A novel blue-emitting polycyclic aromatic system was synthesized via anion-radical coupling. Its efficient direct hydroxylation led to a phenol possessing an intramolecular hydrogen-bond system. Since the energy gap difference between the enol and keto forms of this molecule is very small, characteristic of ESIPT chromophores, bathochromically shifted fluorescence was not observed.
We have developed a highly optimized methodology that allows for the oxidative acetoxylation of a sterically and electronically demanding library of analogues of benzo[h]quinoline. The optimal conditions for the insertion of an OAc group were identified after examining various reaction parameters (solvent, oxidant, catalyst, temperature, time). The conditions identified (Pd(OAc)(2), PhI(OAc)(2), MeCN, 150 °C, 16 h), combined with the hydrolysis of acetates, resulted in the formation of hydroxybenzoquinolines in 27-59% yield, whereas all previously published procedures were ineffective. This synthesis was compatible with diverse functionalities (ester, aldehyde, carbon-carbon triple bond) and, most importantly, worked for sterically hindered analogues as well as for compounds possessing electron-donating and electron-withdrawing substituents at various positions. All the obtained compounds demonstrated excited-state intramolecular proton transfer (ESIPT) manifesting as small fluorescence quantum yields and large Stokes shifts (8300-9660 cm(-1)). The effect of structural variations in eight 10-hydroxybenzo[h]quinoline analogues on absorption and emission properties was studied in detail.
Eight previously inaccessible derivatives of 10-hydroxybenzo[h]quinoline were prepared via a straightforward strategy comprising formation of the benzo[h]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[c]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).
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