We report transient and steady state photoluminescence results along with absorption and NMR data to support the existence of two distinct morphological species in MEH−PPV solutions. NMR data provide evidence for the close packing of polymer chains, a consequence of solvent quality reduction. These data are correlated with optical properties of the aggregated species in poor solvents and the isolated chains in good solvents. We infer that steric hindrance of backbone motions increases effective conjugation length and leads to a spectral red shift in absorption and emission. At the same time, interchain excitations with negligible luminescence can be formed, leading to a dramatic reduction in photoluminescence quantum yield. While spectral changes are observed as packing is induced, we show that interchain state formation and its subsequent back-transfer to excitons are particularly sensitive to the interchain registry of the highly packed chains.

We report measurements of photoluminescence from films of a soluble phenylenevinylene polymer that has prospective importance as the emissive material in light-emitting diodes. We show unambiguously that there is long-lived emission in this material due to excimers and estimate that the quantum yield for excimer formation is as high as 50%. Since excimers in this polymer largely decay nonradiatively at ambient temperature, their prominence serves to drastically reduce the possible efficiency of electroluminescent conjugated polymer devices.

We report measurements of photoluminescence efficiency and time dependence in a high-electron-affinity cyano-substituted derivative of poly(p-phenylenevinylene).We find that the luminescence is much longer lived in solid films than in solutions and interpret this result as emission in the film being fTom an interchain excitation. These results suggest that intermolecular interactions are an important consideration in the design of highly luminescent conjugated polymers. Hg3C60 OC6H~3 CN OC6H, 3 OC6H)3 FIG. 1. The CN-PPV molecule -a high-electron-affinity conjugated polymer. Measurements of luminescence in conjugated polymers are important both for understanding the nature of their excited states, and the operation of electroluminescent devices. Conjugated polymers combine semiconducting electronic properties with favorable processing character-istics. The possibility of chemical tailoring to realize desired properties is a particular advantage of these materials. For example, the energy gap of conjugated polymers may be tuned by chemical substitution or making copolymers. ' A recent molecular engineering innovation has been the development of high-electron-affinity polymers, made by cyanosubstitution of members of the poly(arylene vinylene) family. The particular polymer studied here is a cyanosubstituted poly(p-phenylenevinylene), referred to as CN-PPV and shown in Fig. 1. The increased electron affinity facilitates electron injection from stable electrodes such as aluminum in light-emitting diodes, thereby improving device performance. The alkoxy sidegroups confer solubility in solvents such as toluene and chloroform. In this paper we report measurements of luminescence efficiency and time dependence in CN-PPV.The operation of an electroluminescent device involves several steps: charge injection at the contacts; charge transport through the device; charge capture to form an exciton, which (if singlet) then decays by a combination of radiative and nonradiative decay channels. Photoluminescence measurements enable the last step to be studied directly, and therefore help us to understand the photophysics of luminescent polymers.The picture that has emerged in PPV is that photoexcitation leads to the generation of singlet excitons which then decay by a combination of radiative and nonradiative pathways. The relative rate of the radiative and nonradiative decay processes determines the quantum efficiency of luminescence. For example, if nonradiative decay is very rapid then most of the excitons will decay by this route before they have time to emit so that luminescence will be weak.The measured lifetime of the luminescence v. is shorter than the natural radiative lifetime~R , and can be related to the rate constants for radiative and nonradiative decay kR and kNR by 1/7 = kR+ kNR and the efficiency is then given by bkR/(kR+ kNR) b +/+R (2) where rR = 1/kR is the natural radiative lifetime, i.e. , the lifetime the luminescence would have in the absence of nonradiative decay processes. b is the fraction of absorbed ph...

Squaraines offer significant potential for organic photovoltaics because of their broad absorbance and high extinction coefficients as well as their expected use as mechanistic probes in such devices. In this work steps are taken to develop a comprehensive understanding of the excited state properties of squaraines, based on their molecular structure and the resulting solid state packing. Accurate assignments of the absorption spectral peaks are made based on an essential states model, expanded to include intermolecular charge transfer (ICT). Comparison of simulated spectra with spectra for two symmetric squaraine derivatives confirms that ICT has a major influence on the optical and electronic properties of squaraine aggregates, fully accounting for the origin of the strong panchromaticity of these systems.

Aggregates of quadrupolar DAD-type chromophores are investigated theoretically using the essential states model (ESM) enhanced to include vibronic coupling. On the basis of vibronic signatures in the absorption and photoluminescence spectra, important connections are made to the Frenkel exciton model, which is the basis for defining conventional H-and J-aggregates. In contrast to the exciton theory, the ESM allows for an unusual type of the red-shif ted Haggregate, driven by the large quadrupole−quadrupole interaction. Except for the red shift of the main absorption peak, such aggregates display all of the characteristics of conventional Haggregates, including suppressed radiative decay rates and all of the vibronic signatures in the absorption and photoluminescence spectra consistent with Frenkel exciton models. Red-shifted Hdimers occur for a range of slip angles, which separate conventional H-and J-aggregate behavior, with the slip range diminishing with increasing intermolecular separation. Davydov splitting and vibronic signatures for dimers with nonparallel transition dipole moments are also investigated. The theoretical insight provided herein may be exploited for optimizing optoelectronic device applications for growing families of quadrupolar molecules.