A detailed time resolved investigation of the photoluminescence of a thin tetracene film deposited on highly oriented pyrolytic graphite is presented. In agreement with Lim et al. [Phys. Rev. Lett. 92, 107402 (2004)], we find strong evidence for superradiance: an increase of the relative intensity of the pure electronic transition with respect to the vibronic sideband and a concomitant decrease of the radiative lifetime from 10 to 1.83 ns upon cooling from 300 to 4 K. For lower temperatures, a redshift (approximately 200 cm(-1)) of the free exciton is observed. Previously, this shift was attributed to a structural phase transition. Our time resolved spectra reveal that the spectral shift is related to a dynamical relaxation process which occurs within the first 50 ps.

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Steady state as well as time-resolved fluorescence of phenyl-substituted polyphenylenevinylene (PhPPV), both in solution and in form of spin-coated films has been measured. In solution the fluorescence decays exponentially with a 1/e decay time of ∼700 ps, whereas the 1/e decay in the film is ∼400 ps and the decay approaches a stretched exponential with an exponent of β=0.65. Doping the film with various amounts of trinitrofluorenone (TNF) shortens the decay time without changing the form of the Kohlrausch–Williams–Watts decay law. This is a signature of exciton dissociation in a random polymer matrix through electron transfer to TNF. The data analysis shows that “neat” PhPPV contains a concentration of ⩽ 0.04 wt. % of unidentified electron scavengers. A hypsochromic shift of the cw fluorescence spectra in doped films is interpreted in terms of increasingly frustrated spectral relaxation of singlet excitations within the inhomogeneously broadened distribution of hopping states.

Dendronization has previously been suggested as a means of controlling the level of intermolecular interactions in macromolecular and polymeric compounds. We therefore studied the spectral relaxation dynamics of dendronized and non‐dendronized polyfluorenes at different temperatures. Dendronization strongly affects inhomogeneous spectral broadening, resulting in a blue‐shift and broadening of the otherwise narrow vibronic progression of polyfluorene at low temperature. Using gated spectroscopy we are able to identify emissive keto defects on isolated chains of both dendronized and non‐dendronized polymers, along with a strong aggregation band in the solid state. Surprisingly, the emission from chemical and structural defects is found to be stronger in the case of the dendronized material. The observation of aggregate emission in dendronized polyfluorenes suggests that aggregation in these materials does not occur through linear π‐stacking, but rather through electronic interactions at point contacts between chains introduced by structural kinks along the backbone.

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Microcavities containing a J-aggregate forming organic dye were prepared in the strong-coupling regime. Changing the position of the neat organic films within the microcavity allows a tuning of the giant Rabi splitting observed. We set the Rabi splitting to 25 meV and study the thermal activation of the higher-energy polariton emission under excitation off resonance. This is found to exhibit an Arrhenius activation with an energy corresponding to the polariton branch splitting. A constant offset to the upper polariton is observed at low temperature. Microsecond gated spectroscopy reveals that thermalization of the system occurs on the time scale of milliseconds, which contrasts with the polariton lifetime in the range of picoseconds. Bulk sample heating cannot fully account for the slow polariton thermalization observed in steady-state experiments, suggesting that long-lived macroscopic polariton coherence influences the polariton relaxation.

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