{"id":56,"date":"2021-02-07T14:18:43","date_gmt":"2021-02-07T19:18:43","guid":{"rendered":"https:\/\/sites.temple.edu\/spanogroup\/?page_id=56"},"modified":"2025-03-06T18:06:47","modified_gmt":"2025-03-06T23:06:47","slug":"current-research","status":"publish","type":"page","link":"https:\/\/sites.temple.edu\/spanogroup\/current-research\/","title":{"rendered":"Current Research"},"content":{"rendered":"\t\t
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Figure 1. Variety of packing architectures for conjugated molecules and polymers.<\/h2>\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t
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Figure 2. Simplified view of various excitations.<\/h2>\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/div>\n\t\t\t\t
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Research Description<\/h2>\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t
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\u00a0<\/p>

The Spano Group is primarily interested in modelling the photophysical properties of organic materials, such as molecular aggregates, thin films and crystals comprised of small p-conjugated organic molecules such as tetracene as well as large semiconducting polymers, like polythiophene. Such materials can assemble in a remarkable number of ways \u2013 Figure 1 shows but a few. They can be fashioned into solar cells, light emitting diodes and sensors for a range of commercial applications. The interactions which drive charge and energy transport necessary for device operation can be revealed through the shape of the absorption and photoluminescence spectra, i.e. through spectral \u201csignatures\u201d. The group is particularly interested in how the complex interplay of long-range Coulomb exciton couplings, charge transfer, vibronic coupling, and disorder can be unraveled through the interpretation of well-defined spectral signatures.<\/span><\/p>

In organic materials the simultaneous presence of electronic, vibrational and even photonic degrees of freedom (for example, in optical microcavities) are responsible for the creation of a variety of quasiparticles such as excitons, excimers, polarons, bipolarons, trions and polaritons which contribute in various ways to the optoelectronic response. Figure 2 presents a simplified depiction of some quasiparticle excitations in a polythiophene chain, where each chromophoric unit is a single thiophene unit. In neutral P3HT excitons coupled to intrachain vibrations are responsible for a pronounced vibronic progression in the visible region of the absorption spectrum as well as in the photoluminescence spectrum, details of which are very sensitive to the intermolecular interactions present in aggregates. In oxidized chains, polarons, bipolarons and trions lead to distinctive features in the mid- and near-IR regions of the absorption spectrum, which yield important information about the particle\u2019s wave function.<\/span><\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/section>\n\t\t\t\t

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Fundamental Theory of Excitons and Excimers <\/h2>\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t
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Excitons, which are comprised of a bound electron\/hole pair, are ubiquitous in organic and inorganic semiconductors. In organic molecular aggregates and crystals, the excitons are very tightly bound such that the electron and hole reside on the same chromophore; in this case the electron is viewed as a half-filled LUMO and the hole as a half-filled HOMO. Such excitons are called Frenkel excitons and can effectively transport energy, as required, for example, in a solar cell.<\/p>

The optical response of Frenkel excitons have fascinated scientists since their discovery in the 1930s. A great deal of research has been directed at determining how the properties of such excitons – such as the binding energy, exciton bandwidth, coherence length, etc. \u2013 can be determined through absorption and photoluminescence spectroscopy. The classification of H-aggregates and J-aggregates by Michael Kasha in the 1950s was a significant step forward. Here, the sign of the exciton coupling \u2013 which can then related to the way molecules are packed in the solid phase \u2013 is determined from spectral shifts. In J-aggregates (H-aggregates) the main absorption peak is red-shifted (blue-shifted) relative to an unaggregated monomer.<\/p>

Our unique approach to understanding Frenkel excitons involves vibronic signatures \u2013 those which derive from the coupling between the excitons and vibrations. For a great many conjugate organic molecules the quinoidal-aromatic vibrational mode with an energy of approximately 1400 cm-1<\/sup> is the most strongly coupled, leading to pronounced vibronic progressions in isolated (unaggregated) molecules in solution. However, when molecules interact electronically, the vibronic progression is distorted in a manner which depends on the sign of the electronic coupling. In H-aggregates, the first vibronic peak in the absorption spectrum is attenuated relative to the second peak, by an amount which depends directly on the strength of the coupling. The opposite holds for J-aggregates; here the first peak is enhanced relative to the second peak. Figure 3 shows how the absorption and emission spectra of H- and J-aggregates respond to increasing exciton bandwidth, W.<\/p>

We have also expanded the Frenkel-Holstein approach to encompass intermolecular charge transfer (CT), where electrons and holes can liberate themselves from the confines of Frenkel excitons \u00a0– see Figure 4. Frenkel exciton dissociation into a CT state in which the electron and hole reside on neighboring chromophores, is an important process in molecular p-stacks, for example. The Frenkel-CT Holstein Hamiltonian has been quite successful in describing the photophysics of perylene diimide p-stacks as well as polyacene (tetracene, pentacene) molecular crystals.<\/p>

Current and future research concerns the development of models which include not only the intramolecular vibrations but also intermolecular vibrations which are involved in nonlocal exciton-vibrational coupling. The latter leads to the formation of excimers, in which the excited state is subject to relaxation along an intermolecular coordinate, such as an intermolecular twist, resulting in a generally red-shifted emission spectrum with a significant loss of vibronic structure. One main goal is to accurately identify which types of intermolecular packing leads to efficient excimer formation.<\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t<\/div>\n\t\t\t\t

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Figure 3. Spectral signatures of H- and J-aggregates. Adapted from F.C. Spano, Acc. Chem Res, 43, 429-439 (2010).<\/h2>\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t
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Figure 4. a) pi-stack depicting long-range Coulomb interactions, JC, and short-range CT-mediated interactions, JCT. The Coulomb interactions derive from interacting transition dipoles in b) while CT-mediated interactions derive from a transfer of an electron and hole through a virtual intermediate CT state. te and th are the electron and hole transfer integrals respectively. Adapted from Hestand and Spano, Acc. Chem. Res. 50, 341-350 (2017).


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\u00a0\u00a0\u00a0\u00a0\u00a0 Some Relevant Papers:<\/em><\/h4>