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

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 proton-transfer tautomer (i.e., the keto form) phosphorescence that is maximized at 735 nm (τ<INF>p</INF> = 1.75 μs) was resolved in a 77 K methylcyclohexane (MCH) glass. Further transient absorption and ¹O<INF>2</INF> 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 keto→enol reverse proton transfer in the lowest triplet manifold was estimated to be endergonic by ~7.42 kcal/mol.