Fluorescence quenching in molecular recognition by hydrogen bonding

Abstract

Steady-state fluorescence and single photon timing have been used to study the effect of the presence of hydrogen bonding on the intermolecular quenching of pyrene covalently linked to a guanine-like receptor I by an aliphatic amine (N,N-dimethylpropylamine) covalently linked to cytosine derivative II. By comparing the fluorescence quenching of I by II with that of 10methylpyrene (1-MP) by triethylamine (TEA), as a model system in which no hydrogen bonding can occur, one could possibly analyze the effect of the hydrogen bonding between receptor and substrate as a quenching as it leads to a higher local concentration of donor and acceptor. While the quenching of I by II was observed with an apparent rate constant kq of (1.78 ± 0.10) × 109 M-1 s-1 and (8.72 ± 0.42) × 108 M-1 s-1 in toluene and acetonitrile, respectively, no quenching could be observed in methanol. Upon excitation of 1-MP, no quenching by II could be detected in the same concentration range as used in the quenching of I. Quenching of I and of 1-MP by TEA (≥ 10-2 M) in toluene leads to exciplex formation with maxima centred at 540 and 514 nm, respectively. The rate constants of exciplex formation and dissociation of I with TEA were analyzed using a global compartmental analysis. The following values were obtained for the rate constants: k01 = (9.70 ± 0.01) × 106 s-1, k21 = (1.12 ± 0.003) × 109 M-1 s-1, k02 = (5.24 ± 0.01) × 107 s-1 and k12 = (7.74 ± 0.08) × 106 s-1. Quenching of I by TEA in the presence of III, a hydrogen-bonding system without an alkyl amine substituent, leads to exciplex formation centred at 538 nm. The rate constant values for the exciplex formation and dissociation of I with TEA in the presence of III were: k01 = (9.32 ± 0.08) × 106 s-1, k21 = (9.32 ± 0.003) × 108 M-1 s-1, k02 = (6.16 ± 0.03) × 107 s-1 and k12 = (21.90 ± 0.3) × 106 s-1. The apparent rate constants kq for this system was (7.26 ± 0.56) × 106 M-1 s-1. The observed decrease in the rate of exciplex formation of I with TEA in the presence of III could suggest that the guanine-like moiety in I forms hydrogen bonds with the cytosine-like moiety and this could decrease the electron affinity of I. The rate constant of exciplex dissociation increased, indicating that the exciplex is less stable in the presence of III. Because of the single exponential decay of I in the presence and absence of II and of the agreement between steady-state and transient fluorescence measurements, the information available for quantitative analysis of the association between I and II is limited. © 1996.