Fast and Slow Time Regimes of Fluorescence Quenching in Conjugated Polyfluorene−Fluorenone Random Copolymers:  The Role of Exciton Hopping and Dexter Transfer along the Polymer Backbone

Abstract

The luminescence decay kinetics of polyfluorene copolymers containing fluorenone units randomly distributed along the polymer chain have been studied by steady-state and time-resolved fluorescence techniques in toluene solution. The typical green emission from polyfluorenes containing 9-fluorenone moieties is only observed if the 9-fluorenone group is covalently attached to the polymer. Small-angle neutron scattering (SANS) measurements indicate that, independent of the 9-fluorenone fraction, all the studied copolymers adopt an open wormlike conformation. This prevalent 1-dimensional arrangement confirms that the green emission observed with polyfluorenes is not the result of excimer formation in the typical sandwich-like conformation. Analysis of time-resolved fluorescence decays by the maximum entropy method (MEM) collected at the polyfluorene emission (415 nm) and by global analysis of decays collected at 415 and 580 nm (the 9-fluorenone defect emission wavelength) clearly indicates two different time regimes in the population of the fluorenone defect: one occurring in the time interval of 10 to 30 ps and a second one occurring in the time range from 70 to 200 ps. While the slower process shows a linear dependence with the 9-fluorenone fraction, compatible with a hopping migration process along the polymer chain, the faster process does not show such a dependence and instead suggests a short-range Dexter mechanism. These findings are in agreement with our previous work where the presence of a faster component was suggested.

Long range energy transfer in conjugated polymer sequential bilayers

“…The two faster lifetime components of 48 and 135 ps for the blend at 1.0% are interpreted as the result of intramolecular and/or intermolecular redistribution energy mechanisms due to exciton migration and energy traps before the final PFO radiative state recombination corresponding to the longer decay at 380 ps. 10 Faster decays τ l1 and τ l2 , however, are observed for the bilayer and for the blends at 5.4% and 11.5% MEH PPV concentrations, which is a consequence of the effective energy transfer rate to the MEH PPV sites. The consecutive increase of τ r 1 for the blends is interpreted as a more complex MEH PPV recombination probably due to intermediary intermolecular MEH PPV/PFO complexation states (steric interactions) that would take place with increasing MEH PPV concentration in the blend.…”

The photophysics of singlet, triplet, and degradation trap states in 4,4-N,N′-dicarbazolyl-1,1′-biphenyl

“…The increase in the emission intensity from trap sites looks similar to the effects of keto defects in polyfluorene, 24 where one sees large increases in a LE, broad peak in solid state in comparison to solution and where longer wavelength peaks act as luminescence quencher sites via energy transfer mechanisms. 25 The quenching is so effective that very large changes in emission color of the material may be observed as in CBP. Castex 26 observed the photodegradation under UV light illumination of the carbazole derivative N , N-diethyl-3,3-bicarbazyl, causing the emergence of a new emission band between 400 and 500 nm.…”

Luminescent properties of a novel fluorene organic material

“…Recently, fluorenone-fluorene statistical co-polymers were synthesized. Their solutions exhibited a featureless emission band centred at 535 nm, even at low concentrations, consequently excluding the formation of excimers or aggregates and confirming that keto defects are responsible for the green emission band (24,25). However, the presence of on-chain defects does not completely explain the intensity of green bands in degraded PFs; for this reason, interchain interactions and the formation of fluorenonebased excimers has been proposed by Sims et al (26).…”