Wavelength-resolved studies of forster energy transfer in azobenzene-modified conjugated polymers: The competing roles of Exciton migration and spectral resonance

We report results from wavelength-resolved fluorescence lifetime measurements of azobenzene-modified poly(p-phenylenevinylene) (PPV). The introduction of an azobenzene side chain enables reversible phototriggered modulation of the PPV emission. Intensity modulation is possible because Forster-type energy transfer from the PPV backbone to the azobenzene side-chain acceptors is more efficient for the cis-azobenzene isomer than for the trans. Here we explore how side-chain quenching competes with intrinsic PPV exciton dynamics. By probing the red and blue edges of the PPV emission, we evaluate the photophysical effects of the side chain on different exciton populations. Prior to UV exposure, when the azobenzene side chains are trans, three exciton subpopulations are detected: (i) rapidly diffusing and unquenched, (ii) nondiffusing and quenched, and (iii) low-energy, nondiffusing and unquenched. Exciton diffusion on the far blue edge is extremely efficient and mitigates the effects of side-chain quenching, giving rise to the first population. The second population is due to spectral resonance between side chains and nondiffusing excitons. The third population consists of excitons residing on the longest PPV chain segments. After UV-induced trans -> cis photoisomerization, we observe a greater degree of quenching on the red edge as spectral overlap improves and the lowest-energy chromophores become efficiently quenched. Hence, cis-azobenzene quenching of (iii) contributes greatly to the efficient photomodulation of these PPV derivatives. In addition, rapid exciton migration on the blue edge may help to improve modulation efficiencies by effectively competing with energy transfer to trans-azobenzene. These findings provide improved understanding of the underlying mechanisms of energy transfer to side chains in this important class of polymers.