Anthony C. Capomacchia

Excited state prototropism and solvent dependence of the fluorescence of 3-aminoquinoline

Abstract The effects of solvent polarity and hydrogen-bonding capability on the fluorescence of 3-aminoquinoline have been studied. The failure of the solvent dependence of the absorption spectrum to parallel that of the fluorescence spectrum is attributed to differences in hydrogen bonding in ground and lowest excited singlet states. The large Stokes shifts produced by addition of small amounts of hydrogen-bonding solvents to solutions of 3-aminoquinoline in non-activating solvents suggest the formation of a well defined solute-solvent complex in the excited state. 3-Amino-quinoline is a stronger acid in the excited state than in the ground state with respect to dissociation from the amino group. The same is true for the dication. Fo¨rster cycle calculations, however, indicate that dissociation from the heterocyclic nitrogen is more difficult in the excited state. It is shown that when hydrogen-bonding interactions are present, solvent relaxation effects after excitation are best studied in non-activating solvents rather than in rigid hydrogen-bonding solvents.

Electronic absorption and fluorescence spectrophotometry of quinacrine

Abstract The absorption and fluorescence spectra of quinacrine and its dealkylated derivative, 2-methoxy-6-chloro-9-aminoacridine, were studied as a function of pH and Hammett acidity. The relative stability and high quantum yield of the dication derived from quinacrine make it the most desirable species for fluorimetric determination. This species predominates in the region pH 8-H o -6. The alkylamino side-chain of quinacrine appears to interfere with excited state protonation and dissociation of the dication. Moreover, in the dication, the steric interaction between the side-chain and the 9-amino group may prevent coplanarity between the 9-amino group and the acridine ring, thereby favoring the aminoacridine over the iminoacridan structure.

Lowest excited singlet state pka* values of the isomeric aminopyridines

Abstract Dissociation constants for the various prototropic forms of the three isomeric aminopyridines were determined from shifts in absorption spectra, shifts in fluorescence spectra and variations of fluorescence intensity with pH and Hammett acidity. The pKa* values obtained indicate that charge transfer from the amino nitrogen to the ring nitrogen occurs upon excitation. The excited doubly charged cations of the 2-and 3-isomers are too strongly acidic to equilibrate with their conjugate bases within the lifetime of the excited state. Quenching of aminopyridine fluorescences in basic solutions is attributed to formation of non-fluorescent anions in the excited state. The weak fluorescence of 4-aminopyridine makes fluorimetry unsuitable as a method of analysis for this compound.

Photoluminescence of 6- and 7-aminoquinolines

The fluorescences of 6- and 7-aminoquinoline and 6-dimethylaminoquinaldine were studied in media of different acidities, polarities, hydrogen-bonding capabilities and temperatures. The excited state acidities and solvent dependences of the fluorescences of these compounds are typical of aminoquinolines substituted in the homocyclic ring. The acidities of the doubly protonated cations are so great in the lowest excited singlet state, however, that prototropic dissociation in the excited state occurs even in rigid media at 195°K. The lowest triplet state of the singly charged cation of 6-aminoquinoline is found to be nearly degenerate with the lowest excited singlet state of the same molecule. This is attributed to the great charge-transfer component of the S1←S0 transition which is not present in the T1←S0 transition, and the interaction of the S1 state with the polar solvent.

Electronic absorption and fluorescence of cinchophen, cinchoninic acid and their methyl esters; biprotonic phototautomerism of the singly-protonated species

The pH and Hammett acidity dependences of the absorption and fluorescence spectra of cinchoninic acid (quinoline-4-carboxylic acid), cinchophen (2-phenylquinoline-4-carboxylic acid) and their methyl esters, were studied. The predominant uncharged ground-state species derived from the free acids are zwitterions. Prototropic equilibria are too slow to compete with fluorescence for deactivation of the excited state at hydrogen ion concentrations represented by the pH scale. However, fluorescence shifts accompanying protonation indicate that the carboxyl group is more basic than the ring nitrogen atom in the excited state. In the Hammett acidity range the singly-charged cations of all the compounds studied undergo phototautomerism in the lowest excited singlet state. The rate of this process is acidity dependent. In very concentrated sulphuric acid solutions doubly-charged cations are formed in the excited state but not in the ground state. The intense emissions of these compounds in moderately concentrated sulphuric acid may be suitable for quantitative analysis if great care is taken to control solution acidity.

