Low-Temperature Spectra of the Analogues of 10-Hydroxybenzo[<i>h</i>]quinoline as an Indication of Barrierless ESIPT

The absorption and fluorescence spectra of two analogues of 10-hydroxybenzo­[<i>h</i>]­quinoline (10-HBQ), namely, 1-hydroxy-7-methylbenzo­[<i>c</i>]­acridine (HMBA) and 4-hydroxybenzo­[<i>c</i>]­phenanthridine (HBPA), were studied in <i>n</i>-alkane matrices at 5 K. Considerable energy separation between the onsets of the spectra and broadening of the bands was an indication that intramolecular proton transfer (ESIPT) takes place at such a low temperature. DFT and ab initio methods were used to calculate the electronic transition energies and oscillator strengths and the vibronic structure of the electronic spectra. Shortcomings in our knowledge of the shape of the potential energy surface for ESIPT systems are highlighted in the context of the discussion of the shape of the electronic spectra. The π-expansion of the 10-HBQ chromophore achieved by adding a benzene moiety at various positions adjacent to the pyridine ring led to compounds possessing diverse photophysical properties, ranging from the non-ESIPT strongly fluorescent molecule of 10-hydroxy-1-azaperylene to weakly emitting (or nonemitting) molecules, where ESIPT occurs very efficiently

12-Hydroxy-1-azaperyleneLimiting Case of the ESIPT System: Enol–Keto Tautomerization in S₀ and S₁ States

Absorption, fluorescence, and fluorescence excitation spectra of 12-hydroxy-1-azaperylene (<b>HAP</b>) and 1-azaperylene were studied in <i>n</i>-alkane matrices at 5 K. Two stable tautomers of <b>HAP</b>, each of them in <i>n</i>-nonane embedded in two sites, were identified and attributed to the enol and keto forms. Theoretical calculations of the energy and vibrational structure of the spectra suggest that tautomer <b>A</b>, with the (0, 0) transition energy at 18 980 ± 10 cm^–1 (and 19 060 ± 10 cm^–1 in the high energy site), should be identified as the keto form, whereas tautomer <b>B</b>, with the (0, 0) energy at 19 200 ± 20 cm^–1 (19 290 ± 20 cm^–1), as the enol form. Observation of absorption and fluorescence of both tautomeric forms and lack of large Stokes shift of fluorescence of the keto form classify HAP as the limiting case of the excited-state intramolecular proton transfer system

Excited State Intramolecular Proton Transfer in Electron-Rich and Electron-Poor Derivatives of 10-Hydroxybenzo[<i>h</i>]quinoline

Eight previously inaccessible derivatives of 10-hydroxybenzo­[<i>h</i>]­quinoline were prepared via a straightforward strategy comprising formation of the benzo­[<i>h</i>]­quinoline skeleton followed by C–H acetoxylation at position 10. The occurrence of excited state intramolecular proton transfer (ESIPT) was detected in all cases since emission was observed only from the excited keto-tautomer. Studies on derivatives bearing both electron-donating and electron-withdrawing groups adjacent to the pyridine ring allowed us to identify some design patterns giving rise to NIR emission and large Stokes shifts. For a derivative of 10-hydroxybenzo­[<i>c</i>]­acridine, emission at 745 nm was observed, one of the lowest energy fluorescence ever reported for ESIPT system. On the basis of time-resolved measurements, proton transfer was found to be extremely fast with time constants in the range (0.08–0.45 ps)