Solid-state-concentration effects on the optical absorption and emission of poly(p-phenylene vinylene)-related materials

Abstract

We present measurements of the optical absorption and emission properties of poly(p-phenylene vinylene͒ ͑PPV͒ -related materials focusing on the differences between molecules isolated by dispersion in an inert host and concentrated molecular films. Optical absorption spectra, photoluminescence ͑PL͒ spectra, PL efficiency, and time-resolved PL spectra of dilute blends of PPV oligomers with 2-5 phenylene-phenyl rings are compared with those of dense oligomer and polymer films. In dilute oligomer-poly͑methyl methacrylate͒ ͑PMMA͒ blends with high PL efficiency, the PL decay is exponential, independent of both temperature and oligomer length. This implies that the fundamental radiative lifetime of PPV oligomers is essentially independent of oligomer length. Concentrated spin-cast oligomer films and polymers have a faster and strongly temperaturedependent PL decay that approaches that of the dilute oligomer results at low temperature. The differences in PL decay correspond to changes in PL efficiency. The efficiency of the oligomer-PMMA blend is high and only weakly temperature dependent, whereas that of concentrated films is lower and strongly temperature dependent, decreasing by more than a factor of 3 from 10 to 350 K. The quenching of the PL efficiency in concentrated films is due to migration to extrinsic, impurity related centers as opposed to an intrinsic intermolecular recombination process. The PL spectrum of a dilute oligomer blend redshifts substantially, both as the excitation energy is decreased and as the emission time increases. This spectral redshift is due to disorderinduced site-to-site variation and not to diffusion to lower-energy sites. In contrast, no spectral shift with excitation energy or emission time was observed for dense oligomer films. ͓S0163-1829͑96͒03132-3͔

Correlation of vibrational intensity with fluorescence lifetimes in π conjugated polymers

“…Furthermore the fluorescence lifetimes calculated from the rates are consistent with the documented PPV derivative lifetime measurements [26]. The polymer fluorescence lifetimes can be calculated to be in the order of nanoseconds as is expected of substituted PPV systems [27].…”

Investigations of excitation energy transfer and intramolecular interactions in a nitrogen corded distrylbenzene dendrimer system

“…5 ͑inset͒, does not show fast rise or decay features. We therefore propose conformational changes 42 of the excited dendrimer core to be the origin of the fluorescence dynamics of G0 shown in Fig. 4.…”

“…Delocalization of the excitation could contribute to a significant hindrance to photoisomerization ͑conformation͒ processes thus leading to elongation of the fluorescence decay time. 37,42,44 To probe the intramolecular interaction between stilbene dendrons shell and A-DSB core we compared the fluorescence kinetics for the dendrimer with and without stilbene dendrons: G0, G2. Chloroform solutions with concentrations ϳ5ϫ10 Ϫ5 M were used in this experiment.…”

The effects of supramolecular assembly on exciton decay rates in organic semiconductors

“…The similarities in exciton lifetimes of MOPV derivatives at 90°C are another indication that our experimental temperature is sufficient to dissolve effectively all the stacks and measure the properties of isolated molecules ͑similar lifetimes independent of oligomer length were obtained for a related series of oligomers͒. 4 The differences in exciton lifetimes of MOPV derivatives at 14°C can be explained in various ways, such as differences in stack configuration leading to variations in the magnitude of the intermolecular electronic coupling or the existence of nonemissive trap states with different densities in the three materials.…”

“…3 An important one is to facilitate both intrinsic and extrinsic exciton quenching processes. 4,5 Intrinsic quenching is related to splitting of excitonic states; cofacial alignment of chromophores with sufficiently small separation ͑3-4 Å͒ leads to reduced oscillator strength in the lowest excited-state transition. 6 On the other hand, extrinsic processes can be enhanced by diffusionlimited quenching at either chemical ͑e.g., charge-transfer͒ or structural ͑e.g., internal conversion͒ defects.…”