title: Stereodifferentiation in the intramolecular singlet excited state quenching of hydroxybiphenyl-tryptophan dyads.
abstract: The photochemical processes occurring in diastereomeric dyads (S,S)-1 and (S,R)-1, prepared by conjugation of (S)-2-(2-hydroxy-1,1'-biphenyl-4-yl)propanoic acid ((S)-BPOH) with (S)- and (R)-Trp, have been investigated. In acetonitrile, the fluorescence spectra of (S,S)-1 and (S,R)-1 were coincident in shape and position with that of (S)-BPOH, although they revealed a markedly stereoselective quenching. Since singlet energy transfer from BPOH to Trp is forbidden (5 kcal mol(-1) uphill), the quenching was attributed to thermodynamically favoured (according to Rehm-Weller) electron transfer or exciplex formation. Upon addition of 20% water, the fluorescence quantum yield of (S)-BPOH decreased, while only minor changes were observed for the dyads. This can be explained by an enhancement of the excited state acidity of (S)-BPOH, associated with bridging of the carboxy and hydroxy groups by water, in agreement with the presence of water molecules in the X-ray structure of (S)-BPOH. When the carboxy group was not available for coordination with water, as in the methyl ester (S)-BPOHMe or in the dyads, this effect was prevented; accordingly, the fluorescence quantum yields did not depend on the presence or absence of water. The fluorescence lifetimes in dry acetonitrile were 1.67, 0.95 and 0.46 ns for (S)-BPOH, (S,S)-1 and (S,R)-1, respectively, indicating that the observed quenching is indeed dynamic. In line with the steady-state and time-resolved observations, molecular modelling pointed to a more favourable geometric arrangement of the two interacting chromophores in (S,R)-1. Interestingly, this dyad exhibited a folded conformation in the solid state.
authors: Departamento de Química/Instituto de Tecnología Química UPV-CSIC, Universitat Politécnica de València, Camino de Vera s/n, 46022, Valencia, Spain.
authors: Paula
authors: P
authors: Bonancía
authors: Ignacio
authors: I
authors: Vayá
authors: Dimitra
authors: D
authors: Markovitsi
authors: Thomas
authors: T
authors: Gustavsson
authors: M Consuelo
authors: MC
authors: Jiménez
authors: Miguel A
authors: MA
authors: Miranda
mesh_terms: D001713:Biphenyl Compounds / Q000737:chemistry*; D018360:Crystallography, X-Ray; D008958:Models, Molecular; D015394:Molecular Structure; D010666:Phenylpropionates / Q000737:chemistry*; D011789:Quantum Theory; D013237:Stereoisomerism; D014364:Tryptophan / Q000737:chemistry*
chemical_list: C581359:2-(2-hydroxybiphenyl-4-yl)propanoic acid; D001713:Biphenyl Compounds; D010666:Phenylpropionates; D014364:Tryptophan