Proton-Transfer Tautomerism in 10-Hydroxybenzo[h]quinolines:  Heavy Atom Effects and Non-Hydrogen-Bonded Photorotamer Formation in 77 K Glassy Matrixes

Abstract

The low-lying triplet states of 10-hydroxybenzo[h]quinoline (HBQ) and its halogenated derivatives 7,9-diiodo-10-hydroxybenzo[h]-quinoline (DIHBQ) in an excited-state intramolecular proton-transfer (ESIPT) process have been investigated. For DIHBQ, which is enhanced by the intramolecular heavy atom effect, the protontransfer tautomer (i.e., the keto form) phosphorescence that is maximized at 735 nm (τ p ) 1.75 µs) was resolved in a 77 K methylcyclohexane (MCH) glass. Further transient absorption and 1 O 2 sensitization experiments allow us to deduce the population yield and radiative decay rate of the keto triplet state to be 0.85 and 8.8 s -1 , respectively. Upon increasing the excitation intensity, photolysis reactions were observed for both HBQ and DIHBQ in the MCH glass. The reversibility of the photolysis reaction throughout a thawing and freezing cycle led us to conclude that the rupture of an intramolecular hydrogen bond through the excessive energy dissipated does occur. The product exhibits a non-hydrogen-bonding type of enol emission that would otherwise be inaccessible in the hydrogen-bonded enol form because of the ultrafast ESIPT. Accordingly, relative energy levels in different spin manifolds are established during a proton-transfer cycle. The ketofenol reverse proton transfer in the lowest triplet manifold was estimated to be endergonic by ∼7.42 kcal/mol.

Synthesis, Electrochemistry, Crystal Structures, and Optical Properties of Quinoline Derivatives with a 2,2′‐Bithiophene Motif

“…These bands are related to π‐π* transitions within the quinoline core; the peak maxima are collected in Table 2. These values are in close agreement with the previous results obtained for 10‐hydroxybenzo[ h ]quinolines, for which a peak maximum is centered at 395 nm due to the S 0 →S 1 (ππ*) absorption 43. The absorption bands are slightly shifted to longer wavelengths (redshifted) in 3a → 3b → 6 direction.…”

“…These values are in close agreement with the previous results obtained for 10-hydroxybenzo[h]quinolines, for which a peak maximum is centered at 395 nm due to the S 0 ǞS 1 (ππ*) absorption. [43] The absorption bands are slightly shifted to longer wavelengths (redshifted) in 3a Ǟ 3b Ǟ 6 direction. However, the most important information is that the band position of 3c is considerably redshifted in comparison to all the other studied systems.…”

Theoretical Design of Near-Infrared Fluorescent Sensor for F Anion Detection Based on 10-Hydroxybenzo[h]quinoline Backbone

“…10−13 It gains particular attention for its potential applications as a probe in biological systems and organic light-emitting devices. 14−16 So far, many efforts have been made on improving the photophysical properties and achieved several results. It has been well established both experimentally and theoretically that, in the ground state, only the normal isomer (Enol) exists, while after photoexcitation, ESPT occurs along the intramolecular hydrogen bond O–H···N, forming Keto, from where orange–red fluorescence is observed.…”

Studies of the triplet state of the proton-transfer tautomer in salicylaldehydes

“…Very recently, we have reported the spectroscopy and dynamics of the proton-transfer tautomer 735nm phosphorescence for 7,9-diiodo-10-hydroxybenzo[h]quinoline. The results, in combination with the triplet-state sensitizing singlet oxygen experiment, led us to deduce relative energetics in different spin manifolds during a proton-transfer reaction [22]. To further extend the approach on the spectroscopy and dynamics of triplet states in other ESIPT systems, we have investigated salicylaldehyde (SA) and its halogenated derivative 3,5-diiodosalicylaldehyde (DISA) in the triplet manifold.…”