Hydrogen‐bonding and Proton Transfer Interactions between Harmane and Trifluoroethanol in the Ground and Excited Singlet States

Abstract

A study of the hydrogen‐bonding and proton transfer reactions of the ground and excited states of harmane (1‐methyl‐9H‐pyrido/3,4‐b/indole) and its N9‐methyl derivative with 2,2,2‐trifluoroethanol in cyclohexane is reported. Spectral measurements (UV–visible, Fourier trans‐form IR, steady‐state and time‐resolved fluorescence) show the formation of fluorescent ground‐state hydrogen‐bonded complexes. The results have been interpreted assuming a tautomeric equilibrium between a 1:1 hydrogen‐bonded complex and its 1:2 proton transfer tautomer (hydrogen‐bonding ion pair). Upon excitation to its singlet excited state, the proton transfer tautomer of harmane reacts with an additional 2,2,2‐trifluoroethanol molecule to give a zwitterionic exciplex, which fluoresces at longer wavelength.

β-Carbolines That Accumulate in Human Tissues May Serve a Protective Role against Oxidative Stress

“…Spectral Features of the ␤-Carbolines-The photophysical and photochemical properties of some ␤-carbolines have been studied in the literature (11)(12)(13). Some of these are phototoxic to bacteria and insects; Larson et al (11) have found the ability to kill test organisms to vary in the order I Ͼ II Ͼ IIa, and IIb to be inactive.…”

“…Some of the ␤-carbolines have been shown to be phototoxic to bacteria and insects (11), and attempts have been made to correlate their phototoxicity with their ability to produce reactive oxygen species upon irradiation, i.e. photodynamic or sensitizer abilities (11)(12)(13). On the other hand, there have been reports (14 -16) that some of these Maillard products are antioxidative in nature.…”

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

“…These second complexes, with a stoichiometry of at least 1:2, are called the proton transfer complexes (PTC), N dþ Á Á ÁHÁ Á ÁO dÀ . The existence of a charge separation in the PTC complex has been confirmed by FTIR studies on the fundamental stretching frequency of the hydroxylic group of the alcohols [52]. Thus, upon the addition of the betacarboline derivative, the OH stretching band of HFIP decreases and a continuum absorption in the middle region of the spectra is observed.…”

“…Among the great variety of substrates possessing potential hydrogen bond donor/acceptor sites, the betacarboline ring, 9H-pyrido [3,4-b]indole (BC) [20] (Scheme 19.1), has been widely studied [21][22][23][24][25][26][27][28][29][30][31][32][33]. These rings are the structural units of numerous naturally occurring alkaloids that possess a wide range of biological and pharmacological properties [34][35][36][37][38][39][40][41][42][43][44][45].…”

“…In this sense, and within our interest in the betacarboline photophysics, we have designed a strategy to broach the study of the ground-and excited-state hydrogen bonding of these compounds. It consists in the detailed analysis of ground-and excited-state hydrogen bond interactions of 9H-pyrido [3,4-b]indole (BC), N 9 -methyl-9H-pyrido [3,4-b]indole (MBC), N 9 -methyl-1-methyl-9H-pyrido [3,4-b]indole (MHN) and N 2 -methyl-9H-pyrido [3,4-b]indole (BCA) (Scheme 19.1) with 1,1,1,3,3,3-hexafluoro-propan-2-ol (HFIP) in the low polar aprotic solvent cyclohexane [52][53][54][55][56][57][58]. 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.…”