Final answer:
Observing a biexponential phosphorescence decay of diacetyl suggests the presence of both fluorescence or fast non-radiative processes and true phosphorescence, represented by the very short and the long decay times, respectively. Biexponential decay behavior likely indicates that at least two distinct processes influence the de-excitation of diacetyl molecules, with the instrument response function playing a critical role in the analysis.
Step-by-step explanation:
Observing a biexponential fitting with a long decay time of about 20 μs and a very short time around 30 ns for the phosphorescence decay of diacetyl in acetonitrile can indeed make sense. The short decay component could be attributed to fluorescence or other fast non-radiative processes, while the longer component likely represents true phosphorescence.
The fluorescence from the diacetyl might be quite fast (on the order of nanoseconds), as spontaneous de-excitation typically has a mean lifetime of about 10⁻⁸ s. Conversely, phosphorescence involves the de-excitation of a metastable state with significantly longer lifetimes, up to milliseconds or even hours due to quantum number differences inhibiting quick relaxation.
This kind of dual decay behavior is often observed in molecules with high quantum yield of intersystem crossing, such as diacetyl. When performing a reconvolution fit, it is important to consider the instrument response function (IRF), which often has a width of a few microseconds and significantly influences the decay curve fitting.
The biexponential nature of the decay suggests the presence of at least two distinct processes influencing the de-excitation of excited state molecules.
The very short decay component (around 30 ns) is unusual for phosphorescence and may indicate the presence of a quick quenching process or a measurement artifact, which needs to be explored further, perhaps by considering effects like spillover into UV and/or IR or reabsorption phenomena as these could affect the observed decay times. However, the longer decay time of about 20 μs appears more consistent with typical phosphorescent lifetimes.