New insights on the 7-azaindole photophysics: the overlooked role of its non phototautomerizable hydrogen bonded complexes.

In this paper we explore the formation and the photophysical properties of the scarcely studied open hydrogen bonded aggregates of 7-Azaindole, 7AI. Thus, we have analyzed the influence that the increase of the 7AI concentration and the decrease of the temperature have on the 7AI photophysics. To help the interpretation of the results, the 7AI-Pyridine system has been used as the model for the analysis of the photophysical properties attributable to the open N(pyrrolic) - H · · · N(pyridinic) hydrogen bonded aggregates. Also, the hydrogen bond interactions have been studied by means of the atom in molecule approach from the Bader theory. Experimental and theoretical results support that the formation of open hydrogen bonded aggregates, (-7AI-)n with n ≥ 2 can efficiently compete with that of the profusely studied centro-symmetric cyclic dimer (7AI)2. Moreover, these aggregates suffer a proton-driven electron transfer process that strongly quenches their room temperature fluorescence and, therefore, masks their presence in the 7AI solutions. Therefore, because most of the studies on the 7AI photophysics have been interpreted without considering the existence of such aggregates and, more important, ignoring its quenching process, many conclusions obtained from these studies should be carefully revised.

Spectroscopic study of the ground and excited state prototropic equilibria of 4-azaindole.

The ground and singlet excited state prototropism of 4-azaindole, 4AI, in acid and basic aqueous solutions, inside and outside the pH range, has been systematically studied by using absorption and fluorescence spectroscopic techniques. These studies have thrown light on some interesting aspects on the nature and the photophysics of the 4-AI prototropic species. Thus, the changes of the 4AI absorption spectra reveal the existence of four ground state species; the pyridinic protonated cation, C (pK(a)(C)=7.5±0.1), the neutral molecule, N (pK(a)(N)=15.5±0.5), the pyrrolic deprotonated anion, A, and a previously unnoticed dication, DC (pK(a)(DC)=-4.6±0.4). Besides the emissions of these species, a new fluorescence profile appears in alkaline solutions at around 500nm. This extra band has been ascribed to the neutral phototautomer, NT. What is more relevant to this study is the fact that the position and the intensity of the emission band assigned to the monoprotonated cation are very different from those observed for the normal cation of the 7-azaindole, 7-AI. This together with the fact that for the formation of the DC species a cationic precursor with a quinoid structure must be invoked, have prompted us to assign this cationic emission to the isomeric CI cations. Finally, the excited-state pK(a)s of the prototropic species of 4AI have been theoretically estimated by using the Förster-Weller cycle.

A photophysical study of the α-carboline (1-azacarbazole) aggregation process.

This paper reports a comprehensive photophysical study of the aggregation process of 1-azacarbazole, or α-carboline (9H-pyrido[2,3-b]indole), AC, in low polar aprotic solvents by using absorption, steady state and time-resolved fluorescence spectroscopic techniques. To ascertain the characteristics of the aggregation process we have studied the changes produced by the increase of the AC concentration and the decrease of the temperature on the absorption and fluorescence spectra of the AC monomer. Previously, to aid the interpretation of these results, the hydrogen bonding interactions of the AC monomer with pyridine, PY, and indole, IND, have been also analyzed. The results obtained from these studies reveal that, under our experimental conditions, AC does not form doubly hydrogen bonded cyclic dimers, (AC)(2), but singly hydrogen bonded open dimers, AC-AC, and open higher aggregates, (-AC-)(n). The formation of these species shifts to the red the absorption spectrum of the AC monomer and quenches its fluorescence.

Spectral and photophysical properties of α-carboline (1-azacarbazole) in aqueous solutions.

The absorption and fluorescence spectra of α-carboline, 9H-pyrido[2,3-b]indole, AC, in organic aprotic solvents containing different water proportions and in acid/base aqueous solutions inside and outside the pH range have been examined. In the organic aprotic solvents, the addition of increasing concentrations of water sequentially quenches and shifts to the red the fluorescence spectra of AC. These spectral changes have been rationalized assuming the formation, at the lower water concentrations, of a discrete ground state non-cyclic weakly fluorescent AC hydrate emitting at 376 nm that, upon increasing the water concentrations, evolves to a higher order AC poly hydrate emitting at 397 nm. The changes of the AC absorption spectra in aqueous acid/basic solutions indicate the existence of three ground state prototropic species; the pyridinic protonated cation, C (pK(a) = 4.10 ± 0.05), the neutral, N (pK(a) = 14.5 ± 0.2), and the pyrrolic deprotonated anion, A. Conversely, the changes of the AC fluorescence spectra in these media indicate the existence of four excited state species emitting at 376 nm, 397 nm, 460 nm and 465 nm. Since the emissions at 376 nm and 397 nm satisfactorily match those of the hydrates observed in the organic-water mixtures, they were consistently assigned to two differently hydrated ground state N species. The remaining emissions at 460 nm and 465 nm have been assigned without ambiguity, on the basis of their excitation spectra, to the C and A species, respectively. The excited-state pK(a)s of the prototropic species of AC have been estimated by using the Förster-Weller cycle.

Electronic spectra and photophysics of the α-carboline (1-azacarbazole) monomer.

