Primary optical excitations and excited-state interaction energies in sexithiophene

Abstract

Based on a unique combination of angle-resolved transmission spectroscopy and transmission data at high pressure, we identify the primary photoexcitations and the relevant excited-state interaction energies in a sexithiophene crystal. Optical excitations include charge-transfer excitons and Davydov polaritons. By extrapolation, we predict that in sexithiophene at hydrostatic pressures above 180 kbar, intermolecular excitations are lower in energy than intramolecular ones. The results are representative for a wide class of -conjugated molecular semiconductors because ͑1͒ the pertinent interaction energies and lengths scales are nearly identical and ͑2͒ published data on different molecules are consistent with our interpretation. DOI: 10.1103/PhysRevB.66.113102 PACS number͑s͒: 78.40.Me, 78.55.Kz, 78.66.Qn Linear -conjugated molecules, such as oligothiophenes, oligophenylenes, and oligophenylene-vinylenes, are attracting increasing attention for their use in electronic and optoelectronic devices.1 Despite the high technological interest, the knowledge of their fundamental electronic and optical properties remains inadequate for various aspects. Our interest is to address to the nature and interacting energies of the primary photoexcitations, for which long-lasting and unresolved controversies exist. [2][3][4][5][6] In the crystal, the lowest molecular excitation gives rise to as many intramolecular exciton bands as the number of nonequivalent molecules in the unit cell. The energy difference between the lowest and highest exciton transitions, the socalled Davydov splitting (E DS ), represents a fundamental energy scale, which provides an estimate of the intermolecular interaction strength in the excited state. Experimental optical spectra feature, however, complex patterns, which have precluded an unequivocal evaluation of E DS .In crystals, excitons with small wave vector k couple to photons with the same quasimomentum to yield a new particle called polariton. 7 The latter describes the photonexciton dynamics when the interaction energy (E EPI ) with photons exceeds any line broadening ͑strong-coupling regime͒. The polariton concept is the extension of the weakcoupling limit case, in which photons and delocalized excitations of matter are treated as independent particles. In linear -conjugated molecules, the lowest -* transition exhibits a very large oscillator strength (ӷ1). Therefore, we expect that polaritons, rather than bare excitons, are the primary photoexcitations in the crystal. The ensuing optical response can differ substantially from the one predicted in the weak-coupling regime.Besides intramolecular Frenkel excitons, crystals also support excitations for which the bound electron and hole reside on different molecules ͓charge-transfer excitons ͑CTE͔͒. The energy difference E B between these two kinds of excitations provides an estimate of the intermolecular interaction energy between electrons and holes. Positive ͑nega-tive͒ E B implies that stable exitations have an intramolecular ͑intermolecular...

Absorption and Emission in Quaterthienyl Thin Films

“…[6] Recent experiments of Loi et. al [7] interpret peak A 2 as a charge transfer transition, in conflict with our Frenkel exciton interpretation. Moller et al [26] also argue against a charge transfer origin based on an analysis of their electro-absorption measurements.…”

“…The interaction sums can be partitioned as J(k) + 4pm(l´k à ) 2 /e ¥ m 0 , where m is the number of molecules per unit cell of volume m 0 , and k à is a unit vector along k. [21] Hence, in the long wavelength limit the energy of the longitudinal exciton (l´k à = l) exceeds that of the transverse exciton (l´k à = 0) by 4pm(l´k à ) 2 /e ¥ m 0 which is » 0.8 eV for aT6. [7] An orientationally dependent DS has recently been measured in aT6 crystals [7] as well as highly ordered sexiphenyl films, [23] in both cases slightly exceeding 1 eV when k is close to parallel with the long molecular axis. A direct measurement of the strong H-peak in the crystal limit is difficult in transmission experiments because of the large extinction coefficient.…”

“…Many methods for the synthesis of inorganic nanotubes have been based on high-temperature processes. These include chemical vapor transport, [4] sol±gel process, [5] arc-discharge, [6] solvothermal treatment, [7] and oxide nanowhisker growth. [8] Low-temperature routes such as templated synthesis have also been attempted, [9,10] but it has proved difficult to form single-crystalline nanotubes.…”

Single‐Crystalline Scroll‐Type Nanotube Arrays of Copper Hydroxide Synthesized at Room Temperature

“…These include chemical vapor transport, [4] sol±gel process, [5] arc-discharge, [6] solvothermal treatment, [7] and oxide nanowhisker growth. [8] Low-temperature routes such as templated synthesis have also been attempted, [9,10] but it has proved difficult to form single-crystalline nanotubes.…”

Polariton Effects in Molecular Crystals Studied by Classical and Quantum-Electrodynamic Approaches

“…This situation has changed recently because of the current interest in the physics of oligothiophene crystals, where the lowest electronic transition is very intense. Polaritons were invoked to explain some features of the absorption and electroabsorption spectra of α-sexithiophene [5,6]. A detailed investigation based on classical electrodynamics falsified the newly introduced interpretational paradigm for electroabsorption spectroscopy [7,8], rooted in an erroneous understanding of polariton properties.…”