Fluorescence Quenching by Pyridine and Derivatives Induced by Intermolecular Hydrogen Bonding to Pyrrole-Containing Heteroaromatics

Abstract

Photophysical studies of a series of over 20 compounds based on pyrrole, indole, and carbazole chromophores were carried out in protic, nonpolar, and polar solvents. Absorption spectra revealed the formation of groundstate hydrogen bonding with protic solvent partners. The equilibrium constants were determined by spectrophotometric titration. Strong fluorescence quenching was observed when azaaromatic proton acceptors, pyridine and quinoline, were used as protic solvents. No quenching occurs for nonaromatic protic partners such as dimethyl sulfoxide, morpholine, and piperidine, even though the ground-state equilibrium constants are not smaller. The rates of quenching in pyridine solutions at 293 K span a range from 1.2 × 10 9 to 5.9 × 10 10 s -1 and are sensitive to minor structural variations. The mechanism of quenching involves an electron transfer from a photoexcited chromophore to a hydrogen-bonded partner, followed by a rapid internal conversion (back electron transfer) to the ground state. The quenching rates are larger for systems with stronger hydrogen bonds. This model was confirmed by theoretical time-dependent DFT and semiempirical studies, in which the pattern of excited states of an isolated chromophore was compared with that of a hydrogen-bonded pyridine complex. For the latter, low-lying charge transfer (CT) states were predicted, with an electron transferred to pyridine.

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

“…As a consequence, no phototautomerization was observed for our chromophores in their dimeric forms. 25 In addition to ESPT, our compounds reveal efficient fluorescence quenching by alcohols and by azaaromatic proton acceptors, such as pyridine or quinoline. 5,26 Finally, for molecules with separate moieties containing proton donor and acceptor groups linked by a single bond, such as 2-(2′-pyridyl)-indole and 2-(2′-pyridyl)pyrrole, we observed alcohol-induced syn-anti rotamerization in the ground state.…”

“…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...…”

Ground‐ and Excited‐State Hydrogen Bonding in the Diazaromatic Betacarboline Derivatives

“…In fact, in structurally related systems such as pyrrole or carbazole chromophores, a dramatic quantum yield decrease has been observed in the presence of pyridine [74][75][76][77]. This highly effective deactivation of fluorescent states has been explained by Mataga in terms of curve-crossing dynamics involving a dark charge-transfer state [78].…”

Excited‐State Proton Transfer via Hydrogen‐Bonded Dimers and Complexes in Condensed Phase

“…Besides 7AI , 7-hydroxyquinolines [68][69][70][71][72][73][74][75][76], b-carbolines [77][78][79][80][81] and pyrrole-containing heteroaromatics [82][83][84][85][86] are other prototypes where the ESPT tautomerism is mediated either by self-association or by adding guest molecules (including solvents) forming host/guest types of HB complex. From the structural viewpoint, the proton-donating and proton-accepting sites can be adjacent to each other, such that a complex possessing intact dual hydrogen bonds is formed through a perfectly fitted 1:1 (host:guest) stoichiometric ratio in the ground state.…”

“…From the molecular structure viewpoint, the reaction centre of the ESDPT process in 7AI and its corresponding analogues, such as 6HIQ [54] and pyrrole-containing heteroaromatics [82][83][84][85][86], are essentially the amine/imine proton transfer tautomerism, in which a proton (or hydrogen atom) in the pyrrolic nitrogen transfers to the pyridinic nitrogen, forming an imine-like isomer. Such a process, in general, is highly endergonic in the ground state.…”