.     

Displacement polymerization. III. Synthesis and characterization of polyazoxyarylethers

A new class of polyazoxyarylethers was prepared by nucleophilic displacement polymerization using an activated dichlorocompound and a bisphenol dianion in anhydrous aprotic solvent. Model reactions were studied with 3,3′- and 4,4′-dichloroazoxybenzene and various phenol and thiophenol salts to find out the reaction conditions for polycondensation. IR, ¹H NMR, and elemental analyses were used to establish the structure of the model compounds and the polymers. Thermal analysis indicated that the oxy derivative is less prone to thermo-oxidative degradation than the corresponding thio derivative of polyazoxyarylether. The polymers are crystalline and soluble in halogenated hydrocarbons and polar solvents.     

A 1,3‐Indandione‐Functionalized Tetraphenylethene: Aggregation‐Induced Emission,Solvatochromism, Mechanochromism,and Potential Application as a Multiresponsive Fluorescent Probe

A tetraphenylethene (TPE) derivative substituted with the electron‐acceptor 1,3‐indandione (IND) group was designed and prepared. The targeted IND‐TPE reserves the intrinsic aggregation‐induced emission (AIE) property of the TPE moiety. Meanwhile, owing to the decorated IND moiety, IND‐TPE demonstrates intramolecular charge‐transfer process and pronounced solvatochromic behavior. When the solvent is changed from apolar toluene to highly polar acetonitrile, the emission peak redshifts from 543 to 597 nm. IND‐TPE solid samples show an evident mechanochromic process. Grinding of the as‐prepared powder sample induces a redshift of emission from green (peak at 515 nm) to orange (peak at 570 nm). The mechanochromic process is reversible in multiple grinding–thermal annealing and grinding–solvent‐fuming cycles, and the emission of the solid sample switches between orange (ground) and yellow (thermal/solvent‐fuming‐treated) colors. The mechanochromism is ascribed to the phase transition between amorphous and crystalline states. IND‐TPE undergoes a hydrolysis reaction in basic aqueous solution, thus the red‐orange emission can be quenched by OH^? or other species that can induce the generation of sufficient OH^?. Accordingly, IND‐TPE has been used to discriminatively detect arginine and lysine from other amino acids, due to their basic nature. The experimental data are satisfactory. Moreover, the hydrolyzation product of IND‐TPE is weakly emissive in the resultant mixture but becomes highly blue‐emissive after the illumination for a period by UV light. Thus IND‐TPE can be used as a dual‐responsive fluorescent probe, which may extend the application of TPE‐based molecular probes in chemical and biological categories.     

FLUORESCENCE OF THE COENZYME ANALOG NICOTINAMIDE FORMYCIN DINUCLEOTIDE

Abstract— Emission spectra of unbound reduced nicotinamide formycin dinucleotide (NFDH) revealed the presence of two major conformations of the coenzyme in solution: when examined at excitation wavelengths at or below 307 nm, emission spectra contained peaks at 343 and 447 nm; when excited above 307 nm, an additional maximum appeared at 355 nm and the peak of the dihydronicotinamide emission band shifted from 447 to 440 nm. Both conformers are probably detected at the longer wavelengths since the emission peak at 343 nm was retained. Identical changes occurred in the emission spectra of NFD⁺, however, the dihydronicotinamide emission between 440 and 447 nm was absent. Several mechanisms which may account for the presence of these conformers have been considered. The choice has been narrowed to conformations with ring-ring interactions of the formycin and nicotinamide moieties resulting from (a) formycin tautomerization or (b) heterogeneity of glycosidic bond angles in the structures. The efficiency of intramolecular energy transfer from the formycin to the dihydronicotinamide moiety for free NFDH in aqueous solution was 84% and declined slightly (to 77%) when measured in 1,2-propanediol. NFD⁺ has coenzyme activity for NAD-specific isocitrate dehydrogenase (Plaut et al. , 1979). The emission spectrum of enzyme bound NFDH was altered markedly in the presence of manganese isocitrate; emission intensity at 343 and 355 nm decreased while the emission from the dihydronicotinamide ring at 433 nm increased, when NFDH was excited at 310 nm. This shift in emission intensity was indicative of an increase in energy transfer within the NFDH molecule, caused by a change in coenzyme conformation upon binding to the enzyme-substrate complex.