Journal of Physical Chemistry B, Vol.103, No.43, 9356-9365, 1999
Chain length dependence of intramolecular excimer formation with 1,n-bis(1-pyrenylcarboxy)alkanes for n=1-16, 22, and 32
Time-resolved and photostationary measurements in alkane solvents as a function of temperature are presented for the 1,n-bis(1-pyrenylcarboxy)alkanes (1PC(n)1PC, n = 1-16, 22, 32) with n methylene groups in the chain. For all compounds except 1PC(1)1PC intramolecular excimer fluorescence is observed. From fluorescence decay analysis and quenching data, the rate constant k(a) of excimer formation in the series 1PC(n)1PC is determined as a function of chain length. The value of k(a), as well as that of the excimer-to-monomer fluorescence quantum yield ratio Phi'/Phi, peaks at n = 5 and then gradually decreases with increasing n. These results are in clear contrast to those obtained previously with the 1,n-di(1-pyrenyl)alkanes (1Py(n)1Py), where Phi'/Phi, reaches its maximum at n = 3, with a sharp minimum (no excimer formation) at n = 7. This difference is attributed to a reduced steric hindrance at the excimer configuration (k(a)) and to dimer formation in the electronic ground state between the pyrenylcarboxy end groups in 1PC(n)1PC (Phi'/Phi). Both effects arise from the absence of methylene hydrogens next to the pyrenyl moieties of these molecules, which hydrogens prevent excimer formation in the case of 1Py(7)1Py.