Dual emission of temperature-induced betacarboline self-associated hydrogen bond aggregates

Abstract

A systematic study of the influence of the gradual temperature decrease on the UV-vis absorption and fluorescence emission spectra of betacarboline, 9H-pyrido[3,4-b]indole, BC, and other model systems, such as BC plus N(9)-methyl-9H-pyrido[3,4-b]indole, MBC, and BC plus pyridine, PY, has been carried out in 2-methylbutane, 2MB. These studies have allowed the conclusion that the temperature decrease favours the formation of hydrogen-bonded self-associated BC aggregates. The initial red shifts of the absorption and emission bands and the fluorescence quenching have been ascribed to the formation of hydrogen bond BC dimers with a proton transfer structure, PTC. In these adducts, the fluorescence is quenched by an electron-driven proton transfer process. However, because the quenching rate constant decreases upon decreasing the temperature, the emission intensity later increases without modification of the wavelength maxima. At the lowest temperatures, these dimeric PTC complexes further aggregate. We propose that they form ground state cyclic tetrameric adducts in which both nitrogen atoms of each BC unit are hydrogen bonded. The tautomeric forms of these tetrameric complexes, generated by a quadruple proton transfer, emit dual fluorescence, from its locally excited state, LE, and its intramolecular charge transfer state, ICT.

Ground‐ and Excited‐State Hydrogen Bonding in the Diazaromatic Betacarboline Derivatives

“…The initial temperature decrease, from 258 to $208 K, produces a slight hyperchromic effect Figure 19.8 UV-vis absorption spectra of BC (2 Â 10 À4 M) in 2MB from 298 to 208 K. The inset shows the UV-vis absorption spectra from 258 to 208 K [61]. Reproduced by permission of The Royal Society of Chemistry (RSC) for the European Society for Photobiology, the European Photochemistry Association, and the RSC without significant modifications of the wavelength maxima.…”

“…Finally, at the lowest temperatures, 200 K, only minor modifications of the absorption spectra are observed with the temperature decrease. All these spectral changes are reversible [61].…”

“…However, the fact that the influence of temperature on the absorption spectra of BC is the same as that produced by the addition of pyridine derivatives indicates that dimeric species should be responsible for the observed changes. Moreover, we have also analysed the temperature influence on BC plus MBC and BC plus PY systems in 2MB [61]. For the first system, the MBC concentration was 4 times greater than that of BC to minimize BC self-association.…”

“…Because the last substrates have blocked the pyridinic or the pyrrolic nitrogen atoms by methylation, they make it possible selectively to analyse the hydrogen bond interactions of each one of the centres. On the basis of these results, the photophysics of the hydrogen bond complexes of the parent BC, in which both nitrogen atoms are free for hydrogen bonding, can be safely interpreted [59][60][61].…”

“…Finally, the study in Section 19.5, on the hydrogen bond interactions between BC and pyridines [60] and proper BC self-aggregation [61], provides examples of the PET processes suffered by the excited PTC complexes of these systems.…”