3 research outputs found
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-azaperyleneLimiting Case of the ESIPT System: Enol–Keto Tautomerization in S<sub>0</sub> and S<sub>1</sub> 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<sup>–1</sup> (and 19 060 ± 10 cm<sup>–1</sup> 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<sup>–1</sup> (19 290 ± 20 cm<sup>–1</sup>),
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)