Electronic Structure and Energy Transfer in Solid α‐Sexithienyl

Abstract

Semiconducting conjugated polymers have been widely studied over the last twenty years for their unique electronic and optical properties, which fundamentally reside on the delocalized p-electron structure. In the early stages of the study of conjugated polymers it was thought that the p-conjugated backbone would provide the pathway for coherent electronic transport within the macromolecular unit. Further studies showed, however, that real polymers have a finite distribution of conjugated chain lengths and it became clear that electron transport would ultimately be limited by interchain hopping. Other trapping centers for electron transport are chemical defects of the macromolecular ensemble as well as morphological disorder of the material. It was at this stage that the use of oligomers had the first impulse and was widely considered a valuable route to the understanding of the intrinsic properties of conjugated polymers. In the early nineties the development of organic field effect transistors (FET), based on a-sexithienyl (T 6 ) by Garnier et al. [1,2] and confirmed soon after by Ostoja et al. [3] and Dodabalapur et al. [4], clearly indicated that the availability of chemically pure and ordered solids was the requisite to achieve reasonable working conditions for the FET operation. This finding opened the avenue for the development of the so-called plastic electronics. Disorder moreover is detrimental not only to electric transport but also, to a lesser extent, energy transport. Both charge and energy transport are among the basic steps in the operation of the organic light emitting diodes (OLED) [5,6]. Recombination is followed by energy transfer within the manifold of the excited electronic states leading ultimately to an exciton. The notion of excitons as lowest excitations in conjugated solids is now universally accepted since the emission of OLED is molecular in character as opposed to band-to-band radiative electron-hole recombination. Excitons in molecular crystals were extensively studied over the last decades [7,8]. Anisotropic intermolecular interactions in the ordered molecular solids were shown to allow the formation of collective Bloch electronic excitations (Frenkel excitons) in which the excitation is strongly bound to the molecular entity and therefore is called tight bound exciton as opposed to the weakly bound exciton (Wannier exciton) in inorganic semiconductors. Nevertheless, the excitonic band structure of conjugated oligomers is not known and the relative energy of the lowest singlet Semiconducting Polymers: Chemistry, Physics and Engineering. Edited by G. Hadziioannou and P. F. van Hutten

Size and Shape Controlled Growth of Molecular Nanostructures on Silicon Oxide Templates

“…The height of the island is about 2.5 nm (Figure b), i.e., close to the van der Waals length of the T6 long axis, 2.7 nm. It matches half of the a -axis periodicity in the crystal structure (2.4 nm). , This indicates that the monolayers are growing with the molecules oriented almost normal to the surface, which is consistent with other studies of thiophene oligomer thin film on glass, mica, and silicon oxide. − …”

Energy Transfer Pathways in Light-Harvesting Complexes of Purple Bacteria as Revealed by Global Kinetic Analysis of Two-Dimensional Transient Spectra

“…Therefore under resonance excitation of the X state, the enhancement of the resonance Raman signal will be mostly caused by Herzberg–Teller vibronic coupling. In that case, the selection rules are relaxed if the symmetry of the coupling vibrational mode is the same as the product of symmetries of the weakly allowed resonant state and strongly allowed higher lying excited state . Since S 2 (1B u + ) and X (1B u – ) states have the same symmetry (except for pseudoparity index), the signals observed in 2DVAS at 560 nm excitation are likely due to coupling via totally symmetric a g modes.…”

Femtosecond Differential Transmission Spectroscopy of α-Sexithienyl Thin Film at Low Temperature

“…A broad range of experimental techniques have been used, − and devices including thin film transistors − and polarized light-emitting diodes have been fabricated. Recently, highly ordered T6 thin films have become available through vacuum sublimation, which allows control of the morphology via the deposition parameters. − …”

“…The lowest excited singlet transition has been assigned as parallel to the long T6 molecular axis , and a Herzberg−Teller mechanism controls the vibronic energy distribution. , Recently, the formation of exciton bands and Davydov splitting of the molecular levels, caused by intermolecular interactions, has been observed in T6 in the solid state. , The lowest one-photon allowed transition is the lowest Davydov component a u , which is coincidentally degenerate with the one-photon forbidden transition a g . The overall Davydov splitting is 0.32 eV (2600 cm -1 .…”