The UV-vis electronic absorption and emission spectra of α-carboline or 1-azacarbazole, 9H-pyrido[2,3-b]indole, AC, have been investigated in aprotic solvents. Radiative, k(r), non-radiative, k(nr), rate constants and natural lifetimes, τ(N), of the AC monomer in hexane and acetonitrile, obtained from the experimentally determined fluorescence quantum yields and fluorescence lifetimes, have been compared with those theoretically estimated. The closeness between these experimental and theoretical data, the small Stokes shifts, the mirror image relationship between the absorption and fluorescence spectra and the close correspondence between the absorption and fluorescence excitation spectra, provide good evidences that the emission of AC monomer occurs directly from its lowest singlet excited state. The mono- and multi-parametric analyses of the AC solvatochromism indicate that the polarity-polarizability, the hydrogen bond donor and the hydrogen bond acceptor properties of the solvent preferentially stabilize the singlet excited over the ground state. These analyses also reveal that photoexcitation reinforces the hydrogen bond donor and acceptor properties of the AC, becoming the pyridinic nitrogen atom more basic and the pyrrolic group more acid.

A theoretical study of the hydrogen bond donor capability and co-operative effects in the hydrogen bond complexes of the diaza-aromatic betacarbolines.

In this work, we present a quantum mechanical investigation on the hydrogen bond interactions of N(9)-methyl-9H-pyrido[3,4-b]indole, MBC, and N(2)-methyl-9H-pyrido[3,4-b]indole, BCA, with different hydrogen bond donors. Thus, it has been analysed the influence that the hydrogen bond donor strength and the co-operative effect of the increasing number of donor molecules have on the shape of the potential energy surfaces versus the N···H distances, r(N­H). To rationalize the nature of the interactions, the Bader theory has been applied and the characteristics of the bond critical points analysed. The results show that two different hydrogen bond complexes can be formed depending on the donor capabilities or the number of donor molecules included in the calculations. The topological parameters from the Bader theory are used to justify the statement that the analysed interactions can be classified as weak or partially covalent hydrogen bond interactions, respectively. As experimentally observed, weak hydrogen bond donors form weak hydrogen bond complexes, called HBC. Upon the increase of the donor strength the N···H proton is shifted nearest to the nitrogen atom giving rise to the observation of a stronger hydrogen bond complex, the proton transfer complex, PTC. The most outstanding result of these studies is the fact that the formation of the PTC can also be managed just by changing the number of donor molecules, that is, by a co-operative effect of the hydrogen bonds.

Singlet excited state pyridinic deprotonation of the N9-methylbetacarboline cations in aqueous sodium hydroxide solutions.

The singlet excited state pyridinic deprotonation of the 9-methyl-9H-pyrido[3,4-b]indole, MBC, cations has been studied in aqueous NaOH solutions by absorption, steady state and time resolved fluorescence measurements. This excited state reaction proceeds through a stepwise mechanism involving different ground and excited state hydrogen bonded MBC-(water)n complexes. Thus, in aqueous NaOH solutions of MBC above pH 8, two ground state hydrogen bonded MBC-water adducts, namely PC and PTC, coexist in equilibrium. Upon excitation, the PC behaves as an independent fluorophore, whereas the PTC reacts with water molecules during its excited state lifetime to give the intermediate CL*. This exciplex is the precursor of the excited state cation, C*. In almost all the pH range, C* is practically the only existing species in the singlet excited state of MBC. In concentrated NaOH solutions beyond the pH range, C* deprotonates giving CL* and PTC* species.

Ground and singlet excited state pyridinic protonation of N9-methylbetacarboline in water-N,N-dimethylformamide mixtures.

The ground and the singlet excited state pyridinic protonation of 9-methyl-9H-pyrido[3,4-b]indole, MBC, in water-N,N-dimethylformamide mixtures has been studied by absorption, steady state and time resolved fluorescence measurements. These proton transfer reactions elapse by a stepwise mechanism modulated by different hydrogen bonded adducts and exciplexes formed by water molecules and the pyridinic nitrogen atom of the MBC. Based in the present and previous studies, a general mechanistic Scheme for the ground and the singlet excited state MBC pyridinic protonation has been proposed. Accordingly, in the ground state, upon increasing the water proportion of the water-N,N-dimethylformamide mixtures, a hydrogen bonded complex, HBC, its hydrogen bonded proton transfer complex, PTC, a pre-cationic complex, PC, and the cation, C, are progressively formed. In the excited state, MBC, HBC and PC behave as independent fluorophores. Excited state cations, C*, are mainly formed by direct excitation of the ground state cations and, in minor proportion, by the excited state reaction of the PTC* through the CL* exciplex.

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

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.

Dynamic study of excited state hydrogen-bonded complexes of harmane in cyclohexane-toluene mixtures.

Photoinduced proton transfer reactions of harmane or 1-methyl-9H-pyrido[3,4-b]indole (HN) in the presence of the proton donor hexafluoroisopropanol (HFIP) in cyclohexane-toluene mixtures (CY-TL; 10% vol/vol of TL) have been studied. Three excited state species have been identified: a 1:2 hydrogen-bonded proton transfer complex (PTC), between the pyridinic nitrogen of the substrate and the proton donor, a hydrogen-bonded cation-like exciplex (CL*) with a stoichiometry of at least 1:3 and a zwitterionic exciplex (Z*). Time-resolved fluorescence measurements evidence that upon excitation of ground state PTC, an excited state equilibrium is established between PTC* and the cationlike exciplex, CL*, lambdaem approximately/= 390 nm. This excited state reaction is assisted by another proton donor molecule. Further reaction of CL* with an additional HFIP molecule produces the zwitterionic species, Z*, lambda(em) approximately/= 500 nm. From the analysis of the multiexponential decays, measured at different emission wavelengths and as a function of HFIP concentration, the mechanism of these excited state reactions has been established. Thus, three rate constants and three reciprocal lifetimes have been determined. The simultaneous study of 1,9-dimethyl-9H-pyrido[3,4-b]indole (MHN) under the same experimental conditions has helped to understand the excited state kinetics of these processes.