Fluorescence quenching of sterically-graded pyrene molecules by N,N-dialkylanilines. Exciplexes or locally excited states?

Abstract

Quenching of the excited singlet states of two tetra-arylpyrenes, 1,3,6,8-tetrakis(4-methoxy-2,6-dimethylphenyl)pyrene (PyOMe) and 1,3,6,8-tetraphenylpyrene (TPPy), by a series of N,N-dialkylanilines has been investigated by both steady-state and time-resolved fluorescence measurements, and the results are compared with those from the quenching of fluorescence from pyrene. Specifically, the relative accessibilities of the quenchers to the π-surfaces of the pyrenyl groups are assessed on the bases of the rates and efficiencies of the quenching and the abilities of the constituent molecules to form exciplexes. Access to the pyrenyl group of PyOMe is restricted as a result of the steric congestion imposed by the four ortho-dimethylanisyl rings and the high energy needed to rotate them appreciably; the aryl groups form an imposing ‘picket fence’ around the pyrenyl core. The four phenyl rings around the pyrenyl group of TPPy can be moved much more easily and, as a result, afford much less inhibition to the approach of an aniline quencher. Thus, quenching of the fluorescence from TPPy (or pyrene) by the N,N-dialkylanilines is near the diffusion-controlled limit, and its fluorescence decay can be fitted satisfactorily to mono-exponential functions. The quenching kinetics and strong exciplex emissions for TPPy demonstrate that its excited singlet state behaves similarly to that of pyrene. Quenching of the excited singlet state of PyOMe by the N,N-dialkylanilines is accompanied by much weaker exciplex emissions, and the time-resolved emission from its excited singlet state requires fitting by bi-exponential functions; the steric congestion reduces the energy difference between the locally excited-state and the exciplexes. Furthermore, decay profiles of the exciplexes of PyOMe and N,N-dialkylanilines, as well as associated transient absorption data, indicate that these excited-state interactions involve a distribution of intermolecular orientations and very complex excited-state equilibria. These results are relevant to a variety of weakly complexed systems and demonstrate the importance of concomitant orientational parameters and steric factors to understanding the dynamics of transient bimolecular interactions.