Optically detected magnetic resonance studies on [pi]-conjugated polymers and novel carbon allotropes

摘要

The photophysics of poly(p-phenylene)-type ladder polymers (m-LPPP) and 2,5-dibutoxy poly(p-phenylene ethynylene) (DBO-PPE) films and solutions were studied by X-band photoluminescence detected magnetic resonance (PLDMR). Frequency resolved PLDMR measurements on LPPP, DBO-PPE, poly(3-hexylthiophene) (P3HT), poly(p-phenylene-vinylene) (PPV), and C70 are also reported and discussed;All the polymer samples exhibit three distinct features when excited at wavelengths [lambda]\u3e 458nm: (i) A narrow PL-enhancing spin-1/2 polaron resonance, (ii) broad full- and (iii) half-field spin-1 triplet exciton powder patterns due to the [delta]m s = 1 and [delta]m s = 2 transitions among the triplet sublevels, respectively;The full-width at half maximum (FWHM) of the spin-1/2 resonance in LPPP decreased from film to solution. However, the FWHM of that resonance in PPE was identical at all concentrations. This spin-1/2 resonance is assigned to the magnetic resonance enhancement of the recombination of both interchain and intrachain-intersegment polaron pairs which quench singlet exciton recombination. In solid m-LPPP samples, the aggregate PL gives rise to a proportionally higher magnetic resonance effect than other parts of the PL spectrum;In DBO-PPE and m-LPPP solutions, the triplet resonance decreased with decreasing concentration. This suggests that the triplet state is an intrinsic long-lived (~30[mu]s) trapped state localized on a phenylene ring and stabilized by coupling to a unit of an adjacent chain;Frequency resolved measurements of the lifetime of the species affected by the resonance conditions for all the polymers are described and discussed. The lifetimes appeared to include: (i) fast (9[mu]s \u3c [tau]1 \u3c 40[mu]s) and (ii) slow (575[mu]s \u3c [tau]2\u3c 1868[mu]s) components. The lifetimes increased with increasing concentration of the polymers in toluene solutions. These results can be interpreted to provide support for the interchain/intersegment polaron model or a distribution of lifetimes model.