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

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) × 10^4 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 pentecene in naphthalene and derive in this case at 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)