Quenching Processes in Hydrogen-Bonded Pairs:  Interactions of Excited Fluorenone with Alcohols and Phenols

Abstract

In order to clarify mechanisms of excited state interactions in hydrogen-bonded pairs, we have studied the kinetics of dynamic quenching of singlet and triplet fluorenone by a series of alcohols, phenols, and related compounds, in which hydrogen-bonding power, redox potential, and acidity are systematically varied. In addition, effects of solvent basicity or polarity and deuteration help identify the role of hydrogen-bonding in physical or chemical quenching processes. Alcohols and weak acids, with high oxidation potentials, do not quench the triplet, but quench the singlet at rates which parallel hydrogen-bonding power. This is attributed to a physical mechanism, involving vibronic coupling to the ground state via the hydrogen bond. This is much stronger in the excited state than in the ground state, and provides efficient energy dissipation in the radiationless transition. Phenols, with hydrogen-bonding power comparable to that of the alcohols but with much lower oxidation potentials, quench both singlet and triplet by electron or H-atom transfer, depending on potentials, acidities, and solvent polarity, as shown by formation of anion or neutral fluorenone radicals from the triplet. Rates increase with both decreasing oxidation potential of the phenol and increasing acidity of the incipient cation radical. Quenching proceeds via a hydrogen-bonded complex and is facilitated by proton transfer contributions to the effective excited state redox potential.

Structure and Photophysics of 2-(2‘-Pyridyl)benzindoles: The Role of Intermolecular Hydrogen Bonds

“…58,59,[62][63][64]69 Phototautomerization has also been discovered in complexes of 7AI with alcohols and water. [70][71][72][73][74][75][76][77][78][79][80][81][82][83][84] We have been carrying out detailed studies of bifunctional chromophores based on indole, pyrrole, pyridine, and carbazole units, such as 2-(2′-pyridyl)indoles, 4,5,8,13,18,20,21,31 2-(2′-pyridyl)-pyrrole, 32,85 7-(pyridyl)indoles, 35 dipyrido[2,3-a:3′,2′-i]carbazole, 10,22,25 7,8,9,10-tetrahydro-11H-pyrido [2,3-a]carbazole, 18,22 or 1H-pyrrolo [3,2-h]quinoline. 19,21,22,27,31,86 These molecules, structurally similar to 7AI, differ in the number of bonds between the proton donor (the NH group) and the acceptor (pyridine...…”

Effect of molecular structure and hydrogen bonding on the fluorescence of hydroxy-substituted naphthalimides

“…Most of the studies in this Ðeld have dealt with the e †ect of the hydrogen-bond donors on the Ñuo-rescent properties of aromatic heterocyclic1 and carbonyl compounds.2h6 These molecules form hydrogen-bonded complexes with alcohols and hydroperoxides in the excited state and the hydrogen-bond acts as an efficient vibronic dissipative mode in the nonradiative transition.2h6 On the other hand, coupled electronÈproton movement was found to play a dominant role in the deactivation of excited hydrogen-bonded porphyrins7, 8 and Ru(II)polypyridyl complexes8 as well as in the interaction of excited ketones with phenols. 4,9 Picosecond laser photolysis studies established that excited hydrogen-bond donors, such as aromatic TNÈH or ÈOÈH compounds, are efficiently quenched by pyridine derivatives via non-Ñuorescent hydrogen-bonded complex, in which associated electron and proton displacement facilitates the charge transfer interaction between the two conjugate pelectronic systems.10 However, when phenols form hydrogenbonded complexes with aliphatic amines no charge delocalization is possible along the hydrogen-bond and photoexcitation induces proton transfer. 11 The present paper focuses on the question of how the variation of the molecular structure of the hydrogen-bonding additive inÑuences the Ñuorescent properties and the deactivation mechanism of the excited molecules.…”

A reatividade do estado excitado triplete de 1,3-indanodiona em benzeno

“…A abstração de hidrogênio fenólico por estados excitados triplete de carbonilas normalmente é mais rápida que aquela observada para toluenos, uma vez que fenóis apresentam tanto uma menor energia de dissociação da ligação O-H, comparada à C-H, quanto um menor potencial de oxidação que toluenos. Assim, o mecanismo da reação de abstração de hidrogênio de fenóis deve ocorrer através da formação inicial de um exciplexo triplete, estabilizado por ligação de hidrogênio, seguido por uma transferência seqüencial de elétron e de próton [17][18][19][20][21][22][23][24][25] . 1,4-diazabiciclo[2.2.2]octano (DABCO) suprime de maneira eficiente (k q =5,5x10 9 L mol -1 s -1 , Tabela 1, Figura 4) o estado excitado triplete de 1,3-indanodiona (1) através um processo de transferência de elétron, levando à formação do ânion radical da dicetona, isto é 4, e do cátion radical derivado da amina (Esquema 2).…”