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 func tion 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-mono mer fluorescence quantum yield ratio Phi'/Phi, peaks at n = 5 and then grad ually decreases with increasing n. These results are in clear contrast to t hose obtained previously with the 1,n-di(1-pyrenyl)alkanes (1Py(n)1Py), whe re 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 hin drance at the excimer configuration (k(a)) and to dimer formation in the el ectronic ground state between the pyrenylcarboxy end groups in 1PC(n)1PC (P hi'/Phi). Both effects arise from the absence of methylene hydrogens next t o the pyrenyl moieties of these molecules, which hydrogens prevent excimer formation in the case of 1Py(7)1Py.