TIME-RESOLVED MONOMER AND EXCIMER FLUORESCENCE OF 1,3-DI(1-PYRENYL)PROPANE AT DIFFERENT TEMPERATURES - NO EVIDENCE FOR DISTRIBUTIONS FROM PICOSECOND LASER EXPERIMENTS WITH NANOSECOND TIME RESOLUTION

Monomer and excimer fluorescence decays of 1,3-di(1-pyrenyl)propane (1Py(3)1Py) in n-heptane are measured at different temperatures by using time-correlated single-photon counting (SPC). The decays at 61-degrees-C are triple-exponential with decay times and amplitudes independent of the number of total counts, excitation wavelength, and width of the excitation pulse. At-43-degrees-C and lower temperatures, single-exponential monomer decays are obtained, next to triple-exponential excimer records. Aggregates of 1Py(3)1Py are not detected. These results can be described by the three-state model, DMD, comprising one group of rapidly interconverting excited-state monomers (M*) and two excimers (D*). No evidence for distributed decay times is found down to a time resolution of 0.1 ns/channel and 2000 channels. In support of the DMD model, molecular mechanics calculations backed by H-1 NMR measurements are presented, showing that only two monomer conformers of 1Py(3)1Py are populated in the ground state. The interconversion between these two monomers in the excited state is too fast to be resolved by the present SPC measurements.