Fluorescence transient and optical free induction decay spectroscopy of pentacene in mixed crystals at 2 K. Determination of intersystem crossing and internal conversion rates

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Abstract
In this paper the recently reported fluorescence transient, appearing during the onset of cw dye laser excitation of the energetically lower sites of pentacene in p‐terphenyl at 2 K, is analyzed quantitatively. The role of the lowest triplet state in causing the long (compared to the fluorescence lifetime) transient, is firmly established. For the lower sites a kinetic analysis yields a T1←S1 intersystem crossing (ISC) yield of about 0.4% and an S0←T1 ISC rate of (2.2±0.1) × 104 s−1. Using these data in calculating the S1←S0 transition dipole moment from our earlier OFID experiments yields for the O1 site (proto) μ=0.71±0.24 D. The S0→S1 internal conversion yield is estimated to be 22%. The fluorescence transients of the energetically higher sites of pentacene in p‐terphenyl exhibit a much faster decay, which indicates a drastic increase in ISC yield from S1. Optical free induction decay (OFID) experiments performed on these sites confirm this and imply that the T1←S1 ISC yield for these sites is ≳60%. We further discuss results of an OFID experiment on pentacene in naphthalene and derive in this case a T1←S1 ISC yield of ≲2.8% and an S0←S1 internal conversion yield of 31.5±1.5%. The off‐resonance pumping effect in a 3‐level system was also studied, using the parameter values for pentacene in p‐terphenyl. For the lower sites, with the excitation intensities used (∼1 MW/m2) the part of the inhomogeneous absorption line that is pumped has a width of about 2.5 GHz (effective excitation bandwidth <1 MHz).

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