Proton-Transfer Tautomerism of β-Carbolines Mediated by Hydrogen-Bonded Complexes

Abstract

The carboxylic acid catalyzed excited-state amino-imino tautomerism for β-carboline (β-CB) and its analogues has been investigated. Thermodynamics and microsolvation (i.e., stoichiometry of the complex formation) of various β-CB/acetic acid complexes in nonpolar solvents have been studied by means of absorption and emission titration experiments. Supplementary support of the stoichiometric ratio and structure for the hydrogenbonding formation was provided by molecular design and syntheses of various β-CB analogues incorporating either only one hydrogen bonding site or dual hydrogen bonding sites where interplay between two sites are sterically prohibited. The results in combination with time-resolved measurements and theoretical approaches suggest the 1:2 β-CB/acetic acid complex with a structure of triple hydrogen bonding formation to be responsible for the excited-state proton-transfer tautomerism in cyclohexane. The proton transfer time is beyond the response limit of the detecting system of 15 ps, indicating that only a negligibly small geometry adjustment is required for the guest molecule (i.e., acetic acid) to a correct geometry, i.e., a hydrogen-bond relay configuration, for the triple proton transfer to proceed. In comparison, for the 1:1 β-CB/acetic acid non-hydrogen-bond relayed complexes, amino-imino tautomerism is prohibited during the excited-state lifetime, giving rise to a normal Stokes shifted emission. The results provide detailed ground-state thermodynamics of β-CB HB complexes as well as the dynamics of proton-transfer tautomerism mediated by the hydrogen-bonding structures.

Coupled hydrogen‐bonding interactions between β‐carboline derivatives and acetic acid

“…The bridging protons produce a single signal, implying an exchange process between the different complexes; this effect can be explained only if we consider a cyclic complex. This cyclic structure is in accordance with the stoichiometric ratios and the formation of hydrogen bond proposed by Chou et al 21 This structure is responsible for the ESPT tautomerism observed for these compounds in different solvents using time-resolved measurements, molecular design and syntheses of different beta-carboline derivatives incorporating only one hydrogen bonding site.…”

“…Therefore, this fast excitedstate proton-transfer (ESPT), which takes place through a conduit of relayed hydrogen bonds, is responsible for the emission around 500 nm that traditionally was associated with the fluorescence of zwitterionic species. Nevertheless, like Chou et al, 21 we think 22 -24 that this fluorescence can be attributed to an emission from the tautomer, that we have called D2, resulting from the interaction of N(9)-H and N(2) with two AcOH molecules (Fig. 1).…”

“…Recently, studies 21 were conducted for the simplest BC, norharmane (NOR) (9H-pyrido [3,4-b]indole) ( Fig. 1), in cyclohexane in the presence of trace amounts of a hydrogenbonding donor/acceptor, such as acetic acid (AcOH).…”

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

“…order of magnitude to those reported for the ICT states of other compounds, such as 9-aminoacridinium derivatives and N-phenylpyrrole [68,69]. Interestingly, t 0 ICT is also in excellent agreement with the values reported by different authors for the decay time of the zwitterionic phototautomer of betacarboline [22,28,50].…”

“…Therefore, the simultaneous presence of several species, which in principle can be formed from the same or different precursors, makes it difficult to separate the different equilibria and to characterize the responsible species. This is the main reason why there is not a complete description of the dynamics of these processes and even controversy between different authors [48][49][50][51].…”