Ultrafast spectroscopic studies of organic solids reveal features unexpected within simple noninteracting models for these systems. We consider: (1) organic mixed-stack charge- transfer solids, (2) conjugated polymers, and (3) aggregates of metal-halogen phthalocyanines, and show that in all cases the photophysics is dominated by excitons and bound multiexciton states. Theoretical modeling is simplest for charge-transfer solids, where stable multiexcitons are verified by femtosecond pump-probe spectroscopy as well as two-photon absorption. In conjugated polymers, pump-probe spectroscopy reveals features due to biexcitons as well as low energy charge-transfer exciton. In both charge-transfer solids and conjugated polymers the biexcitons are bound by electron-electron Coulomb interactions. In contrast, the exciton-exciton binding in molecular aggregates can originate from various sources and the magnitude of the biexciton binding energy in these systems is considerably smaller. No evidence for a biexciton in molecular aggregates have found to date. We present the first experimental evidence for a biexciton in the H-aggregate of a metal- halogen-phthalocyanine. Biexcitons, well-established in conventional semiconductors, are therefore characteristic elementary excitations of several different classes of organic solids.
Exact and high order configuration interaction calculations are done on oligomers of poly(paraphenylene) within a molecular exciton basis to investigate high energy excited states relevant in linear and nonlinear optical absorptions. Two different sets of calculations are done. In the first of these, the exciton basis is constructed out of both delocalized and localized highest bonding and lowest antibonding molecular orbitals of each benzene unit. In the second, only the delocalized frontier orbitals of benzene are retained. Fully pictorial descriptions of all correlated eigenstates are obtained. Comparison of the two sets of results indicates that while the lowest energy eigenstates of polyphenylenes can be obtained within effective linear chain models with very large bond alternation, this may not be true for high energy states of the real materials with finite chain lengths. Low energy photoinduced absorption within the model is to an even parity charge-transfer exciton with analog of which occurs also in linear chain polymers. High energy photoinduced absorption, at least in the short oligomers, is to a new kind of two-exciton state that is absent in linear chain systems.
We show that optical absorption spectra of polyphenylenes can be explained only within theoretical models that explicitly include the Coulomb interaction among the (pi) -electrons. We also show that the dominant effect of substitution on the electronic structure of polyphenylenes within Coulomb correlated models is broken spatial symmetry, while broken charge conjugation symmetry plays a rather weak role. The broken spatial symmetry has a subtle, and weak, effect on the optical absorption spectrum. Consequently, optical absorption spectra of unsubstituted polyphenylenes and the substituted derivatives are nearly identical. Comparison of theoretical and experimental absorption spectra leads to the conclusion that the exciton binding energy in a long chain of poly(para-phenylenevinylene) is about 0.9 eV. Such a large binding energy would be in agreement with nonlinear spectroscopic measurements and pump-probe experiments. However, the present work also indicates that the experimental polymers actually consist of short chains with the chain length distribution peaking at about 10 phenylene units. The gaps between the energy levels above the calculated continuum threshold are much too large for transport to be an intrachain process. Photoconductivity may be predominantly an interchain process, and probably measures the dissociation energy of the exciton which is different from the exciton binding energy.
We present a microscopic mechanism of optical nonlinearity in quasi-one-dimensional semiconductors within the context of rigid band Peierls-extended Hubbard models. A detailed configuration space analysis is done to predict the dominant excitation paths. We show that only two channels contribute to the bulk of the optical nonlinearity, even though an infinite number of channels are possible in principle. Most importantly, these channels involve a virtual two photon excited state whose relative energy should be nearly parameter independent in the infinite chain limit. This would imply that the mechanism of optical nonlinearity, as well as the frequency dependence of the third order optical susceptibility, are also largely parameter independent. This universality is a consequence of the one dimensionality alone and remains valid for arbitrary convex Coulomb interactions. These conjectures are confirmed by exact numerical calculations on finite chains that do very careful analysis of finite size effects.
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