The acidity dependence of the fluorescence of β-naphthylamin

Abstract The change of hybridization of β-naphthylamine from sp3 to sp2, subsequent to excitation is found to have little effect upon the entropy of protonation of β-naphthylamine in the lowest excited singlet state. Solvent relaxation of the neutral amine, subsequent to excitation and as a result of dipole-dipole interactions, is shown to play an important role in the energy of the lowest excited singlet state and the pKa∗ value. The rates of protonation and dissociation of excited β-naphthylamine appear to be slightly slower than the rate of fluorescence of the β-naphthylammonium ion.

Acidity Dependences of the Fluorescence Spectra of 1- and 2-Anthroic Acid

The electronic absorption and fluorescence spectra of 1-anthroic acid and 2-anthroic acid were studied throughout the pH range in aqueous buffers and the Hammett acidity range in sulfuric acid. Prototropic equilibrium is established in the lowest excited singlet state, between 1-anthroic acid and the 1-anthroic acidium cation. 1-Anthroic acid is a stronger base in the lowest excited singlet state than in the ground electronic state. Equilibration in the excited state, between the anthroate anions and anthroic acids and between the 2-anthroic acidium cation and 2-anthroic acid fails to occur, the rates of proton exchange in the excited state being too slow to compete with radiative deactivation of the excited state. Measurements of the lifetimes of the excited states of all prototropic species derived from 1-and 2-anthroic acid indicate that the rate constants rather than the concentrations of protonating and deprotonating species in the solvent are responsible for the failure of excited state equilibrium to occur in the latter prototropic reaction. The fluorescences of the three isomerie anthroic acids and their conjugate anions, have been studied in organic solvents and the states from which the luminescences originate identified12. The dissociation constants of the anthroic acids, in their lowest excited singlet state (pKa*) have been calculated3 for the equilibrium between the neutral molecules and their conjugate anions, by means of the Förster cycle4. These pKa* values indicate that the anthroic acids are weaker acids 1 T. C. Werner and D. M. Hercules, J. physic. Chem. 73 (1969) 2005. 2 T. C. Werner and D. M. Hercules, J. physic. Chem. 74 (1970) 1030. 3 E. Vander Donckt and G. Porter, Trans. Faraday Soc. 64 (1968) 3218. 4 T. Förster, . Elektroehem. angew. physik. Chem. 54 (1950) 42. Acidity Dependences of the Fluorescence Spectra of Acids 309 by 3—4 orders of magnitude in the lowest excited singlet state than in the ground electronic state. However, a titrimetric study of the fluorescences of the protonated, neutral and anionic forms of 9-anthroic acid5 has shown that proton exchange in the excited state, between the neutral molecule and anion, takes place too slowly for an equilibrium constant to be measured directly. Thus, the calculation of pKa* for this equilibrium, while yielding a value that is probably a reasonable estimate of the free energy difference between the excited neutral molecule and excited anion, corresponds to a reaction which never actually occurs. The equilibrium between the 9-anthroic acidium cation and the neutral molecule is, however, established in the excited state in solutions some 106 fold less acidic than in the ground state4. Our interest in the anthroic acids stemmed from a desire to use these highly fluorescent materials and derivatives thereof, as probes of the topography of nucleic acid helices in the hope that changes in fluorescence accompanying binding might be qualitatively similar to changes produced by the positive and negative polarizing influences of protonation and dissociation. As a first step it was deemed desirable to study the acidity dependent changes in the fluorescence spectra accompanying the conversions between cations, neutral molecules and anions derived from 1and 2-anthroic acid.