Stereodifferentiation in the intramolecular singlet excited state quenching of hydroxybiphenyl–tryptophan dyads

Paula Bonancía, Ignacio Vayá, Dimitra Markovitsi, Thomas Gustavsson, M. Consuelo Jiménez, Miguel A. Miranda
2013 Organic and biomolecular chemistry  
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. 10 Since singlet energy transfer from BPOH to Trp is forbidden (5 kcal mol -1 uphill), the
more » ... -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 15 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 20 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.
doi:10.1039/c3ob27278h pmid:23381012 fatcat:zqbzyvywljek5nw4tv2z3dbgau